To avoid stress cracking, make sure that the slab is built on a uniformly compacted and well-drained subgrade and that it is thick enough to withstand the type of use it will receive. In residential concrete, 4 inches is the minimum thickness for walkways and patios. One of the most common questions received at ConcreteNetwork, Com concerns the cracks that are developing in freshly poured concrete. The landlord will wonder why it is cracking and if they received a shoddy job.
Plastic shrinkage cracks occur when wind speed, low relative humidity, high ambient temperature, or a combination of the three, cause water to evaporate from a concrete surface faster than can be replaced by bleeding to the surface. The rapid evaporation caused by this cracking can be controlled by windbreaks, shading and surface treatments. Vertical cracks can be caused by the settlement of concrete around the reinforcing bars. Such cracks occur when the concrete near the surface takes a partial setting, while the rest of the concrete can still settle.
Cracking of this type can be identified by a pattern that follows the reinforcement. Therefore, cracks can often be prevented by revibration of concrete. Inclined cracks sometimes appear when coarse particles of aggregate near the surface form a skeleton in which the cement paste can settle and separate. Once again, vibration and revibration will reduce or prevent cracking.
Relatively small movements of formwork in the early stages of hardening will lead to cracks. Swelling or bulging of wood, nail jumping, clumsy or excessive use of vibrators are the common causes of movement of the shape that irregular cracks occur. Wide cracks, which sometimes extend through the slab, occur when the subgrade decreases before the concrete. This can happen because the subgrade is not level and well compacted, or is muddy or otherwise unstable.
Shrinkage cracks cannot always be prevented, but can be controlled by making planes of weakness to establish the direction of cracking when shrinkage occurs. This is done by cutting grooves one-third of the thickness of the slabs, and is done as soon as the concrete is hard enough to withstand the damage of the saw. Cracking, if it occurs, will then occur perfectly below the groove line. If the cut is left much later than the mentioned time point, cracks may appear on the surface suddenly, deviating from the sawing line).
Shrinkage grooves must be cut at logical stress points on the roads, at intervals of ½ to 2 meters (light roads can be marked with a double edger); on roadways or large floors, at intervals of 5 to 7 meters. When the floor is divided into openings by pillars, the joints can be placed from column to column for a better appearance (see illustration). However, they should not be more than 7 meters apart, unless the slabs contain substantial steel reinforcement. Expansion joints, or insulation joints, usually consist of pre-molded tar material 10 mm thick, as deep as the slab and as long as the slab.
They should be placed where paths or driveways meet, and where the slabs meet the base of a column, wall, or any other mass that resists expansion in that direction (see illustration). For a crack control joint to be effective, it must be ¼ deep as the slab is thick. That is, in a typical 100 mm thick slab, the joints should not be less than 25 mm deep; a 150 mm thick slab would require joints 38 mm deep, etc. To minimize the chances of early random cracks, these joints should be placed as soon as possible after pouring the concrete.
If the control joint is not deep enough, the concrete may crack near it instead of inside it (see figure. A good rule of thumb for 100 mm thick residential concrete is to place joints so that they separate the slab into approximately equal square sections, without any joint being more than about 3 meters from the nearest parallel joint. Control joints are shrink joints because they open as concrete shrinks or gets smaller. For concrete floors that require repair, a layer of concrete means tackling repairs on the substrate and covering it with a cement-based layer that does the following;.
If there is something that holds the concrete in place, such as reinforcement, then it will crack around that. Today, Holcim continues to supply essential building materials including aggregates, sand, ready-mix concrete, engineered precast concrete and prestressed concrete solutions throughout Australia. If you use a saw to cut the joints, the cuts should be made as soon as the concrete can withstand sawing without fraying the edges of the saw cut. While concrete is incredibly durable, it is also prone to wear and tear that will manifest as cracks, chips, and even stains over time.
However, given the process involved and the technical aspects of the surface conditions and the choice of the right concrete layer for the best result, it makes more sense for a professional to do the project for you. The covers must be kept continuously moist and a film of water should remain on the surface of the concrete throughout the curing period. Shrink joints can be formed with preformed strips of plastic or chipboard, saw cuts or machined into the plastic concrete. Often, this settlement will cause concrete to crack and lean towards the house, creating a negative slope (see figure.
Read here about methods to cure concrete and understand how your contractor will cure concrete. For smaller projects, such as concrete vases and other decorative items, you can add wire mesh during pouring to improve strength and reduce cracking. While it takes 28 days for concrete to fully cure, the steps you take in the first few days after pouring are the most important to ensure a strong, crack-free surface. .