Apr 2020 GUIDELINES FOR SEASONAL CONCRETE POURING
While concrete contractors can on the most part operate all year round, as seasons change bringing about varying weather conditions, see Sephaku Cement’s top tips and guidelines on seasonal concrete pouring.
Known for its warm and mostly sunny days, South Africa’s weather ranges between Mediterranean to Subtropical climates where conditions are typically influenced by the differences in the county’s varying landscapes. While conditions are rarely extreme, temperatures are known to plummet a few degrees below freezing in winter, both summer and winter rains are encountered depending on the region you are working in and high summer temperatures can see the mercury rise in close vicinity and in some provinces even over the 40°C mark.
Variations in the weather bring about varying ambient conditions that can influence the pouring and curing of Portland cement concrete to follow see guidelines for curing concrete in different South African seasons.
Pouring concrete during a scorching African Summer
South Africa Summer temperatures are usually above 32°C countrywide and, in some regions, often peaking up to 38 °C, temperatures, humidity and wind factors will each play their role in influencing the rate and consistency of how wet concrete cures. While Portland cement concrete is not particularly fussy when it comes to weather conditions, careful attention needs to be paid to keeping the curing speed and process consistent.
Where temperatures climb above 32°C concrete contractors need to take care that the surface of poured concrete are kept cool. To do this, begin by making sure that the materials you are using are kept in a cool place or could be kept cool with a fine mist sprayer whereby for example if aggregate sand and stone has been left to bake in the sun, this will just add to the rise of the concrete temperature above 32°C. Another tip when mixing concrete in hot conditions is to make use of cold water (some contractors often especially when pouring mass concrete like dams where heat of hydration contribute to internal concrete temperatures, even go as far as using ice as a water substitute) and once poured into place keep the concrete surface moist by lightly watering it with the sprinkler or a fine mist.
Pouring concrete under cold conditions
While daytime winter temperatures in South Africa rarely reach below freezing, night-time temperatures in the dead of winter can often plummet into negative numbers on the thermometer. Concrete cures at a slower rate in cooler temperatures which, if kept above freezing are favourable conditions for optimal curing. However, if the ambient temperature reaches below freezing the risk is that the water in wet concrete may freeze before it cures. When water freezes it expands which will result in larger pockets in between the already porous structure of cured concrete and could potentially even crack concrete if it has not reached at least 5-10 MPa. So what’s the issue you may ask? If allowed to freeze, pouring concrete in below 5°C and falling freezing temperature will increase the risk of cracks and fissures as a result of the freezing water in the concrete. To keep curing concrete from freezing in winter begin with mixing combined materials with warm water and conversely to summer tips, let stone and sand aggregates sit in the sun if any, to increase the ambient temperatures as much as possible. Plan concrete pours to take place early as possible to allow strength of 5-10 MPa to be reached before temperatures fall below zero°C. Most contractors will also leave shutters on and insulate the open surfaces with polystyrene when casting mass concrete in the winter to prevent the temperature differential between inside and the concrete surface to go beyond the allowed 17°C. Another tip is to make use of early strength cement that has been blended with various additives to influence the strength class and strength gain pattern of the concrete. Using products such as Sephaku 32,5R or Sephaku 42,5R where the R denotes rapid strength gain Portland cement means that the cement has been engineered to deliver an improved early strength and rapid curing.