Mechanism of Shrinkage Reducing Concrete Admixtures:
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.
Materials of Shrinkage Reducing Concrete Admixtures:
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.
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. The typical materials used for shrinkage compensation in concrete are based on calcium sulfo-aluminate or calcium aluminate and calcium oxide.
Shrinkage reducing admixtures when added to concrete during the batching stage, can significantly reduce both the early and long term drying shrinkage. 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.
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.
In floor slabs, the joint spacing can be increased from 5- 6m. to 10- 15m.due to the reduced movement of the concrete during drying. The risk of the slab curling at joints and/or edges is also significantly reduced.
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.
Some losses in properties are typical with the introduction of these anti-shrinkage agents. Any ill-effects on strength are minimized by the use of High Rate Water Reducers, which provide good workability while allowing reduction of the water content.
These admixtures can be used to great advantage in slabs, bridge decks, structures, and repair work where cracking can lead to steel reinforcement corrosion problems, but maintaining effective air entrainment for resistance to freezing and thawing damage can be difficult with shrinkage-reducing admixtures.
Usage rates vary with batch designs and water content but typically range from 8 to 25%. ACI 223 (Standard Practice for the Use of Shrinkage-Compensating Concrete) specifies the best methods for utilization of this admixture.
About The Author
