ACI Committee 231 defines “early age” as the period afterfinal setting, during which properties are changing rapidly.For a typical Type I portland-cement concrete moist cured atroom temperature, this period is approximately 7 days. Thisdocument, however, includes discussions of early-ageeffects beyond 7 days.
It is important to understand howconcrete properties change with time during early ages andhow different properties are interrelated, which may not bethe same as for mature concrete. It is also important tounderstand how these early-age changes influence theproperties of concrete at later ages. The temperature historyat early ages has a strong effect on whether concrete maydevelop its potential strength.
Poor early-age curing has beendemonstrated to detrimentally affect the strength, service-ability, and durability.Concrete structures change volume due to the thermal- andmoisture-related changes. This may be detrimental becausesubstantial stresses may develop when the concrete isrestrained from moving freely.
This is particularly importantat early ages while the concrete has a low tensile strength. Therefore, the assessment and control of early-age crackingshould be based on several factors, such as age-dependent material properties, thermal- and moisture-related stressesand strains, material viscoelastic behavior, restraints, and environmental exposure.
Temperature control in concrete during the early stages of hydration is essential for achieving early strength as well as ultimate strength and to eliminate or minimize uncontrolled cracking due to excessive mean peak temperature rise and thermal gradients (ACI 207.1R and 207.2R). Of particular importance in determining the risk of early-age cracking of any concrete member is an assessment of the magnitude of the stresses generated in the concrete as a result of restraint to thermally induced movement.
In general, there are two types of restraint: external and internal. External restraints are caused by support conditions, contact with adjacent sections, applied load, reinforcement, and base friction in the case of concrete slabs-on-ground. Internal restraint is a manifestation of the residual stresses that develop as a result of nonlinear thermal and moisture gradients within a cross section.