Following are the Methods of Repair for Dormant Cracks in Concrete:
1. Sealing of Cracks:
Sealing of cracks as stand alone repair should be used in conditions where structural repair is not necessary. Isolated cracks whether extending through the concrete section or partially into it, should be sealed at the concrete surfaces. For this a slot of approx. 25mm wide should be saw cut upto 10mm deep along the crack keeping crack at the center of the slot. The concrete should be chiseled out from between the two saw cut edges and concrete should be further undercut beyond the 10mm depth up to say 20mm depth so that the base width is slightly greater than the surface width. After the slot is thoroughly cleaned, soaked with water for 10 hrs. and surface dried, a bond coat/ primer coat, of an approximate latex bonding compound should be applied. Once the primer becomes tacky, high strength polymer modified cementitious mortar should be filled in the slot, properly tamped and surface finished. Curing compound should be applied as soon as surface becomes touch dry. 7 days wet curing should be done by covering with wet Hessian and polythene sheet.
2. Routing and Sealing of Cracks:
Alternatively a V-groove should be prepared along the crack at the surface ranging in depth from 6 to 25mm and minimum opening at surface of 6mm (Fig. 1)
Fig: Repair of crack by routing and sealing
A concrete saw, hand tools or pneumatic tools may be used. The groove is then cleaned by air blasting, sand blasting or water blasting and dried. A sealant is placed into the dry groove and allowed to cure. The sealant may be any of several materials, including epoxies, urethanes, silicones, polysulphides, asphaltic materials or polymer mortars. A bond breaker may be provided at the bottom of the groove to allow the sealant to change shape, without a concentration of stress on the bottom. The bond breaker maybe polyethylene strip or tape which will not bond to the sealant.
3. Bond Breaking method:
In some cases over bonding (strip coating) is used independently of or in conjunction with sealing. For this an area approx. 25 to 75mm on each side of the crack is sand blasted or cleaned by other means, and a coating (such as urethane) 1 to 2mm thick in a band is applied over the crack. A bond breaker may be used over the crack or over a crack previously sealed (Fig. 2). Cracks subject to minimal movement may be over banded, but if significant movement can take place, sealing must be used in conjunction with over banding to ensure a waterproof repair.
Fig: Effect of bond breaker
4. Epoxy Injection method:
Cracks as narrow as 0.3mm can be bonded by the injection of epoxy successfully in buildings, bridges and other concrete structures. However, unless the cause of the cracking has been corrected, it will probably recur near the original crack. If the cause of the crack cannot be removed and it is not causing reduction in strength of the structure, then either the crack could be sealed with flexible sealant thus treating it as a joint or establish a joint that will accommodate the movement and then the crack should be grouted with epoxy. With the exception of certain moisture tolerant epoxies, this technique is not applicable if the cracks are actively leaking and cannot be dried out. Epoxy injection requires a high degree of skill for satisfactory execution, and the ambient temperature may limit application of the technique.