Abstract
The reasonable inclusion of hybrid fibers can leverage the advantages of each kind of fiber and enhance the frost resistance and flexural toughness of concrete. Previous studies on hybrid steel-polypropylene fiber reinforced concrete (HSPFRC) focused primarily on its mechanics instead of its frost resistance. In this work, the compressive strength, splitting tensile strength, mass loss rate, relative dynamic elastic modulus (RDEM), and flexural toughness of HSPFRC after freezing-thawing (F-T) are studied, and the relative importance of each factor affecting the frost resistance of HSPFRC is quantified by using fuzzy rough set theory. The results show that the inclusion of hybrid fibers has a noticeable effect on the frost resistance of HSPFRC after hundreds of F-T cycles and that the effect on the splitting tensile strength is greater than that on the compressive strength. After 500 F-T cycles, as the steel fiber (SF) content increases, the compressive strength and splitting tensile strength increase by factors of approximately 5 and 4, respectively, the flexural toughness is strengthened, and the mass loss rate is reduced by more than 90%. The addition of polypropylene fibers (PFs) has a relatively small effect on the strength of HSPFRC but reduces the mass loss of HSPFRC by almost 80%. However, the suitability of the RDEM for evaluating the frost resistance of HSPFRC remains uncertain. Quantified by fuzzy rough set theory, the weights of the factors affecting the frost resistance of HSPFRC are 0.50 (number of F-T cycles) > 0.35 (SF content) > 0.15 (PF content), verifying the experimental results.
Highlights
Fiber reinforced concrete (FRC), a new type of high-strength and tough cementitious material, consists of a matrix and reinforcing materials [1]
(1) JSCE steel fiber (SF)-4 method e flexural toughness coefficient suggested by JSCE SF-4 can be calculated as follows: where Epost is the difference between the area under the load-deflection curve at a deflection of (L/m) and that at the initial crack deflection (N·mm); (L/m) is the selected midspan deflection, which is larger than the initial crack deflection, with values of 3 mm, 4 mm, and 5 mm selected in this study; and δpeak is the midspan deflection of the flexural member when it reaches the maximum bearing capacity and is taken as the initial crack deflection in this study
Effect of F-T Cycles. e mass loss of hybrid steel-polypropylene fiber reinforced concrete (HSPFRC) is positively associated with the number of F-T cycles (Figure 6)
Summary
Fiber reinforced concrete (FRC), a new type of high-strength and tough cementitious material, consists of a matrix (i.e., mortar or concrete composite) and reinforcing materials (i.e., metal fibers, inorganic nonmetallic fibers, synthetic fiber, or natural organic fibers) [1]. SFs with a high elastic modulus have a substantial reinforcing effect but are expensive, while PFs with a low elastic modulus are relatively cheap; HFRC mixed with the above two fibers may exhibit excellent mechanical performance and a relatively low cost [20]. The occurrence of frost heave in concrete will result in the accumulation of internal damage and even substantial structural deterioration [27] In this context, appropriate contents of SFs and PFs can have a “positive hybrid effect” and can promote the frost resistance of concrete [26]; an excessive fiber content will cause a “negative hybrid effect” [22]. The flexural toughness of HSPFRC will be evaluated by using the JSCE SF-4 [28], postcrack strength (PCS) [29], and CECS 13-2009 [30] methods, and the applicability of the above three methods will be discussed
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