Date: May 1, 2014
News - Hot Weather Concrete
By Mike Hockenberry, Technical Services Team Member
As a Technical Services team member, I am always concerned about placing concrete in hot weather conditions, and how it can produce a rapid rate of evaporation of moisture from the surface of the concrete and also accelerate set times.
It is important that hot weather be taken into account when planning concrete projects, because of the potential effects on fresh and recently placed concrete. High temperatures alone cause increased water demand, which, in turn, will raise the water-cement ratio and result in lower potential strength. Higher temperatures tend to accelerate slump loss and can cause loss of entrained air. Temperature also has a major effect on the setting time of concrete: concrete placed under high temperatures will set quicker and can, therefore, require more rapid finishing. Concrete that is cured at high temperatures at an early age will not be as strong at 28 days as the same concrete cured at temperatures in the range of 70°F (20°C).
High temperatures, high wind velocity, and low relative humidity can affect fresh concrete in two important ways: the high rate of evaporation may induce early plastic shrinkage or drying shrinkage cracking, and the evaporation rate can remove surface water necessary for hydration unless proper curing methods are employed. Thermal cracking may result from rapid drops in the temperature of the concrete, such as when concrete stabs or walls are placed on a hot day followed by a cool night. High temperature also accelerates cement hydration and contributes to the potential for thermal cracking in massive concrete structures.
In the case of extreme temperature conditions or with mass concrete, the concrete temperature can be lowered by using ice as part of the mixing water. So when placing your order please keep in mind that we do offer ice to our customers.
PLANNING
- Organize the job in advance to have enough concrete workers to avoid delays placing, finishing and curing the concrete.
- Schedule, or consider, early morning or evening placement.
- Work with the concrete producer on the mix design to ensure they deliver a product with the correct slump, strength, and performance properties to meet the job requirements.
- Schedule concrete mixer trucks to avoid waiting time so the concrete will not begin to set.
- Consider modifying the concrete mixture to include set retarders and water reducers, and the lowest practical cement factor.
- Order/schedule needed sunshades, wind breaks, and misting equipment.
PRE-PLACEMENT
- Sprinkle or mist cool water on forms, steel and subgrade before placing.
- However, avoid standing water where you have moistened.
PLACEMENT
- Unload concrete within 60-90 minutes of batching.
- Add ice or chilled water to lower temperature of the plastic mix.
- Minimize the amount of water added on the job. Add water only on arrival at the job site to adjust the slump. Later additions of water should be avoided!