Hot weather concrete tips

Using and placing concrete during the hot summer months present far different challenges than use and placement during cold weather. The summer month effects of temperature, wind, and air humidity can all have a negative impact on the performance of concrete. For purposes of concrete use and placement, “hot weather" can be defined as any period of high temperature during which special precautions need to be taken to ensure proper handling, placing, finishing and curing of concrete. Hot weather problems are most frequently encountered in the summer, but critical drying factors such as high winds and dry air can occur at any time, especially in arid or tropical climates.

Higher temperatures cause water to evaporate from the surface of the concrete at a much faster rate and cement hydration occurs more quickly, causing the concrete to stiffen earlier and improving the chances of plastic cracking occurring. Concrete cracking may result from rapid drops in the temperature of the concrete. This occurs when a concrete slab or wall is placed on a very hot day and which is immediately followed by a cool night. High temperature also accelerates cement hydration and contributes to the potential for cracking in massive concrete structures. Higher relative humidity tends to reduce the effects of high temperature.

Other hot weather problems include increased water demand, which raises the water-cement ratio and yield lower potential strength, accelerated slump loss that can cause loss of entrained air, fast setting times requiring more rapid finishing or just lost productivity.The following tips should help you have a successful placement of concrete during hot weather:

• Organizing/Planning
• Pre-placement
• Placement
• Post-pour

Organizing/Planning

• Organise 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 cement 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!

• Monitor the rate of evaporation using a Nomograph to chart temperature, relative humidity, wind velocity, and the concrete temperature.

Post-pour

• Never add water to concrete that is more than 1.5 hours old.

• Cover placed slabs on grade with a damp bed of sand rather than polyethylene sheeting if a vapor barrier is required.

• Delay or extend setting times by using retarding admixtures.

• Begin barrier or water curing as soon as finishing is complete.

• Begin finishing as soon as the sheen has left the surface of the concrete.

• Start curing as soon as finishing is completed.

• Continue curing for at least 3 days.

• Protect exposed concrete surfaces from drying out by setting up wind breaks, sunshades, water misting equipment or fog sprays while using approved curing membranes.

• Consider adding white pigment to membrane curing compounds to reflect heat away from the concrete surface.

• Protect field test cylinders by keeping them in a shaded area or creating a shaded area to prevent evaporation.

• Chill field curing boxes with ice or refrigeration to maintain the required 60° F to 80° F for the field cylinders.

• Never use accelerators to speed up the setting process!

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