The concrete lining of high-pressure water conveyance tunnels permeates under high-pressure water. Dense and hydrophobic coating can effectively improve the impermeability of concrete. However, the coating exhibits varying impermeability in different high-pressure environments, which can even lead to coating detachment or damage. The objectives of this study are to improve the high-pressure impermeability of concrete by using a polymer coating, and to study the varying impermeability through experiments. This study applied a polymer coating called SCU-SD-SP-II (SSS) to concrete surfaces, and it formed a composite protective layer with an epoxy-modified silicone (EMS) coating. A series of high-pressure impermeability tests were conducted to study the seepage regulation of the coated concrete and the failure mechanism of the SSS coating under cracks in the concrete. The results indicate that the SSS coating has excellent impermeability. Pressurized water of 3 MPa could not permeate the SSS coating with a thickness of 0.5 mm within 24 h. Under both external and internal water pressure conditions, the SSS coatings improved concrete impermeability. Additionally, the average seepage height and relative permeability coefficient of the latter decreased by 49.6% and 71.2%, respectively, compared with the former. After concrete cracking, the SSS coating could withstand 3 MPa pressure on crack surfaces smaller than 1 mm. When the crack width was greater than 2 mm, the SSS coating deformed under 1 MPa pressure. As the pressure increased to 2 MPa or even 3 MPa, the SSS coating was punctured or torn due to stress concentration. This study provides new insights into the impermeability of concrete under high water pressure.

The high water head of some pumped storage power stations will induce the cracking of the concrete lining of their diversion tunnel and the leakage of high-pressure water, which will affect the safety of the tunnel and the surrounding rock. At present, there is no solution to the problem of impermeability of concrete materials after cracking. This paper proposes a composite lining to solve this problem. The composite lining with modified polydimethylsiloxane coating can effectively prevent high-pressure water, but its crack resistance needs to be further studied. Therefore, the tensile mechanical properties, constitutive relationship of modified polydimethylsiloxane impermeable coating, and the crack resistance mechanical properties of modified polydimethylsiloxane impermeable composite lining were studied by laboratory tests and numerical simulations. The results show that the true fracture elongation of the modified polydimethylsiloxane impermeable coating is as high as 118.98%, and its mechanical behavior can be described by a simplified polynomial hyperelastic constitutive model. The in situ stress will affect the crack width of the concrete lining. When the lateral pressure coefficient is less than 1, the crack width decreases with the increase in the lateral pressure coefficient. When the lateral pressure coefficient is greater than 1, the crack width increases with the increase in the lateral pressure coefficient. To prevent the cracking of modified polydimethylsiloxane coating, its spraying thickness needs to increase with the increase in crack width. The ratio of the coating’s thickness to crack width is recommended from 0.162 to 1.930 for internal water pressure from 1 MPa to 10 MPa, respectively. The suggestion provides a reference for designing the impermeable composite lining structure subjected to high internal water pressure.