What are the problems of high temperature when casting concrete ?

Some concrete specifications contain a maximum temperature for the freshly mixed concrete as delivered. Typical values are between80° and95° F as measured by ASTM C1064-86. Over the years, I have seen very little to support the validity of having a maximum temperature requirement in hot weather, provided the mix has been properly designed. ACI305R section3.2.1 states that “concrete can be produced in hot weather without maximum limits on placing temperature and will perform satisfactorily if proper precautions are observed in proportioning, production, delivery, placing, and curing. As part of these precautions, an effort should be made to keep the concrete temperature as low as practical.” Is there a need for maximum temperature limitations on freshly mixed concrete in hot weather? A.: The simple truth of the matter is that concrete placed and cured at a moderate temperature (60° to80° F) will outperform +90° F concrete in strength and durability. If you are looking for superior concrete, control the temperature. The other problem with warmer concrete is cracking. These maximum specified temperatures are necessary to help control early cracking in concrete. Concrete is usually poured during the day when it is warm. Early cracking in slabs and other large concrete structures is often caused by a steep temperature gradient through the concrete caused by the cooling of the surface by the night air when the concrete has very low strength. After hydration starts, concrete will gain in temperature reaching a maximum, which will depend on member thickness, type, and quantity of cement, and so forth. As soon as the concrete begins to cool, the gradient, which depends strongly upon external temperature, will determine the cracking risk of that concrete. During this phase (from24 to72 hours after pouring), if the concrete is able to generate tensile stresses higher than the tensile strength, a crack will appear. Any measure to reduce temperature differences, such as using special low hydration cements, lower cement content, cooling down the compounds before mixing, cooling down concrete during the first hours, or avoiding extreme temperature drops after1 day, will help. One way is to pour during the night, first to reduce the maximum temperature because, during the night, external temperature helps cool the concrete, and second, when the day starts, the temperature increase helps decrease temperature differences and gives the concrete more time to generate tensile strength.

High temperature  leads problems during concrete production and Placement.

1. Reduced  slump retention time

2. Disturbed W/c ratio

3.Cold joint formation

4. Decreased Re-vibrability time 

5. Setting time

6.Choking of  concreting Pumps, crane buckets, chutes etc

 

 

Hot weather creates special challenges for precasters, and technically speaking, there are more obstacles to overcome when placing concrete in hot weather than in the cooler seasons. By understanding how heat, humidity and wind affect the curing of concrete, you can adjust your mix and compensate in a variety of other ways to maintain high quality standards.

High temperature is harmful for heat of hydration. The cement and sand reaction is slowed due to high temperature.

High temperature outside means there is a small temperature gradient between the heat of hydration and outside environment. That will lead to a slower reaction and the concrete will need more time to reach the specified strength.

Due to the high temperature the concrete is very much prone to cracking and loose its strength also. 

Dear Sir,

 

I wish to add a small key note to Engg., reply.

 

As told by Mr. Alex Al Yazour , we are facing the same problem in MSE Wall concrete Panel. Surface cracks are coming over the panels. We did technical study over this with Concrete microscope. Surface crack widths are lower the the maximum limits (Max.0.3mm) as per BS8110-1 The Structural use of Concrete.

 

But some of the panels have deep cracks.

 

Another one issue  cracks formed in pier construction due to high temp (50 degree) shuttering works got heated more than air temperature. 

 

cracks of concrete in high temperature