Special concreting methods

By

Prof. B. L. Rajput

Assistant Professor

NICMAR, Pune

Cold weather concreting

• Effects of cold weather on concrete• Cold weather concreting in following conditions

– Low temperature but above 00c

– Low temperature at the time of concreting but below 00c after concreting

– Temperature below 00c at the time of concreting and during hardening period

– Hardened concrete subjected to alternate freezing and thawing

• Precautions to be taken and methods adopted for concreting in sub-zero temperature

Effects of cold weather on concrete

• Delay in setting and hardening

• Freezing of concrete at early age

• Freezing and thawing

Cold weather concreting in following conditions

– Low temperature but above 00c

– Low temperature at the time of concreting but below 00c after concreting

– Temperature below 00c at the time of concreting and during hardening period

– Hardened concrete subjected to alternate freezing and thawing

Recommended pre-hardening period as per IS-7861(Part 2)- 1979

Specified minimum strength at 28 days for ordinary portland cement MPa

Requisite Pre-hardening period

At stated concrete temperature in hours

20oc 10oc 5oc

M 20 35 53 70

M 25 26 39 52

M 30 22 33 44

M 40 16 24 32

Precautions to be taken and methods adopted for cold weather concreting

• Utilization of heat developed by hydration of cement and practical methods of insulation

• Selection of suitable type of cement• Economical heating of materials of concrete• Admixtures of anti freezing materials • Electrical heating of concrete mass• Use of air entraining agents

General precautions • Wind breakers shall be erected to shield mixing

and batching plant, tarpaulin, plastic sheets and other covering materials may be made available at the site and the steam generating plant or other necessary equipments may be installed and checked for correct functioning

• Concrete will be delivered to the point of placement at not less than 50c. It is necessary to place concrete quickly and cover the top of concrete.

• Before placing concrete remove all the ice, snow and frost.

• During periods of freezing or near freezing conditions, water curing is not necessary as the loss of moisture from the concrete by evaporation will be greatly reduced in cold air conditions.

• Forms shall not be released until concrete has achieved a strength of at least twice the stress to which the concrete may be subjected at the time of removal of formwork.

• During cold weather, inspection personnel should keep a record of the date, time, outside air temperature

• Use formwork which made up of other than steel material. • Heated enclosures are commonly used for protecting concrete when

air temperature are near or below freezing. Enclosures may be heated by steam, steam pipes and other types of heaters. Enclosures may be made up of wood, canvas, fiber insulation board, plywood etc.

• In case of cold weather, it is necessary to cast a number of specimens in a standard way to indicate the potential strength properties of the mix. In addition to control test cubes, it is necessary to cast a number specimens, curing conditions of which are maintained in the same way as that of actual structure.

Hot weather concreting

• Definition

• Effects of hot weather on concrete

• Precautions to be taken

Effects of Hot weather on concrete

• Rapid rate of hydration of cement, quick setting and early stiffening.

• Rapid evaporation of water• Greater plastic shrinkage• Less time for finishing• Absorption of water from the concrete by the sub

grade and formwork• Difficulty in continuous and uninterrupted curing• Difficulty in incorporation of air entrainment

Precautions in Hot weather concreting

• Temperature reduction of concrete• Careful selection of materials• Precautions prior to concreting• Transporting placing and finishing• Plastic cracking• Curing and protection of concrete at an

early age.

Temperature reduction of concrete

• Lowering of temperature of aggregates and water are most effective methods.

• Aggregates should be stockpile in shade. Cooling of aggregates can be done by sprinkling of water. Refrigeration is another method. Aggregates can be kept in water tank or cool air can be circulated through storage bins or insulated tunnels.

• Addition of ice flakes will reduce the temperature of concrete.

Careful selection of material

• Don’t use fine cement.• Don’t use cement with

high C3S and C3A content.• Use minimum possible

amount of cement.• Shape, impurities, grading

and specific gravity of aggregates influence cement content.

• Use maximum size for aggregates

• Slump retention admixtures (Retarders) are most suitable for hot weather concreting.

• Retarder plasticizers

Precautions prior to concreting

• Mixers, chutes, belts, hoppers. Chutes, pump, pump lines, walkways and other equipments which are handling concrete should be shaded.

• Formwork, reinforcement steel and sub grade should be fogged with cool water just prior to concrete being placed.

• For slabs on grade it is recommended to moisten the sub grade a day prior to concreting.

• It is recommended to do the concreting either in early morning or in the evening or at night.

Transporting placing and finishing

• Plan the activities so that job can be finished as early as possible.

• Use adequate number of labours and equipments. Use standby equipments also.

• All the precautions are necessary to maintain the good bond between first layer and second layer.

• Floating and finishing should be done promptly after the water sheen disappears or when concrete is hard enough to support weight of human being.

• Concrete on finishing must be covered effectively to prevent loss of moisture from the concrete.

Plastic cracking

• Cracking occurs when rate of evaporation exceeds 0.5 kg/m2/hr

• Sub grade and formwork is required to be moistened

• Aggregates if dry and absorptive must be moistened

• Keep the temperature of concrete as low as possible by cooling aggregates and mixing water or by using ice.

• Install wind breakers

• Erect temporary shed to protect concrete surface.

• Protect concrete by polyethylene sheets when there is delay between placing and finishing.

Curing and protection of concrete at an early age.

• Formwork should be removed as early as possible without damage o concrete.

• Apply water from top to bottom

• Curing water should not be excessively cooler than concrete

• Do continuous moist curing

• Use of curing compound is better.