International Concrete Abstracts Portal

Editors: Mohamed T. Bassuoni, R. Doug Hooton, and Thanos Drimalas

The papers presented in this volume were included in a three-part session sponsored by ACI Committee 201, Durability of Concrete, about sulfate attack on concrete at the ACI Convention in Philadelphia, PA, on October 23-24, 2016. In line with the practice and requirements of the American Concrete Institute, peer review, followed by appropriate response and revision by authors, has been used.

Deterioration of concrete due to sulfate attack is a complex process characterized by multiple damage manifestations including volumetric expansion, cracking, spalling, softening, and in some cases mushiness. Sulfate attack can generally be classified as internal or external to the cementitious matrix, and the underlying damage modes can be chemical or physical. The scope of papers involves a multitude of theoretical and experimental aspects of different forms of sulfate attack. Readers are urged to critically evaluate the work presented herein, in the light of the large body of knowledge and scientific literature on this durability topic.

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A number of research studies on chemical sulfate attack have been conducted, and considerable disagreement over the mechanisms still exist. They reveal that several factors can influence the severity and type of attack including the concentration of sulfate ions, pH level, temperature, and the nature of the associated cation. However, the biggest challenge that still remains is a performance test method that can determine the sulfate resistance of cement-based systems within a reasonable timeframe. This laboratory experiment – which was part of an extensive doctoral research program – investigated the use of a new vacuum impregnation technique to accelerate the degradation observed during sulfate attack. The mortar bars were immersed in various sulfate solutions and cation types including sodium, magnesium, and calcium sulfate. The results showed an increased rate of linear expansion from the use of the vacuum impregnation technique when compared to the traditional ASTM C1012 method. However, the measured expansion was significantly influenced by the chemical composition of the binders as well the type of sulfate solution used during testing. The microstructural study revealed that the mechanism of expansion seen when using the vacuum impregnation technique was comparable to mechanisms commonly seen in classic cases of external sulfate attack.

The phenomena involving hydrated cement paste and a source of sulfate anion have been extensively studied over the last four decades. The present publication provides an overview of past external sulfate attack studies along with current views on the accuracy of standard methods to evaluate the performance of a concrete mixture in service; illustrating the need to find other means of laboratory testing based on “real” exposure conditions representative of sulfate reaction kinetics encountered in field structures. This study evaluates the efficacy of stresswave propagation testing to detect concrete microstructural disparities related to sulfate-induced damage. While respecting traditional means of inducing an external sulfate attack in the laboratory (complete immersion in a 5% sodium sulfate solution), the experimental study proposed a different methodology for evaluating the extent of sulfate degraded concrete in the laboratory. Over a 2-year exposure term, the extent of degradation of various specimen types, replicating transport mechanisms reminiscent of those seen in the field, were evaluated using ultrasonic pulse velocity. Through statistical analysis, the results discussed demonstrated that the test procedures conducted were reliable for assessing the changes in behaviour observed.

Synopsis: Mortar bars (CSA A3004-C8) were cast with portland and portland limestone cements in combination with various supplementary cementitious materials. The mortar bars were exposed to sodium sulfate solution at 1°C, 5°C, 10°C, and 23°C (34°F, 41°F, 50°F, 73°F); the length change due to external sulfate attack was monitored over time. Mortar cubes were also cast and stored in limewater at 5°C, 23°C, and 38°C (41°F, 73°F, 100°F). The compressive strengths of the mortar cubes were tested at regular intervals to determine the rates of compressive strength gain of the various mortars as a function of curing temperature. The results generally reveal that external sulfate attack is accelerated in cold temperature sulfate exposure, particularly among the mortars with higher supplementary cementitious material replacement levels. The results reveal that the hydration of supplementary cementitious materials is severely diminished upon early-age exposure to cold temperatures, leading to a more permeable pore structure and diminished resistance to sulfate attack. The compressive strength gain of the mortar cubes containing supplementary cementitious materials was retarded at cold temperatures; the impact was much less severe with control mortars. At temperatures ≥10°C (50°F) supplementary cementitious materials greatly enhance resistance to external sulfate attack relative to the control mortars.

This paper presents the potential of carbon fiber reinforced polymer (CFRP) composite sheets for protecting concrete members subjected to sulfate-induced damage. Two types of concrete cylinders with and without CFRP-strengthening are immersed in a 5% sulfuric acid solution for 6 weeks, and their physical and chemical responses are comparatively studied. Also examined is the behavior of the strengthening system consisting of carbon fibers and an epoxy resin in such an environment. Test methods include Fourier transform infrared spectroscopy, thermogravimetric analysis, and destructive mechanical loading in compression. The pH values measured at 0- and 6-week exposure periods indicate that the plain concrete cylinders have actively interacted with sulfuric acid, whereas those specimens with CFRP have not. The damage induced by sulfuric acid appears to be localized at the surface level of the plain concrete, according to the qualitative examinations done by a digital microscope. The chemical responses of the epoxy and CFRP are similar in terms of functional groups, in spite of different absorbance peaks (the carbon fibers retard chemical reactions between the CFRP and sulfuric acid). The thermogravimetric analysis clarifies that the core concrete of the strengthened cylinders is protected by the CFRP, including a gradual decrease in mass with temperature that is contrary to the case of the unstrengthened concrete showing an abrupt change in mass drop. The load-bearing capacity of the plain and strengthened concrete cylinders is reduced by 57% and 23%, respectively, because of the sulfuric acid exposure. A visual assessment on the failed cylinders supports that the concrete core is effectively protected by CFRP-strengthening, although the permeation of sulfuric acid through the CFRP-wrap occurs.