Cracks in concrete can be a major problem for any structure, reducing its strength, durability and aesthetic appeal. In general, a crack makes concrete and structure more vulnerable to external effects, accelerates the aging process, and can immediately reduce the mechanical strength of the structure. Last but not least, cracks reduce a structure's ability to absorb stresses and can cause collapse. 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. 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. Large tree roots can have the same effect on a slab. If a tree is located too close to a slab, growing roots can lift and crack the concrete surface. Always keep this in mind when laying a slab. There are ten ways to help maintain crack-free concrete: 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; 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; cure wet or sealed concrete starting very early; avoid extreme temperatures; protect concrete in service from changes in humidity and temperature whenever possible. If cracks do occur in plastic concrete they can be classified as plastic shrinkage cracking or plastic slump cracking.
In most cases when cracks appear in concrete, the crack can be identified and the cause of cracking established. The main concerns are whether cracks are affecting structural integrity as a result of reduced durability. The ring-shaped mortar samples reinforced with 8 mm diameter steel bar cracked with different crack widths ranging from 0.12 to 0.6.The influence of longitudinal cracks on corrosion protection provided reinforcing steel in high-performance concrete was observed. This increase in volume increases the pressure on the concrete and causes radial cracking as the concrete fails under tensile stresses. Many studies report a wide variety of results on the effect of crack width on steel corrosion in concrete. Contradictory results are also reported on the relationship between crack width and steel corrosion in concrete.
The phenomena of cracking aggravate the process of attack of carbonation chloride which leads to corrosion of reinforcement which reduces durability of structures. If underlying cause is not addressed then repair of crack may be short-term so it is necessary to go through same process again. Concrete surfaces are very vulnerable to cracking both in plastic stage (fig hardening stage) and hardened concrete. In addition shape of crack depth could also influence chloride penetration and corrosion of steel since it depends on concrete cover. This category covers performance of concrete whose shape can no longer be altered without damage including cracks caused by drying shrinkage as well as those resulting from temperature movements that take place in all materials exposed to elements.
