Manual for Concreting

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MANUAL FOR CONCRETING

CONTENT

1. MATERIALS FOR CONCRETE

1.1 Cement

1.2 Aggregates

1.3 Cementitious Materials

1.4 Admixture

1.5 Water for Concrete

2. PROPERTIES OF FRESH CONCRETE

2.1 Workability

2.2 Water / Cement Ratio

3. MIX DESIGN

3.1 Characteristic Strength of Concrete

3.2 Target Mean Strength

3.3 Trial Mix

3.4 Passing Criteria and Compliance

3.5 Prequalification of the Concrete Supplier

3.6 Quality Control

4. ORDER & DELIVERY OF CONCRETE

4.1 Approval on the Concrete Supplier

4.2 Ordering Concrete

4.3 Transporting and Handling and Placing Concrete

4.4 Curing Methods and Control

4.5 Sampling and Testing

5. EVALUATION OF HARDENED CONCRETE

5.1 Strength Evaluation

5.2 Non-destructive Test

5.3 Remedial / Repairing of Concrete Structure

6. APPENDIX

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1. MATERIALS FOR CONCRETE

Concrete is basically a mixture of two components: aggregates and paste. The paste,

comprised of portland cement and water, binds the aggregates (usually sand and gravel or

crushed stone) into a rocklike mass as the paste hardens because of the chemical reaction of

the cement and water (Fig. 1-1). Supplementary cementitious materials and chemical

admixtures may also be included in the paste.

The paste is composed of cementitious materials, water, and entrapped air or purposely

entrained air. The paste constitutes about 25% to 40% of the total volume of concrete. Fig. 1-

2 shows that the absolute volume of cement is usually between 7% and 15% and the water

between 14% and 21%. Air content in air-entrained concrete ranges from about 4% to 8% of

the volume.

1.1 Cement

Different types of portland cement are manufactured to meet various normal physical and

chemical requirements for specific purposes. There are eight types of portland cement specify

in ASTM C150 as follows;

Type I Normal

Type IA Normal, air-entraining

Type II Moderate sulfate resistance

Type IIA Moderate sulfate resistance, air-entraining

Type III High early strength

Type IIIA High early strength, air-entraining

Type IV Low heat of hydration

Type V High sulfate resistance

Some cements are designated with a combined type classification, such as Type I/II,

indicating that the cement meets the requirements of the indicated types and is being offered

as suitable for use when either type is desired.

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Standard Requirements (ASTM C 150)

Optional Requirements (ASTM C 150)

Summary of main BS 5328 recommendations in respect of chloride content

Inspection

Inspection of the material shall be made as agreed upon between the purchaser and the seller

as part of the purchase contract. The cement shall be rejected if it fails to meet any of the

requirements of this specification. At the option of the purchaser, retest, before using, cement

remaining in bulk storage for more than 6 months or cement in bags in local storage in the

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custody of a vendor for more than 3 months after completion of tests and reject the cement if

it fails to conform to any of the requirements of this specification. Cement so rejected shall be

the responsibility of the owner of record at the time of re-sampling for retest. When the

cement is delivered in packages, the words “Portland Cement,” the type of cement, the name

and brand of the manufacturer, and the mass of the cement contained therein shall be plainly

marked on each package. When the cement is an air-entraining type, the words “air-

entraining” shall be plainly marked on each package. Similar information shall be provided in

the shipping documents accompanying the shipment of packaged or bulk cement. All

packages shall be in good condition at the time of inspection.

The cement shall be stored in a weatherproof dry shed with raised boarded floor or in a

properly designed bulk containers. Cement of different manufacturer and of different types

shall be kept separately and shall not be used in the same mix.

1.2 Aggregates

Concreting Sand Rounded Gravel and Crush Stone

Light-weight aggregates Ranged of particle sizes found in aggregates

(Expended clay & shale)

Aggregates make up about 75% of the volume of concrete, so their properties have a large

influence on the properties of the concrete. Aggregates are granular materials, most

commonly natural gravels and sands or crushed stone, although occasionally synthetic

materials such as slags or expanded clays or shales are used. Most aggregates have specific

gravities in the range of 2.6 to 2.7, although both heavyweight and lightweight aggregates are

sometimes used for special concretes. The role of the aggregate is to provide much better

dimensional stability and wear resistance; without aggregates, large castings of neat cement

paste would essentially self-destruct upon drying. In general, aggregates are much stronger

than the cement paste, so their exact mechanical properties are not considered to be of much

importance (except for very high-strength concretes).

For ordinary concretes, the most important aggregate properties are the particle grading (or

particle-size distribution), shape, and porosity, as well as possible reactivity with the cement.

All aggregates should be clean - that is, free of impurities such as salt, clay, dirt, or foreign

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matter. As a matter of convenience, aggregates are generally divided into two size ranges:

coarse aggregate, which is the fraction of material retained on a (5-mm) sieve, and fine

aggregate, which is the fraction passing the (5-mm) sieve but retained on a (0.15-mm) sieve.

BS 882 limits for 20 mm graded and single-sized coarse aggregates.

Typical medium grading of fine aggregate to BS 882.

Aggregates have two prime functions in concrete:

(i) Providing concrete with a rigid skeletal structure

(ii) Reducing the void space to be filled by the cement paste.

Important properties of an aggregate which affect compliance with British Standards and

affect the performance of ready-mixed concrete are Nominal maximum size, Grading and

mean size, Silt, clay or fine dust content, Shape and surface texture, Water absorption,

Relative density, Bulk density and Moisture content.

Other properties which have particular importance for some aggregates or for some special

uses are Chloride content, Susceptibility to alkali-silica reaction, Deleterious materials

content and Moisture movement.

Optional maximum chloride content limits provided in BS 882

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1.3 Cementitious Materials

Supplementary cementitious materials are added to concrete as part of the total cementitious

system. They may be used in addition to or as a partial replacement of portland cement or

blended cement in concrete, depending on the properties of the materials and the desired

effect on concrete. They are used to improve a particular concrete property, such as resistance

to alkali-aggregate reactivity. The optimum amount to use should be established by testing to

determine (1) whether the material is indeed improving the property, and (2) the correct

dosage rate, as an overdose or under-dose can be harmful or not achieve the desired effect.

Supplementary cementitious materials also react differently with different cements.

Supplementary cementitious materials. From left to right, fly ash (Class C), metakaolin (calcined clay), silica fume, fly ash (Class F), slag, and calcined shale.

Fly ash, ground granulated blast-furnace slag, silica fume, and natural pozzolans, such as

calcined shale, calcined clay or metakaolin, are materials that, when used in conjunction with

portland or blended cement, contribute to the properties of the hardened concrete through

hydraulic or pozzolanic activity or both.

A pozzolan is a siliceous or aluminosiliceous material that, in finely divided form and in the

presence of moisture, chemically reacts with the calcium hydroxide released by the hydration

of portland cement to form calcium silicate hydrate and other cementitious compounds.

Pozzolans and slags are generally categorized as supplementary cementitious materials or

mineral admixtures.

Supplementary cementitious materials are added to concrete as part of the total cementitious

system. They may be used in addition to or as a partial replacement of portland cement or

blended cement in concrete, depending on the properties of the materials and the desired

effect on concrete. They are used to improve a particular concrete property, such as resistance

to alkali-aggregate reactivity. The optimum amount to use should be established by testing to

determine (1) whether the material is indeed improving the property, and (2) the correct

dosage rate, as an overdose or under-dose can be harmful or not achieve the desired effect.

Supplementary cementitious materials also react differently with different cements.

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BS 5328 recommendations for use of various cements or combinations of cements with

ggbs or pfa with 20 mm aggregates in concrete subject to sulphate attack

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1.4 Admixtures

Admixtures are those ingredients in concrete other than portland cement, water, and

aggregates that are added to the mixture immediately before or during mixing. Admixtures

can be classified by function as follows:

1. Air-entraining admixtures

2. Water-reducing admixtures

3. Plasticizers

4. Accelerating admixtures

5. Retarding admixtures

6. Hydration-control admixtures

7. Corrosion inhibitors

8. Shrinkage reducers

9. Alkali-silica reactivity inhibitors

10. Coloring admixtures

11. Miscellaneous admixtures such as workability, bonding, damp-proofing, permeability

reducing, grouting, gas-forming, anti-washout, foaming, and pumping admixtures

Liquid admixtures, from left to right: antiwashout admixture, shrinkage reducer, water reducer, foaming agent, corrosion inhibitor, and air-entraining admixture.

Air-entraining admixtures are used to purposely introduce and stabilize microscopic air

bubbles in concrete. Air entrainment will dramatically improve the durability of concrete

exposed to cycles of freezing and thawing. Entrained air greatly improves concrete’s

resistance to surface scaling caused by chemical deicers (Fig. 6-3). Furthermore, the

workability of fresh concrete is improved significantly, and segregation and bleeding are

reduced or eliminated.

Water-reducing admixtures are used to reduce the quantity of mixing water required to

produce concrete of a certain slump, reduce water-cement ratio, reduce cement content, or

increase slump. Typical water reducers reduce the water content by approximately 5% to

10%. Adding a water-reducing admixture to concrete without reducing the water content can

produce a mixture with a higher slump.

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Mid-range water reducers provide significant water reduction (between 6 and 12%) for

concretes with slumps of 125 to 200 mm without the retardation associated with high dosages

of conventional (normal) water reducers. Normal water reducers are intended for concretes

with slumps of 100 to 125 mm. Mid-range water reducers can be used to reduce stickiness

and improve finishability, pumpability, and placeability of concretes containing silica fume

and other supplementary cementing materials. Some can also entrain air and be used in low

slump concretes.

High-range water reducers (HRWR) Types F (water reducing) and G (water reducing and

retarding), can be used to impart properties induced by regular water reducers, only much

more efficiently. They can greatly reduce water demand and cement contents and make low

water-cement ratio, high-strength concrete with normal or enhanced workability. A water

reduction of 12% to 30% can be obtained through the use of these admixtures. The reduced

water content and water-cement ratio can produce concretes with (1) ultimate compressive

strengths in excess of 70 MPa, (2) increased early strength gain, (3) reduced chloride-ion

penetration, and (4) other beneficial properties associated with low water-cement ratio

concrete.

Plasticizers, often called superplasticizers, are essentially high-range water reducers

meeting ASTM C 1017; these admixtures are added to concrete with a low-to-normal slump

and water-cement ratio to make high-slump flowing concrete. Flowing concrete is a highly

fluid but workable concrete that can be placed with little or no vibration or compaction while

still remaining essentially 75mm (3-in.) slump concrete can easily produce a concrete with a

230-mm (9-in.) slump. Flowing concrete is defined by ASTM C 1017 as a concrete having a

slump greater than 190 mm (71⁄2 in.), yet maintaining cohesive properties.

Retarding admixtures are used to delay the rate of setting of concrete. High temperatures of

fresh concrete (30°C) are often the cause of an increased rate of hardening that makes placing

and finishing difficult. One of the most practical methods of counteracting this effect is to

reduce the temperature of the concrete by cooling the mixing water and/or the aggregates.

Retarders do not decrease the initial temperature of concrete. The bleeding rate and bleeding

capacity of concrete is increased with retarders.

Hydration controlling admixtures consist of a two-part chemical system: (1) a stabilizer or

retarder that essentially stops the hydration of cementing materials, and (2) an activator that

reestablishes normal hydration and setting when added to the stabilized concrete. The

stabilizer can suspend hydration for 72 hours and the activator is added to the mixture just

before the concrete is used. These admixtures make it possible to reuse concrete returned in a

ready-mix truck by suspending setting overnight. The admixture is also useful in maintaining

concrete in a stabilized non-hardened state during long hauls.

An accelerating admixture is used to accelerate the rate of hydration (setting) and strength

development of concrete at an early age. The strength development of concrete can also be

accelerated by other methods: (1) using Type III or Type HE high-early-strength cement, (2)

lowering the water-cement ratio by adding 60 to 120 kg/m3 of additional cement to the

concrete, (3) using a water reducer, or (4) curing at higher temperatures. Accelerators are

designated as Type C admixtures under ASTM C 494.

Corrosion inhibitors are used in concrete for parking structures, marine structures, and

bridges where chloride salts are present. The chlorides can cause corrosion of steel

reinforcement in concrete. Ferrous oxide and ferric oxide form on the surface of reinforcing

steel in concrete. Ferrous oxide, though stable in concrete’s alkaline environment, reacts with

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chlorides to form complexes that move away from the steel to form rust. The chloride ions

continue to attack the steel until the passivating oxide layer is destroyed. Corrosion-inhibiting

admixtures chemically arrest the corrosion reaction.

Shrinkage-reducing admixtures have potential uses in bridge decks, critical floor slabs, and

buildings where cracks and curling must be minimized for durability or aesthetic reasons.

Propylene glycol and polyoxyalkylene alkyl ether have been used as shrinkage reducers.

Drying shrinkage reductions of between 25% and 50% have been demonstrated in laboratory

tests. These admixtures have negligible effects on slump and air loss, but can delay setting.

They are generally compatible with other admixtures.

Chemical admixtures to control alkali-silica reaction (ASR) are Lithium nitrate, lithium

carbonate, lithium hydroxide, lithium aluminum silicate (decrepitated spodumene), and

barium salts which have shown reductions of (ASR) in laboratory tests. Some of these

materials have potential for use as an additive to cement.

Natural coloring admixture (pigments) and synthetic materials are used to color concrete

for aesthetic and safety reasons. Red concrete is used around buried electrical or gas lines as a

warning to anyone near these facilities. Yellow concrete safety curbs are used in paving

applications. Generally, the amount of pigments used in concrete should not exceed 10% by

weight of the cement. Pigments used in amounts less than 6% generally do not affect concrete

properties. Unmodified carbon black substantially reduces air content. Most carbon black for

coloring concrete contains an admixture to offset this effect on air. Before a coloring

admixture is used on a project, it should be tested for color fastness in sunlight and

autoclaving, chemical stability in cement, and effects on concrete properties. Calcium

chloride should not be used with pigments to avoid color distortions.

Damp-proofing admixtures are sometimes used to reduce the transmission of moisture

through concrete that is in contact with water or damp earth. The passage of water through

concrete can usually be traced to the existence of cracks or areas of incomplete consolidation.

Sound, dense concrete made with a water-cement ratio of less than 0.50 by mass will be

watertight if it is properly placed and cured.

Permeability-reducing admixtures reduce the rate at which water under pressure is

transmitted through concrete. One of the best methods of decreasing permeability in concrete

is to increase the moist-curing period and reduce the water-cement ratio to less than 0.5. Most

admixtures that reduce water-cement ratio consequently reduce permeability.

Pumping aids are added to concrete mixtures to improve pumpability. Pumping aids cannot

cure all unpumpable concrete problems; they are best used to make marginally pumpable

concrete more pumpable. These admixtures increase viscosity or cohesion in concrete to

reduce dewatering of the paste while under pressure from the pump.

Bonding admixtures are usually water emulsions of organic materials including rubber,

polyvinyl chloride, polyvinyl acetate, acrylics, styrene butadiene copolymers, and other

polymers. They are added to portland cement mixtures to increase the bond strength between

old and new concrete. They are added in proportions equivalent to 5% to 20% by mass of the

cementing materials; the actual quantity depending on job conditions and type of admixture

used. The ultimate result obtained with a bonding admixture will be only as good as the

surface to which the concrete is applied. The surface must be dry, clean, sound, free of dirt,

dust, paint, and grease, and at the proper temperature.

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Concrete Admixtures by Classification

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Concrete Admixtures by Classification (Continued)

* Superplasticizers are also referred to as high-range water reducers or plasticizers. These admixtures often meet both ASTM C 494 and ASTM C 1017 specifications.

1.5 Water for Concrete

Almost any natural water that is drinkable and has no pronounced taste or odor can be used as

mixing water for making concrete. However, some waters that are not fit for drinking may be

suitable for use in concrete. Typically, half the water requirement of concrete is present as

free moisture in the aggregates. In the case of very low workability concrete suitable for

rolling, it is important to ensure that the aggregates do not contain more water than that

needed by the concrete; this may be only 5–7% by mass of the aggregates.

Concern over a high chloride content in mixing water is chiefly due to the possible adverse

effect of chloride ions on the corrosion of reinforcing steel or prestressing strands. Chloride

ions attack the protective oxide film formed on the steel by the highly alkaline (pH greater

than12.5) chemical environment present in concrete. The acid soluble chloride ion level at

which steel reinforcement corrosion begins in concrete is about 0.2% to 0.4% by mass of

cement (0.15% to 0.3% water soluble). Of the total chloride-ion content in concrete, only

about 50% to 85% is water soluble; the rest becomes chemically combined in cement

reactions.

Water containing algae is unsuited for making concrete because the algae can cause an

excessive reduction in strength. Algae in water lead to lower strengths either by influencing

cement hydration or by causing a large amount of air to be entrained in the concrete. Algae

may also be present on aggregates, in which case the bond between the aggregate and cement

paste is reduced. A maximum algae content of 1000 ppm is recommended.

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2. PROPERTIES OF FRESH CONCRETE

Fresh concrete is concrete in a mouldable condition, able to be placed, compacted and

finished by the chosen means. The properties of fresh concrete which are of interest to the

ready-mixed concrete producers and their clients are uniformity, stability, workability,

pumpability, water demand and water/cement ratio, rate of change of workability, and

finishability.

Freshly mixed concrete should be plastic or semi-fluid and generally capable of being molded

by hand. A very wet concrete mixture can be molded in the sense that it can be cast in a mold,

but this is not within the definition of “plastic”— that which is pliable and capable of being

molded or shaped like a lump of modeling clay. In a plastic concrete mixture all grains of

sand and pieces of gravel or stone are encased and held in suspension. The ingredients are not

apt to segregate during transport; and when the concrete hardens, it becomes a homogeneous

mixture of all the components. During placing, concrete of plastic consistency does not

crumble but flows sluggishly without segregation.

In construction practice, thin concrete members and heavily reinforced concrete members

require workable, but never soupy, mixes for ease of placement. A plastic mixture is required

for strength and for maintaining homogeneity during handling and placement. While a plastic

mixture is suitable for most concrete work, plasticizing admixtures may be used to make

concrete more flowable in thin or heavily reinforced concrete members.

2.1 Workability

The term workability covers a wide range of properties;

(1)Mobility (ability for concrete to move around reinforcement and into restricted spaces)

(2)Compactibility (by hand or vibration)

(3)Finishability (of free or moulded surfaces)

(4)Pumpability (for pumped concrete).

The ease of placing, consolidating, and finishing freshly mixed concrete and the degree to

which it resists segregation is called workability. Concrete should be workable but the

ingredients should not separate during transport and handling. The degree of workability

required for proper placement of concrete is controlled by the placement method, type of

consolidation, and type of concrete. Different types of placements require different

levels of workability.

Factors that influence the workability of concrete are:

(1) the method and duration of transportation;

(2) quantity and characteristics of cementitious materials;

(3) concrete consistency (slump);

(4) grading, shape, and surface texture of fine and coarse aggregates;

(5) entrained air;

(6) water content;

(7) concrete and ambient air temperatures; and

(8) admixtures.

A uniform distribution of aggregate particles and the presence of entrained air significantly

help control segregation and improve workability. Properties related to workability include

consistency, segregation, mobility, pumpability, bleeding, and finishability. Consistency is

considered a close indication of workability. Slump is used as a measure of the consistency or

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wetness of concrete. A low-slump concrete has a stiff consistency. If the consistency is too

dry and harsh, the concrete will be difficult to place and compact and larger aggregate

particles may separate from the mix. However, it should not be assumed that a wetter, more

fluid mix is necessarily more workable. If the mix is too wet, segregation and honeycombing

can occur. The consistency should be the driest practicable for placement using the available

consolidation equipment.

Selection of slump for different construction situations

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Selection of slump of flow values for cast-in-place piling.

There are two reasons for the contractor specifying a workability method together with an

appropriate target value, and invoking corresponding compliance limits. They are: (i) To

control workability within a selected range appropriate to the construction conditions and

equipment to be used for transporting, placing and compacting concrete (ii) To control

indirectly the water content and thus water/cement ratio of the concrete. The commonest

method of test for both purposes is that for slump. The commonest specified slumps are

75mm (Normal Mix) and 100 mm (Pump Mix), both with tolerances of ±25 mm.

Compliance requirements of BS 5328 for slump

Generally, concrete will normally be pumpable if the following apply:

(i) Cement content is over 300 kg/m3 or the weight of cement plus fine aggregate below 300

micron is over 1.75×free water content.

(ii) Slump is over 50 mm, preferably target 75 mm

(iii) Nominal maximum aggregate size not exceeding one-fifth of the pipeline diameter, i.e.

20 mm for a 100 mm pipeline

(iv) Fine aggregate content is up to 5% higher than for a normal well designed mix.

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2.2 Water / Cement Ratio

The water-cementitious material ratio is simply the mass of water divided by the mass of

cementitious material (portland cement, blended cement, fly ash, slag, silica fume, and

natural pozzolans). The water-cementitious material ratio selected for mix design must be the

lowest value required to meet anticipated exposure conditions. When durability does not

control, the water-cementitious materials ratio should be selected on the basis of concrete

compressive strength. In such cases the water-cementitious materials ratio and mixture

proportions for the required strength should be based on adequate field data or trial mixtures

made with actual job materials to determine the relationship between the ratio and strength.

ACI strength versus w/c ratio

3. MIX DESIGN

The process of determining required and specifiable characteristics of a concrete mixture is

called mix design. Characteristics can include: (1) fresh concrete properties; (2) required

mechanical properties of hardened concrete such as strength and durability requirements; and

(3) the inclusion, exclusion, or limits on specific ingredients. Mix design leads to the

development of a concrete specification. Mixture proportioning refers to the process of

determining the quantities of concrete ingredients, using local materials, to achieve the

specified characteristics of the concrete. A properly proportioned concrete mix should

possess these qualities:

1. Acceptable workability of the freshly mixed concrete

2. Durability, strength, and uniform appearance of the hardened concrete

3. Economy

Understanding the basic principles of mixture design is as important as the actual calculations

used to establish mix proportions. Only with proper selection of materials and mixture

characteristics can the above qualities be obtained in concrete construction.

The mix design shall ensure that no excessive bleeding occurs. The mix must have the ability

to achieve the specified surface finish. Unless otherwise specified, design mix shall comply

with the specifications below.

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Specification for designed concrete mix Grade 55 50 45 40 35 30 25 20 15 10

Characteristic Strength at 28 days (N/mm2)

55 50 45 40 35 30 25 20 15 10

Minimum cement content in kg/m3

475 425 375 350 350 350 300 270 205 175

Maximum cement content in kg/m3

550 550 550 550 550 550 550 550 550 550

Maximum water/cement ratio

0.4 0.45 0.45 0.5 0.5 0.5 0.55 0.6 0.7 0.8

BS 5328 Limits of chloride content of concrete

3.1 Characteristic Strength

Characteristic strength means that value of the cube compressive strength of concrete fcu

which is normally determined base on 28 days.

The characteristic strength (Mean Strength - 1.64 x Standard Deviation) can be accurately

established by a run of 30 tests over a period, the cement increase to raise the mean strength

to the acceptable level can also be determined with reasonable accuracy.

3.2 Target Mean Strength

In selecting the cement content of a designed mix, a target mean strength has to be selected

that exceeds the specified characteristic strength by a design margin which includes a

statistical constant and the depot standard deviation, in the following way:

Target Mean Strength = Specified Strength + Design Margin

where design margin=depot standard deviation × k

The principle of characteristic strength, adopted by BS 8110, requires k to be 1.64.

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3.3 Trial Mix

Trial mixes are required to verify the mix design for each concrete grade on the project.

When trial mixes are required by consultant, the number of laboratory and/or site mixed

batches shall be specified or agreed with the consultant and supplier using the materials

which are proposed for the work. The workability of each of the trial batches shall be the

same as the proposed supply within the tolerances given below;

BS 5328-4:1990. Workability

The total air content determined from individual samples of concrete taken at the point of

delivery into the construction and representative of any given batch of concrete shall be the

specified total value ± 2.0 %. The mean total air content from any four consecutive

determinations from separate batches shall be the specified value ±1.5 %.

Three cubes shall be made from each batch for test at 28 days. The average compressive

strength of the three cubes tested at 28 days shall exceed the specified characteristic strength

by at least 10 N/mm2.

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3.4 Passing Criteria and Compliance

Unless otherwise specified, cube test result shall comply with the requirement in the below

table.

BS 5328-4 :1990 Characteristic compressive strength compliance requirements

3.5 Prequalification of the Concrete Supplier

Bidders / concrete suppliers should be prequalified prior to the award of a supply contract for

concrete. Where the specified strength has been widely produced for previous projects, a

review of available test data may adequately measure performance. When a strength higher

than previously supplied is specified, or where there is limited experience in the supply of

that strength concrete, a more detailed prequalification procedure should be carried out. This

should generally include the production of a trial batch of the proposed mix proportions.

The trial concrete should be cast into monoliths representative of typical structural sizes on

the project. Fresh concrete should be tested for slump, air content, and temperature. Hardened

concrete should be tested to determine compressive strength and modulus of elasticity based

on standard-cured specimens and on cores drilled from the monolith. Strengths of cores and

standard-cured specimens tested at the same age should be correlated. In massive elements,

core strength may vary with distance from the surface due to different temperature histories.

Therefore, relationships should be established for a specific core depth. If cores need to be

removed during construction, the correlation allows interpretation of core strength results.

The monolith also should be instrumented to determine the maximum internal temperature

and the temperature gradients developed throughout the cross section.

Laboratory and field tests should be performed to evaluate the effects of environmental

conditions on the properties of freshly-mixed and hardened concrete. In particular, slump loss

between the batch plant and the project site should be evaluated to assure adequate slump at

the time of placing. During periods of high temperature or low humidity, it may be necessary

to adjust the concrete mix using retarding or high-range water-reducing admixtures at varied

dosage rates and in different addition sequences.

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In-place strength - If in-place testing is to be used, it is recommended that a correlation with

standard-cured specimens be made at the prequalification trials.

3.6 Quality Control

Quality assurance (QA) - actions taken by the Qualified Person to provide assurance that

what is being done and what is being provided are in accordance with the applicable

standards of good practice for the work. Quality control (QC) - actions taken by a producer or

contractor to provide control over what is being done and what is being provided so that the

applicable standards of good practice for the work are followed.

Comprehensive and timely QA/QC permit confidence in the use of advanced design

procedures, frequently expedites construction, and improves quality in the finished product.

Conversely, the results of poor QA/QC can be costly for all parties involved. QA/QC

personnel must be experienced in their respective duties, including the batching, placing,

curing, and testing of high-strength concrete. QA/QC personnel should be able to provide

evidence of such training or experience, or both.

QA/QC personnel should concentrate their efforts at the concrete plant to ensure consistently

acceptable batching is achieved. For example, QA/QC personnel should ensure that the

facilities, moisture meters, scales, and mixers (central or truck, or both) meet the project

specification requirements and that materials and procedures are as established in the

planning stages. QA/QC personnel should be aware of the importance of batching high-

strength concrete, such as using proper sequencing of ingredients, especially when pozzolans

or ground slag are used. Scales, flow meters, and dispensers should be checked monthly for

accuracy, and should be calibrated every six months. Moisture meters should be checked

daily. These checks and calibrations should be documented. Plants that produce high-strength

concrete should have printed records for all materials batched. Entries showing deviations

from accepted mix proportions are provided with some plant systems.

The QC or QA inspector should be present at the batching console during batching and

should verify that the accepted types and amounts of materials are batched. Batch weights

should fall within the allowable tolerances set forth by project specifications. Moisture

content tests should be repeated after rain and the other tests should be repeated after

deliveries of new batches of materials. High-strength concrete may rely on a combination of

chemical and mineral admixtures to enhance strength development. Certain combinations of

admixtures and portland cements exhibit different strength development curves. Therefore, it

is important for QA/QC personnel to watch for deviations in the type or brands of mix

ingredients from those submitted and accepted.

Substitutions should not be allowed without the prior understanding of all parties. Reference

samples of cementitious materials should be taken at least once per day or per shipment in

case tests are needed later to investigate low strengths or other deficiencies. Sources of

additional mix water such as “wash water” or any “left-over” concrete remaining in the truck

drum prior to batching should be identified. These should be emptied from the truck prior to

batching.

The QA/QC personnel should recognize that prolonged mixing will cause slump loss and

result in lower workability. Adequate job control must be established to prevent delays.

When practical, withholding some of the high-range water-reducing admixture until the truck

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arrives at the job site or site-addition of high-range water-reducing admixtures may be

desirable. Newer high-range water-reducing admixtures with extended slump retention

characteristics may preclude the need for job-site additions of admixture to recover slump.

Truck mixers should rotate at proper agitation speed while waiting for discharge at the site.

Failure to do so may lead to severe slump loss.

When materials are added at the site, proper mixing is required to avoid non-uniformity and

segregation. QA/QC personnel should pay close attention to site mixing and should verify

that the mix is uniform. The concrete truck driver should provide a delivery ticket and every

ticket should be reviewed by the inspector prior to discharge of concrete. Chemical

admixtures can be used to increase workability time. High-range water-reducing admixtures

often are used to increase the fluidity of concrete for a definite time period. QA/ QC

personnel should be aware of that time frame and should know whether re-dosing with

additional admixture is permitted. If the batch is re-dosed, the amount of admixture added to

the truck should be recorded. Addition of water at the job site should be permitted only if this

was agreed upon at the preconstruction meeting and provided that the maximum specified

water-cementitious materials ratio is not exceeded.

QA/QC personnel should verify that forms, reinforcing steel, and embedded items are ready

and that the placing equipment and vibration equipment (including standby equipment) are in

working order prior to the contractor placing concrete. All concrete should be compacted

quickly and thoroughly. The potential strength and durability of high-strength concrete will

be fully developed only if it is properly cured for an adequate period prior to being placed in

service or being subjected to construction loading. Therefore, appropriate curing methods for

various structural elements should be selected in advance. QA/QC personnel should verify

that the accepted methods are properly employed in the work.

High-strength concretes usually do not exhibit much bleeding, and without protection from

loss of surface moisture, plastic shrinkage cracks have a tendency to form on exposed

surfaces. Curing should begin immediately after finishing, and in some cases other protective

measures should be used during the finishing process. Curing methods include fog misting,

applying an evaporation retarder, covering with polyethylene sheeting, or applying a curing

compound.

The inspector should monitor and record ambient temperatures and temperatures at the

surface and center of large concrete components so that the design/construction team can

effectively make any adjustments, such as changes in mix proportions or the use of insulating

forms, during the course of the project. Concrete delivered at temperatures exceeding

specification limits should be rejected, unless alternative procedures have been agreed upon

at the preconstruction meeting. The inspector should ensure that curing procedures are

according to project specifications, particularly those at early ages to control the formation of

plastic shrinkage cracks.

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4. ORDER AND DELIVERY OF CONCRETE

The project team members should establish and maintain procedures to ensure that purchased

products conform to specified requirements. The selection of a supplier should include an

evaluation of the supplier’s capability to process materials in accordance with the

requirements of contract documents and to deliver the materials at a rate consistent with the

project schedule.

4.1 Approval on the Concrete Supplier

Qualified suppliers can be selected based on their successful preconstruction trials. After the

start of construction, further trials are desirable to confirm the field performance of the

submitted and accepted mixes. Further testing may also be required on full-scale mock-ups of

structural sub-assemblages to determine the potential for cracking problems, such as at the

interface between structural elements of different thickness.

Mix design and test reports from the selected concrete supplier shall be submitted to

consultant for approval before proceeding trial mix (if necessary) or use at site.

4.2 Ordering Concrete

Upon the approval on the supplier and mix design, ordering / booking concrete shall be made

one day advance for quantity less than 100 m3 and 3 days advance for more than 100m3

before the casting for large pour in order to ensure the concrete supplier arranged the required

material stock and concrete trucks. Following information shall be stated on the concrete

order form.

(1) Name of Project

(2) Type & Grade of Concrete

(3) Specified Slump

(4) Estimated Quantity

(5) Location & Element to be cast

(6) Concrete quantity per truck to be delivered

(7) Interval of the trucks

(8) Size and numbers of cube to be taken

The delivery ticket needs to include the following as per BS 5328:

(1)Name or number of the ready-mixed concrete plant

(2)Serial number of the ticket

(3)Date

(4)Truck number

(5)Name of the purchaser

(6)Name and location of the site

(7)Grade and full description of the concrete, including any additional items that have been

specified (e.g. minimum cement content and maximum W/C ratio)

(8)Specified workability

(9)Type of cement and limiting proportions of ggbs or pfa, if specified

(10)Nominal maximum size of aggregate

(11)Type or name of admixture, if included

(12)Quantity of concrete in cubic meters

(13)Time of loading

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(14)Space for any additional items that have been specified

(15)Arrival time of the truck

(16)Departure time of the truck

(17)Time of completing the discharge

(18)Water added to meet the specified workability

(19)Extra water added at the request of the purchaser together with the authorizing signature.

(20)Signature of the purchaser or his representative for receipt of the concrete

(21)Safety warning & Conditions of sale.

Following items shall be recorded by site person in-charge on the each concrete delivery

order ticket (DO).

(1) Time of truck arrival to site

(2) Time of truck departure from site

(3) Slump test result of the concrete (sample to be taken immediately after arrival to site

before casting)

(4) Cube No. (if the cube sample is taken)

(5) Location and element has been cast

(6) Signature of consultant’s representative who witnessed

Sample Concrete Booking Sheet

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4.3 Transporting, Handling and Placing Concrete

Transporting

Concrete can be transported by a variety of methods and equipment, such as pipeline, hose,

conveyor belts, truck mixers, open-top truck bodies with and without agitators, or buckets

hauled by truck or railroad car. The method of transportation should efficiently deliver the

concrete to the point of placement without losing mortar or significantly altering the

concrete’s desired properties associated with w/cm, slump, air content, and homogeneity.

Various conditions should be considered when selecting a method of transportation, such as:

mixture ingredients and proportions; type and accessibility of placement; required delivery

capacity; location of batch plant; and weather conditions. These conditions can dictate the

type of transportation best suited for economically obtaining quality in-place concrete.

Vehicles delivering concrete are that it can:

(i) Safely convey a whole range of concrete mixes

(ii) Agitate the mix in the time between mixing and discharging

(iii) Allow the concrete workability to be adjusted, by adding water if necessary

(iv) Distribute the concrete by driving on and around the site and by means of an extended

chute control the height and radius of discharge.

Non-agitating vehicles can be used to deliver mixed concrete that is not prone

to segregation.

Revolving drum—In this method, the truck mixer serves as an agitating transportation unit.

The drum is rotated at charging speed during loading and is reduced to agitating speed or

stopped after loading is complete. The elapsed time before discharging the concrete can be

the same as for truck mixing and the volume carried can be increased to 80% of the drum

capacity.

Truck body with and without an agitator—Units used in this form of transportation usually

consist of an open-top body mounted on a truck, although bottom-dump trucks have been

used successfully. The metal body should have smooth, streamlined contact surfaces and is

usually designed for discharge of the concrete at the rear when the body is tilted. A discharge

gate and vibrators mounted on the body should be provided at the point of discharge for

control of flow. An agitator, if the truck body is equipped with one, aids in the discharge and

ribbon-blends the concrete as it is unloaded. Water should never be added to concrete in the

truck body because no mixing is performed by the agitator.

Use of protective covers for truck bodies during periods of inclement weather, proper

cleaning of all contact surfaces, and smooth haul roads contribute significantly to the quality

and operational efficiency of this form of transportation. The maximum delivery time

specified is usually 30 to 45 min, although weather conditions can require shorter or permit

longer times. Trucks that have to operate on muddy haul roads should not be allowed to

discharge directly on the grade or drive through the discharged pile of concrete.

Handling and Placing Concrete

Placement of concrete is accomplished with buckets, hoppers, manual or motor-propelled

buggies, chutes and drop pipes, conveyor belts, pumps, tremies, and paving equipment. A

basic requirement in all concrete handling is that both quality and uniformity of the concrete,

in terms of w/c, slump, air content, and homogeneity, have to be preserved. The selection of

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handling equipment should be based on its capability to efficiently handle concrete of

proportions most advantageous for being readily consolidated in place with vibrators.

Sufficient placement capacity should be provided so that the concrete can be kept plastic and

free of cold joints while it is being placed. All placement equipment should be clean and in

proper repair. The placement equipment should be arranged to deliver the concrete to its final

position without significant segregation. The equipment should be adequately and properly

arranged so that placing can proceed without undue delays and manpower should be

sufficient to ensure the proper placing, consolidating, and finishing of the concrete. If the

concrete is to be placed at night, the lighting system should be sufficient to illuminate the

inside of the forms and to provide a safe work area.

Arrange equipment so that the concrete has an unrestricted vertical drop to the point of

placement or into the container receiving it. The stream of concrete should not be separated

by falling freely over rods, spacers, reinforcement, or other embedded materials. If forms are

sufficiently open and clear so that the concrete is not disturbed in a vertical fall into place,

direct discharge without the use of hoppers, trunks or chutes is favorable. Concrete should be

deposited at or near its final position because it tends to segregate when it has to be flowed

laterally into place.

If a project involves monolithic placement of a deep beam, wall, or column with a slab or

soffit above, delay placing the slab or soffit concrete until the deep concrete settles. The time

allotted for this settling depends on the temperature and setting characteristics of the concrete

placed, but is usually about 1 h. Concreting should begin again soon enough to integrate the

new layer thoroughly with the old by vibration.

The use of properly designed bottom-dump buckets permits placement of concrete at the

lowest practical slump consistent with consolidation by vibration. The bucket should be self-

cleaning upon discharge, and concrete flow should start when the discharge gate is opened.

Discharge gates should have a clear opening equal to at least five times the maximum

aggregate size being used. Side slopes should be at least 60 degrees from the horizontal.

Internal vibration is the most effective method of consolidating plastic concrete for most

applications. The effectiveness of an internal vibrator depends mainly on the head diameter,

frequency, and amplitude of the vibrators. Vibrators should not be used to move concrete

laterally. They should be inserted and withdrawn vertically, so that they quickly penetrate the

layer and are withdrawn slowly to remove entrapped air. Vibrate at close intervals using a

systematic pattern to ensure that all concrete is adequately consolidated.

As long as a running vibrator will sink into the concrete by means of its own weight, it is not

too late for the concrete to benefit from re-vibration, which improves compressive and bond

strengths. There is no evidence of detrimental effects either to embedded reinforcement or

concrete in partially cured lifts that are re-vibrated by consolidation efforts on fresh concrete

above. In difficult and obstructed placements, supplemental form vibration can be used. In

these circumstances, avoid excessive operation of the vibrators, which can cause the paste to

weaken at the formed surface.

On vertical surfaces where air-void holes need to be reduced, use additional vibration. Extra

vibration, spading, or mechanical manipulation of concrete, however, are not always reliable

methods for removing air-void holes from surfaces molded under sloping forms. Conduct

trial placements to determine what works best with a particular concrete mixture. The use of

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experienced and competent vibrator operators working with well-maintained vibrators and a

sufficient supply of standby units is essential to successful consolidation of fresh concrete.

The equipment and method used for placing mass concrete should minimize separation of

coarse aggregate from the concrete. Although scattered pieces of coarse aggregate are not

objectionable, clusters and pockets of coarse aggregate are and should be scattered before

placing concrete over them. Segregated aggregate will not be eliminated by subsequent

placing and consolidation operations. Concrete should be placed in horizontal layers not

exceeding (600 mm) in depth and inclined layers and cold joints should be avoided. For

monolithic construction, each concrete layer should be placed while the underlying layer is

still responsive to vibration, and layers should be sufficiently shallow to permit the two layers

to be integrated by proper vibration.

Correct and incorrect methods of consolidation

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4.4 Curing Methods and Control

Standard Specification covers requirements for curing the cast-in-place concrete elements

described in the Contract Documents. Specialty concrete and special construction techniques,

or other concrete elements that require the use of curing procedures not discussed in this

specification are not covered by this Specification.

Immediately after placement, continuously keep concrete in moist condition, maintain

specified concrete temperatures, and protect concrete from mechanical injury for the duration

of the initial and final curing periods. Protect the concrete from damaging mechanical

disturbances during the curing period. Protect finished surfaces from damage by construction

equipment, materials or methods, and from damage caused by application of curing

procedures, or by running water. Apply one of the curing procedures listed below.

(1) Moisture Retention : This methods is for curing concrete using plastic sheets, plastic

sheets bonded to water-absorbent fabric, or reinforced paper. Place material on the concrete

surface as soon as possible without marring the surface. Cover all exposed concrete surfaces

and beyond the edge of the concrete surface. Securely tape sheets together or lap them.

Maintain the integrity of the material and the ability to contain the water on the concrete

surface throughout the curing period. Verify that the concrete is continuously wet under the

sheets; otherwise, add water through soaker hoses under the sheets.

(2) Moisture Retention - Liquid Membrane Forming Curing Compounds : This methods

is used for curing concrete using liquid membrane-forming curing compounds. Submit

description of curing procedure to be used, and data demonstrating that proposed materials

meet specification requirements, to include

a)Manufacturer’s technical data including rate of moisture loss at stated application rate and

material safety data sheets (MSDS).

b) Manufacturer’s certification verifying product compliance to volatile organic compound

(VOC) limits.

Apply liquid membrane-forming compounds uniformly and at the rate recommended by the

manufacturer, but at a rate as tested using ASTM C 309. Apply liquid membrane-forming

compounds immediately after final finishing and as soon as the free water has disappeared,

no water sheen is visible, and bleeding has essentially ceased. Keep the concrete surface

moist without standing water. Protect the membrane from damage for the duration of the

curing period. Provide adequate ventilation during the formation of the membrane. Place

curing compounds with an electrical or gasoline-powered sprayer. The use of a hand pump

sprayer, brush, or paint roller for areas less than 200 m2 is permissible.

(3) Addition of Water - Ponding : This methods is used for curing concrete by addition of

water to the concrete surface by ponding or immersion. The temperature of the curing water

shall not be lower than 10 °C cooler than the surface temperature of the concrete at the time

the water and concrete come in contact. Water shall be potable, and shall be free of materials

that have the potential to stain concrete. Execute ponding by building a ridge of earth, sand,

or other material around the concrete and flooding the surface with water. Start ponding on

the concrete surface as soon as possible without marring the surface. Replace water lost due

to evaporation or leakage at a rate sufficient to maintain the pond. Do not allow alternate

wetting and drying of the concrete surfaces. Keep concrete surfaces continuously wet.

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(4) Addition of Water – Fog Spray : This methods is used for curing concrete by addition

of water to concrete surfaces by fog spray. The temperature of the curing water shall not be

lower than 10 °C cooler than the surface temperature of the concrete at the time the water and

concrete come in contact. Water shall be, and shall be free of materials that have the potential

to stain concrete. Equipment shall produce fog spray from an atomizing nozzle with sufficient

velocity to cover the concrete surface. Higher operating pressures and flow rates may be

necessary to deliver the fog spray over long distances. Lower pressure devices are acceptable

for final curing. Direct atomized water spray above the concrete surface to allow the fog to

drift down to the concrete surface. Direct discharge of the atomized water spray onto the

surface of the concrete is unacceptable. Generate sufficient velocity of the atomized water

droplets to reach the extreme edges of the concrete surface. Continue fogging as necessary to

maintain the reflective appearance of the damp concrete. Do not allow the surface to dry, or

to undergo cycles of drying and wetting. Keep the concrete surface damp, but do not

accumulate water until after final set has occurred. Keep the concrete surface continuously

wet. Do not allow alternate wetting and drying of concrete surfaces.

(5) Addition of Water – Sprinkling : This methods is used for concrete curing by sprinkling.

The temperature of the curing water shall not be lower than 10 °C cooler than the surface

temperature of the concrete at the time the water and concrete come in contact. Water shall be

potable, and shall be free of materials that have the potential to stain concrete. Equipment

shall consist of soaker hoses, lawn sprinklers, or a combination thereof. Perform sprinkling

for final curing by using either soaker hoses or lawn sprinklers. Exercise care so the surface

of the concrete is not eroded. Use soaker hoses for initial curing of concrete walls and

columns after time of initial setting and prior to the forms being removed. Place hoses at the

top of walls and columns so water will enter between concrete and form work. Keep the

concrete surfaces continuously wet.

(6) Addition of Water – Water Absorbent Materials : This methods is used for curing

concrete by addition of water to the concrete surface by absorbent materials. The temperature

of the curing water shall not be lower than 10 °C cooler than the surface temperature of the

concrete at the time the water and concrete come in contact. Water shall be potable, and shall

be free of materials that have the potential to stain concrete. Use sand, hay, straw, burlap,

cotton mats, rugs, or earth free of materials that could cause staining of the concrete surface.

Earth materials shall be free of organic matter and particles larger than 25 mm. Uniformly

distribute absorbent materials across the concrete surface. Apply water to the materials so that

the materials are not displaced. Keep the concrete surfaces continuously wet. Do not allow

concrete surfaces to dry or alternate with wetting and drying cycles. Do not place the

materials during the initial curing period. Do not stain the concrete.

4.5 Sampling and Testing

This practice covers procedures for obtaining representative samples of fresh concrete as

delivered to the project site on which tests are to be performed to determine compliance with

quality requirements of the specifications under which the concrete is furnished. This practice

is intended to provide standard requirements and procedures for sampling freshly mixed

concrete from different containers used in the production or transportation of concrete. The

detailed requirements as to materials, mixtures, air content, temperature, number of

specimens, slump, interpretation of results, and precision and bias are in specific test methods.

The elapsed time shall not exceed 15 min. Between obtaining the first and final portions of

the composite sample.

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Transport the individual samples to the place where fresh concrete tests are to be performed

or where test specimens are to be molded. They shall be combined and remixed with a shovel

the minimum amount necessary to ensure uniformity and compliance with the maximum time

limits. Start tests for slump, temperature, and air content within 5 min after obtaining the final

portion of the composite sample. Complete these tests expeditiously. Start molding

specimens for strength tests within 15 min after fabricating the composite sample.

Expeditiously obtain and use the sample and protect the sample from the sun, wind, and other

sources of rapid evaporation, and from contamination.

Make the samples to be used for strength tests a minimum of (0.3 m3). Smaller samples are

not prohibited for routine air content, temperature, and slump tests. The size of the samples

shall be dictated by the maximum aggregate size. Sampling should normally be performed as

the concrete is delivered from the mixer to the conveying vehicle used to transport the

concrete to the forms; however, specifications may require other points of sampling, such as

the discharge of a concrete pump. Sample the concrete by collecting two or more portions

taken at regularly spaced intervals during discharge of the middle portion of the batch. No

samples should be taken before 10 % or after 90 % of the batch has been discharged. Due to

the difficulty of determining the actual quantity of concrete discharged, the intent is to

provide samples that are representative of widely separated portions, but not the beginning

and the end of the load.

Consistency of the concrete shall be checked for every truck of wet concrete and confirm the

slump result is within the specified limit before casting. Slump sampling test shall be in

accordance with BS 1881 and general procedure shall be as follows.

(1) Ensure the internal surface of the mould is clean and damp.

(2) Place the mould on a smooth and horizontal surface. Hold the mould firmly against

the surface.

(3) Fill the mould in 3 layers, each approximately (50 mm) 1/3 the height of the mould.

(4) Tamp each layer with 25 strokes by tamping rod. Ensure that the rod does not forcibly

strike the surface below when tamping the first layer and just passes through the

second and top layers into the layers immediately below.

(5) Heap the concrete above the mould before the top layer is tamped.

(6) After the top layer has been tamped, strike off the concrete level with the top of the

mould by using steel trowel.

(7) With the mould still held down, clean from the surface below any concrete which

have fallen onto it.

(8) Remove the mould from the concrete by raising it vertically, slowly and carefully in 5

to 10 seconds.

(9) Immediately after the mould is removed, measure the slump to the nearest 5mm by

using the rule to determine the difference between the height of the mould and of the

highest point of the specimen being test.

Numbers of cube sample shall be taken as per contract specification depend on the total

concrete volume of each casting. Minimum 6 no’s of 150mm x 150mm x 150mm cube shall

be taken for each casting unless otherwise specified. Two no’s of cube shall be tested at the

age of 7 days and the remaining 4 no’s shall be tested at the age of 28 days. Cube sampling

method shall be as follows.

(1) Place the mould on a level ground

(2) Fill the mould with concrete in layers of approximately 50mm deep.

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(3) Compact each layer by tamping the compacting bar over the cross section of the

mould.

(4) Concrete shall be subjected to a minimum of 35 strokes per layer for 150mm cubes

and 25 strokes for 100 cubes.

(5) After the top layer has been compacted, smooth it level with the top of the mould

using the steel trowel.

5. EVALUATION OF HARDENED CONCRETE

For any particular set of materials and conditions of curing, the quality of hardened concrete

is strongly influenced by the amount of water used in relation to the amount of cement.

Unnecessarily high water contents dilute the cement paste (the glue of concrete). Concrete

does not harden or cure by drying. Concrete (or more precisely, the cement in it) needs

moisture to hydrate and harden. When concrete dries out, it ceases to gain strength; the fact

that it is dry is no indication that it has undergone sufficient hydration to achieve the desired

physical properties.

5.1 Strength Evaluation

Standard acceptance of the cube test result shall be according to the requirement of BS 5328

unless otherwise specified in the contract specification. The average strength of the two cubes

tested at 7 days shall be used as an indication only. If the average strength is less than 75% of

the characteristic strength, shall seek consultant engineer’s instruction and may need to

modify the mix design to increase the mean strength of subsequent concrete. If the average

strength is less than 67% of the characteristic strength, no more concrete shall be placed on

the suspected concrete until the 28 day strength is found to comply with BS 5328 requirement

or unless the consultant engineer specifically approves.

Testing failing to meet these requirements shall require that the hardened concrete be tested

by NDT (Non-destructive Testing) or core sample testing. The core tests shall be deemed to

fail if the in-situ strength is found to be less than 85% of the specified cube strength.

High-strength concretes may continue to gain significant strength after the acceptance test

age, especially if silica fume or fly ash or ground granulated blast-furnace slag are used.

During the evaluations to establish mix proportions, a strength development curve should be

established indicating potential strength over time. However, if questions arise concerning the

load-carrying capacity of a structure, this may be investigated by analysis using core test

results or by load testing. In cases where load testing a structure is not practical, analytical

investigations using the strength results from extracted cores, or in-place tests are more

appropriate. Tests to evaluate the durability of the concrete should be performed separately

on cores other than those used for strength tests.

5.2 Non-destructive Test

Nondestructive tests (NDT) can be used to evaluate the relative strength and other properties

of hardened concrete. The most widely used are the rebound hammer, Ultrasonic Pulse

Velocity(UPV) test, penetration, pullout, and dynamic or vibration tests. Other techniques for

testing the strength and other properties of hardened concrete include X-rays, gamma

radiography, neutron moisture gages, magnetic cover meters, electricity, microwave

absorption, and acoustic emissions. Each method has limitations and caution should be

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exercised against acceptance of nondestructive test results as having a constant correlation to

the traditional compression test; for example, empirical correlations must be developed prior

to use.

The most common NDT methods are rebound hummer, UPV and (Windsor Probe)

penetration test. An NDT program may be undertaken for a variety of purposes regarding the

strength or condition of hardened concrete, including:

• Determination of in-place concrete strength

• Monitoring rate of concrete strength gain

• Location of nonhomogeneity, such as voids or honeycombing in concrete

• Determination of relative strength of comparable members

• Evaluation of concrete cracking and delaminations

• Evaluation of damage from mechanical or chemical forces

• Steel reinforcement location, size, and corrosion activity

• Member dimensions

5.3 Remedial / Repairing of Concrete Structure

The main principles to be applied individually or in combination in cases where it is

necessary to rehabilitate a concrete structure which is subjected to air, is underground or

exposed to water (fresh or salt). Thirty-five methods (M1.1 to M11.3) relate to the 11 main

principles given in Section 5.2. Only methods conforming to the main principles should be

chosen for rehabilitation of damaged concrete structures. Furthermore, any possible

(unintentional) side effect of the actual application should be taken into consideration. These

side effects apply to the structure to be repaired and other (adjacent) concrete structures, as

well as the personnel and the environment.

The principles described in the following are based on the chemical and physical laws that

allow prevention or stabilization of the chemical or physical decay processes in the concrete

or electrochemical corrosion of the surface of the steel reinforcement. As examples of

unintentional consequences of the methods (to be carefully evaluated) the following can be

mentioned:

• Reduction of the moisture of concrete which will normally increase the carbonation rate

of the concrete – other things being equal.

• Surface protection which may encapsulate the water content of the concrete and thus

reduce the adhesion to the surface protection or reduce the frost resistance of the concrete.

• Post-tensioning of the concrete which may introduce local tensile stresses in the concrete.

• Electrochemical methods which may cause hydrogen brittleness in certain types of

reinforcement, alkali reaction in concrete with potential alkali-reactive aggregate, reduced

frost resistance due to encapsulated moisture or, in cases of submerged concrete

structures, corrosion in adjacent structures or containers.

• Limitation of oxygen by surface protection or water saturation which will increase the

possibility of corrosion if the reinforcement in the protected zone is in electrical (i.e.

mechanical) connection with reinforcement in an unprotected zone.

Mutual compatibility of the products and systems for protection, repair and strengthening as

well as compatibility with the substrate of the structure is assumed.

Normally, a rehabilitation task is performed in the following way:

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• First the types of damage should be identified.

• This is obtained through registration, analysis and evaluation of the state of the structure.

• Then the possible principles of rehabilitation should be determined, see Table 6.1.

• Finally, a method should be chosen from among the possible rehabilitation methods, see

Tables 5.2 and 5.3. Technique, economics (and aesthetics) are decisive factors for this

choice. For example, lack of electrical/mechanical connection between reinforcing bars

will be a significant technical defect when using electrochemical methods. This defect

may be remedied, but will put a strain on the economy.

• When the method or methods for rehabilitation have been chosen, the relevant standards

should be selected, see Tables 5.2 and 5.3.

• In conclusion, design and choice of products and systems should be performed and a

work specification for performing the rehabilitation work is made, including inspection,

control, etc.

Table 5.1 Examples of damage and the principles applicable for rehabilitation

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Table 5.2 Overview of principles and methods for rehabilitation of damaged concrete

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Table 5.3 Overview of principles and methods for rehabilitation concrete damaged due

to reinforcement corrosion

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6. APPENDIX

a) Structural Contract Specification of Changi Airport Terminal 1 Upgrading Project

b) Structural General Notes of Changi Airport Terminal 1 Upgrading Project

c) Brochure of HOLCIM

d) Concrete Mix Design from HOLCIM

e) Concrete Mix Design from Pan United

f) Waterproof Concrete Mix Design from HOLCIM & Pan United

g) Precast Concrete Mix Design from Eastern Pretech

h) Admixture Catalogues

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Appendix - a

Structural Contract Specification of Changi Airport

Terminal 1 Upgrading Project

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Appendix - b

Structural General Notes of Changi Airport

Terminal 1 Upgrading Project

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Appendix - c

Brochure of HOLCIM

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Appendix - d

Concrete Mix Design from HOLCIM

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Appendix - e

Concrete Mix Design from Pan United

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Appendix - f

Waterproof Mix Deign from HOLCIM & Pan United

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Appendix - g

Precast Concrete Mix Design from Eastern Pretech

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Appendix – h

Admixture Catalogues

Manual for Concreting

Documents

Transcript of Manual for Concreting