Underwater concrete jetting construction process

Abstract

The application discloses a kind of underwater shotcrete construction technology, including forming a low resistance work area in the underwater side of work surface, concrete is shot from the inside of low resistance work area to work surface, complete reinforcement repair construction;Resistance reducing agent is mixed with compressed gas and then sprayed to form air curtain, for low resistance work area, resistance reducing agent mass concentration is 0.01-0.65%, compressed gas is compressed air with air pressure 0.3~1.5MPa;Interface release agent is added when mixing concrete, its dosage is 0.1~0.5% of the mass of cementitious material, early strength alkali-free accelerator is mixed with pressure gas to form a mixture, after mixing concrete is completed, it is mixed with the above mixture before entering low resistance work area and then sprayed out.The application creates a low resistance work area underwater, reduces the dispersion risk of concrete resistance to water during the spraying process;At the same time, combined with the high cohesion and fast setting performance of shotcrete material, the thickness and compactness of one spraying are improved, and the construction of underwater shotcrete is realized.

Description

An underwater shotcrete construction process Technical Field

[0001] This invention relates to the field of underwater shotcrete construction technology. More specifically, this invention relates to an underwater shotcrete construction process. Background Technology

[0002] In recent years, on the one hand, the number of large-scale infrastructure projects in my country has been increasing, placing higher demands on the durability of concrete; on the other hand, as a large number of buildings in my country enter their aging period, many buildings need to be renovated, repaired, and reinforced, making shotcrete an increasingly important reinforcement and repair material. Marine structures, in particular, suffer severe damage from the corrosive effects of the marine environment, resulting in a service life far shorter than that of terrestrial concrete, leading to a growing demand for repair and reinforcement. Currently, shotcrete technology can only meet its construction requirements in terrestrial environments. In water, due to the high resistance of the medium, the concrete sprayed by high-pressure gas decomposes rapidly upon encountering resistance, causing the concrete paste to fully combine with water and become diluted, losing its bonding properties and failing to adhere to the repair surface, thus failing to achieve the purpose of reinforcement and repair.

[0003] Therefore, the present invention provides a construction process for sprayed concrete, which can not only improve the underwater anti-dispersion ability of concrete and reduce the water resistance in the working space, but also shorten the setting time of concrete, improve the early bond strength with the working surface, and greatly improve the bonding ability between concrete and the repair working surface, thereby realizing underwater wet spraying construction of concrete. Summary of the Invention

[0004] One objective of this invention is to provide an underwater shotcrete construction process that reduces the risk of concrete dispersion due to water resistance during spraying by creating a low-resistance working zone in the underwater area; at the same time, by combining the high cohesiveness and fast setting properties of shotcrete materials, it improves the thickness and density of a single spraying, thereby enabling the construction of underwater shotcrete.

[0005] To achieve these objectives and other advantages according to the present invention, an underwater shotcrete construction process is provided, comprising forming a low-resistance working zone underwater next to the working surface, and spraying concrete from inside the low-resistance working zone onto the working surface to complete the reinforcement and repair construction.

[0006] Preferably, an interface release agent that increases the cohesiveness and hydrophobicity of the concrete is added during mixing, with the amount being 0.1 to 0.5% of the mass of the cementitious material. The resulting concrete has a slump extension of 350 to 500 mm and a slump retention time of 120 to 180 min.

[0007] Preferably, the early-strength alkali-free quick-setting agent is mixed with pressurized gas to form a mixture. After the concrete is mixed, it is mixed with the above mixture before entering the low-resistance working area and then sprayed out. The dosage of the early-strength alkali-free quick-setting agent is 1 to 5% of the mass of the cementitious material.

[0008] Preferably, the drag-reducing agent is mixed with compressed gas and then sprayed out to form an air curtain, creating a low-resistance working area. The drag-reducing agent has a mass concentration of 0.01-0.65%, and the compressed gas is compressed air with a pressure of 0.3-1.5 MPa.

[0009] Preferably, the interface isolating agent is an aqueous solution of hydroxypropyl cellulose grafted copolyacrylamide with a molecular weight of 8 million and a solution concentration of 0.5%; the drag reducing agent is a mixture of dodecyltrimethylammonium chloride and polyvinylurea in a mass ratio of 1:1; the early-strength alkali-free quick-setting agent includes 40% polyaluminum sulfate, 5% modified aluminum hydroxide, 5% modified polyol amine, 0.2% lithium sulfate, 0.3% nano silica solution, 1.0% ultrafine hydrated magnesium silicate, and the remainder is water.

[0010] Preferably, the specific steps include the following:

[0011] Step 1: Water-based concrete mixing; the concrete is extended underwater through a concrete pump pipe to be sprayed.

[0012] Step 2: The concrete pump pipe is connected to the concrete spray gun head. Move the concrete spray gun head to the low-resistance working area and keep it 30-120cm away from the working surface.

[0013] Step 3: Create a low-resistance working zone, 10-50cm away from the working surface. Spray concrete from inside the low-resistance working zone onto the working surface until the reinforcement and repair work is completed.

[0014] Preferably, an interface release agent is added to the concrete during mixing in step one; in step two, the concrete pump pipe and the concrete spray gun head are connected by an annular fixed joint, and the early-strength alkali-free quick-setting agent is mixed with pressurized gas to form a mixture, which enters the annular fixed joint through the quick-setting agent injection pipe, mixes with the concrete, and is then sprayed out.

[0015] Preferably, the low-resistance working area is formed by an underwater shotcrete device, which includes a fixed support, a spiral feeding pipe installed on the fixed support, and a drag-reducing agent injection pipe connected to the spiral feeding pipe. The concrete spray nozzle is located inside the spiral feeding pipe, which is a hollow pipe with several one-way nozzles. The drag-reducing agent injection pipe sprays drag-reducing agent, which is mixed with gas to form an aerosol, from the several one-way nozzles of the spiral feeding pipe to form an air curtain inside the spiral feeding pipe, thus creating a low-resistance working area. The drag-reducing agent injection pipe is connected to a mixing tee, which is also connected to an air inlet pipe and a drag-reducing agent inlet pipe.

[0016] Preferably, the fixed support includes a cross support frame and four slides fixed and perpendicular to the outer end of the cross support frame. The center of the cross support frame is connected to an annular fixed joint through a hollow connecting ring. The spiral feeding pipe is fixedly connected to the four slides. Each of the four outer ends of the cross support frame is provided with a pulley that cooperates with the corresponding slide. The four slides are driven by a drive mechanism to move linearly relative to the plane of the cross support frame to adjust the distance between the spiral feeding pipe and the working surface. The ends of the four slides near the working surface are provided with shock-absorbing rubber airbags and distance detectors.

[0017] Preferably, the air inlet pipe is also connected to a mixing four-way connector, which is also connected to an accelerator feed pipe and a pair of accelerator spray pipes. The pair of accelerator spray pipes are symmetrically connected to an annular fixed joint so that the accelerator is sprayed out after being mixed with the concrete.

[0018] Preferably, the concrete spray gun head has a tapered structure with a gradually decreasing diameter.

[0019] The present invention has at least the following beneficial effects:

[0020] 1. This invention reduces the risk of concrete dispersion due to water resistance during spraying by creating a localized low-resistance working zone underwater; at the same time, it improves the thickness and density of a single spray by combining the high cohesiveness and fast setting properties of the sprayed concrete material; the construction process of this invention can not only improve the underwater anti-dispersion ability of concrete and reduce the water resistance in the working space, but also shorten the setting time of concrete, improve the early bond strength with the working surface, and significantly improve the bonding ability between concrete and the repair working surface, thereby realizing underwater wet spraying construction of concrete.

[0021] 2. This invention uses self-made products for shotcrete construction, and the spraying effect is better compared with other products at home and abroad.

[0022] Other advantages, objectives and features of the present invention will become apparent in part from the following description, and in part from those skilled in the art through study and practice of the invention. Attached Figure Description

[0023] Figure 1 is a schematic diagram of the underwater shotcrete device of the present invention.

[0024] Explanation of reference numerals in the attached figures:

[0025] 101. Concrete pump pipe; 102. Circular fixed joint; 103. Concrete spray gun head; 2. Air inlet pipe; 301. Accelerator feed pipe; 302. Mixing tee; 401. Drag reducer feed pipe; 402. Mixing tee; 501. Pulley; 502. Cross support frame; 503. Slide rail; 504. Shock-absorbing rubber airbag; 601. Spiral feed pipe; 602. One-way nozzle; 7. Distance detector. Detailed Implementation

[0026] The present invention will now be described in further detail with reference to the accompanying drawings, so that those skilled in the art can implement it based on the description.

[0027] It should be noted that, unless otherwise specified, the experimental methods described in the following embodiments are all conventional methods, and the reagents and materials described are all commercially available unless otherwise specified. In the description of this invention, the terms "lateral", "longitudinal", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", and "outer" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0028] This invention provides an underwater shotcrete construction process, which includes forming a low-resistance working zone underwater next to the working surface, and spraying concrete from inside the low-resistance working zone onto the working surface to complete the reinforcement and repair construction. The low-resistance working zone can reduce the risk of concrete dispersion due to water resistance during spraying, thereby enabling smooth underwater shotcrete application.

[0029] In another embodiment, an interface release agent that increases the cohesiveness and hydrophobicity of the concrete is added during mixing. The amount added is 0.1–0.5% of the cementitious material mass, resulting in a concrete slump spread of 350–500 mm and a slump retention time of 120–180 min. Adding the interface release agent to the concrete utilizes the adhesive effect of the long polymer chains and the hydrophobic effect of the surface hydrophobic groups to increase the cohesiveness and hydrophobicity of the concrete, thereby enhancing its drainage capacity.

[0030] In another embodiment, the early-strength alkali-free quick-setting agent is mixed with pressurized gas to form a mixture. After the concrete is mixed, it is mixed with the above mixture before entering the low-resistance working area and then sprayed out. The dosage of the early-strength alkali-free quick-setting agent is 1-5% of the mass of the cementitious material. In shotcrete, the early-strength alkali-free quick-setting agent is added to the shotcrete. The early-strength components in the agent accelerate the hydration of C3A and C3S, while providing nuclei for CSH gel formation. This promotes gel formation and fills the voids in the needle-like ettringite, improving the early strength of the concrete, thereby reducing rebound, increasing the thickness of a single spray, and simultaneously improving the later strength and durability of the concrete.

[0031] In another embodiment, the drag-reducing agent is mixed with compressed gas and then sprayed out to form an air curtain, creating a low-resistance working area. The drag-reducing agent has a mass concentration of 0.01-0.65%, and the compressed gas is compressed air with a pressure of 0.3-1.5 MPa.

[0032] Low-resistance working zone drag reduction involves two methods: adding composite drag-reducing agents and injecting gas. The aim is to utilize the high-molecular polymers in the composite drag-reducing agent to reduce the Chenoroy stress of water, while simultaneously using the cationic surfactants in the composite drag-reducing agent to reduce the surface tension of water. Injecting gas creates an air curtain, forming a low-resistance working zone, reducing the risk of concrete dispersion due to water resistance during spraying. Combined with the high viscosity and rapid setting properties of shotcrete materials, this increases the thickness and density of a single spray application, enabling underwater shotcrete construction. The injecting gas is compressed air at a pressure of 0.3-1.5 MPa, designed to generate bubbles, form an air curtain, and reduce drag.

[0033] In another implementation, the following steps are specifically included:

[0034] Step 1: Water-based concrete mixing, where concrete is extended underwater via concrete pump pipes for spraying; pre-mixed concrete is mixed at the batching plant and transported to the site by mixer trucks for use, and then sprayed by connecting a pump body to a concrete pump pipe.

[0035] Step 2: The concrete pump pipe is connected to the concrete spray gun head. Move the concrete spray gun head to the low-resistance working area and keep it 30-120cm away from the working surface.

[0036] Step 3: Create a low-resistance working zone, 10-50cm away from the working surface. Spray concrete from inside the low-resistance working zone onto the working surface until the reinforcement and repair work is completed.

[0037] In another embodiment, an interface release agent is added to the concrete during mixing in step one; in step two, the concrete pump pipe and the concrete spray gun head are connected by a ring-shaped fixed joint, and the early-strength alkali-free quick-setting agent is mixed with pressurized gas to form a mixture, which enters the ring-shaped fixed joint through the quick-setting agent injection pipe, mixes with the concrete, and is then sprayed out.

[0038] In another embodiment, as shown in Figure 1, a low-resistance working area is formed by an underwater shotcrete device. The underwater shotcrete device includes a fixed support, a spiral feeding pipe 601 mounted on the fixed support, and a drag-reducing agent injection pipe connected to the spiral feeding pipe 601. The concrete spray nozzle 103 is located inside the spiral feeding pipe 601. The spiral feeding pipe 601 is a hollow pipe with several one-way nozzles 602. The drag-reducing agent injection pipe sprays drag-reducing agent mixed with gas into an aerosol from the several one-way nozzles 602 of the spiral feeding pipe 601 to form an air curtain inside the spiral feeding pipe 601, which is the low-resistance working area. The drag-reducing agent injection pipe is connected to a mixing tee 402, which is also connected to an air inlet pipe 2 and a drag-reducing agent inlet pipe 401.

[0039] This application uses several unidirectional nozzles 602 distributed on the surface of the spiral feeding pipe 601 to spray a composite drag-reducing agent and gas into the water. The composite drag-reducing agent reduces the surface tension and Reynolds shear stress of the water. The sprayed gas creates an air curtain, forming a low-resistance working zone. After passing through the low-resistance working zone, the concrete is sprayed onto the working surface and hardens rapidly, completing the reinforcement and repair construction.

[0040] The spiral feeding pipe 601 can extend in an integral cylindrical or divergent spiral shape, with the tail end away from the working surface connected to the drag-reducing agent injection pipe. When the spiral feeding pipe 601 is cylindrical, multiple nozzle sections are evenly spaced laterally. Each nozzle section is perpendicular to the concrete spray gun head 103, and four unidirectional nozzles 602 are evenly spaced on each nozzle section, all of which are set at 45° towards the concrete spray gun head 103. When the spiral feeding pipe 601 extends in a divergent spiral shape, it also has multiple nozzle sections perpendicular to the concrete spray gun head 103, and the unidirectional nozzles 602 are set in the same way as in the cylindrical shape. The spiral feed tube 601 is a corrosion-resistant alloy hard tube with a cylindrical length of 100-150cm, a spacing of 5-25cm between two rings, an inner diameter of 60-100cm for the rings, and an inner diameter of φ10-25mm for the metal tube. Each ring has a one-way nozzle 602 installed at four positions: 45° upwards and 45° downwards. The nozzles have an inner diameter of φ1-3mm and are used to spray drag-reducing agents and pressurized gas.

[0041] The drag-reducing agent feed pipe 401 is a rubber hose with an inner diameter of φ25mm, used to connect the drag-reducing agent pump and the mixing tee 402, which is the delivery channel for the drag-reducing agent. The air inlet pipe 2 has an inner diameter of φ40mm.

[0042] Move the concrete spray gun head 103 to the work area, 30-120cm away from the work surface. Use the distance detector 7 to determine the spraying distance. Adjust the position of the spiral feed pipe 601 to 10-50cm away from the work surface. As the concrete spray gun head 103 moves, the spiral feed pipe 601 will also adjust its distance from the work surface in real time. The purpose is to ensure that the spraying distance and water resistance in the work area are stable, and also to ensure equipment safety.

[0043] In another embodiment, the fixed bracket includes a cross support frame 502 and four slide rails 503 fixed and perpendicular to the outer end of the cross support frame 502. The center of the cross support frame 502 is connected to the annular fixed joint 102 through a hollow connecting ring. The spiral feed tube 601 is fixedly connected to the four slide rails 503. Each of the four outer ends of the cross support frame 502 is provided with a pulley 501 that cooperates with the corresponding slide rail 503. The four slide rails 503 are driven by a driving mechanism to move linearly relative to the plane perpendicular to the plane where the cross support frame 502 is located in order to adjust the distance between the spiral feed tube 601 and the working surface. The ends of the four slide rails 503 near the working surface are provided with shock-absorbing rubber airbags 504 and distance detectors 7.

[0044] The cross support frame 502 is a hollow metal frame with an inner ring and an outer cross. The inner ring is fixed to the annular fixing joint 102. The hollow metal frame with the outer cross connects four slide rails 503, which are parallel to the concrete spray gun head 103. The four slide rails 503 form a cylinder, and their central axis coincides with the central axis of the concrete spray gun head 103. The four slide rails 503 are used to fix and connect the spiral feeding pipe 601, and their central axis also coincides with the central axis of the concrete spray gun head 103. The four slide rails 503 are driven by a drive mechanism to move linearly toward or away from the working surface, thereby adjusting the position of the spiral feeding pipe 601. The length of the slide rail 503 is 110-160cm, and the front end extends 5-15cm beyond the spiral feeding pipe 601. The slide rail 503 is used to fix the spiral feeding pipe 601 and assist its back-and-forth movement. The distance detector 7 is an ultrasonic ranging detector with a range of 0-5m and a response interval of 0.5-2s. It is installed on the outer front end of the slide 503 to detect the distance from the slide 503 to the working surface, assisting in adjusting the spray distance and improving spray quality and efficiency. The shock-absorbing airbag is a hollow FKM rubber airbag with impact resistance, wear resistance, and corrosion resistance. It has a diameter of 2-5cm and is installed at the front end of the slide 503 to buffer the collision with the working surface, protecting the front end of the slide 503 from rigid collision with the working surface and protecting the spiral feed pipe 601. When the slide 503 moves, the spiral feed pipe 601 is always located on the side of the cross support frame 502 closest to the working surface, which is the right side in Figure 1. The ends of the four slides 503 furthest from the working surface are connected as a whole by connectors and driven linearly by a drive mechanism to achieve stable drive. The drive mechanism can be a telescopic cylinder, etc.

[0045] In another embodiment, the air inlet pipe 2 is also connected to the mixing four-way connector 302, which is also connected to the accelerator feed pipe 301 and a pair of accelerator spray pipes respectively. The pair of accelerator spray pipes are symmetrically connected to the annular fixed joint 102 so that the accelerator is sprayed out after being mixed with the concrete.

[0046] The air inlet pipe 2 is a dual-pipe structure, connecting to the mixing tee 402 and the mixing four-way connector 302 respectively. The accelerator feed pipe 301 is a rubber hose with an inner diameter of φ25mm, connecting the accelerator pump to the mixing four-way connector 302 to ensure the accelerator enters the mixing four-way connector 302 normally. The mixing four-way connector 302 is used to connect the accelerator feed pipe 301, the air inlet pipe 2, and a pair of accelerator injection pipes. The air inlet pipe 2 has an inner diameter of φ40mm, and the pair of accelerator injection pipes have an inner diameter of φ32mm. The pair of accelerator injection pipes are rubber hoses, connecting the mixing four-way connector 302 and the annular fixed connector 102. Combined with the annular fixed connector 102, they are used to ensure that the pressurized gas carrying the accelerator is evenly dispersed and added to the concrete pump pipe 101, improving the mixing efficiency of concrete and accelerator and improving the uniformity of shotcrete. An interface release agent is added to the concrete to improve its cohesiveness and hydrophobicity, thereby enhancing the concrete's drainage capacity. Then, the quick-setting agent is mixed with pressurized gas and introduced into the concrete pump pipe 101. After mixing with the underwater non-dispersible concrete at the annular fixed joint 102, it is sprayed out and reaches the working surface after passing through the low-resistance working area surrounded by the spiral feeding pipe 601. The concrete hardens rapidly on the working surface, completing the reinforcement and repair construction.

[0047] In another embodiment, the concrete spray gun head 103 has a tapered structure with a gradually decreasing diameter. The inner diameter of the tail inlet is φ70mm, and the inner diameter of the front outlet is φ40-53mm. The tail is connected to the concrete pump pipe 101 and a pair of quick-setting agent injection pipes through an annular fixing joint 102 for pressurized spraying of concrete and quick-setting agent mixture.

[0048] Underwater shotcrete construction was carried out using the underwater shotcrete device of this application, resulting in multiple comparative examples and embodiments to illustrate that the device and process of this application can achieve the effect of rapid hardening of concrete on the working surface, completing reinforcement and repair construction. Specific details are shown in Table 1 below. Among them, GL-01 is a self-made interface release agent, which is an aqueous solution of hydroxypropyl cellulose grafted copolyacrylamide with a molecular weight of 8 million and a solution concentration of 0.5%; JZ-01 is a self-made modified drag-reducing agent, which is a mixture of dodecyltrimethylammonium chloride and polyvinylurea in a mass ratio of 1:1; SN-01 is a self-made early-strength alkali-free quick-setting agent, with a mass percentage of 40% polyaluminum sulfate, 5% modified aluminum hydroxide, 5% modified polyol amine, 0.2% lithium sulfate, 0.3% nano-silica solution, 1.0% ultrafine hydrated magnesium silicate, and the remainder being water.

[0049] Table 1

[0050]

[0051]

[0052] As can be seen from the test results in Table 1, the interface isolating agent components used in Examples 2, 3, and 6 are the same, but the dosages are different. Compared with Comparative Example 2, it can be seen that the use of interface release agent improves the adhesion between cement paste and aggregate, effectively improving the cohesiveness of concrete and thus reducing the underwater shotcrete rebound rate. Comparing Examples 4, 5, and 7 with Comparative Example 2, it can be seen that as the dosage of composite drag reducer increases, the concrete strength remains unchanged, but the rebound rate initially decreases and then increases. The lowest rebound rate and the largest shotcrete thickness are observed at a dosage of 0.3%, indicating that changes in drag reducer concentration do not affect concrete strength. Appropriate concentrations of drag reducer can significantly reduce water resistance. Further increases in concentration lead to increased viscosity, which, while reducing random Chenoroy stress in water, increases unidirectional resistance, resulting in a higher rebound rate. Comparing the test results of Examples 2 and 7, it can be seen that when the dosage of early-strength liquid accelerator exceeds 3%, the 1-day strength increases, but the rebound rate also increases. This is mainly because the addition of the early-strength liquid accelerator causes rapid hydration and aggregation of the paste, improving its anti-dispersion ability in water. Excessive accelerator leads to excessively rapid hardening of the surface paste, resulting in reduced adhesion between the concrete and the work surface. Based on the above test results, in terms of concrete condition, rebound rate, and compressive strength, the test results of Example 5 are the best.

[0053] Under the process parameters of Example 5, the self-made product of this application was compared with domestic and foreign products. The application effect results are shown in Table 2. It can be seen that the underwater spraying test using different products from other domestic manufacturers was worse than the underwater spraying test using the self-made product.

[0054] in, VM1005 is a long-lasting concrete thickener. -VM1002 conventional concrete thickener -N(Ⅱ) liquid accelerators, A689 and A589T are drag reducers, Anhui Tianrun Chemical Industry Co., Ltd.; SA160 is BASF's early strength alkali-free accelerator.

[0055] Table 2

[0056]

[0057] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and illustrations shown and described herein.

Claims

1. An underwater shotcrete construction process, characterized in that, This includes creating a low-resistance working zone underwater next to the working surface, spraying concrete from inside the low-resistance working zone onto the working surface to complete the reinforcement and repair construction; the drag-reducing agent is mixed with compressed gas and sprayed out to form an air curtain, which is the low-resistance working zone. The mass concentration of the drag-reducing agent is 0.01-0.65%, and the compressed gas is compressed air with a pressure of 0.3-1.5MPa.

2. The underwater shotcrete construction process as described in claim 1, characterized in that, When concrete is mixed, an interface release agent that increases the cohesiveness and hydrophobicity of the concrete is added. The dosage is 0.1% to 0.5% of the mass of the cementitious material. The resulting concrete has a slump spread of 350 to 500 mm and a slump retention time of 120 to 180 min.

3. The underwater shotcrete construction process as described in claim 2, characterized in that, The early-strength alkali-free quick-setting agent is mixed with pressurized gas to form a mixture. After the concrete is mixed, it is mixed with the above mixture before entering the low-resistance working area and then sprayed out. The dosage of the early-strength alkali-free quick-setting agent is 1 to 5% of the mass of the cementitious material.

4. The underwater shotcrete construction process as described in claim 1, characterized in that, The interface isolating agent is an aqueous solution of hydroxypropyl cellulose-grafted copolyacrylamide with a molecular weight of 8 million and a solution concentration of 0.5%; the drag-reducing agent is a mixture of dodecyltrimethylammonium chloride and polyvinylurea in a mass ratio of 1:

1. The early-strength, alkali-free, quick-setting agent comprises 40% polyaluminum sulfate, 5% modified aluminum hydroxide, 5% modified polyol amine, 0.2% lithium sulfate, 0.3% nano-silica solution, 1.0% ultrafine hydrated magnesium silicate, and the remainder is water.

5. The underwater shotcrete construction process as described in claim 1, characterized in that, The specific steps are as follows: Step 1: Mixing concrete on water, extending the concrete through a concrete pump pipe to the underwater surface for spraying; Step 2: Connecting the concrete pump pipe to a concrete spray gun head, moving the concrete spray gun head to a low-resistance working area 30-120cm away from the working surface; Step 3: Forming a low-resistance working area 10-50cm away from the working surface, spraying concrete from inside the low-resistance working area onto the working surface until the reinforcement and repair work is completed.

6. The underwater shotcrete construction process as described in claim 5, characterized in that, In step one, an interface release agent is added to the concrete during mixing; in step two, the concrete pump pipe and the concrete spray gun head are connected by a ring-shaped fixed joint. The early-strength alkali-free quick-setting agent is mixed with pressurized gas to form a mixture, which enters the ring-shaped fixed joint through the quick-setting agent injection pipe, mixes with the concrete, and is then sprayed out.

7. The underwater shotcrete construction process as described in claim 6, characterized in that, A low-resistance working zone is formed by an underwater shotcrete device, which includes a fixed support, a spiral feeding pipe mounted on the fixed support, and a drag-reducing agent injection pipe connected to the spiral feeding pipe. The concrete spray nozzle is located inside the spiral feeding pipe, which is a hollow pipe with several one-way nozzles. The drag-reducing agent injection pipe sprays drag-reducing agent, mixed with gas to form an aerosol, from the several one-way nozzles of the spiral feeding pipe to form an air curtain inside the spiral feeding pipe, creating a low-resistance working zone. The drag-reducing agent injection pipe is connected to a mixing tee, which is also connected to an air inlet pipe and a drag-reducing agent inlet pipe.

8. The underwater shotcrete construction process as described in claim 7, characterized in that, The fixed support includes a cross support frame and four slides fixed and perpendicular to the outer end of the cross support frame. The center of the cross support frame is connected to an annular fixed joint through a hollow connecting ring. The spiral feeding pipe is fixedly connected to the four slides. Each of the four outer ends of the cross support frame is provided with a pulley that cooperates with the corresponding slide. The four slides are driven by a drive mechanism to move linearly relative to the plane of the cross support frame to adjust the distance between the spiral feeding pipe and the working surface. The ends of the four slides near the working surface are provided with shock-absorbing rubber airbags and distance detectors.

9. The underwater shotcrete construction process as described in claim 8, characterized in that, The air inlet pipe is also connected to a mixing four-way connector, which is also connected to a quick-setting agent inlet pipe and a pair of quick-setting agent injection pipes. The pair of quick-setting agent injection pipes are symmetrically connected to an annular fixed joint so that the quick-setting agent is mixed with the concrete and then sprayed out.

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