CoatingsPro Magazine

JAN 2017

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COATINGSPRO JANUARY 2017 99 as a service and maintenance center, and large live loads from vehicles and heavy equipment, brought to light the weak surface and initiated delamination of the coating. e concrete moisture levels were moderate, and on-site observations and pull-off adhesion tests indicated that the moisture in the concrete substrate was not a cause of the delamination of the coating. e epoxy primer installed on the surface of the concrete likely also helped to limit moisture vapor emission and protect the coating system from the concrete moisture. Floor Repairs To address the problem, repairs that accounted for the damaged concrete were discussed. Repairs included the removal of the existing coating system and surface preparation of the concrete slab. A minimum of 5/16 in. (7.9 mm) of the surface of the slab was removed to eliminate the damaged layer prior to installing the new coating. Since it was possible that the energy introduced by surface preparation to remove the existing coating system and damaged concrete layer would again propagate existing fractures and create new fractures, we recommended the least aggressive form of surface preparation to remove the existing coating and damaged surface layer of concrete. Multiple mockups were performed w ith various surface preparation techniques, such as hand scraping and one and two passes of a walk-be- hind shot blasting machine. We also considered hydroblasting, sandblast- ing, epox y injections, and concrete grinding, but all of those were elimi- nated due to various limitations or complications. Tensile pull-off tests on these mockups indicated that areas where the coating was removed, and no shot blasting was performed, had the least amount of damage to the concrete substrate. W hen samples were examined under a microscope, we found that shot blasting induced new microcracks in the concrete and that one pass of the shot blasting did less damage than two passes. However, shot blasting was necessar y to achieve the surface profile needed for application of the replacement coating system. We also tried diluting an initial layer of epoxy primer with a solvent to enhance penetration into the concrete and consolidate the damaged surface. Pull-off adhesion tests on a mockup that included scraping of the coating, two light passes with the shot blasting equipment, and application of a diluted initial epoxy primer layer yielded adequate tensile adhesion results, and this method was selected as the prepa- ration strategy. e original coating system was reinstalled on the properly prepared substrate as determined through the use of mockups. To prevent a large number of cold joints in the coating system, an epoxy coating was installed in a small number of large-area phases. Cold joints are a natural location for debonding to occur, and limiting the number of cold joints is a good practice when installing an epoxy coating. Large-area phases required large sections of the workshop to be taken out of operation. In the end, the flooring system failure discussed here highlights the importance of two key elements of construction: the use of mockups and small-scale testing within a construc- tion project. In this case, the damaged layer of concrete could have been detected through the use of pull-off adhesion tests on a small-scale mockup of the original construction, prior to the full-scale installation of the epoxy/ urethane floor coating system. e damaged concrete could have been addressed during original construction with far less cost and inconvenience to the building occupants. CP Emily R. Hopps, P.E., is a senior project manager in t he bu i ld ing technolog y g roup at t he nat iona l eng ineer ing f ir m Simpson Gumper tz & Heger Inc. (SGH). She is a reg istered professiona l civ i l eng ineer w it h more t han 13 years of e x per ience in bu i ld ing invest igat ion, desig n, repa ir, and med ia- t ion suppor t. Peter E. Nelson, P.E., is a senior pr incipa l and joined Simpson Gumper tz & Heger Inc. (SGH) in 1975. He is t he past head of SGH's Bu i ld ing Technolog y g roup in t he Boston of f ice. He br ings e x tensive e x per ience to t he team in eva luat ing , inspect ing , ana lyzing , and repa ir ing bu i ld ing-enve- lope systems. For more infor mat ion, contact: SGH, w w w.sgh.com Further petrographic examination showed that the concrete was weak and cracked near the surface. The concrete showed paste characteristics. It was concluded that the delaminated coating was due to a damaged upper layer of concrete. Car Dealership Floor Failure

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