Concreting in hot weather provides a set of unique challenges for contractors and is only possible at high temperatures if special protective measures are provided.
These must be in place from the start of concrete production to the end of curing. It is dependent on the outside temperature, air humidity, wind conditions, fresh concrete temperature, heat development, dissipation and the dimensions of the element.
The fresh concrete itself must not be hotter than +30 °C during placing and installation without protective measures.
Working with non-retarded concrete can become a problem at air temperatures higher than 25°C. Hydration is the chemical reaction of the cement and water. It begins immediately when cement is mixed with water and continues through stiffening to setting and hardening of the cement paste. This reaction is accelerated at elevated temperatures. As a result of early stiffening placing the concrete is no longer possible. The normal countermeasure is the use of a set retarder.
Durable concrete should not only be characterised by high compressive strength, even more important is its impermeability especially in the areas near the surface. The lower the porosity and the denser the hardened cement paste near the surface, the more durable the concrete will be. To achieve constant and consistent concrete quality, appropriate and correct curing is essential. The following curing measures contribute to this:
- Generally protect from adverse climatic influences (direct sun, wind, rain, etc.)
- Prevent vibration (after finishing)
- Use a curing agent
- Keep damp/mist or spray if necessary
- Maintain the curing time relevant to the temperature
This diagram gives an illustration of the amount of water evaporation per m² of concrete surface under different conditions. As can be seen from the figure (arrow marking), at air and concrete temperatures of 20°C, relative air humidity of 50% and an average wind speed of 20 km/h, 0.6 litres of water per hour can evaporate from 1 m² of concrete surface. At concrete temperatures higher than air temperature and with widening temperature differences, the rate of water evaporation increases significantly.
To ensure durability, several measures have to be undertaken to protect the fresh concrete, particularly from premature drying due to wind, sun, low humidity, etc and extreme temperatures including fluctuations between daytime highs and night time lows. Protection from premature drying is necessary so that the strength development of the concrete is not affected by water removal. The consequences of too early water loss are:
- Low strength in the parts near the surface leading to reduced durability
- Tendency to dusting
- Occurrence of both early age and long term shrinkage cracks