rt_060_01 guide for cfrp systemsx

TECHNICAL RECOMMENDATION Guide for the Design of Externally Bonded

FRP Systems for Strengthening TECHNICAL DEPARTMENT

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DRIZORO S.A.U. C/Primavera, 50-52. 28850 Torrejón de Ardoz-Madrid (SPAIN) Tel./Phone: +34 916766676 – Fax: +34 916776175

e-mail: [email protected] – Web site: www.drizoro.com

nº 6003176 / 6003176-MA



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INDEX

1. INTRODUCTION …………………………………………………………………………………. 1

1.1. BACKGROUND...……………………………………………………………………………… 1

1.2. SCOPE AND LIMITATIONS …....………………………………………………… …………. 3

2. MATERIALS FOR FRP STRENGTHENING 4

2.1. WET LAY-UP SYSTEMS .…………………….………………………………….…………… 6

2.1.1. DRIZORO® WRAP System 2.1.2. DRIZORO® CARBOMESH System 2.1.3. DRIZORO® WRAP QUADRIAXIAL System

2.2. PRECURED SYSTEMS.................................................................................................... 8

2.2.1. DRIZORO® COMPOSITE System 2.2.2. DRIZORO® CARBOROD System

2.3. PRIMER ........................................................................................................................... 9

2.3.1. MAXPRIMER® C

2.4. LEVELLING PUTTY……….…………………………………………………………………. 10

2.4.1. MAXEPOX® CP

2.5. RESINS FOR WET LAY-UP SYSTEMS…………………………………………………… 11

2.5.1. MAXEPOX® CS 2.5.2. CONCRESEAL® CARBOFIX

2.6. ADHESIVES FOR PREFORMED SYSTEMS..........…………………………………………… 13

2.6.1. MAXEPOX® CARBOFIX



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3. BASIS OF DESIGN ……………………………………………………………………………… 14

3.1. BASIC REQUIREMENTS……………………………………………………………………... 14

3.2. DURABILITY REQUIREMENTS ………………………………………………………… ...... 15

3.3. GENERAL PRINCIPLES FOR STRENGTHENING DESIGN….. .………………………… 15 3.3.1. Limit states 3.3.2. Ultimate Limit States (ULS) 3.3.3. Serviciability Limit State (SLS) 3.3.4. Durability limit state

3.4. ACTIONS……………………………………………………………………………………….. 18 3.4.1. Partial factors and design loads 3.4.2. Combination of actions

3.5. MATERIALS ……………………………………………………………………………………. 19 3.5.1. Concrete and steel. Design strength and part ial safety coefficients 3.5.2. Partial coefficient for FRP materials

3.6. STRENGTHENING LIMITS IN CASE OF FIRE ………...………………………………….. 21

4. FLEXURAL STRENGTHENING ……………………… ……………………………………. 23

4.1. INITIAL SITUATION..……………………………………………………………………… ….. 23

4.2. ANALYSIS AT FLEXURAL ULTIMATE LIMIT STATE (ULS ) …………………………… 24 4.2.1. Strain regions

4.3. FLEXURAL CAPACITY FOR SECTIONS WITH REINFORCED WITH FRP.................

27 4.3.1. Failure by peeling-off of composite material 4.3.1.1. Mode 1 & 2. Peeling-off at the end anchorage and at flexural cracks 4.3.1.2. Mode 3. Debonding by diagonal shear cracks 4.3.1.3. Mode 4. Debonding by irregularities and roughness of the concrete surface 4.3.2. Design of flexural anchorage length

4.4. ANALYSIS AT SERVICEABILITY LIMIT STATE (SLS……… ……… …………………... 34 4.4.1. Service stress limitation 4.4.2. Service train limits (Defection control) 4.4.3. Cracking limit state (Crack control)

4.5. DUCTILITY…. ………………………………………………………………………………….. 38

5. SHEAR STRENGTHENING……………………...………………………………………….... 39

5.1. SHEAR CAPACITY FOR SECTIONS REINFORCED WITH FR P…………………..……

40 5.1.1. Caculus of V u1 5.1.2. Calculus of V u2



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6. CONFINEMENT STRENGTHENING ……………………....……………………………. 42

6.1. AXIAL CAPACITY FOR FRP-CONFINED ELEMENTS SUBJE CT TO COMPRESSIVE FORCES…………………………………………………………………….

42

6.1.1. Confinement lateral pressure

6.1.1.1. Circular section: Coefficients kH, kV, ρR

6.1.1.2. Rectangular section: Coefficients kH, kV, ρR

6.1.1.3. Coefficient by continuous wrapping: kα

6.1.1.4. Design strain for FRP: εRd,conf

6.2. DUCTILITY OF FRP-CONFINED ELEMENTS UNDER COMBI NED BENDING AND AXIAL LOAD …………………………………………………………………………….

48

7. INSTALLATION, MONITORING AND CONTROL FOR THE APP LICATION......... 49

7.1. PREPARATION OF SUBSTRATE ……………………………………………………… ….. 49

7.2. WET LAY-UP SYSTEMS: SHEETS.…………………………………… ………… ………… 50 7.2.1. Primer application 7.2.2. Putty application 7.2.3. Cutting of carbon fibre sheets 7.2.4. Under/overcoat resin application, and carbon fibre sheet applications 7.2.5. Top-coating application

7.3. WET LAY-UP SYSTEMS: FABRICS....…………………………………. ………………… 54 7.3.1. Cutting of the carbon fibre fabrics 7.3.2. Cement-based application and carbon fibre fa bric application 7.3.3. Top-coating application

7.4. PRE-CURED SYSTEMS: LAMINATES ................. ......................................................… 56 7.4.1. Primer application 7.4.2. Cutting of pre-cured laminates 7.4.3. Epoxy structural adhesive application and pr e-cured laminate pieces application

7.5. CUT-IN SYSTEMS: COMPOSITE BARS.........………………… …………………………..

58 7.5.1. Cutting of carbon fibre pre-cured bars 7.5.2. Epoxy structural adhesive application and pr e-cured bars application

7.6. QUALITY CONTROL DURING INSTALLATION........... .....……………………………….. 59 7.6.1. Semi-destructive tests

7.6.1.1. Pull-off tests

7.6.1.2. Shear tearing test

7.6.2. Non destructive tests

8. DESIGN EXAMPLE………………………………………………………………….......... 49

8.1. FLEXURAL STRENGTHENING OF A RECTANGULAR REINFO RCED CONCRETE BEAM ………………………………..…………………………………………………………..

49



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1. INTRODUCTION

DRIZORO S.A.U., founded in 1977, is, today, a leading independent Spanish company, within the Industry of Construction Chemicals in the world.

Its international character makes it present in all five continents and in over forty countries

where quality, price and service is crucial to maintain these markets, among which are some of the most developed of the world as America, Sweden, United Kingdom, China or Australia, which in 1988 earned her the "Export Award of the Chamber of Commerce and Industry of Madrid."

For over thirty years, DRIZORO S.A.U. has developed its industrial activity based on research,

development, manufacture and marketing of building products that has allowed the standard in the sector in areas as significant as the waterproofing and restoration and rehabilitation of concrete structures.

Its strong commitment to quality, safety and the environment led to the introduction in 1997 of a

system of quality management based on the criteria of the UNE-EN-ISO 9002 standard, thus beginning a process of continuous improvement which has led to the implementation of Integrated Management System, supported by the award of the Certificate of Quality based on guidelines established in the UNE-EN-ISO 9001 and Environmental Management Certificate according to UNE-EN-ISO 14001, commitment to maintain in the future to ensure quality standards and services in accordance with the requirements of the market and society.

On the other hand, as a core value is personal attention, technical advice and training of

applicators and customers, allowing us to reach the maximum quality and guarantee the implementation of our systems to the final customer.

The continuing commitment to research and development of new products and systems, we can

offer solutions to market high quality and latest technology, backed by a proven and tested experience under adverse conditions throughout the entire world geography.

In this spirit of technological innovation, technical support and customer service, it has been

created this "Guide for the Design of Externally Bonded FRP Systems for Strengthening", in order to provide the best tool for understanding and sizing structural strengthening based on the application carbon fibre composite.

1.1. BACKGROUND

The strengthening through the use of carbon fibres is carried implementing with great success

for over 25 years. At first, the high costs of manufacturing and marketing led to its application was directed primarily to markets where the use of high technology, was part of the value added product.

Carbon fibre has been used with great success in making materials of very high mechanical

stresses and maximum lightness. Since its implementation in the aerospace industry, to the automotive industry, through its

implementation in the manufacture of sporting goods and consumer products, carbon fibre has always



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been very successful, which have led to significant improvements and increases in production and a large drop in manufacturing costs.

All these factors combined with the large number and diversity of projects and research

conducted worldwide in search of new building materials capable of bearing the greatest challenges that each day requires actual engineering, has made the carbon fibre becomes a real and competitive alternative in the strengthening and design of structures.

On the other hand, the high costs of labour and the need for actions on structures in use and

rehabilitation makes it advisable to use this type of strengthening. With the implementation of strengthening by applying carbon fibre sheets, you get to save a

portion of the costs of labour and aids in addition to allowing implementation of strengthening in structures in use, quickly and easily.

The high strength carbon fibre, combined with its lightness and ease of installation, make it an

alternative to traditional metal systems strengthening structures, with the added advantage of eliminating the problem of corrosion.

Likewise, the use of minimum material thickness for strengthening allows not alter the

dimensions of the structural elements, preserving and maintaining the standards of design and functionality of the original structure.

The strengthening system based on the use of carbon fibre sheet structure, was developed

after years of research in Japan in 1984. After considering all possible forms, including sheets, woven and moulded shapes, structural engineers concluded that the carbon fibre in its simplest form, a sheet of unidirectional carbon fibre, provided the best and most reinforced flexible as possible.

The system of carbon fibre strengthening, namely DRIZORO WRAP, was the first in the world

to be used for strengthening of structures under construction. Since then it has been commonly used in Asia, Europe, USA and Canada among others. The system has been applied to increase the flexural capacity of beams, slabs and columns, shear in beams, columns and walls, pillars compression, and to improve the ductility of columns.

DRIZORO WRAP products and application process becomes a cost-effective alternative to

conventional systems repair and strengthening of existing concrete structures. The use of carbon fibre reinforced polymer reinforcing structures is relatively new. Some rules

for the calculation of structures have been built with external strengthening calculation of carbon fibre order to standardize the design of such strengthening.

1.2. SCOPE AND LIMITATIONS This Guide is based on extensive research on existing technical literature, test results externally

bonded FRP (Fibre Reinforced Polymer) system. And a review of the design recommendations developed by the International Federation for Structural Concrete (FIB), the American Concrete Institute (ACI) and the National Research Council of Italy (CNR) and embodied in the following documents:



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• FIB, Technical Report, bulletin 14: Externally bonded FRP strengthening for RC structures, July 2001.

• ACI 440.2R-02 Guide for the Design and Construction of Externally Bonded FRP Systems

for Strengthening Concrete Structures. 11 de Julio de 2002 • CNR-DT 200/2004: Guide for the Design and Construction of Externally Bonded FRP

Systems for Strengthening Existing Structures. Italian Design Guidelines. Roma 13 de Julio de 2004.

This Guide aims to provide an overview of the strengthening of concrete structures by using

carbon fibre, by explaining the basic equations for calculating the most common cases of flexural, shear and axial strengthening.

DRIZORO, S.A.U.



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2. MATERIALS FOR FRP STRENGTHENING

This chapter describes the physical and mechanical properties of composite materials composed of carbon fibre, and those features that affect its use in reinforcing concrete structures. It also shows the effects of factors such as use and time, temperature and humidity on the properties of the composites.

Strengthening systems based on composite materials are marketed in a variety of forms (unidirectional, bidirectional, quad-directional fibre sheets, preformed laminates and bars, etc.). Factors such as fibre volume, fibre type, resin type, fibre orientation, three-dimensional effects, and quality control during production, play an important role in the characteristics of a composite material. Strengthening systems with composite materials can be classified according to how markets and their method of installation. The backup system should be selected based on the successful transfer of structural loads and the ease and simplicity of implementation. Application systems of composite materials more common, form of execution, are:

• Wet lay-up Systems. • Pre-impregnated Systems (Prepeg Systems) • Preformed Systems (Precure Systems)

The Wet lay-up systems consist on wet dry tissue sheets of carbon fibre unidirectional or multi-

directional which are impregnated with a resin at job site. Resin saturation, together with the primer, a compatible levelling putty, if requiered, are used to bond sheets of carbon fibre fabric to the concrete surface. Thus, wet lay-up installation systems are saturated formed and cured in place wherein they are applied and, in this sense, they are analogous to a concrete poured on site. The three most common wet lay-up installation systems are:

• Dry tissue sheets of unidirectional carbon fibre, where resistant fibres are arranged in the

same predominantly. • Dry tissue sheets multidirectional carbon fibre, which has become resistant fibres oriented in

at least two plane directions. • Dry tissue dry carbon fibre on spools that wind mechanically to the concrete surface. The

dry fabric is impregnated with resin in the same place during the winding operation.

The Prepeg Systems consist of pre-impregnated unidirectional fibre sheets or multidirectional fibre sheets with a saturating resin in the manufacturer's facilities. Pre-impregnated systems adhere to the concrete surface with or without application of additional resin, depending on the specific system requirements. Pre-impregnated systems are saturated at the factory but the curing is carried out on site, such as wet lay-up systems installation, usually by additional heating for curing. The three most common types of pre-impregnated are:

• Pre-impregnated sheets of unidirectional carbon fibre, where resistant fibres are arranged in

the same predominantly.



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• Pre-impregnated sheets of multi-directional carbon fibre, where resistant fibres are arranged in at least two oriented plane directions.

• Pre-impregnated carbon fibre fabric on reels that are wound mechanically to the concrete surface. Thus dry fabric is impregnated with resin in the same place during the winding operation.

Finally, Preformed Systems consist of a wide variety of pieces with different shapes made on

the manufacturer facilities. In general, an adhesive is used, along with a primer and a compatible levelling putty, if requiered, to adhere the preformed elements of concrete surface. Preformed systems are analogous to precast concrete. The three most common preformed systems are:

• Plate laminates of unidirectional fibres, supplied, usually rolled into thin strips. • Perform bars of unidirectional fibres supplied individually or in rolls or multi-way mesh.

• Special shapes “Shirts, L, and others forms” supplied as a preformed longitudinal segments

that can be opened and placed around columns or other concrete elements



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2.1. WET LAY-UP SYSTEM

2.1.1. DRIZORO® WRAP DRIZORO WRAP is a system for repair and strengthening of concrete structures based on

flexible sheets of unidirectional high strength, high modulus of elasticity carbon fibres, and three products based on epoxy resins specially designed to adhere to the above fibres to the concrete substrate. Thus, the system is to generate "in situ" laminar system perfectly fitted to the geometric characteristics and mechanical characteristics of the element to be reinforced. Its large capacity and light resistance, as well as its versatility and ease of application provide the ideal characteristics to repair damaged structures and/or strengthen existing structures or changes of use or project execution errors. It is available in three different types of carbon fibre sheets to fit the requirements of each case: DRIZORO® WRAP 200, DRIZORO® WRAP 300 and DRIZORO ® WRAP HM.

Table 2.1.- Technical Data for DRIZORO®WRAP SHEET

Name

CARBON FIBRE SHEET DRIZORO® WRAP 200

DRIZORO® WRAP 300

DRIZORO® WRAP HM

Appearance Sheet composed of black unidirectional carbon fibres Thickness (mm) 0,111 0,167 0,163 Weight (g/m2) 200 300 300 Tensile modulus (N/mm2) 230.000 440.000 Tensile strength at break (N/mm2) 4.500 4.400 3.360 Elongation at break (%) 1,9 0,76 Guaranteed tensile strength (N/mm2) 3.400 2.400 Guaranteed rupture strain (%) 1,5 0,55

2.1.2. DRIZORO® CARBOMESH System DRIZORO® CARBOMESH is a carbon fibre fabric of high tensile strength and elastic modulus,

arranged in two orthogonal directions (0º / 90º), designed for repair and strengthening of reinforce concrete, masonry, brick, wood and steel. It is bonded to the substrate to be strengthened with the CONCRESEAL® CARBOFIX cement-based mortar of low modulus or by using the MAXEPOX® CS high performance epoxy-based adhesive, forming a laminar composite "in situ" system perfectly adhered, and adapted to the substrate geometry and mechanical characteristics of the element to be strengthened. It is available in two weights to fit the requirements of each case: DRIZORO® CARBOMESH 160 y 210.



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Table 2.2.- Technical Data for DRIZORO® CARBOMESH

Name CARBOMESH 160 CARBOMESH 210

Appearance and colour Fabric composed of black bi-directional carbon fibres

Equivalent thickness for fabric 0/90º (mm) 0,04/0,04 0,06/0,06 Weight (g/m2) 160 ± 5% 210 ± 5% Tensile modulus (N/mm2) 230.000 Tensile strength at break (N/mm2) 4.900 Elongation at break (%) 2,1 Guaranteed tensile strength (N/mm2) 3.400 Guaranteed rupture strain (%) 1,5

2.1.3. DRIZORO® WRAP QUADRIAXIAL System DRIZORO® WRAP QUADRIAXIAL is a carbon fibre fabric of high tensile strength and elastic

modulus, arranged in four orthogonal directions (0º/45º/90º/135°), designed for repair and strengthening of reinforce concrete, masonry, brick, wood and steel. It is bonded to the substrate to be strengthened by using a high performance epoxy-based adhesive, forming a laminar composite "in situ" system perfectly adhered, and adapted to the substrate geometry and mechanical characteristics of the element to be strengthened.

Table 2.3.- Technical Data for DRIZORO® WRAP QUADRIAXIAL

Name WRAP QUADRIAXIAL 380

Appearance and colour Fabric of black four-directional carbon fibre Equivalent thickness for fabric 0º/45º/90º/135° (mm) 0,053/0,053/0,053/0,053

Weight (g/m2) 380 ± 5% Tensile Modulus (N/mm2) 229.000 Tensile strength at break (N/mm2) 5.100 Elongation at break (%) 2,26 Guaranteed Tensile Strength (N/mm2) 3.400 Guaranteed rupture strain (%) 1,5



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2.2. PRECURED SYSTEMS

2.2.1. DRIZORO® COMPOSITE System DRIZORO® COMPOSITE is a pre-formed laminate composed of unidirectional carbon fibres,

embedded in a epoxy resin matrix and conformed by pultrusion process. Its high tensile strength, lightness and easy to use, provides and efficient system for strengthening of reinforce concrete, masonry, brick, wood and steel subject to tensile stress due to flexural loads. It is bonded to the substrate to be strengthened with the MAXEPOX® CARBOFIX structural epoxy-based adhesive.

Table 2.4.- Technical Data for DRIZORO® COMPOSITE

Name COMPOSITE 1405 COMPOSITE 1410

Appearance and colour Laminate of black unidirectional carbon fibre

Width (mm) 50 100 Thickness (mm) 1,4 Content of carbon fibre 68% Tensile modulus (N/mm2) 165.000 Tensile strength at break (N/mm2) 2.200 Elongation at break (%) 1,30 Guaranteed tensile strength (N/mm2) 2.000 Guaranteed rupture strain (%) 1,2

2.2.2. DRIZORO® CARBOROD System DRIZORO CARBOROD is a circular rod composed of unidirectional on carbon fibres

embedded in a epoxy resin matrix and conformed by pultrusion process. Its high tensile strength, lightness and easy to use, provides and efficient system for strengthening of reinforce concrete, timber and masonry structures. It is bonded to the substrate to be strengthened with the MAXEPOX® CARBOFIX structural epoxy-based adhesive, or with the MAXFIX E anchoring resin.

Table 2.5.- Technical Data for DRIZORO® CARBOROD

Name

CARBOROD 308 310 312

Appearance Black rod with rough finish surface Diameter (mm) 8 10 12 Cross-section area (mm2) 50 78 110 Tensile modulus (N/mm2) 150.000 Tensile strength at break (N/mm2) 2.000 Elongation at break (%) 1,33 Guaranteed tensile strength (N/mm2) 1.800 Guaranteed rupture strain (%) 1,2



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2.3. PRIMER 2.3.1. MAXPRIMER® C

MAXPRIMER® C is a two-component, solvent-free colourless epoxy-based liquid formula, specially designed for priming and consolidation of substrates prior to application of strengthening system based on composite.

It is available in two different versions: • MAXPRIMER® C –S. For applications with temperatures from 15 ºC to 35 ºC. • MAXPRIMER® C -W . For applications with temperatures from 5 ºC to 15ºC.

Table 2.6.- Technical Data for MAXPRIMER® C

Name MAXPRIMER® C -S MAXPRIMER® C -W Optimum temperature range (°C) 15 – 35 5 – 15

Appearance and colour Main agent Pale yellow liquid Hardener Brown liquid

A:B mixing ratio (by weight) 4:1

Specific gravity 25ºC Main agent 1,15 ± 0,01 1,13 ± 0,01 Hardener 0,96 ± 0,01 0,97 ± 0,01

Pot life (minutes) 30 ºC 90 - 23 ºC 130 18 15 ºC > 180 40 5 ºC - 130

Tack-free time (hours) 30 ºC 8,0 - 23 ºC 11,0 3,0 15 ºC 17,0 7,0 5 ºC - 15,0

Adhesive strength to concrete (N/mm2) >2,5 (breaks concrete)



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2.4. LEVELLING PUTTY 2.4.1. MAXEPOX® CP

MAXEPOX® CP is a two-component, solvent-free, epoxy-based liquid formula, specially designed for patching and levelling of voids, honeycombs, small cracks, and other unevenness of substrates prior to application of strengthening system based on composite.

It is available in two different versions: • MAXEPOX® CP –S. For applications with temperatures from 15 ºC to 35 ºC. • MAXEPOX® CP -W. For applications with temperatures from 5 ºC to 15ºC.

Table 2.7.- Technical Data for MAXEPOX® CP

Name MAXEPOX® CP -S MAXEPOX® CP -W Optimum temperature range (°C) 15 – 35 5 – 15

Appearance and colour Main agent White putty Hardener Black putty





Adhesive strength to concrete (N/mm2) >2,5 (breaks concrete)



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2.5. RESIN FOR WET LAY-UP SYSTEMS 2.5.1. MAXEPOX® CS

MAXEPOX® CS is a two-component, solvent-free epoxy-based liquid formula, specially designed for bond and saturating the carbon fibres for wet lay-up systems such as DRIZORO® WRAP, DRIZORO® CARBOMESH and DRIZORO® WRAP QUADRIAXIAL .

It is available in two different versions: • MAXEPOX® CS –S. For applications with temperatures from 15 ºC to 35 ºC. • MAXEPOX® CS -W. For applications with temperatures from 5 ºC to 15ºC.

Table 2.8.- Technical Data for MAXEPOX® CS

Name MAXEPOX® CS -S MAXEPOX® CS -W Optimum temperature range (°C) 15 – 35 5 – 15

Appearance and colour Main agent Green and thixotropic liquid Hardener Brown liquid





Curing time (days) 30 ºC 5 - 23 ºC 7 5 15 ºC 14 7 5 ºC - 14

Tensile strength (N/mm2) > 29 Flexural strength (N/mm2) > 39 Tensile shear strength (N/mm2) > 9,8 Adhesive strength to concrete (N/mm2) >2,5 (break concrete)



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2.5.2. CONCRESEAL® CARBOFIX

CONCRESEAL® CARBOFIX is a one-component, polymer-modified cement-based mortar with very low tensile modulus, specially designed for bonding of composite systems: carbon fibres, glass fibres or vegetable fibres.

Table 2.9.- Technical Dates for CONCRESEAL® CARBOFIX

Appearance and colour Grey powder

Maximum size, (mm) < 0,5 Density for powder, (g/cm3) 1,35 ± 0,1 Density for mixed and fresh mortar, (g/cm3) 1,85 ± 0,1 Mixing water, (%, by weight) 19 ± 1 Application and curing conditions Minimum application temperature for substrate and ambient, (ºC) > 5 Open time for mixture at 20 ºC and 50 % R.H., (min) 20 – 30 Initial/Final setting time at 20 ºC y 50 % R.H., (h) 3 – 4 / 5 - 6 Curing time at 20 ºC and 50 % R.H., (d) 28 Cured product characteristics Compressive strength at 28 days, EN 12190 (N/mm2) ≥ 20 Flexural strength at 28 days, EN 196-1 (N/mm2) ≥ 5,5 Tensile modulus at 28 days, EN 13412 (N/mm2) < 10.000 Adhesion to concrete at 28 days, EN 1542 (N/mm2) ≥1,5 Reaction to fire, EN 13501-1 (Class) A1



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2.6. ADHESIVES FOR PREFORMED SYSTEMS

2.6.1. MAXEPOX® CARBOFIX MAXEPOX CARBOFIX is a two-component, solvent-free, epoxy-based structural adhesive,

specially designed for bonding of the preformed composite laminates such as DRIZORO ® COMPOSITE or the carbon fibre bars such as DRIZORO ® CARBOROD .

Table 2.10.- Technical Data for MAXEPOX CARBOFIX

Product characteristics MAXEPOX CARBOFIX

Appearance and colour Component A White putty Component B Black putty

A:B mixing ratio (by weight) 2:1 Solid content (%, by weight) 100 Appearance and colour for A+B Grey putty Density for mixture A+B at 20 °C (g/cm 3) 1,74 ± 0,1 Application and curing conditions Temperature/Relative Humidity of application (°C / %) > 10 / < 85 Open time (minutes) 30 ºC 15

20 ºC 40 10 ºC 55

Tack-free time at 20 ºC (hours) 5 - 8 Total cuing time at 20 ºC (days) 7 Cured product Characteristics Compressive strength at 7 days and 20ºC (MPa) 80 Flexural strength at 7 days and 20ºC (MPa) 60 Tensile strength at 7 days and 20ºC (MPa) 30 Elongation at break point at 7 days and 20ºC (%) 0,39 Compressive modulus at 7 days and 20ºC (MPa) 4.450 Flexural modulus at 7 days and 20ºC (MPa) 7.750 Adhesion on concrete at 7 days and 20ºC (MPa) > 2 Coefficient of lineal thermal expansion (PPM/K) 62 ± 11 Water absorption (%) 0,08 Hardness Shore D 80



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3. BASIS OF DESIGN

The subject of this chapter regards FRP strengthening of existing reinforced and pre-stressed structures, as well as, masonry structures for which building code requirements are not met. The same principles also apply to existing structures made out of steel and timber, not included in this document. The following is assumed:

• The choice and the design of the strengthening system are made by an appropriately

qualified and experienced engineer.

• The installation phase is carried out by personnel having the appropriate skills and experience.

• Proper supervision and quality control is provided during installation.

• Construction materials are used as specified in the following.

The FRP strengthening system shall be designed to have appropriate strength, and meet serviceability and durability requirements. In case of fire, the strength of the selected FRP system shall be adequate to the required period of time. The FRP strengthening system shall be located in areas where tensile stresses are to be carried out. FRP composites shall not be relied upon to carry compressive stresses. 3.1. BASIC REQUIREMENTS

Design of FRP strengthening system shall be performed in compliance with the following principles:

• The risks to which the structure can be subjected shall be accurately identified, removed or attenuated.

• The strengthening configuration shall not be very sensitive to the above risks.

• Strengthening systems shall survive the occurrence of acceptable localized damages.

• Strengthening systems collapsing without warning shall be avoided.

• The above defined basic requirements can be considered met if the following are satisfied:

• Suitable materials are chosen.

• Design is properly performed, with a careful choice of the construction details.



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• Quality control procedures are defined for design and construction relevant to the particular project.

If FRP strengthening concerns structures of historical and monumental interest, a critical

evaluation of the strengthening technique is required with respect to the standards for preservation and restoration. The actual effectiveness of the strengthening technique shall be objectively proven, and the adopted solution shall guarantee compatibility, durability, and reversibility.

3.2. DURABILITY REQUIREMENTS

A strengthening application shall be designed such that deterioration over the design service life of the strengthened structure does not impair its performance below the intended level. Environmental conditions as well as the expected maintenance program need to be carefully addressed

Durability is of fundamental relevance and all the operators involved in the FRP-based

strengthening processes shall pursue such requirement. To ensure durability to FRP strengthened members the following shall be taken into account:

• Intended use of the strengthened structure.

• Expected environmental conditions.

• Composition, properties, and performance of existing and new materials.

• Choice of the strengthening system, its configuration, and construction details.

• Quality of workmanship and the level of control.

• Particular protective measures (e.g., fire or impact).

• Intended maintenance program during the life of the strengthened structure.

Special design problems (regarding environmental issues, loading, etc.) shall be identified at the design stage to evaluate their relevance for a durability point of view, assign proper values of the conversion factors and take the necessary provisions for protection of the adopted FRP system

3.3. GENERAL PRINCIPLES OF THE STRENGTHENING DESIGN 3.3.1. Limit states Design with FRP composites shall be carried out both in terms of Serviceability Limit state (SLS) and Ultimate Limit State (ULS), as defined by the current building code.

Limit states are defined as those situations for which, when exceeded, shall be considered that the structure does not meet any of the functions for which it was designed.



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Limit States are classified as: • Ultimate Limit State • Serviceability Limit States

• Durability Limit State Verification that a structure does not exceed any of the above limit states defined in any of the

situations of the project, considering the design values of actions, of the characteristics of the material and geometric data.

The procedure to verify a limit state, is to deduce, on the one hand, the effect of actions on the

structure or part of it and, secondly, the response of the structure to the limit situation under study. Limit State will be ensured if verified, with a sufficient level of reliability that the structural response is not less than the effect of the applied actions. To determine the effect of actions must be considered the combined design actions and the geometric data and a structural analysis should be performed.

Verification of one limit state may be omitted provided that sufficient information is available to prove that it is satisfied by another one. 3.3.2 Ultimate Limit States (ULS)

The name of Ultimate Limit States includes those that cause structural failure, loss of balance, collapse or breakage of the same or part of it. As Ultimate Limit State should be considered those due to:

• Failure by excessive plastic deformation, breakage or loss of stability of the structure or part

of it.

• Loss of balance in the structure or part of it, considered as a rigid body

• Failure to progressive deformation or cracking under repeated loading.

Structures and structural members strengthened with FRP shall be designed to have design strength, Rd, at all sections at least equal to the required strength, Ed, calculated for the factored load and forces in such combinations as stipulated in the current building code. The following inequation shall be met:

Rd ≥ Sd (Eq. 3-1)

Where: Rd: Design value of the structural response. Sd: Design value of action effects.

For the evaluation of Equilibrium Limit State should be satisfied the condition:



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Ed,stab ≥ Ed,destab (Eq. 3-2)

Where: Ed,stab: Design value of the effects of stabilizing actions. Ed,destab: Design value of the effects of destabilizing actions. Fatigue Limit State is related to the damage that a structure may suffer as a result of repeated variables solicitations.

When verifying the Fatigue Limit State shall satisfy the condition:

RF ≥ SF (Eq. 3-3) where: RF: Design value of the fatigue resistance SF: Design value of the effects of fatigue actions 3.3.2. Serviceability Limit State (SLS)

The name of Serviceability Limit States includes all those states in which the requirements of

functionality comfort or appearance required are not satisfied. When verifying the serviceability limit states shall satisfy the condition:

Cd ≥ Ed (Eq. 3-4) where: Cd: Permissible limit value for the limit state to be verified (strain, vibration, crack width, etc.). Ed: Design value of the effect of actions (stress, vibration level, crack width, etc.). 3.3.3. Durability limit state

Durability limit state is produced by chemical and physical actions, different to loads and actions of the structural analysis, which can degrade the concrete or reinforcement to unacceptable limits.

Verifying the durability limit state is to verify that the following condition is satisfied:

tL ≥ td (Eq. 3-5) where: tL: Time needed for the agent to produce an aggressive attack or significant degradation. td: Design value of performance life.



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3.4. ACTIONS

3.4.1 Partial factors and design loads The design values should be obtained by using the characteristic values, in combination with partial factors, in accordance to the current Standard and conveniently integrated in the present Standard as far as the tensile strength of FRC is concerned.

The characteristic value of an action can be determined by a mean value, a nominal value or, in cases where statistical criteria is set by a value, which has a certain probability of not being exceeded during a reference period, which has account of the performance life of the structure and the duration of the action.

As safety factors of actions for the verifications of Ultimate Limit States shall be adopted the values in Table 3.1, whenever the applicable specific local regulation does not set other criteria.

In general, for permanent action, obtaining favourable or unfavourable effect is determined by

considering all the actions of the same origin with the same coefficient, as shown in Table 3.1.

Table 3.1.- Safety partial factors for actions in the Ultimate Limit State.

ACTIONS Persistent o transient situation Accidental situation Favourable

effect Unfavourable

effect Favourable

effect Unfavourable

effect Permanent γG = 1,00 γG = 1,35 γG = 1,00 γG = 1,00

Pre stressed γP = 1,00 γP = 1,00 γP = 1,00 γP = 1,00 No constant value

and permanent γG* = 1,00 γG* = 1,50 γG* = 1,00 γG* = 1,00

Variable γQ = 0,00 γQ = 1,50 γQ = 0,00 γQ = 1,00 Accidental - - γA = 1,00 γA = 1,00

As partial safety factors of actions for verification of serviceability limit states adopting the

values in Table 3.2, whenever the applicable specific local regulation does not set other criteria.

Table 3.2.- Partial safety factors for actions in the Serviceability Limit State.

TYPE OF ACTION Favourable effect Unfavourable effect

Permanent γG = 1,00 γG = 1,00

Pre-stressed

Pre stressed steel reinforcement γP = 0,95 γP = 1,05

Post stressed steel reinforcement γP = 0,90 γP = 1,10

No constant value and permanent γG* = 1,00 γG* = 1,00

Variable γQ = 0,00 γQ = 1,00



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3.4.2. Combination of actions

For each one of the studied situations are set out the possible combinations of actions. A combination of actions is a set of consistent actions to be considered simultaneously acting for a particular verification.

Each combination, in general, will consist of permanent actions, a determinant variable action and one or more concomitant variables actions. Any of the variables can be decisive.

For different design situations, combinations of actions are defined in accordance with the criteria set out in current building code both Ultimate Limit States and serviceability limit states.

3.5. MATERIALS

3.5.1. Steel and concrete. Design strength and part ial safety coefficients.

The characteristic values of the strength of materials are the quantiles corresponding to a probability 0.05.

For the study of Serviceability Limit States shall be taken as partial safety factors values equal

to the unity. Design values of material properties in the Ultimate Limits States are obtained from the

characteristic values divided by a partial safety factor. The values of partial safety factors of the materials are listed in Table 3.3.

Table 3.3.- Partial safety factors for the materials in the Ultimate Limit State

Design situation Concrete Steel Persistent o transient situation γc = 1,50 γs = 1,15

Accidental γc = 1,30 γs = 1,00 Considered as resistance of the steel fyd ; the value

s

ykyd

ff

γ=

(Eq. 3-6)

where; fyk is the characteristic elastic yield. These given expressions, are valid for both tension and compression.

For concrete, the characteristic design strength, fck, is the value that is adopted in the project as compressive strength as a basis for calculations. The estimated characteristic strength, fc, est, is the estimated value for the actual characteristic strength from a finite number of standardized



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compressive strength test results, on samples taken on the job site. Normally called, characteristic strength. The design strength, fcd, takes the value:

c

ckcd

ff

γ=

(Eq. 3-7)

The value of average tensile strength of concrete, fct, m, can be estimated, in the absence of test results, as:

3 2

ckm,ct f30,0f = (Eq. 3-8)

The characteristic tensile strength for concrete, fct, k, can be expressed as:

m,ctk,ct f70,0f = (Eq. 3-9)

The factors in Table 3.3 are not applicable to the verification of Ultimate Limit State Fatigue, or

the fire resistance testing.

In structural elements subjected to repeated significant variables actions, may be necessary to verify that the effect of these actions does not compromise their safety during the expected period of service. In normal structures, generally it is not necessary to verify the state limit.

The safety of a structural element or detail regarding fatigue is guaranteed if the condition

established in Section 3.3.2., is accomplished. Concrete and steel shall be verify separately. In the case of concrete, it must be limited the maximum values of compressive stress produced

by both normal stress and tangential stress (compression struts), due to permanent loads and overloads that cause fatigue. For elements under shear stress without shear steel reinforcement, it also limited the strength capacity due to the effect of fatigue.

The maximum values of compressive stress and shear strength are defined according with existing experiments or, where appropriate, contrasted with the contrasted criteria set in the technical literature. In the case of steel, the maximum stress, ∆σsf, due to overloads that produce fatigue should be below to the fatigue limit, ∆σd = 150 MPa



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3.5.2. Partial factors for FRP materials

For serviceability limit state, a value of 1.0 is assigned to all partial factors, except where otherwise indicated.

For Ultimate Limit States, values to be assigned to the partial factor, indicated by γm for FRP materials, suggested in Table 3.4, as a function of the FRP failure mode; completed by the partial factor indicated by γRd, suggested in Table 3.5, as a function of the resistance model and the partial factor, and the partial factor as a function of the ambient, indicated by ηa in Table 3.6.

Table 3.4.- Partial safety factors of materials in Ultimate Limit States.

Type of failure Wet lay-up systems Composite rods and laminates

Compound failure γm = 1,25 γm = 1,25 Bonding failure γm,d = 1,35 γm,d = 1,50

Table 3.5.- Partial safety factors of materials as in mechanical theory

Mechanical Theory

Simple or composite flexural stress

Torsion and shear stress Confinement

γRd = 1,00 γRd = 1,20 γRd = 1,10

Table 3.6.- Safety factors as in ambient conditions

Ambient conditions

Indoor Outdoor Aggressive ambient ηa = 0,95 ηa = 0,85 ηa = 0,85

In the case of Fatigue Ultimate Limit State, performance of FRP systems under fatigue

conditions, need to be taken into consideration as well. Such performance depends on the matrix composition and, moderately, on the type of fibber. In unidirectional composites, fibbers usually have few defects; therefore, they can effectively delay the formation of cracks. The propagation of cracks is also prevented by the action of adjacent fibbers.

To avoid failure of FRP strengthened members under continuous stress or cyclic loading, values of the conversion factor for long term effects, as a safety additional factor is taken ηl = 0,80.

3.6. STRENGTHENING LIMITATIONS IN CASE OF FIRE

FRP materials are particularly sensitive to high temperatures that may take place during fire. When the room temperature exceeds the glass transition temperature of the resin (or the melting temperature in the case of semi-crystalline materials), both strength and stiffness of the installed FRP system are reduced. In case of FRP applied as external reinforcement to concrete or masonry members, exposure to high temperature produces a fast degradation of the bond between



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the FRP system and the support. As a result, degradation of the strengthening effectiveness and debonding of FRP composite may take place.

With regard to fire exposure, mechanical properties of FRP strengthened members may be

improved by increasing the thickness of protective coatings. It is suggested to employ coating capable of reducing the spreading of flames as well as smoke production. It is also recommended to employ protective coating systems provided with official certificates. Further specifications on the application of protective coating systems are reported in Section 4.8.

In order to prevent collapse of the FRP strengthened structure, as long as further information on the actual performance of coatings and resins under fire exposure is not available, it is recommended to keep low the FRP contribution to the member capacity.



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4. FLEXURAL STRENGTHENING

Flexural strengthening is necessary for structural members subjected to a bending moment larger than the corresponding flexural capacity. Flexural strengthening with FRP materials may be carried out by applying one or more laminates or one or more sheets to the tension side of the member to be strengthened. The strengthening is very effective with low steel reinforcement ratios.

4.1. INITIAL SITUATION

The effect of the initial load prior to strengthening should be considered in the calculation of the

strengthened member. Based on the theory of elasticity and with Mo the service moment (no load safety factors are applied) acting on the critical RC section during strengthening, the strain distribution of the member can be evaluated. As Mo is typically larger than the cracking moment Mcr, the calculation is based on a cracked section (see Figure 4-1). If Mo is smaller than Mcr, its influence on the calculation of the strengthened member may easily be neglected.

Figure 4.1 Initial situation

Based on the transformed cracked section, the neutral axis depth xo can be solved from:

( ) ( )o1s2o2s2o xdnAdxA)1n(bx

21 −=−−+ (Eq. 4-1)

where n = Es/Ec.

As1

As2

b

h d

d1

d2

xo

εco

Mo

εo



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6

bhfM

GPa 200E

(MPa) 8ff

f8500E

2ctm

cr

s

ckcm

3cmc

=

=+=

=

(Eq. 4-2)

where fcm is the concrete average compressive strength at 28 days. Thus, the concrete strain εco at the top fibre can be expressed as:

coc

ooco IE

xM=ε (Eq. 4-3)

where Ico is the moment of inertia of the transformed cracked section:

( ) ( )2o1s

22o2s

3o

co xdnAdxA)1n(3

bxI −+−−+= (Eq. 4-4)

Based on strain compatibility, the concrete strain εo at the extreme tension fibber can be derived as:

o

coox

h ox−= εε (Eq. 4-5)

This strain equals the initial axial strain at the level of the FRP EBR, needed for the evaluation of the strengthened member.

4.2. ANALYSIS AT FLEXURAL ULTIMATE LIMIT STATE (ULS )

Flexural design at ULS of FRP strengthened members requires that both flexural capacity, MRd , and factored ultimate moment, MSd, satisfy the following inequation:

RdSd MM ≤ (Eq. 4-6)

ULS analysis of RC members strengthened with FRP relies on the following fundamental

hypotheses: • Cross-beam sections, perpendicular to the beam axis prior to deflection, remain still plane

and perpendicular to the beam axis after deflection. • Perfect bond exists between FRP and concrete, and steel and concrete. • Concrete does not react in tension.



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• Constitutive laws for concrete and steel are accounted for according to the current building code.

• FRP is considered a linear-elastic material up to failure. FRP strengthening is effective for

low steel reinforcement ratios (e.g., steel yielded at ultimate); the rules hereafter reported refer exclusively to this situation.

It is assumed that flexural failure takes place when one of the following conditions is met:

• The maximum concrete compressive strain, εcu as defined by the current building code is reached.

• The maximum FRP tensile strain, εRd, is reached; εRd can be calculated as follows The maximum bending stress is determined by the value of the strain in some specific fibres in

the section, defined in the maximum strain region. The value of the ultimate concrete strain, εcu, is 0,0035, If the characteristic concrete strength, fck, is lower than 50 MPa; the value of the tensile

maximum steel strain, εmáx, in the design shall be limited to 0,01; and the maximum tensile strain of the composite material shall be calculated as following;

= Rddm

RkaRd ,min ε

γεηε (Eq. 4-7)

where εRk is the characteristic strain at failure of the adopted strengthening system; ηa is the safety factor as per ambient type; γm is the partial safety factor for materials; and εRdd is the maximum strain due to intermediate debonding as defined in Section 4.1.5. Generally, the one that prevail.

The shear capacity of the strengthened member shall be larger than the shear demand corresponding to the examined case. If deemed necessary, shear capacity shall be increased according to the provisions of Section 5.

Because a member strengthened with FRP is generally loaded at the time of FRP application, the existing strain in the structure before FRP strengthening takes place shall be taken into account, according to Section 4.1.1.

4.2.1. Strain regions As stated above, materials reinforced with composite carbon fibber are most effective in

sections with low amounts of steel reinforcement and when used in areas subject to section tensile stress. The strain regions where reinforcements are effective are as follows (see Figure 4.2):

Region 1: Simple or compound tensile stress, where the entire section is in tension. The deformation lines turn around the point R, a new pivot corresponding to an elongation of the reinforcement of composite material equal to εRd.



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Region 2: Simple or compound bending, where the concrete does not reach the ultimate bending deformation. The deformation lines turn around the point R. Region 3: Simple or composite bending where the lines of deformation turn around the point B corresponding to the ultimate flexural strain of concrete εcu defined in Section 4.1.2. The elongation of tensile steel reinforcement is greater than the corresponding yield strength of

steel, εyd, and the elongation of the carbon fibber composite is less than εfd. Region 4: Simple or composite bending where the lines of deformation turn around the point B. The elongation of tensile steel reinforcement is less than that for yield strength of steel, εyd. In this region, the efficiency of the composite reinforcement is very low.

Figure 4.2.- Strain regions

R

B

1 2

3

4

εyd

εcu

εRd εo



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4.3. FLEXURAL STRENGTH CAPACITY OF REINFORCEMENT SE CTIONS WITH COMPOSITE MATERIALS. When checking the strain regions 2 and 3, in which the RFP is efficient, the flexural analysis of

FRP strengthened members can be carried out by using strain compatibility and force equilibrium. The stress at any point in a member must correspond to the strain at that point; the internal forces must balance the external load effects (see Figure 4.3).

Figure 4.3.- Analysis of flexural ULS Calculation of neutral axis depth, x: Forces equilibrium:

RRRyd1s2ss2scd EAfAEAx8,0.bf85,0 εε +=+ (Eq. 4-8)

Momentum equilibrium:

)d4,0(EA)x4,0h(EA)x4,0d(fAM 22ss2sRRRyd1sd −+−+−= εε (Eq. 4-9)

Strain compatibility:

yd2ss2

c2s fE ;x

dx≤ε

−ε=ε (Eq. 4-10)

εo

As1

As2

b

h d

d1

d2

x

fcd

Md

fyd

fR

AR

εc

εs2

εRd

εs1



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ocR xxh εεε −−= (Eq. 4-11)

s

ydc1s E

f

xxd ≥−ε=ε (Eq. 4-12)

In these regions, two types of failure can be observed, depending on whether the ultimate FRP strain (region 2) or the ultimate concrete compressive strain (region 3) is reached.

Region 2: Tensile failure of FRP composite: RdR εε =

Region 3: Compressive failure of the concrete: 0035,0c =ε

4.3.1. Failure by peeling-off of composite material s

Bond is necessary to transfer forces from the concrete into the FRP, hence bond failure modes have to be taken into account properly. Bond failure in the case of EBR implies the complete loss of composite action between the concrete and the FRP reinforcement, and occurs at the interface between the EBR and the concrete substrate. On the other hand, localised debonding, means a local failure in the bond zone between concrete and EBR. In this case the reduction in bond strength between concrete and FRP reinforcement is limited to a small area, e.g. a loss in bond length of 2 mm next to a crack in a flexural member. Therefore localised debonding is not in itself a failure mode which will definitely cause a loss of the load carrying capacity of a member with EBR.

When localised debonding propagates, and composite action is lost in such a way that the FRP

reinforcement is not able to take loads anymore, this failure is called peeling-off. If no stress redistribution from the externally bonded FRP reinforcement to the embedded reinforcement is possible, peeling-off will be a sudden and brittle failure.

Bond failure may occur at different interfaces between the concrete and the FRP reinforcement, EBR such as carbon fibre fabric or laminated and it is applicable to reinforced concrete beams, subject to flexural and shear stress.

The loss of adhesion between FRP and concrete may concern both laminates or sheets applied to reinforced concrete beams as flexural and/or shear strengthening. As shown in Figure 4-1, debonding may take place within the adhesive, between concrete and adhesive, in the concrete itself, or within the FRP reinforcement (e.g. at the interface between two adjacent layers bonded each other) with different fibre inclination angles. When proper installation is performed, because the adhesive strength is typically much higher than the concrete tensile strength, debonding always stakes place within the concrete itself with the removal of a layer of material, whose thickness may range from few millimetres to the whole concrete cover.



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Figure 4.4. Type of debonding failure. 1) Debonding in concrete; 2) Concrete and adhesive failure; 3) Adhesive failure; 4) Adhesive and composite failure; 5) Composite FRP delaminating.

Debonding failure modes for laminates or sheets used for flexural strengthening may be classified in the following four categories, schematically represented in Figure 4-5.

• Mode 1 (Laminate/sheet end debonding) • Mode 2 (Intermediate debonding, caused by flexural cracks) • Mode 3 (Debonding caused by diagonal shear cracks) • Mode 4 (Debonding caused by irregularities and roughness of concrete surface)

Figure 4.5.- FRP flexural strengthening: Debonding failure modes. 4.3.1.1. Mode 1 and 2. Peeling-off at the end anchorage and at flexural cracks

Treatment of anchorage peeling-off and at flexural cracks may be done according to various

approaches, which are described briefly in the following. In the first, shall proceeds to verify the final anchorage, limiting the tensional stress in the

carbon fibber reinforcement. The end portions of the FRP system are subjected to high interfacial shear stresses for a length of approximately 100-200 mm. First, the anchorage must be verified

Mode 1 Mode 2 Mode 3 Mode 4

g+q

Cracked zone

Composite

Adhesive

Concrete

Rebar 1

2 3 4

5



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based on the law of shear stress for the interface between composite and concrete. Then apply a tensional stress limitation to carbon fibre composite to prevent the adhesion failure. Note that the CFRP tensional stress for the failure due to the lack of adherence is not a fixed value but depends on a certain number of parameters, including moment-shear ratio, the steel stress and crack distribution.

In this approach peeling-off is treated in a unified way both at the end anchorage and at any point along the FRP-concrete interface based on the interface shear stress –slip law and the envelope line of tensile stresses in the FRP (Niedermeier 2000). The main advantage of this approach is that peeling-off at the end and at flexural cracks is treated with the same model, whereas the main disadvantage is its complexity, which makes it difficult to apply as a practical engineering model.

According to the third approach (Matthys 2000), two independent steps should be followed (as

in the first one). In the first, the end anchorage should be verified based on the shear stress – slip constitutive law at the FRP-concrete interface. And in the second it should be verified that the shear stress along the interface, calculated based on simplified equilibrium conditions, is kept below a critical value (the shear strength of concrete). One disadvantage of this approach is the treatment of the same – in principle - phenomenon (peeling-off at the FRP end and far from it) with different models and another one is that it is based on a stress distribution for a homogeneous, uncracked beam. However, one major advantage is the simplicity of application in practical problems.

In this design guide, in order to avoid the debonding on the anchorage length, it has been

chosen to follow the approach of limiting the deformation of the CFRP at ultimate limit state to a certain value, beside to verify the final anchorage with methods mainly based on fracture mechanics and bond shear stress (following the criteria of Pichler 1993, Täljsten 1994, Holzenkämpfer 1994, Neubauer & Rostásy 1997, Niedermeier 2000),which are gathered in many different jobs , Guides and others technical documents, with limits from 0,0065 to 0,0085.

Anchorage verification is based on the model that relates the bond shear stress to the slip (see

Figure 4.6).

Figure 4.6.- Model for interfacial shear stress – slip relations of EBR (Holzenkämpfer 1994).

8

6

4

2 τb: In

terf

acia

l she

ar s

tres

s (M

Pa)

sf: Slip (mm)

ΓFk: Fracture energy

fctm: 2 MPa

0 0.10 0.20 0.30 0.40

Sf,0

Sf,1

τf,1



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Bond models such as that described above may be used for calculating anchoring forces,

NR,max, and optimum anchorage length, Lb,opt.

ctm

RRopt,b f2

tE)mm(L = (Eq. 4-13)

ctmRRRbmax,R ftEbk64,0)N(N α= (Eq. 4-14)

1

4001

206.1 ≥

+

−=

R

R

b bb

b

k (Eq. 4-15)

where:

• α is a reduction factor, approximately equal to 0,9, to account for the influence of inclined

cracks on the bond strength in without sufficient external shear reinforcement and in slabs, and α =1 in beams with sufficient internal and external shear reinforcement and in slabs;

• kb is a geometric factor expressed by equation (4.15) which relates reinforcement width bR and total width of the element b, with bR/b ≥ 0,33 (if bR/b < 0,33; bR/b = 0,33 must be taken); and

• fctm is the concrete average tensile strength (see Equation 3.8).

For bond lengths Lb < Lb,opt, the ultimate bond force was calculated according to Equation 4.16:

−=

max,b

b

max,b

bmax,RR L

L2

LL

NN (Eq. 4-16)

For laminate/sheet end debonding assuming that the provided bond length is equal to or larger

than the optimal bonded length, the ultimate design strength, fRdd, can be calculated as follows:

R

FkR

cd,m

Rdd tE21

fΓ

γγ= (Eq. 4-17)

ctmckbFk ffk03,0=Γ (Eq. 4-18)

The specific fracture energy, FkΓ , of the FRP–concrete interface may be expressed as above

(Equation 4-18), and where; γm,d is the partial safety factor indicated in Table 8; γc is the partial safety factor for concrete , as indicated in Table 7; kb is a geometric factor expressed by equation (4.15); fctm and fck are the concrete average tensile strength and the concrete characteristic strength. This equation may be used for both flexural and shear stress.



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An increase in anchorage length above Lb,opt, does not result in an increase in resisting tensile stresses. This is due to the limitation of fracture energy. For anchorage lengths lower than Lb,opt, (that is Lb < Lb,opt) the maximum tensile stress is described by Equation 4-19;

−=

max,b

b

max,b

bRddLb,Rdd L

L2

LL

ff (Eq. 4-19)

To prevent debonding failure by bending cracks, the variation of tension in the composite, ∆σR,

between two close cracks should not exceed a certain limit value ∆σR,lim. This value depends on the adhesion between concrete and carbon fibber composite, on the distance between cracks in the concrete and on the level of tension in the reinforcement. In a shorter way, the maximum tensional stress that can be transferred between the concrete and reinforcement in Ultimate Limit State is fRdd,2:

⋅= RddR

ctmckR

c2,Rdd f3;

t

ffE23,0minf

γ (Eq. 4-20)

4.3.1.2. Mode 3. Debonding by diagonal shear crack. For members where shear stresses are predominant compared to flexural stresses, a relative

displacement between the edges of the crack is displayed. Such displacement increases normal stress perpendicular to the FRP laminate responsible for FRP debonding. Such a debonding mechanism is active irrespectively of the presence of stirrups. Collapse due to debonding by diagonal shear cracks is peculiar of four-point-bending laboratory tests; it is not common for field application where the applied load is distributed over the beam’s length. For heavily strengthened beams with low transverse reinforcement, debonding usually generates at the end plate section due to peeling (see Figure 4.7).

Figure 4.7.- Debonding by diagonal shear crack.



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4.3.1.3. Mode 4. Debonding by irregularities and roughness of the concrete surface. Localized debonding due to surface irregularities of the concrete substrate may propagate and

cause full debonding of the FRP system. This failure mode can be avoided if the concrete surface is treated in such a way to avoid excessive roughness.

4.3.2. Design of flexural anchorage length

As studied in Section 4.1.5., Laminate/sheet end debonding depends on a number of parameters such as location and type of cracks (shear or flexural cracks), uneven concrete surface, as well as stress concentration close to the anchorage zone.

The maximum distance a to avoid debonding shall be computed by equating the ultimate design

strength fRdd, from Equation (4.17), for Lb ≥ Lb,opt, to the stress calculated at ULS in the FRP system, at a distance a + Lb from the support. If the available bond length is Lb<Lb,opt, then it should be equated to fRdd in Equation (4.19).

When the end of the FRP system is close to the member supports, where shear forces may

induce inclined cracking, the moment to be taken into account in item (2) shall be evaluated by increasing the design moment as follows:

)cot1(d9,0VaVM sd1sd α−⋅=⋅= (Eq. 4-20)

Where Vsd is the factored shear force in the beam support, a is the angle of existing transverse

steel reinforcement, and d is the member effective depth (see Figure 4.8).

Figure 4.8.- Shifting of bending moment diagram.

When special anchoring devices used to avoid FRP debonding at the termination points are

employed, it shall be permitted to neglect provisions of Section 4.4.5.1. Such anchoring devices need to be guaranteed based on proper experimental tests. Experimental tests need to be conducted for the material intended for such application (adhesives and reinforcing fibres), for the specific system used (transverse bars embedded in concrete, U-wrap with FRP sheets, etc.), for construction procedures as recommended by the manufacturer, for surface preparation, and for the expected environmental conditions.

a Lb

a1



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4.4. ANALYSIS AT SERVICEABILITY LIMIT STATE (SLS)

This section deals with the following serviceability limit states:

• Stress limitation. • Deflection control • Crack control

Other serviceability limit states may be relevant in particular situations, even though they are not

listed in this document.

At SLS the following items need to be checked:

• Stresses need to be controlled to avoid yielding of tensile steel and creep phenomena in both concrete and FRP.

• Deflections should not attain excessive values such as to prevent the normal use of the

structure, induce damage to non-structural members, and cause psychological disturbance to the users.

• Excessive cracking could significantly reduce the durability of structures, its functionality, its

aspect and decrease bond performance of the FRP-concrete interface.

Design at SLS can be carried out considering all materials having a linear-elastic behaviour for both uncracked and cracked transformed section conditions. Existing strain at the time of FRP installation shall be accounted for. The principle of superposition can be used for design. Design assumptions are as follows:

• Linear-elastic behaviour of all materials. • Cross-beam sections, perpendicular to the beam axis prior to deflection, remain still plane

and perpendicular to the beam axis after deflection. (Navier Hypothesis) • Perfect bond exists between steel and concrete, and concrete and FRP.

Upon the expressed above, the calculus is made with the transformed uncracked or cracked

section before and after of the reinforcement, see Figure 4.9.



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Figure 4.9.- Linear elastic analysis of cracked section.

From the equilibrium of forces and strain compatibility, the depth of the neutral axis x is obtained from the following:

+−+−=−−+ x1hAn)xd(An)dx(A)1n(bx

21

c

oRR1ss22ss

2

εε

(Eq. 4-21)

( ) ( )dhx

xdAndh

x

dxA)1n(

3x

hbx21

ME

1ss22

2ss2

kcc

−−−−−

−+

−=ε (Eq. 4-22)

Neglecting the steel reinforcement in compression (As2 = 0) and assuming h/d � 1.1 (mean effective depth of the steel and FRP reinforcement ≈ 1.05d), Equation (4-22) can be written as:

−=

3x

d05,,1bx21

ME

2

kccε (Eq. 4-22b)

or, based on the equations in Section 4.5:

ok

0

c

o

xx

MM≈

εε

(Eq. 4-22c)

The moment of inertia of the cracked section Ifis, is given by:

εo

As1

As2

b

h d

d1

d2

x

fcd

Mk

fyd

fR

AR

εc

εs2

εRd

εs1

Ns2

Nc

Ns1

NR



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( )2RR

21ss

222ss

3fis xhAn)xd(An)dx(A)1n(bx

31

I −+−+−−+= (Eq. 4-23)

4.4.1. Service Stress Limitation

Stress at service in the FRP system, computed for the quasi-permanent loading condition, shall satisfy the limitation σR ≤ ηfRk, where fRk is the FRP characteristic strength at failure and η is the conversion factor as suggested in Section 3.5.2. Service stress in concrete and steel shall be limited according to the current building code, σc ≤ 0,60 fck,j, where fck,j is the characteristic strength at age j when; σs ≤ σe = fyd/Es.

Assuming that M0 is the bending moment acting on the member prior to FRP strengthening, and

assuming that M1 is the bending moment acting after FRP strengthening, the stress due to the combined moment M = M0 +M1 can be evaluated as follows:

;xh

IW;

xdI

W;xd

IW;

xI

W;xI

W

resistance of Modulus

WM

nσ :composite FRPin Stress

WM

WM

nσ :steel the in Stress

WM

WM

σ :concrete the inStress

1

1supR1,

1

1infs1,

0

0infs0,

1

1supc1,

0

0supc0,

infR1,

1Rs

infs1,

1inf

s0,

0ss

supc1,

1supc0,

0c

−=

−=

−===

=

+=

+=

(Eq. 4-24)

When the existing applied moment, M0, is such to produce cracking in the concrete member, (M0 > Mcr), neutral axis determination as well as values of the moment of inertia I0 and I1 shall be calculated with reference to cracked transformed section for both unstrengthened and strengthened conditions.

4.4.2. Service strain limits (Deflections control) The strain (deflections) limit state is satisfied if movements (deflections or spins) in the structure

or structural elements are below of the maximum limit values. Verification of the strain limit state will be made in the cases in which the strain may cause the

decommissioning of the structure or structural element due to functionally, aesthetic or others. The strain analysis should be done for the servicing conditions according to the specific subject

and according to the actions combination criteria. The total strain in a concrete element is the sum of the different partial strains along time due to

the applied loads, to creep and shrinkage in the concrete and to relaxation of steel reinforcement.



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Deflection is made up of the instant deflection and the differed deflection, due to permanents loads. Deflections shall not exceed the establish limits in the current building code.

In beams and slabs, deflections verification may not be necessary when the ratio span/depth of

a studied element, equals to the indicated value in the Table 4.1. In lightweight slabs or beams in “T” section, in which a ratio width of a wing beam/web beam higher than 3, the slenderness L/d should be multiplied by 0,8.

Table 4.1.- Ratio L/d in reinforced concrete beams and slabs under simple bending

STRUCTURAL SYSTEM L/d

K

Strongly steel reinforcement

elements ρ=1,5%

Poorly steel reinforcement

elements ρ=0,5%

Simple supported beam. Simple supported one way slab or waffle slab

1,00 14 20

Continuous beam 1 supported in one end. Continuous one way slab1,2 supported in one end.

1,30 18 26

Continuous beam 1 supported in both ends. Continuous one way slab or waffle slab1,2

1,50 20 30

Exterior areas and corners in slabs without beams on isolated supports.

1,15 16 23

Interior areas and corners in slabs without beams on isolated supports.

1,20 17 24

Cantilever 0,40 6 8 1 One end is considered continuous if the momentum is equal or higher to 85% of the fixing perfect momentum.

2 In one way slabs, the given slenderness are referred to the minor span. 3 In slabs on isolated supports (pillars), the given slenderness are referred to the major span.

In the specific type of floor beam slab with a span less than 7 m and in pre stressed hollow core

slabs, with a span less than 12 m and with over loads not higher than 4 kN/m2, will not be necessary to verify the deflection, if the total depth h is higher than the minimum hmin given by:

421

21min 6

L;

7q

;C

Lh === δδδδ

(Eq. 4-25)

where q is total load (kN/m²) L is the calculus distance for frameworks (m); and C is a coefficient from Table 4.2.



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Table 4.2.- Coefficient C

Floor beam slab type Load type Type of item

isolated edge Interior

Reinforced beams With walls and partitions 17 21 24

Roofs 20 24 27

Pre-stressed beams With walls and partitions 19 23 26

Roofs 22 26 29 Pre stressed hollow

core slabs* With walls and partitions 36 -- --

Roofs 45 -- -- (*) Pre stressed elements designed in such way (less frequent combined actions) that never reach the cracking momentum.

4.4.3. Cracking Limit state (Crack control)

At SLS, crack width shall be checked to guarantee a proper use of the structure and to protect

steel internal reinforcement.

Crack width limitations for FRP strengthened structures shall satisfy the requirements of the current building code. 4.5. DUCTILITY

The linear analysis with restricted redistribution requires a ductility conditions in the critical

sections that may guarantee the necessary redistribution for the adopted stress conditions. For flexural members, ductility is a measure of the member capability of evolving in the plastic

range; it depends on both section behavior and the actual failure modes of the overall structural member.

For FRP strengthened members, greater ductility is ensured when failure takes place due to crushing of concrete. The collapse due to FRP rupture leads to brittle failures.

Regardless of the type of cross section, ductility is mainly controlled by the member failure mode. It can be considered totally absent if debonding starts prior to any other failure mechanism.



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5. STRENGTHENING IN SHEAR

Shear strengthening is deemed necessary when the applied factored shear force is greater than the corresponding member shear capacity. The latter shall be determined considering the contributions of both concrete and steel transverse reinforcing bars when available.

Shear strengthening shall be verified at ULS only. Shear strengthening of RC members using FRP may be provided by bonding the external

reinforcement with the principal fibre direction as parallel as practically possible to that of maximum principal tensile stresses, so that the effectiveness of FRP is maximized. For the most common case of structural members subjected to lateral loads, that is loads perpendicular to the member axis (e.g. beams under gravity loads or columns under seismic forces), the maximum principal stress trajectories in the shear-critical zones form an angle with the member axis which may be taken roughly equal to 45º. However, it is normally more practical to attach the external FRP reinforcement with the principal fibre direction perpendicular to the member axis (see Figure 5.1).

Figure 5.1.- Shear strengthening

Shear strengthening is realized by applying one or more layers of FRP material externally

bonded to the surface of the member to be strengthened. External FRP reinforcement can be applied in a discontinuous fashion, with gaps between following strips, or continuously, with strips next to each other (see Figure 5.1).

Discontinuous strengthening

aR sR

Continuous strengthening

Type A Type B Type C Type D



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Strengthening may also be lateral setting, as showed in Figure 5.1; type A: Side bonding only, type B: U-wrapped without upper anchorage, type C: U-wrapped with upper anchorage or type D: Fully wrapped. Obviously, the best results are obtained when the element is fully wrapped, being in order of efficiency the types D, C and B besides the type A which is of a minimum efficiency and should be avoided.

The strengthening type C, or U-wrapped with upper anchorage are consider equivalents to the

type D or fully wrapped.

5.1. SHEAR CAPACITY IN SECTION REINFORCED WITH FRP

The shear limit state shall be reached in two ways; when maximum oblique compression in the

web beam is reached or when maximum tensile stress is reached. Thus, it is necessary to verify that both conditions are accomplished:

Rsucu2ud

1ud

VVVVV

VV

++=≤≤

(Eq. 5-1)

Where; Vd is the effective shear stress; Vu1 is resistance to shear by oblique compression in the

web beam, and Vu2 is the shear strength by tension in the web beam. Verification of oblique compression in the web beam Vd ≤ Vu1 is performed on the edge of

support and not on its axis. Verification for the tension in the web beam Vd ≤ Vu2 is performed to a section located at a

distance of one effective depth from the edge of the support.

5.1.1. Calculus of V u1

The maximum shear stress by oblique compression of the web beam is obtained from the following expression;

θαθ

20cd11ugcot1

gcotgcotdbKfV

++

= (Eq. 5-2)

where;

- f1cd is the compressive strength of concrete equal to 0.6 fcd, - b0 is the minimum net width of the element, - K coefficient depending on the axial force, K = 1.00 for non pre stressed structures or

without axial compression, - α is the angle between reinforcement with the axis of the element, and - θ is the angle between compression struts and the axis of the piece (see Figure 5.2).



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Figure 5.2.- Model of struts and ties.

5.1.2. Calculus of V u2

The shear stress due to the maximum tensile stress in the web of the beam is;

( )

( )

( )R

RRef,R

RdRu

cdl3/1

cvlc

0cu

d,ysu

Rsucu2u

sa

gcotgcott2fd9,01

V

'15,0f10015,0

dbV

fAgcotgcotsenzV

VVVV

θβγ

σαρξγ

ψ

Σθαα αα

+=

+=

+=++=

(Eq. 5-2)

where;

- VSU is the contribution of transverse reinforcement of the web to the shear stress resistance; - VCU is the concrete contribution to shear resistance; - VRU is the contribution of CFRP reinforcement to shear resistance; - Aα is the area per unit length of each group of steel reinforcement that form an angle with

the axis of the element (Figure 5.2); - fyα,d is the design strength of steel reinforcement Aα; - θ is the angle between the concrete compression struts and the axis of the element (see

Figure 5.2), shall be the same value as for verification of the maximum shear stress by oblique compression in the web of the beam 0,5 ≤ cotg θ ≤ 2,0;

- α is the angle of steel reinforcement with the axis of the element (see Figure 5.2); - z is the mechanical arm, which in simple bending, and in the absence of more precise

calculations can be adopted the approximate value z = 0.9 d; fcv is the effective shear resistance of concrete in N/mm2 with value fcv = fck with fcv no higher than 15 N/mm2 in the case of reduced control of the concrete;

- fck is the compressive strength of concrete in N/mm2; - ψ depends on the angle of inclination of the cracks, for cracks at 45 ° has a value of 1.0, - b0 is the minimum net width of the element;

θ

α

β



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- b is the angle between the FRP reinforcing strips with the axis of the element, usually 90 degrees;

- aR is the width of the FRP reinforcing strips, with 50 mm ≤ aR ≤ 250 mm; - sR is the separation between FRP reinforcing strips aR ≤ sR ≤ min (0,5d; 3aR;aR+200 mm); - γRd is the safety factor for FRP reinforcement type defined in Table 3.5, equal to 1.20; and

fR,ef is the design tension stress of carbon fibre reinforced, expressed as:

006,0

11,0;1043,0min

11,0

,

3

≤

=

=

=

−

efR,

efR,

RRcm

0,30

R

2/3cm

Ru

0,56

R

2/3cm

efR,

0,30

R

2/3cm

RuefR,

ρ(GPa);E(MPa);f

E

f

E

f

:B) (Type ingstrengthenU in sectionsFor

E

f

:C)and D(Types wrappedtotally sectionsFor

εε

ρε

ρε

ρεε

RefR

RR

R

Ef

ratioingstrengthen

x (Eq. 5-3)



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6. STRENGTHENING OF ELEMENTS BY CONFINEMENT

Confinement is generally applied to the elements in compression, with the aim of improving capacity or, in case of seismic action, to increase its ductility. Traditional techniques for strengthening are based on confinement by steel jackets. It is well known that confinement increases the strength of concrete, ductility, and also prevents slippage and buckling of longitudinal strengthening. In case of seismic loads, current techniques for improving the capacity is usually based on the increase of confining pressure on the complete element. This technique can also be useful in areas of joints of steel.

Composite materials have a significant advantage over steel. While steel maintains a constant confining pressure after yield limit, the composite has an elastic behaviour to failure and therefore exert their action on the concrete containment differently.

The value of the capacity of confinement depends on the transverse strain of concrete, which in

turn is affected by confining pressure. Therefore, the value of confinement capacity for concrete using composite materials must take into account both the interaction between the transverse strain of concrete and containment system.

6.1. AXIAL CAPACITY FOR FRP-CONFINED ELEMENTS SUBJE CT TO COMPRESSIVE

FORCES Confinement of reinforced concrete element can be realized with FRP sheets disposed along

the element perimeter as both continuous or discontinuous external wrappings. The increase of axial capacity and ultimate strain of FRP-confined concrete depends on the applied confinement pressure. The latter is a function of the element cross section and FRP stiffness.

For axial strain values, εc, up to 0,2%, the stress in the confined concrete is only slightly greater

than that exhibited by unconfined concrete. For axial strain value larger than 0,2%, the stress-strain diagram is not linear and the slope of the corresponding σ/ε curve gradually lowers up to a nearly constant value. In the linear branch of the diagram, the confined concrete gradually loses its integrity due to widespread cracks. Failure of reinforced concrete confined element is attained by fibre rupture. However, beyond a critical value of the axial strain, the FRP-confined element may be linked to a recipient with very flexible walls filled with incoherent material. Beyond that threshold it loses its functionality since it can only carry small or negligible transverse forces. As a result, failure of the FRP-confined reinforced concrete element is said to be reached when FRP strain equal to 0,4% is attained.

If the adopted FRP system is not initially pre-stressed, it exerts a passive confinement on the

compressed element. The confinement action becomes significant only after cracking of the concrete and yielding of the internal steel strengthening due to the increased lateral expansion exhibited by the strengthened element. Prior to concrete cracking, FRP is practically unloaded.

Design at ULS of FRP confined elements requires that both factored design axial load Nd and

factored axial capacity, NR, satisfy the following inequation:

Rd NN ≤ (Eq. 6-1)



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For non-slender FRP confined elements, the factored axial capacity can be calculated as follows:

ydsRd

d,cccR fA

fAN +

γ= (Eq. 6-2)

where:

- Ac represents the member cross-section area; - fcc,d represent the design strength of confined concrete as indicated bellow); - γRd is a practical factor from Table 3.5; - As and fyd represent area and yield design strength of the existing steels strengthening. The design strength, fcc,d, of confined concrete shall be evaluated as follows (see Figure 6.1.):

3/2

cd

efR,1

cd

d,cc

f

f6,21

f

f

⋅+= (Eq. 6-3)

where:

- fcd is the design strength of unconfined concrete; and - f1,efR is the effective confinement lateral pressure as defined as follows.

Figure 6.1.- Stress-strain relationship for FRP-confined concrete

σc

fcd

εco εccu

fccd

εcu



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6.1.1. Confinement lateral pressure

Confinement using composite is function of effective confinement lateral pressure that is equal to confinement lateral pressure modified by a group of coefficients of element cross section shape (circular, square or rectangular), FRP configuration (continuous or discontinuous) and system used for installation (continuous wrapping or discontinuous wrapping).

The effective confinement lateral pressure is indicated in the following equation:

conf,RdRRVHlefRefR,1 E21

)kkk(fkf ερ⋅=⋅= α (Eq. 6-4)

where:

- fl is the confinement lateral pressure (see Figure 6.2); - kefR is a coefficient of efficiency for FRP, product of different coefficients kH, due to cross-

section shape (see Figure 6.3), kV, due to vertical efficiency (see Figure 6.4), y kα, type of system used for wrapping;

- ρR is the strengthening geometric ratio; - ER and εRd,conf is the Young modulus of elasticity of the FRP in the direction of fibres, and

reduced FRP design strain, respectively.

Figure 6.2.- Confinement lateral pressure

The formula for coefficients kH and kV, as well as, ρR, depend on the cross-section shape, (circular o rectangular) and shall be expressed as follow:

6.1.1.1. Circular section: Coefficients kH, kV, ρR

R

RRR

RR

2

RRV

H

Dsbt4

2/D)bs(;D2

)bs(1k

1k

=ρ

≤−

−−=

=

(Eq. 6-5a)

fl fl



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where tR, bR and sR represents FRP thickness, width, and spacing, respectively (se Figure 6.4.), and D is the diameter if the circular cross-section. In case of continuous wrapping ρR=4tR/D;

6.1.1.2. Rectangular section: Coefficients kH, kV, ρR

R

RR

R

RR

RR

V

H

bhs

hbbt

hbsh

bsk

bh

rhrbk

)(2

2/)(;2

)(1

;3

)2()2(1

2

22

+=

≤−

−−=

≥<≤−+−−=

ρ

mm 20 r mm;900hb, 2;b/h

(Eq. 6-5b)

Where:

- b y h are the dimension indicated in Figure 6.3; - r is the corner radius that shall satisfy r≥20 mm; - tR, bR and sR represent FRP thickness, width, and spacing, respectively (see Figure 6.4), for

a totally wrapped element ρR=2tR(b+h)/(bh).

45º

º

CFRP

bR

sR

s’

Figure 6.4 Elevation view of circular element confined with FRP strips

con bandas de refuerzo distantes s

45º

45º

b

b’=b-2r

h’=h-2r h

Confined area

Figure 6.3 Rectangular cross-secsectionsecciones

r



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6.1.1.3. Coefficient for continuous wrapping: kα

5,0º45

10

:

1

12

=→==→=

+=

α

α

α

αα

αα

k

k

tgk

R

R

R

R

45º at wrappingcontinuous For

wrappingousdiscontinu For

section and wrappingcontinuous betteen angle (Eq. 6-6)

6.1.1.4. Design strain for FRP: εRd,conf

Design strain for composite used in confinement shall be expressed as follow:

= 004,0;min,

conf

Rk

aconfRd γεηε (Eq. 6-7)

where:

- εRk is the guarantee strain for FRP; - γconf, is a factor from Table 3.4 and Table 3.5 (γm=1,25 –failure compound-, γR=1,1); and - ηa is a coefficient from Table 3.6.



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6.2. DUCTILITY OF FRP-CONFINED ELEMENTS UNDER BENDI NG AND AXIAL LOADS FRP-confinement may also be realized on concrete elements under combined bending and

axial load; confinement will result in a ductility enhancement while the element axial capacity can be only slightly increased.

Unless a more detailed analysis is preformed, the evaluation of the ultimate curvature of a FRP

confined concrete element under combined bending and axial load may be accomplished by assuming a parabolic-rectangular approach for the concrete stress-strain relationship, characterized by a maximum strength equal to fcd and ultimate strain, εccu, computed as follows:

c

ckcd

cu

0c

cd

efR,1ccu

ff

0035,0

0020,0

f

f015,00035,0

γ=

=ε=ε

+=ε

(Eq. 6-8)

Where:

- fcd is the design strength of unconfined concrete; and - f1,efR is the effective confinement pressure from Section 6.1.1.

Figure 6.5.- Parabolic-rectangular approach diagram

σc

fcd

εco εcu εccu



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7. INSTALLATION, MONITORING AND CONTROL OF APPLICATION

Several aspects influence the effectiveness of the strengthening of composite materials used as

external systems bonded to the strengthening of concrete. Besides those mentioned in previous chapters, the surface preparation and installation of the compound are discussed in this section. The relative importance of each of these issues depends on whether there is flexural or shear strengthening or confinement. For example, some tests on the quality of the substrate can be omitted for containment applications or when the composite system is anchored by a proven system.

This chapter describes the tests can be performed for quality control and inspection and

maintenance for the strengthening system after installation. The type and number of tests to perform will depend on the importance of the application.

7.1. PREPARATION OF SUBSTRATE For maximum effectiveness of this type of strengthening is absolutely essential to have a clean

and sound concrete surface, because if not the adhesion of the adhesive or resin will be weak and the effectiveness of strengthening will not be total. Surface preparation is carried out to provide a ideal surface for maximum effectiveness of the FRP.

The first thing to do is to temporarily remove all electrical conduits, pipes, etc., to prevent having

a free surface on which to apply the strengthening. These elements must be identified fully displaced to return it to its correct position at the end of the work.

The quality control of surface preparation involves the determination of concrete conditions, the elimination of any damage or loose area of concrete, cleaning and corrosion protection of existing rebars, and finally the preparation of substrate to receive the strengthening system selected.

Before the application of strengthening with composite materials, will test the strength of the concrete substrate. In any case, the concrete compressive strength should not be less than 15 N/mm2. The CFRP strengthening is not considered effective for concrete with compressive strength below this value. Some tests should be carried out for quality control throughout the area to be strengthened.

The concrete substrate may have suffered physical and chemical, mechanical or impact deterioration. The deteriorated concrete shall be removed from all damaged areas. The removal of the concrete in poor condition allows the evaluation of existing steel reinforcing bars. Corroded reinforcing steel must be protected against corrosion to prevent a possible source of deterioration of concrete repair.

Once all deteriorated concrete was removed, and have taken appropriate measures to prevent further corrosion of the existing rebar, as well as all other phenomena that cause degradation of the concrete (i.e., water leaks), proceed with the repair of concrete by using a structural repair mortar without shrinkage such as MAXREST® or MAXRITE® range (500/700/S/F/HT). On the other hands, unevenness of the concrete surface more than 10 mm shall be levelled.



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In addition, fissures bigger than 0,50 mm should be injected with low viscosity epoxy resin such as MAXEPOX® INJECTION before strengthening.

Once control of the quality of the substrate has been carried out, the deteriorated concrete has been removed, the concrete section has been restored, and the existing steel bars have been adequately treated, we proceed to the proper surface preparation, starting with the removal of the surface layer of concrete grout, dust and debris, previous coatings, oil, grease, surface curing agents, foreign particles, paints and any other material that may adversely affect the adhesion of the system. In this operation it is best to use the sand blasting because of the fact, in addition to clean, provides a very suitable surface roughness for bonding adhesive, or mechanical grinding using a grinder equipped with a diamond disc to provide a degree of roughness at least 0,3 mm. This level of roughness can be seen visually or for more accuracy can be measured by laser profilometer or an optical measuring device profile.

All edges, corners and sharp edges that can cut the composite material should be rounded or chamfered a minimum radius of 20 mm. The curvature of the corners can be checked by a metal template.

All the dust produced in the above operations should be removed from the concrete surface by vacuum, compressed air or pressurized water. Using this latter system, the strengthening should not be applied while the concrete is wet.

Once prepared the support, wait for it to dry and check the moisture content is below 4%, i.e. dry surface, unless otherwise stated

7.2. WET LAY-UP SYSTEMS: SHEETS For the installation of carbon fibre sheets, in which the fibres are very compact, so special

epoxy-based formulas are needed to moisten the fibres saturating for produce the CFRP composite. The steps are:

• Primer application. • Putty application (if necessary to complete the preparation of the substrate).

• Cutting of carbon fibre sheets according to the established work plan based on the

strengthening project.

• Resin under/overcoat application and carbon fibre sheet application.

• Top coating application

7.2.1. Primer application

Primer penetrates into the concrete surface to increase the strength of concrete and to improve

the bonding between the concrete and the carbon fibre sheet. Two types of primer can be applied.



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Selection depends on the substrate / ambient temperature of application. So that, “cool season” primer, –W, is recommended for use in the temperature with the range of 5°-15 °C, while the “warm season” primer, –S, is recommended for use in the temperature with the range of 15°-35 °C.

Prior to the MAXPRIMER®-C primer application, check and confirm the on-site conditions which

can affect the DRIZORO® WRAP System application: ambient temperature (greater or equal to 5°C) and presence of moisture on surface (less than 4%).

MAXPRIMER®-C is mixed using an electric hand mixer or spatula in clean container at a weight

ratio of main agent/hardener of 4:1 to produce a product with uniform consistency. The mixed quantity should be controlled to ensure that it is all used within the pot life period. Do not use any mixed primer that has exceeded the specified pot life.

Using a clean roller or brush, apply one or two layers, in a uniform manner, with a total

consumption from 0,10 to 0,35 kg/m2 (standard 0,25 kg/m2). The applied primer must become tack-free (no longer sticky) when touched before progressing onto the next stage.

7.2.2. Putty application If after primer application, concavities, gaps or pinholes up to 6 mm are seen on the surface,

MAXEPOX® -CP putty must be applied to smooth the concrete surface prior to the application of the resin and the carbon fibre sets.

Prior to the MAXEPOX®-CP putty application, check and confirm the ambient temperature

(greater or equal to 5°C) and that primer has becom e tack-free to the fingertip. If more than 7 days have passed since the primer application then, the primer surface must be roughened with sandpaper and wipe clean before putty application. Two types of putty can be applied. Selection depends on the substrate/ambient temperature of application. So that, “cool season” putty, –W, is recommended for use in the temperature with the range of 5°-15 °C, while the “warm season” putty, –S, is recommended for use in the temperature with the range of 15°-35 °C.

MAXEPOX®-CP is mixed using appropriate tools, in clean container at a weight ratio of main agent/hardener of 2:1 to produce a product with uniform consistency (paste) and colour (grey). The mixed quantity should be controlled to ensure that it is all used within the pot life period. Do not use any mixed putty that has exceeded the specified pot life.

Using a clean trowel o spatula, apply to all indents, defects or pinholes larger than 1 mm, with a

total consumption from 0,5 to 1,5 kg/m2. Generally, the putty is applied to small areas, but it can also be applied to level uneven concrete surfaces. The applied putty must become tack-free (no longer sticky) when touched before progressing onto the next stage.

7.2.3. Cutting of carbon fibre sheets Before proceeding to the MAXEPOX® -CS under-coating resin application, preparation of the

carbon fibre sheets must be taken. Using a scissor or utility cutter, cut a sheet to the specified length according to the drawing and work plan. It is recommended that prepared sheets have a maximum length in the range from 4 to 6 meters for easy of handling and to prevent wrinkling.



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Sheets must be handled and stored with care to prevent it from becoming contaminated or damaged. Contact with water is strictly prohibited. Sheets must not be folded, or rolled up without a central core.

Particular care should be taken when handling DRIZORO® WRAP HM (High Modulus) sheets,

as its fibres are particularly prone to damage.

7.2.4. Under/overcoat resin application, and carbon fibre sheet application

MAXEPOX®-CS under-coating resin functions as an adhesive to bond the carbon fibre sheet to

the concrete surface. The resin impregnates into the carbon fibres, which when cured, will form the laminate to strengthen the concrete element.

Prior to the MAXEPOX®-CS under-coating resin application, check and confirm the on-site conditions: ambient temperature (greater or equal to 5°C) and that primer, putty, or both have become tack-free to the fingertip. If more than 7 days have passed since the primer, putty application, or both then, the surface must be roughened with sandpaper and wipe clean before putty application. Two types of primer can be applied. Selection depends on the substrate / ambient temperature of application. So that, “cool season” primer, –W, is recommended for use in the temperature with the range of 5°-15 °C, while the “warm season” primer, –S, is recommended for use in the temperature with the range of 15°-35 °C.

MAXEPOX®-CS is mixed using an electric hand mixer or spatula, in clean container at a weight ratio of main agent/hardener of 4:1 to produce a product with a liquid uniform consistency and green colour. The mixed quantity should be controlled to ensure that it is all used within the pot life period. Do not use any mixed resin that has exceeded the specified pot life. Using a clean roller or brush, apply one or two layers, in a uniform and adequate manner to the surface of the primed concrete, with a total consumption from 0,40 to 0,50 kg/m2 (depending of carbon fibre type). Relatively large quantities of resin must be applied to areas of curvature when compared to that applied to flat concrete surfaces.

Immediately after MAXEPOX®-CS under-coating resin application, the carbon fibre sheets must be applied (within 20 minutes). The sheets are applied by pressing then onto the resin. The sheets should be smoothed longitudinally by hand, either from one end to the other. After the sheets have been attached, any air trapped between the sheets and concrete substrate is removed by gently and firmly pressing an “air removal roller” over the length of the sheets. This will allow the resin to impregnate into the carbon fibres. Do not roll against fibre direction since this may misalign or damage the fibres.

When connecting two sheets in the fibre longitudinal direction, the sheets are recommended overlap by at least 15 cm. When applying two sheets next to each other (side by side), a 1,25 cm overlap is recommended to ensure that there is no surface uncovered by carbon fibre.

Wait approximately 30 minutes to allow the resin impregnate into the carbon fibre sheet (If ambient temperature is 10 °C or below, wait 1 h), b efore proceeding to the next stage. The sheets application and MAXEPOX® -CS over-coating resin application must be completed in the same day.

Prior to the MAXEPOX®-CS over-coating resin application, check and confirm the on-site

conditions: ambient temperature (greater or equal to 5°C) and that under-coating resin has become



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tack-free to the fingertip. MAXEPOX®-CS is mixed according to above procedure. The mixed quantity should be controlled to ensure that it is all used within the pot life period. Do not use any mixed resin that has exceeded the specified pot life.

a) Single-layer application. Using a clean roller or brush, apply one o two layers, in a uniform and adequate manner to the carbon fibre sheet, with a total consumption from 0,2 to 0,3 kg/m2 (depending of carbon fibre type). Apply following the fibre longitudinal direction to prevent misalignment of the fibres. Wait 30 minutes to allow the resin to impregnate the fibre and then DRIZORO® WRAP System application is complete.

b) Multi-layer application. Where multiple layers of carbon fibre sheets have been specified, the

standard quantity of over-coating resin for mid-layers is the sum of the over-coating resin (0,2-0,3 kg/m2) and the under-coating resin (0,4-0,5 kg/m2) for the next layer, that is 0,6-0,8 kg/m2 per layer applied at once.

Alternatively, the over-coating resin and the under-coating resin may be applied in separate operations according above procedures, If more than 7 days have passed since the previous resin application, then the over-coated surface must be roughened with sandpaper and wiped clean before the next resin application. If the over-coating resin is not for the last layer of carbon fibre sheet, apply the next layer within 20 minutes of applying the resin. If this is the last layer of sheet applied, wait 30 minutes to allow the resin to impregnate the sheet and then, DRIZORO® WRAP System application is complete. When multiple layers are applied in one day, a maximum of 3 layers to the vertical surface, and a maximum of 2 layers to horizontal surface is recommended to be applied in any one day in order to prevent slippage or separation. In several days application, the top layer each day must be finish with the overcoating-resin. The next day, the under-coating resin is applied on top and the DRIZORO® WRAP System application process continues as indicated above. In case of a large surface area that requires multiple layers; overlapping size should be taken into consideration.

Curing time for MAXPRIMER®-C primer and MAXEPOX®-CP levelling putty depends on

substrate/ambient temperature. It varies from 3,5 to 7 hours and from 3 to 5 hours for primer and putty respectively.

Allow the MAXEPOX®-CS to cure and bond with strengthen the concrete surface. Depending on

the type of resin used (-W/-S) and ambient temperature, both complete curing and full load transfer occurs in 5 to 14 days.

If the substrate/ambient temperature is less than 5 °C, then heating apparatus such as a

spotlight or heater may be used to increase them. It should be noted that high a substrate/ambient differential temperature means a danger of condensation or dew forming.

If an air pocket is found, test the swell by tapping the surface with a hard object. This is necessary to confirm that swell is an actual air pocket and not a natural formation of the concrete surface. Inject resin into the air pocket and completely fill the air pocket with resin. Allow the injected resin to cure.



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7.2.5. Top-coating application

DRIZORO® WRAP System is extremely durable to weather conditions (heat, humidity, freeze/thaw cycles, marine environment), many acids and chemicals, gasoline, fuel and UV rays. From an architectural and aesthetic point of view, an application of a finish coating (weather resistant, finishing, or both mortar) is recommended to provide further protection against impacts, fire, weather conditions and mischief-makers. DRIZORO coatings or mortars are suitable products for these purposes:

• Exposed areas to UV rays or direct sunlight: MAXURETHANE® 2C. • Exposed areas to mechanical impacts: MAXREST®, CONCRESEAL® PLASTERING . To

improve the bonding property of these mortars over DRIZORO® WRAP System , silica sand (1,0 to 2,0 mm diameter – DRIZORO® SILICA 1020 ) should be spread over the before the over-coating resin hardens. A standard spraying quantity is 1 kg/m2.

• Decorative and protective finishing: MAXSHEEN®, MAXSHEEN® ELASTIC, MAXSEAL ®

FLEX, MAXQUICK ®

7.3. WET LAY-UP SYSTEMS: FABRIC

For installation of wet lay-up system based on fabric such as DRIZORO® CARBOMESH ,

wherein the fibres are separated forming an open grid mesh. In this cases can be used both saturating epoxy-based formula, referred to in paragraph 7.2, such as cement-based mortars modified with polymers to create the CFRP composite. In the case of using a cement-based mortar, such as CONCRESEAL® CARBOFIX , the steps are:

• Cutting of carbon fibre fabrics according to the established work plan based on the


• Cement-based mortar application and carbon fibre fabric application.

• Top coating application

7.3.1. Cutting of carbon fibre fabrics

Before proceeding to the installation, preparation of the carbon fibre fabrics must be taken.

Using a scissor or utility cutter, cut a fabric to the specified length according to the drawing and work plan. It is recommended that prepared fabrics have a maximum length in the range from 4 to 6 meters for easy of handling and to prevent wrinkling.

Fabric pieces must be handled and stored with care to prevent it from becoming contaminated or damaged. Contact with water is strictly prohibited. Fabric pieces must not be folded, or rolled up without a central core.



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7.3.2. Cement-based application and carbon fibre fa bric application

For bonding of the DRIZORO® CARBOMESH carbon fibre fabric, the CONCRESEAL®

CARBOFIX cement-based mortar with low modulus can be used.

A 25 kg bag of CONCRESEAL® CARBOFIX requires from 4,5 to 5,0 litres of water, depending on existing ambient conditions and desired consistency. Pour the required amount of water in a clean container and then slowly add CONCRESEAL® CARBOFIX to the liquid and mix, using a slow speed electric drill (400-600 rpm) fitted with a disc mixer for about 2-3 minutes to obtain a smooth, lump-free and homogeneous mortar of dry consistency. Do not overmix and allow the mixture to rest 5 minutes to fully wet out all the powder, and remix briefly before applying. Mix only the amount of material that can be place in about 20-30 minutes. After this time, mortar will have started its setting and will no longer be workable. To restore the workability, remix the mortar but never add more water.

Once substrate has been prepared, dampen thoroughly the entire surface to be coated with

clean water, avoiding the formation of puddles. Allow excess water to drain away, and then start the application once the surface acquires a matte appearance. If it is dry, proceed to saturate it with water again.

Apply one uniform and homogeneous layer of CONCRESEAL® CARBOFIX with a recommended consumption from 3 to 4 kg/m2, taking into account that application thickness is about 2 mm. Then, place the carbon fibre fabric pieces into the fresh previously applied mortar layer ensuring that fabric is completely embedded. Finally, apply a new layer of CONCRESEAL® CARBOFIX with consumption from 3 to 4 kg/m2, i.e. a thickness of about 2 mm. In case of application of several fabrics, repit the above-mentioned procedure as requiered.

For confinement of pillars and strengthening of walls, a fabric overlapping must be carried out in the longitudinal and transverse direction of about 10 cm in order to maintain the continuity of the strengthening.

Do not apply in rain or when rain, contact with water, condensation, dampness and dew is expected within 24 h after the application.

The optimum temperature range for application is from 10 ºC to 30 ºC. Do not apply with

substrate and/or ambient temperature is at or below 5°C, or when temperatures are expected to fall bellow 5 °C within 24 h after application. Do not a pply to frozen or frost-covered surfaces. For applications at hot temperatures (>35 °C), low rela tive humidity and/or windy conditions, i.e. summer time, surface must be wet thoroughly with plenty of water prior to application. Avoid also applications in areas exposed directly to the sunlight with high temperatures.

Prevent rapid drying of the CONCRESEAL® CARBOFIX application, and protect it from

extreme heat and direct sunlight exposure to maintain its moisture for at least 2 hours after the application, spraying a fine mist of water, without causing the washing or by using polythene sheeting or damp burlaps. Curing procedures should be observed mainly with high temperature (>30 °C), direct exposure to sunlight, and wind or low humidity (<50%) conditions. Total curing time for CONCRESEAL® CARBOFIX Is about 28 days. Do not allow to bear loads before full curing time.

When epoxy-based products are used, ambient and surface temperature must be at least 3 ºC

higher than dew point. Do not apply with R.H. higher than 85 %. Measure the relative humidity and



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dew point before applying the product. With low temperatures, high humidity levels or both, use dry and warm air in order to get the suitable conditions, such as with an electric powered air blower system.

7.3.3. Top-coating application

DRIZORO® WRAP System is extremely durable to weather conditions (heat, humidity, freeze/thaw cycles, marine environment), many acids and chemicals, gasoline, fuel and UV rays. From an architectural and aesthetic point of view, an application of a finish coating (weather resistant, finishing, or both mortar) is recommended to provide further protection against impacts, fire, weather conditions and mischief-makers. DRIZORO coatings or mortars are suitable products for these purposes:

• Exposed areas to UV rays or direct sunlight: MAXURETHANE® 2C. • Exposed areas to mechanical impacts: MAXREST®, CONCRESEAL® PLASTERING . To

improve the bonding property of these mortars over DRIZORO® WRAP System , silica sand (1,0 to 2,0 mm diameter – DRIZORO® SILICA 1020 ) should be spread over the before the over-coating resin hardens. A standard spraying quantity is 1 kg/m2.

• Decorative and protective finishing: MAXSHEEN®, MAXSHEEN® ELASTIC, MAXSEAL ®

FLEX, MAXQUICK ®

7.4. PRE-CURED SYSTEMS: LAMINATES

For installation of the re-cured systems externally bonded, an epoxy-based structural adhesive

is requiered:

• Primer application. • Cutting of carbon fibre pre-cured laminates according to the established work plan based on

the strengthening project.

• Epoxy-based structural adhesive application and pre-cured laminate application.

7.4.1. Primer application

Primer penetrates into the concrete surface to increase the strength of concrete and to improve

the bonding between the concrete and the carbon fibre sheet. Two types of primer can be applied. Selection depends on the substrate / ambient temperature of application. So that, “cool season” primer, –W, is recommended for use in the temperature with the range of 5°-15 °C, while the “warm season” primer, –S, is recommended for use in the temperature with the range of 15°-35 °C.



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Prior to the MAXPRIMER®-C primer application, check and confirm the on-site conditions which can affect the DRIZORO® WRAP System application: ambient temperature (greater or equal to 5°C) and presence of moisture on surface (less than 4%).

MAXPRIMER®-C is mixed using an electric hand mixer or spatula in clean container at a weight

ratio of main agent/hardener of 4:1 to produce a product with uniform consistency. The mixed quantity should be controlled to ensure that it is all used within the pot life period. Do not use any mixed primer that has exceeded the specified pot life.

Using a clean roller or brush, apply one or two layers, in a uniform manner, with a total

consumption from 0,10 to 0,35 kg/m2 (standard 0,25 kg/m2). The applied primer must become tack-free (no longer sticky) when touched before progressing onto the next stage.

7.4.2. Cutting of carbon fibre pre-cured laminates Before proceeding to the installation, preparation of the carbon fibre pre-cured laminates must

be taken. Clean thoroughly the laminate surfaces with MAXEPOX ® SOLVENT before use. Allow to dry and cut the laminate according to the lengths specified in the design drawings, with a suitable grinder using a duct tape in the cutting zone.

Pre-cured laminate pieces must be handled and stored with care to prevent it from becoming contaminated or damaged. Contact with water is strictly prohibited. Pieces must not be folded.

7.4.3. Epoxy-based structural adhesive application and pre-cured laminate pieces application For bonding of carbon fibre pre-cured laminated, the use of the MAXEPOX CARBOFIX

structural epoxy-based adhesive is recommended. MAXEPOX® CARBOFIX is supplied as a pre-weighed two-component set. Premix the

components separately, and then the hardener, component B, is poured into the resin, component A. In order to ensure the proper reaction of the two components make sure all of component B is added. Mixing manually for small quantities of product, or preferably using a low speed drill (300-400 rpm. maximum), fitted with a mixer suitable for epoxy liquids for about 2-3 minutes until achieving a homogeneous product in colour and appearance. Scrape the sides and the bottom of the container several times during mixing to ensure complete mixing. Do not mix for prolonged period nor use high-speed mixer, which may heat the mixture or introduce air bubbles. Check Technical Data Table for product pot life (40 minutes at 20° C for a 5 kg se t). This value increases with lower temperatures or small quantities of mixture, and reduces with higher temperatures or mixing bigger quantities.

Apply on DRIZORO ® COMPOSITE a layer of MAXEPOX ® CARBOFIX with a thickness of about 1 to 3 mm and spread it with curved spatula achieving greater thickness on the middle and decreasing on the edges. Apply a similar layer on the structure surface wherein laminate will be bonded.

Place DRIZORO ® COMPOSITE on the substrate within the open time of the adhesive and

press the laminate using a hard rubber roller to force the adhesive overflow on both sides, ensuring total saturation between surfaces of the laminate and the substrate and avoiding entrapped air voids. Finally, remove any excess of adhesive with a spatula.



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Do not apply in rain or when rain, contact with water, condensation, dampness and dew is

expected within 24 h after the application, and protect the application against contact with water until the total curing of the material. Optimum application temperature range is from 10 ºC to 35 ºC. Do not apply with substrate and/or ambient temperature below 10 °C, or when are expected to fall bellow 10 °C within 24 h after application.

Do not apply to frozen or frost-covered surfaces. Ambient and surface temperature must be at

least 3 ºC higher than dew point. Do not apply with R.H. higher than 85 %. Measure the relative humidity and dew point before applying the product. With low temperatures, high humidity levels or both, use dry and warm air in order to get the suitable conditions, such as with an electric powered air blower system.

Temperatures above 30 ºC lead a quick-setting between components and heat production, so the pot life is greatly reduced. In this case, before applying the system, store products at temperatures between 15 °C to 20 °C and plan previously the work s.

Allow the MAXEPOX ® CARBOFIX structural adhesive for DRIZORO ® COMPOSITE to cure for at least 7 days at 20 °C and 50% R.H. before puttin g into service. Minimum temperature during the full curing time must be higher than 10°C. Applicat ions at lower temperatures, high humidity and/or poor ventilation require longer drying and curing times. Do not allow to bear loads before full curing time.

7.5. CUT-IN SYSTEMS: COMPOSITE BARS For the installation of pre-cured bars, such as DRIZORO CARBOROD , placed into grooves on

concrete surfaces (cut-in systems), an epoxy-based structural adhesive is required: • Cutting of carbon fibre pre-cured bars according to the established work plan based on the


• Epoxy-based structural adhesive application and pre-cured bars application.

7.5.1. Cutting of carbon fibre pre-cured bars

Before proceeding to the installation, preparation of the carbon fibre pre-cured bars must be

taken. Clean thoroughly the laminate surfaces with MAXEPOX ® SOLVENT before use. Allow to dry and cut the laminate according to the lengths specified in the design drawings, with a suitable grinder using a duct tape in the cutting zone.

Pre-cured bars pieces must be handled and stored with care to prevent it from becoming contaminated or damaged. Contact with water is strictly prohibited. Pieces must not be folded.



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7.5.2. Epoxy-based structural adhesive application and pre-cured bars application For bonding of carbon fibre pre-cured bars, the use of the MAXEPOX CARBOFIX structural

epoxy-based adhesive is recommended. MAXEPOX® CARBOFIX is supplied as a pre-weighed two-component set. Premix the

components separately, and then the hardener, component B, is poured into the resin, component A. In order to ensure the proper reaction of the two components make sure all of component B is added. Mixing manually for small quantities of product, or preferably using a low speed drill (300-400 rpm. maximum), fitted with a mixer suitable for epoxy liquids for about 2-3 minutes until achieving a homogeneous product in colour and appearance. Scrape the sides and the bottom of the container several times during mixing to ensure complete mixing. Do not mix for prolonged period nor use high-speed mixer, which may heat the mixture or introduce air bubbles. Check Technical Data Table for product pot life (40 minutes at 20° C for a 5 kg se t). This value increases with lower temperatures or small quantities of mixture, and reduces with higher temperatures or mixing bigger quantities.

Open a groove or drill hole into the concrete or masonry by suitable means and provide an open roughened texture. Size should be about 1,5 times the diameter of the carbon fibre rod to be set. Care must be taken not to cut any other existing embedded element; bars, tendons, ducts, etc. Finally, thoroughly clean the inner surface at the hole or groove using a vacuum and/or compressed air. Surface must be free of paints, coatings, efflorescence, greases, oils, demoulding agents, dust, gypsum, etc.

Apply MAXEPOX ® CARBOFIX into the groove, or MAXFIX® -E into the borehole checking that there is no occluded air at the bottom. Place DRIZORO ® CARBOROD pressing slightly while the structural adhesive is still fresh, i.e. within its open time, and ensuring it is fully saturated by the epoxy adhesive of the groove. Finally, apply additional layers of MAXEPOX ® CARBOFIX to cover completely all around the rod. Remove excess adhesive before hardening.

Do not apply in rain or when rain, contact with water, condensation, dampness and dew is expected within 24 h after the application, and protect the application against contact with water until the total curing of the material. Optimum application temperature range is from 10 ºC to 35 ºC. Do not apply with substrate and/or ambient temperature below 10 °C, or when are expected to fall bellow 10 °C within 24 h after application. Do not apply to f rozen or frost-covered surfaces. Ambient and surface temperature must be at least 3 ºC higher than dew point. Do not apply with R.H. higher than 85 %. Measure the relative humidity and dew point before applying the product. With low temperatures, high humidity levels or both, use dry and warm air in order to get the suitable conditions, such as with an electric powered air blower system. Temperatures above 30 ºC lead a quick-setting between components and heat production, so the pot life is greatly reduced. In this case, before applying the system, store products at temperatures between 15 °C to 20 °C and plan previously the works.

Allow the MAXEPOX ® CARBOFIX structural adhesive or MAXFIX® -E anchoring resin for DRIZORO ® CARBOROD to cure for at least 7 days at 20 °C and 50% R.H. b efore putting into service. Minimum temperature during the full curing time must be higher than 10°C. Applications at lower temperatures, high humidity and/or poor ventilation require longer drying and curing times. Do not allow to bear loads before full curing time.



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7.6. QUALITY CONTROL DURING THE INSTALLATION Quality control during FRP installation should include at least one cycle of semi-destructive tests

for the mechanical characterization of the installation itself, and at least one non destructive mapping to ensure its uniformity.

7.6.1. Semi-destructive tests Both pull-off tests and shear tearing tests may be carried out. Semi-destructive tests shall be

carried out witnesses and, where possible, in non-critical strengthening areas at the rate of one test every 5 m2 of application, and in any case, not less than 2 per each type of test.

7.6.1.1. Pull-off test This test is used for assessment of the properties of restored concrete substrate; it is carried out

by using a 20 mm thick circular steel plates with a diameter of at least 3 times the characteristic size of the concrete aggregate nor less than 40 mm, adhered to the surface of the FRP with epoxy adhesive

After the steel plate is firmly attached to the FRP , it is isolated from the surrounding FRP with a

core drill rotating at a speed of at least 2500 rpm; particular care shall be taken to avoid heating of the FRP system while 1-2 mm incision of the concrete substrate is achieved.

FRP application may be considered acceptable if at least 80 % of the test (both test in case of

only two tests) return a pull-off stress not less than 0,9-1,2 MPa provided that failure occurs in the concrete substrate.

7.6.1.2. Shear tearing test The test is particularly significant to assess the quality of bond between FRP and concrete

substrate. It may be carried out only when it is possible to pull a portion of the FRP system in its plane located close to an edge detached from concrete substrate. FRP applications may be considered acceptable if at least 80 % of the tests (both in the case of two tests) return a peak tearing force not less than 24 kPa.

7.6.2. Non destructive tests

Non destructive tests may be used to characterize the uniformity of FRP applications. The most common tests are:

• Test carried out by a technician hammering the composite surface and listening to the sound from the impact, indicating how the bond between FRP layers and concrete substrate is.

• High-frequency ultrasonic testing. • Thermo-graphic tests. • Acoustic emission tests

Guide for the Design of Externally Bonded

DRIZORO S.A.U. Tel./Phone: +34 916766676

e-mail:

8. DESIGN EXAMPLES

In this chapter, some strengthening design examples using carbon fiber composite are provided:

- Flexural strengthening of a

8.1. FLEXURAL STRENGTHENING OF A RECTANGULAR REINFORCED CONCRETE BEAM

8.1.1. Geometric al and mechanical data A simply supported rectangular

mm = As1 = 1.470 mm2), and upper;below figure). Characteristic yield strength for sstrength for concrete fck=30 MPa. increase in its live-load-carrying requirementsbeam still has sufficient shear strength to resist the new required shear strength and meets the deflection and crack-control serviceability requirements. Its flexuto carry the increased live load.

Figure 8.1.- Schematic of the idealized simply supported beam with FRP external

As1

As2

b

h d

d1

d2


FRP Systems for StrengtheningTECHNICAL DEPARTMENT

DRIZORO S.A.U. C/Primavera, 50-52. 28850 Torrejón de Ardoz-Madrid (SPAIN)Tel./Phone: +34 916766676 – Fax: +34 916776175 mail: [email protected] – Web site: www.drizoro.com

DESIGN EXAMPLES

In this chapter, some strengthening design examples using carbon fiber composite are

Flexural strengthening of a rectangular reinforced concrete beam.

FLEXURAL STRENGTHENING OF A RECTANGULAR REINFORCED CONCRETE

al and mechanical data

rectangular with cross-section (60 cm x 30 cm) reinforced (upper; 2Φ = 12 mm = As2 = 339 mm2) concrete

Characteristic yield strength for steel fyk=500 MPa, and characteristic compressive 30 MPa. Beam is located an exterior area and it

carrying requirements. An analysis of the existing beam indicates that the beam still has sufficient shear strength to resist the new required shear strength and meets the

control serviceability requirements. Its flexural strength, however, is inadequate

Schematic of the idealized simply supported beam with FRP external

Guide for the Design of Externally Bonded FRP Systems for Strengthening

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Madrid (SPAIN)

In this chapter, some strengthening design examples using carbon fiber composite are

FLEXURAL STRENGTHENING OF A RECTANGULAR REINFORCED CONCRETE

reinforced (lower; 3Φ =25 concrete beam of 7 m length (see

characteristic compressive it is subjected to a 42%

An analysis of the existing beam indicates that the beam still has sufficient shear strength to resist the new required shear strength and meets the

ral strength, however, is inadequate

Schematic of the idealized simply supported beam with FRP external reinforcement



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Length of the beam: L (m) 7,0 Section width of the beam: b (mm) 300 Section height of the beam: h (mm) 600 Effective depth (distance to the centroid of the tension steel reinforcement: d (mm) 540 Characteristic compressive strength of concrete: fck (N/mm2) 30 Characteristic yield strength of steel reinforcement: fyk (N/mm2) 500 Area of lower (tensile) steel reinforcement: As,1 (mm2) 1.470 Area of upper (compressive) steel reinforcement: As,2 (mm2) 339 The ultimate moment for the above rectangular cross-section reinforced concrete beam before

placing FRP external reinforcement is: Ultimate moment without FRP: Mu,sr (kN·m) 314,0

8.1.2. Loadings and corresponding moments Summarized in Table are the existing and new loadings and associated midspan moments for the beam.

Loadings / Moment Existing loads Anticipated loads Dead loads: g (kN/m) 14,6 14,6 Safety partial factor for dead (permanent) loads, γg 1,35 1,35 Live loads: q (kN/m) 17,5 24,8(1) Safety partial factor for variable (live) loads, γq 1,5 1,5 Unfactored loads: p=g+q (kN/m) 32,1 39,4 Unstrengthened load limits: (kN/m) --- 51,3(2) Dead-load moment: Mg,k (kN·m) 89,4 89,4 Live-load moment: Mq,k (kN·m) 107,2 151,9 Service-load moment: Mk = Mg,k + Mq,k (kN·m) 196,6 241,3 Factored moment: Md = Mg,k*γg + Mq,k*γq (kN·m) Actual: 281,5 Design: 348,6

(1) 42% increase in its live-load-carrying requirements; 17,5 kN/m * (1,0+0,42) = 24,8 kN/m (2) Unstrengthened load limit:

M�p�L2

8 � �

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�� 314

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�

8.1.3. Mechanical characteristics for FRS systems The existing reinforced beam should be strengthened with any FRP system described in

following table, specifically, a) two 300 mm wide sheets, or b) two 50 mm wide plies bonded to the soffit of the beam using the wet lay-up system or the procured system technique, respectively.



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Type and class of FRP strengthening system Wet lay-up system

(unidirectional sheet) Precured systems

(unidirectional laminate)

Name DRIZORO® WRAP 300

DRIZORO® COMPOSITE 1405

Thickness per sheet / ply: tR (mm) 0,167 1,4 Width of sheet / ply (mm) 300 50 Number of sheets / plies: nR 2 3 Modulus of elasticity, ER (N/mm2) 230.000 165.000 Tensile strength at break: fRu (N/mm2) 4.400 2.200 Ultimate rupture strain: εRu (%) 1,9 1,6 Characteristic rupture strain: εRk (%) 1,50 1,33

The maximum tensile strain of the composite, εRd shall be calculated as following:

= Rddm

RkaRd ε

γεηε ,min (Eq. 4-7)

where εRk is the characteristic strain at failure for the adopted strengthening system; ηa is the safety factor as per ambient type (See Table 3.6); γm is the partial safety factor for materials (See Tables 3.4 and 3.5; γm= γmd * γRd = 1,35 or 1,50 * 1, and εRdd is the maximum strain due to FRP debonding as defined in Section 4.2. Generally, the one that prevail

Rua

RRR

ckRdd

tEn

f εηε 9,041,0 ≤⋅⋅

=

Type and class of FRP strengthening system Wet lay-up

systems (sheet)

Precured systems (Laminate)

Name DRIZORO® WRAP 300


ηa for exterior exposure (Table 3.6) = 0,85 0,85 γm (Table 3.4) = 1,35 1,50 ηa·εRk/γm (%) = 0,94 0,75

0,9·ηa·εRu (%) = 1,45 1,22 εRdd (%) = 0,81 0,47

Maximum tensile strain of the composite: εRd (%) (Eq. 4-7) 0,81 0,47



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8.1.4. Initial situation The effect of the initial load prior to strengthening should be considered in the calculation of the strengthened member. Based on the theory of elasticity and with Mo the service moment (no load safety factor are applied) acting on the critical RD section during strengthening, the strain distribution of the member can be evaluated.

MPa200.000E

MPa 28.577388.500·f8.500·E

MPa 2,896300,30f0,30f

MPa 38 830 8ckfcmf

s

33cmc

3 23 2ckmct,

====

===

=+=+=

kN·m 52,16

0,3·0,62,896·106

hbfM

232ctm

cr =⋅=⋅⋅

=

Based on the transformed cracked section, the neutral axis depth xo can be solved from:

( ) ( )o1s2o2s2o xdnAdxA)1n(bx

21 −=−−+ (Eq. 4-1)

wherein ns = Es/Ec = 200.000/28.577 = 7,0

Mean concrete tensile strength: fct,m (MPa) (Eq. 3-8) 2,896 Modulus of elasticity of concrete: Ec (MPa) (Eq. 4-2) 28.577 ns = Es/Ec 7,0 Cracking moment: Mcr (kN·m) (Eq. 4-2) 52,1 Initial moment = Dead load moment: Mo = Mg,k (kN·m) 89,4 Neutral axis depth: xo (mm) (Eq. 4-1) 157,8

As1

As2

b

h d

d1

d2

xo

εco

Mo

εo



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Moment of inertia of the transformed cracked section:

( ) ( )2o1s

22o2s

3o

co xdnAdxA)1n(3

bxI −+−−+= (Eq. 4-4)

Moment of inertia of the cracked section transformed to concrete: Ico (mm4), (Eq. 4-4)

1915·106

Base on strain compatibility, the concrete strain εo at the extreme tension fibber, ie. strain on the

soffit, can be derived as:

o

ocoo x

xhεε

−= (Eq. 4-5)

coc

ooco IE

xM=ε (Eq. 4-3)

Initial strain on the soffit, εo (%) 0,00072

This existing state of stain on the soffit, i.e wherein FRP is placed, is necessary in order to

calculate the stress level in the reinforcing section.

8.1.5. Calculate of strengthening

εo

As1

As2

b

h d

d1

d2

x

fcd

Md

fyd

fR

AR

εc

εs2

εRd

εs1



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Calculation of neutral axis depth, x: Forces equilibrium:

Fc + Fs comp = Fs trac + FRmax

RRRyds1s2ss2cd εEAfAεEAx0,8bf0,85 +=+⋅⋅⋅⋅ (Eq. 4-8)

Momentum equilibrium:

Md = Ms trac + MR + Ms comp

Md = As1 fyd (d - 0,4x) + AR ER·εR (h - 0,4x) + As2 Es·εs2 (0,4x – d2) (Eq. 4-9) Strain compatibility:

s

yd2cs2 E

f ;

x

dxεε ≥

−= (Eq. 4-10)

ocR εx

xhεε −−= (Eq. 4-11)

s

ydc1s E

f

xxd ≥−ε=ε (Eq. 4-12)

In these regions, two types of failure are considered:

Region 2: Tensile failure of FRP composite: εR = εRd = 0,0081 (DRIZORO® WRAP 300) εR = εRd = 0,0047 (DRIZORO® COMPOSITE 1405)

Region 3: Compressive failure of concrete: εc= 0,0035 Procedemos al cálculo suponiendo que tanto el acero traccionado como el comprimido han

plastificado y que el refuerzo está trabajando en su máxima deformación, i.e. Region 2 (Tensile falure of FRP composite: εR = εRd).

RRRRRRRRmaxR,

yds2comps,

yds1s,trac

cdc

·ε)·E·w·t(n εEA F

N 147.3911,15500

· 339fAF

N 639.1301,15500

· 1.470fAF

4.080·x300·0,8·x1,530

·0,85x0,8bf0,85F

==

==⋅=

==⋅=

=⋅=⋅⋅⋅⋅=

Fc + Fs comp = Fs trac + FRmax

0,85·fcd·0,8·x + As2·Es·εs2 = As1·fyd + ARERεR



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DRIZORO WRAP 300:

FRmax = AR·ER·εR = (nR·tR·wR)·ER·εR = (2·300·1,4)·165.000·0,0081= 186.725 N

4.080·x + 147.391 = 639.130 + 186.725

xn est = 166,3 mm DRIZORO COMPOSITE 1405:

FRmax = AR·ER·εR = (nR·tR·wR)·ER·εR = (2·50·0,167)·230.000·0,0047= 161.898 N

4.080·x + 147.391 = 639.130 + 161.898

xn est = 160,2 mm

CALCULATE OF M u,R DRIZORO® WRAP 300


Number of sheets / plies, nR 2 3 Width of the sheet / ply, (mm) 300 50

Fs,trac (N)= 639.130 639.130 Fs,comp (N)= 147.391 147.391 Fc (N/mm)= 4.080·x 4.080·x FR,max (N)= 186.725 161.898

Estimated neutral axis depth: xn,est (mm)= 166,3 160,2

Ms trac = As1 fyd (d - 0,4x)

MR max = AR ER·εR (h - 0,4x) Ms comp = As2 Es·εs2 (0,4x – d2) Mu est R = Ms trac + MR + Ms comp



Ms,trac (kN·m)= 302,6 304,2 MR,max (kN·m)= 99,6 86,8 Ms,comp (kN·m)= 1,0 0,6

Mu,est,R (kN·m) = 403,2 391,5



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Strain compatibility:

0,217200.000

1,15500

E

f ;

x

dxεε

s

yd2cs2 ==≥

−= (Eq. 4-10)

ocR εx

xhεε −−= (Eq. 4-11)

0,217200.000

1,15500

E

f;

xxd

εεs

ydcs1 ==≥−= (Eq. 4-12)

Verification DRIZORO® WRAP 300


εR (%) = 0,81 0,47 εc (%) (Eq 4.11) = 0,34 < 0,35 →OK 0,20 < 0,35 →OK εs2 (%) (Eq. 4.10) = 0,22 ≥ 0,217 →OK 0,12 →NO εs1 (%) (Eq. 4.12) = 0,76 ≥ 0,217→OK 0,47≥ 0,217→OK

For DRIZORO® COMPOSITE 1405, a new value for the strain in compressed steel reinforcement, εs2, is proposed (0,12 % → 0,0012):

Fs comp = As2·Es εs2 = 339· 200.000 0,0012 = 83.341 N



Number of sheets / plies, nR

3 Width of the sheet / ply, (mm) 50

Fs,trac (N)= 639.130 Fs,comp (N)= 83.341 Fc (N/mm)= 4.080·x FR,max (N)= 161.898

4.080·x + 83.341 = 639.130 + 161.898

xn est = 160,2 mm

Estimated neutral axis depth: xn,est (mm)= 175,9



Ms,trac (kN·m)= 300,2 MR,max (kN·m)= 85,7 Ms,comp (kN·m)= 0,9

Mu,est,R (kN·m) = 386,8



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8.1.6. Analysis at Serviceability Limit State (SLS)

From the equilibrium of forces and strain compatibility, the depth of the neutral axis x is obtained

from the following:

+−+−=−−+ x1hAn)xd(An)dx(A)1n(bx

21

c

oRR1ss22ss

2

εε (Eq. 4-21)

okc

o

xM

M x0≈εε (Eq. 4-22c)

Loadings / Moment Anticipated loads Initial moment = Dead-load moment: Mo =Mg,k (kN·m) 89,4 Live-load moment: M1 =Mq,k (kN·m) 151,9 Service-load moment: Mk = Mg,k + Mq,k (kN·m) 241,3

Neutral axis depth: xo (mm) (Eq. 4-1) 157,8

DRIZORO® WRAP 300


Number of sheets/plies, nR 2 3 Width of the sheet / ply, (mm) 300 50

Neutral axis depth (Eq. 4-21): x = x1 (mm)= 149,8 144,0 Moment of inertia of the cracked section: Ifis,1 (mm4)= 2.155.·106 2.248·106

εo

As1

As2

b

h d

d1

d2

x

fcd

Mk

fyd

fR

AR

εc

εs2

εRd

εs1

Ns2

Nc

Ns1

NR



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( )2RR

2s1s

22s2s

3fis,1 xhAnx)(dAn)d(x1)A(nbx

31

I −+−+−−+=

Moment of inertia of the cracked section: Ico (mm4) = 1.915·106

Stress at service in the FRP system, computed for the quasi-permanent loading condition, shall satisfy the limitation σR ≤ ηfRk, where fRk is the FRP characteristic strength at failure and η is the conversion factor as suggested in Section 3.5.2. Service stress in concrete and steel shall be limited according to the current building code, σc ≤ 0,60 fck,j, where fck,j is the characteristic strength at age j when; σs ≤ σe = fyd/γs.

SERVICE STRESS (MPa)

Concrete: σc = 0,60 fck,j = 30·0,6 = 18,0

Steel: σs = fyd/γs.500/1,15= 434,8

DRIZORO® WRAP 300 DRIZORO® COMPOSITE 1405

η·fRk (MPa): 0,85 3.400=2.930 0,85·2.000=1.700 Assuming that Mo is the bending moment acting on the member prior to FRP strengthening, and

assuming that M1 is the bending moment acting after FRP strengthening, the stress due to the combined moment M = M0 + M1 can be evaluated as follows:

;xh

IW;

xdI

W;xd

IW;

xI

W;xI

W

resistance of Modulus

W

Mnσ :composite FRPin Stress

W

M

W

Mnσ :steel in Stress

W

M

W

Mσ :concrete inStress

1

1R1,

1

1tracs1,

0

0tracs0,

1

1c1,

0

0compc0,

R1,

1Rs

is1,

1i

s0,

0ss

c1,

1

c0,

0c

−=

−=

−===

=

+=

+=

(Eq. 4-24)

Verification

DRIZORO® WRAP 300

DRIZORO COMPOSITE

1405

Stress at service for concrete (Eq.4-24): σc (MPa)= 17,9 < 18,0

Ok 17,1 < 18,0

Ok

Stress at service for steel (Eq.4-24): σs (MPa)= 317,3 < 434,8

Ok 312,1 < 434,8

Ok

Stress at service for FRP composite : (Eq.4-24) σR (MPa)= 255,3 < 2.930

Ok 177,8 < 1.700

Ok



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8.1.7. Service strain limits (Deflections control)

L/d = 7.000/540 = 13 < (20-14)·(1.470·100/(50·300)-0,5)+14 = 15,9 It is not necessary the flexa verification.

rt_060_01 guide for cfrp systemsx

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