What is plastic cracking in concrete?
This term refers to cracking which occurs while the concrete is still in a plastic state. The phenomenon is most common in hot, dry climates and can usually be attributed to settlement of the mix in the form or over-rapid drying. Depending on the mix proportions, any water added in excess of that needed for hydration of the cement may bleed from the mix after placing and compacting and appear as a film over the surface. If this bleeding is excessive and the concrete is plastic enough, the mix will then settle in the form to an amount equivalent to the volume of the displaced water. However if the mix is placed under warm, dry conditions the surface will often stiffen first and the settlement will then cause cracks to appear over any areas of restraint, such as reinforcing bars or formwork ties. If at all possible these plastic cracks should be closed by revibration of the concrete in the areas affected. Prevention is always better than cure, however, and the trouble can be avoided by using a mix designed to give minimum bleeding. This will be a low-slump concrete (not greater than 3 1/2-inch) with well graded aggregates and a high degree of workability for minimum water requirement. A modern air-entrained mix, made with high-quality aggregates, eliminates bleeding in concrete almost entirely. Over-rapid drying means, of course, that proper curing is not being practiced. In hot weather the rate of evaporation of moisture from the surface of the concrete can be excessive and plastic cracking may then take place because of the sudden volume change in the partially hardened mix. These cracks can occur at random over large areas, such as slabs or paving, but are more likely to appear around holes in flat slabs or at points where reinforcement changes section such as at laps or stop-off points. Proper curing to prevent water from being evaporated from the surface will prevent the trouble.