Throughout the life of concrete, water has central importance:
-It is an essential ingredient in the mixing, curing, and Harding of concrete;
-Its exchange with the surroundings causes hardened concrete to shrink, swell and possibly crack;
-Its presence in hardened concrete influences the strength and creep;
-It plays a central role in deterioration caused by frost action or alkali-silica reactions.
Obviously, control of water is important to concrete. This article gives an overview of some of the opportunities offered by the use of super absorbent polymer (SAP) for achieving that control. Parts of the article are sourced from a previous publication.
When SAP is exposed to water, they swell, and when subsequently subjected to drying, they reversibly shrink. These key properties to drying, they reversibly shrink. The key properties can actively be used in relation to concrete.
Influence on Strength
An SAP can ensure very efficient internal water curing, which is defined as “incorporation of a curing agent serving as an internal reservoir of water, gradually releasing it as the concrete dries out.”7 Internal water curing has been used for decades to promote hydration of cement and to control the shrinkage of concrete
From a strength point of view, the addition of SAPs to concrete has two opposite effects: while the SAP generates voids in the concrete and thus reduces strength, the internal water curing provided by the SAP enhances the degree of hydration and thereby increases the strength. Which of these two effects is dominant depends on the water-cement ratio (w/c), the maturity of the concrete, and the amount of SAP addition. The total effect seems to be described well with existing models, such as the gel-space ratio concept. In particular, at a high w/c (>0.45), SAP addition has very little effect on hydration and therefore generally reduces compressive strength. At a low w/c (<0.45), SAP addition may increase the compressive strength.
The shrinkage of concrete due to loss of water to the surroundings is a well-known cause of cracking both in the plastic and in the hardened state. This type of cracking can effectively be mitigated by slowing down or preventing water loss. By acting as a water source, SAPs may potentially be used in relation to this, but these types of shrinkage are basically surface-related phenomena and it may be difficult to focus the action of the SAP towards this interface. Autogenous shrinkage is a phenomenon that is closely connected with high-performance concrete. Autogenous shrinkage may lead to cracking and affect the strength, durability, and aesthetics of concrete. This has been a technological challenge that has limited the use of high-performance concrete.SAP added to a concrete mixture during mixing permits active control of geometric and thermodynamic properties of the water phase. The water in the formed SAP inclusions is essentially free water, and the size and shape of the inclusions are governed by the initially added SAP particles. Water entrainment can thus be considered engineered water phase distribution.
SAPs may also be used as a means to engineer the pore structure of cementitious materials. During cement hydration, the SAP particles shrink and leave gas-filled voids. This can potentially be used for controlled air entrainment to improve the frost resistance of concrete. The method normally used for entraining air is connected with a number of significant technological difficulties, including coalescence of air bubbles in the fresh concrete, loss of air during vibration or pumping, and problems with compatibility between air-entraining admixtures and high-range water-reducing admixtures. The use of SAPs offers the possibility of actively controlling the entrained air in the hardened concrete, including the total air content, the spacing of the air bubbles, and the size (and even the shape) of the individual cavities–we refer to this as engineered air-entrainment of concrete.
The addition of dry SAP during mixing results in a considerable change in the rheology of fresh concrete if extra water is not added to compensate for the SAP absorption. For example, with water absorption of around 15 g/g dry SAP, the addition of just 0.4% SAP relative to the cement weight will lead to a lowering of the free w/c by 0.06. This change in w/c will cause the yield stress to triple and the plastic viscosity to increase by 25% for concrete with an initial w/c of 0.4. In addition to this pure water-binding effect, a further increase in the yield stress and plastic viscosity will be caused by the physical presence of the swollen SAP particles. If the thickening effect caused by the SAP is unwanted, it may be mitigated by the addition of plasticizing admixtures. For example, the thickening effect associated with the absorption of water by the SAP may be particularly useful for wet-mix concreting, a process that can have a number of technological difficulties. To allow normal concrete mixtures to be pumped, a high slump is needed. However, to minimize rebound and to allow a proper buildup thickness during concreting, a low slump is needed. In practice, the producer of the concrete may need to balance on a knife blade and keep the slump at 80 mm (3 in.).In addition to exercising precise control of the fresh concrete slump, it is normally necessary to add a set-accelerating admixture at the nozzle. Unfortunately, the set-accelerating admixture leads to marked reductions in long-term compressive strength. The concept of using SAPs to adjust the slump of wet-mix shotcrete has been tested in practice. Dry SAP was added to the nozzle and its rapid uptake of water was shown to create a viscosity change during placing, which allowed the buildup of thick layers without the use of set-accelerating admixtures. The latter benefit is particularly intriguing because it is very difficult to control the air entrainment of placed shotcrete. As shotcrete is placed, major changes in the air-void structure will occur if it is based on a normal, air-entrained admixture. With SAPs, however, it is possible to accurately design the final air-void structure, which will be unaffected by the pumping and placing procedure.