Hairline Crack in Concrete – Causes, Repair and Prevention


Hairline cracks are formed commonly in the freshly placed concrete. These cracks are formed due to the plastic shrinkage of concrete. As the name implies, these cracks are very small about 0.003inch (0.08mm) in width and can be very shallow.

Fig 1: Hairline cracks in Concrete

Causes of Hairline Cracks in Concrete

The main causes for hairline cracks in concrete are due to the plastic shrinkage. Itis causes when the fresh concrete losses the moisture content very rapidly within its plastic state.

causes of these cracks can be various factor such as 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,

  1. Water loss due to absorption by subgrade, formwork or aggregate.
  2. Bleeding and sedimentation
  3. Poor workmanship and negligence
  4. Alkali-Aggregate reactions.
  5. Type and content of Portland cement and supplementary cementitious materials (fly ash, silica fume,etc.)
  6. Type and dosage rate of chemical admixtures
  7. Construction operations like screeding and finishing operations.

Preventation 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.

  1. Use of fog nozzles to saturate the air above
    the surface.
  2. Use of plastic sheeting to cover the surface
    between the final finishing operations.
  3. Use of wind breakers to reduce the wind
    velocity, sunshades to reduce the surface temperature are also helpful.
  4. Dampen the subgrade and formwork, ensuring
    that any excess water is removed prior to placing concrete
  5. Applying a sprayable evaporation retardant
  6. Use of microfibers in the concrete mixture to
    increase the tensile capacity of the plastic concrete.
  7. Starting of curing process
    as soon as possible after finishing.

Repair of Hair Line
Cracks in Concrete

The steps involved in the repair of hairline cracks in concrete are,

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. Applying 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 5-10mins to ensure penetration.

4. Curing

Proper curing must be ensured to complete hardening of sealant. The warmer the temperature, the faster the cure, the lower the temperature, the slower the cure.

** Precautions

Repairing hairline cracks can be challenging because making the crack disappear takes skill, patience, 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.

Read More: Causes and Types of Cracks in Masonry Buildings and their Repair Methods

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