It is generally recognized that when concrete has to be mixed and placed in a very hot or very cold climate, care must be taken to ensure that the concrete is not damaged or adversely affected by environmental climatic conditions. At temperatures below freezing, for example, the newly placed concrete may be damaged by the formation of ice within its pore structure. In very hot climates, the concrete may harden prematurely, preventing it from compacting and ending properly, or the temperature of the concrete can rise to the point where thermal cracking occurs as it cools. However, it may not be recognized so well that even at moderate air temperatures, strong dry winds can cause the concrete to dry prematurely and crack.
There are some fixed rules on what constitutes hot or cold weather with respect to concreting operations. NZS 3109 Concrete Construction discusses the range from 5 ° C to 30 ° C and AS 1379 Concrete Specification and Manufacturing to be within the range of 5 ° C to 35 ° C at the point of delivery. Cautions will always be necessary when ambient air temperatures are outside this range.
Concrete in hot weather
The effects of high temperatures can be summarized as follows:
• Shorter adjustment times and early stiffness
• Increased hardening rates
• Possible loss of strength of 28 days
• Increased tendency for plastic retraction
• Difficulty in laying and finishing
• Danger of cold joints - A cold joint is formed when plastic concrete is placed against the concrete it has fixed and hardening began.
However, they may be needed at air temperatures within this range, at less than 10 ° C or above 30 ° C, say. At lower temperatures, the concrete, without danger of freezing, may take an unduly long time to gain its specified resistance. At higher temperatures, particularly if accompanied by hot dry winds, plastic cracks may occur and premature hardening of the concrete.
Precautions should be taken for hot concreting when the ambient temperature is expected to exceed 30 to 35 ° C.
These precautions may consist of one or more of the following practices:
• Forms of damping, reinforcement and subbase
• Erecting wind breaks and umbrellas to protect exposed concrete surfaces
• Cooling of concrete ingredients
• (During the transport of wet concrete) cooling containers, pipes, conduits, etc.
• Complete transport, placement and finishing of concrete as fast as possible
• Informed use of delay retarding additives (to counter the premature hardening of the fresh mixture)
• Immediately after the initial finishing operation, spray a thin film of aliphatic alcohol on the exposed concrete
• Surface: to limit evaporation and help control plastic shrinkage cracking (this must be repeated
• any subsequent operation until final finishing)
• Immediate curing after finishing the final finish
• Wet curing to control concrete temperature
• Restrict the placement at night when ambient temperatures are generally lower.
Concrete in cold weather
The main effects of low temperature on newly placed concrete are:
• A decrease in the speed at which the concrete is established and gains strength, with the consequent increase in the time necessary to finish the concrete;
• (at temperatures below freezing) physical damage to the concrete in the form of scaling or bursting of the surface, and cessation of hydration.
Precautions that can be taken to protect concrete in cold weather may consist of one or more of the following practices:
• Provide heaters, insulation materials and enclosures if temperatures below zero are expected
• Use of high-strength early cement
• Heat the raw materials (the temperature of the concrete when placed in the forms must be higher than 5 ° C)
• Do not place concrete on frozen ground
• Ensure means to maintain adequate curing temperatures (when using GP type cement (general purpose portland)
• the temperature of the concrete must be maintained at 20 ° C or higher for 3 days
• Insulation of the concrete (a thick insulation blanket is usually sufficient protection for pavements).