Shotcrete design and installation compliance testing: early strength, load capacity, toughness, adhesion strength, and applied quality.
Spokane, WA: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2015-107, (RI 9697), 2015 Mar; :1-108
The National Institute for Occupational Safety and Health (NIOSH) conducted a research study to document and develop safe practices for the use of shotcrete as ground support in underground mines, particularly in underground metal mines operating in weak host rock. Shotcrete is the generic name for a mixture of cement, sand, fine aggregate, and water that is applied pneumatically and compacted dynamically under high velocity. The objective of this research is to reduce mine worker fatalities and injuries resulting from rockfall accidents. Although the information, techniques, and technology covered in this publication will impact both the mining and construction sectors, the primary audience is the mining industry with a focus on underground metal mines operating in weak ground conditions. The information and practices covered in this publication relating to the use of shotcrete can be put to use by mining professionals towards improving mine design and ground control plans. The guidance and practices reported in this document will help safety auditors, mining companies, and shotcrete suppliers in improving their shotcrete product specifications and the performance of ground support systems, evaluating ground control plans, and assessing shotcrete quality control. Ground control safety can be improved by providing these groups with a better understanding of the use of shotcrete in weak rock conditions, field test methods and equipment for measuring the strength properties of shotcrete directly at the mine site, and a practical means of conducting quality control during shotcrete applications. Development of Portable Test Machines: NIOSH researchers developed three portable test machines for determining shotcrete strength properties directly at the mine site. These portable test machines can be used to measure the early-age compressive strength of the shotcrete, the flexural load capacity and toughness of the shotcrete, and the installed quality and bond strength of the shotcrete that is applied to underground entries; this enables the test machines to be used to verify safe re-entry times. Onsite testing of as-placed shotcrete allows the mine personnel and shotcrete supplier to determine if the shotcrete is performing to design specifications. Using these shotcrete test machines directly at a mine site allows the support capabilities of the shotcrete to be evaluated in terms of the specific ground conditions, support methods, mining spans, and entry dimensions at the mine. As a result, mine design decisions regarding the use of shotcrete can be made from a much more informed position. Ultimately, this enhanced onsite knowledge of shotcrete strength properties and the quality of shotcrete application techniques can result in better ground support system designs and procedures, thereby reducing the number of fatalities and injuries associated with groundfall accidents. Shotcrete Characteristics and Application: When the shotcrete is applied in-cycle to underground mine surfaces as part of the ground support system, it becomes important to quantify when mining can safely resume under the material. As part of the overall ground support system, shotcrete is typically sprayed on the surface of an underground opening to stabilize the ground and prevent raveling. Shotcrete is also applied at lower water-to-cement ratios than concrete and develops its own unique strength characteristics. In addition, the quality of the applied shotcrete, the competency of the underlying rock, and the load-carrying capability of the shotcrete once cured are of critical importance. The significant shotcrete characteristics examined in this report are: slump, compressive strength, tensile strength, early strength, adhesion strength, and flexural strength. Of these shotcrete characteristics the engineering strength properties that are determined using early strength, adhesion strength, and flexural strength testing methods are the focus of this report. The significant shotcrete tests to determine characteristics examined in this research study are: 1. Slump Test - used to determine (wet) uncured shotcrete consistency. 2. Compression Test - used to measure cured shotcrete compression strength. 3. Tensile Test - used to measure cured shotcrete tensile strength. 4. Early Strength Partial-beam Test - used to measure shotcrete cure strength development over time. 5. Overcoring and Direct-tension Pull Test - used to measure cured shotcrete adhesion strength. 6. Round Determinate Panel Flexure Test - used to measure cured shotcrete load capacity and toughness.