Plastic shrinkage cracks form while the concrete is still soft and finishable. These cracks appear on horizontal exposed surface (top of slab/flooring/raft).
When do plastic shrinkage cracks start appearing?
The plastic shrinkage cracks appear in the surface of concrete within a few hours of placing, sometimes very soon after screeding and frequently before trowelling.
What are the causes for plastic shrinkage cracks?
Plastic shrinkage cracking occurs when fresh concrete is subjected to a very rapid loss of moisture. It is caused by a combination of factors, such as
- Air and concrete temperature
- relative humidity
- and wind velocity
These factors can cause high rates of evaporation in either hot or cold weathers.
How plastic shrinkage cracks are formed?
Plastic shrinkage cracks occur when the rate of evaporation of moisture from the surface exceeds the rate at which moisture is being supplied to it. Moisture is supplied to the surface by ‘bleed water’.
What is the pattern, length and depth of the plastic shrinkage cracks?
Cracks may form a random pattern or may appear parallel to one another. They range in length from 25 mm to 2 meters, but are usually 300 to 600 mm long. They are spaced from a few centimeters to as much as 3m apart. Plastic shrinkage cracks begin as shallow cracks, but can become deeper if not attended in time.
Are plastic shrinkage cracks detrimental to the strength of the structural element?
Plastic shrinkage cracks are unsightly, but rarely impair the strength of the structural element. It is best to avoid plastic shrinkage cracks forming or to identify that they have formed while the concrete is still plastic and to rework the surface to close the cracks using a wooden float.
What are the remedial measures to be adopted to avoid plastic shrinkage cracks?
Plastic shrinkage can be avoided/minimized by adopting the following remedial measures:
The practical measures include:
- Proper finishing operations, viz., floating and trowelling- Delayedfinishing
- Proper curing procedure. Light sprinkling of water after 30 minutes of finishing the concrete surface.
- Covering finished surfaces with polythene sheet or wet gunny bags within 10 to 20 minutes.
- The use of sprayed curing compounds/fog-spray immediately after finishing.
- Provision of windbreaks to reduce wind velocity.
- Areas where ambient temperatures and wind velocity are high, concrete operations can be planned in the evening or atnight.
How to repair plastic shrinkage cracks once the concrete has hardened?
Once the concrete has hardened, there is little that can be done. If the cracks had not penetrated to the level of the reinforcement and are also not wide at the surface, it is better to tolerate them. But if the cracks had penetrated to the level of the reinforcement and deeper and are more than 2 mm wide, then they have to be repaired. The cracks have to be chased out and filled using a suitable epoxy grout/filler.
What are Plastic Settlement Cracks?
Plastic settlement cracks form while the concrete is still soft and vibratable. The plastic settlement cracks tend to have a regular pattern and occur over reinforcement, particularly in deep members. They may also occur over steps in formwork, such as in a ribbed floor.
When do plastic settlement cracks start appearing?
They may become visible very early even while finishing is proceeding, but are often not noticed until some hours after placement.
How plastic settlement cracks are formed?
After concrete is placed, it bleeds. If there is no restraint this merely produces a slight lowering of the concrete surface. However, if the concrete is locally restrained from settling (eg. by a reinforcing bar, hollow core, duct or insert) while the adjacent concrete continues to settle, there is the potential for a crack to form over the restraining element The amount of settlement tends to be proportional to the depth of concrete. The deeper the section the greater the chances for settlement.
How to distinguish between plastic shrinkage and plastic settlement cracks?
They are different from plastic shrinkage cracks by their distinct pattern which typically mirrors the pattern of the restraining elements such as the reinforcement. They tend to roughly follow the restraining element like reinforcing bars, or changes in the concrete section.
What is the pattern, length and depth of the plastic shrinkage cracks?
They can be quite wide at the surface, tend to extend only to the reinforcement or other restraining element and taper in width to thatlocation.
Are plastic shrinkage cracks detrimental to the strength of the structural element?
In exposed situations, plastic settlement cracks may increase the risk of corrosion of the reinforcement and pose a threat to durability.
What are the practices to be followed to minimise the risk of plastic settlement cracking?
Concrete
- Use mixes with lower bleeding characteristics.
- Increase the ratio of cover to reinforcing bardiameter. This is done by increasing the cover or decreasing the size of thebars.
Construction
- Wet the subgrade before placing concrete to avoid excessive water loss from the base of the concrete.
- Set all formwork accurately and rigidly so that it will not move during concrete placement.
- Place concrete in deep sections first (including columns) and let it settle prior to placing and compacting the top layers. Vibration of subsequent layers should penetrate the previous layer (ensuring that the two layers blend together).
- Fully compact the concrete.
- Cure the concrete promptly and properly.
How to repair plastic settlement cracks?
- The most effective repair is to close the cracks shortly after formation by re-vibration and reworking the surface while the concrete is still plastic.
- Careful timing is essential to ensure the concrete re-liquefies under the action of the vibrator so that the cracks are fully closed.
- Re-vibrate too soon and cracks may reform;re-vibrate too late and the bond to the reinforcement may be damaged.
- Mechanical re-trowelling of the surface may be sufficient to close the cracks and compact the concrete around the reinforcement provided the cover is not too great, but the best result is where this is combined with some form of vibration.
- Caution needs to be exercised in the use ofre-trowelling alone since it may just form a skin (which can fracture with subsequent shrinkage, thermal or traffic impacts) over the cracks but not close them. If used it must be done as soon as the cracks becomeevident.
Author
About Author – Author’s areas of research include Concrete Technology, Non- Destructive testing of RC structures and Repair and Rehabilitation of RC structures. He is involved in more than 150 consultancy projects for various Government, Public Sector and Private agencies, including Defense and Nuclear departments.
