Rise in the curing temperature speeds up the chemical reactions of hydration and affects the early strength of concrete without any ill effect on the later strength. However a higher temperature during placing and setting, although increases the early strength, but affects adversely strength beyond about 7 days onwards. The explanation for this may be that a rapid initial hydration forms products of hydration of poorer physical structure, probably more porous, thus a large proportion of the pores always remain unfilled.
From gel/space ratio rule this will lead to a lower strength compared with a less porous, though slowly hydrating paste in which a high gel/space ratio will eventually be reached.
This adverse effect of high early temperature on the later strength of concrete has been explained by Verbeck and Helmuth as follows:
In their view the rapid initial rate of hydration at higher temperatures retard the subsequent hydration and produces a non-uniform distribution of the products of hydration within the paste. The reason for this may be that at the high initial rate of hydration there is insufficient time available for the diffusion of the products of hydration away from the cement grain and for a uniform precipitation in the interstitial space. Thus as a consequence, a high concentration of the products of hydration is built-up in the vicinity of the hydrating grains. This retards the subsequent hydration of the grain and affects the long term strength adversely.
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The effect of temperature during the first two hours after mixing on the development of strength of concrete has been shown in Fig. 12.3. The range of temperature varies from 40°F to 115°F (4°C to 46°C). Beyond the age of two hours all specimens were cured at 70°F (21°C). To prevent the movement of moisture, specimens were sealed. Specimens cured during first 24 hours at 36°F (2°C) and then at 64°F (18°C) were found 10% stronger than those cured at 64°F (18°C) from the very beginning. Fig. 12.3 is based on price’s results.
The temperature at the time of placing concrete also influences its strength. It has been observed that an increase of 9°F (5°C) decreases the strength by 19 kg/cm2. The influence of curing temperature on strength of concrete tested after curing at 1 day and 28 days.
The temperature at the time of testing also has been found to influence the strength of concrete. Specimens cooled to 20°C (68°F) for two hours before testing showed’ no deleterious effect except temperature above 150°F or 65°C.
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Specimens cast and stored at different temperatures for a period of 28 days and thereafter at 23°C, a higher temperature was found to result in a higher strength during the first few days after casting, but after 7 to 28 days situation changed radically as shown in Fig. 12.4. From the Fig, it will be seen that early high temperature leads to a lower strength for a given total maturity than when heating is delayed for a week or is absent totally. Specimens cured at temperatures between 4°C to 23°C for 28 days showed higher strength than those cured at 42 to 49°C. Even the concrete cast at 4°C (40°F) and stored at – 4°C (25°F) for 28 days and then at 23°C (73°F) was found stronger after 90 days from similar concrete stored at 23°C continuously Fig. 12.4.
Klieger’s tests have shown that there is an optimum temperature during the early life of concrete that will lead to the highest strength at a desired age. Using ordinary Portland cement for laboratory made concrete the optimum temperature has been found as 55°F (13°C). For rapid hardening cement it is 40°F (4°C). However beyond the initial period of setting and hardening the influence of temperature continues within limits i.e., higher temperature accelerates development of strength.