Construction principle of concrete dry spraying

As far as I know, the construction method of shotcrete hanging formwork began in the construction site in the late 1990s and early 2000, but so far, no relevant norms or formal regulations have been seen to stipulate and popularize this method. According to the field observation, whether dry spraying or wet spraying, its products have serious defects.

The characteristics of shotcrete technology determine its advantages, that is, the formwork supporting process is omitted, the construction speed is fast, the mixture is convenient to transport, the effect is quick, and the strength can reach the level of ordinary concrete. At the same time, because high-pressure air is used as the conveying medium, the material exits the muzzle at a high speed, so coarse particles can be embedded in the bottom material, which makes it have strong binding force, and the sprayed layer formed by the correct construction method is not easy to form a layered structure. However, due to the lack of formwork support, the high-speed solid-gas mixed flow directly acts on the rock or soil layer at the beginning, and a large number of coarse aggregates fall off due to rebound, which leads to a large deviation between the initial spray level and the design gradation, and "wastes" some aggregates. As a result, someone "invented" the method of hanging formwork and spraying concrete.

Due to the high initial velocity of the mixture when spraying concrete, it generally reaches15 ~ 20m/s. When the mixture contacts solid surfaces, such as rocks and hard soil layers, fine aggregate and cement binder are easy to adhere to the spraying surface, while coarse aggregate is easy to rebound and fall when colliding with hard spraying surface due to its large particle energy. Under the condition of not hanging the mold, the falling objects will become waste products, but the compactness of the successfully bonded spray coating can be guaranteed, thus ensuring the strength of the spray coating. Because the thickness of the spray layer is usually not large, even if the mold is hung, it is inevitable that the solid-gas mixed flow will not rebound on the spray surface, and the rebound substances are usually concentrated on one side of the template, which actually forms a poorly graded spray layer. Moreover, the template limits the injection angle of the spray gun, which easily leads to the non-compact layer, thus leading to the defect of low local strength.

Whether dry spraying or wet spraying, the defects of hanging formwork construction are obvious, and the only advantage is to "save" rebound materials and reduce costs, which may be the reason why hanging formwork spraying construction is widely used in practice. After demoulding, there are a lot of honeycomb pits on the surface of spray coating, and some construction units will cover the spray coating again to cover up these defects. However, if on-site drilling and coring inspection and adhesion test are carried out in the hanging mold injection area, it is easy to find the fact that the qualified rate is low.

Some people think that it is wrong to use hanging formwork wet spraying method to be equal to or similar to pouring normal concrete! Normal concrete must be vibrated and compacted, and it is obviously impossible to vibrate by hanging formwork wet spraying, or it has no intention of vibrating at all. Even if pumping construction is used for normal concrete, the speed of material entering the warehouse is much lower than that of spraying construction, and it will not rebound due to high speed, thus avoiding the separation of a large number of aggregates; Aggregate separation is inevitable in spray construction, so it needs to be considered in mix design.

In a word, there are three main hazards of hanging formwork spraying technology: first, coarse aggregate lacking binder is wrapped in engineering structure, forming low strength defects; Second, the template limits the spraying angle, which makes the dense spraying layer unable to be dense, resulting in insufficient strength; Thirdly, it takes precious time to hang the formwork, which slows down the shotcrete layer construction that should be quick and effective, and reduces the significance of shotcrete. As engineers and technicians, we must resolutely resist the hanging formwork spraying construction!