Hybrid effects of carbon fibers on mechanical properties of Portland cement mortar Xiang Shu, Ryan K. Graham, Baoshan Huang ⇑ , Edwin G. Burdette Department of Civil and Environmental Engineering, The University of Tennessee, Knoxville, TN 37996, USA article info Article history: Received 26 August 2014 Accepted 4 October 2014 Available online 13 October 2014 Keywords: Portland cement composite Carbon fiber Reinforcement Hybrid effect Size Mechanical properties abstract This study investigates the effects of carbon fiber with different sizes on the mechanical properties of Portland cement mortar through laboratory testing. To achieve the goal, Portland cement mortars rein- forced with hybrid-sized carbon fibers were tested for potential improvement in mechanical properties. The sizes of carbon fibers used in the study include micro-length (in micrometers), macro-length (in millimeters), and a combination of both. The properties of the mortar mixes were evaluated based on the results from indirect tension test, semicircular notch fracture test, mortar cube compression test, as well as unit weight and specific gravity tests. The laboratory test results show that the hybrid fiber mix exhibited superior tensile performance to the microfiber mix. Although all three types of fiber reinforcement improved the pre-peak load energy absorbing capacity of the mortar, only hybrid fiber blend improved the mortar’s fracture resistance. The test results also showed that the hybrid fiber mix had lower air-to-cement mortar ratio than the macrofiber mix. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Short discontinuous fibers are widely used to improve the ten- sile and bending performance of brittle materials, such as concrete [1–4]. Fibers are able to improve the properties of cement-based materials because they supply additional outlets for energy absorption. If a crack forms in a fiber-reinforced cementitious com- posite due to tensile forces and fibers are available to bridge the gap, in order for the crack to propagate, additional energy must be supplied to break the fiber–matrix bond, allowing fiber pull- out to occur, or fiber yielding or rupture must take place [1–3]. The underlying factor in determining the mode of failure is embed- ment length: there is a critical embedment length above which the fiber will rupture and below which pull-out will occur [2,3]. To fur- ther improve the reinforcement effect, hybrid fibers of different types or different sizes have been gaining popularity in recent years [5–11]. Carbon fibers in particular have garnered much attention. Com- pared to the unreinforced materials, cement composites containing carbon fibers with lengths on the order of millimeters [12,13] to one centimeter [12–14] have shown superior tensile strength [12–15], flexural strength [12–14], and flexural toughness [13]. Further enhancements to the mechanical performance of cement- based materials can be achieved by utilizing multiple sizes of fiber. Lawler et al. [5] performed flexural tests on cement mortars con- taining 0.5-mm diameter, 30-mm long, hooked steel fibers and 22-lm diameter, 21-mm long, straight steel fibers, and reported that the hybrid-fiber mix had superior peak and post-peak strength than the single-fiber mixes. Betterman et al. [6] preformed direct tension tests on cement-mortars reinforced with polyvinyl alcohol fibers with lengths of 4, 7, or 12-mm and aspect ratios on the order of 300. A hybrid-fiber mix composed of 4-mm and 12-mm fibers, each at 2% volume fraction, achieved higher peak stress and strain energy density at 6000-le than did any of the three single-fiber mixes. In a preliminary study by the authors [16], the indirect tension (IDT) tests were employed to compare the tensile performance of a Portland cement mortar reinforced with one of two lengths of car- bon fiber. While all fibers had similar diameters, the shorter fibers (microfibers) had lengths measured on the micron scale and the longer fibers (macrofibers) had an average length of 0.631-cm. The results showed that the microfibers increased the pre-peak load performance of the mortar but that they were less efficient in doing so than the macrofibers. However, even though the microfibers were not optimal for improving the tensile properties of the mix, the microfiber-reinforced mixes were much more workable than the macrofiber-reinforced mixes. Additionally, a comparison of specific gravities showed that the microfiber mixes were denser than the macrofiber mixes. Since fibers tend to increase the air content of the host mix [7,15], the specific gravity http://dx.doi.org/10.1016/j.matdes.2014.10.015 0261-3069/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +1 865 974 7713; fax: +1 865 974 2669. E-mail address: bhuang@utk.edu (B. Huang). Materials and Design 65 (2015) 1222–1228 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes