Factors Influencing the Properties of Wet Shotcrete
Wet shotcrete is a mixture of cement, aggregate, accelerator, silica fume, and water, projected by pneumatic or other power at high velocity on a surface. The technology of wet shotcrete has advantages such as high construction mechanization level and high efficiency. It provides concrete with excellent mechanical properties and has gradually replaced dry shotcrete and has been widely used in water infrastructure, hydraulic engineering, underground construction, etc. However, problems like low pumpability, high rebound rate, low strength may occur if the mix ratio and construction process are not well controlled. And the factors that affect the problems are discussed below.
Factors influencing the pumpability and strength of wet shotcrete
The amount of cement is the core factor that influences the pumpability and strength of wet shotcrete. Adding more cement can ensure the workability and higher strength of the concrete, though more cohesiveness will cause poor pumpability; Whereas reducing cement content increases friction, reduces both pumpability and strength. The addition of ultra-fine materials is an effective way to reduce the cement content, which ensures not only fluidity but also strength. Besides, the type and composition of cement, the compatibility of cement and admixtures, etc., all have a great influence on the strength of wet shotcrete.
For materials, using limestone and other high-strength crushed stones will increase the strength of concrete by more than 25% compared with using low-strength raw stones such as mudstone. The fineness of sand and gravel also influences the pumpability and strength: The finer the sand and gravel, the better the pumpability, the lower the strength; Whereas the coarser the aggregate, the easier it will lead to segregation and a decrease in the strength of concrete. Regarding shapes of aggregate, those with irregular shapes will affect the lubricating layer of cement slurry when conveying through a hose.
Silica fume and fly ash
Silica fume is very fine particles (More than 95% of the particles are less than 1 μm.) that can fill the voids among cement particles. It enhances the bonding strength of cement paste and aggregate, improves the compressive strength of concrete, reduces the penetration of chloride and water, thereby improving the durability and chemical resistance of shotcrete. But because silica fume has a strong water absorption capacity, the pumpability decreases with the increase of silica fume content. Fly ash has strong hydrophobicity which makes up for the loss of pumpability caused by adding silica fume. Adding silica fume in tandem with fly ash can improve strength, maintain good pumpability, and low cost. The dosage of silica fume is generally controlled within 40 kg/m3.
Underground engineering requires shotcrete to have both high early strength and a thick spray layer, so it is usually necessary to add an accelerator. The type and dosage of accelerators have a direct effect on the early strength and final strength of shotcrete. Currently, common liquid accelerators include silicate-based, aluminate-based, and alkali-free. The pH value of silicate-based accelerators is above 11. With this type, the thickness of the first layer is relatively small, a high dosage rate will lead to a fast initial setting time, which will greatly reduce the final strength and durability of the concrete. Aluminate-based accelerators have a pH above 12, if the dosage rate is too high, they will participate in the hydration of cement, but will reduce the final strength (by 30%) and durability of concrete. Alkali-free accelerators have good strength development characteristics and a positive effect on the final strength and durability. The appropriate type of accelerators should be selected according to construction operation, technical requirements, compatibility of cement and actual conditions. The content of the accelerator is inversely proportional to the initial setting and final setting time of concrete, but the addition of the accelerator is not conducive to the improvement of the long-term strength of concrete.
A large amount of cement and water in wet shotcrete is not conducive to improving the strength of concrete. Adding superplasticizers can reduce water consumption and ensure the fluidity of slurry, which is beneficial to the improvement of concrete strength and maintaining good pumpability. Superplasticizers can reduce water consumption by up to 30%. Naphthalene sulfonate superplasticizers have good adaptability to cement, little effect on setting time, but cause high slump loss, and have limited effect on the increase of concrete strength. Polycarboxylate superplasticizers have the characteristics of greatly increasing the strength of concrete, low dosage required, high water reduction, improving fluidity and slump retention of fresh concrete, strong adaptability to cement, low shrinkage, environmental friendliness.
Concrete reinforcing fibers
Adding concrete reinforcing fibers to wet shotcrete can not only improve the tensile, shear, flexural, and bonding strength of shotcrete, but also effectively prevent the cracking of concrete due to shrinkage, improve the impermeability and frost resistance of layers. With concrete fibers, a slurry has a higher viscosity, which reduces the concrete rebound, and can also increase the thickness of the layer, and it will not leave voids behind welded wire mesh.
Steel fiber reinforced shotcrete can be evenly sprayed on a rock surface with higher energy absorption, better bond, support, and quality. However, the addition of steel fibers reduces the fluidity, affecting the pumpability of the concrete. The size and dosage of steel fibers should be determined according to the actual situation. In a certain range, the reinforcing effect of steel fiber increases with the increase of the aspect ratio. If the length is too short, the reinforcing effect is not obvious. Whereas if the length is too long, the concrete pumpability will be poor, the construction will be difficult and the quality of the mixture will be affected. Fibers with small diameters are easy to bend, and those with large diameters have limited effect.
Water-cement ratio will influence pumpability, concrete early strength and strength development, and determines the durability and resistance to chemical corrosion of concrete. Studies have found that the water-cement ratio required for Portland cement in the hydration process is only 0.22. And the water-cement ratio in the experiment is recommended at 0.40 – 0.45. A large amount of water will lead to the increase of porosity after water evaporation, resulting in low strength, high permeability, long setting time, segregation being likely to occur. Whereas a small water-cement ratio is not conducive to workability, pumpability, and strength improvement.
Factors influencing the rebound of wet shotcrete
The key factors affecting the rebound of wet shotcrete include the water-cement ratio, the dosage of the accelerator, the distance and angle between the nozzle tip and the sprayed surface. The substrate, quality of accelerator, operation proficiency will also result in different rebound rates.
The initial and final setting time of shotcrete is proportional to the water-cement ratio with the same dosage of an accelerator. For wet shotcrete with a high water-cement ratio, the rebound can only be reduced by increasing the dosage of the accelerator, which not only increases the material cost, but also reduces the final strength of shotcrete. Small water-cement ratio, low workability, the uneven mixture will also increase the rebound rate. According to the test, wet shotcrete slurries with a water-cement ratio between 0.40 and 0.45 have a low rebound rate and high strength.
Dosage of accelerator
Adding accelerators to wet shotcrete can reduce the setting time and the rebound loss. The dosage of alkali-free accelerator is generally 6% to 8%. Low dosage will cause spalling especially on the overhead surface. Whereas a too fast setting time due to a high dosage will cause aggregate rebound. A test shows that the relationship between the dosage of the accelerator and the rebound rate is a U-shaped curve.
The distance and angle between the nozzle tip and the sprayed surface
A small distance between the nozzle tip and the sprayed surface will increase the rebound rate. Whereas with a large distance, the weaker impact force leads to worse compaction and spalling.
According to experience, the smaller the angle between the nozzle tip and the sprayed surface, the higher the rebound rate, so the best angle is 90°.