Pullout test of a rebar from an axisymmetric concrete specimen yields the bond‐slip relationship characterizing the rebar‐concrete interfacial constitutive relationship. It is assumed that inclined compressive forces are developed between the rebar ribs and the concrete during pullout. The components of the inclined forces that are perpendicular to the rebar axis create a ring action in the concrete producing radial compressive stresses and circumferential tensile stresses. The ring action develops circumferential strains on the specimen outer surface that are the subject of the present investigation. Although these strains are usually not measured, they may enhance understanding of the ring action in concrete during a pullout test, which is an important component of the bond resistance mechanism. The present study aims at investigating these circumferential strains and at examination of their dependence on different parameters. The cylindrical specimens were made of concrete without coarse aggregates to provide a relatively homogeneous material and assure the specimens axisymmetric behavior and coherent results. The investigation examines the effects of several major parameters (concrete strength, specimen diameter governing the confinement level and the number of ribs in the bonded length). Attaching a number of strain gauges along the specimen envelope to measure the circumferential strains allows investigation of their longitudinal variation during the experiment.

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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.

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