Figure 3.21 illustrates the stress induced when concrete is subjected to early-age temperature history. After placing a large mass of concrete, the temperature rise due to the heat of hydration generated causes the interior of the concrete to become hotter than the surface layers, where heat is lost to the atmosphere unless the mass is completely insulated. The consequent differential expansion results in the interior section being restrained by the outer section generating compressive stresses in the interior and tensile stresses in the surface section. These stresses are relieved to a significant extent by creep, as illustrated in Fig. 3.22, since at this stage creep is large because the concrete is young and of low maturity. At peak temperature, the interior section will be in relieved compression, while the surface could crack if the relieved tensile stress exceeds the tensile strength of the young concrete (Fig. 3.22).
As the concrete starts to cool, the inner section now tends to contract more than the outer section so the effect of this internal restraint is to reduce the compression in the inner section and reduce the tension or close any cracks in the outer section. Creep will again relieve the stresses but to a lesser extent than during the heating cycle because the concrete is now more mature. Eventually, as the temperature of the concrete approaches the ambient temperature, the stress in the inner section could change from compression to tension, with a risk of cracking, while the outer section will be in compression.
In the foregoing situation, it is apparent that creep could be a cause of potential cracking because of too much early relief of compressive stress during the heating cycle compared with tensile stress relief during the cooling cycle. Internal restraint may be quantified in terms of the restraint factor, R, which is defined as:
It is important to note that the tensile creep rupture strength, fcr, which determines the condition for cracking in the cases of early age thermal cracking and restrained drying shrinkage cracking, is less than the value determined in the laboratory under short-term loading and typically it is taken as 60% of the short- term strength (see Section 3.2.4).