Active cracks in concrete are a type of crack that continues to expand in length, width, and depth over time. They typically arise due to factors such as overloading and thermal expansion, such as those caused by freeze-thaw cycles. Various methods are available to repair these cracks, including drilling and plugging, stitching, external prestressing, and flexible sealing.
However, repairing active cracks is a complex process since the underlying cause of the crack formation must also be addressed. If the root cause of the crack development is not addressed, it is possible that new cracks may form near the repaired active cracks due to uncontrolled propagation. In other words, if the factors contributing to the initial crack formation are not resolved, the repaired cracks may not last long before new cracks emerge. Therefore, it is crucial to identify and address the underlying issue to ensure long-term success in repairing active cracks in concrete.
Methods of Repairing Active Cracks in Concrete
Different methods to repair active cracks in concrete structures are explained briefly in the below section.
1. Drilling and Plugging through Crack
A technique that can be utilized for addressing cracks that appear in straight lines involves drilling a hole in close proximity to the crack and filling it with grout. By doing so, the grout forms a key that effectively locks the crack, preventing it from spreading any further. Not only does this method prevent any leaks or loss of soil, but it is also cost-effective and requires less time to complete.
Fig.1: Drilling and Plugging
A potential solution for repairing cracks in concrete involves filling a drilled hole with epoxy mortar or another epoxy formulation, along with reinforcement bars. The size and length of the bars are chosen in advance to stitch the cracks across, and a hole with a diameter of 50 to 75mm is drilled to intersect the crack and provide enough repair material to support the loads on the key.
If the primary concern is water tightness rather than load transfer, a low modulus resilient material can be used to fill the drilled hole. However, if both water tightness and load transfer are necessary, the first hole is filled with grout and the second hole is filled with a resilient material. This procedure should effectively repair the crack and provide the necessary structural support or water tightness as needed.
2. Stitching of Cracks
The stitching method is considered a straightforward and effective way to repair cracks. The process involves drilling holes at strategic points across the cracks to create entry and exit points. U-shaped metallic staples are then passed through the holes and securely anchored at each end. Grout or epoxy can be used to further secure the ends. The result is a durable repair that can withstand normal wear and tear. The process is illustrated in Figure 2.
Fig.2: Stitching of concrete cracks
3. External Prestressing
The post-tensioning method is a technique used to prevent flexural cracks in reinforced concrete structures from further widening or to correct them completely. This method works by applying compression force to the tendons in the concrete. The tendons are anchored to an anchoring device that is attached to the beam. This provides additional residual compressive force which helps to prevent further cracking.
To carry out this method, tie-rods are employed which are anchored to the anchoring device. The tie-rods are then tensioned, which creates a compressive force that helps to offset the forces acting on the concrete. This force helps to prevent the flexural cracks from getting wider or can even completely correct them.
The anchoring device used in this method is critical as it must be able to withstand the forces created by the tensioning of the tie-rods. Figure-3 illustrates the arrangement of the tie-rods and the anchoring device attached to the beam. The tie-rods are anchored to the anchoring device, and then the tension is applied to create the compressive force needed to prevent further cracking or correct existing ones.
Overall, the post-tensioning method is an effective technique for sealing flexural cracks in reinforced concrete structures. By providing the necessary compression force through the use of tie-rods and an anchoring device, this method can help prevent further damage and improve the overall durability of the structure.
Fig.3: Post Tensioning Cracked Beam
When considering the use of external prestressing as a means of correcting cracks in a structure, it is important to thoroughly examine any potential negative effects it may have. To do so, approximate stress checks should be conducted. By conducting these checks, it is possible to determine if the use of external prestressing may cause any adverse effects on the structure. Such effects could include further cracking or weakening of the structure. Therefore, it is crucial to carefully evaluate the potential risks before implementing external prestressing as a crack correction method.
4. Flexible Sealing
The flexible sealing method is a technique used for healing active cracks. It involves the use of a bond breaker, as illustrated in figure-4. This method aims to prevent the further propagation of the crack by filling it with a flexible material. The bond breaker serves as a barrier between the flexible material and the substrate, ensuring that the material can move independently of the surface it is applied to.
The goal of the flexible sealing method is to create a sealed crack that can withstand movement without re-cracking. The flexible material used in this method must be able to stretch and compress without breaking or losing its adhesion to the substrate. By using a bond breaker, the flexible material is allowed to move freely, reducing the stress on the material and the substrate.
Overall, the flexible sealing method is a practical approach to repairing active cracks. It offers a simple and cost-effective solution to prevent the propagation of cracks, which could lead to more severe damage if left unaddressed. With the use of a bond breaker, the flexible material can provide long-lasting protection to the substrate, ensuring that the crack is healed and the structure remains intact.
Fig.4: Flexible Sealing
When faced with active cracks in a structure, it is important to assess whether there is a need to enhance the flexural or tensile strength across the crack before selecting a repair method. If strength restoration is deemed necessary, it is advised to also consider the installation of an expansion joint in close proximity to the repaired crack. This is done to prevent the formation of any future cracks in the immediate vicinity of the repaired crack.
The decision to improve the strength across the crack can have a significant impact on the selection of the most appropriate repair method. A thorough evaluation of the structure and the crack’s characteristics is necessary to determine whether such an improvement is needed. If it is deemed necessary, a repair technique that can effectively restore the strength across the crack should be selected.
Furthermore, installing an expansion joint in the vicinity of the repaired crack can help to prevent the development of future cracks in the same area. This is important, as the occurrence of additional cracks can significantly compromise the structure’s integrity and lead to more extensive repair work. Therefore, the installation of an expansion joint should be given careful consideration when planning the repair of an active crack.