What is the cause of a plastic shrinkage crack in RC slabs? How can we solve this poblem?

Causes and preventions:

1.            An excessive amount of entrapped air held within the concrete by a higher percentage of material passing the # 30, #50 and #100 sieves resulting in sticky or tacky concrete.

2.            Insufficient vibration during compaction that does not adequately release entrapped air or over use of vibration.

3.            Avoid placing a slab directly on polythene film or other vapor barriers. Use a 100 mm (4 inches) layer of compactable, drainable fill (not sand). A crusher run material usually graded from 38 mm to 50 mm (1 ½” to 2 “) down to rock dust, is suitable.

4.            Avoid overworking the concrete, especially with vibrating screeds, jitterbugs, or bull floats. Overwork causes aggregate to settle and bleed water and excess fines to rise.

5.            Don’t attempt to seal the surface too soon. Use a wooden bull float on non-air entertained concrete to avoid sealing. Magnesium or aluminum tools should be used on air entertained concrete.

6.            Use proper finishing techniques and proper time during and between finishing operation.

7.            Reduce evaporation over slab by using a fog spray slab cover.

8.            Avoid using air contents over 3% for interior slabs.

9.            Minimize the mix water content by maximizing the size and amount of coarse aggregate and use low shrinkage aggregate.

10.          Use the lowest amount of mix water required for work-ability.

11.          Avid Calcium Chloride admixtures.

12.          Provide contraction joints at reasonable intervals, 30 times the slab thickness

13.          Provide isolation joints to prevent restraint from adjoining elements of structure.

thanks for the invite  i agree with mustaq answer

The shrink occurs according to the moisture of the concrete became less, the vibration not enough during pour the  RC slab , also the % of the air contents and exceed of water  affect the durability of the concrete and strenght .

We can solve this problem by avoid adding admixtures hurt the  concrete such as Calcium Chloride or any materials lead to the concrete became weak, by high humidity and lower temperature help structure frame to become strong not shrink.

When a concrete's tensile strength is exceeded by an applied stress, a crack forms in the concrete. Concrete has a relatively low tensile strength compared to its compressive strength and experiences a variety of volumetric changes depending on environmental conditions, curing conditions, and applied stresses. In general, the "gel" structure of the cementitious paste in concrete undergoes swelling when it is wetted and shrinkage when it is dried. The ability of the concrete to resist cracking depends primarily on 1) the magnitude of shrinkage strains due to carbonation shrinkage and drying shrinkage, 2) the stress induced in the concrete, 3) the stress relief associated with creep and relaxation, and 4) the tensile strength

Prevention

There are several corrective procedures listed below to reduce the risk of experiencing plastic shrinkage cracks.

1. 1. Moisten subgrades and forms to prevent absorption.
   2. Dampen dry aggregates that are absorptive.
   3. Reduce the temperature of the concrete by

1. 1. Precooling aggregate with water.
   2. Cooling the cement.
   3. Using chipped ice to cool mixing water.
   4. Shading aggregates, water tanks, and lines.

1. 1. Avoid overmixing.
   2. Place concrete early in the morning or late afternoon.
   3. Construct temporary walls to reduce wind velocity.
   4. Provide sunshades for concrete.
   5. Reduce time between placing and start of curing by working efficiently during construction.
   6. Use evaporation retardant (usually polymers).
   7. Use fog sprays to keep the humidity high and the air temperature low.

Shrinkage cracks in concrete occur due to change in moisture of concrete. Most of the building materials like concrete, mortar, burnt clay bricks are porous in their structure in the form of inter-molecular space. They expand when they absorb the moisture and shrink when they dry. This is the main cause the concrete shrinks on drying. Shrinkage of concrete is an irreversible process.

It depends on the rate of evaporation from the surface of the concrete .

The surface dries up while the inner concrete is wetter. The slab should be covered with a water retaining sheet.

For the cracks, applying non shrink grout or epoxy resin seals the cracks.

I agree with Mr Mushtaq answer.

After placing concrete it tends to settle and moisture start evaporating. Rapid loss of moisture caused by a combination of factors, which include air and concrete temperatures, relative humidity and wind velocity at the surface of the concrete. This is the main cause of plastic shrinkage cracks in RC slabs. It is more prominent in hot and cold weather.

Following measures can be taken to prevent it:

1. Proper and early start of curing.

2. Use of curing compound on the surface. 

3. Dampening subgrade, sunshade, windbreaker, fog nozzle, plastic sheet to cover, evaporation retardant (e.g. aliphatic alcohol within one hour of concrete placement).

4. Avoiding congestion of rebars; rigid design of forms; proper vibrating needle penetration;  

5. The use of the lowest possible slump.