CoatingsPro Magazine

JAN 2017

CoatingsPro offers an in-depth look at coatings based on case studies, successful business operation, new products, industry news, and the safe and profitable use of coatings and equipment.

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COATINGSPRO JANUARY 2017 97 laser screed. Project records showed the average air temperature during place- ment was 40° F (4.4° C), the average temperature of the concrete during placement was 59° F (15.0° C), and the average slump measurement of the concrete was 5 inches (12.7 cm). After the concrete was placed, it was finished using a motorized trowel up to 12 hours after initial placement and covered with another layer of the 6-mil (152.4 microns) poly for a 72-hour period to cure. Concrete testing reports indicated that the average 28-day compressive strength of the concrete was 5,320 psi (36.7 MPa). e service and maintenance center was heated for a period of two weeks after the slab was cast to facilitate curing, after which time no climate control was in place until the heating, ventilation, and air condition- ing (HVAC) system was installed. W hen it was time to install the floor coating, the contractor measured the moisture-vapor emission rate (MVER) of the slab as outlined in ASTM F1869: Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subf loor Using Anhydrous Calcium Chloride, with results of approx- imately 6 lbs./1,000 sq. ft. (2.7 kg/92.9 m²) in 24 hrs. In situ concrete relative humidity tests per ASTM F2170: Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes showed an average concrete internal relative humidity (RH) of 88 percent. Based on these results, the following 60-mil (1,524.0 microns) floor coating system was selected (from top to bottom): • Clear urethane topcoat • Epoxy color coat • Epoxy basecoat with broadcast sand • Epoxy primer e floor coating system was installed during the summer. e surface of the concrete floor slab was shot blasted prior to installation of the epoxy primer. An in situ mockup floor assembly was never installed or tested during the original installation. Investigation e investigation into the causes of the delamination consisted of two key components: pull-off adhesion tests and a petrographic analysis of the concrete slab-on-grade. Pull-off adhesion tests (as outlined in ASTM D7234: Standard Test Method for Pull-Off Adhesion Strength of Coatings on Concrete Using Portable Pull-Off Adhesion Testers) revealed an average tensile strength of 260 psi (1.8 MPa), with five out of six samples having a cohesive failure plane within the upper layer of the concrete, consistent with obser- vations of the delamination plane at failure areas. A petrographic analysis was also conducted on core samples from the project site. Two concrete core samples — one from an area that had not experienced any visible delamination and the other extracted near an area of visible delamination — were evaluated. Several small pieces of delaminated coating were also taken for review. e petrographic examination revealed fractures near the surface of the concrete, through both concrete paste and aggregate materials. Epoxy primer was found within these fractures, confirming that they existed prior to the application of the epoxy floor coating system. Based on these observations, it was determined that the delamination of the epoxy coating system was the result of a weak surface zone of the concrete. To investigate the delaminating areas, pull-off adhesion tests and a petrographic analysis were completed at failure areas. The investigation into the causes of the delamination consisted of two key components: pull-off adhesion tests and a petrographic analysis of the concrete slab-on-grade. Car Dealership Floor Failure

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