Plastic Shrinkage Cracking

Plastic Shrinkage Cracking

Plastic shrinkage is the reduction in concrete volume which occurs when water is removed from compacted concrete before it sets. Water can be removed from plastic concrete (concrete which has not yet set) either by evaporation or by being absorbed by dry surfaces such as soil or dry concrete onto which the fresh concrete is placed. When the shrinkage is restrained either by formwork or by other concrete, the concrete can crack. This is called plastic shrinkage cracking and typically occurs in floor slabs and pavements, where water is lost from the exposed surface of the concrete. At a high rate of evaporation, the concrete at the surface loses water and decreases in volume, while the inner concrete maintains its volume. This difference in shrinkage results in the formation of cracks in the surface concrete. The rate at which water evaporates from the concrete is increased by higher concrete temperatures; higher ambient temperatures; lower ambient relative humidities and higher wind speeds.

Plastic shrinkage cracking can occur in both reinforced and unreinforced concrete and the cracks are normally straight, discontinuous, and parallel and are usually placed 0.3-2m apart. Plastic shrinkage cracks appear about two hours after concrete compaction but they are often only visible the following day. The cracks may penetrate the full depth of the concrete. Plastic shrinkage cracking can be minimised by employing activities which reduce water loss from concrete. These activities include: pre wetting the surface onto which concrete is being placed; covering the compacted concrete with plastic sheeting; using fog spray above concrete surface or by carrying out concreting work during the cooler part of the day. If cracks are noticed before the concrete sets, they can be closed by revibrating the concrete. 

References:

• Addis, B. (2008). Fundamentals of Concrete.
  Midrand, Cement and Concrete Institute.
• Kellerman, J and Crosswell, S. (2009).
  Properties of Fresh Concrete. In: Owens,
  G (ed) Fulton’s Concrete Technology. Midrand,
  Cement and Concrete Institute, pp 83-96.

Image source:https://www.researchgate.net/figure/Plastic-shrinkage-cracking-in-concrete_fig2_333880951

Compiled By: Marha Mwatile