Factors affecting adhesive strength of different thin spray-on liners
Thin spray-on liners, a new form of rock support, are receiving increasing attention by various mines around the world. Various liner materials are currently being developed and tested. They can be generically classified as multi-component polymeric materials and have applications as a replacement for either wire mesh or shotcrete. Where adequate adhesive bond to a substrate exists, thin spray-on liners have the potential to transfer or carry the dead weight of loose rock in contact with liners to stable or unfractured rock surfaces that also maintain liner contact. This paper examines factors that can affect the adhesive strength between a liner and rock surface, including choice of liner material and rock substrate.
The test method is a slight modification of other methods that have been used for thin spray-on liners and is based on direct pull-off of a 33 mm diameter elevator bolt that is affixed to a liner using a strong epoxy. The liner is over-cored using a 35 mm diameter, thin-wall diamond-coring bit. The elevator bolt is then glued to the liner using a two-component epoxy. After the epoxy sets, the test specimen is clamped to a tensile loading machine and the elevator bolt is pulled at a rate of 2 mm/min., until the specimen fails. This results in specimen failure within 60 seconds of application of the load. The adhesive strength is calculated by dividing the maximummeasured load by the over-core area.
Adhesion tests were conducted to assess the impact on adhesion of the presence of oil, water, and dust on a substrate, and the roughness and grain size of different substrates. Other factors, such as curing time, loading rate, and liner thickness, were also considered. Most tests were performed using Tekflex as the liner material, although five other liner products were also tested. Adhesion tests on five other liner products showed an order of magnitude range in adhesive strengths between 0.2 and over 2.4 MPa. Some liner products are weak in tension and give low adhesive strengths. This will likely preclude their use in rock support applications.
Adhesive strengths of about 2 MPa can be achieved with Tekflex under optimal conditions. Other liner products were either relatively weak in tension and gave lower adhesive strengths compared to Tekflex, or had adhesive strengths that were similar or higher than Tekflex. A product for use as a thin spray-on liner for rock support applications probably requires a tensile strength greater than about 2 to 3 MPa. Good adhesion to a substrate requires even higher liner tensile strength. Where the rock surface is contaminated with dust or the rock is weak in tension, it may be difficult to reach adhesion strengths of 1 to 1.5 MPa. Based on the performance of different substrates, it appears that the rock’s tensile strength probably needs to exceed 2 MPa to ensure good adhesion.
Long-term creep tests showed that the adhesive strength could drop by at least 50% when the liner carries load for about a month. As the thickness of the liner increases, the adhesive strength to a substrate also decreases. These laboratory test results have implications for liner design in the field, in that design values for adhesion probably need to be significantly reduced from those measured by the various adhesion tests that have been used to date. This new insight on lower adhesion associated with long-term sustained loading and thicker liners warrants further investigation to assess whether this phenomenon holds true under field conditions.