Shrinkage compensating concrete is an expansive concrete. It is expanded in an early stage before the shrinkage of the concrete takes place. The expansion of concrete will counteract the effects of concrete shrinkage.
Figure no: 1 comparing shrinkage compensating concrete with normal concrete. We can notice that the concrete size will be reduced for normal concrete, and if the concrete is constrained, tensile stresses will be developed, and concrete will crack. Concrete compensating cracks will expand, as shown in figure no:1, steel will be in tension while concrete is in compression.
We know that the concrete is excellent in resisting compressive forces. The size of the concrete will be reduced once the shrinkage crack takes place. The concrete size will go back to the original size with a slight difference, as shown in figure no: 1. The expanding of concrete will prevent the developing of cracks due to concrete shrinkage.
Shrinkage compensating concrete can be produced by the using of expansive cement such as types K, M, S, and O. another way is by adding expansive admixture to normal Portland cement. The cement expands by the growth of expansive hydration products such as ettringite, monosulfoaluminate (AFm), and MH.
These products will expand right after the concrete setting in moist conditions. These hydration products have a good crystal structure and a large volume of expansion, they will expand the volume of concrete by the growth of the crystals. The expansion of these products should occur under the moist condition and before the concrete matures.
If the expansion takes place when concrete matures, the hardened concrete will suffer damages and cracks under the pressure of volume increase. Also, an Air-entraining agent can be used to expand the concrete volume due to air-entrained voids.
It is more convenient to produce expansion concrete by adding expansion agents to ordinary Portland cement. The dosage of expansion agents can range from 5-15% of cement weight. The main characteristic of cement in which expansion can be measured using the restrained frame shown in figure no:2.
To measure the restrained expansion rate, expansion concrete will be casted around the restrained frame, and the expansion rate will be measured after demolding. The test is designed to reflect the actual condition of the concrete in the field by considering the minimum reinforcement. The experiments show that the restrained expansion rate can reach (11-18)*10^-4 for expansion agent of 12-15%.
The properties of fresh and hardened shrinkage compensating concrete is different from typical concrete. The bellow points clarify some of the shrinkage compensating concrete properties.
- Workability: fresh shrinkage compensating concrete tends to be stiff but highly cohesive. The slump loss of concrete at 32 C or higher temperatures is high, and this can result in a reduction of concrete strength and expansion. Therefore chilled water should be used to reduce concrete temperature to less than 29 C. quick stiffening and setting of concrete under high temperature will result in plastic shrinkage cracks and more serious problems than typical concrete.
- Strength: for type K expansive cement, concrete usually prepared with w/c of 0.40 to 0.65. The paste matrix of concrete is denser, and the interface between aggregate and paste is stronger. Certainly, the compressive concrete is enhanced, and it is much higher than ordinary Portland cement concrete. Adding an expansion agent to concrete will improve concrete compressive strength. The degree of improvement of concrete compressive strength differs for a different concrete grade. For example, there is no noticeable increase of 28 days of concrete compressive strength for 30 MPa concrete. However, there is a 15% to 20% increase of 28 days concrete compressive strength for 60 MPa concrete.
- Durability: shrinkage compensating concrete possesses higher durability. The higher durability stems from the dense concrete paste and strong aggregate paste interface. Therefore the permeability of concrete is reduced, and the ingress of chloride or any other aggressive chemicals is reduced.