Jun 2018 GUIDE TO CONCRETING THIS WINTER
Sephaku Cement discusses concrete temperature considerations, how surrounding air temperature influences cement hydration and offers a guide on how to ensure cold weather doesn’t impacts negatively on your concrete projects in winter.
Fortunately most parts of South Africa don’t experience extreme weather in winter, however it is not uncommon for a number of areas throughout the country to encounter short bursts of below freezing temperatures during cold snaps brought on by cyclical cold fronts.
So what do we define as “Cold Weather” for concreting?
Weather considered to be “cold” for concreting is characterised by the following conditions:
- Where cold weather conditions lasts for a period of three consecutive days or more
- Average daily temperatures are lower than five degrees Celsius
- Where the ambient air temperature is lower than ten degrees Celsius and falling for longer than a twelve hour period (day or night)
Concrete freezes at zero degrees Celsius and the effects of early freezing can be detrimental to any interior or exterior concrete project, early freezing of concrete can lead to the following results:
- Adversely affects the strength of the concrete by interrupting its matrix and make up
- Slows the curing time which negatively impacts the hydration reaction required by the cement to gain strength (Cracking and or drying out before sufficient strength is gained)
So how can we prevent concreting projects from being unfavorably impacted by below freezing weather conditions during winter?
The answer? Influence the concrete mix to encourage early hydration and heat gain.
Once concrete has gained about 5MPa compressive strength (load bearing per square mm), the hydration reaction during this time has absorbed enough moisture into the mix not to cause any damage to the cement’s construct should it freeze after this point. To speed up the rate at which cement cures (Hydration Reaction) in cold weather the following approaches can be applied.(take care to protect drying out of retarded concrete)
- Mix the concrete using hot water in cold weather conditions. Aim for a temperature between 18 and 20 degrees Celsius to encourage a more swift heat gain and hydration reaction in cold weather.
- Hugely effective and relatively affordable, add an accelerator such as calcium chloride to speed up the hydration reaction of cement during cold winter months. The accelerating admixture reduces the setting time and speeds up early strength development of the concrete which is typically slowed down by cold weather (CACL can only be used where there is no steel and or reinforcing involved)
- Non-chloride accelerators can also be effectively used to speed up the setting time of cement and will not result in the corrosion of embedded steel reinforcement such as entrenched rebar. These accelerators, while more costly than calcium chloride are also effective for decorative concrete projects in that they won’t mottle or discolour the setting cement.
- Make sure that when placing the concrete, the bed and subgrade that it’s poured onto is not frozen. Frozen subgrades impede setting times which as we have already discussed, slows the concrete’s rate at which it gains strength with the risk of freezing to take place before hydration is successfully achieved.
- Start concreting early in the morning to attain a set and strength gain before the onset of colder night temperatures.
- Change type or content of cement per cubic meter to improve the set time by lowering the unit water content. By adding concrete, preferably high-early strength concrete, by doing so this helps to hydrate more rapidly, generally with a rule of 1 bag for 1 hour.
- Cover concrete with plastic sheeting/ polystyrene sheeting after setting to keep heat of hydration temperatures inside for as long as possible. (Significantly retarded concrete by low temperatures can easily dry out before complete hydration can take place, very much effecting final strength of the concrete)
Overcoming cold weather concreting is both possible and effective by following these simple solutions. With the objective being to prevent curing concrete from freezing for up to 48 hours even the most elaborate concrete construct will cure before cold weather can get the better of your concreting project.