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Ground Support With Sprayed Concrete

A new type of sprayed concrete using coarse-grained aggregates and cement with special additives to accelerate the hardening of the concrete has been developed in Europe.

Known as “shotcrete” it has found increasing application as a means of ground support for underground excavations in Europe and North America.

Its use in underground mines has been largely experimental. It was found that it could be used as a substitute for more conventional methods of ground support under normal underground ground conditions but that under adverse situations, such as talc schist and very wet conditions, it was not possible to apply it successfully.

Use of shotcrete as a means of ground support in underground mines is expected to increase. Sprayed cement with plastic types of additives is underway that may further increase the scope of its application. Sprayed concrete associated with wire mesh is already finding wider application in underground excavations.

Application of Shotcrete

There was two methods of mixing coarse-aggregate shotcrete, namely wet-mix and dry-mix involves the mixing of all the concrete constituents with water and the pumping the thick mixture through the delivery hose to the nozzle, where additional air is added and the material is sprayed onto the subject surface. The dry-xix process allow an easier introduction of accelerators generally admixtures of water-soluble, thus accelerating the hydration process. Accelerators have been developed that enable the concrete to adhere to rock surfaces and to set under a heavy flow of water.

The wet-mix machines have not yet been developed to the stage where they can practically handle aggregates that are larger than 3/4 in. These types of machines are mainly used for underground stabilization rather than for support in poor ground. Amachine of this type is the true Gun-All Model H, distributed by mining equipment company, and which is in relatively common use for underground applications where a thin coating of concrete up to about 2in. thick and having an aggregate of about 1/2 in. maximum size is required for relatively dry condition.

The Supporting Function of Shortcrete

Shotcrete can be used either as a structural or as a non-structural support. Weak to plastic rocks and cohesionless soils require the application of a rigid, competent structure to prevent the ground from loosening and flowing into the opening. This may be achieved by applying 4 or more inches of shotcrete.

In more competent rocks, it may be used to joints and fractures to prevent the lesser rocks movements that trigger rock pressures and failures. The shotcrete is applied 2 to 4 in. thick on the rough rocks to fill cracks and hollows to create an almost flat surface and to eliminate notch effects, only a thin application is required on smooth surfaces. In this case, the intimately bonded concrete matrix acts as a glue to hold the keys and wedges that support the larger pieces of rock and ultimately the tunnel arch. This type of application is common in Sweden, where design of tunnel support based on shotcrete is very popular because of its effectiveness and low cost.

The shotcrete can also be used on the form of a thin sheet to protect newly –excavated rock surfaces from attack and deterioration by air and water. In this form, it is a continuous flexible memebrane against which the atmospheric pressure may act as a support.

A Comparison of Gunite and Shotcrete

Coarse-aggregate shotcrete differs from similarly mixed and applied gunite in that the shotcrete is a true concrete containing coare (up to 1.25 in) stone in its aggregate, while gunite is commonly a cement sand mortar. The shotcrete differs from gunite in application and function in the following ways:

1) The gunite tends to form a thin cover the rock, but shotcrete if applied immediately after blasting will supply both a seal and a support to stabilize a new rock surface. The strong shotcrete-rock bond is thought to be due to the action of the specially developed accelerating admixtures which do not allow the concrete to slough away from the rock surface the peening effect of the large aggregate particles on the finer particles and the design of the shortcreting machines used.

2) The shotcrete uses large (up to 1.25 in) aggregate which may be mixed with cement and sand at its inherent moisture content without the expensive drying that is often required with gunite. It can also be applied in thickness of up to 6 in. in one pass, whereas gunite is necessarily restricted to thickness of not over 1 inch. Thus the shotcrete quickly becomes a strong support as well as a stabilizer of rough open ground.

3) The accelerating admixtures used in shotcreting aid it in achieving a bond with the rock, even though the shotcrete may actually be weaker than conventional concrete of similar mix proportions but with less accelerator. It is waterproof and characterized by high early strength (about 200 psi in one hour), due not only to the admixtures but also to the degree of compaction received from impact velocities of 250-500ft. per sec. and to tis low water/cement ratio (about 0.35). Shotcrete, with special additives, can transform a rock of minor strength into a stable one, and weak to plastic rocks sprayed with it can remain stable with only a few inches of shotcrete support. Because of its creep properties, shotcrete can sustain significant deformation over months or years without failure by cracking.

Post time: Dec-06-2021