CoatingsPro Magazine

JAN 2017

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Page 98 of 116

98 JANUARY 2017 COATINGSPROMAG.COM Based on our in-house laboratory analysis and the project information provided, we hypothesized that the most likely cause of the concrete sub-surface damage was the placing and finishing of the concrete while it was semi-plastic, instead of while it was plastic. Placing and finishing the concrete while it was in a semi-plastic state could be due to the mix design, temperatures, time delays, or a combination of these. is leads to subsurface tearing within the concrete, with fractures remaining after the concrete hardened. Further testing confirmed the mode of failure. e further petrographic analy- sis revealed consistently weak and cracked near-surface concrete and an increase in slag content in the concrete subsurface. e concrete exhib- ited paste characteristics normally associated with concrete mixtures having a moderately low to moderate water-to-cementitious material (w/ cm) ratio; however, we found evidence of slightly higher w/cm ratios within the upper ⅛‒¼ in. (3.2–6.4 mm) of the concrete. We observed horizontal and diagonal fractures, or microcracks, in the upper 1/16‒5/16 in. (1.6–7.9 mm) of the concrete substrate in all of the cores, including those from areas that were not mechanically prepared and coated. Our petrographic analy- sis found separation voids in the near-surface layer of the concrete that traverse around, rather than through, aggregate particles. We also observed vertical hairline fractures in the upper 1/16‒5/16 in. (1.6–7.9 mm) of the concrete surface. Conclusions Delamination of the epoxy coating was the result of a damaged upper layer of the concrete substrate, which was weakened prior to the application of the epoxy coating system. A number of conditions contributed to the weakened concrete surface, including the concrete mix design, the finishing process, the curing process, and surface preparation. • Concrete Mix Design: Higher slag content in the near-surface region as compared to the body of the concrete can reduce the strength in the near-surface region. An increased concentration of slag in the near-surface region of the concrete can also lead to an increased amount of bleed water during finishing, which typically produces a weakened zone or voids in the surrounding concrete paste. The higher w/cm ratio at the surface is due to bleed water and contributed to a weakened slab surface. • Finishing Process: We found microcracks in the concrete from spaces adjacent to the service and maintenance center that were not mechanically prepared or coated. The only operation done over these areas and the coated areas that could create the observed types of cracks is finishing of the concrete slab. The types of microcracks we found in our laboratory, separation voids consist- ing of tears, are usually associated with finishing, and in particular finishing of the slab too late (i.e., when it is semi-plastic) and/or the use of ride-on power trowels. • Curing Process: The orientation, pathway, and frequency of the vertical microcracks we observed in the concrete are indicative of crazing cracks from very early age drying shrinkage that probably occurred during or shortly after the final finishing operations. Although these types of crazing cracks are typical, the presence of these cracks provides additional weakness in the surface of the slab. • Surface Preparation: The degree of surface preparation (shot blast- ing) observed on the concrete we examined is considered light to moderate; however, the petrographic evidence indicates that the surface preparation resulted in both paste and aggregate fractures. Aggressive surface preparation can cause micro- cracking (i.e., bruising). In this case, shot blasting resulted in bruising of this concrete surface because it was already weakened by the mix design, finishing, and curing processes. The majority of the observed damage is due to the shot blasting aggravating existing defects. Of these four condi- tions, placing and/or finishing while the concrete was semi-plastic was the primary cause of the sub-sur- face damage observed. Localized impact from tools or parts dropped on the floor during ordinary usage Petrographic examination showed fractures near the surface of the concrete with epoxy primer showing within the fractures. This suggested that the fractures were present before the coating was applied. Car Dealership Floor Failure

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