Types and Remedies of Cracks in Concrete

There are various type of cracks which can develop in a concrete structure. These can be superficial or critical to the integrity of the structure and can have various causes, and accordingly various remedies. Depending upon the type of cracks, the situation can be assessed and remedial measures taken accordingly. A simple method to check whether cracks are superficial or deep is to strike the cracked concrete with a hammer. If the hammer makes a ringing noise then cracks are superficial while if the noise is dull, then it means that delamination has occurred in concrete and crack is deep.

Determining what crack width is acceptable and when cracks become problematic is a subjective topic with no clear cut and precise answer. However, following criteria can be used as a guideline for cracks appearing in a structure after being in service for a long time.

If cracks are less than 1/16 inch (1.5mm) in width, and do not penetrate deep into the concrete then they can be totally ignored as these are normal superficial cracks.

Cracks which are 1/16 (1.5mm) inch to 1/4 (6.3mm) inch in width need to be assessed. If these are not deep and are static i.e. they are not lengthening or widening with time then they can also be ignored.

If cracks are wider than 1/4 (6.3mm) inch, they need to be properly assessed by a structural engineer and remedial measures should be adopted as per structural requirement.

Non Structural Cracks

If the cracks are very fine hairline cracks then it might not be a problem as RCC will crack to transfer the load to steel reinforcement and this is normal behaviour of RCC. These types of cracks are tight and hardly visible and do not pose any problem to the structural integrity of the members. These cracks don't require any treatment unless they start to move or widen. Cracks which are tight and do not allow water to penetrate and which are not deep enough to extend to the full depth of the section usually do not require any treatment. However, if required, they can be treated by application of crack filler which will seal them and will not allow moisture to penetrate the cracks.

Shrinkage Cracks

Shrinkage in concrete is of two types i.e. Drying Shrinkage and Plastic Shrinkage. Drying Shrinkage cracks are caused by loss of capillary water in concrete which happens when concrete is not cured properly and it dries up prematurely. These cracks are caused due to tensile stresses in concrete as it dries and shrinks. Shrinkage in concrete can be controlled by using Type-K (Non-Shrinkage cement). Drying Shrinkage cracks are usually structurally not critical. They can be treated by preparing the cracks and injecting epoxy into the cracks to seal them.

However, Plastic Shrinkage cracks can be more critical as these can penetrate to full depth of concrete structure. They are caused by rapid loss of water from fresh concrete, or due to settlement of aggregates in the concrete. These cracks usually occur in slabs which have large surface area from where water can rapidly evaporate. Escaping of water from surface can be prevented by covering concrete with PE sheet, or by wet curing of concrete.

Structural Cracks

If cracks continue to grow in length and width then it is sign for trouble. It means the member is over stressed. The location of cracks will indicate what the failure mode of structural member is. In beams, for example, there can be mainly two types of cracks i.e. shear cracks, or flexural cracks as shown in the figure below.

If these types of cracks appear in a beam then it will need retrofitting in the form of jacketing, or load on the beam may have to be reduced to reduce the stress within allowable limits. Shear failure of beam can be brittle and without any warning, while if the beam is correctly designed then flexural failure should be ductile.

Beam may exhibit compression failure cracks which are due to incorrect design of beam. If beam is over reinforced, then this type of failure will occur. Cracks will appear in the compression zone of beam accompanied by crushing of concrete.

Repair of such cracks may be done by providing additional compression reinforcement such that balanced design is achieved, and jacketing of beam.

There can also be another type of cracking in structure which is caused by rusting of reinforcement. These are horizontal in direction and can be on the sides or bottom/top of the Beam or underside of Slab. Their depth will be up to the concrete cover. If this is the case then concrete cover needs to be removed and rust must be removed from all around the reinforcement, then member should be repaired by applying rust inhibitor and polymer or cement based repair material or epoxy coating.

Another type of cracking can be due to Alkali-Silica Reaction (ASR). This happens when aggregates contain some minerals which react with alkalies present in concrete. Aggregates should be tested for ASR, or low alkali cement should be used if use of reactive aggregates is unavoidable. These take the form of alligator cracks with appearance of white bi-product of reaction in the cracks as shown below:

These types of cracks are most difficult to deal with. The alkali-silica reaction will continue as long as there is alkali and reactive silica present in cement, and as long as they are in contact with each other. Once any of these conditions cease to exist the reaction will stop. It might be the case that although initially cracks appearing in the structure are very few but potential for ASR to continue is large, which will mean that cracking will continue and eventually become large. Hence, depending upon the extent of damage, and potential for ASR to continue, repair work can be carried out by removing the damaged concrete and jacketing the member. In worst case, structural member may have to be demolished and re-built.