WEATHER OR NOT

Rain or shine. What happens to your concrete mix when weather goes to the extremes?



What does a cold beer on a hot day have in common with the reaction of fresh concrete in the heat of the day? In the late 1980’s, a technical industry professional drew a parallel to how such weather causes water to condense on the outside of the glass, but how exactly does a concrete mix react to heat versus cold, and in the growing shifts to more volatile weather patterns, to extreme heat and cold?

Concrete should generally not be placed in weather conditions that exceed 40°C or below 5°C and falling, without additional precautions having been put in place.

Being based in Southern Africa, let’s start with the effect of heat on the concrete mix as it effects the outcome of concrete work through factors including evaporation, condensation, ambient temperatures, exothermic reaction, and friction. Friction during mixing can generate enough heat to raise the concrete temperature 5°C in 30 minutes and in hot weather, as the concrete sets up, slump decreases rapidly and more mixing water is needed, which can contribute to lower strength.

In a white paper by the International Journal of Science and Research (IJSR), authors Shubham Giri Goswami and Ritesh Kamble confirm that an ambient temperature above 40°C and lack of a protected environment for concrete placement and finishing can contribute to difficulty in producing quality concrete. They add that “adverse site condition can adversely impact the quality of concrete when the temperature differential from ambient to internal temperature of a cast member approaches approximately 20°C.”

There are five effects of hot weather in concreting: accelerated setting, increased tendency to crack, strength reduction, rapid water evaporation in the curing stage, and difficulty in air content  as well as joint cutting when needed. Accelerated setting and a lack of control of air content lead to greater difficulty in workability.

The American Concrete Institute (ACI) Guide to Hot Weather Concreting describes what happens to the materials in hot weather:

“Hot weather accelerates the cement hydration reaction and increases the water demand and rate of slump loss of a given concrete mixture. As a result, it is often more difficult to properly adjust mixture proportions, deliver, place, consolidate, and finish concrete during hot weather. These increase the likelihood of retempering, which can lower strength dramatically, increase the potential for cracking and increase the variability in concrete quality over the course of a large job. Hot weather conditions also accelerate the rate of surface evaporation, which can increase plastic shrinkage cracking, lead to crusting and other finishing problems and make curing and finishing more challenging.”

It is also important to understand that while concrete mixed, placed and cured at higher temperatures secures higher strength early, at 28 days on the strength tends to be lower.

When it comes to cold weather conditions, construction professionals face the risk of major damage that can set in such as a delay in setting time when concrete temperature falls below 5°C. Compressive strength can reduce by 50% if fresh concrete freezes and the slab can/will crack.

Two tips for curing concrete under wintry conditions are to maintain the right water-cement ratio, which should not be over 0.40 under freezing conditions, and to use propane heaters or polyethylene enclosures to raise the ambient temperature. An easier way is to introduce heat to the mixing materials and/or to used “hot water” A third consideration is to “control chloride ions by using fly ash, silica fume and furnace slag.”[1]

Whilst South Africans are used to building in hot weather, the country’s recent unpredictable and persistent rainfall could have put a damper on projects. Whether your build is in the coastal rainy season or inland’s summer afternoon downpours, you can put measures in place when pouring concrete to avoid dusting, scaling and compromising concrete strength.

For example, never work the rainwater into the concrete surface and do not broadcast dry cement into the concrete to soak up surface water as this weakens the top layer and impairs the finish. Rather wait until the rain stops and use a float to push the water off the edge of the slab before you start finishing. For interior slabs try to place concrete after the interior has been closed in. Other practical tips include always covering the concrete with plastic sheeting and making sure you seal the edges.

Once your concrete is poured, also keep an eye out for ponding, which can happen if there is excess water on the surface of the concrete.

After a day of giving it your all on site, it may be time for an ice cold beer! As no matter what weather extremes we may face in our construction projects, in any weather, we can take on each challenge because we have the toughest of teams who – just like concrete – have what it takes to stand strong as we pour new foundations for our kusasa (tomorrow) together. 

[1] International Journal of Science and Research (IJSR) Concreting in Extreme Weather Condition  Shubham Giri Goswami, Ritesh Kamble