With the continuous progress of the production technology of wet spraying machinery and related materials and equipment, wet sprayed concrete will be widely used as a mature construction method.
1. Overview of sprayed concrete technology
1.1 Sprayed concrete process principle and characteristics
The concrete with the accelerator is sprayed on the surface of the rock or structure with a jet machine, so that the rock or structure is strengthened and protected. The compactness and overall control of sprayed concrete is different from the principle of vibration compaction of ordinary cast concrete. The sprayed concrete is compacted by high-speed jets, and its compactness is closely related to the jet process. Sprayed concrete construction is simple, eliminating the need for formwork, pouring and demoulding processes, the construction progress is faster, and the compactness is higher than ordinary concrete, and the strength and impermeability are good.
1.2 The main role of sprayed concrete
(1) Fill the exposed joints and cracks to improve the overall strength and stability of the surrounding rock.
(2) Concrete is a kind of support method that can contact large areas of surrounding rocks firmly.
(3) Through the adhesion and shear force of the contact surface, the external force acting on the sprayed concrete is dispersed on the surrounding rock to prevent local block loss, and a bearing ring is formed near the tunnel wall surface to control the stress release of the surrounding rock.
(4) Fill the concave surface of the surrounding rock to prevent stress concentration and strengthen the weak layer. The earth pressure is transmitted to the steel support and anchor rod. After excavation, the wall surface is covered in time to prevent weathering of surrounding rocks.
2. Wet shotcrete construction technology
2.1 Construction preparation
(1) Check the contour size of the sprayed surface and trim it to conform to the design. If there are loose or broken parts, it must be cleaned.
(2) Water proof and drainage treatment should be done in advance in areas with obvious water seepage.
(3) Wash the rock surface with high pressure wind and water before spraying, and clean the weak and broken rock. The flushing starts from the tunnel vault and is washed from the top to the bottom of the side wall in order.
(4) Prepare scaffold or working platform and iron gauze cover.
(5) Check whether the machinery, pipelines, safety valves and pressure gauges are intact and carry out trial operation.
(6) Mark the design spray thickness on the vault, side wall or rock surface.
2.2 Technical points of wet spraying operation
(1) When concrete is mixed in the order of sand → crushed stone → cement → water, full-automatic metering force mixer is used for mixing, and the construction ingredients are strictly operated according to the mixing ratio; after adding the compound pumping agent, the mixing is performed for more than 2 minutes. Over 4 minutes, so as not to affect the quality of the concrete.
(2) When spraying concrete, spray from the bottom of the tunnel to the top and move in an S curve to prevent the rebound material from contaminating the washed rock surface and affecting the quality of sprayed concrete; After final setting, if it is sprayed again more than 1 hour after the final setting, the concrete surface should be cleaned by feng shui. The tunnel starts spraying from the bottom of the side walls on both sides and closes to the centerline of the vault to complete a test spray of a ring of sprayed concrete.
(3) Wind pressure should be strictly controlled when spraying concrete. If the wind pressure is too small, the spraying force will be weakened, which will not meet the requirements for the compactness of the sprayed concrete, or even meet the design and specification requirements. The subsequent strong impact blows away, and both will increase the rebound of the aggregate, thereby directly increasing the rebound. Generally, the initial wind pressure ≥0.5 MPa is required to start the operation, and the wind pressure is adjusted according to the discharge of the nozzle. The side wall wind pressure should be controlled between 0.3 and 0.5 MPa, and the arch part should be controlled between 0.4 and 0.65 MPa.
(4) When spraying concrete, the appropriate initial shot thickness should be determined according to the type of surrounding rock and hydrogeological conditions: Type II surrounding rock is divided into 3 times, type III surrounding rock is divided into 2 times, and type IV surrounding rock is divided into 1-2 times; Using wet spraying, the thickness of the initial sprayed layer was determined to be 4 to 5 cm, and the thickness of the second sprayed layer was more than 10 cm.
(5) When spraying without steel mesh, the nozzle should be as close as possible to the rock surface at an angle of 90 °, and should be biased towards the newly sprayed part (the inclination angle should be controlled within 10 °), maintaining a distance of 0.8 to 2 m to ensure the compactness of the concrete. The sprayed surface on which the steel mesh is hung, the fuel injection is slightly inclined, and the distance should be controlled within the range of 0.6 to 1.0 m.
(6) The spray head should run uniformly along the spiral trajectory, divided into sections, generally not more than 6 m, from top to bottom, press a circle for half a circle, and move slowly, each circle diameter 15 ~ 20 cm; if the spray surface is uneven, it should First spray the pits to level.
(7) Water spray curing should be carried out 1 hour after the final setting of sprayed concrete to keep the sprayed surface moist and the curing time should be no less than 7 days.
2.3 Precautions in wet shotcrete construction
(1) Before spraying, the concrete wet sprayer and the feeding pipe must be lubricated with cement slurry. After the spraying is finished, the concrete remaining in the pipeline is rinsed with water.
(2) Strictly control the wind pressure of the working machine in the range of 0.15 ～ 0.2 MPa.
(3) Strictly control the water-cement ratio, with the degree of faint gloss on the surface of the freshly sprayed concrete.
(4) For the treatment of water seepage areas, when the amount of water is not large, blow it with compressed air before spraying the concrete, and spray the concrete mix and mix according to normal construction procedures; when the amount of water is large, reduce the water appropriately when starting spray The amount of spraying is from far to near, temporarily increase the amount of accelerator (≯5%) and gradually close. After the water is stopped, it must be closed with sprayed concrete with normal accelerator setting.
(5) In the spraying process, the sprayer should always observe the nozzle condition. Once the pipe is blocked, the assistant should immediately contact the operating driver, stop the machine, shut off the wind, and check whether the pipeline is unblocked.
(6) If the local sprayed concrete is too thick, the protruding concrete must be removed before hanging the waterproof board. Because this situation may break the waterproof board, it is more likely to form a point contact between the initial support and the secondary lining, causing local stress.
Concentration causes cracks in the surface of the secondary lining.
(7) The shot firing time from the shot to the next cycle should be determined through tests, generally not less than 4 hours. The concrete should be inspected after shot firing. If cracks occur, the shot firing interval or blasting parameters should be adjusted.
(8) After the concrete has set, the strength of the sprayed concrete needs to be tested. In the strength test, the cutting core method should be used to take samples to approximate the true condition of the sprayed concrete. If the sampled concrete cannot meet the design requirements, it should be
Take timely measures to remedy.
(9) When spraying concrete, pay attention to observe the rock surface from the arch waist to the vault, strengthen labor protection, and prevent the concrete and rock surface stones from falling and hurting under the impact of high-pressure sprayed concrete.