Possibly the one aspect of concrete production that is key to successful project completion is that of temperature control. Lack of understanding and attention to this has caused – and continues to cause –severe problems on some of the World’s largest dam projects. Some contractors fail to address the subject of cooling properly at time of bidding, and so have no budgeted amount included in their bid to cover the cost of cooling. Some think, mistakenly, that merely by using chilled mix water, low temperature concrete production is possible. Others fail to understand that there are limits of how much ice can be used in the mix – allowing for admixtures and entrapped water in aggregates.The ‘old school’ contractors who have built conventional gravity and /or arch dams well understand the need for cooling systems, whereas some contractors new to mass concrete, in particular RCC, are still learning from their mistakes.
Much care has to be taken in the selection of a cooling system, to ensure that it can work effectively with the selected mixing plant to achieve the required placing temperature, taking into consideration the method used to deliver the concrete from the plant to the dam.
As the size of RCC dams has increased, the maximum permitted concrete temperatures have become more critical, to minimize cracking. The placing temperature can only be determined when the mix design is known (particularly the pozzolanic material types and amounts used) and the joint spacing is determined.
Cooling systems typically use ICE and/or CHILLED WATER in the mix and/or WET BELT and/or AIR COOLING of coarse aggregates, depending on the amount of heat that has to be removed from the concrete in order to achieve the required placing temperature of the concrete.
5°C chilled mix water instead of natural water temperature can reduce concrete temperature by only 2-3ºC. 6.5kg of ice will reduce 1m3 of concrete by approximately 1°C. If an ex-mixer temperature of less than 20°C is required, then almost certainly it will be necessary – in addition to using ice in the mix – to pre-cool one or more sizes of coarse aggregate.
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