Analysis of Unbonded Capping Materials Used in Determining the Compressive Strength of Concrete Masonry Prisms Jacob Ballard  
Civil Engineering

ABSTRACT Compression testing is often used as a quality control measure in concrete masonry production and construction. Caps are used on masonry prisms to eliminate surface imperfections and provide uniform load distribution. Currently bonded caps, specifically gypsum and sulfur, are the only accepted method for Concrete Masonry Unit (CMU) prism capping. Preparing bonded caps is time consuming and labor intensive. The use of reusable or unbonded caps could dramatically reduce specimen preparation time and labor while still providing accurate results. This project researches the viability of several unbonded caps as suitable for compressive strength testing. Selection of cap material was based on multiple criteria, including compressive strength, surface bonding, thickness, Poisson’s ratio and hardness. Hydrocal Gypsum cement was chosen as the control method and was compared against recycled rubber, neoprene, fiberboard and laminated foam. The rubber and neoprene pads were restrained against lateral expansion using a steel confining apparatus to avoid development of lateral tensile forces at the prism-cap interface. Results indicate that the rubber caps provide nearly identical strengths compared to the control, with a 3% reduction in observed strength. The fiberboard caps also produced comparable strengths to the control with a 5% increase in observed strength. The repeatability of these methods is evident from the calculated coefficient of variation. The neoprene cap results had a small coefficient of variation (3.18), but strengths were 13% greater than the control’s. The significant increase in strength was likely a product of high confining stresses at the prism ends. The laminated foam exhibited the lowest strength with a 27% reduction compared to the gypsum. Based on results, rubber caps of a durometer hardness of 65 with proper confinement could be adopted as a suitable alternative to hard capping. Keywords: Jacob Ballard, gypsum capping, soft capping, unbounded capping, neoprene cap, board cap, reusable cap, masonry compression test, masonry prisms, rubber cap. More



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