What Is a Jet Pipe?

Sprayed concrete is a construction method of pouring fine stone concrete with a pressure spray gun. Commonly used for lining tunnels, linings of thin-walled structures such as walls, ceilings, or other structures, and protective layers of steel structures. Shotcrete is a method in which pre-mixed cement, sand, stones, water and a certain amount of admixtures are loaded into a sprayer, which is sent to a spray head and mixed with an accelerator using high-pressure air. Or the surface of concrete.

Shotcrete dry mix

The dry method is to

In the case of split spraying, the re-spraying should be performed after the final coagulation. If spraying is performed one hour after the final setting, the sprayed surface should be washed with water. If there are over-digging or cracks and low depressions, the spray should be smoothed first, and then spray normally.

It is strictly forbidden to point the sprinkler at personnel.

During the spraying process, if there is a problem such as pipe blocking, wind or power failure, the water valve should be closed immediately, and the nozzles should be placed downward to prevent water from flowing into the feeding pipe;

, sprayers should wear dust masks, latex gloves and glasses.

After spraying is completed, spray the layer within seven days and spray water once per shift. After 7 days, spray water once a day for 28 days of continuous maintenance.

The driver of the shotcrete machine must be specially trained to be familiar with the performance structure and working principle of the shotcrete machine, and be able to eliminate general faults and perform daily maintenance and maintenance.

The air inlet of the shotcrete machine is well sealed to prevent air leakage from blowing dust; the exhaust port is unobstructed, and the exhaust gas is discharged smoothly; the shotcrete pipe joint is firm, well sealed, and neatly arranged. Personal wear dust mask, dust concentration should not exceed 6mg / m3 ^[2]

First, wet spraying technology should be preferred for sprayed concrete.

2. The raw materials of sprayed concrete shall meet the following requirements:

1. Portland cement or ordinary portland cement shall be used for shotcrete, and special cement may be used if necessary.

2. The fineness modulus of the fine aggregate used for sprayed concrete should be greater than 2.5, and the particles in the sand less than 0.075mm should not be greater than 20%.

3. The maximum particle size of coarse aggregate used for sprayed concrete should not be greater than 16mm. When short fibers are used, the maximum particle size should not be greater than 10mm, and continuous particle size should be used.

4. Accelerators should be used with stable quality products, which should have good compatibility with cement, and performance indicators should meet the requirements of relevant standards. The content of the accelerator should not be greater than 5% of the cement. 5 Sprayed concrete can be mixed with other admixtures according to needs, and its content can be determined through experiments.

3. The mix design of shotcrete shall be selected through tests according to the properties of the raw materials, the spray process and the design requirements, and shall meet the following requirements:

1. The rubber-bone ratio should be 1: 4 ~ 1: 5.

2. The ratio of water to glue should be 0.40 ~ 0.50.

3. The sand rate should be 45% ~ 60%.

4. The amount of cement should not be less than 400kg / m.

4. The sprayed concrete should be stirred with a forced mixer, and the mixing time should not be less than 1.5 mm. When using fibers, the stirring time should be determined by field homogeneity tests. The sprayed concrete mix should be used with mixing, and the parking time should not be greater than 30mm. During transportation and storage, it must not be exposed to rain, water, and sundries.

Fifth, before spraying the concrete, the surface to be sprayed shall be treated, and the equipment and wind, water, electricity and other pipe lines shall be inspected, and the test run.

6. The sprayed concrete operation shall meet the following requirements:

1. It should be sprayed in sections and layers from bottom to top, and the length of each section should not be greater than 6 m. When there are large depressions on the rock surface, the depressions should be sprayed first. The concrete surface sprayed on the rock surface shall be free from slip and fall. The thickness of one shot can be determined by field test according to the shot position.

2. The nozzle should be perpendicular to the spray surface, and the distance should be 0.6m ~ 1.8m. The nozzle should move continuously and slowly in a circular and circular motion, and the thickness of the spray layer should be uniform.

3. For layered spraying, after the concrete of the previous layer has set for 1 hour, spray the surface of the sprayed layer with wind and water first.

4. The rebound rate of concrete should be checked during spraying. The rebound rate of sprayed concrete shall not be greater than 15%, and the arch shall not be greater than 25%.

5. The ambient temperature of the spraying operation shall not be lower than 5 ° C; the sprayed concrete working area shall have good ventilation and lighting conditions. The dust content during spraying operation shall not be greater than 2mg / m.

6. After the spraying operation is completed, the thickness detection should be performed in time, and the non-compliance should be promptly refilled.

7. Steel fiber shotcrete construction should meet the following requirements:

1. When using steel fiber sprayed concrete for initial support, the thickness of shotcrete should be determined according to the geological conditions of the surrounding rock; the toughness indicators of the surrounding rock's geological conditions, deformation magnitude and engineering type should be met. Spray layer thickness should not be greater than 150mm.

2. The steel fiber used for sprayed concrete should determine its construction parameters according to the characteristics of the spraying process and meet the following requirements:

1) Steel fibers must not have obvious rust and oil stains and other impurities that prevent the adhesion of steel fibers to cement; the total weight of impurities such as adhesion pieces, severely rusted fibers, and rust powder contained in steel fibers due to poor processing does not Should exceed 1% of the weight of the steel fiber.

2) The cross-section diameter (or equivalent diameter) of the steel fiber should be 0.3mm ~ 0.8mm, and the length should be 20mm ~ 35mm, and should not be greater than 0.7 times the inner diameter of the conveying hose and nozzle. The deviation should not exceed ± 5% of the nominal length.

3) The tensile strength of steel fiber shall meet the design requirements.

4) The design of steel fiber content should take into account the different rebound rates of steel fiber concrete components during spraying, and the actual content of steel fibers in steel fiber concrete sprayed on the rock surface should be taken as the basis. The actual steel fiber content of steel fiber sprayed concrete should not be greater than 78.5kg / m (volume ratio 1.0%). The minimum content can be selected according to the aspect ratio of the steel fiber with reference to Table 8.4.7.

Table 8.4.7 Minimum actual content of steel fibers in steel fiber concrete

Aspect ratio of steel fiber	40	45	50	55	60	65	70	75	80
Minimum actual content (kg / m)	65	50	40	35	30	25	20	20	20
Minimum actual volume ratio	0.83	0.64	0.51	0.45	0.38	0.32	0.25	0.25	0.25

5) The strength grade of steel fiber sprayed concrete should meet the design requirements.

Aggregates for steel fiber sprayed concrete should be graded continuously. The maximum particle size of coarse aggregate should not be greater than 10mm, and the sand rate should not be less than 50%. The raw materials of steel fiber sprayed concrete should be added with active admixtures such as slag powder or fly ash. When design is not required, the amount of slag powder can be 5% ~ 15% of the weight of cement, and the amount of fly ash can be 15% ~ 30% of the weight of cement. The choice of the amount of admixture should be determined through experiments. Steel fiber shotcrete should be used with alkali-free quick-setting admixtures. The dosage should be determined according to tests, usually 2% to 5% of the cement dosage. If high-efficiency water-reducing or plasticizers are incorporated, their varieties and dosages should be passed. Determined by test and should be tested by field test spray. 3. The mix design of steel fiber sprayed concrete should follow the following principles:

1) Determine the water-binder ratio according to the compressive strength requirements of steel fiber shotcrete.

2) Determine the amount of steel fiber according to the bending toughness ratio and bending tensile strength requirements.

3) Determine the amount of water, cement and admixtures according to workability and conveying performance. The sand rate is determined according to the particle size and gradation of the aggregate, the fineness and workability of the sand.

4. The mixing of steel fiber sprayed concrete should meet the following requirements:

1) The tolerance of steel fiber weighing is ± 1% by weight.

2) The mixing process of steel fiber sprayed concrete should ensure that the steel fibers are dispersed evenly in the mixture without agglomeration. It is preferable to use the method of mixing the steel fibers, cement, and coarse and fine aggregates first with water and wet mixing. The mixing time must not be less than 1.5mm; you can also use the method of putting cement, coarse and fine aggregate and water first, and dispersing and adding steel fibers during the mixing process. If necessary, use a steel fiber spreader to evenly disperse the mixture into the mixture. Must not be grouped.

3) The mixing time of steel fiber concrete should be determined by the field homogeneity test, and it should be extended by 1mm ~ 2mm compared with the mixing time specified for ordinary concrete. When using the stirring method of dry mixing first and then adding water, the dry mixing time should not be less than 1.5min, and the stirring time should not be less than 3min.

5. Another layer of cement mortar with a thickness of 10mm should be sprayed on the surface of steel fiber sprayed concrete, and its strength level should not be lower than that of steel fiber sprayed concrete.

8. The construction of synthetic fiber shotcrete should meet the following requirements: 1. Polypropylene fibers should be used for synthetic fibers in shotcrete.

2. Synthetic fibers should have good acid resistance, alkali resistance and chemical stability, and be modified to have good dispersibility without clumping.

3. The tensile strength of synthetic fiber should meet the design requirements; when there is no design requirement, the length should be 12mm ~ 19mm.

4. The amount of synthetic fiber should be determined through experiments. In the absence of special requirements, the commonly used dosage is 0.8kg / m ~ 1.2kg / m.

5. The stirring time should be 4mm ~ 5mm. Random samples are taken after the mixing is completed. If the fibers have been uniformly dispersed into monofilaments, the concrete can be put into use. If there are still bundled fibers, the mixing time must be extended at least 30s before use.

6. The water-binder ratio of sprayed synthetic fiber concrete should be 0.35 ~ 0.45.

Nine, sprayed concrete curing should meet the following requirements: 1. After spraying concrete is set for 2 hours, it should be sprayed with water for a period of not less than 14 days.

2. Do not spray water when the temperature is below 5 ° C.

X. The working site of sprayed concrete should have anti-freezing and heat insulation measures; the ambient temperature of sprayed concrete operation and the temperature of the mixture entering the spraying machine should not be lower than 5 ° C.