Faineng YENGYANG

Improvement of Crack Resistance in Shotcrete Reinforced with Basalt Fibers

Takumi SHIMOMURA

Shotcrete is widely used in applications such as tunnels and slope stabilization, but it sometimes generates shrinkage cracks, which reduce the water-tightness and durability of the shotcrete. This study focuses on the use of basalt fibers and wollastonite to prevent cracks and to reduce crack width in shotcrete. The crack resistance of shotcrete incorporating these materials was investigated through restrained shrinkage tests.
Three mixture types of mortar were tested: plain (N), basalt fiber-reinforced (B), and a combination of basalt fiber and wollastonite (BW), all with a water-to-cement ratio of 55%. Ring type specimens were prepared using steel rings with a height of 15 cm and an outer diameter of 40.6 cm. To observe varying shrinkage behaviors and cracking patterns, two concrete thicknesses were adopted: 21.8 mm (Series 1) and 46.8 mm (Series 2). After a seven-day curing period, strain changes were measured under controlled environment at 20°C and 60% humidity.
The measurement results strain was missed during the initial four days of drying. Shrinkage cracks occurred during this unmeasured period. Even if differences in cracking dates were observed among the mixtures during these four days, such variations would be limited to a few days at most. Therefore, it was concluded that the addition of fibers does not effectively prevent crack initiation or significantly delay the timing of the first crack.
However, a comparison of total crack widths on the 19th day revealed that mixtures B and BW exhibited markedly smaller crack widths than the plain mixture (N) in both series. Observations of the crack surfaces confirmed that exposed basalt fibers provided a "bridging effect," anchoring the concrete on both sides of the crack and restricting its expansion. While the specific contribution of wollastonite was difficult to verify visually, the results clearly demonstrate that basalt fiber reinforcement, while unable to suppress crack initiation, is highly effective to reduce crack width.