Civil Engineering: Temperatures of Concrete | what is the minimum and maximum temperature of concrete?

The simple truth of the matter is that concrete placed and cured at a moderate temperature (60° to 80° F or 15.56 C° to 26.67°) will outperform +90° F or 32.33 C°, concrete in strength and durability. If you are looking for superior concrete, control the temperature. The other problem with warmer concrete is cracking.

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Effects of high temperatures on the concrete

Mechanical properties of concrete:

Compression strength, tensile strength and longitudinal deformation modulus are mechanical properties that have their values reduced when the concrete is submitted to high temperatures. According to Paul on [1], when concrete is submitted to temperatures up to 150°C, its strength is not altered, but for higher temperatures tensile strength begins to decrease. This loss in strength can reach 70% for temperatures close to 600ºC due to gel dehydration and the increase of micro-cracking, Table [1].

Compression strength does not significantly change up to about 300°C.

However, at this temperature and above, a significant reduction begins, with a loss around 20% according to Almeida [2].

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Tests performed by Galle to & Meneguini [3], confirming Almeida[2], have shown that conventional concrete heated to 300°C and slowly cooled had a 24% loss in compression strength in relation to its original, unheated strength.

The reduction in compression strength of concrete submitted to 600°C is approximately 50%, according to Petrucci [4].

Neville [5] justifies this reduction with the occurrence of a progressive degradation of concrete submitted to this temperature.

Cracking intensifies in joints, in imperfectly compacted areas or, in the case of reinforced concrete, on the planes of steel rods, which, after being exposed begin to conduct heat and accelerate the harmful effect of the high temperature on the concrete.

Working with cylindrical test bodies, 10 cm diameter x 20 cm height, heated to 300°C, cooled both slowly and rapidly, Galle to & Meneguini [3] obtained reductions, respectively, of 4% and 21% only on the longitudinal deformation modulus in relation to test bodies that had not been submitted to high temperatures.

These results significantly differ from results shown in Table [1].

There is a great difference between the results obtained by the various researchers on this matter.

Malhotra [6] justifies this difference as a result from factors such as: differences in acting stresses and humidity conditions of concrete while in the heating process, differences in the exposure time to high temperatures, differences in physical and mechanical properties of aggregates, to mention a few.

A factor that has a major influence on the effect of high temperature on concrete mechanical properties is the cooling speed.

The utilization of water in a fire, for instance, is similar to quenching, causing a great strength reduction as a result from intense temperature gradients created in the concrete, Figure [1].

It is important to note that part of the decrease in mechanical properties as a result from heating can be recovered with concrete re_hydration.

According to Canova’s [7], if the concrete temperature is not higher than 500°C, it can be subject to rehydration later, which can help in recovering up to 90% of its initial strength after one year.

Therefore, one cannot generalize results obtained by the various researchers. One should take into account all factors pointed out by each of them in order to have a correct interpretation of the various results.