The effect of local rebar geometry on global strain measurements in pull out tests

Abstract

Strain gauges are the common means to measure local strains in structural elements. It is well known that the strain at a given coordinate on the boundary of a structural element depends on the geometry, boundary conditions, loading system, and material properties. The precise location of the strain gauge along a line is not considered critical to obtaining a quality strain measurement at a specific location, wherever a continuous variation of strain along this line is expected, and a slight deviation of the gauge location will yield similar measurements. In a recent study, precise measurement of the circumferential strains on the outer face of cylindrical concrete specimens was carried out during pullout of a central rebar. Analyzing the measured data indicated that a slight shift in the strain gauge location on the order of millimeters only may considerably affect the measured strain. These controlled pullout tests highlighted the pronounced effect of the relative distance between the rebar rib and the strain gauge location on the strain magnitude and its variation with the increasing slip. This effect has not been reported before. It means that a slight difference in the strain gauge location yields different strain–slip relationships. This new finding may affect the interpretation of the measured strains. Attention should be given to the exact strain gauges' locations and detailed rebar ribs locations to properly interpret the measured data.

Predicting the circumferential strains on a cylindrical concrete specimen during a pull‐out test

“…This three‐dimensional description of the ring action in the pull‐out problem is different from the plane strain disc response to the action of an inner evenly distributed pressure that is assumed in other models, where the radial compressive stresses and tangential tensile stresses and strains are independent of the longitudinal coordinate. The two‐dimensional model cannot consider the strain in the longitudinal direction as well as the longitudinal variation of the circumferential strains as observed in the experimental measurements 8 and in the qualitative analysis above.…”

“…Figure 3 shows that the normalized circumferential strain variation is insensitive to the small band width size. Therefore, a 5‐mm bandwidth, which is about the rib width in the experiments, 7,8 will be adopted in the following analysis. Figure 3 also shows the influence zone of a single pressure band, which is equal to about a one‐cylinder radius at each side of the pressure band.…”

“…In a recent study 7,8 series of cylindrical specimens with different diameters (100, 150, and 200 mm) were tested and measurements of the circumferential strains at different locations along the specimens exterior cylindrical surface were conducted at different load and slip levels.…”

“…This is likely due to non‐uniform radial rebar‐concrete interaction pressure. Hence, the common two‐dimensional analysis of circular discs that is described above cannot represent the concrete cylinder behavior in the longitudinal direction. Circumferential strain gauges that were placed beyond the bonded length also measured circumferential strains 7,8 and their readings decrease with increasing distance from the end of the bonded zone. This finding leads to the conjecture that the radial compressive force of a rib induces circumferential strains along a certain length in the longitudinal direction which determines its influence region.…”

An 18-Month Analysis of Bond Strength of Hot-Dip Galvanized Reinforcing Steel B500SP and S235JR+AR to Chloride Contaminated Concrete

“…Two first processes play an important role for bond strength between the smooth rebars. On the other hand, the mechanical interaction has a fundamental significance for the bond strength with respect to ribbed steel [ 10 ]. Thus, the bond stress for smooth steel rebars was only from one-third to one-half of stresses determined for the ribbed rebars [ 11 ].…”