Loading rate effect on crack velocities in steel fiber-reinforced concrete

• Crack velocity in an SFRC under a wide range of loading rates was measured.
• The order of magnitude of loading rates changes from 10−3 to 103 mm/s.
• At quasi-static loading condition the crack velocity almost keeps constant at 10−4 m/s.
• The main crack propagates with decreasing velocity at other loading rates.
• Thecrack initiation strain rate varies from 10−6/s to 101/s.

This paper presents the recent experimental results aimed at studying the loading rate effect on the crack velocity in steel-fiber reinforced concrete. Three-point bending tests were performed by adopting either a servo-hydraulic testing machine or a drop-weight impact instrument. Four strain gauges glued along the ligament of the beam were used to measure the crack velocity and, the fifth one was bonded right in front of the notch root for detecting the crack initiation strain rate. Six different loading rates were applied, from 10−3 mm/s to 103 mm/s, i.e., a low loading rate range (3.33 × 10−3 mm/s, 0.10 mm/s and 3.33 mm/s) and a high loading rate range (8.85 × 102 mm/s, 1.77 × 103 mm/s and 2.66 × 103 mm/s). At quasi-static loading condition, 3.33 × 10−3 mm/s, the crack velocity almost keeps constant at 10−4 m/s, while the main crack propagates with decreasing velocity at other loading rates. Furthermore, the crack initiation strain rate is proportional to the loading rate, varied from 10−6/s to 101/s when the loading rate is less than 1.77 × 103 mm/s.