Issue 41

L. Zhang et alii, Frattura ed Integrità Strutturale, 41 (2017) 356-368; DOI: 10.3221/IGF-ESIS.41.47 359 Figure 1 : Dosage change curves of initial and final setting time The liquid accelerator has different setting time with different dosages. Fig. 1 shows that this liquid accelerator has the best accelerating effects when the dosage is 2%. The initial and final setting time is 1min30s and 3min20s respectively, which meets the requirements of first-class goods stipulated in the Standard. With increasing dosage, the setting time is delayed on the contrary. Indicating that the liquid accelerator has an optimal dosage, it is not that more accelerators make better accelerating effects. Whether the accelerator can quickly condense and harden cement also depends on accelerator adaptability to cement mineral compositions and gypsum types instead of all depending on accelerator dosage. As there are many interacting internal and external factors affecting accelerator adaptability to cement, the mechanism is very complex. Therefore, the optimal accelerator dosage can only be the result of the adaptability test conducted on the used cement and accelerator in a proper water-cement ratio range and at a certain ambient temperature. A dosage lower or higher than the optimal will prevent the accelerator from playing its proper role in shotcrete application. (2) Measurement of the setting time of cement with different water-cement ratios According to the requirements of the Standard, keep 2% dosage of liquid accelerator unchanged, the water-cement ratio ranges from 0.30 to 0.50. Fig. 2 shows the initial and final setting time of cement with different water-cement ratios. Figure 2: Water-cement ratio change curves of initial and final setting time. According to Fig. 2, as the water-cement ratio increases, the initial and final setting time is prolonged. Indicating that the greater the water-cement ratio, the worse the accelerating effects. In dry-mix shotcrete, the water-cement ratio is controlled by adjusting the water volume by the shotcrete manipulator based on his observation. The mixture water consumption in dry shotcrete should not only make the shotcrete have better compaction and adhesiveness but also reduce resilience materials. A too large water-cement ratio usually causes shotcrete to fall off while a too small water- cement ratio causes stratification to mixtures. In wet-mix shotcrete, the water-cement ratio is specific and needs to meet properties such as the slump and pumpability. (3) Impact of the accelerator with different dosages on the strength of cement mortar 50 100 150 200 250 300 350 1,5 2,0 2,5 3,0 3,5 4,0 Setting time/s Dosage of liquid accelerator/% Initial setting time Final setting time 50 100 150 200 250 300 350 0,30 0,35 0,40 0,45 0,50 Setting time/s Water-cement ratio Initial setting time Final setting time