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What is Shrinkage-Reducing Admixtures of Concrete?
Shrinkage-reducing concrete admixtures encourage the concrete to expand at approximately the same rate as usual drying shrinkage contracts it. The net length shift of the hardened concrete is sufficiently minimal to avoid shrinkage cracks. Typically, admixture used for shrinkage-reducing is made of polyoxyalkylene alkyl ether or a similar composition.
When applied to concrete at the stage of mixing batching, this type of admixture will greatly minimize both short-term drying shrinkage in addition the long-term one; reductions of up to 50% are possible. This reduction is accomplished by addressing the cause of drying shrinkage occurring in the pores of cement paste.
It should be noted that this form of admixture should not be mixed up with products used for compensating shrinkage, which are typically applied at a concentration of greater than 5% to cement and work by initiating a reaction that is expansive within the cement paste to compensate for the consequences of drying shrinkage.
Components of Shrinkage-Reducing Mixtures
Admixtures for shrinkage reduction are primarily composed of ethylene glycol by-products. These organic fluids are diametrically opposed to the majority of other admixtures, which are dependent on water.
Usually, the admixtures are 100 percent active and soluble in water. They have a distinct aroma and a specific gravity less than one.
Reduced Shrinkage Mechanism
Admixtures that reduce shrinkage have a special mechanism of action. After positioning, compacting, finishing, and curing the concrete, excess water evaporates from the surface, forming an air/water interface or “meniscus” inside the pores of the hardened cement paste.
Due to the extremely high surface tension of water, it exerts a stress on the internal porewalls of the pores where the meniscus developed.
This stress manifests as a pulling force acting inwards, which tends to constrict the capillary or pore. Thus, the capillary volume is decreased, resulting in shrinkage of the hardened cement paste surrounding the aggregates, resulting in an overall volume reduction.
The shrinkage-reducing admixtures work by interfering with the chemistry of surface of the air/water interface in the pores of cement paste, thus lowering the surface tension and thereby lowering shrinkage as water gets evaporated from the concrete.
Dosage
Usage rates vary according to batch design and water content, but usually range between 8% and 25%. The required dose would be determined by the desired shrinkage, the acceptable degree of set retardation, and curing conditions.
Impacts on Fresh Concrete
When 2% of shrinkage-reducing admixture is applied to the concrete without modification, the slump value increases.
It can slightly slow the hydration rate and add an hour to the setting time.
It has an impact on the amount of air in the concrete mixture. When concrete reducing and air entraining admixtures are combined, a higher amount of air entraining admixture needs to be applied to obtain the specified air material.
Impacts on Hardened Concrete
- Reduce shrinkage by about 30%–50%. This reduction is dependent on the atmospheric conditions, the composition of the concrete mix, and the type of materials incorporated.
- It results in a decrease in the compressive strength of concrete. It is demonstrated that incorporating 2% of a shrinkage-reducing admixture results in a 15% reduction in intensity after 28 days. Strength loss can be compensated for by lowering the water-to-cement ratio or by adding a superplasticizer.
- Thermal cracking is minimized.
- It prevents the concrete slab from curling.
Pros
- The shrinkage-reducing concrete admixture’s ability to reduce cracks enables it to decrease both the contraction at joints and curling.
- Decreases the amount of cracks in systems prone to leakage.
- Reduce prestress failure in prestressing applications.
- A shrinkage-reducing admixture in concrete compensated for the effect of high-shrinkage aggregates. In such situation, a comparison in terms of cost between shrinkage-reducing concrete admixture and high-quality aggregate need to be performed to determine the most cost-effective alternative.
- Shrinkage reducing admixtures may be used in circumstances where shrinkage cracking may result in reliability issues or where the presence of a huge number of shrinkage joints is unwanted for both economic/technical reasons.
- As concrete is applied to reinforce or restore existing structures, shrinkage-reducing admixtures can help minimize the risk of cracking in a highly constrained setting.
Typical Applications
- Precast architectural elements
- Concrete applied pneumatically
- Structures that store water
- The majority of horizontal slab applications, including floors, roofs, and parking decks.
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