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Reasons Affecting The Quality Of Shotcrete Spraying


In order to make the concrete sprayed by the shotcrete have a good quality, to ensure that the concrete structure has no original defects when forming, so as to ensure the durability of the concrete, the following reasons are analyzed to affect the injection quality of the sprayer during the operation of the shotcrete. We analyze from the following aspects:


1. Impact of rock surface cleanliness


Before spraying concrete, it is necessary to keep the rock surface to be sprayed without loose rock and clean and free of oil and dirt. At the same time, it is necessary to use hanging steel mesh and anchor rod in the development section of surrounding rock joint to strengthen the overall support effect of the sprayed concrete layer and the surrounding rock.


2. Determination of the mix ratio of on-site shotcrete


According to the test, the mixing ratio is adopted in the water seepage or dripping section. In order to ensure the quality of the rock face with large local water seepage or dripping, change the mix ratio. Only fine aggregate is used without coarse aggregate, add 6% quick-setting agent, first spray 2cm thick and then sprayed according to the normal mix ratio. In a wet or dry section. The injection is performed using a mixing ratio.


3. Impact of raw materials


The cleanliness of the material is important for spraying.


4. Groundwater impact


For the drip or seepage of the rock surface, the blind pipe can be specially set up for refilling or direct sealing. From the field test situation, it is difficult to achieve the effect of no leakage and leakage. This is mainly because the initial injection groundwater distribution is not concentrated, and it is difficult to find the water outlet point for larger water inrush. Therefore, it is more suitable for groundwater and high waterproofing requirements. A layered spray method is employed.


5. The effect of the water-cement ratio:


For wet-sprayed concrete, the water-cement ratio is too large, the quick-setting effect is worse, the rebound amount is increased, the concrete strength is low; the water-cement ratio is small, the construction is difficult to spray, the cement hydration is insufficient, the strength is low, and the rebound is increased big.


6. Effect of construction temperature


The cement setting time increases with the increase of temperature. When the temperature reaches 30 °C, the concrete after the test admixture of the accelerator is unfavorable in the final setting time and the late strength. The temperature of the shotcrete is declared at 15~25°C.


7. Effect of spray layer thickness


The spray layer is too thick, and under the action of its own weight, the spray layer will be cracked and cause a large piece of slump: when it is too thin, most of the coarse aggregate will rebound. Only the mortar is left in the spray layer, which will affect the spray effect and quality. After field test, the spray thickness of the arch is 6cm and the side wall is 8cm.


8. Wind pressure and water pressure


If the wind pressure is too high, the jetting speed is too high and the rebound amount is increased. If the wind pressure is too small, the jetting force will be weakened, and the compactness of the concrete will affect the strength; if the water pressure is too small, the construction quality will be affected. The test was carried out using a wind pressure of 200 N/cm 2 and a water pressure of 210 N/cm.