Bond Strength of Plain Strand Cablebolts

Adhesion and Bond Strength

In the context of steel reinforcement of rock or concrete materials, adhesion describes a bonding mechanism (Farmer, 1975; Littlejohn and Bruce, 1975) in which a pseudo-chemical bond develops at the steel/cement interface which is brittle (no residual bond after rupture) and independent of confining pressure (stress normal to the interface).

Typically, for regular carbon steel and cement grouts with W:C in the range of 0.35 - 0.5, this adhesion or shear resistance is equivalent to 1 to 3 MPa. Over the surface area of a 15.2 mm diameter cable, this is equivalent to a capacity of 10 kN over a 20 cm length of grouted cable. Unfortunately, this adhesion is exceeded after less than one fifth of a millimetre of relative slip (Fuller and Cox, 1975; Hyett et al., 1992; Nosé, 1993). As such, it is unlikely that adhesion can act simultaneously over any appreciable embedment (grouted) length and rarely accounts for any significant percentage of the instantaneous pullout resistance (bond strength). In fact, as the cable is loaded and begins to slip at the cable/grout interface, a wave of localized adhesion failure propagates down the cable away from the loading site.

Adhesion is thereby rapidly removed from the system as this initial bond is broken and is not considered hereafter as a load transfer mechanism. Slip, dilation, friction and bond strength The helical, multi-wire nature of the cable surface creates a negative relief of equivalent geometry in the hardened grout. After adhesion is removed from the interface, the cable slips with respect to the grout annulus. If rotation of the cable during pull-out is prevented, a geometric mismatch occurs between the cable flutes and the corresponding grout ridges. This mismatch increases with increasing relative slip as illustrated in Figure 2.6.4.

As the grout ridges must ride up and over the cable wires, the grout compresses in the confined borehole and thus generates a normal pressure on the grout/steel interface. Friction (pressure dependent shear strength) thus develops along this interface providing resistance to further slip. This interaction is called dilation. Dilation is limited in the extreme by the absolute scale (height) of the grout ridges. In reality, dilation pressures develop to the point where these ridges crush, reducing the maximum dilation to less than 0.1 mm for plain strand cable (Diederichs et al., 1993).

Dilation is the key to cablebolt performance and is a complex process which is dependent on grout stiffness, rock stiffness and grout strength. This relationship will be explored in the next section.

Bond Strength and Load Transfer

Before proceeding with a discussion of bond strength, it is necessary to understand the process by which load is transferred from the rockmass to the cable via the shear resistance at the cable-grout interface. As the rock slips with respect to the cable, shear stresses (load/unit area) are generated at the interface. As these shear stresses accumulate along the length of the cable due to the addition of incremental rock loads, the tension in the steel strand increases (for an unplated cable) from zero at the face to a maximum at some point into the borehole.

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