CONCRETE OR SHOTCRETE ?????????

CONCRETE OR SHOTCRETE

                                                                               The process of shotcreting (rock bolt support) consists of the following stages: preparation of underground working space surface, installation and assembling of support elements (anchors, arches, expanded lath), cover lamination, cover protection, quality management. 

The acceptable distance between heading and shotcreting lining is defined in project according to particular engineering and geological conditions. Immediate shotcreting after heading allows achieving the best results. In weatherproof ground with above-average competence the distance between heading and shotcreting is allowed to be 50 m. In bad-weather ground with above-average competence shotcreting must be done immediately. When tunnel construction is performed by means of drill-and-blast method the shotcreting is done after face cleaning. The distance between heading and shotcreting is forbidden in strong-weather ground, where local falls, intensive decompaction and fracturing can take place. When the heading is performed by means of drill-and-blast method the first shotcrete layer is sprayed on the crown and available part of walls immediately after blasting, ventilation and wall trimming. The last layers are sprayed after ground cleaning.

For tunnel excavation consolidation in stiff natural-moisture-content clay continuously reinforced shotcrete with reinforcing fiber and steel bars are used. Steel bars are installed along the excavation crown and are fixed in face toothing on one side and between ground and shotcrete of previous heading on another side.

It is recommended to apply admixture for acceleration of setting and hardening for shotcreting on water-bearing ground.

When shotcrete is sprayed on ground surface with the temperature below zero it is necessary to remove ice, blow the ground off with pressure air and sand-blasting machine if necessary in advance.

The shotcrete can be sprayed immediately or step-by-step, which is defined by the project according to development rate of rock pressure and work technique features.

Shotcreting of armored coating is performed satisfying the following requirements: reinforcing fabric must have cubicles 100*100 minimum, wire size - 2-4 mm, fabric must be at distance over 20 mm from rock surface. The fabric is retained against working space surface with the help of framework fixed on shear connectors. Special assembling anchors with length of 0.4-0.6 m are installed in case of strong edge roughness.

Dry pack for shotcrete is centralized-prepared on mechanic concrete mixing stations with element-metering accuracy stated by all-Union State Standard 7473-76. It is allowed to prepare dry pack in the face when the volume of works is restricted and when the delivery of mixture to the face in hermetical containers is impossible to be performed in time. The maximum term of safekeeping of dry pack from the moment of preparation to the moment of use is two hours.

The shotcrete is placed layer-by-layer, the coating thickness - 5-6 sm. It is allowed to increase the thickness of layers when quick-hardening mixtures are applied on condition that newly-laid material doesn't slough and its gravity in construction is more than 2.2 g/sm³.

For shotcrete placing special cars are used, where by means of "dry" technology the mixture of cement, sand and gravel batched in equal proportions is squeezed out with pressure air by means of flexible hose to the nozzle atomizer, where the mixture is moisturized with water. When the "wet" technology is used, prepared concrete is delivered to the machines.

Shotcreting has high strength properties; resistance of its extension is 10 per cent more in comparison with standard concrete. The cover of high strength, density and waterproofing capacity is formed thanks to impact laying. The other advantages of shotcreting are good adhesion to ground and reinforcement, manufacturability, reduction of cement consumption.

Shotcrete application allows decreasing face thickness and out-turn, excluding the necessity of formwork and mechanizing the concrete work process totally. Moreover, manufacturing content decreases by 2 times, and lining cost reduces by 30-40% in comparison with cast-in-place lining.