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Evaluating the Structural Performance and Durability of CNT and Polyethylene Fiber-Reinforced Concrete

Al Araj, Rashad R.
A Master of Science thesis in Civil Engineering by Rashad R. Al Araj entitled, "Evaluating the Structural Performance and Durability of CNT and Polyethylene Fiber-Reinforced Concrete," submitted in November 2016. Thesis advisor is Dr. Adil Al-Tamimi and thesis co-advisor is Dr. Noha Hussein. Soft and hard copy available.
Concrete, despite being one of the most produced materials in the world, still has weaknesses and drawbacks. The major concern of cementitious materials in structural applications is their quasi-brittle behavior, which causes them to crack and lose strength and durability. Recent studies have incorporated micro-fibers in concrete mixes to control crack growth at the micro-level. However, new insights will help control cracks at the nano-level by incorporating nano-reinforcement. Carbon nano-tubes (CNTs) are promising nano-engineered materials that could be a perfect fit for this role. However, no major implementation of CNTs has yet been witnessed in this field. Therefore, this research studies the effects of incorporating CNTs, polyethylene (PE) fibers, and their combination on the structural performance and the durability of concrete. In this study, four different concrete mixes were prepared (control, control+ CNTs, control+ PE fibers, and control+ CNTs+ PE fibers), and a total of forty specimens (cubes, cylinders, and prisms) were tested. Structural performance was assessed in terms of compressive and flexural strengths. Durability was assessed in terms of chloride penetration into concrete using rapid chloride permeability test (RCPT). Furthermore, microstructures of concretes were studied with scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX). The study also included finite-element modeling for the novel concrete (incorporating both CNTs and PE fibers) in ABAQUS, and the model was validated against the experimental results for future applications. While no major impacts on compressive strengths were witnessed, ductility in compression was enhanced by up to 50%. Likewise, results from the 3-point-bending tests showed that the flexural strengths were enhanced by up to 194%. Similarly, up to 398% improvement in flexural strain gain was recorded. As for the durability, RCPT results were opposite to expectations, since mixes incorporating CNTs had more than double the chloride permeability to that of the control mix. However, based on literature, it is concluded that RCPT most probably fails to be an accurate test to measure the chloride permeability of concretes containing CNTs. The fresh properties of concrete were not considerably affected by the addition of CNTs, PE fibers, or their combination. However, mixes incorporating PE fibers had around 50% less workability than the control mix.
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