The main component of the bond capacity in deformed bars (rock bolts) is mechanical interlocking, which after onset of slip, mobilizes the shear strength of the grout against the steel ribs. Similar to any rough interface, the most important factor controlling the shear strength at bolt-grout interface is confining pressure. Many examples exist in which a properly performed bolt at initial stages of installation has failed later due to stress decreases caused by nearby excavations. Due to the importance of confining pressure on bond capacity in deformed bars (rock bolts) an investigation is performed using a modified triaxial Hock cell to facilitate application of a constant radial confining pressure to a grouted bolt while pulling it axially. During the test, axial load and displacement of the bolt as well as the radial dilation of the grout is recorded using a data acquisition system. In this paper, details of the laboratory test results are presented and discussed and conclusions are made.
The idea of using steel bar as a means of reinforcement goes back to 1877 where Hyatt referred to the possibility of using Iron combined with Portland cement to overcome the weakness of the concrete under tensile flexure. This initiated plenty of experimental studies starting as early as 1900 which revealed that in most of the cases, the failure is not due to the excessive tension in the bar, it rather causes by slip of this. Therefore bond was recognized as a critical parameter in reinforced concrete design. The use of bar as a means of reinforcement in geotechnical and tunneling projects is however delayed. The first use of bolt (wooden type though) was reported by Beyl (1945) to be in a longwall mine in 1912. The first use of rock anchor in its present form was in Cheurfas Dam in Algeria in 1934. Due to the success of bolting fundamental studies on bolting action was continued both in mining and construction fields in parallel. One of the earliest comprehensive studies in civil field was dated back to 1940 by Gilkey et al. The importance of different parameters effecting bond capacity and the mechanism of load building up in smooth and deformed bars was studied in numerous pullout and beam tests. in the mining field Rabcewicz (1955) and Panek (1956 a.b,c, I962a,b) studied the importance of limiting displacement and creating suspension and beam building effects for bolts. Littlejohn and Bruce (1977) continued these studies and detailed the bond failure mechanisms in deformed bars.
Bond may be defined simply as the gripping effect of an annulus (usually concrete or cement) with an embedded length of a steel bar (smooth or deformed) to resist forces tending to slide the bar longitudinally. There are contradictory ideas about whether the bond is a property of annulus or of the bar. However, there is no doubt that the properties of both (for the annulus, compressive strength and confinement and for the bar.
