The paper explains the basic types of shrinkage: carbonation shrinkage, plastic shrinkage, temperature shrinkage, chemical shrinkage, autogenous shrinkage, and drying shrinkage.
Thermal contraction is the "shrinking" of substances as they cool. Changes in temperature change the physical properties of substances- each substance does this on a different level, but with changes in temperature, substances either expand or contract. This is due to their energy levels on an atomic level.
Shrinkage of concrete, drying shrinkage in particular, is inevitable; and because of restraint, cracking can occur. However, with good concreting and construction practices, shrinkage and subsequent cracking can be minimized.
Carbonation shrinkage occurs when the concrete is exposed to air containing carbon dioxide. When the hardened concrete is exposed to this type of air, the weight increases, and the concrete undergoes irreversible carbonation shrinkage.
the consumption of free water by chemical reaction so as to leave insufficient water to cover the solid surfaces and cause a decrease in the relative humidity of the system.
Factors Affecting Shrinkage in Concrete:
- Cement Content: As a general rule, the increase in the richness of concrete mix which leads to drying shrinkage.
- Water Content:
- Aggregate:
- Curing:
- Presence of Excessive Fines in Aggregates:
- Chemical Composition of Cement:
- Temperature:
- Humidity:
Factors Affecting Shrinkage of Concrete
- Humidity (Drying Condition)
- Water Cement Ratio.
- Hardness of Aggregates.
- Moisture Movement in Concrete.
- Type of Coarse Aggregates.
- Shape of Aggregates.
Drying shrinkage happens mostly because of the reduction of capillary water by evaporation and the water in the cement paste. The higher amount of water in the fresh concrete, the greater the drying shrinkage affects. Concrete shrinkage occurs mostly due to the evaporation of the mixing capillary water.
Low shrinkage or shrinkage-compensating concrete is high performance concrete that makes use of a controlled expansive chemical reaction to compensate for drying shrinkage and minimize cracks due to internal stresses.
Drying shrinkage (often, simply shrinkage) is the reduction in volume of hardened concrete due to loss of moisture by evaporation. In shrinkage, the loss is due to difference in the relative humidity of concrete and the environment, in creep it is due to sustained applied stress.
Concrete slabs can shrink as much as 1/2 inch per 100 feet. This shrinkage causes forces in the concrete which literally pull the slab apart. Cracks are the end result of these forces. The bottom line is a low water to cement ratio is the number one issue effecting concrete quality- and excess water reduces this ratio.
The mixture shrinks and hardens as it dries. Thin-set mortar is less likely to crack if it dries slowly, but if you need it to dry more quickly to avoid damage to it, you can speed up the process.
Changes in the temperature of concrete can cause cracking. As the concrete gets hotter, it expands; as concrete gets cooler, it contracts. This is similar for many other materials you have probably learned about. If the concrete wants to expand or contract but can't, it will most likely crack.
Creep Coefficient: is defined as the ratio of creep strain to elastic coefficient. Creep Coefficient = ε cr/ε E Creep and shrinkage of concrete are influenced by a large number of factors, which may be divided into intrinsic factors and extensive factors.
It is possible to seal the cracks by some such means as epoxy injection. But the usual result is to accentuate their presence. If the cracks are in fact plastic shrinkage cracks, the chances are that they won't get any worse and that the best thing you can do for their appearance is to leave them as they are.
Divide the amount of shrinkage by the original size to find the shrinkage rate. In the example, divide 2 by 8 to get 0.25. Multiply the shrinkage rate by 100 to find the shrinkage as a percentage. In the example, multiply 0.25 by 100 to get 25 percent.
Rapid drying of the surface of the plastic concrete causes it to shrink and crack, but the cracks are not always evident during finishing operations and may not be discovered until the next day. Plastic shrinkage cracks may form in a random manner or be roughly parallel to each other.
Shading aggregates, water tanks, and lines.
- Avoid overmixing.
- Place concrete early in the morning or late afternoon.
- Construct temporary walls to reduce wind velocity.
- Provide sunshades for concrete.
- Reduce time between placing and start of curing by working efficiently during construction.
Serious defects, such as water ingress or faulty heating are obviously dealt with as soon as they happen. Whilst plaster cracking when shrinking is an issues often brought up by clients, plaster is actually designed to shrink, so is not technically a defect!
Shrinkage cracks and "hairline" cracks (cracks that are less than 1/16" wide) common occurrences in poured foundations. Shrinkage cracks occur naturally in poured concrete during the curing process as the concrete is losing its moisture.
