Silica fume is a kind of high performance mineral admixture, which is mainly used to improve the strength and durability of concrete, and can replace Portland cement. With the use of silica fume, the performance of dry mixing and wet mixing can be greatly improved.
The effect of silica fume on Shotcrete
With the use of silica fume, the performance and production technology of shotcrete can be improved:
1 reduce the rebound in dry mix of the sprayed concrete, thereby improving the economic benefits of the material.
2 without coagulant can improve the top and vertical surface of the monolayer thickness, thereby improving the production efficiency.
3 in the tidal zone, the cohesive force and anti erosion strength of piles and piles can be improved.
4 to reduce the permeability and improve the durability of freezing and thawing.
5 the early and late stage of the injection of concrete and the flexural and compressive strength were improved.
6 chemical erosion of chloride, nitrate, sulfate, acid and alkali aggregate reaction are enhanced by the resistance.
7 in chloride environment, the high resistivity and low permeability of the concrete in the chloride environment reduce the corrosion of steel and steel wire mesh.
Two, silica fume concrete construction method
Sprayed concrete is a cement and aggregate in the mortar or concrete mixture, compressed air jet in a surface, the shotcrete has two kinds of basic construction methods, namely, wet mix method and dry mixing method. In these two construction methods, the silica fume admixture is easy to be added.
Dry mix construction methods are as follows:
There are 1 large bags of mixture of cement, aggregate, silica fiber good pre mixed, general weight is 1000kg.
2 with cement and aggregate in the factory to transport dry mix mixture, and silica fume and fiber mixing machine at the site in transportation investment.
3 volume ratio of the weight of the site, or the mixture of silica fume or mixed cement with water.
4 the slurry is added to the nozzle.
In wet mix construction method, the method of adding silica fume is as follows:
1. Transport mixture, and ready mix concrete, cement, admixture and aggregate together, in the center of the mixing plant or dry mix plant by volume
Ratio of adding silica fume.
2 from the factory shipped ready mixed concrete, silica fume is according to the bag of ingredients on site.
3 adding dilute slurry in batch feeding equipment.
In the following circumstances, the best use of dry mix of spray concrete:
1 the site is far from or difficult to approach, providing a wet mix of sprayed concrete, for example, in the application of some mines and the repair of offshore structures.
2 the need to periodically provide a small amount of sprayed concrete, for example, in the traffic of a busy highway or railway tunnel to repair. In recent years, wet concrete has been widely used, and it has the following advantages compared with dry mixing method:
1 through the plant ingredients can better control the water cement ratio, in the dry mixing method, the water cement ratio is controlled by the control of the nozzle of the workers.
2 the amount of rebound is less, injection rate and productivity are higher, so the cost is reduced.
3 mixing less dust, greater uniformity.
Three different kinds of silica fume are used in the wet mixing of dry mix. (1) non compacted silica fume; (2) the compacted low density silica fume; (3) the high density of the compacted high density silica fume.
The average dosage of silica fume was 13% (cement). Using a sulfonated naphthyl superplasticizer to control the wet mix shotcrete water cement ratio. Mix the dry sprayed concrete without superplasticizer, due to the mixture in water is mostly in the injection mouth added, water with the cement and silica fume reaction contact time is too short the that in in mixture injection to prior to the actual surface of consolidation to less than effective reduction in water. With a spray of concrete pump to spray the wet mix of concrete, dry mix in the mixing of the rotary cylinder feeding into the spray gun, the water content is 4% ~ 3%.
Three, silica fume concrete performance
1 the thickness of bond failure and the loss of resilience in the sprayed concrete mixed with silica fume can improve the adhesion and cohesion of the bonded surface. Therefore, the thickness of the sprayed concrete in the top and vertical surfaces can be greatly increased.
In the study of wet mix concrete, the failure of the top surface of the concrete is 90mm, and the maximum value of the low density silica fume is 280mm. The thickness of the top surface of the adhesive is usually dry mixed with the concrete, if the maximum value of the dry mixed with the non compacted silica fume is 380mm, and the top surface of the dry mixed with the non - doped silica fume is only 65mm.
Dry mix of the top surface of the concrete rebound, no silica fume of the sprayed concrete is 42.7%, the incorporation of silica fume can be reduced to 21.4%. The amount of springback in the vertical plane: the non silica fume concrete is 45.5%; the addition of silica fume can be reduced to 22.8%. In all of the sprayed concrete, the rebound percentage of wet mix is lower.
2 compressive and flexural strength
The compressive strength of wet concrete is obviously improved after the silica fume is used. In the 63d, the compressive strength of the concrete without silica fume was 44MPa, while the average compressive strength of the sprayed concrete with silica fume was 66.1Mpa, which increased by 50%.
The same is the same as the dry mixed concrete, that is, the compressive strength of silica fume is higher than that of non silica fume.
The flexural strength of wet concrete mixed with dry mixing is also the way,