Impact resistance of fiber-reinforced concrete – A review
Introduction
Concrete has been widely used as a construction material in combination with deformed steel reinforcing bar (rebar) and prestressing strand. Since they have great compressive strength, the steel rebar or strands are only adopted in zones in which tensile or shear stress occur, which have been called reinforced concrete (RC) or prestressed concrete (PSC) elements. The enhanced tensile or shear resistance of RC and PSC leads to their successful use as structural elements under quasi-static loading conditions. However, in recent years, civil structures or buildings have frequently been exposed to extreme loading conditions, such as impacts, blasts, and fire from a variety of sources, including terrorist attacks. Although ordinary RC and PSC structures are successfully used under static conditions, they are insufficient under extreme loads because of the poor energy absorption capacity and brittle nature of concrete, which lead to its fragmentation. To overcome the drawbacks of plain concrete under impacts and blasts, researchers [[1], [2], [3]] have suggested concrete strengthened with continuous textiles, discontinuous short fibers, external fiber-reinforced polymer, etc. Among others, the inclusion of discontinuous fibers made of materials such as steel, polymer, carbon, and basalt, has been most widely adopted by researchers because of its several advantages: (1) they are easy to include in concrete mixtures, (2) they are effective in enhancing concrete's toughness under impact or blast by fiber bridging, and (3) they are more cost effective than other methods.
Concrete that contains discontinuous fibers with random orientation is called fiber-reinforced concrete (FRC). The randomly orientated fibers can effectively resist crack propagation and widening in the cement matrix, improving the post-cracking ductility of concrete under both static and impact loads. The fibers’ effectiveness in enhancing post-cracking ductility depends on their bond performance, which is affected by factors such as the number of fibers per unit area, fiber orientation, fiber shape and aspect ratio, matrix strength, etc. Thus, to properly design FRC for practical application to civil structures and buildings, the factors affecting post-cracking ductility must be comprehensively investigated. The comprehensive mechanical properties and developments of FRCs including various fiber types (i.e., steel, glass, synthetic, and carbon fibers) in static conditions were reviewed by Brandt [4]. If the properties of FRCs are independent of the loading rate, the previous review paper [4] can provide useful information to researchers and engineers who are interested in using FRCs for protective structures under extreme loads. However, unfortunately, plain concrete and FRCs are both very sensitive to loading rates (i.e., strain or stress rates), exhibiting totally different behaviors under impact as compared to static conditions, thus requiring a new review of the state of the art on the impact resistance of FRC. In this paper, several important points regarding the impact resistance of FRCs are addressed as follows: (1) a summary of current impact testing methods; (2) some limitations and solutions of current impact testing methods; (3) the general impact behaviors of FRCs regardless of fiber type, (4) the specific impact response of FRCs by fiber type, i.e., steel, polymer, carbon, basalt, and natural sources; and (5) the comparative impact resistances of FRCs by fiber type, which suggests the best ones for use in protective structures. Finally, we examine the effect of supplementary cementitious materials (SCMs), which are now widely used in eco-friendly concrete mixtures, on the impact resistance of FRCs.
Section snippets
Types of impact test methods
Several types of impact test methods are available worldwide, as shown in Fig. 1. Kim et al. [5] categorized the high strain-rate test methods into four classes: (1) methods based on potential energy, in which a large mass freefalls onto the specimens (i.e., the drop-weight impact, Charpy, and Izod tests); (2) methods based on kinetic energy, in which a mass strikes the specimen very rapidly (i.e., the gas gun and fiber pullout impact tests); (3) methods in which hydraulic machines deform
Common properties of FRC under impact
The strength of FRC is sensitive to the rate of loading [34], mainly because (1) crack growth resistance is enhanced (greater stress intensity factor KI) with increasing crack velocity under impact [35] and (2) the crack path is altered and shortened with increasing loading rate because crack propagation is significantly slower than applied stress [36]. Increased crack velocity was successfully observed from Bindiganavile and Banthia [35] using a contoured double-cantilever beam test under
Dynamic crack growth resistance of FRC
In contrast to ordinary concrete, FRC can provide higher closing pressure at crack surface due to fiber bridging effect, acting behind the propagating crack tip where fibers undergo bond-slip process [49] and mitigating the stress intensity factor [50]. The fracture process of FRCs is thus more complex, and a sophisticated model is required to simulate it properly. Previously, cohesive crack model [51] and J-integral [52] were applied to model the fracture behavior of FRC, but these are
Impact resistance of various types of FRC
FRC exhibits significantly improved impact resistance compared to plain concrete. Due to the fiber bridging effect at crack surfaces, fiber reinforcement is effective in improving the energy absorption capacity of concrete under impact. However, as indicated by Banthia et al. [21], the improvement depends on the fiber type and geometry; as a result, the impact resistance of FRC must be analyzed by fiber type and geometry. This section comprehensively summarizes and analyzes the impact
Steel fibers vs. polymeric fibers
Bindiganavile and Banthia [6] examined the pullout resistance of a straight polyolefin (PO) fiber, a sinusoidally deformed PP fiber, and a flat-end steel fiber in concrete with a 28-day compressive strength of 40 MPa. A higher pullout resistance was measured when using the steel fibers up to a COD rate of 2000 mm/s. The bond strengths of all polymeric fibers increase with loading rate, but the steel fiber showed a decrease in bond strengths at very high loading rates (COD rates of
New findings on the strain-rate sensitivity of FRCs
Fig. 12a shows the relationship between the DIF on compressive, tensile, and flexural strengths and strain-rate of FRCs with various fiber types and volume fractions. It was obvious that the least loading rate sensitivity was obtained for the compressive strength, while the highest sensitivity was found from the tensile strength. The flexural strength term provided an intermediate rate sensitivity because it is subjected to both the compressive and tensile stresses in the cross section. This
The effect of supplementary cementitious materials on the impact resistance of FRC
Alhozaimy et al. [99] investigated the effect of SCMs, such as fly ash, silica fume, and slag, on the impact resistance of FRC that includes 0.1 vol% PP fibers. Replacing a portion of cement with SCMs deteriorated the impact resistance of plain concrete. However, replacing a portion of cement in PP FRC with SCMs led to great improvement in its impact resistance, because pozzolanic action improved the bond performance of PP fibers in the cement matrix. The impact resistance at failure of PP FRC
Conclusion
This paper comprehensively reviewed the impact resistance of ordinary FRC with various fiber types. Based on literature reviews, several important findings were obtained, and the following conclusions could be drawn from the above discussions.
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The strain-rate sensitivity of FRCs differs according to loading condition, matrix strength, and saturation. Tensile impacts lead to the highest rate sensitivity, followed by flexural and compressive impacts. Matrix strength also affects the strain-rate
Acknowledgements
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2017R1C1B2007589).
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