Sometimes concreting is required in Special Cases such as cold weather, hot weather, deep lifts, mass concrete, along coastal areas, underwater concreting etc. For these special cases, following requirements should be considered before placement of concrete.
Concreting in deep lifts
Placing of concrete in lifts exceeding 3m in columns and 2m in walls is in the category of deep lifts. Before commencement of concreting in deep lifts, the suitable method of placing concrete has to be selected. The placement of concrete is preferably done by tremie chute or any other suitable method.
In structures of heavy/complicated reinforcement or in complicated form works, the sufficient number of windows in the form works are provided to check the placement and compaction of concrete in different stages. Such windows is closed as soon as the concreting reaches the bottom level of the same.
Concreting under water
When it is necessary to deposit concrete under water, it is done in accordance with Clause 13.2 of IS:456 – 2000.
Cold weather concreting
When conditions are such that the ambient temperature expected is 4.50C or below during the placing and curing period, the work is carried to conform to IS: 7861 (Part-II).
Hot weather concreting
When concreting in very hot weather the it is carried as per IS:7861 (Part-I) and stagger the work to cooler parts of the day to ensure that the temperature of wet concrete used, especially in massive structure, does not exceed 380C.
Positive temperature control by methods like pre-cooling, post cooling or cooling of concrete by circulating cold water through small embedded pipe lines inside concrete, if required is undertaken.
Concreting in the area exposed directly along sea coast
This special requirements of concreting along sea coast is carried out as per specifications in clause 13.3 of IS:456 – 2000.
Concreting in large pours (mass concrete)
The aim of controlling the concreting in large pours (or mass concrete) is to reduce cracking caused by shrinkage due to heat of hydration. The suitable method of placement of concrete is selected for the site condition. The maximum height of lifts will depend on the type of cement used. The use of cement having low heat of hydration, could allow greater lifts.
All the necessary arrangements like precooling of aggregates, cooling of fresh concreting by passing cold water through pipes placed inside the concrete or such other measures at least 48 hours before the placement of concrete should be available. Also, temperature at least 24 hours prior to placement of concrete should be recorded.
The minimum interval between concreting of successive lifts, separated by horizontal construction joint, should be six days. The minimum interval between the concreting of adjacent pours separated by vertical construction joints shall be three days.