Vital factors to consider for cold weather concreting
28 May 2019 | Web Article Number: ME201914600
By Bryan Perrie
WITH colder weather now fast approaching, concrete producers – despite South Africa’s relatively mild winters – sometimes have to contend with the challenge of concreting in cold weather, especially in mountainous regions.
Weather in the concrete industry is regarded as cold when the ambient temperature falls below 5 degrees Celsius. This does not generally happen during the day in most parts of South Africa which means expensive preventative measures such as heating aggregates are seldom necessary.
The effect of concrete freezing at early ages depends on whether the concrete has set, and what strength the concrete had attained when freezing took place. If concrete which has not yet set is allowed to freeze, an increase in the overall volume of the concrete occurs due to the expansion of water, especially in the capillary pores. When thawing takes place, the concrete will set with an enlarged volume of pores which will reduce strength and durability.
- If freezing takes place after the concrete has set, but before it has gained sufficient strength (about 3 to 5 MPa), expansion associated with the formation of ice will cause disruption of the microstructure and irreparable loss of strength and durability.
When the concrete has achieved a compressive strength of at least 3 to 5 MPa, it can resist a freezing cycle without damage because it has a higher resistance to the pressure of ice and because a large part of the mixing water will either have combined with the cement or will be located in gel pores and therefore unable to freeze.
Consideration should be given to the following when concreting in cold weather:
- Cement type: Because of their slower setting and rate of strength gain, the use of highly extended cements or the partial replacement of CEM I cement with significant amounts of either ground-granulated blast furnace slap (GGBS) or fly ash (FA) is not recommended. It may be advantageous to use CEM I 42, 5R or 52.5N cement in preference to 42.5 N or 32.5 cements;
- Aggregate protection: Water in aggregate may be prevented from freezing by covering stockpiles with tarpaulins. If aggregates are likely to become frozen or contain ice and snow, they may have to be heated with steam injection or hot air blowers. When using steam heating, adequate draining must be provided. Typically, the aggregate should be heated to between 10 and 20 degrees Celsius;
- Lagging of water pipes: All water pipes must be adequately lagged to prevent supply pipes from freezing, or even bursting;
- Heating the mixing water: The most common and easiest way to heat concrete is to heat the mixing water but care must be taken not to exceed 60 to 70 degrees Celsius. At higher temperatures, flash setting of the cement and reduced workability may occur;
- Batching and mixing: Adequate protection must be given to the batching and mixing plant as the concrete temperature can drop significantly if the equipment is very cold.
- Concrete temperature: The minimum concrete temperature as mixed should be higher in colder conditions but may be reduced for concrete placed in larger sections. For air temperatures between -18 and 0 degrees Celsius, recommended concrete temperatures vary between 7 and 18 degrees Celsius;
- Transporting and placing: As significant heat losses occur during these processes, they must be carried out quickly. Unless the concrete is adequately protected, methods of transport such as conveyors and chutes are not recommended.
Finally, the main requirement in cold weather concreting is to prevent heat loss of the freshly placed concrete. So, under no circumstances should water-curing methods be used. Heat may be retained by using insulated forms, covering exposed surfaces with insulating materials, or erecting covers with internal heating. Combustion type heating under covers should be avoided.
Formwork and props must be left in place longer than for normal weather, and pedestrian and vehicular traffic will also have to be kept off slabs for longer than usual.
- Bryan Perrie is MD of The Concrete Institute