Investigations on corrosion and mechanical properties of a 20 year old ground anchor exhumed at a power station site

Abstract

This paper presents the observed findings and laboratory test results of an anchor exhumed from Manwan Hydropower Station, China, that was installed 20 years ago. The prestressed cables are 25.6 m long with a working load of 1000 kN. It consists of eight strands, and each strand consists of seven wires. The anchor was installed using the “single protection” technique (i.e., the steel strands without greased sheath and were backfilled directly with cement mortar). The anchor was unearthed by excavating a 1.5 m × 1.7 m tunnel. Visual inspection shows that the steel strands in the free and fixed lengths are basically stainless except for a few places where the grout failed to cover the anchor completely, thereby allowing direct contact of the strands with air. The magnitude of rebound of the tendon during the advancement of the tunnel face was simultaneously measured indicating a gradual release of bonding force, which was up to 62% of the prestressed load. This shows that the existence of the bonding force is provided by the bonding between the strand and the grout, while this bonding does not exist in the “double protection” system. The results of the mechanical test confirm that the tensile strengths in all the 33 steel wires exceed the requirement as specified by a current Chinese Standard. The concentrations of various chemicals also satisfy the requirements as specified by the China State Bureau of Quality Technical Supervision. The results of the X-ray diffraction test show that the stains taken from the surface of the strand are composed of FeO and FeOOH, both being oxide products of iron. This shows that exposure of the material to air is an important condition to initiate corrosion. By following the ASTM International G1-03 test procedure, the average corrosion rate is found to be between 3 × 10−4and 6 × 10−4mm/year. This investigation generally supports the use of unprotected steel strands in rock anchor.