In order to comprehensively investigate the influence of the thickness of thin spray-on liners on their adhesive strength with different surrounding rock environments, pull-off tests were employed to measure the adhesive strength of thin spray-on liners of different thickness on the surface of various substrates (granite, sandstone, concrete, and coal), which were used to simulate different surrounding rock environments. The results indicated that the failure mode was mainly liner material–substrate adhesion failure when granite, sandstone, or concrete was used as the substrate, while substrate failure was observed to occur more frequently when coal was used as the substrate. The variation rules of the adhesive strength ( σ) of thin spray-on liners, along with liner thickness ( t), for different surrounding rock environments were obtained: [Formula: see text] for granite, [Formula: see text] for sandstone, [Formula: see text] for concrete, and [Formula: see text] ( t < 4) and [Formula: see text] ( t≥4) for coal. Furthermore, analysis of the standard deviation of experimental adhesive-strength data suggested that the variation in adhesive strength decreased in the following sequence: coal > concrete > sandstone > granite. The achievements of the current study provide a theoretical basis for the on-site determination of thin spray-on liner thickness requirements, as well as the scientific and rational design of liner material structure.
Polyethylene terephthalate (PET) fiber from waste plastic bottles is an environmentally friendly fiber that can improve the mechanical properties of wet-mix shotcrete. The main objective of this paper is to investigate the effects of PET fiber parameters on pumpability, shootability, and mechanical properties of wet-mix shotcrete. For this purpose, the orthogonal test based on three factors and three levels was conducted. The width, length, and content of PET fiber were selected as the experimental variables. The tests of wet-mix shotcrete reinforced by PET fibers were carried out, along with properties tests such as slump, pressure drop, build-up thickness, compressive strength, and splitting strength. The results showed the change trend of shotcrete performances with increasing PET fiber parameters (width, length, and content). According to the orthogonal test, PET fiber parameters were determined (1 mm width, 20 mm length, and 5 kg/m3 content). Furthermore, relationships between slump and pumpability and shootability were explored, as well as the relationships between pumpability, shootability, and mechanical properties. It was found that pressure drop and compressive strength had the strongest negative linear relationship among all fitting relationships. We hoped that this study could contribute the useful information for the application of wet-mix shotcrete mixed with PET fibers.
Compared to conventional shotcrete, thin spray-on liner (TSL) has several advantages, including high efficiency, good adhesion, and low dust content. In order to study the underlying mechanism of polymers on thin spray-on liner materials, three polymers were tested in this study. They were mixed into the shotcrete materials to conduct viscosity, pulling, flexural strength, and compression tests. The correlations between the viscosity of the materials and their other mechanical properties were analyzed. The optimal mixing ratio obtained by the tests was at a polymer–cement ratio of 15%. The material had better mechanical properties overall when modified with a vinyl acetate-ethylene (VAE) copolymer emulsion. When a VAE emulsion was used to modify cement mortar, the results showed that the VAE modified mortar had the best viscosity performance of any of the tested emulsions at the same polymer–cement ratio. Because of the adhesion of the modified mortar on a marble wall, when the viscosity was 2,300–5,800 mPa·s, the slurry was evenly distributed and did not sag on the wall surface. The VAE emulsion was affected by its own viscosity, and the pull-out strength was also enhanced as the viscosity increased. As the viscosity of the VAE emulsion increased, its flexural strength initially increased and then decreased, whereas its compressive strength decreased linearly. This study provides a theoretical basis for the development and application of thin shotcrete materials.
In order to adapt to dry and wet environment in mine roadways and improve the performance of sealing materials under different environmental conditions, this study selected the materials based on their ability to spray and found that the materials with a viscosity of 1500–6000 MPa·s can achieve a balance between performance of spray and wall-hanging performance. Next, by selecting high-elastic polyacrylate emulsion and ordinary Portland cement as the film-forming agent and gel, respectively, mix proportion factors of the nonreactive thin spray-on liner (TSL) were analyzed by using the Taguchi method. Moreover, fluidity, setting time, sealing properties, compressive strength, bending strength, and bonding strength of those materials were measured. It was disclosed that addition of an appropriate amount of polyacrylate emulsion can enhance the effects of fibers. By observing the morphologies of the materials’ bonding surfaces and comparing the factors influencing bonding strength between both dry and wet walls, it was uncovered that the adhesive property of cement-based sealing materials was mainly affected by the water-cement ratio. Eventually, bonding strength and sealing properties were selected as primary optimization indexes, while setting time, bending strength, and compressive strength were selected as secondary optimization indexes. The appropriate proportions of two types of TSLs for dry and wet walls were (W/C = 50%; P/C = 6%; F/C = 1%; T/C = 0.2%) and (W/C = 45%; P/C = 6%; F/C = 1%; T/C = 0.4%), respectively. A prepared TSL can be used for filling rock fractures in mine roadways. In addition, TSL exhibited a great performance for dry and wet walls under different environmental conditions. The present study may provide an insightful guidance and reference for the investigation of TSL sealing materials applied in the mine roadways.
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