contextocw.snu.ac.kr/sites/default/files/note/week11_structural... · 2019-03-15 · • the above...

Context

High-Performance Concrete (HPC)

Lightweight Concrete

Fiber Reinforced Concrete (FRC)

Polymer Concrete (PC)

High-Performance Concrete

• Terminology

• High-strength concrete

• High-performance concrete

• High elastic modulus

• High flexural strength

• Low permeability

• Improved abrasion resistance

• Good durability

• High flowability

• Great progress on concrete technology in the last 50 years through

the use of

• High-range water-reducing admixtures (superplasticizers)

• Viscosity modifying admixtures


Benefits of using HPC

• For high-rise buildings: increase strength/weight ratio

• ↓thickness of columns

• Translate to more usable space

• ↑stiffness of high-rise buildings

• Reducing lateral sway caused by wind loading

• Increase the comfort level of occupants

• ↓ dead load of structures

• Structures maybe built on soil with marginal load-

carrying capacities.

• For severe exposure conditions: increase density,

reduce water permeability

• Improve durability

• Increase service life

• Reduce maintenance cost


• HSC: 60 - 105 MPa (characteristic cube strength)

• Water/cement: 0.30 - 0.35 or even lower

• Cement content: about 450 – 550 kg/m3


HPC Principles

• Concrete is a composite material

• Hardened cement paste

• Aggregate

• Interface (transition) zone

• Concrete failure under compressive load

• Will always develop in the weakest part of these three phases

• In ordinary concrete

• Within mortar or along the interface between the mortar

and coarse aggregate particles

• In HPC

• May go through coarse aggregate particles


HPC Principles (cont’d)

• Improving the strength of hydrated cement paste

• Reduce porosity

• Reduce pore size, particularly large pores

• Decrease the grain size of hydration products

• The above can be achieved by reducing w/cm with the help of

superplasticizers and the use of supplementary cementing

materials


HPC Principles (cont’d)

• Improving the strength of the ITZ

• Reduce w/cm

• Use supplementary cementing materials such as silica fume

• Search for strong aggregates

• If the aggregates, particularly the coarse ones, are not strong

enough, they can become the weakest links within the concrete

• The strength of natural aggregates depends on the nature of the

parent rock

• Selection of stronger aggregates

• Petrographic analyses

• Introduce the aggregate in a HPC,

and to look at the fracture surface

after compression failure


Effect of Silica Fume (SF) and Nano-silica (NS)

• Both SF and NS are nano-sized highly reactive silica, but

the average primary particle size of the NS is generally

much smaller than that of SF.

• Mechanisms by which silica fume modifies cement paste,

mortar, and concrete are investigated extensively, and are

summarized in ACI Committee 234 report.

• Physical effect

• Chemical effect

• Microstructure modification

• These mechanisms are also applicable to NS.


Mechanisms by which Silica Fume Modifies Cement Paste,Mortar, and Concrete


• Physical effect

• Accelerate cement hydration by providing high amount of

nucleation sites

• Act as reactive filler which reduced bleeding and increases

packing density of solid materials

• Chemical effect

• Highly reactive pozzolanic material

SiO2 + Ca(OH)2 +H2O → C-S-H


• Microstructure modification

• Reduce porosity

• In cement paste

• In ITZ between the paste and aggregate

• Reduce CH crystal size in ITZ

• Increase density of cement paste and improve bonding between

paste and aggregate

Mechanisms by which Silica Fume Modifies Cement Paste,Mortar, and Concrete (cont’d)

Compressive Strength of HPC


• Unlike ordinary concrete, the w/c “law” is only valid until the concrete

aggregate becomes the “weakest” link within the HPC.

• Select stronger aggregate

• Rapid rate of strength development at early age

• 15 MPa within 12 hrs, 30 MPa within 24 hrs

• The hydration reaction can be delayed in HPC due to the use of

high dosages of SP, but when hydration starts it develops

rapidly.

Tensile Strength


• As the compressive strength ↑, ft/fc ↓

Elastic Modulus of HPC


• Equation recommended by ACI Committee 363

Ec = 3320 sqrt(fc) + 6900 MPa

21 < fc < 83 MPa

• It is recommended to measure E directly for HPC rather

than rely on empirical equations.

Stress-Strain Curve of HPC


• Ordinary concrete

• High-performance concrete

• Steeper

• More linear to a higher stress-strength ratio

• Fracture: more brittle

Mechanical Properties of HPC


• Shrinkage

• Drying shrinkage

• Autogenous shrinkage

• Creep

• Are generally lower

compared with ordinary

concrete

• Poisson ratio

• Limited data are available

• Is generally ≤ that of

ordinary concrete

(Ngab et al. 1981)

Durability of HPC


• Due to low w/c and improved ITZ

• HPC has low permeability, excellent durability to various

physical and chemical attacks

• Due to higher cement content

• Thermal cracking can be a durability problem

• Expected temperature rise: 12-14 °C for every 100 kg/m3

cement

• How to reduce rapid temperature rise?

• Use pozzolanic materials to replace some cement

• Use ice to replace part of the mixing water

• High-strength concrete usually has good abrasion resistance

Applications of HPC


• High-rise buildings

• Bridges

• Offshore-oil platforms

• Pavements

• Application for reducing abrasion-erosion damage

…

Water Towel Place - Chicago (1968)


• In 1960, the highest strength available in Chicago market was 30 MPa concrete with 100 mm slump.

• John Albinger asked for permission to cast a 40 MPa concrete column at no extra cost in a building with 30 MPa concrete.

• He used same tactics and asked for permission to cast a 50 MPa concrete column at no extra cost in a building with 40 MPa concrete.

• The same approach was employed with 60 MPa concrete.

• This opened door for the design of Water

Tower Place

• 60 MPa concrete for columns of lower floors.

• Strength is progressively reduced to 30 MPa for the columns of top floors

Two Union Square – Seattle (1989)


• Seattle, windy city

• To minimize the lateral deflection

• A rigid composite structure with steel tubes confining concrete was designed

• Design requirements

• strength = 90 MPa

• E = 50 GPa

• To meet requirement of E = 50 GPa

• Coarse aggregate imported from Canada

• w/c ≤ 0.22• Compressive strength = 130 MPa


The Petronas Towers- Kuala Lumpur (1998)

Burj Khalifa – Dubai(completed in Jan 2010)

• 451.9 m• 80 MPa used for

the columns of lower floors

• 828 m• Tallest building

in the world• Up to 586 m,

RC structure• Above 586 m,

steel structure


Kinzua Dam, Western Pennsylvania, USA


• Compared to steel, the low strength/weight ratio for

concrete presents an economic problem in the

construction of

• Tall buildings

• Long-span bridges

• Floating structures

• Two approaches to improve the strength/weight ratio

• Increase strength

• Lower the unit weight of the concrete


Classification of Lightweight Concrete

• Density of concrete can be reduced by replacing some

solid materials by air voids

• In aggregate: lightweight aggregate concrete

• In cement paste: cellular concrete, autoclaved aerated concrete

• Between coarse aggregate particles (fine aggregate omitted):

no-fine concrete

• Mainly used in load bearing walls in domestic buildings

and in-filling panels in framed structures.

• It is normally not used in reinforced concrete

• Strength < 14-15 MPa


Classification Unit Weight (kg/m3)

Ultra-lightweight concrete < 1200

Lightweight concrete 1200 < UW < 1800

Normal-weight concrete ~2400

Cellular Concrete (mortar), Autoclaved Aerated Concrete


• Applications: masonry units

• Production

• High-pressure steam curing (180 °C)

• Compressive strength (0.3-7 MPa)

• Law dry density (300-1100 kg/m3, typically < 800 kg/m3)

• Low thermal conductivity (0.1-0.3 W/m. K) compared with

ordinary concrete (1.5 to 3.5 W/m. K)

• Can be handled easily

Lightweight Aggregate Concrete (LWAC)


• Types of LWA

• Natural LWA: pumice, scoria

• Manufactured LWA

• Natural materials (expanded clay, shale, slate, perlite)

• Industrial by-products (sintered fly ash, expanded slag)

• Standard for LWA used for structural applications

• BS-EN 13055-1: 2002

• ASTM C 330

LWAC vs. NWAC.What are the differences?


• Definition of LWAC

• In many codes, LWAC is defined as concrete having

oven-dry density of < 2000 kg/m3

• Characteristics of LWAC

• Low unit weight

• Low thermal conductivity, high insulating capacity

• Moisture transport between aggregate and cement paste

• Interfacial zone

• Brittleness

• Failure mode

• Permeability

Characteristics of LWA


• Particle density

• LWA < 1800 kg/m3

• NWA ~2600 to 2700 kg/m3

• Strength

• Relationship between the

strength of LWAC and the

particle density of LWA

• Pore structure

• Total porosity

• Pore size distribution

• Structure near surface

• Water absorption

• Low in LWA with smooth surface

• Relatively high in crushed LWA

with open pores

Influence on Interface Transition Zone (ITZ)


• NWAC

• More porous ITZ relative to bulk cement

paste matrix

• Interfacial zone improves with the

reduction in w/c and use of some

pozzolanic materials

• LWAC

• Pore system is only partly open, some inner

pores are not connected to surface

• Partly open structure allows absorption of

water into aggregate particles, and the

water can be used for internal curing

• Dense ITZ due to water absorption of LWA

• Better bonding of LWA and cement paste

• Less microcracking in the ITZ caused by

drying shrinkage due to lower E of LWA

Failure Mode


• NWAC: fracture pass around the coarse aggregate

• LWAC: fracture pass through the coarse aggregate

• LWAC is more brittle than NWAC

Mechanical Properties of LWAC


• Compressive strength

• Strength of LWAC is generally determined by the characteristics

of LWA (strength, porosity, pore size distribution, etc.)

• Tensile strength

• For given fc, the tensile strength of LWAC is lower than that of

corresponding NWAC

• Stress-strain relationship of LWAC

• Ultimate strain is higher that NWAC

• Curve is more linear

• Modulus of elasticity

• Lower modulus of elasticity that NWAC

(Mindess 2003)

Shrinkage


LWAC generally has lower shrinkage rate and value at early age, but

the ultimate shrinkage is higher that that of normal weight concrete.

• Lower shrinkage of LWAC

at early age

• Attributed to water

absorbed of LWA,

“internal curing”, reduce

drying & autogenous

shrinkage

• Higher ultimate

shrinkage of LWAC

• Lower E of the LWA has

less restraint effect on

shrinkage.

Other Propertied of LWAC


• Thermal properties

• Thermal expansion

coefficient of LWAC similar

to NWAC

• Lower thermal conductivity

• LWA reduce the rate of

heat transfer, improve

thermal insulation

• Abrasion resistance

• LWAC not suitable for heavy

wear

Fresh Concrete Property


• LWAC has smaller properties in the plastic state as NWAC

• Slump should be limited to ~100 mm

• Too high slump cause segregation of the coarse aggregate

• Density of mortar matrix: typically 2.0 – 2.2 kg/l

• Air entrainment is desirable to improve workability, reduce segregation

• Viscosity modifying admixtures can be used to reduce segregation

Transport of Water and Ions in LWAC


• Difference of LWAC and NWAC in resistance to water

and chloride ion penetration

• Porosity

• Interfacial transition zone (ITZ)

• Internal curing effect

• Microcracking frequency

• Transport properties of LWAC vs NWAC depend on

which of the above factors are dominant.

• Connectivity of pore system is of primary importance

• Quality of paste matrix in LWAC is more important.


LWAC vs NWAC inresistance to water andchloride ion penetration

Porosity andpermeability

Effect of Cumulative LWA Content and Comparison of Concretes with Similar Strength


Effect of Cumulative LWA Content and Comparison of Concretes with Similar Strength (Cont’d)


Comparison of concretewith similar strength

LWAC has lower water permeability and higherresistance to Cl-ionpenetration than theNWAC with similar 28-dstrength.

Effect of cumulativeLWA content

Resistance of LWAC to Cl-

penetration decreases withincrease in cumulative LWA.

Water permeability showsimilar trend.

Applications of LWAC


• High-rise buildings

• Smaller dimensions of structural elements such as

columns and beams

• Less requirements for foundations

• Bridges (conventional and floating)

• Longer spans

• Offshore-oil platforms

…

Guggenheim Museum, Bilbao, Spain


Heidrun Tension Leg Offshore Oil Platform, Norway


• Water depth 345 m

• Design service life 60 years

• World first tension leg platform with concrete hull

• Durable concrete, maintenance cost will be less that a steel structure

• Slump = 250 mm

• Wet density ~ 1900 kg/m3

• 7-day strength = 70 MPa

• 28-day strength = 79 MPa

Bank of America Corporate Center, Charlotte, USA


Bridge


Fiber Reinforced Concrete (FRC)

Fiber Reinforce Concrete (FRC)

• Geometry of fiber-reinforced materials

• Historical perspective

• BC horse hair

• 1990 Asbestos fibers

• 1960 FRC

• 1970 search for asbestos replacement

• 1970 Steel FRC, glass FRC, etc.

Fiber Reinforced Concrete (FRC) (cont’d)


(Mindess et al 2003)


• Fiber reinforcement is not a substitute for conventional steel

reinforcement

• Reinforcing bars are used to increase the load-bearing capacity of

structural concrete members

• Fibers

• effective for crack control

• improve behavior of concrete under blast and impact loading,

in seismic applications

• ACI 544-3R-84 states

• In structural members where flexural and tensile loads will occur, such as in beams, columns, suspended floors, the conventional reinforcing steel must be capable of supporting the total tensile load.

• In applications where the presence of continuous reinforcement is not essential to the safety and integrity of the structure, e.g. pavements, the improvements in flexural strength can be used to reduce the section thickness

Fiber Reinforced Concrete (FRC) (cont’d)

Definitions


• Aspect ratio

= fiber length/equivalent fiber diameter

(equivalent D is the D of a circle having the same cross-sectional area

as the fiber)

• Typical aspect ratio: 50 – 150

• Orientation factor (or fiber efficiency factor)

• Efficiency with which randomly oriented fibers can carry a tensile

force in any one direction, ~0.2 to 1.0

• First crack strength

• Stress corresponding to the load at which the load-deflection curve

of the FRC first exhibits a significant non-linearity


• Critical length lc• Length above which the fiber will

fracture rather than pull out when

a crack intersects the fiber at its

mid point

lc = (σfu r) / τfu

where

σfu = ultimate fiber strength

r = fiber radius

τfu = max. frictional shear stress

2πrlτ/2 ⇔ πr2σ

lτ ⇔ rσ

l τfu > r σfu, fiber fracture

l τfu > r σfu, fiber pull out

Fiber fracture

Fiber pull out


Definitions (cont’d)


• Water-filled porous spaces around fibers due to

• Bleeding of water around fibers

• Inefficient packing of ~10 μm cement

particles in the zone out to about 50 μm

from the fiber surface

• How to increase the fiber-matrix bond?

• Reduce w/c, use silica fume

• Deforming fibers along their lengths or

at the end

Fiber-matrix Bond




• Principal role of fibers –

bridge across cracks

• Typical load-deflection curve

• A: matrix crack, first crack

strength

• FRC does not break

immediately after “A”, can

continue to sustain load

after that

• Failure takes place

primarily due to fiber

pull-out or debonding

in FRC

• Increased toughness for

FRC

Mechanics of Fiber Reinforcement



• Stress field around an

advancing crack in FRC

• Traction-free zone: crack is

wide enough for all the

fibers to be pulled out

• Fiber bridging zone:

stresses are transferred by

frictional slip of the fibers

• Microcracked matrix

process zone: aggregate

interlocking to transfer

some stress within the

matrix itself

Mechanics of Fiber Reinforcement (cont’d)


• General considerations

• Addition of fibers to plain concrete reduce workability

• Loss of workability is proportional to the volume of the fibers

• Aspect ratio of fibers also affect workability (curling up or balling)

• Compromise in the selection of fibers and the design of FRC

mixtures

• In general, the steel fiber content in concrete is < 1% by vol. of

concrete with max. aspect ratio of 100 (polymer based fibers

< 0.5%)

• Effect of max. aggregate size

• Recommend: max. aggregate size < 19/20 mm

• For the evaluation of workability of FRC, Vebe test

• Fiber content ↑, Vebe time ↑

• Fiber length (or L/D) ↑, Vebe time ↑

• Workability of FRC can be improved by air entrainment, SP, higher

cement content

Mix Proportion for FRC


Mix Proportion for FRC (cont’d)


• Strength and toughness

• Modest strength increase

may occur

• Toughness and impact

resistance increased even

when low E fibers such

as nylon and

polypropylene have been

used

• The improvement in

strength and toughness

depends on volume and

type of fibers

Properties of FRC

(Balaguru & Shah 1992)


Properties of FRC (cont’d)

• Elastic modulus, creep, and drying shrinkage

• Inclusion of fibers in concrete does not have significant effect on E,

drying shrinkage and compressive creep, though they tend to

reduce crack width during shrinkage

• Fibers can reduce plastic shrinkage

• Tensile creep is reduced slightly

• Durability

• Steel fibers

• surface rust is inevitable

• but the fibers in the interior usually remain uncorroded

• Glass fibers

• can not be used in Portland cement mortars or concretes due

to chemical attack by alkaline cement paste


Properties of FRC (cont’d)

• Impact resistance

• Increased dramatically by the addition of fibers

• Steel and carbon fibers are more effective than synthetic fibers

• Fatigue strength

• Improved by the addition of fiber

• Plain concrete: after 2 million cycles of loading, flexural

fatigue strength = 55% of static strength

• Fiber reinforced concrete: fatigue strength = 65-90% static

strength, due to the inhibition of crack extension by fibers


Applications of FRC

• Pavement, highway, and

airport applications

• Shotcrete for rock slope

stabilisation, tunnel lining,

structural repair

• For resistance to impact

and dynamic loading

• For reducing plastic

shrinkage

(polypropylene fibers)

• Prefabricated structural

members, thin sheets,

cladding members

Precast segments with FRC used for

tunnel lining of Gold Coast Desalination

Plant at Tugun, Australia.


Strain-hardening Fibre-reinforced Cement-based Composites (SHCC)

• Also called as Engineered Cement Composites (ECC)

• Moderate tensile strength (3–8 MPa),

• Pseudo strain-hardening tensile behavior of ultra ductility

• ductility of SHCC is not due to plastic deformation but due to the

formation of multiple micro-cracks

• material is progressively damaged in strain hardening range.

• Exhibit superior crack width and spacing control in pseudo strain-

hardening phase.


SHCC VS. UHPFRC

• Typical fibers used in SHCC

• Polyvinyl alkohol (PVA)

• Steel+polyethylene


SHCC VS. UHPFRC (cont’d)

SHCC (or ECC)

• Moderate tensile strength but significant ductility (up to and beyond 3% of tensile strain)

• In SHCC cracks of small, controlled width arise over a wide range in strain, extreme ductility leads to large pseudo-plastic zones, containing multiple cracks.

• Resists long term moisture and chloride diffusion through crack control to fine widths.

HPFRCC

• High tensile strength, flexural strength (25–60 MPa), and extremely high compressivestrength (180–240MPa), but reach these strengths at moderate strain levels,

• In UHPFRC the cracks are generally localized in areas of weakness or positions of maximum internal forces instructural elements

• Usually has a dense matrix, which is highly resistant to capillary suction.


Water Permeability – Crack Width

• Diffusion coefficient

versus

• crack width for

mortar deformed

under bending load,

• deflection of SHCC

and steel mesh

reinforced mortar



• Polymer concrete (PC)

• formed by aggregate and polymerizing a monomer (no other

bonding material)

• Latex modified concrete (LMC) (PMC)

• made by replacing a part of the mixing water with a latex

(polymer emulsion)

• Polymer-impregnated concrete (PIC)

• produced by impregnating a hardened Portland cement concrete

with a monomer and subsequently polymerizing the monomer in

situ

• Due to the high material cost and cumbersome production

technology, the use of polymer concrete is limited, except for LMC

which has been used extensively for repair and rehabilitation of

structures


Latex Modified Concrete

• Latex – a dispersion of polymer particles in water

• Generally contains ~50% by wt of spherical and very small

(0.01 to 1 μm in diameter) polymer particles held in

suspension in water by surface-active agents

• Specially formulated to be compatible with high alkaline

environment of concrete

• Type of latex



Classification of Polymers

• Thermoplastic polymers• Composed of long polymer

chains produced by joiningmonomers together

• Typically plastic and flexible

• Soften when heated to hightemperatures

• May be recycled

• Thermosetting polymers• Composed of long polymer chains with cross links to form 3D network

structure • Generally more rigid, stronger, but more brittle, than thermoplastics• No fixed melting temperature and cannot be easily reprocessed after cross-

linking reaction

• Elastomers• Have intermediate structure with some cross linking of the polymer chains• Have the ability to elastically deform by enormous amounts without

permanent change in shape


Latex Modified Concrete

• Typical LMC contains 15-20%

polymer (solid basis) by weight

of cement

• The hardening takes place by

cement hydration, drying or

loss of water

• Short time moist curing

(<48 hrs)

• dry curing is mandatory

• polymer film coats the

cement hydration

products, capillary pores,

aggregate-paste interface(Mindess et al 2003)


Mechanical Properties

• compressive strength comparable to Portland cement concrete if cured

properly

• better tensile strength than OPC concrete(Mindess et al 2003)


Mechanical Properties (cont’d)

• Lower E and higher strain at failure

• Greater nonlinearity of stress-strain curve

• Good bonding with old concrete

• Low drying shrinkage and creep

Polyvinyl

acetate


Mechanical Properties (cont’d)

• Loss of strength when LMC is immersed in water

• Adsorption of water by polymer film weaken the integrity of the film



Durability and Applications

• Good resistance to the penetration of water and aggressive solutions

• Reduced w/c

• Polymer film lining capillary pores, ↓ permeability

• Reduced microcracking due to improved tensile strength

• Applications

• Overlay for bridge decks and parking decks

• Repair work (Mindess et al 2003)



• Polymer concrete

• Examples of polymer used in PC

• polyester

• epoxy

• methyl methacrylate (MMA)

• Maximize aggregate content to reduce cost

• grading of the aggregate is important - dense packing

• Properties of polymer concrete

• Depend on the type and amount of the polymer used

• Typical mechanical properties (Table 11-14)

• High initial strength and elastic modulus

• Good chemical resistance, low permeability

• Rapid curing at ambient temperature from –18 to + 40 °C

• Applications: corrosive environment, industrial floors, repair work,

sewer pipes


Durability and Applications

• Higher compressive strength

• Higher tensile strength

• Higher rate of strength gain (within hours vs days for LMC)

PC LMC

polyester Polymerized

MMA

Control

concrete

LMC

containing

20% styrene

butadiene,

air cured

polymer/

aggr = 1/10

polymer/

aggr = 1/15

Moist

cured

Air

cured

Compressive strength, MPa 125 140 40 32 34

Tensile strength, MPa 14 10 3.7 2.2 4.3

Flexural strength, MPa 35 21 7.5 4.3 10.0

E, (GPa) x10-6 35 39 24 - 11

Source: PC from ACI SP-40, LMC from ACI Committee 548, and PC from Dikean,

in Progress in concrete Technology

contextocw.snu.ac.kr/sites/default/files/note/week11_structural... · 2019-03-15 · • the above...

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