Sprayed concrete

Sprayed concrete, shotcrete (dry sprayed) or gunite (wet sprayed) are methods of rapidly applying concrete to soffits or vertical surfaces. It can be used over patches and to overlay metal mesh anodes for cathodic protection as described in Section 7.6. It is sometimes applied as a temporary cosmetic repair in the Northern USA and Canada when concrete has spalled or is in danger of spalling. As it does nothing to slow the corrosion rate it is comparable in effectiveness to patch repairing, and may suffer from the same incipient anode problem. 

Wet mix spraying of mortars is becoming more popular as proprietary mixes have been developed. The consistency of the mix is easier to control in wet mix application if segregation does not occur in the mixer before spraying. Application rates are higher and it is often possible to finish the surface of a wet sprayed mix. This is far more difficult with dry missed shotcrete. 

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With damage of a large area, the concrete is removed down to the uppermost layer of the reinforcement then the first layer of sprayed concrete is applied (see Fig. 19-3a). The anode is fixed to this layer and is followed by a second layer of sprayed concrete. With large areas of old concrete which are still solid, the anodes can be attached to this and finally embedded with sprayed concrete (see Fig. 19-3b). The anodes must supply the required protection current and be structurally robust. The bond between the old concrete and sprayed concrete must not be damaged during the spraying process or in operation. 

Fig. 19-3 Methods of installing anodes (a) Old loose concrete, two sprayed concrete layers, (b) solid concrete, one layer of sprayed concrete.

There are different concrete replacement systems available for renovating reinforced concrete structures. They range from sprayed concrete without polymer additions to systems containing conducting polymers (PCC-mortar). Since with the latter alkalinity is lower, more rapid carbonization occurs on weathering [59] and the increased electrical resistivity has to be taken into account, so that with cathodic protection only sprayed concrete should be used as a repair mortar. 

It has to be remembered that hydration processes and moisture exchange occur with old concrete when sprayed concrete is applied. Both processes can affect the potentials so that the protection current should only be switched on 4 weeks after... 

Although the natural zeolites are widely used (around 4 million tpa) they are not particularly valuable as commercial catalysts. This is due to a number of factors including natural variations in crystal size and porosity as well as the actual small pore size, which limits their synthetic usefulness. Natural zeolites do, however, find widespread use in applications such as removal of heavy metals from water, odour removal and building materials e.g. cavity grouting and sprayed concrete). 

Silica fume when used in conjunction with superplasticizers plays an important role in improving the cohesion of the shotcrete mix, allowing for the build-up of the sprayed concrete on vertical surfaces, within a short period, without sloughing off. In addition the combination of these admixtures has provided ease of pumping, better compaction and greatly reduced the rebound, thus minimizing waste and cleaning time. Typical mixture proportions and properties for wet-mix superplasticized shotcrete are presented in references 69 and 72. 

There are, however, several important niche applications for CAC. Most notably, CACs are used as linings to sewers and mine tunnels. Calcium aluminate cements are resistant to chemical attack from sulfate-producing bacteria that thrive in sewer systems (especially in warmer climates), and sprayed concrete linings to sewers have been shown to resist degradation for periods up to 30 years. The high impact and abrasion resistance of CAC also makes it suitable as a lining material for ore tunnels in mines and because CAC sets rapidly, it can be sprayed onto tunnel walls (as shotcrete ) and even used as a tunnel lining. 

According to the former analysis, a supporting scheme is selected for the deep soft rock tunnel in the coal mine. Specifically, the combination of anchor cable, anchor bolt, steel belt, steel mesh and sprayed concrete is applied for the comprehensive supporting in the tunnels. For the sections with severe floor heaving, concrete buffers should be used and forepoling bolts should be applied to support the fractured roof... 

One of the first commercial, proprietary anodes for decks was a flexible cable with a conductive plastic round a copper condnctor. The cable was woven across the deck and then a concrete overlay applied. It was also used on substructures with a sprayed concrete overlay. This system was very popular in the 1980s but unfortunately the condnctive plastic started to fail after about five year s of service. This led to attack of the copper and, in some cases, expansion of the plastic which delaminated and spalled the concrete overlay. This system is no longer available. 

The mesh has also been mounted in permanent form work and then grouted up by pumping in concrete or mortar from above or below. This avoids the quality control problems of sprayed concrete but gives more engineering problems and can increase the cost of the permanent form work. 

Excavated areas may be filled with the same material used to apply the anode, for example, sprayed concrete on substructures or the concrete... 

Excavated areas may be filled ivith the same material used to apply the anode, e,g, sprayed concrete on substructures or the concrete overlay on decks. In the case of the titanium mesh, special fixings will hold the anode in po,sition as the concrete fills around it,... 

As consideration of the element type choice for the rock mass and the sprayed concrete layer, the LST element with 6 nodes is used to simulate the rock mass, the BEAM6 element with 6 nodes is used to simulate the sprayed concrete layer, and the CO JO element is used to fault, which is established based on the theory of Katona s discontinuity (Goodman, R.E., 1972). The governing equation for the CO JO is shown in equation (1). 

According to the geological report on the Zipingpu hydraulic project, the physical parameters and the mechanical parameters of the surrounding rock mass, the sprayed concrete layer, fault and its fillings are listed in Table 1. 

Rock mass Fault and its filling Sprayed concrete... 

From the results of numerical test, the displacement, the stress, the plastic region of the surrounding rock mass, the inner force and the stress in the sprayed concrete layer are analyzed in this section. 

The influence on the internal force of sprayed concrete layer... 

The influence of fault on sprayed concrete layer focuses mainly on two aspects, the first aspect is to change internal force of sprayed concrete layer, and another is to change the distribution of inner force along the tunnel periphery. In evaluation of the safety status of sprayed concrete layers, the... 

Figure 7. Axial force of sprayed concrete layer (a) no fault (b) with crown fault (c) with shoulder fault (d) with sidewall fault.

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