Why do they Formed? What  are the reasons? 

How do I minimize it?



PLASTIC SHRINKAGE CRACKS 


1. These cracks are visible on the surface of fresh concrete immediately after it is poured and while it is still in a plastic stage.

Plastic shrinkage cracks, cracks, shrinkage cracks
Plastic Shrinkage Cracks

 2. These cracks are most likely appeared on the slabs.

3. They are usually random to each other and making shape of a tree.

4. They will usually start appearing from 30 minutes to six hours.

5. These cracks do not intersect the perimeter of the slab.




THEIR FORMATION & REASONS

1. They most likely formed, when HIGH EVAPORATION rates cause the concrete surface to dry out before it has set. The water on the surface evaporate rate become faster due to hot weather.

2. The WATER present inside the surface of the concrete, means present in the concrete not on the concrete, start making TENSILE FORCES between the particles of cement and aggregate develop in the CONCRETE Surface layers.

3. Now, in the above situation, if CONCRETE Surface starts to harden and develops the enough TENSILE Strength to bear these TENSILE Forces, then no cracks will be formed.

4. But, in case the surface of the concrete dries quickly then these Tensile Forces are responsible for the Plastic Shrinkage Cracks on the concrete surface.


INFLUENCE THE STRENGTH OF CONCRETE?

These cracks are ugly and rarely ruin the strength and long life of concrete floors and pavements.


HOW TO MINIMIZE THESE CRACKS?

  • To avoid the plastic shrinkage cracks, it is very important to know the weather condition before to pour the concrete. High temperature or windy weather may increase the rate of evaporation rate which can be a caused of cracks.
  • When adverse weather exists, make a temporary wind break arrangement to reduce the wind velocity over the surface of concrete. In case, if there is more sun then provide sun shades to control the temperature of surface.
  • If arrangements cannot be done, then schedule the casting later afternoon or early evening. During the summer days when temperature is very high then try to plan the pour in the early morning to have better control on concrete temperatures.
  • In a very hot and dry days, use of Fog Sprays to deliver the mist into the air above the concrete surface which reduces the rate of evaporation.
  • The role of sub grade is also very important, we need to ensure that the subgrade should be dampened, not freestanding water on form work. The reinforcement and shuttering both should also be dampened.
  • Sufficient manpower with equipment should be arranged in advance so that concrete should be placed and finished promptly. Cover the concrete with moisture-retaining covering if there is a delay in concrete placing.
  • Spraying evaporation retardant after the screeding  process and then troweling can be done to avoid cracks.
  • Start curing the concrete surface by spraying the water, cover the surface with wet burlap, or by spreading the curing compound.
  • If plastic shrinkage cracks should appear during final finishing, the masons can close all these by refinishing it.






Tricks / Hints to remove these cracks

  • Wet the sub-grade and shuttering when high evaporation rate is expected.
  • Provide Fog sprays and wind breakers to avoid loss of moist from surface.
  • Cover the concrete surface with wet jute bags.
  • Start curing as soon as finishing work is completed.