1- DRYING SHRINKAGE / RESTRAINED SHRINKAGE CRACKS
Hardening concrete expands slightly with a gain in moisture and contracts with a loss in moisture. As drying takes place, concrete shrinks. Where there is no restraint, movement occurs freely and no stresses or cracks develop. If the tensile stress that results from
restrained drying shrinkage exceeds the tensile strength of the concrete, cracks can develop. Random cracks may develop if joints are not properly provided or there are no joints as in tunnel-formwork system and the concrete element is restrained from shortening. Shrinkage may continue for a number of years, depending on the size and shape of the concrete mass. Higher surface-to-volume ratios in concrete elements, considering shear walls in this project, show higher shrinkage. Those cracks may start to occur from holes and corners where stress concentration is high and from some certain details where sudden section changes and irregularities in the elements take place.
These cracks are independent from insufficient compressive strengths of concrete. Adversely, more drying/restrained shrinkage cracks could be occurred in high concrete classes because of the brittle behavior of concrete. Thus, high strength concrete is more crack-sensitive.
These type of cracks can also be seen in basement shear walls restricted by rigid columns without proper jointing in any other building constructed with conventional system or in jointless walls of swimming pools or water deposits. The similar principles is also valid in industrial concrete floor applications.
FACTORS AFFECT DRYING SHRINKAGE / RESTRAINED SHRINKAGE CRACKS
•The most important controllable factor affecting shrinkage is the amount of water per unit volume of concrete. Shrinkage can be minimized by keeping the water content of concrete as low as possible.
•One of the most important factors is cement content. The more cement is used in the concrete, more shrinkage cracks are occurred.
•Shrinkage can also be minimized by keeping the total coarse aggregate content of the concrete as high as possible. High fine aggregate contents, or use of small-size coarse aggregate, will increase shrinkage. Aggregates in concrete, especially coarse aggregate, physically restrain the shrinkage of hydrating cement paste. Drying shrinkage is also dependent on the type of aggregate. Hard, rigid aggregates are difficult to compress and provide more restraint to shrinkage than softer, less rigid aggregates. As an extreme example, if steel balls were substituted for ordinary coarse aggregate, shrinkage would be reduced.
•The amount and type of curing can affect the rate and ultimate amount of drying shrinkage. Curing compounds, sealers, and coatings can trap free moisture in the concrete for long periods of time, resulting in delayed shrinkage. Wet curing methods, such as fogging or wet burlap, hold off shrinkage until curing is terminated, after which the concrete dries and shrinks at a normal rate.
2- PLASTIC SHRINKAGE CRACKS
Plastic shrinkage cracks appear in the surface of fresh concrete soon after it is placed and when it is still plastic. Plastic shrinkage cracks are caused by rapidly evaporating surface moisture. While some evaporation occurs all the time, its rate is increased by high ambient air temperature, elevated fresh concrete temperature, low humidity, and strong winds.
All four of these factors are in full force on warm summer days. Evaporation may be so strong, it actually pulls moisture from the concrete. Cracks develop when the upper concrete layer hardens before all the excess water from the fresh concrete can reach the surface. The pull of the moisture is stronger than the concrete, resulting in tensile stresses which become cracks.
FACTORS AFFECT PLASTIC SHRINKAGE CRACKS
•High wind velocity increases plastic shrinkage cracks.
•Low relative humidity increases plastic shrinkage cracks.
•High air temperature increases plastic shrinkage cracks.
•High concrete temperature increases plastic shrinkage cracks.
•Low water content and lower water/cement ratio in concrete decrease such cracks.
•Curing conditions are very effective on plastic shrinkage cracks. Inadequate curing may not prevent cracks.
•Evaporation from the concrete surface increases with increasing ambient temperature. Plastic shrinkage crack amount depends to evaporation speed of water.
3- SETTLEMENT CRACKS
Settlement cracks generally occur on the surface of slabs, beams, foundations or any other reinforced elements. They can be seen just over the reinforcement within concrete cover. There’s two type of movement in fresh concrete. When coarse aggregates move to deep of the section, water bleeds and tries to go over the surface. Reinforcement bars which are close to surface resist to this movement and then concrete cover which is not settled yet loses its tensile strength and crack develops. Thus, all cracks can be seen on the surface along the reinforcement bars close to surface.
FACTORS AFFECT SETTLEMENT SHRINKAGE CRACKS
•High fine aggregate content and high cement content reduces settlement cracks.
•Sufficient vibration and good compaction prevents settlement cracks.
•Settlement cracks can be reduced by keeping the water content of concrete as low as possible.