The dramatic improvements in spraying technology has allowed shotcrete to become the first choice ground support in many underground mines the world-over. In addition, the increased use of wet-mix fiber-reinforced shotcrete has accelerated, and been spurred along by improvements in machinery, admixtures, fibres and understanding the way shotcrete behaves as an underground support element.
Shotcrete is “aggregate conveyed through a hose and pneumatically projected at high velocity on to almost any surface”. Shotcrete is usually an all-inclusive term that can be used for both wet-mix and dry-mix versions.
Shotcrete undergoes placement and compaction at the same time due to the force with which it is projected from the nozzle, unlike conventional concrete that is first placed and vibrated in a second operation. Shotcrete is more dense, homogeneous, strong and waterproof than any other process.
Sprayed concrete is an excellent tool for stabilisation and support of structures in a very short time and for concrete applications without using any moulds. Over the past years, sprayed concrete has replaced the traditional methods of lining tunnel profiles and has become very important in stabilising the excavated tunnel section.
Sprayed concrete refers to a number of components of a complete technology, that is, the material sprayed concrete, sprayed concreting process and the sprayed concrete system. The sprayed material concrete refers to a concrete mix design that is determined by the requirements of the application and the specified parameters.
Sprayed concrete process encompasses the dry process sprayed concrete and wet process sprayed concrete. The main mix requirements focus on the workability and durabilityand they are high early strength, the correct set concrete characteristics, user friendly workability, good pumpability, good sprayability and minimum rebound.
Applications for Shotcrete
Any underground tunnelling must be supported and Shotcrete is the quickest, strongest support available. Any loose rock requires support, every tunnel opened should be supported, explains Wayne Treges, Owner of Gunnite International.
Shotcrete is used in many different types of projects whether it is above-ground or underground buildings. In mining, the main uses are focused on excavation stabilisation, temporary and permanent arch lining, while in tunnelling- it is for primary support and finished linings. In addition, they are also used in the restoration and repair of bridges, foundations, walls, parking garages, canals and silos, and in new construction, buildings, pools, tanks and domes.
Uses of shotcrete are widespread, including reinforcement of slopes, sidewalks, piers, and lateral support on foundations and embankments. Corrosion supports, silos and shafts with high strength (75mpa) wear linings.
Sprayed concrete is used in all areas of tunnel construction- for road/ rail tunnels, water drainage and underground structures, in addition to slope stabilisation. In underground construction, because high stresses would often be exerted on the newly placed excavation stabilisation and lining predetermined deformation of the excavated section is often allowed and only then is the stabilisation given a non-positive seal.
Sprayed concrete is the perfect material for excavation stabilisation. Its unique flexibility in the choice of application thickness, material formulation, output capacity, very early strength development and the ability to respray at any time makes sprayed concrete the complete material for excavation stabilisation.
The final lining of a tunnel is the permanently visible visiting card of the tunnelling contractor. The exception is a final lining with panelling. Inner lining concrete and sprayed concrete are both used for a durable final lining. The higher the specifications for the evenness of the concrete finish, the more likely it is that a lining of structural concrete with interior ring forms will be used. Formed interior finishes are also considered to be aesthetically superior.
A rock bolt is a long anchor bolt, for stabilizing rock excavations, which may be used in embankment, lateral support, tunnels or rock cuts. It allocates load from the unsteady exterior, to the confined and much sturdier interior of the rock mass.
Rock bolts are inserted 500mm/ 1000mm apart, what about in between them? That is where shotcrete comes in, rock bolt (anchor) mesh overlaid or (steel/ polyfibre in mix) then Shotcrete continues, covering the whole unsafe area, further explains, Mr Treges.
Wet Sprayed Concrete
Wet sprayed concrete is always used when high concrete quality is specified and high output is required, and it is by far the most popular in mechanical tunnelling. It works with high output capacity, substantially improved working conditions in the spraying area and higher durability due to controlled mixing water quality.
Advantages of Sprayed Concrete
Wet sprayed concrete is the more modern and efficient method. It leads to increased spraying output, up to 25 m3 / hour in some cases, rebound level is reduced by a factor of two to four, substantially improved working conditions due to less dust generation, reduced wear costs on the spraying equipment, low air requirement during spraying and higher quality installed sprayed concrete.
Dry Sprayed Concrete
Dry sprayed concrete is always used when smaller quantities and outputs are required and high very early strength is essential, for instance for preliminary sealing against high water penetration with Shotcrete.
Advantages of Dry Sprayed Concrete
The advantage of dry sprayed concrete lie in its flexibility, and it enjoys high very early strength for preliminary sealing or stabilising, almost unlimited holding time of silo stored material and there is no concrete waste.
Rockcrete Equpment (Pty) Ltd
Rockcrete Equipment (Pty) Ltd was founded in 1992 by team that had a total of eighty-five years’ experience in the industry.
The ROCKCRETER 95, ROCKCRETER 65 and ROCKCRETER 45 are the leading Gunite / shotcretemachines in the marketplace. We strive to improve on the operation, quality and excellence of the equipment, whilst still keeping it simple.
The ROCKCRETER C70 WET SHOTCRETE PUMP and the ROCKCRETER TSL GROUT PUMP and the ROCKCRETER ANTLIA PISTON PUMP have recently been added to our range of equipment.
Rockcrete Equipment also supplies the MAYCO CONCRETE PUMPS. These pumps are designed and built in the USA and are well suited for our industry at very competitive prices.
A full range of spares are kept in stock at all times.
Additional services which we offer are, concrete additives and training with certification.Because of our extensive experience in the industry we are able to offer assistance with local tenders and hands on assistance with on-site problems which may occur.
We are committed to Quality and Service and our impeccable record speaks for itself, in that all of our clients that have supported us from inception are still with us today, and more being serviced every day.