Hot weather concreting
Problems generally associated with hot weather of summer can really happen any time of year. High winds, low relative humidity and solar radiation can produce rapid evaporation of moisture at the surface of the concrete. Concrete placed at high temperatures will set quicker and will need more rapid finishing. A high rate of evaporation at the surface may cause plastic shrinkage and dry shrinkage. Crusting of the surface while the underlying concrete is still movable, is a sign that evaporation is taking place. Precautions such as the use of wind breaks, cooling with ice in concrete and evaporation retarders should be considered. Other things which are necessary include: have adequate manpower to finish and cure, use white pigment curing compound or other method to hold in moisture after setting. Dampen the sub-grade before placing concrete.
Cold weather concreting
The rate of cement hydration is highly dependent on the concrete temperature. We provide heated mix water in colder weather to decrease the concrete setting time and increase overall rate of strength gain. Accelerators such as calcium chloride and non chloride accelerators are available and used to speed the setting time for certain job conditions and specification. Calcium Chloride can darken the color of the concrete and may react with steel reinforcement. So if these things are a factor, the non chloride is the product to use.
Concrete that is exposed to cycles of freezing and thawing and in contact with water should be air entrained. Slabs of fresh concrete should be protected from freezing until it attains at least 3500 psi compressive strength and should never be placed on frosted or frozen sub-grade. Insulated blankets or straw and poly-film are used to retains heat during curing.
Curing means keeping the water in concrete where it can do its job. Improper curing can cut the strength of concrete by 50%. Keeping the water in allows it to combine with the cement to form a tough glue, which means the concrete shrinks less, cracks less, dusts less and is stronger and more durable.
Curing must start as soon as possible after concrete hardens. Membrane curing compounds are the most practical and widely used. They can be sprayed or rolled on. They hold water in. Water sprayers are sometimes used but must be continuous. Recommended practices calls for minimum of seven (7) days duration of curing or until 70% specified compressed strength is attained.