Effect Of Internal Curing On The Shrinkage And Permeability Of Self-compacting Concrete

In the construction industry, self-compacting concrete(SCC) has grown significantly due to its technical advantages resulting from the elimination of mechanical compaction. Compared with conventional concrete, SCC contains higher content of fine aggregates, higher volume of paste, larger quantities of high-range water-reducing admixtures. Therefore, SCC exhibits higher shrinkage deformations than conventional concrete, which will result in greater risk of shrinkage cracking. Internal curing can be used for mitigating the autogenous shrinkage of SCC. Meanwhile, the existence of the porous internal curing materials will change the rheological, mechanical and permeable properties of SCC.In this paper, fine lightweight aggregate(or lightweight sand, LWS) and superabsorbent polymer(SAP) were used as internal curing materials. Lightweight aggregate A and B used in the experiment had different water absorption, grain shapes and mechanic properties. There were three different particle sizes of water absorbent resins of sodium polyacrylate series used. Influence of wet internal curing materials on the rheological properties, compressive strength, early-age autogenous shrinkage, and drying shrinkage, water absorption, carbonation and chloride-ion penetration of self-compacting concrete(SCC) was mainly investigated.Research results have shown that the absorption and desorption of water by SAP in fresh mortar, and subsequently the effect of SAP addition on its rheological behaviour, depend on the availability of free water in the mix. With the increasing of the dosage of internal curing agent and additiona l water, more free water were available in fresh mortar releasing from SAP, so as to keep mortar flowable for a purposed time. The particle size of SAP influences their absorption and desorption kinetics. For the same amount of internal curing water, the l arger particle of SAP, while slowed at the beginning, continued to absorb water over the entire time of the experiments, leading to a steady and most pronounced increase of plastic viscosity value for fresh mortar.When natural sand is replaced by the same volume of LWA, the weakest component of SCC is not the cement matrix or the interfacial transition zone(ITZ) but the LWAs. Therefore, replaced volume fraction and properties of LWA dominate the compressive strength of SCC. When the improvement of the hydration degree of cement matrix could compromise the negative effect of SAP holes, the compressive strength of SCC with incorporation of SAP and extra water can keep close with the plain group. The compressive strength of SCC reduced slightly with the incre asement of the particle size of SAP.The introduction of pre-soaking fine lightweight aggregate could reduce early-age autogenous shrinkage of SCC. The early-age autogenous shrinkage of SCC decreased significantly with the wet fine lightweight aggregate in creasing. Compared to specimens with incorporation of LWA A, LWA B were shown to be less effective in reducing the autogenous shrinkage strain of SCC. Pre-soaked LWA had little effect on the drying shrinkage of SCC. As the dosage of SAP and extra water inc reasing, the early-age autogenous shrinkage of SCC reduced gradually. SAP with the particle size of 75-125 ?m could mitigate early-age autogenous shrinkage and dry shrinkage of SCC most effitively.The introduction of internal curing materials improved the hydration degree of cement matrix, which increased the density of SCC. On the other hand, porous internal curing materials led to increasing total porosity of SCC, which reduced the permeability resistance of SCC.