1. Control of Construction Process and Raw Materials 1.1 Before the construction process is sprayed, the rock surface should be washed clean, and the surface of the broken rock should be cleaned. Spraying operations should be carried out in sections. The length should not exceed 6 m. The injection sequence should be from bottom to top. The latter injection should be carried out after the final setting of the concrete at the previous time. If it is sprayed more than 1 hour after the final setting, the concrete surface is cleaned with feng shui. The thickness of a side wall is 4~6cm, and the arch is 2-4m. After spraying for 2-4h, it should be sprinkled and maintained. The general curing time should be 7-14d. After the concrete is sprayed, the time for firing the next cycle should be determined through experiments. It is generally not less than 4h, the concrete should be inspected after blasting. If cracks occur, the firing interval or blasting parameters should be adjusted.
1.2 Raw material control (1) Mixing water: In the project, drinking water is mostly used as mixing water, and acidic water with a pH value of less than 4 and water with a sulfate content (SO4, a) exceeding 1% of water content have effects on the normal condensation of cement. Hardened hazardous substances must not be used in water.
(2) Cement: In order to ensure the solidification time of shotcrete and the compatibility with the quick-setting agent, the cement used should have high strength, good impermeability and durability, and 4S5 and above ordinary Portland cement should be preferred. Slag Portland cement and pozzolanic Portland cement are used. Early-strength cement should be used in tunnels with complex geological conditions. Strength identification tests should be conducted before use. Cement must not be damped or agglomerated during storage. Cement of different specifications and different manufacturers must not be mixed.
(3) Aggregate: The strength of concrete not only depends on the strength of the aggregate, but also depends on the bonding strength of the cement slurry and the aggregate. At the same time, the rougher the surface of the aggregate is, the higher the interfacial bond strength is. Therefore, the ratio of the gravel is used. Good with pebbles. Experiments have shown that within a certain range, the smaller the aggregate size, the more uniformly distributed the higher the concrete strength, and the decrease in the maximum aggregate size of the aggregate not only increases the bonding area between the aggregate and the cement slurry, but also reduces the harmful gas around the aggregate. Thinning of the film makes it easy to mix evenly, thereby increasing the strength of the concrete.
(4) Activity: Admixture: Cement hydration is a gradual development process. In 28d age, the actual utilization rate of cement accounts for only 60% to 70%, while Ca0 in the other unhydrated cement Ca(OH)2 is generated after water contact, which causes volume expansion and adversely affects the later strength. The use of active admixtures to replace partially unhydrated cement not only can reduce costs, but the most important one is the use of active materials. The active ingredients (mainly Si02 and AL203) and the cement hydration product Ca(OH)2: undergo secondary reactions to form hydrous calcium silicate and aqueous calcium aluminate. The newly formed hydration products not only increase the strength of the shotcrete and Compactness, but also improve its antifreeze, impermeability, corrosion resistance and other properties, this type of active admixture is mainly fly ash and zeolite powder, dosage is about 10% -20% of the weight of cement.
(5) Additives: In order to reduce the water consumption, reduce the rebound rate and the dust rate, and make the shotcrete early-curing early, an additive must be used. The accelerating agent that meets the quality requirements and has little harm to the human body should be used. Before the accelerating accelerator is added, the compatibility test between the accelerator and the water should be done and the cement paste rapid setting effect experiment should be taken. Note the accelerating agent effect experiment. The initial setting time should not exceed 5min, and the final setting time should not exceed 10min. Keep the quick-setting coagulant dry without deterioration. The purpose of adding the accelerator in the shotcrete is to make the shotcrete meet the design requirements and promote early strength. Generally, the best dosage of quick-setting admixture is about 2% to 4% of cement weight. In actual use, the arch can use 2% to 4%, and the side wall can use 2%. Excessive mixing amount is not good for sprayed concrete. This is because the accelerating agent accelerates the setting rate of the shotcrete, but it also prevents the water from evenly diffusing in the cement, causing part of the water to be entrapped in the congealed cement, forming pores after hardening, and the other part of the cement is not adequate. The hydration reaction proceeds and shrinks, causing cracks. In addition, the admixture of accelerator should be uniform.
2. Design and control points of shotcrete mix ratio The mix ratio of shotcrete is different from that of ordinary concrete, and it needs to be selected according to its construction process. It mainly describes the design method of mix ratio of C and sprayed concrete. In order to reduce the rebound rate, a higher sand rate is required. The increase in the sand rate means that the total area of ​​the aggregate increases. This requires more cement to wrap the surface of the aggregate to meet the strength requirements of the shotcrete. The more easily sprayed concrete shrinks and cracks, the more the cost increases. Therefore, first determine the amount of cement, according to experience the amount of cement should be 375 a 450kg / m;, then determine the sand rate, should choose coarse sand or medium sand, sand rate should be 45% to 55%, sand rate is too high or too low Cause blockage, once again determine the water-cement ratio, water-cement ratio should be 0.4-0.5, water-cement ratio is too small will produce dust, large rebound, low adhesion, spray layer will produce dry spots, sand nest and other phenomena, The water-cement ratio is too low to result in low strength, quick settling effect, splashing, slippage, and slumping. In addition, it is necessary to pay attention to the temperature of the construction environment, the type of surrounding rock walls, and the construction level of the construction team to make corresponding adjustments.
The composite effect of superplasticizer, silica fume, quick-setting agent and incorporation of volcanic ash or slag Portland cement was studied. The production of high-performance shotcrete (HPS) by wet spraying has the following advantages: (1) low alkali corrosion; (2) High workability and low slump loss; (3) Low rebound rate; (4) Early strength and high late strength; (5) Strong durability.
2.1 Materials and mix ratio 2.1.1 Cement high-strength portland cement is generally superior to mixed cement in its performance as a shotcrete due to its faster hydration rate. Volcanic ash and slag portland cement have high durability, low heat of hydration, and have good performance on crack self-shrinkage and shrinkage. In pozzolanic cement, 35% fly ash can be used instead of Portland cement; in slag cement, 50% slag can be used instead of Portland cement.
2.1.2 Silica fume uses an encrypted silica fume to improve adhesion to the substrate and reduce aggregate rebound.
2.1.3 Superplasticizers Commercially available 30% by mass liquid light acrylic vinegar (CAE) is used as a superplasticizer. The water-cement ratio is 0.42 to 0.44 and the slump is 210-220mm. . Polysulfonate-based superplasticizers can also be used.
2.1.4 Accelerating Agents Two different commercial accelerators are used. One commonly used accelerator is sodium silicate (30%), and the other alkali free accelerator is (SO4), which is the main component. The aqueous solution (60%), when used in the latter, reduces the risk of alkaline corrosion during construction due to the absence of alkali.
2.1.5 The aggregate used fine sand (0 to 4mm), coarse sand (4 to 6mm), and gravel (6 to 8mm) in a volumetric ratio of 65%, 30%, and 5%.
2.1.6 Concrete Mixtures Two benchmark blends with no accelerators are prepared. The main differences are the type of cement (42.5 pozzolan cement and 42.5 slag cement). Each concrete is mixed for 30 min. The amount of quick-setting admixture (water glass or alkali-free quick-setting admixture) is 8%-12% or 6%-7%, respectively, of the cement content.
2.2 Test results Due to the use of superplasticizers, the slump loss within 50 min is negligible. This means that the slump loss is negligible before adding the accelerator to the nozzle, and the pump can be supplied to the injection equipment in a reliable manner. Send concrete with good workability, shotcrete output up to 20m3/h. When the superplasticizer content is 1% to 2%, although the slag cement concrete is slightly lower than the pozzolan cement concrete, its initial collapse Degree (220mm) or slightly higher than the pozzolan cement concrete slump (210mm) in the construction of the non-reinforced tunnel, due to the composite superplasticizer, silica fume and quick-setting agent, the admixture of good adhesion, all the research The rebound rate of shotcrete is only 2% to 3%.
3. The maintenance of shotcrete curing measures is an important part of shotcrete construction. Under normal curing conditions, the strength of concrete increases with age, and the reason is that the water of the cementitious material is cut off. The hydration speed is related to the ambient temperature and humidity. Because the temperature inside the tunnel is high due to frequent blasting and poor ventilation, the air surrounding the shotcrete is relatively dry, and the internal temperature of the concrete caused by the hydration heat is high. The surface water quickly evaporates, which in turn causes water to continue to evaporate in the “capillaries†of the water stone. The contact time between cement and water in shotcrete is short and the range is limited. The degree of cement hydration is lower than that of ordinary concrete. The coagulation process of shotcrete is also a process of further hydration of cement. The hydration reaction of cement must occur under the condition of water. Cement hydration cannot continue because cementite lacks moisture, and it also causes shrinkage due to capillary action in concrete. At this time, the shotcrete strength is still very low. The tensile stress caused by shrinkage will crack the concrete, destroy the concrete structure, affect the continuous growth of the concrete strength, and stop the hydration so that the hydrate cannot be further filled into the cement pores. It will also affect the impermeability of concrete.
Sprayed concrete has been widely used in tunnel construction. The quality of shotcrete directly affects the structure of force, protection, durability, etc. Therefore, for shotcrete construction, quality control system must be used to strengthen quality awareness through various measures. Enhance quality management and formulate operating procedures to make shotcrete more and more important in tunnel protection.
1.2 Raw material control (1) Mixing water: In the project, drinking water is mostly used as mixing water, and acidic water with a pH value of less than 4 and water with a sulfate content (SO4, a) exceeding 1% of water content have effects on the normal condensation of cement. Hardened hazardous substances must not be used in water.
(2) Cement: In order to ensure the solidification time of shotcrete and the compatibility with the quick-setting agent, the cement used should have high strength, good impermeability and durability, and 4S5 and above ordinary Portland cement should be preferred. Slag Portland cement and pozzolanic Portland cement are used. Early-strength cement should be used in tunnels with complex geological conditions. Strength identification tests should be conducted before use. Cement must not be damped or agglomerated during storage. Cement of different specifications and different manufacturers must not be mixed.
(3) Aggregate: The strength of concrete not only depends on the strength of the aggregate, but also depends on the bonding strength of the cement slurry and the aggregate. At the same time, the rougher the surface of the aggregate is, the higher the interfacial bond strength is. Therefore, the ratio of the gravel is used. Good with pebbles. Experiments have shown that within a certain range, the smaller the aggregate size, the more uniformly distributed the higher the concrete strength, and the decrease in the maximum aggregate size of the aggregate not only increases the bonding area between the aggregate and the cement slurry, but also reduces the harmful gas around the aggregate. Thinning of the film makes it easy to mix evenly, thereby increasing the strength of the concrete.
(4) Activity: Admixture: Cement hydration is a gradual development process. In 28d age, the actual utilization rate of cement accounts for only 60% to 70%, while Ca0 in the other unhydrated cement Ca(OH)2 is generated after water contact, which causes volume expansion and adversely affects the later strength. The use of active admixtures to replace partially unhydrated cement not only can reduce costs, but the most important one is the use of active materials. The active ingredients (mainly Si02 and AL203) and the cement hydration product Ca(OH)2: undergo secondary reactions to form hydrous calcium silicate and aqueous calcium aluminate. The newly formed hydration products not only increase the strength of the shotcrete and Compactness, but also improve its antifreeze, impermeability, corrosion resistance and other properties, this type of active admixture is mainly fly ash and zeolite powder, dosage is about 10% -20% of the weight of cement.
(5) Additives: In order to reduce the water consumption, reduce the rebound rate and the dust rate, and make the shotcrete early-curing early, an additive must be used. The accelerating agent that meets the quality requirements and has little harm to the human body should be used. Before the accelerating accelerator is added, the compatibility test between the accelerator and the water should be done and the cement paste rapid setting effect experiment should be taken. Note the accelerating agent effect experiment. The initial setting time should not exceed 5min, and the final setting time should not exceed 10min. Keep the quick-setting coagulant dry without deterioration. The purpose of adding the accelerator in the shotcrete is to make the shotcrete meet the design requirements and promote early strength. Generally, the best dosage of quick-setting admixture is about 2% to 4% of cement weight. In actual use, the arch can use 2% to 4%, and the side wall can use 2%. Excessive mixing amount is not good for sprayed concrete. This is because the accelerating agent accelerates the setting rate of the shotcrete, but it also prevents the water from evenly diffusing in the cement, causing part of the water to be entrapped in the congealed cement, forming pores after hardening, and the other part of the cement is not adequate. The hydration reaction proceeds and shrinks, causing cracks. In addition, the admixture of accelerator should be uniform.
2. Design and control points of shotcrete mix ratio The mix ratio of shotcrete is different from that of ordinary concrete, and it needs to be selected according to its construction process. It mainly describes the design method of mix ratio of C and sprayed concrete. In order to reduce the rebound rate, a higher sand rate is required. The increase in the sand rate means that the total area of ​​the aggregate increases. This requires more cement to wrap the surface of the aggregate to meet the strength requirements of the shotcrete. The more easily sprayed concrete shrinks and cracks, the more the cost increases. Therefore, first determine the amount of cement, according to experience the amount of cement should be 375 a 450kg / m;, then determine the sand rate, should choose coarse sand or medium sand, sand rate should be 45% to 55%, sand rate is too high or too low Cause blockage, once again determine the water-cement ratio, water-cement ratio should be 0.4-0.5, water-cement ratio is too small will produce dust, large rebound, low adhesion, spray layer will produce dry spots, sand nest and other phenomena, The water-cement ratio is too low to result in low strength, quick settling effect, splashing, slippage, and slumping. In addition, it is necessary to pay attention to the temperature of the construction environment, the type of surrounding rock walls, and the construction level of the construction team to make corresponding adjustments.
The composite effect of superplasticizer, silica fume, quick-setting agent and incorporation of volcanic ash or slag Portland cement was studied. The production of high-performance shotcrete (HPS) by wet spraying has the following advantages: (1) low alkali corrosion; (2) High workability and low slump loss; (3) Low rebound rate; (4) Early strength and high late strength; (5) Strong durability.
2.1 Materials and mix ratio 2.1.1 Cement high-strength portland cement is generally superior to mixed cement in its performance as a shotcrete due to its faster hydration rate. Volcanic ash and slag portland cement have high durability, low heat of hydration, and have good performance on crack self-shrinkage and shrinkage. In pozzolanic cement, 35% fly ash can be used instead of Portland cement; in slag cement, 50% slag can be used instead of Portland cement.
2.1.2 Silica fume uses an encrypted silica fume to improve adhesion to the substrate and reduce aggregate rebound.
2.1.3 Superplasticizers Commercially available 30% by mass liquid light acrylic vinegar (CAE) is used as a superplasticizer. The water-cement ratio is 0.42 to 0.44 and the slump is 210-220mm. . Polysulfonate-based superplasticizers can also be used.
2.1.4 Accelerating Agents Two different commercial accelerators are used. One commonly used accelerator is sodium silicate (30%), and the other alkali free accelerator is (SO4), which is the main component. The aqueous solution (60%), when used in the latter, reduces the risk of alkaline corrosion during construction due to the absence of alkali.
2.1.5 The aggregate used fine sand (0 to 4mm), coarse sand (4 to 6mm), and gravel (6 to 8mm) in a volumetric ratio of 65%, 30%, and 5%.
2.1.6 Concrete Mixtures Two benchmark blends with no accelerators are prepared. The main differences are the type of cement (42.5 pozzolan cement and 42.5 slag cement). Each concrete is mixed for 30 min. The amount of quick-setting admixture (water glass or alkali-free quick-setting admixture) is 8%-12% or 6%-7%, respectively, of the cement content.
2.2 Test results Due to the use of superplasticizers, the slump loss within 50 min is negligible. This means that the slump loss is negligible before adding the accelerator to the nozzle, and the pump can be supplied to the injection equipment in a reliable manner. Send concrete with good workability, shotcrete output up to 20m3/h. When the superplasticizer content is 1% to 2%, although the slag cement concrete is slightly lower than the pozzolan cement concrete, its initial collapse Degree (220mm) or slightly higher than the pozzolan cement concrete slump (210mm) in the construction of the non-reinforced tunnel, due to the composite superplasticizer, silica fume and quick-setting agent, the admixture of good adhesion, all the research The rebound rate of shotcrete is only 2% to 3%.
3. The maintenance of shotcrete curing measures is an important part of shotcrete construction. Under normal curing conditions, the strength of concrete increases with age, and the reason is that the water of the cementitious material is cut off. The hydration speed is related to the ambient temperature and humidity. Because the temperature inside the tunnel is high due to frequent blasting and poor ventilation, the air surrounding the shotcrete is relatively dry, and the internal temperature of the concrete caused by the hydration heat is high. The surface water quickly evaporates, which in turn causes water to continue to evaporate in the “capillaries†of the water stone. The contact time between cement and water in shotcrete is short and the range is limited. The degree of cement hydration is lower than that of ordinary concrete. The coagulation process of shotcrete is also a process of further hydration of cement. The hydration reaction of cement must occur under the condition of water. Cement hydration cannot continue because cementite lacks moisture, and it also causes shrinkage due to capillary action in concrete. At this time, the shotcrete strength is still very low. The tensile stress caused by shrinkage will crack the concrete, destroy the concrete structure, affect the continuous growth of the concrete strength, and stop the hydration so that the hydrate cannot be further filled into the cement pores. It will also affect the impermeability of concrete.
Sprayed concrete has been widely used in tunnel construction. The quality of shotcrete directly affects the structure of force, protection, durability, etc. Therefore, for shotcrete construction, quality control system must be used to strengthen quality awareness through various measures. Enhance quality management and formulate operating procedures to make shotcrete more and more important in tunnel protection.
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