Micro-silica fume is high-performance mineral admixture with volcanic ash, which is mainly used to improve the strength and durability of concrete and can replace micro-silica fume Portland cement. The performance of dry-mix and wet-mix shotcrete can be greatly improved by using micro silica fume, such as the application in rock mass stability, tunnel lining and underground structure repair, and excellent performance can be obtained.
The effect of shotcrete can be improved by using silica fume:
1. The springback of dry-mixed shotcrete is reduced, and the economic benefit of the material is improved.
2. The single-layer spray thickness on the top and vertical surfaces can be increased without coagulant, thus improving the production efficiency.
3. When repairing piles and breakwaters in tidal areas, the cohesion and erosion resistance of piles and breakwaters can be improved.
4. the anti freeze and thaw durability is improved due to reduced permeability.
5. the flexural and compressive strength of shotcrete is increased at the early and late stages.
6. Chemical attack on chloride, nitrate, sulfate, acid and alkali-aggregate reactions enhances resistance.
7. The high resistivity and low permeability of silica fume (minecrete) shotcrete reduce the corrosion of steel bar and steel wire mesh in the concrete restoration project under the chloride environment.
Construction method of micro silicon powder shotcrete
Shotcrete is a mixture of cement and aggregate mortar or concrete. Shotcrete is sprayed on surface at high speed with compressed air. There are two basic construction methods for shotcrete: wet mixing and dry mixing. In these two construction methods, silica fume additives are easy to add.
The construction method of dry mixed silica fume is as follows:
1. A super-large bag mix of fiber pre-mixed with cement, aggregate and microsilica powder, usually weighing 1000kg.
2. Micro-silica fume, cement and aggregate are dry-mixed into transport mixer in the factory, and silica fume and explosion-proof fiber are put into the transport mixer.
3. use the weight standard volume ratio at the construction site, add bagged silica fume or mixed cement.
4. add micro silica powder slurry into the nozzle.
In wet mix construction, the methods of adding silica fume are as follows:
1. Transport mixtures, just like ready-mixed concrete, with cement, admixtures and aggregates, in the central mixing plant or dry mixing plant add micro-silica fume by volume ratio.
2. transporting concrete from ready mixed plant, the microsilica powder is placed in bags.
3. add slurry to batch feeding equipment.
Dry mix shotcrete is best used in the following situations:
1. It is difficult to provide wet-mix shotcrete when the site is far away or difficult to approach, such as in some mines application and repair offshore structures.
2. A small amount of shotcrete should be provided periodically, such as repairing in busy road or railway tunnels.
In recent years, wet shotcrete has been widely used. Compared with dry shotcrete, wet shotcrete has the following advantages:
1. The ratio of water-cement can be well controlled by the plant ingredients. In the dry mixing construction, the ratio of water to cement is controlled by the workers who master the nozzle.
2. less springback, higher injection rate and higher productivity, thus reducing the cost.
3. mixing with less dust and greater uniformity.
Three kinds of micro silica fume products are used in wet mixing and dry mixed concrete:
(1) non densified silica fume;(2) densified low density micro silica fume;(3) densified high density silica fume.
In the mix proportion design of Silica fume (minecrete) shotcrete, the average dosage of silica fume is 13% (according to the quality of cement). Dry-mix shotcrete does not require superplasticizers, because most of the water in the mixture is added into the nozzle, and the contact time between water and cement, silica fume is too short to effectively reduce water before the mixture is actually consolidated on the surface.