Rock bolt corrosion – an experimental study

Abstract

The effect of long term exposure of full size bolts to corrosive environments is presented. A special test rig was used to test four bolts under different loading conditions. Four X-grade identical profile bolts, each 21·7 mm core diameter (23·7 mm full diameter) were subjected to prolonged corrosion testing using acid sulphate water. The pH value of the circulated water varied between 3·4 and 4·3. The corrosion exposure test period lasted 3·5 years. Two bolts were axially loaded to 10 t and 20 t force respectively, the third bolt was subjected to a 360 Nm torsion load and the fourth bolt was left unstressed to act as a reference bolt. After the test period ended, the bolts were stripped of their corroded coatings and weighted for weight loss. The diameter of each bolt was subsequently measured, and the loaded bolt samples were first tested non-destructively for tensile cracks and then tested for tensile failure. No cracks were found on post-corrosion bolts tested non-destructively. The failure strength reduction on all four post-corroded bolts was significant, varying between 21 and 39%. The onset of corrosion was not confined to the water targeted mid-section length of the bolt, however, the most severe corrosion occurred at the anchored ends of the bolts, possibly attributed to the combination of several types of corrosion which may have been also compounded with increased surface area of the threading at the bolt ends and excessive aeration corrosion due to increase in air movement.