The role of shotcrete
Like concrete dry/wet jets, shotcrete has two construction processes: “dry spray” and “wet spray”. The dry spray method is to grind cement, sand and stones with a maximum particle size of less than 25 mm, and then mix them in a certain proportion, put them into a spray machine, and use compressed air to transport the dry mixture along the pipeline to the nozzle, mix it with water and High-speed injection of 40 to 60 m / s to the work surface.
The wet spray method is to pre-mix the raw materials with water and spray them. In the construction of shotcrete, due to the collision of cement particles and aggregates, continuous extrusion, and the use of a smaller water-cement ratio, the concrete has sufficient compactness, high strength and good durability.
1. Supporting role
Shotcrete has certain physical and mechanical performance requirements, among which compressive strength is one of its main properties, and the general compressive strength can reach 20 MPa, so it can play the role of supporting ground pressure. The concrete formed by layering is dense, and the compressive and shear strength is higher than that of concrete poured under the same conditions. During the spraying construction, a certain amount of quick-setting admixture is added to make the concrete condense fast, and the early strength is high. It can follow the excavation working face and play the role of supporting the surrounding rock in time to make the pressure relaxation zone caused by the blasting of the surrounding rock. Not too big a development.
2. filling effect
The dry mix is transported by compressed air and has a high velocity at the exit of the spray gun. It can well fill the cracks and joints of the surrounding rock and fill the rock face of the pocket. The concrete filled in the cracks or cracks not only has a high cohesive force, but also increases the friction between the fractures of the rock layers and acts as a "wedge", so that the originally separated rock mass can be closely combined. Being a whole effectively prevents the relative movement between the rock masses and enhances the support capacity of the surrounding rock itself.
The shotcrete is directly and tightly bonded to the rock, so it can completely isolate the contact between the air and the surrounding rock, and can avoid the surrounding rock slab and roof due to weathering and deliquescence. At the same time, due to the concrete filled in the surrounding rock fissure, The original filling in the depth of the crack will not reduce the strength due to weathering. As a result of the insulation, the surrounding rock maintains its original stability and strength.
The concrete layer formed by high-speed spraying onto the rock surface has high adhesion and high strength. The concrete and the surrounding rock are tightly combined to transmit various stresses on the joint surface. The results of the isolation and filling effects improve the stability of the surrounding rock and its own supporting ability, thus forming a common mechanical homogeneity between the shotcrete layer and the surrounding rock, and transforming the surrounding rock load into a rock bearing structure. The role has fundamentally changed the weaknesses of various supportive negative support in the past.