Summer’s high temperatures can cause problems with concrete placement and finishing so it is important to take precautions to minimise the potential adverse effects when placing concrete in hot weather conditions.
Bryan Perrie, MD of The Concrete Institute, offers some guidelines on minimising the effect of hot weather concrete:
The first option to be considered in exceptionally hot weather conditions is whether to postpone the placement of concrete. It is often better to wait than risk costly repairs, or even replacement of defective work and dissatisfied clients seeking compensation. If work is to proceed, proper planning from careful selection of materials to procedures for hot weather work is essential if risks are to be minimised.
As stated, planning for hot weather conditions is essential because of the potential effects on fresh and recently placed concrete. For plastic concrete, these include increased water demand and risk of plastic shrinkage cracking, greater slump loss, faster setting and difficulty in controlling entrained air content. For hardened concrete, the risks include lower strength, reduced durability and increased drying shrinkage.
Because aggregates can be the hottest part of the mix, they have the greatest effect on the initial temperature of the freshly mixed concrete. However, the temperature of the aggregates is difficult to control. It may help to shade stockpiles from the sun and keep them wet with sprinklers and cold water but keeping aggregates cool is not an easy task.
The mix water is easiest to cool, particularly by adding crushed ice to it.
The temperature of the cement does not usually contribute much to the temperature of freshly mixed concrete because of its low specific heat and relatively small mass in the mix. Liquid nitrogen, injected into the concrete while mixing, may be useful. Latent thermal energy on vaporisation to gas can cool the concrete substantially without adverse effect on the treated concrete. This process is usually economical only on major projects involving construction of large concrete elements.
Adding some admixtures to the mix can help in hot weather conditions. Water reducers (plasticisers) will reduce the water content and aid workability. Set-retarders can provide more time to place and finish flatwork but beware: the retarders can make the surface look ready for finishing but the concrete below may still be plastic from the retarder. This can lead to cracking of the finished surface and affect the uniformity of the surface finish.
Selection of a particular cement type may also help. Slower hydration cements with lower rate of heat development can provide extra time for placing and finishing while reducing the concrete temperature and the risk of thermal cracking upon cooling of the concrete.
Painting silos and readymix trucks white or silver can also help reduce concrete temperatures.
In hot weather concreting, problems can arise when site personnel are not aware of the effect of weather conditions, or if weather conditions change during the placing and finishing of the concrete. The builders and sub-contractors should be ready for all possibilities and when hot weather conditions are likely consult the concrete supplier as early as possible. It will also be useful to have standby equipment and manpower for all stages and use the largest size and amount of coarse aggregate possible for the job.
In determining the slumps of the concrete, consider scheduling concreting for the cooler parts of the day, or even do night placement if possible. Plan the locations of construction joints ahead of time with hot weather contingencies in mind and consider spacing contraction joints at slightly smaller intervals than when concreting at lower temperatures. Even consider using sunshades or windbreaks and the use of high-pressure mist sprayers during placing of slabs on the ground or for pavement construction.
As concrete will set more rapidly and have a shorter finishing time in hot weather, perform all operations rapidly – but don’t finish slabs prematurely such as while bleed water is still on the surface.
It is essential that all surfaces be kept continuously moist by curing the concrete. As drying, even intermittently, can produce drying shrinkage and crazing type cracking on the concrete surface, curing should start immediately after the slab has been finished, and is particularly important during the first day after placement and in hot or windy conditions.
Curing methods include ponding with water, use of wet hessian or cotton mats, continuous spray mist, covering with plastic sheeting or spraying on curing compounds. Adequate curing of the concrete must still be provided once final finishing has been completed.