Shrinkage-reducing concrete admixture promote expansion of the concrete at about the same volume that normal drying shrinkage is contracting it. The net change in length of the hardened concrete is small enough to prevent shrinkage cracks. Shrinkage reducing concrete mixture is typically comprised of polyoxyalkylene alkyl ether or similar composition.
When such admixture is added to concrete during the batching stage, it can significantly reduce both the early and long-term drying shrinkage; reductions of 30 to 50% can be achieved. This is achieved by treating the ‘cause’ of drying shrinkage within the capillaries and pores of the cement paste.
This type of admixture should not be confused with shrinkage compensating materials which are normally added at above 5% on cement and function by creating an expansive reaction within the cement paste to treating the ‘effects’ of drying shrinkage.
Contents:
Materials
Shrinkage reducing admixtures are mainly based on ethylene glycol derivatives. These organic liquids are totally different to most other admixtures, which are water-based solutions.
The admixtures are normally 100% active and are water soluble. They have a characteristic odor and a specific gravity of less than 1.00.
Mechanism of Shrinkage Reduction
The mechanism by which shrinkage reducing admixtures operate is unique. When excess water begins to evaporate from the concrete’s surface after placing, compacting, finishing and curing, an air/water interface or “meniscus” is set up within the capillaries or pores of the cement paste of the concrete.
Because water has a very high surface tension, this causes a stress to be exerted on the internal walls of the capillaries or pores where the meniscus has formed.
This stress is in the form of an inward pulling force that tends to close up the capillary or pore. Thus, the volume of the capillary is reduced leading to shrinkage of the cement paste around the aggregates, leading to an overall reduction in volume.
The shrinkage reducing admixtures operates by interfering with the surface chemistry of the air/water interface within the capillary or pore, reducing surface tension effects and consequently reducing the shrinkage as water evaporates from within the concrete.
Dosage
Usage rates vary with batch designs and water content but typically range from 8 to 25%. Selection, of an appropriate dosage would depend on the level of shrinkage desired, the degree of set retardation that is acceptable, and curing conditions.
Effects on Fresh Concrete
- Increase slump value when 2% of this admixture is added to concrete mixture without adjustment.
- It might retard rate of hydration slightly and extend setting time by an hour.
- It influences air content of concrete mixture. If concrete reducing admixture and air entraining admixture is added together, greater quantity of air entraining admixture should be used to achieve designated air content.
Effects on Hardened Concrete
- Reduce shrinkage by 30 to 50%. This reduction is influenced by the ambient conditions, mixture design, and the materials used.
- It causes the reduction of concrete compressive strength. It is demonstrated that, blending 2% of shrinkage reducing admixture would lead to the reduction of strength by 15% at 28 days. Strength reduction can be offset by reducing water to cement ratio or adding high range water reducing admixture.
- Reduce thermal cracking.
- It decreases curling of concrete slab.
Advantages
- The capacity of shrinkage reducing concrete admixture to minimize cracks would allow it to decline both joint contraction and curling.
- Reduces number of cracks in leakage susceptible structures.
- Decrease the loss of prestress in prestressing applications.
- Shrinkage reducing concrete admixture offset the influence of high-shrinkage aggregates. In this case, cost comparison between cost of shrinkage reducing concrete admixture and good quality aggregate should be made to specify economic option.
- Shrinkage reducing admixtures can be used in situations where shrinkage cracking could lead to durability problems or where large numbers of shrinkage joints are undesirable for economic or technical reasons.
- Where new concrete is used to strengthen or repair existing structures, shrinkage reducing admixtures can reduce the risk of cracking in what can be a highly restrained environment.
Applications
- Architectural precast
- Pneumatically applied concrete
- Water-retaining structures Majority of horizontal slab applications such as floors, roofs, and parking decks.