Concrete cracking

Causes of cracking

It has long been recognized that the main reason for concrete cracking or delamination in reinforced concrete structures is due to the stresses caused by restrained shrinkage and/or movement. At any time, whether in fresh or hardened concrete, if the tensile stress builds up to pass the tensile strength of concrete, cracking occurs to relieve the tensile stress build up. Strain release can be both on a large (macro) and small (micro) scale and is not limited to certain areas, direction or causes.

Thus, if the tensile stresses do not reach the tensile strength, the concrete will not need to release the strain and will not crack. Early cracking can be classified in two time frames or time of appearance:

• 10 mins to 3 hours
• 30 mins to 6 hours

Early cracking is mostly due to plastic settlement followed by plastic shrinkage cracks. Both forms of cracking are present within the early stages of casting concrete. It occurs before the concrete completely sets or hardens, normally within the first half hour to six hours of placing and compaction. 

Plastic settlementPlastic settlement

Plastic settlement occurs after the concrete has been compacted, where the tendency is for the solid/larger particles in the concrete matrix to settle. Bleeding water is a result of the settling mixing water being displaced to the concrete surface. Cracking, due to settlement, will continue until the concrete stiffens enough to refrain this movement.

If there is no restraint, settlement rarely causes any problems or cracks. Restraint in concrete can be induced by the following:

• A change in sectional depth such as a junction between a web and a flange in a T-beam or coffer slab.
• Top reinforcement in beams or slabs
• Links or stirrups in columns
  Form ties
• Bridging of coarse aggregate between form face and narrow sections

In horizontal reinforcing, settlement directly above the steel is far less than below the steel, thus plastic settlement cracks often form immediately in the vicinity of the reinforcing steel. On flat slabs the crack pattern may virtually map the positioning of the reinforcing or top steel. Because plastic settlement cracking is associated with bleeding, decreasing the bleed potential will reduce the amount of settlement. Revibrating workable concrete will recompact the concrete and overcome settlement cracking.

In horizontal reinforcing, settlement directly above the steel is far less than below the steel, thus plastic settlement cracks often form immediately in the vicinity of the reinforcing steel. On flat slabs the crack pattern may virtually map the positioning of the reinforcing or top steel. Because plastic settlement cracking is associated with bleeding, decreasing the bleed potential will reduce the amount of settlement. Revibrating workable concrete will recompact the concrete and overcome settlement cracking.Plastic Shrinkage

Plastic Shrinkage is not confined to slabs, can also occur in both reinforced and unreinforced horizontal slabs and pavements. The principal cause of these cracks are rapid removal of water from the concrete matrix. Cracking in concrete slabs occur when the exposed surface’s rate of evaporation exceeds the amount of bleed water. Surface water is lost and leads to a decrease in volume of the placed concrete. The change in volume causes the concrete to shrink. Water evaporation would increase due to the following:

• High concrete temperatures
• High ambient temperatures
• Lower ambient relative humidity
• High wind velocities

Water loss can be due to dry subgrade or absorbent formwork in direct contact with the concrete. The aggravated water loss and the restraint shrinkage by the subgrade; formwork or reinforcing will increase the likelihood of plastic shrinkage cracking.

Typically plastic shrinkage cracks are roughly straight but discontinuous and parallel and can be spaced from 0.3m to 2m apart. They normally do not extend to the edge of the slab but can penetrate through the depth.

They could appear within 2 hours after compaction but are most commonly noticed the following day. If the rate of evaporation exceeds 0.5 kg /m²/hr, exposed concrete surfaces should be covered and kept moist.

Drying shrinkage cracking is not limited to warm weather but can also occur in cool weather with high wind velocities.

References:

• Goodman, J.  Mixing Water. Fulton’s Concrete Technology 9th Ed. Midrand: The Concrete Institute, 2009. Properties of fresh concrete, Page 91 – 92.
• Newman, J; Ban Seng Coo. Advanced Concrete Technology – Concrete Properties. Elsevier, Butterworth Heinemann; 2003. Plastic and thermal cracking, Page 2/3 – 2/17.
• Newman, J; Ban Seng Coo. Advanced Concrete Technology – Testing and Quality. Elsevier, Butterworth Heinemann; 2003. Diagnosis, inspection, testing and repair of reinforced concrete structures, Page 6/4 – 6/11.

Compiled and written by Jurgens Nel