Effects of sediment mineral compositions on the combined erosions of cavitation and abrasion of concrete material

Most of the rivers in China are sediment-laden ones. The high-head flood release structures are generally concrete structures in sediment-laden rivers. The damage can vary in the high-velocity flows. Especially, the surface of the flow passage is damaged quickly under the action of abrasion and cavitation erosions, which are caused by the sediment-laden flow on flood release structures. There is a serious influence on the safe operation of hydraulic structures. For example, the Yellowtail Dam spillway tunnel in the United States and the overflow dam of Zhexi Hydropower Station in China suffered serious abrasion and cavitation damages. It is a high demand to clarify the effects of sediment concentration and sediment grain size on abrasions of hydraulic machinery. However, it is still lacking in the different mineral compositions (quartz, feldspar, dolomite, mica, and pyroxene) on the abrasion and cavitation erosions of concrete in the high-velocity flow. In this study, the sediment mineral compositions were selected at different sediment concentrations to be experimentally studied in a self-developed small circulating water tunnel. The water tunnel was composed of a water tank, multistage centrifugal pump, Venturi working section, electromagnetic flowmeter, control valve, and circulating pipeline. The water tank was composed of an inner and outer cylinder. The inner cylinder accommodated the sediment-laden water flows, and the outer was for the cooling water. The sediment-laden water flows in the inner cylinder were pumped into the circulating water pipeline using a hydraulic pump, and then returned back to the inner cylinder via the throttle valve, Venturi working section, back pressure valve, and electromagnetic flowmeter. The low-velocity water flow in the circulating pipeline was formed by the high-velocity water flow in the throat after passing through the contraction section, where the cavitation occurred. Cavitation water flow entered into the expansion section, where the cavitation erosion occurred, due to the collapse of cavitation bubbles under the action of the pressure rise. Firstly, sediment-laden water flows with different sediment mineral compositions (quartz, feldspar, dolomite, mica, and pyroxene) and different sediment concentrations ((S=2.5, 12.0, and 20.0 kg/m³) were prepared in the circulating water tunnel. Sediment concentrations of the sediment-laden flows were measured with an infrared-suspended solid analyzer (Model 3150 and TSS). There are seven measuring points at the bottom of the Venturi working section, of which measuring point 1 was at the throat (cavitation zone), and the measuring points 2-7 were equidistantly at the expansion section (abrasion zone). The measuring points were connected with the pressure sensors, and the pressures of each measuring point in the cavitation and abrasion zones were real-timely acquired by the pressure data acquisition system (Model YE6263). Secondly, a concrete specimen placement box was set at the top of the expansion section to install the concrete specimen. The lower surface of the specimen was connected with the high-velocity water flow. Concrete specimens with the same strength were used in the test. The water-cement ratio was 0.375 and the cement-sand ratio was 2.5. At the age of seven days, the abrasion and cavitation erosion tests were carried out for four hours under the working conditions of the same grain size, different sediment concentrations and mineral compositions. The concrete specimen was removed per hour of the test and then dried in the electric blow-drying box. The high-precision electronic balance was used to weigh and record the surface morphology of the abrasion. The abrasion amount was represented by the weight loss of concrete specimen per hour. The experimental results showed that the pressure at the measuring point in the cavitation zone decreased with the increase in the sediment concentration under the condition of median diameter (d₅₀=0.150 mm) and throat flow velocity V=38.6 m/s. The pressure at the measuring point in the abrasion and cavitation erosion zone increased with the increase in the sediment concentration, indicating the occurrence of cavitation, abrasion and cavitation erosion. As the same sediment concentration, the pressure at the measuring point in the cavitation zone decreased with the increase in the mineral Mohs hardness, while the pressure at the measuring point in the abrasion zone increased with the increase in the mineral hardness. Remaining unchanged both the mix proportion and the age of concrete specimens, the degree of abrasion and cavitation erosions of concrete specimens aggravated with the increase in sediment concentration, indicating the linear relationship. The aggravation of abrasion and cavitation erosions also developed with the increase in the mineral hardness