It’s that time of year – the weather is cold, yet construction continues. When pouring concrete in cold weather extra challenges emerge. The most noticeable effect is the light-dark discoloration of concrete that may occur. The color of the concrete can appear inconsistent and sometimes as though there are dark components throughout the concrete.
Why does this occur?
The combination of low temperatures and a high relative humidity can result in a light-dark discoloration of the concrete surface. A low fresh concrete temperature (in combination with low outside temperatures) retards the hydration of concrete. The mineral phases that occur during hydration crystallize on the concrete surface. This could cause a dense, darker appearing concrete surface that sometimes appears even glass-like. To avoid this, a higher fresh concrete temperature (15-20°C or 59-68°F) could be used or the formwork could be insulated.
Sometimes, to avoid this discoloration may involve using either hot water or accelerators in the mix. Are you using the appropriate mix for the temperature and relative humidity? Remember, concrete mixes that offer a very slow setting could result in the light-dark discoloration.
Additionally, encasing and heating the concrete could be another possibility. When doing so, a drying-out of the panels or the concrete surface is necessary.
Was the concrete compacted for too long?
If there is too much vibration of the concrete, the concrete could slightly segregate. When factors such as a smooth panel and a not completely stable concrete are combined, a slightly increased occurrence of light-dark discoloration could result.
How high was the building component?
High walls are more likely to show light-dark discoloration because of the higher fresh concrete pressure.
Do you know what type of plywood was used?
Smooth, non-absorbing panels encourage the effect of light-dark discoloration. During cool/cold weather conditions wooden panels should be preferred to steel formwork, as panels made of wood feature a lower thermal conductivity and with it a higher insulating effect. The best way to prevent from discoloration is to work with alternative thermoplastic plywood.
The recommended formwork to lower the risk of discoloration with cold weather concrete is Doka’s Xlife plywood – an alternative thermoplastic plywood.
, Manager of the Concrete Laboratory at Doka, is an industry expert and has been responsible for concrete testing and research within Research & Development Department for more than 7 years. Holding a Master’s degree in Civil Engineering, she started her professional career with Doka in 2006 as a Structural Engineer.