Abstract
The objective of this study is to measure the crack propagation speed in three types of self-compacting concrete reinforced with steel fibers loaded under four different loading rates. Central-notched prismatic beams with two types of fibers (13 mm and 30 mm in length), three fiber volume ratios, 0.51%, 0.77% and 1.23%, were fabricated. Four strain gages were glued on one side of the specimen notch to measure the crack propagation velocity, a fifth one at the notch tip to estimate the strain rates upon the initiation of a cohesive crack and the stress-free crack. A servo-hydraulic testing machine and a drop-weight impact device were employed to conduct three-point bending tests at four loading-point displacement rates, the former to perform tests at 2.2 m/s, 22 mm/s and the latter for those at 1.77 m/s, 2.66 m/s, respectively. With lower fiber contents, smooth mode-I cracks were formed, the crack speed reached the order of 1 mm/s and 20 m/s. However, crack velocities up to 1417 m/s were obtained for the concrete with high content of fibers under impact loading. This value is fairly close to the theoretically predicted terminal crack velocity of 1600–1700 m/s. Numerical simulations based on cohesive theories of fracture and preliminary results based on the technique of Digital Image Correlation are also presented to complement those obtained from the strain gages. In addition, the toughness indices are calculated under all four loading rates. Strain hardening (softening) behavior accounting from the initiation of the first crack is observed for all three types of concrete at low (high) loading rates. Significant enhancement in the energy absorption capacity is observed with increased fiber content.
Highlights
Crack propagation in concrete is the main mechanism of material failure and this process is often complex, especially under dynamic loading
We summarize the beams to be tested under three-point bending configuration under four loading rates in Table 2, the ones glued with strain gages will be used to measure parameters related with external loading, Pmax and Pini, the strain rates upon crack intitiation, εi and εic, the energy absorption, I5, I10, I20, R5,10 and R10,20, as well as the crack speed, VSG
The effect of fiber content (0.77%, 0.51% and 1.23%) and loading rate (2.2 μm/s, 22 mm/s, 1.77 m/s and 2.66 m/s) on the mode-I crack advancing velocity is explored using the technique of strain gages and complemented with the digital image correlation (DIC) measurement and numerical simulations
Summary
Crack propagation in concrete is the main mechanism of material failure and this process is often complex, especially under dynamic loading. Knowing at what speed and in which direction a crack can propagate under different loading conditions is of great importance. Different experimental techniques such as strain gages [1,2,3,4,5,6,7,8,9,10], acoustic emission [11] and digital image correlation (DIC) [12,13,14] have been employed to measure crack propagation velocity in concrete. The earliest measurement of crack velocity in plain concrete goes back to the work of Curbach and. Regarding fiber-reinforced concrete (FRC), Mindess et al [15] measured crack velocities of up to 190 m/s for Materials 2020, 13, 4053; doi:10.3390/ma13184053 www.mdpi.com/journal/materials
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