Effect of Internal Curing as Mitigation to Minimize Alkali-Silica Reaction Damage
By Mengesha A. Beyene, Jose F. Munoz, Richard C. Meininger, and Carmelo Di Bella
Newswise — In this study, the potential of internal curing from the use of prewetted lightweight aggregate (LWA) instead of the more common use of low-alkali cement to reduce ASR damage was investigated. Common practices in the construction industry to avoid potential damage from alkali-silica reaction (ASR) in concrete are: testing the aggregates for potential reactivity, using low-alkali cement (0.60 percent Na2O equivalent maximum), limiting alkali content in concrete, using supplementary cementitious materials (SCMs), using lithium salts as admixtures, or any combination of these. However, the absence of low-alkali cement in North America or other parts of the world, high cost of hauling low alkali cement from one region to another, and decreasing availability of unreactive aggregates is a growing problem of the industry. Additionally, the current reliable alkali reactivity test for aggregates (ASTM C1293) takes a relatively long time (at least one year) to complete the test. Therefore, the question arises, why not use internal curing for its potential to reduce ASR damage in concrete?
Internal curing refers to the use of prewetted lightweight aggregate (LWA) or other water-filled inclusions, including prewetted crushed returned concrete fines, superabsorbent polymers, and prewetted wood fibers that can provide curing water throughout the concrete.
Commonly, a portion of the conventional aggregate used in the mixture is replaced by a pre-wetted LWA. While the multiple benefits of internal curing using LWA, including reducing early-age cracking due to internal desiccation, mitigating autogenous and drying-shrinkage cracking, reducing plastic shrinkage and thermal cracking, and reducing water absorption, are well recognized, the role of LWA in reducing ASR damage in a concrete is not well understood. Therefore, in this study, the potential of internal curing from the use of prewetted LWA in reducing ASR damage was investigated. The study was conducted by comparing internally cured concrete with companion plain concrete (control sample) cured in similar conditions and for the same time. The two concretes are the same except that a portion of the sand was replaced by prewetted LWA in the internally cured concrete. Concrete mixtures in both concrete types consist of type I cement with Alkali expressed as Na2O=0.86 percent, crushed stone coarse aggregate consisting of limestone and dolomitic limestone, and fine aggregate is natural sand, which consists mainly of quartz with lesser amounts of chert, minor amounts of sandstone, granitic rocks and feldspar, and carbonate rocks.
The primary goal of this research was to investigate and evaluate if internal curing can mitigate damage from ASR in a concrete when high-alkali cement, high cement content, and potentially reactive aggregates are used. The second goal of the research was to investigate whether voids in the LWA have contributed in mitigating ASR damage by providing relief through ASR gel migrating into the LWA void structure, thus reducing swelling pressure.
Quantitative paste characterization, including image analysis of BSE images, quantitative fluorescent intensity assessment, resistivity measurements, and qualitative analyses using SEM-EDS and PLM, showed that internal curing improved paste quality through a quantitative reduction in paste porosity and unhydrated cement. The degree of ASR cracking and damage in the internally cured concrete cylinders with partial replacement of the natural sand with prewetted LWA coarse sand was minimal and much more localized than the plain concrete cylinders. The reduction in cracking and distress in the LWA-bearing concrete cylinders is attributed to formation of dense and less permeable paste microstructure due to increased hydration from the use of prewetted LWA. There was strong evidence of a higher confinement capacity of the paste in the LWA concrete specimens than in the control, as well as a denser microstructure proved by SEM analysis and smaller crack propagation confirmed by extensive petrographic analysis. The dense and less permeable paste microstructure and reduced interfacial transition zone (ITZ) should have reduced the rate of fluid ingress and movement, which in turn reduced the rate of ASR reaction and crack propagation. No ASR gel was observed in the pore spaces of LWA particles in the 2.5 -year-old concrete and, therefore, it is unlikely that the pores in the LWA contributed to the reduced ASR damage in the LWA concrete.
This study clearly showed the potential for using prewetted LWA in reducing/mitigating ASR damage in a concrete. With partial replacement of aggregate, it may be possible to mitigate ASR and associated damage when high cement content and potentially reactive aggregates are used.
 ASTM International: ASTM C1293-08b(2015) “Standard Test Method for Determination of Length Change of Concrete Due to Alkali-Silica Reaction”
The research can be found in a paper titled “Effect of Internal Curing as Mitigation to Minimize Alkali-Silica Reaction Damage,” published by ACI Materials Journal, May-June 2017.