Selecting shotcrete composition for Vostoktsvetmet mines

According to the results of the pilot trials in mines of Vostoktsvetmet, the combination shotcrete lining ensures stability and safety of underground excavation in complicated ground conditions. The paper presents the information on the conformity of the shotcrete mixture components to the standards. The compositions for the wet-mix shotcreting are selected. It is shown that inert materials used in the compositions have considerable influence on the properties of shotcrete mixes and shotcrete linings. The cost per unit of basic process materials (mixture components) is calculated in terms of Orlov Mine.

Compression strength class of concrete should be B25 in line with [2] and B22.5 as per [3]. In B25 class, the average compression strength of concrete at the age of 28 days is 32.06 MPa, and recommended cement grade is 400. In concrete class В t = 1.6, the average axial tension strength and bending tension at the age of 28 days is 2.06 MPa, and recommended cement grades is 20.
Based on the generalized experience of use of different cement grades from different manufacturers in East Kazakhstan, it is concluded that cement of Semey Cement Plant sometimes fails to exhibit the rated specifications as against cement of Bukhtarma Cement Company. Furthermore, Bukhtarma cement has higher strength characteristics than Semey cement as follows from practice of backfill preparation and use.
The laboratory tests on conformity between shotcrete mixes and standards and the selection of wet-mix shotcrete have been carried out in the Certification Testing Laboratory of the East Kazakhstan State Technical University, in a special construction and backfilling laboratory at Ridder-Kazzinc, BASF-Central Asia and Normet Company. The compression and bending testing results for different cement grades are given in Figure 1.
The best conformity with the strength standards is demonstrated by cement grades CEM II/A-Sh 42.5 N (Bukhtarma Cement Company) and PTs400-D20 (Semey Cement Plant). The cement grades have almost the same compression strength characteristic, while CEM II/A-Sh 42.5 N cement grade has the increased bending strength, which is one of the basic properties of shotcrete). The cement grades have approximately equal costs, too. Shotcrete mix filler may be: -sand as per state standards GOST 8736, GOST 26633 and GOST 9757; -break stone or grave by GOST 8267; -light fillers by GOST 9757. Sand and gravel mix should have characteristic described below: -size modulus not less than 2 (in case that size modulus is less than 2, special experimentation equipment is required; -moisture content to 7%; -limit content of clayey particles and fine powder dust to 3%; -clot clay content to 0.25%; -content of grains smaller than 0.14 mm in size to 10%; -limit content of particles larger than 8 mm no more than 10%. Particles larger than 8 mm in size should be eliminated while sifting, storing and processing of fillers.
The best conformity with the listed requirements is demonstrtaed by sand manufactured by Nonmetallic Materials Plant, except for a slight excess of standard contents of clayeye and powder particles by 1.7%.
Making of trial batches of mixes using blast-furnace granulated slag, manufacture of specimens and the tests of all rated quality indexes has been carried out in the special construction and backfilling laboratory at Ridder-Kazzinc [4,5]. Characterization of compositions for wet-mix shotcrete lining is given in the table 1. Based on the tests in the Ridder-Kazzinc laboratory, the curves of compression strength and cement consumption at water flow rate of 190 l/m 3 have been plotted for for different mixes (CEM II/A-Sh 42.5 N by Bukhtarma, sand and break stone by Nonmetallic Materials Plant) in Figure 2.
It is seen in the figure that the strength of concrete 7 days old is 27.43 MPa, which is nearly the required strength at the age of 28 days (28.83 MPa); the concrete acqures the required compression strength at the age pof 28 days even at the cement consumption of 360 kg/m 3 . The tests of concrete batches with blast-furnace granulated slag show that this mix of concrete becomes stronger less intensive. When granulated blast-furnace slag is used as inert material, the wanted strength is not reached in concrete 28 days old (at binder consumption to 440 kg/m 3 ).
By the data of tests of shotcrete bars 40×40×160 mm, the tensions strength of concrete and cement consumption are related (see Figure 3).
On the basis of the laboratory tests, the accepted design for the wet-mix shotcrete is:     Additional expenditures in shortcrete mix preparation may be reinforcement fibers. Concrete reinforced with fibers preserved structural integrity owing totally to stress redistribution inside a specimen and due to high plasticity.

Conclusion
Based on the performed tests, the compositions of wet-mix shotcrete are recommended, and it is shown that inert materials have an essential influence on properties of shotcrete mixtures and shotcrete lining, which implies that the final selection of shotcrete mix components should be made after additional full-scale testing of shotcrete properties.