Then, the interfacial bond-slip curves obtained from the test are compared with several existing bond-slip models; on this basis, considering the corresponding relationship between the interface strain-slip curve and the bond-slip model at different stages, an improved trilinear bond-slip model is proposed for this kind of interface.
According to the strain data measured in the test, the slip amount of each measuring point can be obtained according to (2) and (3), and the strain-slip curve of the microsection bond interface of the long anchor sample is shown in Figure 16.
It can be seen from Figure 16 that the strain-slip distribution trend of the bolt samples is similar, both showing a certain nonlinear variation trend.
Correspondence between Strain-Slip Curve and Bond-Slip Model.
It is assumed that the linear segments of the strain-slip curve have the following relationship:
where [epsilon](s) and s are the interface shear strain and its corresponding slip, respectively; [[epsilon].sub.1] is the maximum strain in the linear phase of the strain-slip curve; and [s.sub.1] is the slip amount corresponding to [[epsilon].sub.1].
As shown in Figure 19(a), the strain-slip curve corresponding to the linear softening section of the bond-slip model is nonlinear.
 used a simple mathematical function as given in (1) which was proved to represent the strain-slip ([epsilon]-s) relationship with a good accuracy when compared with the experimental results.
where A and B are experimental parameters obtained from the nonlinear regression between the plots of strain-slip relationship.
The predicted ultimate loads were calculated using (5) where the parameter A was determined by regression analysis of the experimental strain-slip relationship given by (3) for each individual case.
This could be possibly due to the fact that the strain-slip ([epsilon]-s) relationship in case of the ambient exposure is different from that of the moisture exposure.
In Figure 6, typical measured strain-slip data at 0, 6, 12, and 18 months and their corresponding regressed curve fittings are shown for SBB system.