Precautions to minimize Plastic Shrinkage Cracks:

Pravin Wakhure

Staff member
Sep 7, 2017
6
13
Pune
Precautions to minimize Plastic Shrinkage Cracks:


The Plastic Shrinkage cracks occur on the surface of freshly placed concrete during finishing or soon after.
In plastic concrete bleed water surfaces due to gravity. If the accompanying settlement is restricted by formwork or reinforcement , cracking may occur. This type of cracking is popularly known as Plastic Settlement Cracking.
Both of types of cracks occur when the rate of evaporation of surface moisture exceeds the rate at which bleed water is rising through the concrete. It is obvious that the water in the mix will travel to the surface due to a known phenomenon of “Capillary Action” and as soon as it reaches to surface it will dried off due to factors like heat, wind and low humidity and we can see the cracks on surface. The most important thing to reduce the evaporation of water from the mix.
Key Factors causing plastic shrinkage cracks:
a. High Winds produce rapid surface evaporation; an increase in wind speed from 0 to 10 mph will increase the rate of evaporation 4 times.
b. High Slump Concrete increases shrinkage. Excess water can be expected to increase shrinkage approximately in proportion to its percentage of the total mix water.
c. High Air & Concrete Temperatures increase rate of evaporation and set, a 20 degree temperature increase can double the rate of evaporation. In our case that in Pune the temperature gradient is 15 to 20 degree celcius.
d.Low Relative Humidity can increase rate of evaporation by five times.
e.Dry Sub grade in hot weather can absorb too much water from slab.
We suggest following required procedures to be followed at site for reduction of plastic / shrinkage cracks.
  1. Dampen form-work, sub grade and reinforcement.
  2. Sprinkling of water onto a concrete slab that has lost its initial sheen of bleed water is perhaps the best method of curing as long as all concrete is covered with water.
  3. Early curing can be done by means of mist – spray to compensate the loss of surface moisture. Early curing is of the essence to avoid plastic shrinkage cracking in concrete.
  4. Polythene sheeting should be placed on a slab as soon as the concrete is hard enough to prevent surface damage or marking.
  5. Exposed edges and corners should be well fixed down to avoid wind and draughts drying the concrete surface.
  6. The use of Hessian, kept damp by regular spraying, can be an effective method of curing. This method of curing has disadvantage that the hessain must be kept wet at all times which entails constant monitoring. Dry areas will do more harm than not curing at all. Therefore, this method of curing should be used only when it can be monitored for 24 hrs a day.
  7. Polythene sheets can also be used to cover the slab. It is also an effective method of curing.
  8. Retamping and finishing the cracks where necessary.
  9. Use of fibers in concrete.
 

HockeyPuck

Active Member
Feb 3, 2019
119
90
Chicago, Illinois (USA)
Just a quick comment on:
8. Retamping and finishing the cracks where necessary.
If you are at the point where you are seeing shrinkage cracks developing before you've gotten your curing measures established, you might have a bigger problem on your hands. When newly-placed, un-set concrete is showing signs of early cracking, there is a high likelihood that your concrete is "Flash Setting" on you,which means it is drying out too rapidly. Smoothing out visible cracks on the surface might help in closing up the surface and making it look like the issue has been solved, but, there's a high likelihood that the cracking might be deeper in the mass whereby you are simply masking the problem by just closing the surface - Don't be surprised if the cracks redevelop as the concrete ages..... Cheers!!!
 
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