holcim strength performance passion technical manual cement concrete holcim vietnam ltd 1st edition 2013

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,

Holcim (Vietnam) Ltd.

1 st edition 2013

Cement & Concrete

Technical Manual

C2013, Holcim (Vietnam) LtdAll 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

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Special permission must be requested in writing to

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

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.

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 technical.service-vnm@holcim.com

Yours sincerely,

Pieter Keppens

Technical Marketing Manager

5 Production and transport 37

7 Concreting in hot weather 41

2 Sulfate resistant Portland cement 50

3 Sulfate resistant blended cements 51

2 Cement for massive structures 52

3 Concrete for massive structures 53

2 Production and use of high strength concrete 55

2 Production of very flowable / self-compacting concrete 57

2 Cement for treated aggregates 58

3 Testing procedure for cement treated aggregates 59

4 Optimization of cement treated aggregates 61

Chapter III

1 Plastic settlement cracks 65

2 Plastic shrinkage cracks 66

1 Chloride-induced corrosion of the steel reinforcement 70

2 Attack by sulfates from seawater 71

C Recommendation for limiting values of concrete composition 97

Chloride - induced corrosion in sea water 97

Aggressive chemical environments 97

1 Cement

General

Cement 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

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 conformity

The 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 system

Most 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 monitoring

In-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 life

If 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

A Components of concrete

Holcim recommendation

For general use concrete, standard cement offers

the best supply security for any project:

• TCVN 6260:2009 – PCB 40

• ASTM C1157:2008 – GU

2 Mixing water

Water 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

General

Mixing 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 water

According to TCXDVN 302:2004, mixing water must meet these following requirements:

• Does not contain oil scum and oily film

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

Purpose of mixing water

Maximum Level (mg/l)

SolubleSalt

Sulfate Ion (SO4-2)

3 Fine Aggregate

Grading

Fine 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 Impurities

Fine 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 Impurities

Impurities 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-μm)

Akali-Silica Reaction

For 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.2

Organic 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

4 Coarse aggregate

General

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

Characteristics

The most important characteristics of coarse aggregates are:

• Specific gravity

• Bulk density (unit weight) and moisture content

• Mineral composition, grain shape, and surface texture

Total moisture None Less than potential absorption Equal to potential absorption Greater than absorption

Fig I.3

The moisture

state of

aggregate

Bulk density (unit weight) and moisture content

Bulk 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

Aggregate type Specific Gravity (kg/m 3 ) Aggregate Material Application

Standard aggregate 2700 River or glacial deposits; crushed stone Reinforced and non-reinforced concreteHeavy aggregate >3000 Barite (heavy spar), iron ore, granulated steel Concrete for radiation protectionLightweight aggregate < 2000 Expanded clay, polystyrene Insulating concrete, concrete topping, sloped concreteHard aggregate > 2500 Quartz, corundum, silicon carbide Hard concrete slabs, abrasion-resistant concrete

Adhesive 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.)

Grading

The 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 texture

Porous 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

Desirable

Less Desirable

Flaky Elongated

5 Admixtures

Definition and classification

Concrete 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)

Dosage

Admixtures 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 admixtures

According 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:

Fig I.7 - Admixture used in concrete.

6 Additions

Fibers

Polypropylene 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 fume

Silica 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

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

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 pigments

Inorganic 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

hardened concrete

1 Composition of Concrete

Concrete 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 mixing

In 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) ratio

A 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

Choosing the water/cement ratio

An 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 concrete

With 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 concrete

A 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.18

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

Fig I.19

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

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:

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)

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

Table I.4

Effect of various factors on workability and strength of concrete

Fig I.20 Apparatus to

determine slump

2 Workability

To 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 <

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 12350-2The apparatus used in the slump test are: mold, tamping rod, measuring equipment (Fig I.20):

Workability compressive 28-day

strengthChange

• 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

- 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:

60-80 Elements with intense vibration: Precast elements, concrete pavement

Concrete placed by bucket

100-160 Elements with good vibration (compaction needles): column, slab, beams etc

Concrete 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

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.25

Structure with dense reinforce steel

Fig I.26

Transfer beam

Fig I.24 - Determine slump flow for fresh concrete

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

Handle Clip

Bottom Plate

200mm

200mm

700mm 40mm

3 Concrete strength

One 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.33

Equipment for capping specimen and the specimen after capping and testing  

Shape & size specimen (mm) Correlation factor

Cube specimen

71,4 x 143 & 100 x 200 1.16

200 x 400 1.24

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 strength

The 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

Load

c Assessment of compressive strength test results

Test methods for sampling & testing

General 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

Table I.7

Test methods for making, curing and sampling concrete specimen

Fig I.38

Satisfactory failure of cylinder specimens

Fig I.39

Unsatisfactory failure of cylinder specimens

Fig I.37

Unsatisfactory failure of cube specimens

F E

Following causes can lead to unsatisfactory failure of the specimen:

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 results

The 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

Cube 150mm

1 set = 2 specimens

Cube 150mmCylinder 300x150mm

Not less than 1 set for each

V ≤ 40m3: 1 / 10m3

V ≤ 80m3: 1 / 20m3

V ≤ 200m3: 1 / 50m3

First 50m3: 3 setThen 1 set / 150m3

Take 2 or more specimens per set

Testing fmin : lowest strength

f’cr : 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 f'c ≤ 35 MPa:

individual strength test

≥f‘c - 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 above Criteria 1 (Rolling average):

First 2 samples: favg ≥ fck +1First 3 samples: favg ≥ fck +2Any consecutive 4 samples:

favg ≥ fck + 3

Criteria 2 (Individual sample):

All valid samples: f ≥ fck - 3

For C7.5 to C15 Criteria 1 (Rolling average):

First 2 samples: favg ≥ fck

First 3 samples: favg ≥ fck+1Any 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

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)

4 Special characteristics

a 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

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 content

Air 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.40

illustration of the pressure method for air content

A rule of thumb

1% 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 valve Pump

Petcock B Petcock A

c Bleeding

Bleeding 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 concrete

After 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.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

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 Permeability

To 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

The overdosage of admixture may delay the

setting time of concrete up to 1 day or even

longer

Fig I.45

The test method

to determine the water

permeability of concrete

Table I.11

Estimation of water permeability base on concrete grade

Fig I.46 - Water permeability test machine

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) penetrability Chloride Ion

5 Production and Transport

Dosage of the components

The 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 components

The 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 duration

The 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.

6 Placing and Compaction

Conveying and depositing

In 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 Structure Workability 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

Bucket Column, beam and floor… in highrise building 8 - 14 cm Fig I.51

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

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

Compaction

Good 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 compaction

Selecting 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.55

The 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

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