Case 3

Remedial (see also Remedial)

Cracks can be live or dead cracks. Live cracks must be treated as movement joints whereas dead cracks can be grouted or covered over.

If cracks are deemed to be only surface level (utilising ultrasonic pulse velocity), cosmetic repair methods can be used. These include:

• Epoxy injection
• Routing and sealing
• Pressure grouting
• Dry packing
• Autogenous healing
• Flexible sealing
• Stitching

The most commonly used methods for non-structural cracks are epoxy injection and pressure grouting. While flexible sealing and stitching are used mainly for active cracks. Flexible sealing not only seals the crack but also allows continued movement at the location by allowing the crack to function as a movement joint. Stitching, on the other hand, does not close the crack but prevents it from growing further.

For cracks that are less fine (less than 0.2mm wide) and hidden from public view, the process or autogenous healing should be allowed to take place. Autogenous healing relies on the inherent ability of the concrete to heal itself, it is applicable for closing dormant cracks in moist environment. Concrete repair by stitching is able to aid in autogenous healing of concrete.

Repair using epoxy injection

One of the most versatile, crack repair method is epoxy injection. This method has been successfully used in buildings, bridges, dams etc., due to following characteristics:

• Outstanding adhesive qualities to materials such as concrete and steel
• Resistance to a wide range of acids and alkalis and other chemicals, except acids such as nitric which have high oxidizing characteristics
• Rather vulnerable to organic solvents
• Low shrinkage when the compound course and changes from the liquid to the solid state
• High coefficient of thermal movement compared with concrete
• High compressive, tensile and flexural strength
• Appreciable loss of strength at temperatures over about 80oC
• High rate of gain of strength which can be varied to suit the particular application

The site conditions under which the resin will be applied must be stated to the formulator. Unless specially formulated, most epoxide resins must be applied to dry surfaces and the ambient air temperature and relative humidity during application must be kept within fairly narrow limit.

Repair using polyurethane injection

Polyurethane (PU) grouting is specially designed for providing water control in static joints or cracks in concrete structures. The grout material, when mixed with a certain amount of accelerator, will react with water or moisture and expand to form a tough, closed-cell rubber that is essentially unaffected by corrosive environment.

Polyurethanes are products of the petrochemical industry. They are very durable when exposed to external conditions and retain their gloss well. They can be obtained as elastomers, solid and rigid materials or flexible coatings. Major cracks or localized porous concrete can be repaired by this method.

In the application, PU reacts upon with the water in the slab and expands its volume and thus fills the cracks or pores in the slab. Therefore, it acts as a sealer over the cracks. The repairing process can be completed within half a day. Accelerator agents can be used to speed up the setting time of the grout. A finishing coat may be applied after the remedial work is completed.