Super absorbent Polymer as an internal curing agent for reducing early-shrinkage of traditional and high performance concretes

Concepts The introduction of SAP (superabsorbent polymer) into traditional concrete does not seem to provide improvements in performances. In order to preserve the same strength obtained without SAP addition, no additional water to pre-saturate SAP grains was added; the required workability was obtained by adding a higher rate of superplasticizer admixture. On the contrary, with the high performance concrete (HPC), a slight shrinkage reduction was achieved. Unfortunately, without a preliminary SAP saturation, the slump loss was too high, and the increase in compressive strength was not enough to override the penalization due to SAP addition. Motivation and Objectives Cement materials with low water/cement (w/c) or low water/binder (w/b) ratios are vulnerable to early age cracking due to autogenous deformation caused by self-desiccation. A decade ago, it was suggested and demonstrated that this problem can be solved by mixing superabsorbent polymers (SAP) into fresh concrete, thereby establishing small reservoirs of internal curing water. When a new component like SAP is introduced into the concrete matrix, it becomes important to investigate how this component influences concrete properties such as workability and mechanical properties. In addition, one can examine whether a partial reduction in early hygrometric shrinkage can be achieved, other than the reduction in autogenous shrinkage. Results and Discussion The actual work is related to concretes with two different water-cement ratios, 0.4 and 0.53: the lower ratio is the upper limit of HPC while the higher one is typical of traditional concretes. The rate of SAP addition to the mixture was 0.25% or 0.50% by weight of cement. In order to verify the effect of this admixture, a reference concrete and a concrete with SRA (Shrinkage Reducing Admixture) were also produced. Other researchers have hypothesized the amount of water absorbed by SAP during mixing and casting and they have added some water in the mixture. We preferred not to predict the absorption amount and we did not modify the nominal composition. The slump loss was preserved by increasing the superplasticizer rate. In the case of traditional concrete, the resulting decrease in the w/c overcame the penalization ascribed to SAP addition, and it lead to the same compressive strength as the reference concrete. As regards to HPC, the strength recovery was not complete and the loss in workability, by using a correct (permissible) superplasticizer rate, was not avoided.