Concrete is prone to shrinkage due to its inherent properties, with cement paste alone able to shrink by up to 1%. However, the presence of aggregates in the mixture reduces this volume change to about 0.06%. Shrinkage also occurs due to the hydration of cement, although wetting can cause some expansion in the concrete. Despite this, concrete will continue to shrink throughout its lifetime, albeit at a reduced rate. The combination of shrinkage and restraint, particularly in thicker sections of concrete, causes tensile stresses to develop and can result in cracking. Differential shrinkage between the surface and interior concrete can cause tensile stresses, leading to cracks that may deepen over time.
Long-term measurements on large reinforced concrete bridge structures have shown that the strain due to drying shrinkage after five years is approximately 30 x 10-6. However, the tensile strain capacity of hardened concrete is between 80 to 150 x 10-6, indicating that long-term drying shrinkage alone could not initiate cracks induced by no-load. Nevertheless, it still plays an important role. Other factors that affect the shrinkage of concrete include temperature history, curing, relative humidity, and the ratio of volume to exposed surface. Sound aggregates, with low shrinkage properties, can help to reduce the shrinkage of concrete.
Figures 1 and 2 depict examples of shrinkage cracks in concrete structures, with one being a concrete tunnel and the other being a pre-stressed concrete girder.
Preventive Measures for Long Term Shrinkage Cracks:
To minimize the water content in construction, it is important to use a plasticizer to compensate for any workability issues that may arise due to the reduced water content. Additionally, it is beneficial to utilize the highest possible aggregate content in order to minimize the quantity of cement required. Eliminating external restraints, such as by using a smooth polythene sheet on the sub grade for base slabs, can also be helpful in achieving the desired results. Finally, using sufficiently close spaced reinforcement, such as a spacing of generally 15 cm in slabs and walls, can further aid in the construction process.
Repair Method:
Sealing and grouting are two methods used to address cracks, with the specific technique chosen depending on the width of the crack. If the crack is relatively narrow, sealing is typically the preferred option. This involves filling the crack with a specialized sealant that will prevent any further water or air infiltration, which can cause additional damage over time. Sealing is a relatively simple process that can be accomplished quickly and easily, making it a cost-effective solution for many small cracks.
However, if the crack is wider than what sealing can effectively address, then grouting may be necessary. Grouting is a more involved process that involves injecting a specialized material, typically a cement-based product, into the crack to fill it completely. This provides a more permanent solution, as the injected material will dry and harden, creating a new barrier that prevents any further movement in the area. While grouting can be a more expensive option, it is often necessary for wider cracks that require a more extensive repair.
Ultimately, choosing between sealing and grouting depends on the size and severity of the crack. A trained professional can help assess the damage and recommend the most appropriate course of action to ensure the problem is addressed effectively and efficiently.