SHOTCRETE TECHNOLOGY

The use of shotcrete for the support of underground excavation was pioneered by the civil engineering industry. In recent years the mining industry has become a major user of shotcrete for underground support. An important area of shotcrete application in underground mining is in the support of 'permanent' openings such as ramps, haulages, shaft station and crusher chambers. The incorporation of steel fiber reinforcement into the shotcrete is an important factor in this escalating use, since it minimizes the labor intensive process of mesh installation. 

Shotcrete is the generic name for cement, sand, and fine aggregate concretes which are applied pneumatically and compacted dynamically under high velocity.

Dry Mix Shotcrete

Figure 1: Simplified Sketch of a Typical Dry Mix Shotcrete System (After Mahar et al, 1975)

 Figure 2: Allentown Air Powered Rotary Gunite Dry Gun
(Source: http://www.awagnerco.com)

As illustrated in Figure 1, the dry shotcrete components, which may be pre-dampened to reduce dust, are fed into a hopper with continuous agitation. Compressed air is introduced through the delivery hose. Water is added to the mix at the nozzle. Gunite, a proprietary name for dry-sprayed mortar used in the early 1900's, has fallen into disuse in favor of the more general term shotcrete.

Wet Mix Shotcrete

 Figure 3: One Typical Type of Wet Mix Shotcrete Machine (After Mahar et al, 1975)

In this case the shotcrete components and the water are mixed (usually in a truck mounted mixer) before delivery into a  positive displacement pumping unit, which that delivers the mix hydraulically to the nozzle where air is added to project material onto rock surface.

The final product of either the dry or wet shotcrete process is very similar. The dry mix system tends to be more widely used in mining, because inaccessibility for large transit mix trucks and because it generally uses smaller and more compact equipment. This can be moved around relatively easily in an underground mine environment. The wet mix system is ideal for high production applications in mining and civil engineering, where a deep shaft or long tunnel is being driven and where access allows the application equipment and delivery trucks to operate.

 Figure 4: Wet Mix Shotcrete Process

(Source: American Shotcrete Association)

Figure 5: Wet Mix Shotcrete with Robotic Equipment

(Source: http://miningandconstruction.com)

Steel Fibre Reinforced Micro Silica Shotcrete

Of the many developments in shotcrete technology in recent years, two of the most significant were the introduction of silica fume, and steel or polypropylene fibre reinforcement.

 Figure 6: Polypropylene Fibres

Figure 7: Steel Fibres in Packaging

Figure 8: Steel Fibre Types on The North American Market (mm)

(Source: Ater Wood et al, 1993)

Silica fume or micro silica is a by-product of the silicon metal industry and is an extremely fine pozzolan. Pozzolans are cementitious materials which react with the calcium hydoxide produced during cement hydration. Silica fume, added in quantities of 8 to 13% by weight of cement, can allow shotcrete to achieve compressive strengts which are double or triple the value of plain shotcrete mixes. The result is an extremely strong, impermeable and durable shotcrete. Other benefits include reduced rebound, improved flexural strength, improved bond with the rock mass and the ability to place layers up to 200 mm thick in a single pass because of shotcrete's 'stickiness'. However, when using wet mix shotcrete, this stickiness decrease the workability of the material and superplaticizers are required to restore this workability.

Steel fibre reinforced shotcrete was introduced in the 1970s and has since gained world-wide acceptance as a replacement for traditional wire mesh reinforced plain shotcrete. The main role that reinforcement plays in shotcrete is only called to carry significant loads once the rock surrounding an underground excavation deforms. This means that unevenly distributed non-elastic deformations of significant magnitude may overload and lead to failure of the support system, unless that system has sufficient ductility to accommodate these deformations.

Typical steel fibre reinforced, silica fume shotcrete mix designs are summarized in Table 1. These mixes can be used as a starting point when embarking on a shotcrete programme, but it may be necessary to seek expert assistance to 'fine tune' the mix designs to suit site specific requirements. For many dry mix applications it may be advantageous to purchase pre-mixed shotcrete in bags of up to 1,500 kg capacity, as illustrated in Figure 9.

Table 1: Typical Steel Fibre Reinforced Silica Fume Shorcrete Mix Design

(Source: After Wood, 1992)

Figure 9: Bagged Pre-Mixed Dry Shotcrete Components

Wood et al (1993) have reported the results of a comprehensive comparative study in which all of the fibres shown in Figure 8 were used to reinforce shotcrete samples which were then subjected to a range of tests. These tests showed that addition of steel fibres to silica fume shotcrete enhances both the compressive and flexural strength of the hardened shotcrete by up to 20%. A significant increase in ductility was also obtained in all the tests on fibre reinforced samples, compared with plain samples.


Mesh Reinforced Shotcrete

While steel reinforced shotcrete has been widely accepted in both civil and mining engineer, mesh reinforced shotcrete is still widely used and preferred in some applications. In very poor quality, loose rock masses, where adhesion of the shotcrete to the rock surface is poor, the mesh provides a significant amount of reinforcement.

Kirsten (1992, 1993) carried out a comprehensive set of laboratory bending tests on both mesh and fibre reinforced shotcrete slabs. He found that the load carrying capacity of the mesh and fibre reinforced shotcrete samples were not significantly different, but the mesh reinforced samples were superior in bending with both point loads and uniformly distributed loads.

Chainlink mesh, used in any underground mining excavations to support loose rock, is not ideal for shotcrete application because it is difficult for the shotcrete to penetrate the mesh. On the other hand, weldmesh, tightly pinned against the rock face as illustrated in Figure 11, is generally ideal for shotcrete applications. Typically the weldmesh should be made from 4 mm diameter into a 100 mm x 100 m grid. This type of mesh is strong enough for most underground applications and the sheets are light enough to be handled by one man.

Figure 10: Chainlink Mesh

Figure 11: Weldmesh

Note:
It is true to say that shotcrete is a science in its own right and an indispensable element of modern rock support technology in all subsurface construction. However, the technology is far from complete and new innovation in this field are intensifying.