Shrinkage compensating concrete is used to minimize cracking caused by drying shrinkage in concrete slabs, pavements, and structures. Drying shrinkage is the contraction caused by moisture loss from the hardened concrete. It does not include plastic volume changes that occur before setting when surface evaporation exceeds the concrete bleeding rate, and length or volume changes induced by temperature, structural loads, or chemical reactions.
The amount of drying shrinkage that occurs in concrete depends on the characteristics of the materials, mix proportions, placing methods, curing, and restraint. When apavement, floor slab, or structural member is restrained by subgrade friction, reinforcement, or other portions of the structure during drying shrinkage, tensile stresses develop.
While Portland cement concretes normally possess tensile strengths in the range of 300 to 800 psi (2.1 to 5.5 MPa), drying shrinkage stresses are often large enough to exceed the tensile strength of the concrete, resulting in cracking.
Significant early age drying shrinkage stresses can occur before these tensile strengths are developed. Furthermore, because of the probable existence of additional stresses imposed by loads, temperature changes, settlement, etc., the inherent tensile strength of the concrete cannot be relied on to resist shrinkage stresses. The frequency and size of cracks that develop in many structures depend on the amount of shrinkage and restraint.
Shrinkage compensating concrete is proportioned so the concrete will increase in volume after setting and during early age hardening. When properly restrained by reinforcement or other means, expansion will induce tension in the reinforcement and compression in the concrete. On subsequent drying, the shrinkage, instead of causing a tensile stress that might result in cracking, merely reduces or relieves the expansive strains caused by the initial expansion of the shrinkage-compensating concrete.
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