holcim technical manual english

Holcim (Vietnam) Ltd.1st edition 2013

Cement & Concrete

Technical Manual

Strength. Performance. Passion.

3

Copyright

C2013, Holcim (Vietnam) Ltd

All rights, including the partial re-print of parts or entire section of the book in Vietnamese version

and/ or English version (including photo copy, micro copy, CD-Rom, or any other way of copying and

presenting it in public), the storage in date centers and the translation, are reserved to the authors.

Special permission must be requested in writing to Holcim (Vietnam)

Authors

Technical consultant team

Holcim (Vietnam) Ltd

A special thank to Silvia Vieiria Mcs, PhD Holcim Group Support Ltd

Publication

1st edition 2013 in Vietnamese

1st edition 2013 in English

Disclaimer

Alone the complete standards referred hereto serve as reference. They can be sourced at the respective

organizations. Holcim (Vietnam) is not liable for misapplication and/or interpretation of the content

of this manual.

Imprint

5

About Holcim (Vietnam) Ltd.

Founded in 1912 in the tiny Swiss village of Holderbank, Holcim is

one of the world leading cement and construction materials

companies. Holcim operates in more than 70 countries across all

continents and employs around 90,000 people world-wide. Today

Holcim has become synonymous of leadership in the supply of

cement and aggregates (crushed stone, sand and gravel), as well as

readymix concrete and construction-related services.

Holcim (Vietnam), founded in 1993, has the unique network of 4

cement plants in south Vietnam at Hon Chong, Hiep Phuoc, Cat Lai,

Thi Vai, to guarantee the best supply security for each project. To

meet the requirements of every application, Holcim Vietnam has

researched and developed a wide range of cements that offer the

optimal solution for every project.

Established in 2005, Holcim Beton has developed into a leading

readymix supplier in southern Vietnam, offering its customers high

quality, innovative products and services. Over the last years,

Holcim Vietnam has worked with leading national and

international contractors and developers as the preferred partner in

projects in southern Vietnam.

7

Preface

To develop Vietnam in the 21st century and to meet the requirements of modern society, many high rise buildings and infrastructure projects, like ports, roads, bridges are being designed and constructed by national and international developers, designers and contractors.

These structures are expected to be in service for long time, sometimes for 100 years, with low maintenance costs. The durability of concrete as building material is a key element for long lasting projects. This Technical Manual offers an overview of good practices in concrete as well as an overview of relevant Vietnamese and international standards.

A better understanding of cement/concrete standards can make it easier for designers, consultants and contractors to choose the type of cement and concrete, suitable for their specific project. With good concrete practice at the jobsite, the high quality building material concrete will be molded and transformed into long lasting concrete structures, to build Vietnam for future generations.

As the different standards are complex to summarize and the construction industry changes quickly in Vietnam, it is possible that there are inaccuracies in this Technical Manual. We are looking forward to any feedback or input for improvement on [email protected].

Yours sincerely,

Pieter KeppensTechnical Marketing Manager

8 Index

Chapter ICement & Concrete 11A. Components of concrete 11

1. Cement 112. Mixing water 123. Fine aggregate 134. Coarse aggregate 145. Admixtures 166. Additions 17

B. From fresh concrete to hardened concrete 201. Composition of concrete 202. Workability 233. Concrete strength 274. Special characteristics 335. Production and transport 376. Placing and compaction 387. Concreting in hot weather 418. Pumped concrete 439. Curing 4510. Influence of formwork 47

Chapter IIApplications with specific requirements 49A. Infrastructure 49

1. Introduction 492. Cement for infrastructure 49

B. Aggressive environments 501. Introduction 502. Sulfate resistant Portland cement 503. Sulfate resistant blended cements 51

C. Massive structure 521. Introduction 522. Cement for massive structures 523. Concrete for massive structures 53

D. High strength concrete 541. Introduction 542. Production and use of high strength concrete 55

E. Very flowable and self-compacting concrete 561. Introduction 562. Production of very flowable / self-compacting concrete 57

F. Cement treated aggregates 581. Introduction 582. Cement for treated aggregates 583. Testing procedure for cement treated aggregates 594. Optimization of cement treated aggregates 61

9

Chapter IIICauses and prevention of concrete defects 62A. Segregation of concrete 63

B. Cracking 641. Plastic settlement cracks 652. Plastic shrinkage cracks 663. Surface crazing 674. Drying shrinkage cracks 675. Early thermal cracking 68

C. Carbonation and corrosion of reinforcement 69

D. Degradation in seawater environment 701. Chloride-induced corrosion of the steel reinforcement 702. Attack by sulfates from seawater 713. Preventive measures 71

E. Chemical attack 721. Classification 722. Preventive measures 73

F. Alkali Aggregate Reaction 74

G. Fire Resistance 751. Concrete in fire 752. Preventive measures 75

Chapter IVOverview of cement & concrete standards 76A. Cement 77

Vietnamese standards TCVN 77American standards ASTM 83European standards EN 86

B. Concrete 89Vietnamese standards TCVN 89American standards ASTM 91European standards EN 93British standards BS 95

C. Recommendation for limiting values of concrete composition 97Chloride - induced corrosion in sea water 97Aggressive chemical environments 97

Reference 98

11

1. CementGeneralCement is a hydraulic binder a material that hardens after being mixed with water, either in the air or under water. The hardened cement paste is water-resistant and possesses high strength. For all concrete without specific requirements, the type of cement generally used in Vietnam is a blended Portland cement, type PCB 40, according to the Vietnamese standard TCVN 6260. For plaster/mortar in rural areas, PCB30, a lower strength class, is sometimes used as well.

Several types of blending materials are used, like limestone, puzzolan or slag, depending on the locally available materials.

International standards, comparable to TCVN 6260, are: American Standard ASTM C1157: type GU

(General Use)

European Standard EN 197-1: CEM II/A or CEM II/B 42.5

Other types of cement, which are used worldwide, like

Ordinary Portland Cement OPC (TCVN 2682, ASTM C150, EN 197-1 CEM I)

Blast Furnace Slag cement (TCVN 4316, ASTM C1157, EN 197-1 CEM III)

are not available in Vietnam as general use cement.

The test methods of the TCVN standard are very close to the EN standard, with the correction of testing temperature (27oC instead of 20oC), to take the local climate conditions into account.

The ASTM standards use a completely different set of testing methods and the requirements cannot be compared to the TCVN/EN standards. In Vietnam, several 3rd party laboratories are equipped to test cement according to TCVN & ASTM, but not according to the EN standard.

Testing cement quality and conformityThe quality and conformity of Vietnam cements are assured through three types of control:

Control of the product in the plant

An certified quality-management system

External monitoring

Control of the product in the plant At each step of the cement production, from the quarry to cement delivery, material specimens are collected for analysis. Gap-free monitoring of production ensures uniform, high-quality cement. The testing methods for cement are described in standard TCVN 6017:1995 and ISO 9597:2008.

Quality management systemMost cement plants have established a quality management system and all are certified according to the ISO 9001:2008 series of standards. Some cement plants also have a testing center in series of VILAS according to ISO 17025. This ensures that all operational processes are standardized, traceable, and transparent.

External monitoringIn-house testing is supplemented by external monitoring. External monitoring is carried out by a testing institute accredited for testing cement. In the south part of Vietnam, the most referenced external monitoring is Quality Assurance and Testing Center 3 (QUATEST 3). From November 2012, every cement in Vietnam has to carry the CR quality label. Cement storage and shelf lifeIf cement is stored unprotected for a long time, it absorbs moisture, which leads to lumps and may reduce the strength development. If lumps can be crushed between the fingers, the loss of strength will be negligible.

Cement can be stored for a limited time in silo or bags. Bag cement is best stored in dry shelter. Bags stacked temporarily outdoors must be placed on timber sleepers for ventilation. The plastic cover must not be allowed to contact the cement bags, because condensation would wet the bags.

Chapter I: Cement & ConcreteA. Components of concrete

Holcim recommendationFor general use concrete, standard cement offers the best supply security for any project:

TCVN 6260:2009 PCB 40

ASTM C1157:2008 GU

12

2. Mixing waterWater for mixing concrete and mortar must comply with TCXDVN 302:2004 or ASTM C1602. Water that meets these requirements, can be used for washing aggregate and curing concrete sample. According to these standards, drinking water can be used as mixing water without testing. Water from rivers and canals is in most cases not appropriate to make concrete. The use of seawater in reinforced concrete is strictly forbidden.

GeneralMixing water is the total amount of water contained in fresh concrete. It is the sum of: The water added directly to the mix The surface moisture of the aggregates The water content of the concrete admixtures

and additions, if applicable(silica fume, pigment in suspension, etc.)

Mixing water has two functions in concrete technology. It is required for hydration of the cement, and for the production of a plastic concrete that can be well compacted.

Requirements for mixing waterAccording to TCXDVN 302:2004, mixing water must meet these following requirements: Does not contain oil scum and oily film Organic content < 15mg/l 4 < pH < 12.5 Color free Depending on the type of concrete, sulfate and

chloride content must follow the requirements in Table I.1 (TCXDVN 302 : 2004).

Chapter I: Cement & Concrete A. Components of concrete

Table I.1 - Limit sulfate and chloride content in mixing water for different purpose

Purpose of mixing water

Maximum Level

(mg/l)

Soluble

Salt

Sulfate Ion

(SO4-2)

Chloride Ion

(Cl-)Insoluble rest

1. Pre-stressed concrete. 2000 600 350 200

2. Reinforced concrete. 5000 2000 1000 200

3. Non-reinforced concrete. 10000 2700 3500 300

13Chapter I: Cement & Concrete A. Components of concrete

3. Fine Aggregate GradingFine aggregate shall consist of natural sand, crushed sand, or a combination thereof. For concrete production, fine aggregates must comply with TCVN 7570 : 2006 or ASTM C33 (Standard Specification for Concrete Aggregates). In the south of Vietnam, 3 sources of fine aggregates are used in concrete (FM = fineness modulus):

Sand from Dong Nai river : FM = 2.40 (good not available in significant quantity)

Sand from Mekong river : FM = 1.1 -1.6 (too fine)

Manufactured (crushed) sand : FM = 4.0 (too coarse)

Usually when the sand is very fine, the mix is un-economical because the increase of water demand will lead to the increase of cement. When it is very coarse, the mix is harsh and unworkable because there are so much voids between the grains and the cement paste can not fill the voids. According to ASTM C33, a reference for good sieve curve of fine aggregates for concrete is like Fig I.1.

In the south of Vietnam, sand compliant to ASTM C33 cannot be found. The current practice is to combine Mekong sand with manufactured sand, to reach the best performance.

Organic ImpuritiesFine aggregate must be free of deleterious amounts of organic impurities. Fine aggregates that contains many organic impurities, will lead to delay in concrete setting, loss of strength and durability of concrete.

Fine aggregate should be tested before use on organic impurites according to standard TCVN 7572-9 : 2006 or ASTM C40 (Standard Test Method for Organic Impurities in Fine Aggregates for Concrete). When a sample has a color darker than the standard color, or Organic Plate No. 3, the fine aggregate under test contains possible injurious organic impurities. It is advisable to perform further tests before approving the fine aggregate for use in concrete.

Other ImpuritiesImpurities like silt, dust, clay content also have a disavantage effect on concrete. It should be tested before use for concrete according to standard TCVN 7572-8 : 2006 (Standard test method for silt, dust, clay content) or ASTM C117 (Standard Test Method for Materials Finer than 75-m).

Akali-Silica ReactionFor concrete that is subjected to wetting, extended exposure to humid atmosphere, or contact with moist ground (for example, foundations, bridges, tunnels,), the aggregates (both fine and coarse) shall not contain any materials that are deleteriously reactive with the alkalies in the concrete to cause Alkali Aggregate Reaction. This expansive reaction can create cracks in the concrete, which reduces both the concrete strength and the durability. Potential Alkali-Silica Reactivity of Aggregates should be tested according to standard TCVN 7572-14:2006 (Determination of alkali silica reactivity ) or ASTM C289 (chemical method), ASTM C1260 or ASTM C227 (mortar bar method).

Fig I.2Organic impurities test using organic plate.

Fig I.1 - Good sieve curve of fine aggregate for concrete

10

0

20

30

40

50

60

70

80

90

100

9.50

10.0 1.0Sieve openings (mm)

Fine limit (ASTM C33)CombinationCoarse limit (ASTM C33)Pa

ssin

g (%

)

0.1

4.75 2.36 1.18 0.60 0.30 0.15

Manufactured sand

Mekong sand

14

4. Coarse aggregateGeneral Coarse aggregates form the skeletal structure of the concrete and must comply with TCVN 7570 :2006 or ASTM C33 (Standard Specification for Concrete Aggregates).

CharacteristicsThe most important characteristics of coarse aggregates are: Specific gravity

Bulk density (unit weight) and moisture content

Mineral composition, grain shape, and surface texture

Purity

Grading (grain size distribution) and aggregate fractions (range of sizes)

Soundness

Table I.2 Classification of

aggregates by specific gravity

State Ovendry Air drySaturated surface dry

(SSD)Damp or wet

Total moisture NoneLess than potential

absorption

Equal to potential

absorption

Greater than

absorption

Fig I.3The moisture

state of aggregate

Bulk density (unit weight) and moisture contentBulk density is the weight of loosely poured material per unit of volume. It is greatly influenced by moisture content of the aggregate (Fig I.3). Thus the two characteristics, bulk density and moisture content, are closely related. Test method of bulk density according to TCVN 7572-6 : 2006 or ASTM C29 (Standard Test Method for Bulk Density and Voids in Aggregate).

The moisture state of aggregates can change between ovendry and wet aggregates, depending on the situation.

Specific gravity The aggregate specific gravity is the ratio of the weight of a given volume of aggregate to the weight of an equal volume of water. Aggregate specific gravity is needed to determine weight-to-volume relationships and to calculate various volume-related quantities such as voids in mineral aggregate. The test standard for coarse aggregate specific gravity and water absorption is the TCVN 7572-4 : 2006 or ASTM C127 (Determination of apparent specific gravity, bulk specific gravity and water absorption).

Aggregate type Specific Gravity (kg/m3) Aggregate Material Application

Standard aggregate 2700River or glacial deposits; crushed stone

Reinforced and non-reinforced concrete

Heavy aggregate >3000Barite (heavy spar), iron ore, granulated steel

Concrete for radiation protection

Lightweight aggregate < 2000 Expanded clay, polystyreneInsulating concrete, concrete topping, sloped concrete

Hard aggregate > 2500Quartz, corundum, silicon carbide

Hard concrete slabs, abrasion-resistant concrete


15

PurityAdhesive impurity on coarse aggregate surface, such as dust from degraded rock, reduces concrete quality, for example, by disturbing setting properties and reducing the contact area between aggregate and cement paste. It is suggested to wash coarse aggregate before use in concrete (Fig I.4.).

GradingThe grading and maximum size of coarse aggregate is an important parameter in concrete mix. The grading of aggregate is measured according to TCVN 7572-2 or ASTM C136 (Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates)

Grading, or the distribution of grain sizes along with surface texture, specific surface, and grain shape of coarse aggregate greatly determines the water requirement, and thus is one of the most important characteristics.

The maximum size of aggregate (Dmax) is the smallest sieve size, through which at least 90% the aggregate would pass. The maximum size of aggregates is limited by the application. It depends on: the distance between reinforcement, size of elements, and pumpability of concrete. The choice for maximum size of aggregate follows the Fig I.5.

The use of smaller aggregates increases the water demand, increases the cement content to meet the same strength.

Mineral quality, grain shape, and surface texturePorous or overly soft aggregate (for example degraded rock) impairs the quality of concrete. Grain shape largely determines the compactability and water requirement of concrete, as does grading and surface texture (Fig I.6).

A cubical grain shape is good for concrete mix, it decreases the water requirement and increases workability of concrete. In contrast, non-cubical, grain shape (elongated and flaky- aggregate particles having a ratio of length to thickness greater than a specified value) will increase water demand and decreases the workability of concrete. Non-cubical grain shape content is measured according to TCVN 7572-13 (Determination of elongation and flakiness index of coarse aggregate).

Fig I.5 - The choice for maximum size of aggregate

Fig I.6 Grain shapes of aggregate

Rounded Irregular Angular

Desirable

Less Desirable

Flaky ElongatedFlaky Elongated

Fig I.4 - Screening and washing aggregate in a gravel plant

I- Dmax < 3d/4

Dmax < e/5Dmax < f/5

Dmax < a/3

Dmax < 1/3 diameter of hoseor 37.5mm

For pumped concrete

c

f

c

e

a

d

dDmax < 3c/4

II-

III-

IV-


16

5. AdmixturesDefinition and classificationConcrete admixtures are chemical substances that are added to concrete to change, through chemical and/or physical action, some of its properties, such as workability, setting, hardening.

In Vietnam, the performance requirements for different types of admixtures comply with standards TCVN 8826 : 2011 or ASTM C494 (Standard Specification for Chemical Admixtures for Concrete).

DosageAdmixtures are added to concrete mainly in liquid form and in very small amounts. The dosage is generally about 0.4 to 2% in relation to the weight of cement. In certain cases the amount will be recommended by the manufacturer. If the dosage exceeds about 1%, the water introduced with the admixture, must be considered as part of concrete mixing water. Too low dosage can reduce significantly the desired effect, and too high dosage can produce unwanted effects such as retarded setting or loss of compressive strength.

The most important and common types of admixturesAccording to ASTM C494, there are seven types of admixture (from type A through type G). In Vietnam, three types are commonly used:

a/ Water reducing and retarding admixture.

This type of admixture, based on lignosulphonate, can be used at dosage 0.4 - 0.6% to reduce the quantity of water required (6% - 12%).

Water reducing admixtures require less water to make a concrete of equal slump which improves the concrete strength, or increase the slump of concrete at the same water content.

Retarding admixture is useful for concrete that has to be transported over long distances, requires a long slump retention and to retard the setting time of concrete when placed at high temperatures.

b/ Mid-range water reducing admixture.

This type of admixture, based on napthalene sulfonate, can be used at dosage 0.7 1.2% to decreases the water requirements by about 15 25%.

Mid-range water reducers allow larger water reduction to increase strength or slump/slump retention at jobsite. They can achieve a specific consistency and workability at a greatly reduced amount of water. As with most types of admixtures, napthalenes can significantly delay the initial setting time of concrete, depending on the admixture formulation.

c/ High-range water reducing admixture

This type of admixture is based on polycarboxylate base. Common dosages are between 0.8 1.8%, depending on the supplier recommendation. This type of admixture can reduce the quantity of mixing water required (20 - 35%) to produce concrete with high consistency, better workability and high strength. The optimal dosage needs to be determined based on the particular concrete mix and specific requirements.

Other type of admixtures

Many other types of admixture for concrete are available:

Accelerators

Air entrainer admixture

Corrosion inhibitor

These specific admixtures are rarely used in Vietnam. More information can be found from different admixture suppliers.

Fig I.7 - Admixture used in concrete.


17

6. AdditionsFibersPolypropylene fibers are organic fibers, used in concrete to prevent plastic shrinkage cracks. About 0.7kg - 1kg of fibers is required per m3 of concrete (Fig I.8).

Steel fibers, uniformly distributed in concrete, improve certain mechanical characteristics, particularly ductility (toughness) and tensile strength. The efficiency of steel fibers greatly depends on their length, diameter, and shape. The main use of steel fibres is in industrial floors, to replace the steel mesh in the concrete (Fig I.9).

Glass fibers are used to reinforce thin prefabricated sections. Using glass fibers is tricky; it requires the experience of a recognized expert (Fig I.10).

Silica fumeSilica fume (Fig I.11), also known as silica dust or microsilica, possesses a high pozzolanic activity due to extreme fineness and very high amorphous silica content. Silica fume dosages of 5 to 10% by weight of cement can produce permanent improvement of concrete characteristics:

Reduction of concrete porosity, thus improvement of durability; increased resistance to salts, sulfates, and other aggressive chemicals.

Carbonation progresses slower, thus reinforcement is better protected against corrosion.

Contributes to concrete strength; allows the production of high-strength concrete (80-100MPa)

Fig I.8 Polypropylene fibers

Fig I.9 Steel Fiber

Fig I.10 Glass fiber, cut and bundled

Fig I.11 Silica fumeCaution

Adding silica fume to a concrete mix reduces the workability and changes the rheologic characteristics (flow characteristics)! Adequate workability can be achieved by adding special superplasticizers.

As silica fume is very fine, the homogeneous distribution into the concrete is an important issue that requires specific attention. If the silica fume is not well distributed into the concrete, its efficiency in increasing strength and durability will be reduced.


18

Other mineral additions (puzzolan, fly ash)In many countries, high quality fly ash, a by product from thermo power plants, is commonly used in concrete, as this is an active puzzolan that contributes to the strength of the concrete. In Vietnam, the use of both puzzolan (Fig I.12) and fly ash (Fig I.13) is mainly limited to Roller Compacted Concrete (RCC) in hydraulic dams. The available fly ash is not suitable for flowable concrete, due to its:

High loss of ignition (= unburned coal)

High water demand

Issues with admixture compatibility

Unstable quality, with limited quality control.

Inorganic pigmentsInorganic pigments are used to dye concretes and mortars (Fig I.14). Oxide pigments are virtually the only ones that can meet the demanding criteria of stability and grading. Pigments have no chemical effect on concrete. Because of their high fineness, they increase the concrete water demand. This can be counteracted by adding a highrange water reducer. Pigment dosage, usually a few percent measured by weight of cement, depends on the desired color intensity. Amounts are recommended by the suppliers.

Producing flawless colored concrete surfaces requires great experience. Uniformly colored, bright concrete surfaces can be achieved only with a completely homogeneous concrete mix using white cement and light colored sand. The color of the gravel is not so important.

Any residue of colored concrete must be completely removed from mixers, transport vehicles, and conveyor equipment, so that subsequent batches of concrete are not contaminated. Even the best pigments cannot prevent the color of concrete from fading somewhat over time.

Fig I.14 - Concrete products made using white portland cement colored with pigments

Fig I.12 Puzzolan

Fig I.13 Fly Ash


19Cement & Concrete Concrete component

20 B. From fresh concrete to hardened concrete

1. Composition of ConcreteConcrete is a composite material that consists essentially of fine and coarse aggregates, glued together by the cement paste. Aggregates occupy 60-75% of the concrete (measured by weight or by volume, as Fig I.15 and they are important constituents from a technical and economical point of view. Aggregates play a central role in concrete strength and durability.

But the picture looks a bit different when we consider the so-called internal surface area, that is, the combined surfaces of all the particles in concrete. Measured in this way, the dominant component in concrete is clearly cement and the cement paste is fundamental in defining many concrete characteristics.

Concrete mixingIn proportioning the constituents of concrete, or determining the so-called concrete mix or mix design, the producer is primarily concerned with optimizing concrete's:

Workability

Strength

Production cost

Durability

Importance of the water/ cement (w/c) ratioA central characteristics of concrete, and one that largely determines its performance, is the water/cement ratio, or w/c ratio (Fig I.16).

The relationships between the w/c ratio and required characteristics of concrete are well known in practice. Thus, the designing engineer usually specifies the w/c ratio when he specifies the type of concrete.

Fig I.15 - Composition of Concrete

Fig I.16 - Influence of the w/c ratio on concrete properties

21

Choosing the water/cement ratioAn appropriate w/c ratio will depend primarily upon environmental exposure and the loads the concrete construction will be carrying (Fig I.17). Recommended maximum w/c for different exposure conditions are given, for example, in the EN 206 or in ACI 318.

Minimum cement content in concreteWith sufficient cement in concrete, enough calcium hydroxide is formed during hydration that the high alkalinity and low porosity achieved in the concrete will reliably protect the steel reinforcement from rusting. On the other hand, overly large amounts of cement in concrete increases the possibility of cracks due to shrinkage and increased heat of hydration.

According to EN 206, reinforced concrete with a maximum aggregate size of 32mm should normally contain at least 300kg cement per m3 compacted concrete. The dosage may be reduced to 250 kg/m3 only if the constructed element is permanently protected from environmental action and other forms of attack.

TCXDVN 327:2004 - Concrete and Reinforced Concrete Structures Requirements of Protection from Corrosion in Marine Environment requires:

The European standard EN206 increases the minimum cement content to the environmental conditions (refer chapter IV.C)

Low porosity in concreteA well-designed aggregate mix with a smooth grading curve produces concrete with good workability and high cohesion, with a high resistance to segregation. The hardened concrete will have low permeability, which gives it good durability (Fig I.18 and I.19).

Fig I.17 Influence of the w/c ratio on 28-day compressive strength of concrete

Fig I.18Poor filling of void spaces, high permeability concrete with only one size of aggregate (schematic)

Fig I.19Good filling of void spaces, low permeability concrete with a smooth grading curve (schematic)

Area Minimum cement content (kg/m3)

No direct contact 350

Direct contact 400

Table I.3 - Minimum cement content depend on environmental exposure (TCXDVN 327)

Chapter I: Cement & Concrete B. From fresh concrete to hardened concrete

22 Chapter I: Cement & Concrete B. From fresh concrete to hardened concrete

Proportioning the mix by absolute volume

In practice, the proportions of each constituent of a concrete mix are determined by calculating their absolute volumes. The unit volume of each component is calculated based on 1m3 (1000l) of compacted concrete, and obtained by dividing the mass of each component by the specific gravity

Example:

Volume (m3)=Mass (kg)

Specific Gravity(kg/m3)

Specification: Cement dosage 325 kg/m3

Water/Cement ratio 0.48 Plasticizer 1% based on cement mass ( = ~ 3 kg)Assumption: Normal porosity 1.5% entrapped air (=15 l)

Component Mass (kg)Specific Gravity (kg/m3)

Unit volume (m3)

Cement 325 3,100 0.105

Mixing water 156 1,000 0.156

Plasticizer 3 ~ 1,000 0.003

Entrapped air - 0.015

Subtotal 484 0.279

Dry aggregate 0.721 x 2,700 = 1947 2,700 1 - 0.279 =0.721

Fresh concrete 484 + 1947 = 2431 2,431 1

1) Mixing water = water added + moisture of aggregates. The number through indicate the sequence of the calculation.

To calculate the actual amount of aggregate necessary, the water contained as moisture in the aggregate (generally 4 to 6% for sand and 1 to 3 % for gravel) must be added for each fraction. Subtracting the moisture contained in all the aggregates from the total mixing water gives the necessary amount of water to be dispensed.

The unit volume of entrapped air bubbles (generally 1 to 2 %) as well as the volume of entrained air must also be considered in proportioning the mix by absolute volume. The example shows a method of calculating the dry aggregate amount and the fresh concrete density.

Influence of other factors on the workability & strength of concrete

Besides admixtures, many other factors influence concrete workability. Changing one or more of these factors changes not only the workability, but also other characteristics of concrete, for example strength. Table I.4 shows how various changes in concrete constituents and mix affect the consistence and 28-day compressive strength of concrete.

23

Table I.4Effect of various factors on workability and strength of concrete

Fig I.20 Apparatus to determine slump

2. WorkabilityTo achieve a high quality concrete structure, the method of placing and compaction as well as the shape of the concrete element and reinforcement arrangement, should be considered to select the workability of the concrete.

The concrete workability affects the speed of placement and the degree of compaction of concrete. Inadequate compaction may result in the reduction in both strength and durability of concrete.

Different test methods for workability are available including slump, Vebe time, flow table, etc. The choice of the test method depends on the concrete workability and its application.

To get reliable results, each test method for workability should be applied within its test range (EN206):

TCXDVN 374:2006 specifies:

For too dry concrete: the vebe time > 50 second.

For dry concrete: the vebe time > 5 second and < 50 second.

For plasticized concrete: The slump from 10 to 220 (mm)

For super-plasticized concrete: the flow from 260 400 (mm)

a. Slump test :The slump test is the most well-known and widely used method to characterize the workability of fresh concrete. This simple test is used at the job sites to quickly determine whether a concrete batch should be accepted or rejected.

The slump test measures the ability of concrete to flow under its own weight, without vibration. This method is suitable for medium to high workability concrete with slump ranging from 10 to 210 mm (EN 206).

The test method is widely standardized throughout the world:

TCVN 3106

ASTM C143

EN 12350-2

The apparatus used in the slump test are: mold, tamping rod, measuring equipment (Fig I.20):

Workability28-day

compressive strength

Change

Slump 10 mm and 210 mm;

Vebe time 30 sec and > 5 sec;

Flow diameter > 340 mm and 620 mm.

More rounded aggregate

Smoother grading

positive influence negative influence no significant influence

More crushed (angular) aggregate

More mixing water

Higher concrete temperature

Use of a superplasticizer

Use of an air entrainer

Use of a retarder


24

- In EN and TCVN standards, the slump is the vertical difference between the top of the mould and that of the highest point of the slumped test specimen.

- In ASTM standard, the slump is the vertical difference between the top of the mould and the displaced original center of the top surface of the specimen.

The slump test is only valid if the concrete cone stays visible and symmetrical (true slump). If the concrete cone shears (shear slump), the test needs to be done again. If it fails again, the slump test is not applicable for the concrete (EN 12350-2)

Depending on the application of concrete, the following slump values are recommended:

Slump Range (mm) Application Illustrated photo

60-80Elements with intense vibration: Precast elements, concrete pavement.Concrete placed by bucket

100-160Elements with good vibration (compaction needles): column, slab, beams etc.Concrete placed by bucket or pump

180-200

Elements with low vibration level: Bore piling Retaining wall Core wallConcrete placed by bucket or pump

Fig I.22 - Determine Slump conform to ASTM standard

Fig I.23 - True and shear slump shape

Fig I.21 - Determine Slump conform to TCVN and EN standard

True Slump Shear Slump


Table I.5Slump range for

different applications

25

b. Slump flow: The slump flow test method is used to determine workability of very flowable concrete with a very high slump. At this high slump > 200mm, normal concrete has the tendency to segregate, which impacts the concrete quality significantly. To reach a high quality concrete at very high workability, the mix design needs to be specially developed to avoid segregation and achieve the required stability.

Two types of concrete can be distinguished (see Chapter II.E):- Very flowable concrete (slump flow: 450- 650mm)

- Self Consolidating Concrete (SCC), also known as Self Compacting Concrete (slump flow > 650mm).

This test uses the same equipment as the slump test, but the diameter of the concrete spread is measured.

The test method to determine slump flow is ASTM C1611 or EN 12350-8. In ASTM standard, there are two ways to measure slump flow of concrete:

- Upright mold

- Inverted mold

The upright mold (same way as the slump test) is popularly used in Vietnam. Slump flow is the average of the largest diameter of circular spread of the concrete and the circular spread of the concrete at an angle approximately perpendicular to diameter above.

Concrete with high workability is used for structure with dense reinforced steel such as transfer beam, core walls, pile cap, etc or for the areas that are difficult to reach for compaction.

c. VEBE test: For semi-dry concrete with a low workability, the use of the Vebe test is recommended. The Vebe time is the time needed to level and compact fresh concrete in Vebe consistometer and ranges from 5s to 30s (EN 206). Some typical applications are:

- Roller compacted concrete (RCC) for hydraulic RCC dams

- Base layers of roads, container ports

- Precast products: concrete pipes

Fig I.25Structure with dense reinforce steel

Fig I.26Transfer beam

Fig I.24 - Determine slump flow for fresh concrete


26

The freshly mixed concrete is packed into a similar cone used for the slump test. The cone stands within a special container on a Vebe table, which is vibrated at a standard rate after the cone has been lifted. The time taken for the concrete to be compacted is measured.

General standards which are used to determine Vebe time:

- TCVN 3107:1993,

- EN 12350-3,

- ASTM C1170.

In Viet Nam, two methods have been applied: TCVN 3107 and EN 12350-3 to test Vebe time of semi-dry concrete. Basically, both of standards are similar. However, EN standard is more detailed than TCVN.

d. Flow table test:The flow table test measures the workability of concrete under the impact of compaction energy. Generally, in Viet Nam, EN 12350-5 standard is used to test flow table of fresh concrete.

To perform the test, the cone mold is placed in the center of the plate and filled in two layers, each of which is compacted with a tamping rod. The plate is lifted by the attached handle at a distance of 40 mm and then dropped a total of 15 times. The horizontal spread of the concrete is then measured.Fig I.28 - Apparatus

to measure Vebe time

Moving Vertical Rod

Rotating Arm

Slump Cone

Container

Vebe Table

Clear Plastic Disk

Mold

Top Plate

Hinge

HandleClip

Bottom Plate

200mm

200mm

700mm

40mm

30mm


Fig I.27RCC for hydraulic

dams

Fig I.30 - Apparatus to determine flow table

Fig I.29 - Flow table test for fresh concrete

27

3. Concrete strengthOne of the most important characteristics of concrete is the strength, as strength is an important input parameter to the design of the concrete structure. Concrete is a very strong material when it is used in compression and it is however, less resistant to tension.

There are different ways to measure the concrete strength, such as compressive strength, flexural strength, and tensile strength tests.

a. Compressive strength:Compressive strength is the capacity of a material or structure to withstand axially directed pushing forces. When the limit of compressive strength is reached, the concrete fails and breaks.

The compressive strength of concrete is the most common performance parameter used by the engineer in designing building and other structures. The compressive strength is measured in cylindrical (150x300mm) or cubical (150mm) concrete specimens that are casted, compacted, cured and tested in standard conditions.

The type of specimen, as well as sampling method, curing and testing, are specified in the following standards:

- TCVN 3105 :1993 & TCVN 3118:1993

- BS EN 12390-2 & EN 12390-3

- ASTM C31 & ASTM C39

To obtain accurate test result with cylinder specimens, the cylinder should be capped with a thin layer of stiff Portland cement or sulfur paste which is permitted to harden and cure with the specimen in accordance with ASTM C 617.This capping method has to be done carefully, especially for high strength concrete.

The compressive strength is conventionally determined on specimens tested at 28 days age. For particular applications, for example mass concrete, RCC etc, the concrete strength can be specified at later ages, like 56 or 90 days.

In case early strength is required, to remove the support frame or formwork, or to prestress the concrete the compressive strength at earlier ages (1 day, 3 days etc) are commonly specified in addition to the 28 days strength.

Sometimes, other specimen sizes are used the following correlation factors can be appied to recalculate into the standard size specimen (cube 150mm):

(source: TCXDVN 3118:1993)

Fig I.31 - Cube and cylinder specimens

Fig I.32 - Specimens in a compression-testing machine:cube and cylinder specimens

Table I.6 - The correction factor to recalculate into the standard size specimen (cube 150mm)

Fig I.33Equipment for capping specimen and the specimen after capping and testing

Shape & size specimen (mm) Correlation factor

Cube specimen

100x100x100 0.91

150 x 150 x 150 1,00

200 x 200 x 200 1.05

300 x 300 x 300 1.1

Cylinder specimen

71,4 x 143 & 100 x 200 1.16

150 x 300 1.2

200 x 400 1.24


28

In Vietnam, the concrete is classified based on grade and class of hardened concrete.

Grade of hardened concrete (TCXDVN 239:2006)The grade of concrete is the mean compressive strength in MPa, tested on 150 x 150 x 150mm cube samples, which are casted, compacted, cured and tested according to the standard at the age of 28 days. Grade of concrete is prefixed with letter M.

Class of hardended concrete (TCXDVN 356:2005)The class of concrete is the compressive strength of concrete which the reliable probability is 0.95. Class of concrete is prefixed with letter B.B = M(1 1.64v)

With:v variable strength coefficient

Accoding to the European standard EN 206, the concrete is classified based on compressive strength at 28 days of 150mm diameter by 300mm cylinders (fck,cyl) or 150mm cubes (fck, cube). Example: C30/37 is interpreted as follows:

C stands for concrete

30 is the characteristic strength, determined using test cylinders (d=150mm, h=300mm),

37 is the characteristic strength, determined using test cubes measuring 150mm.

EN 206 defines 16 concrete classes, ranging from C 8/10 to C 100/115.

In American standard system, there are two main standards for concrete: ASTM C94 Standard specification for ready-mixed concrete and ACI 318 - Building Code Requirements for Structural Concrete and commentary. The ASTM/ACI standards do not classify concrete based on compressive strength.

b. Flexural strengthThe flexural strength of concrete is measured by loading 150x150mm concrete beams with a span length at least three times the depth. The flexural strength is expressed in MPa and is determined by standard test methods ASTM C78 (four-point loading), ASTM C293 (three-point loading) or EN 12390-1.

Flexural strength is about 10 to 20 percent of compressive strength depending on the type, size and volume of coarse aggregate used. However, the best correlation for specific materials is obtained by laboratory tests for given materials and mix design. The flexural strength of specimens shall be prepared and cured in accordance with ASTM C42 or Practices C31 or C192 or EN 12350-1 and EN 12390-2.

Pavements are normally designed to achieve a targeted flexural strength. Therefore, laboratory mix design based on flexural strength tests may be required, or a cement content may be selected from past experience to obtain the required flexural strength. Sometimes it is used for field control and acceptance of pavement or slab. Very few use flexural testing for structural concrete.

Depending on actual use, it may be necessary to specify the flexural strength at different ages such as: 3 days, 7 days, 28 days and 56 days.

Fig I.34 - Four point loading

Fig I. 35 - Three point loading

1/2 Load 1/2 Load

Load


29

c. Assessment of compressive strength test results

Test methods for sampling & testingGeneral methods for the making of the concrete specimen, their curing and testing are summarized in below table:

The below 3 steps are very important to assure the reliability of the result:

The sampling of the concrete and the making of the concrete specimens shall be done properly, so that the concrete cubes are representative of the concrete batch. This procedure is sometimes neglected in some job sites, which may lead to low strength of the concrete specimen.

The curing in water tanks specific attention needs to be given to the transport of concrete cubes at early age. A careless handling can impact their final strength.

Finally, the compressive strength of the concrete specimen is determined in the laboratory. Experience shows that the skill of laboratory staff can have a significant impact on the final test result. Special attention is required for the loading speed of the concrete specimen.

EN 12390 3: 2002 defines the shape of satisfactory and unsatisfactory specimens (cube and cylinder) after the compressive strength test as shown beside:

When the specimen shows an unsatisfactory failure, the obtained result will not represent the true compressive strength of the concrete.

Characteristic EN TCVN ASTM-ACI

Making EN 12390-2 TCVN 3105:1993 ASTM C31

Curing EN 12390-2 TCVN 3105:1993 ASTM C31

Compressive strength EN 12390-3 TCVN 3118:1993 ASTM C39

Fig I.36Satisfactory failure of cube specimens

Table I.7Test methods for making, curing and sampling concrete specimen

Fig I.38Satisfactory failure of cylinder specimens

Fig I.39Unsatisfactory failure of cylinder specimens

Fig I.37Unsatisfactory failure of cube specimens

1

4

7 8 9

5

2 3

6

A B C D

HGFE

I J K


30

Following causes can lead to unsatisfactory failure of the specimen:

Specimens Cause

Cube

The surface of the cube is not flat and parallel

The cube is not positioned centrally in the test machine

The fresh concrete has segregated during compaction

Cylinder

The capping method is not suitable or well-done

The cylinder is not positioned centrally in the test machine

The fresh concrete has segregated during compaction

Compressive machine

The compression plates are not flat

Excentric loading of the test machine

Inappropriate measuring range (20-80 max load)

Assessment of test resultsThe test results from cube or cylinder specimen are primarily used to determine that the delivered concrete mix meets the strength requirements specified in the technical specification.

Strength test results may be used for quality control, acceptance of concrete, or for estimating the strength in a structure for scheduling construction

operations such as formwork removal or for evaluating the adequacy of curing and protection provided to the structure.

The test results on concrete specimen, to meet the required grade of concrete according to a specific standard, are evaluated as follows:

TCXDVN 356:2005

TCXDVN 374:2006

TCVN 4453:1995

ASTM C94:2005 BS 5328:1990 EN 206-1:2000

Type of sample

Cube 150mm

1 set = 3 specimens

Cylinder 300x150mm

For a strength test, at least two standard test specimens shall be made from a composite sample secured

Cube 150mm

1 set = 2 specimens

Cube 150mm

Cylinder 300x150mm

1 set = 3 specimens


Table I.8Posible causes of

unsatisfactory failures

Table I.9Asessment of

test result

31

Method of Sampling

Foundation: 1set/100m3

Foundation under machinery: 1set/50m3

Frame and thin structure: 1set /20m3

Base and sub-base: 1set/200m3

Mass pour: V < 1000m3: 1set/250m3

V 1000m3: 1set/ 500m3

Not less than 1 set for each 115m3

number of set required:

V 40m3: 1 / 10m3

V 80m3: 1 / 20m3

V 200m3: 1 / 50m3

First 50m3: 3 set

Then 1 set / 150m3

Take 2 or more specimens per set.

Testing fmin : lowest strength specimen

fmed: median strength specimen

fmax: highest strength specimen

1 = fmax - fmed ; 2 = fmed - fmin

fc : the specified compressive strength.

fcr : the average compressive strength.

fmin: lowest strength specimen

fmax: highest strength specimen

fcm = (fmax + fmin) / 2

Measure compressive strength of the specimens.

fmin: strength of the specimen with lowest strength

fmax: strength of the specimen with highest strength

fcm = average strength of all specimens

Compliance checking

If 1 and 2 are both less than 15% of fmed, then favg = (fmin + fmed + fmax)/3

If either 1 or 2 is larger than 15% of fmed, then favg = fmed

If (fmax fmin) / fcm > 15% then the sample was invalid.

Otherwise, f = fcm

If (fmax fmin) / fcm > 15% then the sample was invalid.

Otherwise, f = fcm

Compressive strength

assessment

favg fck

fmin 85% x fck

The average of 3 consecutive strength tests shall be equal to or greater than specific strength-f'c

If f'c 35 MPa:individual strength test

fc - 3.5(MPa)

If f'c > 35 MPa: individual

strength test 0.9f 'c

When meeting failure case, refer to section 19 ASTM C94-2005.

favg = average strength of all valid sample.

For C20 or aboveCriteria 1 (Rolling average):

First 2 samples: favg fck +1

First 3 samples: favg fck +2

Any consecutive 4 samples:

favg fck + 3

Criteria 2 (Individual sample):

All valid samples: f fck - 3

For C7.5 to C15Criteria 1 (Rolling average):

First 2 samples: favg fck

First 3 samples: favg fck+1

Any consecutive 4 samples:

favg fck + 2 Criteria 2 (Individual):

All valid samples: f fck - 2

favg = average strength of all valid samples

Criteria 1 (Rolling average):

favg fck + 4

Criteria 2 (Individual sample):All valid samples:

f fck - 4


32

d. Comparison of strength between different standards:Every standard has its own system to evaluate the compliance of the test result to the requirement of the standard.

It is very difficult to compare the standards. In principle, it is not recommended to translate one

standard into a different standard. To assure the compliance to the design, the concrete should be tested according the standard set (TCVN, ASTM, EN, BS), used for the design.

The following graph provides an indication how TCVN, EN and BS are related in terms of cube strength (not to scale).

C25/30 C30/37 C35/45 C40/50

EN 206

C30 C35 C40 C45 C50

BS

TCVN

M300 M350 M400 M450 M500


Table I.10 Comparison of

strength between different standards

in terms of cube sample

33

4. Special characteristicsa. Concrete density The density of both fresh and hardened concrete is of interest to the engineers for different reasons including structural design and impact on compressive strength.

By choosing suitable aggregates and mix design, the density of concrete can be increased significantly (heavy concrete) or reduced (light-weight concrete).

For fresh concrete:The density plays an important role in controlling concrete yield (compared to the mix design) at readymix batching plant. Typical readymix concrete density varies from 2200 2500kg/m3 (TCXDVN 374:2006), depending on the aggregate type and mix design.

Based on the density of compacted fresh concrete, plant operators are able to check if the mix design is over- or under yielding: this means that the mix design gives more or less than 1m3 concrete after compaction. Fresh concrete density test method complies with ASTM C138; EN 12350 6; TCVN 3108:1993.

For hardened concrete:Before testing the compressive strength, the density of concrete samples (cube, cylinder) should be checked and compared with the mix design to confirm the sampling, compaction, presence of entrained air.

Example: A mix design shows that the density of concrete is 2450 kg/m3; however, the hardened concrete sample only measures 2370 kg/m3 .The strength of this sample will be much lower than the design strength. Hardened concrete density is determined either by simple dimensional checks, followed by weighing and calculation or by weight in air/water buoyancy methods (comply with EN 12390-7).

b. Air contentAir content of concrete is also an important characteristic to indirectly assess the quality of concrete.

Fresh concrete always contains a significant amount of air bubbles. One of the main reasons to compact the concrete is to remove them. If the concrete is not well compacted, some air will remain in the concrete, reducing the strength significantly.Normally, a typical compacted concrete will have air percentage varies from 0.5 2.5%. Concrete with high slump usually has lower air content than low slump concrete. Besides, the plasticizer/super plasticizer admixture can increase the air content in concrete, which may lead to lower strength.

In some cases, the air content in the concrete is increased with an air-entraining admixture up to 4-6%, to improve the resistance of the concrete against deterioration caused by freeze-thaw. For the tropical climate in southern Vietnam, air entrained concrete is normally not used for this purpose.

Air content test method is complied with ASTM C231, TCVN 3111:1993

Fig I.40illustration of the pressure method for air content

A rule of thumb1% excessive air reduces the concrete strength by 4-5%.


Extension tubing for calibration checks

Clamping device

Bowl

Air chamber

Air bleeder valve

Pressure gage

Main air valvePump

Petcock B

Petcock A

34

c. BleedingBleeding is a particular form of segregation, in which the water from the concrete appears on the surface of the concrete. Bleeding is predominantly seen in very wet mixes with high workability. Excessive bleeding can have a negative impact on the quality of the concrete: Dusty surface, linked to cement particles that are

carried to the top of the concrete layer

Discolorations of the concrete surface

Reduction of the bond between large aggregates / steel bars and mortar.

Not all bleeding is harmful for the concrete. A limited amount of bleeding protects the concrete surface against plastic shrinkage, in hot and windy weather.

For concrete floors, the bleeding of concrete is a very important characteristic:

A limited bleeding reduces the risk of early cracking

Too much bleeding water delays the finishing of the concrete floor and can lead to delamination problems

The bleeding of concrete can be reduced by:

Lowering the water/cement ratio

Intense and uniform mixing

Adapting the sand fraction of the concrete

Increasing the cement content in the mix

Bleeding of concrete test method is specified in ASTM C232 (or TCVN 3109:1993). Bleeding of concrete is determined by the percentage of water coming out the concrete.

d. Setting time of concreteAfter cement and water are mixed, they react chemically, the concrete sets and changes to the hardened state. Concrete setting time is defined as the time taken for the concrete to change from the fresh to the hardened state. Setting time of concrete is defined by 2 two parameters: (ASTM C403 Test method for setting time of concrete):

Initial set: the period time from mixing until the penetration resistance of equals 500psi (3.5 MPa).

Final set: the period time from mixing until the penetration resistance equals 4000psi (27.6 MPa).

Fig I.42Concrete

bleeding meter

Fig I.43Apparatus to determine the setting time of concrete

Fig I.44 - Diagram to determine the setting time of concrete

Fig I.41 - Bleeding of fresh concrete (good and bad)

0

1000

2000

3000

4000

5000

180 210 240 270 300 330 360 390 420

Pene

trat

ion

Resi

stan

ce, p

si

Elapsed Time, min

Final Setting

Initial Setting Outlier


35

The setting time of concrete should not be confused with the slump retention or early strength of the concrete. These three characteristics are very different properties of concrete, even if they sometimes move in similar directions.

The setting time is heavily influenced by the type of admixture, as some plasticizers act as a retarder for concrete.Thus, for specific application with different setting time requirement, the admixture (compatible with cement, dosage) and concrete workability (slump, flowability, mixing water) should be controlled very carefully.

e. PermeabilityTo determine the durability of concrete, the concrete permeability is more important than the compressive strength.

There are two types of concrete permeability, frequently used in Vietnam:

Water permeability for water-tightness of concrete

Chloride permeability for concrete in aggressive environment (seawater, brackish water)

Permeability to Water:

For specific structures which directly get in contact with water such as : basement for high rise building, dams, dikes, the water tightness of concrete is required, in addition to strength.

The concrete to permeability to water is classified into 6 levels: B2, B4, B6, B8, B10 and B12 and the testing method is specified in TCVN 3116:1993.

The level for permeability to water is the maximum water pressure for which water has not gone through 4 in 6 test samples.

In general, concrete with a higher strength will have a lower water permeability. So from the grade of concrete, the level of permeability to water can be estimated.

Concrete Grade Estimated Level of Water Permeability

30 B6

35 B8

40 B10

45 - 50 B12

1

3

2

4

6 5

4 4 4 4 4 4

4

WarningThe overdosage of admixture may delay the setting time of concrete up to 1 day or even longer.

Fig I.45The test method to determine the water permeability of concrete

Table I.11Estimation of water permeability base on concrete grade


Fig I.46 - Water permeability test machine

36

Permeability to chlorides The permeability of concrete to chloride ions is an important indicator to measure the durability of concrete in aggressive environment. At a low chloride permeability, the steel reinforcement will be protected against the chloride-linked pitting corrosion and the durability of concrete will be increased.

The method to measure the rapid chloride permeability of concrete is specified in ASTM C1202 or TCXDVN 360:2005.

The test method consists of monitoring the amount of electrical current which passes through 51 mm thick slices of 102 mm nominal diameter cores or cylinders during a 6 hours period. The total charge passed, in coulombs, has been found to be related to the resistance of the specimen to chloride ion penetration.

As the ASTM C1202 specification, the rapid chloride penetration ability of concrete is classified into 5 levels:

Charge passed (coulombs)

Chloride Ion penetrability

> 4000 High

2000 4000 Moderate

1000 2000 Low

100 1000 Very low

< 100 Negligible

The chloride permeability of concrete can be improved by:

Using blended cements, with a high percentage of blended material

Reduction of water/cement ratio, to make a more compact concrete

Efficient compaction and curing

Fig I.47The rapid

chloride permeability test

equipment

Table I.12: Classification of the rapid chloride penetration ability of concrete


37

5. Production and Transport Dosage of the componentsThe production of concrete is closely linked to the technology and equipment used. The task of dosage is to dispense the components of the concrete mix aggregate, cement, additions, mixing water, admixtures in controlled amounts, to produce the specified mix proportions with great accuracy. Two systems are used, dosage by volume and dosage by mass. Dosage by mass gives more accurate results. Every batching plant must establish sequencing for adding the material through systematic pretests. Sequencing is critical for:

The dispersion The mixing effect The optimal effect of admixtures Plant efficiency Mechanical wear

Mixing the componentsThe mixer must blend the separate components into a homogeneous mix. The mixer must also satisfy the following requirements and tasks:

High mixing intensity

Short mixing duration

Dispersion of the cement and the additions

Optimal coating of the aggregates with fines mortar (fines paste)

Fast discharging

Low wear

At ready-mix plants the paddle mixer is the most common type, used discontinuously for mixing single batches. Each type of mixer requires a minimum batch size, below which the quality of the fresh concrete is reduced.

Mixing durationThe duration of mixing depends on the type of mixer (drum or paddle mixer). Mixing duration should be determined by testing.

If a small additional dosage of water is necessary during mixing to achieve the specified concrete consistence, the mixing duration must be appropriately extended. Plotting homogeneity of the mix as a function of mixing duration gives a curve that increases rapidly at first and asymptotically approaches the ideal line as mixing advances (Fig I.48)

Readymix concrete should be brought to the construction site immediately after production at the concrete plant and placed without delay in order to preserve quality. There is a certain danger of segregation during transport, so truck mixers are used when the concrete is of highly plastic consistence, for long hauls, or when traffic conditions are poor.

During the trip, concrete must be protected from rain, exposure to sun, wind blast, and the like. Depending on the prevailing weather conditions on the day of concreting, suitable measures should be taken (covering the concrete, reducing the temperature of fresh concrete, etc.).

For delivery by truck mixer, the concrete should be mixed an additional one to two minutes after arrival on site and immediately before pouring. Adding more water should be avoided, because such additions are uncontrolled and the water cannot be mixed in thoroughly. If the delay becomes too long, the concrete may be used only for less critical applications (fill, lean concrete, etc.).

Fig I.48 Homogeneity of the mix as a function of mixing duration


Definition: Mixing duration = Wet-mixing duration starts when all components are in the mixer.

38

6. Placing and CompactionConveying and depositingIn Vietnam there are three main means of conveying used: chute, bucket and pump. Depend on local circumstances, kind of structure, workability of fresh concrete, economy and progress of project , the method of conveying will be chosen. Show in table I.13

Method of conveying StructureWorkability of concrete

(Slump)Picture

Chute

Some small structures like foundation, ground slab, floor...

8 -10 cm Fig I.49

Bore piling > 18 cm Fig I.50

BucketColumn, beam and floor in highrise building

8 - 14 cm Fig I.51

Pump Floor slab, foundation... 12 - 18 cm Fig I.52

Fig I. 49

Fig I.50

Table I.13 Method of conveying

Fig I.51 Fig I.52


39

Delivery volume and placing capacity must be coordinated. Concrete should be deposited at a constant rate, in horizontal layers of uniform thickness. To prevent segregation, the concrete should not be dropped more than 50 to 70 cm. Drop heights greater than 1,5 m require the use of a drop chute or feed hose.

CompactionGood compaction is the prerequisite for durable concrete. The advantages of well-compacted concrete are:

Higher density

Improved durability

Good compressive strength

Better bond between reinforcement and concrete

Method of compactionSelecting the best method of compaction will depend on the workability of the concrete and the reinforcement density/rebar spacing of the element. The most common effective method of compaction is vibrating. Vibrating is most often done with internal vibrators (poker-type vibrators) or external vibrators (form vibrators or surface finishers with surface vibrators).

Vibration almost completely overcomes the internal friction between the aggregates. The separate particles move closer together, and entrapped air escapes to the surface in the form of air bubbles (the content of entrapped air after compaction is about 1.5 % by volume). The voids become filled with fines paste and the fresh concrete is consolidated under its own weight.

Effective range of electrical high-frequency vibrator heads (Table I.14).

Experience shows that a frequency of about 12,000 CPM is best for normal concrete. The vibration frequency should be increased (up to 18,000 CPM) for fine-aggregate concretes.

Fig I.53 - Segregate concrete because of too high drop

Fig I.54 - Honeycomb on concrete

Fig I.55The structure with good compaction

Table I.14 Reference values for the effective range diameter and spacing of insertion points

Diameter of vibrator head

(mm)

Effective range diameter

(mm)

Spacing between inserrtion

points (cm)

< 40 30 25

40 bis 60 50 40

> 60 80 70


40

Rules for good compaction The vibrator head should be quickly immersed in

the concrete, held briefly at the lowest point and slowly extracted. The concrete surface must close behind. If the surface no longer closes, either the consistence is too stiff, the concrete has already begun to set, or the duration of vibration has been insufficient. Spacing between the insertion points should be uniform.

The vibrator head should not be used to distribute the concrete.

Vibration should be stopped when a thin film of fine mortar forms on the surface and larger air bubbles surface only occasionally.

The insertion points should be spaced close enough that the effective range diameters of the vibrator overlap.

If concrete is deposited in several layers fresh on fresh, the vibrator head should extend through the layer to be compacted and about 10 to 15 cm into the underlying layer of fresh concrete. This ensures a good bond between the two layers (Fig I.56).

Rule of thumb Spacing between insertion points = 8 to 10 times the diameter of the vibrator head

Fig I.57Spacing between insertion points,

depositing fresh on fresh

Fig I.56 - Concrete compaction by vibrating method

Rightinsertion point

Wrong

150

mm

1-2xD 8-10 D

II

IIII

IIIIII

8-10 D

300-

400

mm


41

Fig I.58Aggregate shading

Fig I.59Cooling concrete by liquid nitrogen

7. Concreting in hot weather Vietnam is a country located in hot climates, it effects directly to the placing and quality of concrete. With hot weather, the workability of fresh

concrete drops faster so the placing of concrete becomes harder. In spite of warnings not to add extra water to the mix on the construction site, this pratice is still often used to improve consistence. Water addition at the jobsite increases the w/c ratio, lowers the strength and durability of the concrete. It can lead to strength failures at the project.

To keep the drop in concrete strength due to hot weather within narrow limits, the temperature of fresh concrete should be controlled carefully. Some projects in Vietnam require the tempera-ture of fresh concrete from 30 to 32oC.

In addition to the decrease in strength and durability, higher concrete temperatures produce other negative effects:

Faster hydration of the cement causes faster setting of the concrete or even premature setting greatly impairing workability, to the point of making the concrete unworkable.

The concrete, specifically the surface layer, dries out faster especially under strong winds, intensive sun, and low relative humidity.

Water loss must be prevented by curing. If water is lost, plastic shrinkage will occur cracks (see Chapter III.B). Additionally, cement hydration will remain incomplete. This further reduces final strength in the prematurely dehydrated outer layer, which further impairs durability.

Methods of controlling the temperature of concrete The temperature t of fresh concrete can be

roughly estimated using the formula:

Base on this formula, controlling the temperature of aggregate and water has the highest impact on the temperature of concrete. The effect of cement temperature to fresh concrete temperature is relatively small.

Methods of lowering the temperature of fresh concrete: Cooling the aggregate by shading or spraying

with water (*) Cooling the mixing water with ice or water

chiller (*) Cooling the concrete mix with liquid nitrogen(*) The amount of mixing water is to be reduced accordingly.

Rule of thumb10 liters of extra mixing water per m3 concrete causes a 10-percent drop in 28-day strength.

tconcrete = 0,7 taggregate + 0,2 t water + 0,1 t cement


42

Concreting in hot weather requires good planning and preparation The delivery of fresh concrete must be well

coordinated with the concreting work so that it can be poured without delay.

Sufficient equipment and personnel must be planned so that the concrete can be placed and compacted without delay.

The substrate and forms must not extract water from the fresh concrete. Forms should be moistened before pouring the concrete (Fig I.60). But excessive soaking of forms and substrate should be avoided; no puddles should form.

If the conditions for successful concreting at high temperatures cannot be achieved for any reason, concrete work must be rescheduled to a cooler hour of the day, for example at night.

Retarders can be used to largely eliminate the disadvantages of fast cement hydration, but they do little against premature setting of concrete. Retarders also require extended curing times, as they increase the risk of plastic shrinkage cracks.

Placing and compaction The shortest waiting time and fastest possible

placement of fresh concrete are the cardinal rules.

The contractors personnel should be familiarized with the special aspects and requirements of concreting at high temperatures.

If sudden stops cannot be avoided, any concrete in the truck and in the delivery equipment must be protected from the effects of direct wind and sun. Truck mixers can be hosed down on the outside with water.

Adding extra water on the construction site is to be strictly prohibited. Compliance with this rule must be checked.

Fig I.60 Wetting the forms


43

8. Pumped concreteApplication RangeThe use of pumps is recognized as a modern and efficient method of transport and placing concrete. Pumped concrete can be used for practically any construction task, and is particularly useful when high performance in placing is required or when the pouring location is poorly accessible. In general, there are two types of concrete pumps: stationary and mobile.

Requirements for pumped concretePumped concrete is pushed like a plug through a pipeline. The key is to keep the concrete from segregating under the forces acting upon it.

Cement Practically any standard cement is suitable for use in pumped concrete. A fresh concrete that can be efficiently moved through a pipeline should have a cement content of at least 320 kg/m3.

Aggregate mix Experience shows that increasing the fines ( 0.125mm, including cement) to about 400 kg/m3 considerably improves pumpability without compromising durability of the hardened concrete.

Thanks to improvements in pump design, the grain shape of coarse aggregate has only a minor influence on pumpability.

AdmixturesThe rules that apply to using admixtures in concrete also apply to pumped concrete. It should be kept in mind when using air entrainers, that fresh concrete with an air content greater than 4% can reduce the delivery capacity of concrete pumps.

Consistence Pumped concrete must have a plastic to soft workability.

The required workability can depend greatly on the characteristics of the sand, and must be adjusted when necessary as indicated by pretests.

Fig I.62 Casting a large concrete floor slab. Mobile pump fed by a truck mixer

Fig I.61 - Pump concrete by mobile pump


44

Tips for pumping concrete A smooth process must be ensured by good

planning between the concrete pump operator, the building contractor, and the concrete supplier.

The setup and operation of the pumps is the responsibility of the pump operator.

The rate of delivery and the delivery rating of the concrete pumps should be suited to the working capacity of the crew placing the concrete.

The concrete should be delivered to the concrete pump with truck mixers to prevent any segregation. Hopper trucks or silo trucks may be used for short hauls.

The construction contractor is responsible for the proper placement and curing of the concrete.

About 0.52.0m3 of a cement-rich mortar serves as a lubricating mix to prime the pumping system. This material may not be used as structural concrete.

Safety aspects of using concrete pumps

Delivering and placing pumped concrete can be dangerous.

The following must be ensured:

Formwork for walls and columns must be strong enough to handle the increased pressure of pumped concrete.

No overhead power lines should be in the working area.

The load-bearing capacity of the pump platform must be adequate. Directives of the pump personnel must be strictly followed.


45

9. Curing Purpose and objectivesThe purpose of curing is to protect concrete from water loss and harmful influences during the early hardening period. Compressive strength alone does not guarantee durability; the concrete must also be dense. Especially in the surface layer, hardened cement paste with high density and low-as-possible permeability is very important.

This gives better resistance to carbonation and other types of attack. Curing includes all the measures taken to protect freshly placed, young concrete while it develops adequate strength. The chief objectives of curing is to protect the concrete from:

Evaporation due to wind, sun, dry cold

Extreme temperatures (cold or heat) and rapid temperature change

Heavy rain

Early influences of foreign substances (oil etc.)

Premature dryingProtection against premature moisture loss is especially important. Protective measures must be taken immediately after concrete is placed.

The consequences of premature water loss in the surface layers are: Heavy plastic-shrinkage cracking (see Chapter III)

Low strength

Tendency to surface dusting

Lower density and durability

Faster corrosion of steel reinforcement

Lower abrasion resistance

Preventive measures Leaving forms in place

Covering with a membrane (Fig I.63)

Wrapping with insulating material (Fig I.64)

Covering with water-retaining fabrics (burlap, geotextiles)

Application of a liquid curing compound (Fig I.65)

Continuous spraying with water

Keep under water

A combination of these measures

Fig I.65

Fig I.63

Fig I.64


46

Rate of dryingThe rate of drying depends on:

air temperature

concrete temperature

relative humidity

wind speed

Typical effects of these factors are shown in Fig I.66 and Fig I.67 and Fig I.68 shows the correlations among the factors mentioned. The chart can be used to estimate the rate of drying.

Fig I.66 - Influence of water retention on strength development in the surface layer of concrete

Fig I.67 - shows the correlations among the factors mentioned. The chart can be used to estimate the rate of drying.

Testing age [days]

kept constantlymoist

notkept moist

kept moistfor 7 days

10

10

20

30

40

3 7 28 90

Com

pres

sive

str

engt

h [N

/mm

2 ]

Time [hours]

unprotected concrete,wind speed 20 km/h

unprotected concrete,wind speed 10 km/h

concrete protectedwith a curing compound

00

1

2

3

4

6 12 18 24

Plas

tic

shri

nkag

e [m

m/m

]

Fig I.68 - Chart for calculating the rate of drying of exposed concrete

surfaces. Example illustrated: air temperature: 28C relative humidity:

50% concrete temperature: 28C wind speed: 5m/sec. result: rate of drying =

0.8 kg/m2 hr.

relative humidity

ambient temperature (0C)

10 20 30

20

0

410

8

6

4

2

0

3

2

1

1015

20

25

30

40

'C%

35

40

60

80

100

wind speed

m/sec

rate of drying (kg/m2 hr.)

concrete temperature


47

10. Influence of FormworkFormwork plays an important role in a successful construction project. It gives the concrete surface its form, texture, and color. It gives the concrete structure, correct dimensions, and the proper form. Formwork often does not receive the attention it deserves.

Selection of formsThe construction contractor usually selects forms based on the following criteria:

Building structure / construction task

Specified surface quality of the concrete

Number of potential reuses

Labor required for erection

Thermal insulation characteristics

Price

Common facing materials for forms Raw, rough-cut wooden boards treated wooden

sheets

Plastic-laminated forms (polyester, polystyrene, linoleum, elastomers, etc.)

Steel, aluminium

Requirements for forms

Dimensional accuracy

Watertightness (Fig I.69)

Stiffness, no deformation

Cleanness

Low adhesion to hardened concrete (Fig I.70 and Fig I.71)

Attractive surface texture (Fig I.72)

Form typesAbsorptive forms usually produce a smooth, closed concrete surface, because they absorb surplus water and air bubbles. The face of wooden forms should include only boards which have been used for an equal number of times, because the absorption capacity of the wood decreases with each use, which has an effect on the color of the concrete surface. Raw wooden boards should be coated with cement paste before initial use to remove the wood sugar that disturbs hydration of cement. This coating also evens out any variations in absorbency of the boards (Fig I.73).

Fig I.72 Example of a successful textured concrete surface

Fig I.73 Non-uniform absorbency of wood used in forms affects the concrete surface

Fig I.71 Concrete skin adhered to wooden forms

Fig I.69 Results of a leaky form

Fig I.70 Peeled-off concrete skin


48

Non-absorptive, water-repellant forms promote the partial accumulation of mortar paste. This leads to irregularities in the color of the concrete surface (clouding). Strong surface segregation can lead to reduced durability (see Chapter III.A Segregation of Concrete). Thus for exposed surfaces it is advantageous to use absorptive forms or water-conducting form liners of polypropylene fibers, etc

Form-release agentsForm-release agents make it easier to loosen the form faces from the concrete surface. At the same time, they protect and preserve the form material. They are to be applied thinly and uniformly, normally before the reinforcement is put in place. Surplus chemical should be wiped away with a cloth (Fig I.74). Staining, and irregular gray color of the concrete surface, can frequently be traced to improper application of a form-release agent.

Fig I.74 Effect of form-release agents on the concrete surface:

- Left: surplus form-release agent removed with a cloth

- Right: excessive form-release agent used


49

A. Infrastructure

Chapter II: Applications with specific requirements

1. IntroductionTo support the growth of the economy in Vietnam, both public and private funds invest important amounts of capital into infrastructure projects, like roads, bridges, dams, ports, tunnels, power plants

As this infrastructure is the backbone of the economy, the design life of these projects is significantly longer than normal buildings (houses, schools). With proper maintenance, a bridge should be used for at least 50 years up to 100 years and even longer!

To meet this long service life, the concrete for infrastructure projects requires special attention for durability, with careful selection of the concrete components.

2. Cement for infrastructureIn south Vietnam, the cement, used for infrastructure, is Blended Portland Cement (PCB40), compliant to TCVN6260, ASTM C1157 with low alkali content (Na2O-eq < 0.60%) to prevent alkali aggregate reaction.

The alkali-aggregate reaction or concrete cancer in laymens terms is a reaction between aggregates, the alkali in the concrete and water to form an expansive gel that creates cracks in the concrete. This reaction is a very slow process over years, but can become visible in 5 to 10 years after construction.

In case of aggressive environment (presence of chlorides, sulphates, seawater,), additional precautions have to be taken (see chapter II.B).

Holcim recommendationCement PCB 40 according to TCVN 6260:2009 or ASTM C1157 - GU, with low alkali content (Na2O-eq M 0.60%)

50

1. Introduction To assure the long life of the construction in aggressive environments, special care has to be taken for the concrete: cement choice, mix design, placing and compacting, and last but not least, curing.

A key element is the choice of cement, as concrete can be exposed to different aggressions:

Sulfates in the environment attack the cement matrix (C3A cement mineral) and create cracks in concrete

Chlorides penetrate into the concrete pores and can lead to the dangerous pitting corrosion of steel reinforcement of the structure

Other aggressive elements (low pH, acids,..) can attack the cement matrix, by dissolving its constituents

For aggressive environments, 2 main types of cement are generally specified:

Sulfate resistant portland cement (only for sulfate attack)

Sulfate resistant blended portland cement

2. Sulfate resistant Portland cementMain characteristic of sulfate resistant Portland cement is a lower C3A content, a specific cement mineral, as this component will react with sulfates in the environment to ettringite, that expands in the concrete pores to create tensions and cracks in the concrete.

The C3A-content of cement can only be measured on Ordinary Portland Cement (OPC). For blended cement, the addition materials will change the chemistry of cement and the calculated C3A content (based on the Bogue formula C3A = 2,65 Al2O3 1,692 Fe2O3) is not valid any more.

This type of cement complies to following standards:

TCVN 6067

ASTM C150 Type II (Medium Sulfate MS) or type V (High Sulfate HS)

BS 4027

The maximum value of C3A depends on the standard used:

By limiting the C3A mineral, sulfate resistant Portland cement offers protection to sulfate attack from the environment only. It does not offer additional protection to chloride penetration or other aggressive elements (low pH, acids), compared to a concrete with general use cement PCB40.

B. Aggressive environments (sulfate, seawater ...)

Note: According TCVN 6067, OPC type II (comply with ASTM

C150) does not classify as sulfate resistance cement

holcim technical manual english

Documents

southern vietnam

south vietnam

holcim vietnam ltdall

holcim beton

readymix concrete

durability of concrete

supply of cement

international standards