Details about Concreting under Special Conditions
Concreting under Special Conditions
There are three type of concreting under special conditions, Namely
Work in Extreme Weather Conditions
Under Water Concreting
Concrete in Sea Water
Work in Extreme Weather Conditions: During hot and cold weather, the concreting shall be done as per the procedure set out in IS 7861 (Part–I)-1975 and IS 7861 (Part II)-1981 respectively. Concreting shall not be done when the temperature falls below 4.5ºC. In cold weather, the concrete placed shall be protected against frost. During hot weather, it shall be ensured that the temperature of wet concrete does not exceed 38ºC.
Under Water Concreting: Concrete shall not be deposited under water if it is practicable to de- water the area and place concrete in the regular manner. When it is necessary to deposit concrete under water, the methods, equipment, materials and proportions of the mix to be used shall be submitted to and approved by the Engineer-in-Charge before the work is started.
Under-water concrete should have a slump recommended in 4.2.2. The water-cement ratio shall not exceed 0.6 and may need to be smaller, depending on the grade of concrete or the type of chemical attack. For aggregates of 40 mm maximum particle size, the cement content shall be atleast 350 kg/m3 of concrete.
Degree of Workability
Blinding concrete: shallow sections: Pavements using pavers
See Note 1
Mass concrete: Lightly reinforced sections in slabs, beams, wall, columns, : floors
Hand placed pavements: canal lining; Strip footing
Heavily reinforced sections in slabs, beams, walls, columns:
Slip form work: Pumped concrete
See Note 2
Note 1: In the ‘very low’ category of workability where strict control is necessary, for example, pavement quality concrete, measurement of workability be determination of compacting factor will be more appropriate than slump ( see IS 1199) and a value of compacting factor of 0.75 to 0.80 is suggested.
Note 2: In the ‘very high’ category of workability, measurement of workability by determination of flow will be appropriate (see IS 9103).
Concrete in Sea Water: Concrete in sea-water or exposed directly along the sea-coast shall be at least M20 Grade in the case of plain concrete and M30 in case of reinforced concrete. The use of slag or pozzolana cement is advantageous under such conditions.
Special attention shall be given to the design of the mix to obtain the densest possible concrete: slag, broken brick, soft lime stone, soft sandstone, or other porous or weak aggregates shall not be used.
As far as possible, preference shall be given to precast members unreinforced, well-cured and hardened, without sharp corners, and having trowel-smooth finished surfaces free from crazing, cracks or other defect; plastering should be avoided.
No construction joints shall be allowed within 600 mm below low water-level or within 60 mm of the upper and lower planes of wave action. Where unusually severe conditions or abrasion are anticipated, such parts of the work shall be protected by bituminous or silico-fluoride coatings or stone facing bedded with bitumen.
In reinforced concrete structures, care shall be taken to protect the reinforcement from exposure to saline atmosphere during storage, fabrication and use. It may be achieved by treating the surface of reinforcement with cement wash or by suitable methods.