Investigation of Polypropylene Fiber Reinforcement in Concrete Under Heating

REU: Underground Infrastructure REU | AUTHOR: Janell Prater – University of Louisville

MENTORS: Clay Naito, Spencer Quiel, Aerik Carlton – Lehigh University

ABSTRACT

In heat-induced concrete spalling, polypropylene fiber reinforcement is a well-known spalling prevention technique in fire scenarios. In literature, the mechanical properties effects of polypropylene fibers in concrete when exposed to thermal loading is fairly well documented. The theory in literature is that while of course, any reinforcement type would offer increased strength, polypropylene is special in its ability to increase permeability and reduce pore pressure buildup. When exposed to high temperatures, concrete experiences high pressure buildup due to water vaporization within the material. Especially due to concrete’s high density and low porosity, it is difficult for water vapor to escape and dissipate through the material, resulting in high pore pressure. When this pressure exceeds the tensile capacity of concrete, it is prone to spalling. Since polypropylene has a melting point of about 160°C, it is believed that polypropylene fibers melt and create micro-channels where the solid fibers once were. These micro-channels act as a network of escape routes for water vapor to dissipate throughout the specimen. The general takeaway, therefore, is that polypropylene fibers increase permeability and reduce pore pressure buildup after experiencing melting point, therefore offsetting spalling. In this project, cylinders were cast using a formerly used tunnel liner mix design. These were then broken into 6 different temperature groups: room temperature, 150°C, 200°C, 400°C, 600°C, and 800°C. Each cylinder group was placed in an oven and heated to their respective goal temperature with a heating rate of 1°C/min. They were then held at goal temperature for 1 hour and left to cool back to room temperature again at a controlled rate of 1°C/min. Post heating, compressive strength and modulus of elasticity testing was performed. Visual observations were recorded and photographed. A trendline of steady decrease in compressive strength and modulus of elasticity as temperature rose was recorded. When tested to failure during compressive strength testing, a plethora of solid polypropylene fibers were observed inside specimens of goal temperatures 200°C and below, compared to no solid fibers being found in specimens of goal temperature 400°C and above. Testing will continue in the future with using a permeability testing apparatus to measure the permeability of specimens from each temperature group. Microscopy is also planned for the future to look more closely at where the polypropylene fibers are in each temperature group.