Shrinkage is one of the main causes of cracking. As concrete hardens and dries, it shrinks. This is due to the evaporation of excess mixing water. The wetter the concrete mix, the greater the shrinkage.
Excess water in the concrete mix can also increase the likelihood of cracking. When placing concrete, avoid adding additional water to the mix. Excess water will evaporate from the concrete, leading to increased shrinkage. Be sure to choose the right concrete mix for your project.
When a service pipe hole is drilled through the concrete, the coring process can cause damage to the concrete structure by creating cracks. The core hole can often provide a direct path for water to pass through the concrete structure. Plastic shrinkage cracks Probably the most common reason for early cracks in concrete is plastic shrinkage. When concrete is still in its plastic state (before hardening), it is full of water.
This water takes up space and makes the slab a certain size. As the slab loses moisture during curing, it becomes slightly smaller. As concrete is a very stiff material, this shrinkage creates stresses in the concrete slab. As the concrete shrinks, it is dragged along its granular sub-base.
This impediment to its free movement creates a stress that can literally pull the slab apart. When the stress is too great for the hardened concrete, the slab cracks to relieve the stress. Especially in hot climates, shrinkage cracks can appear as soon as a few hours after the slab has been poured and finished. Like a balloon, heat causes concrete to expand.
As the concrete expands, it pushes against anything in its path (a brick wall or adjacent slab, for example). When neither has the ability to flex, the force of expansion can be enough to cause the concrete to crack. Cracks that are identified as small and hairline (less than 0.3 mm wide) are generally considered acceptable as part of a minor settlement. V-cutting along the crack line and then sealing it with the appropriate sealing material will allow movement and protect against further crack expansion and possible water ingress.
This is achieved by properly preparing the subbase, ensuring that the concrete is not too wet, using reinforcement where necessary, and properly placing and spacing crack control joints and expansion joints. This repair option is used to stop water leaks and involves injecting a highly water-reactive resin into cracks under pressure. If concrete is placed on top of a poorly compacted trench, the void created by the subsidence can cause a crack along the unsupported concrete slab (see figure. Reduced alkalinity of the concrete, either by carbonation or by the ingress of chlorides (salts), are the most common causes of concrete corrosion.
In most cases where cracks appear in concrete, it is possible to identify the crack and establish the cause of cracking. One of the most common mistakes made by DIYers starting out in the concrete world is to add too much water to the dry concrete mix to facilitate mixing, which leads to weak concrete and a high risk of cracking. Cracks in concrete can range from non-structural and unsightly, to detrimental to the structural integrity and safety of a building. However, a qualified and licensed professional should be consulted to classify the severity of the crack and properly investigate the cause of the crack and determine the most effective repair method if necessary.
Do not overdig and backfill with a few centimetres of soil; the soil that is put back in will eventually settle and cause the slab to crack. Apart from appearance, crazing cracks do not usually affect the structural integrity of the concrete, but can lead to further deterioration. Unless the structure in question allows movement of its members without excessive stresses, extensive cracking can often occur. Controlling the evaporation rate of the drying surface is the main key to avoiding plastic shrinkage cracking.
Careful timing is essential to ensure that the concrete re-sets under the action of the vibrator and the cracks close completely.