Reasons for the rebound of shotcrete and its control measures

Reasons for rebound

1. The unsatisfactory shaping of the smooth surface of the tunnel results in irregular rock faces of the tunnel and different rebound angles, resulting in staggering rebound and crossover, which increases the rebound rate.(2) The sand ratio of the mix ratio is small, and the sand ratio should be 50~60%. When the concrete sand ratio is less than 50%, it is easy to block the pipe and cause high concrete rebound; when the sand ratio is higher than 60%, the concrete strength decreases and shrinks. Increase.(3) The addition amount of accelerator is small or uneven, causing the shotcrete to be blown off by the high-pressure wind before it reaches the setting state.(4) The spray speed (wind pressure), the best spray speed (wind pressure) is not grasped.(5) The operator has not grasped the spray angle and spray distance well.(6) The thickness of one-shot is too thick, which exceeds the weight of the concrete and falls under the influence of wind pressure.(7) The sprayed concrete water is not clean, oily, or dirty.

Rebound control countermeasures

The influencing factors of rebound come from many aspects: such as injection pressure, nozzle angle, setting time of accelerator, optimal content of accelerator and adding method, quality of rock surface, proficiency of sprayer, the thickness of spray layer, and so on. Welcome to visit this site: to get more information about concrete.

Under normal circumstances, the concrete flow rate at the nozzle is determined, so the impact force on the interface depends on the distance between the nozzle and the interface. If the distance is too small, the layered shape will not be formed, because the shotcrete will be blown away by the subsequent strong impact as soon as it touches the interface; if the distance is too large, the shotcrete cannot meet the bonding and compaction requirements due to insufficient impact.. Either of the two will cause a sharp increase in the amount of rebound because there is very little concrete that can be sprayed onto the rock surface. On the contrary, if the distance between the nozzle and the rock surface is adjusted according to the flow rate of the concrete at the nozzle, the rebound can be controlled to a minimum range. How to accurately grasp the nozzle distance is very important for the sprayer.

There is no doubt that the structure and hardness of the interface have a great influence on the rebound. Normally, the rebound angle is equal to the incident angle. If the interface is smooth and the nozzle remains perpendicular to the interface, this can reduce the rebound. If you encounter irregular rock faces, such as uneven rock faces, the rebound angles of each small surface are different, resulting in staggering rebound and collision. Therefore, the rebound caused by an uneven interface is greater than that of a smooth interface. This illustrates the importance of keeping the nozzle perpendicular to the interface. Although it may sometimes be achieved, the sprayer should try his best to overcome difficulties and strive to reduce rebound. This can be confirmed by closely observing the spray surface during spraying. When the sprayed material hits a hard rock surface, such as rock or concrete, within a few seconds of hitting the rock surface, the coarse aggregate particles cannot adhere. The reason is that the buffer layer has not yet formed. Only after the thin layer composed of aggregate and cement slurry can the coarse aggregate be gradually absorbed. Using sodium lignosulfonate chemical is a good way to improve concrete workability.

The amount of rebound depends on the thickness of the spray layer. Almost all the coarse aggregates rebounded at the beginning of spraying. When a certain thickness of the sprayed layer is formed, the rebound will gradually become smaller. Another factor that determines the rebound is the speed of the concrete from the nozzle to the interface. The harder the interface, the more rebound particles. However, the impact speed should not be too low, otherwise, the compression strength and adhesion will be insufficient and the rebound will increase again; the quick-setting admixture must be added uniformly, and the content of the quick-setting admixture should be ensured or appropriately increased according to the mixing ratio In order to facilitate the construction, some construction parties randomly added or even did not add accelerators, which caused a large amount of concrete rebound; In summary, it shows that concrete rebound is caused by multiple factors and is unavoidable, and it also shows the importance of the sprayer.

Kingsun is a professional concrete admixtures manufacturer and supplier.