Circumferential strains of a concrete specimen in a pullout test

Abstract

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.

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

“…These results are important both from the theoretical point of view, as this effect has not been known before, and from the practical point of view, as this effect should be taken into consideration to properly interpret measured strain data. An attempt to understand this local effect motivated the present study, which complements our recent study …”

“…Hence, the maximum possible strain would be even larger if the gauge location would have been closer to the rib. The development of a smaller strain value in the specimen with lower tensile strength is also due to the fact that radial splitting crack are developed in this specimen as part of the overall damage . In addition, a different slope of the descending branch is observed, resulting from the development of a different damage mechanism; The comminuted concrete around the rebar in the lower tensile strength specimen does not allow unloading of the circumferential deformation thus leading to a constant strain at larger slips, as detailed in the previous article …”

“…For tensile strength that is lower than the theoretically expected value () the strain–slip curve does not develop a postpeak descending branch and remains at a constant strain level at postpeak slips. In the case where the specimens' tensile strength is higher (), a postpeak descending branch is developed (As also detailed in a previous article). It is worth noting that these two curve types are clearly observed in Tepfers's results although this distinction was not diagnosed and emphasized in the study (Figure ). Normalized curves of the circumferential strain–slip relationships are similar regardless of the location of the strain measurement (i.e., on the relative distance between the strain gauge and the rib), the position of the rib relative to the pullout force point or the specimen diameter. These curves do differ, however, depending on the concrete tensile strength. The radial pressure exerted by a rib at the rebar–concrete interface, has an influence zone in the longitudinal direction along the outer face of the specimen.…”

“…The circumferential strain–slip curves are shown in solid and dotted lines and the bond stress–slip curve is shown in dashed lines. The magnitude of the circumferential strains is affected by the level of confinement, and therefore higher circumferential strains are obtained at a lower confinement level, where the concrete cover is relatively smaller. At higher confinement, where the concrete cover is thicker, the circumferential strain is relatively smaller.…”

“…The measured circumferential tensile strains on the specimen outer surface result from these radial stresses developed during the pullout process. In the previous study, global aspects of the measured circumferential strains were identified and discussed. Two curve forms of the strain–slip relationship were identified: Form A curve, which is characterized by an ascending branch until a maximum strain is reached; beyond the peak strain the strain magnitude is descending.…”