Long Term Shrinkage Cracking of Concrete:
Shrinkage is the inherent property of cement paste, which in pure form may shrink up to 1%. Aggregate provides internal restraint that reduces the magnitude of this volume change to about 0.06%. Shrinkage also occurs partly due to hydration of cement. On wetting concrete tends to expand but to a lower extent as compared to original shrinkage. Concrete would continue to shrink during its lifetime albeit at a very reduced rate.
If the shrinkage of concrete could take place without restraint, the concrete would not crack. The combination of shrinkage and restraint cause tensile stresses to develop in the concrete, leading to cracking. In thicker section of concrete, tensile stresses are caused by differential shrinkage between the surface and the interior concrete. The larger shrinkage at the surface
causes cracks to develop that may, with time, penetrate deeper into the concrete.
Long term measurements on some large reinforced concrete bridge structures have shown that the strain due to drying shrinkage after 5 years was about 30 x 10-6. As the tensile strain capacity of hardened concrete is in the range 80 to 150 x 10-6, it is clear that long term drying shrinkage alone could not initiate the no-load-induced cracks. However it certainly plays an important role.
The shrinkage of a particular concrete mix is also affected by additional factors such as temperature history, curing, relative humidity and ratio of volume to exposed surface. Sound aggregates for concrete have low shrinkage and the more quantity of it is present in concrete smaller would be the shrinkage. Fig. 1 & 2 shows shrinkage cracks in concrete tunnel and Pre-Stressed Concrete girder respectively.
Fig. 1: Lateral Cracks in Tunnel Ceiling
Fig: 2: Cracks in Girder
Preventive Measures for Long Term Shrinkage Cracks:
Minimum water content, use of plasticizer for compensating workability due to lesser water, use of highest possible aggregate content and hence smaller quantity of cements, eliminate external restrains (e.g. smooth polythene sheet on the sub grade for base slab), sufficiently close spaced reinforcement (e.g. generally 15 cm in slabs & walls).
Sealing & grouting as necessary depending on the width of crack.