With the arrival of summer, concreting in the scorching heat can become a challenge. This year in April, South Florida broke records for high temperatures. Hot weather concreting without taking proper measures can have many adverse consequences on many projects, and affect more than just the timeline. When the cement combines with water it crystallizes around the amassed surrounding particles, helping the concrete set. When it sets the concrete generates heat, and if the concrete is hot it sets faster due to the rapid reaction of the crystals. The strength of the set concrete suffers as a result of this increase in concrete temperature.
The general rule of thumb is that if the concrete is hotter by 18° more than normal, the compressive strength of the concrete will be 10% lower. The actual temperature of the concrete mixture will ultimately depend on the cement content, the temperature of the materials in the mixture, the temperature of the transport equipment used for the concrete, conditions and temperature in the site for concrete placement. Ambient temperature between 77 to 95 degrees Fahrenheit may be considered as hot weather concreting. So, do plan precautions against these hot weather challenges in advance so that the concreting process can continue without incident.
- Moisture Loss
Hot weather causes rapid evaporation from the freshly mixed concrete and surface drying, which may even lead to surface shrinkage. Water is essential when pouring concrete, so to prevent quick water loss, it is advisable to wet down dry substrates before the pouring process. The concrete surface can also be protected during placement with plastic sheeting to preserve the moisture in the concrete mixture.
- Rapid Slump Loss
In hot weather, there is faster slump loss from warmer air temperatures, which can result in lower concrete strength, increased permeability and cracking. Therefore precautions should be taken to reduce temperature in all equipment being used such as pump lines, mixers etc. by being kept in the shade or protected with wet burlap and using refrigerated water in concrete batching.
- Faster Setting Time
The hotter the ambient temperature, the faster concrete will set, which can reduce the time available for placing and finishing the concrete, making it a cause for concern. The setting time of a concrete mixture is reduced by about 50 percent with an increase of 20 degrees Fahrenheit. The reduced setting time due to hotter concrete temperatures can result in shrinkage cracking, strength reduction, and potential for thermal cracking too. Hydration control admixture, cool concrete materials and proper concrete consistency can help rapid placement to slow down setting time.
- Plastic Shrinkage Cracking
Air temperature, low relative humidity, concrete temperature and wind velocity can increase the rate of evaporation of surface moisture increasing the potential for shrinkage cracking. Dry subgrades can absorb water from the mix resulting in cracks later. Also if there is difference in temperature of one part of the concrete mass from another, then the likelihood of cracks developing increase too. This can be avoided by keeping the concrete temperature low, moistening substrates or form works, adding fibers to the concrete mix, setting up shades & windbreaks, and finally using temporary coverings for concrete slabs after placing and before finishing.
Here is a checklist of some additional precautions you can take during warm weather concreting:
Order and only place ready-mixed concrete than you have time to finish and cover
Use plastic sheets and evaporation retarders to prevent surface drying in the concrete mixture.
Have enough provisions for labor and placement equipment to minimize time required for placing and finishing the concrete rapidly. Delays in hot weather conditions will lead to slump loss and further increase concrete temperatures
To raise relative humidity in the air it may be necessary to fog or mist the area above the concrete placement or even spraying piles to cool them
Adjust the time of concrete placement in summer heat, so that you can capitalize on cooler early morning or evening temperatures
Ensure that in extremely hot days, ready-mix trucks carry additional superplasticizer for slump loss
Avoid over-vibration: 5 to 15 seconds of vibration should give the desired compaction
Begin appropriate curing process as soon as possible after the final concrete finishing has been completed.
With the proper precautions in place it is possible to control the effects of hot weather when placing and finishing concrete jobs to a high quality.