Methods for preventing and reducing concrete damages caused by oxidation of iron sulfide in aggregates

Aggregates containing iron sulfide are vulnerable to oxidation reactions, leading to severe concrete distress in the form of expansion that causes significant map cracking and progressive reduction of the stiffness and strength of the concrete, pop-outs, and unsightly rust stains. Solving these issues is extremely important in the modern construction industry. Numerous approaches, such as aggregate screening, alternative mix design, and protection/repair techniques, can be used to achieve the intended goal. The guidelines available in the existing standards and articles to overcome the issues associated with concrete containing iron sulfide-bearing aggregates are inadequate for practicing engineers. Therefore, this study was split into four phases to address these unsolved issues. The first phase of this study mainly focuses on developing an appropriate screening approach. A novel formulation that consists of three chemical components, i.e., 20 wt % DTPA, 4 wt % K2CO3, and 3% KOH, was formulated to detect the Fe ions. Fe ions can be recognized by the reddish-brown color changes within 5 min after the iron sulfide-contaminated aggregates are added to the developed novel chemical mixture. The formulated chemical mixture's interaction with iron sulfide aggregates also results in significant weight loss. The developed test method was compared with the available screening techniques, and the developed method outperformed the existing methods. A test method to detect sulfur's presence and to compute the equivalent reactive SO3 was also developed and verified with different aggregates. Appropriate performance evaluation test methods are required to formulate protection and repair measures guidelines. Unfortunately, the current standards and specifications do not comprehensively include such test techniques. Therefore, the study's second phase attempted to develop a set of test methods to evaluate the performance of concrete containing iron sulfide-bearing aggregates. These methods include acceleration of the oxidation process, evaluation of expansion, quantitative evaluation of rusting, and assessment of the severity of pop-outs and map cracks. An accelerated mortar bar test was developed to evaluate the expansion. A storing temperature of 60oC, 80% RH, and wetting and drying cycles of 7 cycles per two weeks (6% NaOCl or H2O with oxygen supplied by pumping the air through a pump while submerged) were found to be effective for accelerating the oxidation process in the laboratory. The digital image processing method based on the 'Otsu technique' was used to evaluate the rusting quantitatively. Moreover, the pop-out frequency (n) and the average area of pop-outs (A) were used to characterize the pop-out, and these parameters were evaluated based on the developed image-processing algorithm. The severity of the map crackings was also assessed using the average crack width and the total length of cracks per unit area, which were measured using an automated image processing technique.The proposed evaluation techniques were used in the third phase of the study to evaluate the effectiveness of surface treatments as a protective measure (application to newly constructed structures) with distinct functional mechanisms, such as hydrophobic impregnation, pore-blocking surface treatment, and surface coating, in reducing damage caused by the iron sulfide-bearing aggregate. All of the examined surface treatments, except for hydrophobic impregnation, reduced the rate of oxidation and associated damage. Nonetheless, the degree of protection was influenced by the types of surface treatment. The surface treatment system with a crystalline waterproof system covered with an acrylic polymer coating, and the epoxy coating system exhibited outstanding performance in reducing the oxidation rate. The fourth phase of the study was conducted to see if the findings of the protective measures could still be used for repair work (damaged structures). It was discovered that the tested surface treatments in the investigation into repair methods displayed a similar trend to that seen in the study on protective measures. Hence, the formulated guidelines for the protection measures can be applied to repair the affected concrete structures. Repairing cracks and pop-outs before using the suggested techniques are recommended.