Hairline cracks are commonly observed in freshly placed concrete and their occurrence is owed to the phenomenon of plastic shrinkage. As the name implies, these cracks are very small- about 0.003 inches (0.08mm) in width and can be very shallow.
Causes of Hairline Cracks in Concrete
The main cause behind the generation of hairline cracks in concrete is plastic shrinkage which is the rapid depletion of moisture from the fresh concrete within its plastic state.
The causes for these cracks to show up can be due to the variations in air temperature, concrete temperature, relative humidity and wind velocity at the surface of the concrete. These factors can cause high rates of evaporation in either hot or cold weather.
The other factors that accelerate the chances of hairline cracks are:
- Water loss due to absorption by subgrade, formwork or aggregate.
- Bleeding and sedimentation
- Poor workmanship and negligence
- Alkali-Aggregate reactions.
- Type and content of Portland cement and supplementary cementitious materials (fly ash, silica fume,etc.)
- Type and dosage rate of chemical admixtures.
- Construction operations like screeding and finishing operations.
Prevention of Hairline Cracks in Concrete
There are many methods and techniques to prevent this type of crack in case of rapid loss of moisture due to hot weatherand dry winds.
- Use of fog nozzles to saturate the air above the surface.
- Use of plastic sheeting to cover the surface between the final finishing operations.
- Use of to reduce the , sunshades to reduce the surface temperature are also helpful.
- Dampen the subgrade and formwork, any excess water is removed prior to placing concrete
- Applying a sprayable evaporation retardant
- Use of microfibers in the concrete mixture the tensile capacity of the plastic concrete.
- Starting of curing soon as possible after finishing.
Repair of Hairline Cracks in Concrete
The steps involved in the repair of hairline cracks in concrete are as follows:
1. Cleaning of Crack
This is the first and foremost step in the repair of hairline cracks in the concrete. Inspection of the crack is done and if any oil or stains have penetrated the crack and have discoloured the concrete, the stains will have to be addressed before repairing the crack.
2. Selection of Sealant
The sealant is a filler material that is penetrated into the crack. The ideal sealant must have low surface tension and low viscosity. These two characteristics allow the liquid sealer to penetrate deeply into the crack.
Crack repair materials for concrete are often polymer-modified cement grouts, silicone or latex caulking, or epoxies. Polymer-modified cement grout is the typical material used to fill pinholes and voids in concrete during manufacture. If cracks occur before the concrete is sealed, this is often the material used to repair them.
3. Application of Sealant
The selected sealer is applied on the substrate by using a squeegee and/or in combination with a roller. The application process must be carried out for at least 5-10 mins to ensure penetration.
Proper curing must be ensured to complete hardening of sealant. The warmer the temperature, faster the curing, lower the temperature, the longer it will take.
Repairing hairline cracks can be challenging because making the cracks vanish takes skill, forbearance, practice and a good eye for colour matching. One definition of a hairline crack is a crack that hasn't opened up.
That means there is very little room to fill it with anything but a very low-viscosity liquid. Since most hairline cracks don't affect the structural integrity of a countertop, sealing and concealing the crack are the main goals of a repair.