Construction industry services and technology provider Hilti has improved on its powder-actuated DX 450 shotcrete-testing system to suit the mining sector.
After seeing the scope to increase its client portfolio, Hilti presented the DX 450 to a copper mine in the Democratic Republic of Congo. Hilti is currently working with the mine to implement the system for shotcrete testing.
The DX 450 – which has been used in the construction sector since its launch in 2011 to determine the early strength of sprayed concrete – will for the first time be used in a mining application in Africa.
Testing of shotcrete is important for quality assurance when the shotcrete is applied on site. A sufficiently accurate knowledge of the concrete’s strength is needed under safety requirements stipulated by mines and construction firms. The strength development has to be proven during the entire construction process.
The DX 450 shotcrete-testing system is used to determine early uniaxial compressive strength of concrete to determine safe re-entry times for mineworkers underground to continue working.
The DX 450 comprises a powder actuated tool, M6x thread studs and cartridges, with the tensile tester driving a nail into the shotcrete at a certain distance. The depth of penetration gives an indication of the strength of the installation based on a calibration curve. The tensile strength of the nail used is then determined with a pull-out tester.
Shotcrete, a sprayed concrete, is used as a reinforced lining for tunnels in underground mines, and is the primary lining of a tunnel or underground installation that is also responsible for waterproofing the tunnel. It can be used as the only application or in conjunction with a sprayed membrane, and is also responsible for the stabilisation of underground mining tunnels.
The thick layers of shotcrete – sprayed overhead and on vertical surfaces in the construction of tunnels – needs to be fast setting and maintain a high early strength. To achieve these properties, special binders or typically binders, such as cements in combination with accelerators, are used.
Notably, shotcrete is often uniquely accountable for the waterproofing of structures underground. Once a stretch of shotcrete has been sprayed, testing and measuring have to be timed from two minutes to three hours in accordance with the build-up of strength in the sprayed concrete. This allows for an accurate continuous graph of strength development to be obtained, with anywhere between 10 and 15 samples being taken using the DX 450.
“Regular sampling enables you to carry out trend analysis graphs where you can monitor the shotcrete application over time,” says Hilti mining field engineer Charon Maseka.
She adds that the DX 450 – which falls under its gunite strength determination equipment range – has depth indicators to assist in gauging the thickness of the shotcrete applied. This complies with regulation 14.1 (6) of the Mine Health and Safety Act, which requires that a viable quality assurance system is in place. “This ensures that the support units used at the mine provide the required performance characteristics for the loading conditions expected.”
While research and development for all Hilti’s products is conducted at its head office in Schaan, Liechtenstein, Maseka says input on the local environment and requirements is often relayed from the South African branch, in Midrand, to the international team.
She adds that, while the DX 450 would nominally be used by rock engineers and geotechnical engineers on site, the testing process can be carried out by anyone who is sufficiently trained in its application.
“We are hoping to see a standardised method of determining quality of assurance on site,” says Maseka.
“If successfull, the DX 450 can help other mining clients test various applications, such as backfill, grout and shotcrete, and get quick results on site in addition to third-party testing,” she concludes.