It is generally well recognised that when concrete has to be mixed and placed in either very hot or very cold weather, it is necessary to take precautions to ensure that the concrete is not damaged or adversely affected by the ambient weather conditions. At temperatures below freezing, for example, freshly placed concrete may be damaged by the formation of ice within its pore structure. In very hot weather the concrete may stiffen prematurely, preventing it from being compacted and finished properly, or the temperature of the concrete may rise to the point where thermal cracking occurs as it cools. It is perhaps not so well recognised, however, that even at moderate air temperatures, strong dry winds can cause concrete to dry out prematurely and to crack.
There are a few fixed rules on what constitutes hot or cold weather in respect of concreting operations. NZS 3109: 1997 Concrete Construction discusses the range 5°C to 30°C and AS 1379: 2007 Specification and Supply of Concrete to be within the range of 5°C to 35°C at the point of delivery. Precautions will always be necessary when ambient air temperatures lie outside this range.
Concreting in Hot Weather
The effects of high temperatures can be summarised as follows:
- 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
They may well be necessary, however, at air temperatures within this range, at less than 10°C or more than 30°C, say. At the lower temperatures, the concrete, whilst in no danger of freezing, may take an excessively long time to gain its specified strength. At the higher temperatures, particularly if accompanied by hot dry winds, plastic cracking and premature stiffening of the concrete may take place.
Precautions for hot-weather concreting should be initiated when the ambient temperature is expected to exceed 30 to 35°C. These precautions may consist of one or more of the following practices:
- Dampening forms, reinforcement and sub-base
- Erecting wind breaks and sunshades to protect exposed concrete surfaces
- Cooling concrete ingredients
- (During transport of wet concrete) cooling containers, pipelines, chutes, etc
- Completing the transporting, placing and finishing of concrete as rapidly as is practicable
- Informed usage of set-retarding admixtures (to counter premature stiffening of the fresh mix)
- Immediately following the initial finishing operation, spraying a fine film of aliphatic alcohol over the exposed concrete
- surface - to limit evaporation and help control plastic shrinkage cracking (this should be repeated as necessary during
- any subsequent operations up to final finishing)
- Immediate curing after final finishing is complete
- Moist curing to control concrete temperature
- Restricting placing to night time when ambient temperatures are generally lower.
Concreting in Cold Weather
The prime effects of low temperature on freshly placed concrete 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;
- (at temperatures below freezing) physical damage to the concrete in the form of surface scaling or bursting, and the cessation of hydration.
Precautions which may be taken to protect the concrete in cold weather may consist of one or more of the following practices:
- 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 (when using Type GP (general purpose portland) cement
- the temperature of the concrete should be maintained at 20°C or above for 3 days
Insulating the concrete (a thick insulating blanket is often sufficient protection for pavements)
Preplan carefully to avoid delays at all stages.
Have standby equipment and manpower for all stages.
Pay attention to speed of application, effectiveness and duration of curing arrangements.
Schedule night-time placement if possible.
Preplan carefully to ensure adequate equipment and manpower available especially if there is a likelihood of temperatures below 0°C.
Use water reducing retarding admixtures in the concrete.
Reduce the temperature of the concrete by (in order of effectiveness):
- reducing temperature of aggregates
- using liquid nitrogen injections in the mixed concrete
- reducing temperature of mixing water
- using cement with lower heat of hydration
- reducing temperature of cement
Reduce the setting time of the concrete by (in order of effectiveness):
- heating mixing water (maximum 70°C)
- using (chloride-free) accelerating admixture
- using higher cement content
- using high-early-strength cement
Batching, Mixing & Transporting
Shade batching, storage and handling equipment or at least painting with reflective paint.
Discharge transit mixer trucks as soon as possible.
Placing & Compacting
Shade reinforcement, formwork and subgrades if possible and spray with water.
Ensure that slabs have minimum 'fronts' to which concrete is added.
Place concrete in walls and deep beams in shallow layers.
Use burlap covers if there is any delay between load deliveries.
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.
Finishing & Curing
Use sunshades and windbreaks to lengthen finishing time (or, if hot/dry winds present, to control plastic shrinkage cracking).
For flatwork, use aliphatic alcohol after initial screeding if hot/dry winds present.
Use re-vibration to correct plastic shrinkage cracking.
Use water curing as the preferred method for at least 24 hours.
Maintain concrete temperature until safe strength reached by means of form insulation, insulated covers or heated enclosures.
Delay striking of formwork for as long as possible.
Avoid thermal shocks and temperature variations within a member. This includes not using cold water for curing, and removing protective measures gradually.
Table 1. Controlling the Effects of Hot & Cold Weather