Issue 41

L. Zhang et alii, Frattura ed Integrità Strutturale, 41 (2017) 356-368; DOI: 10.3221/IGF-ESIS.41.47 366 Figure 13 : Field dry-mix shotcrete. Field Shotcrete Test (1) Cement: 425# ordinary Portland cement. Fine aggregate: used in the test is hard medium-coarse sand with the fineness modulus greater than 2.5. The proportion of particles with the diameter less than 0.075 mm among sand cannot be exceed 20%. Coarse aggregate: used in the test is crushed stone with the particle diameter ranging from 5 to 10 mm. (2) The concrete mix proportion of cement, sand, to stone as 469kg : 912kg : 912kg, the water-cement ratio is designed to 0.45. (3) The dosage of the liquid accelerator is designed to 1.5% during spraying on the side wall and that is designed to 2.5% during spraying on the vault. Given a larger dosage, the cohesiveness of shotcrete becomes better and slurry shedding can hardly happen. The dosage is controlled by the high-pressure dosing device. This liquid accelerator is used together with a matching shotcrete method. Based on the original dry-mix shotcrete technology, a high-pressure accelerator dosing device is added, which can provide 1.2 MPa pressure to pump the liquid accelerator to the high-pressure water pipe and accurately control the dosage of the liquid accelerator. The mixture of the liquid accelerator and water contacts with the concrete mix at the nozzle and are sprayed to the surface together. Using the new liquid accelerator and matching spraying technology, the dust and resilience in the tunnel are relatively little. The pull canvas is adopted to measure resilience and the resilience rate is 8%. The sprayed surface has bright luster, without slurry shedding. Fig. 14 shows the liquid accelerator matching spraying technology. Fig.15 shows the new field shotcrete technology. Figure 14 : The liquid accelerator matching spraying technology. Figure 15 : New field shotcrete technology. During field spraying concrete, the jet molding test is conducted to measure the strength of shotcrete. The specimen is a 150 mm 3 cube template that is grounded flush after tunnel shotcrete. Then, it is maintained in the lab incubator and the 1d, 7d, and 28d uniaxial compressive strength are measured, as listed in Tab. 9. The test results show that shotcrete using this liquid accelerator has a higher compressive strength.