# Cyclic Behavior of Long Concrete Interfaces Crossed by Steel Screws
## 5 Test Results
### Steel Strains  
Figure 10 presents typical diagrams of the recorded steel strains (positive sign for tension) in function of the crack width. For the sake of clarity of the graphs, the measurements of the strain gauges on the left side of the five screws closer to the actuator are presented. The measurements for the remaining five screws were similar. It is well known (see also [2,19,20]) that when an interface is subjected to shear, strains develop in the interface reinforcement due to the joint action of two distinct mechanisms, namely, friction and dowel action. Indeed, shear slip at the interface causes opening of the crack due to the asperities of the interface. As the crack at the interface occurs and is gradually increasing (for increasing shear slip), the interface reinforcement develops tensile strains (and stresses) due to its pullout. The tensile stresses of the reinforcement are equilibrated by compressive stresses on the interface. These stresses, known to be the result of the clamping effect, multiplied by a friction coefficient depending on the characteristics of the interface, constitute the contribution of the friction to the shear resistance of the interface. In parallel, the reinforcement crossing the sheared interface is deforming, and strains develop (of opposite sign in the compressed and tensioned zones of the connector under bending), while compression is exerted by the interface reinforcement to the concrete, close to the deformed portion of the interface reinforcement. Figure 10: Steel strains measured with strain gauges “1 L” up to “5 L”, vs. the crack opening for all (a–c) long interfaces. The diagrams are plotted up to a crack opening corresponding to shear slip equal to 1.05 mm. Figure 9: Shear slip versus crack opening (lateral dilatancy—average of all ten transducers) curves for all (a–c) long interfaces. The reaction to this compression constitutes the contribution of the deforming connectors to the shear resistance of the interface, a mechanism termed dowel action. The strains plotted in Figure 10 are due to both shear-resisting mechanisms. Strain values higher than 3.45% denote the yield of the screws (Table 1). It is interesting to observe that in all three specimens, the strains developed in the screws due to the joint action of the two shear transfer mechanisms exceeded the yield strain of the steel. This result seems to be unexpected, due to the quite small embedment length of the anchors to the concrete and, by way of consequence, to the expected limited contribution of the clamping effect, known to generate friction and be the main mechanism contributing to the transfer of shear across interfaces. The arrangement of the strain gauges (Figure 5) in the part of the screw that protrudes to the new concrete allowed to calculate the contribution of each mechanism to the strains measured by the strain gauges. In order to evaluate the contribution of each mechanism to the development of tensile strains in the screws, the following procedure (described in detail in [17]) was applied, also illustrated in the example of Figure 11. The distribution of the steel strains at a screw's cross-section was the result of the combination of the uniformly distributed tensile strains due to pullout and the strains due to dowel action (triangular, with opposite values at the two ends). The strain measured at the most tensioned screw side represents the sum of the pullout strain and the positive dowel strain. The strain measured at the least tensioned screw side represents the algebraic sum of the pullout strain and the negative dowel strain. By applying this procedure, the plots of Figure 12 were derived. Those plots show that the contribution of dowel action prevailed, even for rough interfaces.