Concrete is prone to shrinkage, which is an inherent property of cement paste. In its pure form, cement paste can shrink up to 1%, but the presence of aggregates reduces this volume change to about 0.06%. Shrinkage also occurs due to the hydration of cement. Although concrete tends to expand when wet, it continues to shrink throughout its lifetime, albeit at a reduced rate. If concrete could shrink without restraint, it would not crack. However, the combination of shrinkage and restraint leads to the development of tensile stresses in concrete, which can cause cracking.
In thicker sections of concrete, tensile stresses arise due to differential shrinkage between the surface and the interior concrete. The larger shrinkage at the surface causes cracks to develop, which may deepen over time. Long-term measurements of large reinforced concrete bridge structures have revealed that the strain due to drying shrinkage after 5 years is about 30 x 10-6. However, as the tensile strain capacity of hardened concrete ranges from 80 to 150 x 10-6, it is clear that long-term drying shrinkage alone cannot initiate no-load-induced cracks, although it does play a significant role.
Several factors can affect the shrinkage of concrete, including temperature history, curing, relative humidity, and the ratio of volume to exposed surface. Aggregates with low shrinkage rates are suitable for concrete, and the more quantity of aggregates present in concrete, the smaller the shrinkage. Figure 1 and 2 depict examples of shrinkage cracks in a 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:
To ensure optimal concrete strength and workability, there are several factors to consider. One such factor is the minimum water content required to achieve the desired results. Additionally, the use of plasticizers can compensate for the reduced water content, helping to maintain workability.
Another important consideration is the aggregate content. Using the highest possible aggregate content can reduce the amount of cement required, which can help to minimize costs and reduce the environmental impact of the project.
It’s also important to eliminate external restraints that could impact the performance of the concrete. For example, placing a smooth polythene sheet on the subgrade can help to ensure the base slab is properly supported and protected from the elements.
Finally, reinforcement is critical to ensure the strength and durability of the finished product. In slabs and walls, it’s generally recommended to use close-spaced reinforcement, typically spaced around 15 cm apart, to provide the necessary support and reinforcement.
Repair Method:
When it comes to repairing cracks, the necessary steps will depend on the width of the crack. In some cases, sealing may be enough to address the issue. This involves applying a sealant to the crack to prevent any further damage. However, in other cases, grouting may be necessary. Grouting involves filling the crack with a mixture of cement and water to provide a more substantial fix. It’s essential to determine the appropriate solution for each crack, as using the wrong approach can lead to further damage or future repairs.