Cracking of hardened concrete

When working with concrete, the ease of placement is of great importance, and therefore adequate workability and consolidation of concrete often results in more water being added than needed for the hydration of cement. As the needed water is used in the concrete matrix as it hardens, it results in a reduction of volume, known as shrinkage. When the volume reduction occurs while the concrete is in the initial stages of setting, still fresh, it is known as plastic shrinkage. When volume reduction takes place after hardening primarily due to moisture loss, it is known as drying shrinkage. 

DRYING SHRINKAGE

Drying Shrinkage occurs due to the reduction of volume caused by loss of water, which occurs both physically (evaporation) and chemically (hydration). Physical loss of water due to evaporation does not affect the homogenous nature of fresh concrete. Thus, the structure of the mass is not affected and shrinkage up to this stage is acceptable. The amount and rate of water loss by evaporation becomes important only after the hardening process. Drying shrinkage cracks has no typical pattern and tend to form at right angles to the direction of restraint of shrinkage movement. Because the cracks can form after the concrete has gained designed strength, these cracks can pass through weak aggregate.

Loss of water by chemical reaction in the hardening process is more complex. Water and cement becomes a solid through the chemical process known as hydration. This does not lead to a loss of mass but is however accompanied by a change in volume. Water loses about 25 percent of its volume when reacting with the different elements in the concrete matrix. In wet mixes, chemical shrinkage is generally far less important than physical shrinkage, whereas on dry mixes chemical shrinkage becomes increasingly more important. In severe cases it may be necessary to reduce or limit evaporation, or having to replace the water used up in the hydration process.

CRAZING

Crazing is the development of a network of fine cracks or fissures (spider web cracks) on the surface of concrete or mortar. The cracks do not exceed more than 3 mm and are likely to be noticed on steel troweled surfaces. Crazing can also be referred to as shallow maps or pattern cracking. These cracks develop at an early age and are apparent the day after placement and will continue for at least the first week. The cracks will normally be more visible once the surface has been saturated and begins to dry.

Although crazing does not affect the structural integrity, it negatively impacts the aesthetics. Cracking occurs because the shrinking surface is restricted by the sub layer.  Cracks are normally caused by:

• Poor or inadequate curing. 
• Intermittent curing, where the concrete surface dries out before it is saturated again.
• Excessive floating, when the cement paste accumulates on the surface leading to a crust to form.
• Wrongful application of a dry-shake.
•  Floating finish was done too early, while bleed water was still present on the concrete surface.