Abstract
Hydraulically expansion-type rock bolts made of steel pipe (hereinafter referred to as rock bolt) have often been used for the construction of tunnels in the natural ground with plentiful spring water or areas where the natural ground is readily displaced by tunnel excavation. Rock bolts hot-dip coated with Zn-6%Al-3%Mg alloy and without coating have been installed in Hida Tunnel on the Tokai Hokuriku Expressway to evaluate the durability under spring water conditions. These rock bolts were collected annually by over-coring method, one, two and three years after the installation and their corrosion behavior was examined.
The corrosion rate of rock bolts without coating was found to be nearly linear and therefore the corrosion is thought to be controlled by the diffusion of dissolved oxygen. In the case of rock bolts coated with Zn-6%Al-3%Mg, on the other hand, there exists little corroded area with gray appearance as well as somewhat corroded area with dark gray appearance. However, the corrosion rate of coating in dark gray area decreased with progress of time and became substantially zero after two-year service. The outermost surface of the rock bolt after three-year service has been covered uniformly with Zn-Si-O composite which was estimated to be zinc silicate by analyzing chemical compositions of spring water obtained at the test site. This zinc silicate layer formed on corrosion products may act as a further anti-corrosive barrier and hinder further progress of corrosion.
