Sometimes, concrete works has to be carried out in special conditions such as extreme weather, underwater, under sea water and in aggressive soils. These special concreting methods are discussed.
Concreting Under Special Conditions
1. Concrete Work in Extreme Weather Conditions - Hot and Cold ClimatesConcreting operations done at atmospheric temperature above 400 C, need special attention. IS 7861(part-1)-1975 gives the recommended practices that would result in concrete possessing improved characteristics in the fresh as well as hardened state. Good practices of concreting require special care with respect to following:
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 DesignUse low cement content and cements with low heats of hydration. Use approved admixtures for reducing the water demand or for retarding 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 uniform mixing.
- Period between mixing and delivery should be kept to a minimum.
Placement and Curing:
- Prior to placing concrete formwork, reinforcements and subgrade should be kept cool by spraying with cold water first. If possible, concreting may be restricted to evenings and nights.
- Placement and finishing should be speedy.
- Immediately after compacting and finishing, concrete should be protected from evaporation of moisture.
2. Under-water ConcretingInspection of concrete during placement under-water is difficult. Therefore, it is essential to evaluate the proposed mix proportions, inspect the equipment and review preparation prior to the start of underwater concreting. 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 the type of the chemical attack. For aggregates of 40mm maximum particle size, the cement shall be atleast 350kg/cubic m of concrete. Cofferdams or forms shall be sufficiently tight to ensure still water if practicable, and in any case to reduce the flow of water to 3m/min through the space into which concrete is to be deposited. Cofferdams or forms in still water shall be sufficiently tight to prevent loss of mortar through the walls. Dewatering 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. Concrete cast under water shall not fall freely through the water. Otherwise it may be leached and become segregated. Concrete shall be deposited by, 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. The methods to be used for depositing concrete under water shall be one of the following:
- Direct placement with pumps
- Drop bottom bucket
3. Concrete in SeawaterIn addition to the grade of concrete specified, it will be necessary to control the minimum cement contents and the maximum water-cement ratio. Portland slag cement may be used but it will be necessary to seek specialists 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 seawater. Construction joints are potentially weak and the problems of durability are accentuated in the zone subject to alternate drying and wetting that is, between upper and lower planes of wave actions. IS 4082-1977 recommends a coat of cement water over the reinforcing steels stored in coastal areas.
Fig: Concreting Under Water
4. Concrete in Aggressive Soils And WaterThis refers to concrete placed in soils and waters, containing sulphates, nitrates and other salts which may cause deterioration of concrete. Naturally occurring aggressive chemicals such as sulphates of sodium and magnesium, are sometimes found in soils and waters. Sea water is mildly aggressive to concrete because of soluble sulphates it contains. The decomposition of sulphide minerals contained in colliery waters may cause the formation of F2SO4 which can cause severe sulphate attack. Durability problems may arise also when concrete is exposed to acids. Two types of precautions given in the code:
- Those in the proper attention to the concrete itself will provide sufficient immunity.
- Those in which additional precautions are to be taken to prevent contact between the aggressive chemicals and the concrete.