What is Shrinkage Compensating Concrete?
Shrinkage compensating concrete is the most efficient and extensively used material in the recent years used to eradicate or minimize cracking which is caused due to drying shrinkage in RCC structures. Drying shrinkage is basically decrease in volume caused by moisture loss in the process of hardening of concrete, which ultimately results in crack formation.
Shrinkage Compensating Concrete can be defined as an expansive concrete or a concrete consisting of an expansive cement or expansive admixture, which induces expansion whilst hardening and consequently offsets the contraction during drying shrinkage. When the expansive concrete is restrained accurately with appropriate reinforcement, it will expand to a value equal or slightly higher than the foreseen amount of drying shrinkage.
Types of Expansive Materials
Expansive component is the key ingredient of shrinkage compensating mechanism. These are added to a dosage of 5.0 to upto 15% by cement weight. The mix proportioning is adjusted accordingly when these additive are used. Various types of expansive materials are as follows:
Type K cement:
It is also referred to as Klein cement. It comprises of C4A3S (C=CaO, A=Al2O3, and S=SO3), calcium sulphate (CaSO4) and lime CaO.
Type M cement:
It is constituted by blending mixes of Portland cement, calcium-aluminate cement (CA and C12A7), and calcium sulphate proportioned in accordance with the requirement.
Type S cement:
It is primarily Portland cement consisting of large amount of Tricalcium aluminate (C3A) and gypsum (Calcium Sulphate (CS)).
Admixtures
Expansive admixtures are made of clinker, alumstone, anhydrous gypsum.
Mechanism of Shrinkage compensating concrete
Shrinkage compensating concrete is designed to work co-jointly with internal restraints, reinforcements. Compressive stresses will be induced in the concrete as well as tension in the steel restraints during time of expansion of the concrete. This results in off-setting of tensile stresses and negative strains induced in the concrete by drying shrinkage. Further, a residual compression will always remain in the concrete, eradicating the risk of shrinkage cracking.
Expansion is achieved by huge amount of ettringite formed, as soon as the type K cement hydrates. Ettringite is nothing but a mineral crystal that forms rapidly when Portland cement and water is mixed. The crystal nature of ettringite takes up a significantly huge amount of volume in concrete. This property of ettringite results in the primary expansion of concrete after setting.
Once the concrete starts expanding, it simultaneously bonds with the reinforcements. This will induce tension in the steel while the concrete is subjected to compression. Once the curing is completed, the shrinkage compensating concrete will shrink like normal Portland cement concrete. First the shrinkage releases pre-compression and thus the induced tensile stresses in concrete are relieved.
Advantages of Shrinkage Compensating concrete
Minimizes Cracking
As we are aware that concrete is a brittle material and the drying shrinkage stresses are more than enough to surpass the tensile strength of the concrete. This results in the formation of cracks. Drying shrinkage cracks affect RCC structures as they are often cause of early deterioration and loss of serviceability. Hence the most evident advantage of shrinkage compensating concrete is that it eliminates cracking by minimizing the drying shrinkage.
Reduces Curling
Due to the effects of the curling process, the floor flatness will not decline. Removing bleed water helps in keeping up the integrity of the WC ratio at the surface of the concrete. This results in efficient enhancement in abrasion and impact resistance. It also resists laitance and other debris in the concrete from being brought up to the surface.
Eliminates control joints
The first figure represents a slab which will experience curling at the control joints, panel movement and joint deterioration due to the movement. Whereas the second figure represents a slab that will function as a single unit and provide positive load transfer across the slab.
Cost Effective
Shrinkage compensating concrete provides best value solution to construction industry professionals. It helps them to achieve basic objectives of building a sustainable and economical building. Specifically, Type K cement offers a very high performance, cost effectiveness, and valued solution which enhances the project potential. Type K cement consists of suitable additive, local Portland cement, and aggregates blended together proportionally to form concrete. The additive used in Type K cement is the most economical material used, even in remote locations. It also imparts a cost effective way to produce Type K shrinkage compensating concrete with enhanced efficiency.
Increases stability of the concrete
The advancement in the hydration mechanism due to the ettringite crystals formed during hydration, effectively resolves volume change, restraint to shortening challenges, reduces permeability, and assures long lasting stability of the concrete.
Application of Shrinkage compensating concrete
There are extensive applications of Shrinkage compensating concrete across the construction industry, especially in China. These expansive cement concrete are in use ever since 1960’s.
Shrinkage compensating concrete is used in various construction industrial aspects such as Water and Sewage structures, Treatment plants, Water Storage tanks, Bridge decks, beneath ice rinks and stadiums, Spillways, Exhibit halls, Parking garages, Industrial floor slabs, Cooling tower basins, Swimming pools, Airport taxiways and runways, Floor without joints, and Containment vessels