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By

E.Ram Sai

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Concrete placed under difficult situations special care isto be taken.

We dontwant to stop the work under any circumstances.

Changes in temperature will effect the concrete.

The initial rate of development of strength can be

expected to be faster in hot weather and slower in coldweather.

If proper precautions are not taken the cement may beleached or the aggregates may become segregated.

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Special Circumstances

Hot Weather Concreting Cold Weather Concreting

Underwater Construction

Constructions of Piles

Diaphragm Walls

Underground Construction

Concrete Construction in Marine Environment

Miscellaneous Concreting

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Hot Weather Concreting

concreting done or placing at atmospherictemperature above 40C.

IS 7861(part-1)-1975 gives the recommended

practices that would result in concrete possessing

improved characteristics in the fresh as well ashardened state.

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Effects of Hot Weather on Concrete

Accelerated Setting: High Temperature increases the initial setting

of concrete accelerated.

Rapid Hydration: Higher temperatures result in rapid hydration of

cement, increased evaporation of mixing water, greater demand of

mixing water, and large volume changes resulting in cracks.

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Reduction in Strength:High temperature results in the increase of thequantity of mixing water to maintain the workability with consequent reduction

in strength.

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Increased Tendency to Crack: Either before or after hardening,

plastic shrinkage cracks may form in the partially hardened concrete

due to rapid evaporation of water.

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Affected Curing:

It is difficult to retain moisture for hydration and

maintain reasonably uniform temperature conditions

during the curing period.

Difficulty in control of Air Content in Air-Entrained

Concrete:

This adds to the difficulty of controlling workability.

For a given amount of air-entraining agent, hot

concrete entrains less air than concrete at normal

temperatures.

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Recommended Practices and Precautions

Temperature control of concrete ingredients Aggregates- stored under shade or cooled by water.

Water- used in the form of ice or in near freezing temperatures.

Cement- Temperature restricted to 770 C .

Mix Design

Use low cement content and cements with low heats ofhydration.

Use approved admixtures for reducing the water demand or forretarding the set.

Production and Delivery: Temperature of concrete at the time of placement should be

below 400 C.

The mixing time should be held at minimum, subject to uniformmixing.

Period between mixing and delivery should be kept to a

minimum.

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Placement and Curing: Prior to placing concrete formwork, reinforcements and

sub grade should be kept cool by spraying with cold

water. If possible, concreting may be restricted to

evenings and nights.

Placement and finishing should be speedy.

Fresh concrete, as soon as laid, should be covered by fog

sprays, wet burlap, cotton mats, or other similar means.

Immediately after compacting and finishing, concrete

should be protected from evaporation of moisture.

Continuous curing for a period of not less than 10 days

should be done without fail since volume changes due to

alternate wetting and drying promote development of

surface cracks.

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MOIST CURING

Covering the Concrete

Surface

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Cold Weather Concreting

Any operation of concreting, whether mixing, placing, casting, etc., done

at about 50C atmospheric temperature or below is termed as ColdWeather Concreting.

It is essential to keep the temperature of the concrete above a minimum

value before placing.

Proper care should be taken on concrete in cold climates because it has

to sustain several problems.

IS 7861(part-1I)-1981 gives the recommended practices that would result

in concrete possessing improved characteristics in the fresh as well as

hardened state.

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Effects of cold weather on concrete

Delayed Setting:

When the temperature falls to about 50C or below, the

development of concrete strength is retarded compared with

the strength that is developed at normal temperatures.

Forming of Ice-Lenses:

Severe damage may occur if concrete, which is still in theplastic state, is exposed to low temperature ice lenses are

formed.

Due to ice lenses expansion occurs within the pores of the

mixed and poured concrete.

Stress Due to Temperature Differential:

Large temperature differentials within the concrete members

may promote cracking and have harmful effects on the

durability.

Such differentials are likely to happen in cold weatherconditions at the time of removal of forms.

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Freezing of Concrete at Early Age:When concrete is exposed

to freezing temperatures, there is a risk of concrete suffering

irrevocable loss of strength and other qualities, that is,

permeability may increase and the durability may be impaired.

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Impact on Strength: Concrete produced at lower

temperatures normally develops strength at a slower

rate.

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Recommended Practices and Precautions

Temperature control of concrete ingredients Aggregates- heated by steam or hot blowers.

If coarse aggregates are free of frozen lumps, ice and/or snow, onlysand should be heated up to a temperature of about 400C.

Water - can be used up to boiling point temperature.

Cement - Heat of Hydration should be more.

Mix Design- The Mix design should separately be done for thecold weather conditions and additives if any should be used onlyon results of the extensive study of the trial mixes made on themix design.

Production and Delivery: Making sure that the concrete is delivered to the point of

placing at not less than 100C, followed by placing quickly andinsulating.

If concrete is delivered to the point of placing at not less than

5

0

C, placing quickly and providing continuous heating to theconcrete externally.

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Placement and Curing: Ensuring that the concrete temperature is not less than

50C at the time of placing it.

Making sure that the concrete is not placed against a

frozen sub-grade or against reinforcement or forms

covered with snow or ice.

Covering the top of concrete surface as well as the forms

with insulating material.

During freezing or near freezing conditions, water curing

is not necessary.

Low-pressure, wet-steam curing provides the best means

of both heating the enclosure and curing the concrete.

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Underwater Construction

Situations: Basements, Sewerage and Marine works.

Inspection of concrete during placement under-water is

difficult.

Underwater concrete should have a slump of 100 to 180mm.

The water cement ratio should not exceed 0.6 and may need

to be smaller, depending upon the grade of concrete or thetype of the chemical attack.

For aggregates of 40mm maximum particle size, the cement

shall be at least 350kg/m3of concrete.

For aggregates of 20mm maximum particle size, the cementshall be at least 380-390 kg/m3of concrete.

The void content of the coarse aggregates should be kept as

low as possible. The code assumes a maximum void content

of 55 percent.

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Placing concrete by Tremie

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Placing concrete by Bucket

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Placing concrete by Direct Pumping

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Placing concrete by Constructing form work

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Concrete Repair underwater

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Precautions

Concrete cast under water shall not fall freely through the water.

Otherwise it may be leached and become segregated.

Concrete shall be deposited continuously until it is brought to the

required height.

While depositing, the top surface shall be kept as nearly level as

possible and the formation of seams avoided. De-watering by pumping shall not be made while concrete is

being placed or until 24 hours thereafter, otherwise it may disturb

the concrete and may lead to undesirable results.

No tamping, ramming or compaction of concrete underwater.

The concrete mix should be rich and have 10% - 30% extra

cement

No construction joint is allowed within 600mm below the water

level.

The aggregates should be properly graded to reduce the voids toa minimum.

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Construction of Piles by Dry Installation

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Construction of Piles Franki Process

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Construction of Piles Franki Process

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Construction of Diaphragm Walls

Diaphragm walls can be used in underpasses as abutment walls to

support the superstructure load and transfer them to the soil. Theyare generally used as load-bearing elements and retention systems.

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Underground Construction

Situations : Underground water tank

Trench

Sewerage System laying

Deep Excavation in site

Foundation works

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Types of Cofferdams

Single Sheet

pilling

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Concrete Construction in Marine Environment

The amount of Chlorides and sulphates in the mix should be reduced

to a minimum.

Cement Factor: 400 kg/m3, W/C ratio should be less than 0.45

Portland slag cement may be used but it will be necessary to seekspecialists advice.

Precast members are to be preferred because then it will be possible

to achieve dense concrete and eliminate those with porous or

defective concrete by inspection before installation.

Unreinforced elements should be used if practicable, as reinforcing

steels are susceptible to corrosion caused by chlorides present in

sea water.

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Construction joints are potentially weak and the problems of

durability are accentuated in the zone subject to alternate drying andwetting that is, between upper and lower planes of wave actions.

No construction joints shall be allowed within 600 mm below low

water-level or within 600 mm of the upper and lower planes of wave

action.

In reinforced concrete structures, care shall be taken to protect the

reinforcement from exposure to saline atmosphere during storage

and fabrication.

Aggregates should be non reactive and sound.

IS 4082-1977 recommends a coat of cement water over the

reinforcing steels stored in coastal areas.

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Miscellaneous Concreting

Bubble Deck Slab

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Self Compaction Concrete

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