Concrete does not require much water to achieve maximum strength. When the soil freezes, it can sometimes rise many centimeters before thawing and re-settling. This movement of the soil caused by the freezing and thawing cycle is a huge factor contributing to the cracking of concrete. If the slab cannot move freely with the ground, the slab will crack.
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 filled with water. This water takes up space and makes the slab a certain size. As the slab loses moisture during curing, it becomes a little smaller.
Because concrete is a very rigid material, this shrinkage creates stress in the concrete slab. As concrete shrinks, it crawls through its granular sub-base. This impediment to their free movement creates a tension that can literally separate the slab. When the stress becomes too great for the now hardened concrete, the slab cracks to relieve stress.
Especially in hot weather, shrinkage cracks can occur as early as a few hours after the slab has been poured and finished. Cracks in concrete are common and develop when stresses in concrete exceed its strength. Cracks are often caused by normal shrinkage of concrete as it hardens and dries. Cracks in concrete can range from being non-structural and unsightly, to being detrimental to the structural integrity and safety of a building.
Shrinkage is one of the main causes of cracking in hardened concrete. In drying shrinkage, the volume of concrete gradually decreases and, if the component is constrained against free movement, tensile stresses develop, causing cracks. There are several causes of cracks in concrete. Cracks caused before hardening are due to construction movement, settlement shrinkage and setting shrinkage.
Cracks caused after hardening are due to chemical reactions, physical movements, thermal changes, stress concentrations, structural design and accidents. Concrete itself contains many elements that affect cracks. It has been found that the more water is used, the greater the tendency to crack, as water increases shrinkage and reduces strength. The amount of cement is also important; in general, richer concretes crack more.
The mineral composition, shape, surface texture and classification of the aggregate variously affect the required proportions, thermal coefficient, drying shrinkage, stiffness, creep and strength of concrete. Some blends can also affect cracking due to their effects on contributing factors such as hardening rate, shrinkage, and creep. There are ten ways to help maintain crack. Design the structure taking into account the degree of restriction during the drying or cooling of concrete.
Provide and support competent inspection. Use materials known to have a good service history with respect to cracking, regardless of shrinkage or other evidence in single-contributing causes. Use minimum cement content consistent with design requirements. Use the minimum water content necessary for workability; do not allow consistencies in excess of moisture.
Place concrete evenly and take into account early settling in forms, around reinforcement, on slopes and elsewhere. Cure wet or sealed concrete, starting very early. Avoid extreme temperatures. And finally (protect concrete in service from changes in humidity and temperature whenever possible, such as filling, shading or coating).
This is the main cause of cracking in hardened concrete. This cracking occurs close to restrictions due to volume changes in concrete. When concrete is exposed to moisture, it swells, and when exposed to air with relatively low humidity, it shrinks. If shrinkage could occur without the use of reinforcing bars, cracking would not occur, but in most cases, structural support requirements make this impossible.
The gradual and slow time-dependent deformation of the concrete structure under sustained loads is known as creep. It can generate excessive stress and provoke the development of cracks. These plastic shrinkage cracks are usually shallow and usually 1 to 2 mm wide, which means you cannot repair them with the injection method. Deeper sections of concrete lead to greater separation between sediment and water, so it is important to ensure that all surface restrictions are covered properly to reduce the number of cracks.
Plastic slump cracks can also occur in forms that involve a sudden change in the depth of the concrete, as it settles more in the deep sections than in the shallow ones, forcing it to crack at the change point. If the steel reinforcing bars are close to the surface and are not sufficiently covered with concrete, the concrete bends back around the restriction and cracks at the vertex. The key to successful crack repair is to understand the causes of cracks and also whether the cracks are dormant or active. The product reacts quickly with water, chasing the water present in the crack and begins to foam and expand, filling the entire crack, resulting in a strong bond with concrete and a flexible waterproof seal that prevents future water leakage.
Thermal stresses often cause cracks in mass concrete structures, the main cause of temperature differentials being the influence of the heat of hydration on the change in volume. Crack sizes range from microcracks that expose concrete to efflorescence, to larger cracks caused by external load conditions. If a tree is located too close to a concrete slab, growing roots can lift and crack concrete (see figure. The pressure causes the concrete to form cracks near the steel which over time will lead to more extensive cracking as rust builds up until the concrete begins to peel off the reinforcing steel bars (peeling of the concrete) and exposing the corroded reinforcing steel rods.
This leads to stresses greater than the tensile strength of concrete and early thermal cracks appear. Due to the alkaline nature of cement, it reacts with carbon dioxide (CO) present in the atmosphere, resulting in an appreciable increase in the volume of materials, which ultimately leads to cracking. Because concrete cannot shrink in a corner, stress will cause concrete to crack from the point of that corner (see figure. Eliminating or limiting any of these conditions eliminates or reduces corrosion of the steel reinforcement of concrete, reducing the risk of cracking.