Hot Conditions for Concrete

The temperature on days when placing concrete can change the properties of concrete completely. It is essential the temperature be moderate and not near extreme temperatures. On hot days when curing, the heat causes excessive moisture from the concrete to evaporate. This can ruin the entire curing process because the concrete loses too much water.

The effects of high temperatures are:

• Shorter setting times and early stiffening
• Increased rates of hardening
• Possible 28 day strength loss
• Increased tendency for plastic shrinkage
• Difficulties in placing and finishing
• Danger of cold joints - a cold joint is formed when plastic concrete is placed against concrete that has set and commenced hardening.

Precautions for hot-weather concrete should be initiated when the ambient temperature is expected to exceed 27°C. These precautions may consist of one or more of the following practices:

• Dampening forms, reinforcement and subbase
• Making wind breaks and sunshades to protect exposed concrete surfaces
• Cooling concrete ingredients
• (During transport of wet concrete) cooling containers, pipelines, chutes, etc
• Informed usage of set-retarding admixtures (to counter premature stiffening of the fresh mix)
• Immediate curing after final finishing is complete
• Moist curing to control concrete temperature
• Restricting placing to early morning or even at night, when ambient temperatures are generally lower.

Any combination of high air temperature, low relative humidity and wind velocity will adversely effect the quality of fresh or hardened concrete. Because it's impractical to recommend a restrictive maximum temperature due to the many variables involved, it is advised that at some temperature between 23°C and 38°C, there is a limit at which favorable conditions do not exist.

Typical concrete temperatures for various relative humidities pose potential plastic shrinkage cracking.

 

 

 

 

Due to the rapid slump loss of concrete placed in hot weather, preparations must be made to place, consolidate and finish the concrete at the fastest possible rate. Formwork and concrete delivery systems (mixers, belts and pump lines) are susceptible to excessive heat during the summer months and should be shielded or cooled prior to coming in contact with the concrete. The maximum depth of the concrete should be shallower in hot weather to assure coverage with the previous layer and prevention of cold joints.

Flatwork is particularly susceptible to drying due to wind at low humidities and should be properly protected or cured to help minimize the effects of the hot weather.

Following basic concrete guidelines, it is preferable to use water curing as the primary method for at least 24 hours, and preferably for the first seven days. This will help you achieve the maximum strength and durability from the curing process that is possible. If it is unfeasible to water cure the concrete, a quality curing compound should be sprayed or rolled over the surface of the concrete to protect the concrete during the curing period.

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Cold Conditions for Concrete

The effects on freshly placed concrete in cold conditions are:

• A decrease in the rate at which the concrete sets and gains strength, with a resultant increase in the time taken to finish the concrete.
• Physical damage to the concrete in the form of surface scaling or bursting, and lack of, or no hydration whatsoever.

Preplan carefully to ensure adequate equipment and manpower available - especially if there is a likelihood of temperatures below 0°C.

You can reduce the setting time of the concrete by:

• heating mixing water (maximum 70°C)
  
• heating the aggregates
  
• using (chloride-free) accelerating admixture
  
• using higher cement content
  
• using high-early-strength cement.

Other steps to take when preparing to pour concrete in cold weather conditions:

• Thaw frozen subgrades and heat frozen forms (particularly steel) before placing concrete.
  
• Warm, insulate or enclose handling and placing equipment.
  
• Avoid delays in handling and placing.

Precautions to protect the concrete in cold weather by:

• Providing heaters, insulating materials, and enclosures, if sub-zero temperatures are expected
  
• Using high-early-strength cement
  
• Heating the raw materials (the temperature of the concrete when it is placed in the forms should be above 5°C)
  
• Not placing concrete on frozen ground
  
• Ensuring means of maintaining suitable curing temperatures. The temperature of the concrete should be kept in a warm environment for a minimum of three days.
  
• Insulating the concrete (a thick insulating blanket is often sufficient protection for pavements).
  
• Avoid temperature variations. This includes not using cold water for curing, and removing protective measures gradually.