How to Repair Active Cracks in Concrete?

Active cracks in concrete are live cracks which expand in length, width, and depth over time. These cracks are formed due to overloading and thermal expansion e.g. cracks due to freeze-thaw. Common methods to repair active cracks are drilling and plugging, stitching, external prestressing and flexible sealing of cracks.

Repairing active cracks is difficult because if the cause of the crack development is not fixed, a new crack may form next to the repaired active cracks due to their uncontrolled propagation.

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

This method can be applied to the cracks that appear in straight lines. In this method, a hole is drilled near to the crack and this is filled with grout. This grout forms the key to lock the crack preventing it from further propagation. Leakages and loss of soil are prevented by the grout. This method is cost-effective and consumes less time. 

Fig.1: Drilling and Plugging

Another way of plugging the drilled hole is filling it with epoxy mortar or any epoxy formulation with reinforcement bars placed in the drilled hole. The bars used are of predetermined length and size such that it stitches the cracks across. 

The procedure involves drilling a hole of 50 to 75mm diameter depending on the width of crack following the crack location. The hole must be large enough to intersect the crack along its full length and provide sufficient repair material to structurally take the loads exerted on the key.  

If water tightness is primary need over structural load transfer, then the drilled hole can be filled by a resilient material of low modulus. If both properties are required, the first hole is filled with grout and the second hole is filled with a resilient material.

2.  Stitching of Cracks

Stitching of cracks seems to be simple and a durable method. In this method, holes are drilled in a way that entry and exit points are made across the cracks. Through the holes, a number of U-shaped metallic staples are passed through and at the ends, the holes are anchored strongly. Grout or epoxy can be used to anchor the ends.

Stitching of cracks is shown in figure-2 below. 

Fig.2: Stitching of concrete cracks

3.  External Prestressing

Post-tensioning method can be employed to seal flexural cracks in reinforced concrete. This can either arrest the cracks from further widening or correct it completely. The method provides compression force in order to compensate the tendons and then additional residual compressive force. 

This method requires anchorage of the tie-rods to the anchoring device attached to the beam as shown in figure-3

Fig.3: Post Tensioning Cracked Beam

Any adverse effect of external prestressing on the structure as a part of crack correction must be studied by approximate stress checks.

4. Flexible Sealing

Another method for healing active cracks is the use of the flexible sealing method. This method makes use of bond breaker as shown in figure-4 below.

Fig.4: Flexible Sealing

Before choosing a repair method for active cracks, it must be determined whether it is necessary to improve the flexural or tensile strength across the crack. If strength must be restored, it is recommended to install an expansion joint near to the repaired crack in order to avoid the occurrence of future cracks nearby the corrected one.