buildings Article Absorption and Strength Properties of Short Carbon Fiber Reinforced Mortar Composite Md. Safiuddin 1,2,3, * , George Abdel-Sayed 3 and Nataliya Hearn 3   Citation: Safiuddin, M..; Abdel-Sayed, G.; Hearn, N. Absorption and Strength Properties of Short Carbon Fiber Reinforced Mortar Composite. Buildings 2021, 11, 300. https://doi.org/10.3390/ buildings11070300 Academic Editor: Karim Ghazi Wakili Received: 8 June 2021 Accepted: 6 July 2021 Published: 8 July 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Angelo DelZotto School of Construction Management, George Brown College, 146 Kendal Avenue, Toronto, ON M5T 2T9, Canada 2 Department of Civil Engineering, Faculty of Engineering and Architectural Science, Ryerson University, 350 Victoria Street, Toronto, ON M5B 2K3, Canada 3 Department of Civil and Environmental Engineering, Faculty of Engineering, University of Windsor, 401 Sunset Avenue, Windsor, ON N9B 3P4, Canada; geosayed@yahoo.com (G.A.-S.); drhearn@gmail.com (N.H.) * Correspondence: msafiuddin@georgebrown.ca or safiq@yahoo.com; Tel.: +1-416-415-5000 (ext. 6692) Abstract: This paper presents the water absorption and strength properties of short carbon fiber reinforced mortar (CFRM) composite. Four CFRM composites with 1%, 2%, 3%, and 4% short pitch-based carbon fibers were produced in this study. Normal Portland cement mortar (NCPM) was also prepared for use as the control mortar. The freshly mixed mortar composites were tested for workability, wet density, and entrapped air content. In addition, the hardened mortar composites were examined for compressive strength, splitting tensile strength, flexural strength, and water absorption at the ages of 7 and 28 days. The effects of different carbon fiber contents on the tested properties were observed. Test results showed that the incorporation of carbon fibers decreased the workability and wet density, but increased the entrapped air content in mortar composite. Most interestingly, the compressive strength of CFRM composite increased up to 3% carbon fiber content and then it declined significantly for 4% fiber content, depending on the workability and compaction of the mortar. In contrast, the splitting tensile strength and flexural strength of the CFRM composite increased for all fiber contents due to the greater cracking resistance and improved bond strength of the carbon fibers in the mortar. The presence of short pitch-based carbon fibers significantly strengthened the mortar by bridging the microcracks, resisting the propagation of these minute cracks, and impeding the growth of macrocracks. Furthermore, the water absorption of CFRM composite decreased up to 3% carbon fiber content and then it increased substantially for 4% fiber content, depending on the entrapped air content of the mortar. The overall test results suggest that the mortar with 3% carbon fibers is the optimum CFRM composite based on the tested properties. Keywords: carbon fibers; fiber content; mortar composite; strength properties; water absorption 1. Introduction Cement-based mortar is widely used because of its many beneficial properties, in- cluding good compressive strength, high fire resistance, ease of application, and low cost. Unfortunately, cement mortar possesses very low tensile and flexural strengths, limited ductility, and little resistance to cracking. It exhibits an elastic brittle behavior under tensile stresses. The primary reason for such weakness of cement mortar is its inability to resist the initiation and growth of cracks due to relatively low tensile strength [1]. Internal mi- crocracks are inherently present in cement-based composites, such as mortar and concrete. When loaded, these microcracks propagate and gradually connect to each other. Due to poor tensile strength, the connectivity of microcracks increases and results in macrocracks that lead to the brittle fracture in cement-based composites. This inherent deficiency can be overcome by adding fibers, as they arrest the cracks and allow much larger deformation beyond the peak stress [2]. Many studies have reported that short fibers substantially Buildings 2021, 11, 300. https://doi.org/10.3390/buildings11070300 https://www.mdpi.com/journal/buildings