When pouring concrete, the ambient temperature is critical. If the measures that should be taken when pouring concrete in hot weather are not implemented, concrete strength will be negatively affected. Concrete hardens as a result of a chemical reaction, and the speed of this reaction increases as the temperature increases. At high temperatures, rapid hydration results in relatively high initial strength. However, the rate of evaporation from the surface of exposed concrete is also higher, so long-term strength and durability generally declines. Therefore, ambient temperature is an important consideration when pouring concrete, and the precautions we outline in this article should be taken.
How Does Hot Weather Affect Concrete?
According to the TS 1248 standard (Rules for preparing, pouring and curing concrete – in abnormal weather conditions), hot weather is defined as three consecutive days of temperatures averaging more than 30ºC. Hot weather has a negative effect on concrete quality and characteristics. It presents difficulties in terms of mixing, placing and curing.
Most of these problems are related to a higher rate of hydration and water in the concrete mixture evaporating more rapidly. Cement hydration heat depends on ambient temperature and the temperature of the concrete, cement components and fineness, mineral additives used as binders and the use of other chemical additives. The temperature of concrete during placement affects concrete setting speed, strength, durability and the formation of plastic shrinkage cracks, thermal cracking, and drying contraction.
The initial strength of concrete poured in hot weather is higher than that of concrete poured under normal weather conditions. However, the strength at 28 days and longer is lower under these conditions.
If the concrete is cured for the first 24 hours after placement at 38ºC, the 28-day strength of a sample tested according to TS EN 12390-2 is 10%-15% lower than the 28-day strength of concrete cured under normal conditions. Therefore, it is critical that test samples made in hot weather be properly cured.
When the proper precautions are not taken to control concrete performance at high temperatures, the rise in concrete temperature will have the following negative consequences even if the right materials and blend proportions are selected.
- The amount of water required to achieve the specified slump value increases depending on the amount of time that elapses between concrete preparation and placement in the forms at the construction site. See Figure 1 for the effect of increasing temperature on the water requirement of concrete.
Figure 1. Effect of temperature increase on the water requirement of concrete (U.S. Bureau of Reclamation 1975).
- Increasing the amount of water without increasing the binding substance in the same proportion will reduce setting time, strength and durability.
- Slump loss begins after the initial mixing procedure and leads to difficulties in transportation and placement.
- It increases the likelihood of plastic shrinkage cracks in dry weather conditions. Durability is compromised due to the formation of cracks.
- There are difficulties checking the amount of air entrained in the concrete.
- What occurs as a result is rapid hydration that increases the temperature difference between the internal and external parts of concrete masses as well as a temperature increase in the internal portions. The difference in temperature can result in thermal cracks.
- Because of different hydration rates or different water-binding ratios, surface imperfections such as a cold joint or color differences may occur.
How Should Concrete be Prepared, Poured and Cured in Hot Weather?
Depending on weather conditions, the making, pouring, maintenance, protection and placement of concrete must be managed in accordance with the TS EN 206 concrete standard, or the TS 13515 and TS 1248 standards.
In situations where no measures are taken to prevent negative effects, the temperature of fresh concrete must not exceed 35°C (TS 13515).
Concrete samples taken at the construction site should be protected from the effects of hot weather for 1 day after being removed from the forms, and should be kept covered to prevent the loss of water, and if possible, should be kept at a maximum of 25°C.
Pouring Concrete in Hot Weather: What Measures Should be Taken?
The procedures that will be carried out when pouring concrete in hot weather depend on how high the concrete temperature is, how low relative humidity is, how fast the wind is blowing, the type of structure and the properties of the materials being used. Everything related to selecting and procuring the proper materials, mixing, placement, protection, curing and sampling must be planned in detail beforehand. In order to ensure stronger concrete when pouring in hot weather, it is better to use the proper mineral additives or cement with a low hydration heat with appropriate chemical additives than it is to increase the amount of cement. The heat from the components in the concrete mixture have a direct effect on the temperature of fresh concrete, so these materials should be checked rigorously.
- The temperature of the cement that is used should not exceed 80 °C in situations where no precautions are taken at the moment it is added to the concrete.
- The temperature of the mixing water should be as low as possible.
- The temperature of the aggregate is also important because it comprises the largest volume in the concrete mixture. Aggregate should be protected from direct sunlight, or the temperature should be lowered by spraying it with water.
Pouring concrete should be avoided in extremely hot and dry weather, and placement should be done in the cooler part of the day (at night or early morning). Forms and steel reinforcements that are exposed to direct sunlight before the concrete is poured heat up quickly and should be cooled prior to pouring concrete by spraying with water as these elements will be in direct contact with the fresh concrete. Dry wood forms should be avoided as they accelerate water loss. It may also help to paint transmixers white or use thermal insulation. Concrete should be poured as soon as it reaches the construction site. Avoid keeping it waiting or mixing the concrete for long periods of time.
Extra water should not be added to the concrete due to hot weather and rapid evaporation. Adding water will have a negative effect on concrete strength. Accelerated water loss in fresh concrete will cause plastic shrinkage cracks, so the surface of the concrete should be kept moist at all times. After the concrete is poured, the curing conditions are especially important in the first hours and first two days.
In the summer months, the curing time for concrete should be at least one week. Proper curing can prevent sudden water loss from the surface of the concrete. The surface of the concrete should be constantly monitored, and the curing procedures should be determined based on the evaporation rate of the water. The curing blanket for the concrete surface can also be kept constantly moist with chemical curing applications. Leaving joints at the proper intervals and/or increasing the amount of steel reinforcement is an effective way to minimize and limit the size of shrinkage cracks.
Why Does Pouring Concrete in Hot Weather Require Special Measures?
We can summarize the procedures that should be implemented and the precautions that should be taken to completely prevent problems that might potentially occur when pouring concrete in hot weather.
- Use concrete materials and mixing ratios that have already been recorded as adequate under hot weather conditions
- Minimize and control the temperature of the fresh concrete
- Select a concrete slump value that allows quick placement and easy compaction
- The transportation, placement, compaction and surface leveling of the concrete should be done as soon as possible
- Schedule concrete placement for night or day based on the weather conditions
- Protect concrete from loss of moisture during placement and the curing process
- Prepare a workflow plan through dialog with the concerned parties before starting a concrete pour in hot weather
References Used: