Free-end Slip Research Articles

Corrosion influence on bond in reinforced concrete

The bond between reinforcing steel and the surrounding concrete can be deteriorated by corrosion. Pullout tests were carried out to evaluate the effects of corrosion on bond and bond–slip behavior, for a series of specimens with varying reinforcement corrosion levels between 0% and 9%, and for specimens with and without stirrups that provide confinement. Specimens with both smooth and deformed bars were tested. The tests were designed to provide the data required to assess the bond properties, including the ultimate bond strength and free-end slip for various degrees of corrosion under pullout loads. The specimens were tested in an MTS testing machine on which loads, slips and displacements were recorded. Some conclusions have been reached based on the test results.

Evaluation of the bond properties between concrete and reinforcement as a function of the degree of reinforcement corrosion

In order to investigate the effect of reinforcement corrosion on the bond properties between concrete and reinforcement, the pullout tests were conducted using reinforcements embedded in concrete specimens, which were corroded by an accelerated electric corrosion method. A finite element method (FEM) analysis was also carried out on the basis of the results of the pullout tests. The maximum bond strength ( τ max) and the bond rigidity ( D s) of specimens decreased in proportion to the increase of corrosion percentage (Δ w), respectively. Also, the curves of bond stress–free end slip could be analyzed by the FEM, if the τ max and D s were determined as a function of corrosion percentage. The equations for calculating the maximum bond strength and the bond rigidity necessary for an FEM analysis of RC members with corroded reinforcements were obtained by the experiments and the FEM analysis of the pullout tests.

Effect of reinforcement corrosion on bond strength

The effect of reinforcement corrosion on the bond strength between steel and concrete was investigated. The bond behaviour of reinforced concrete elements, including the ultimate bond strength, free-end slip, and the modes of failure in precracking, cracking and postcracking stages was studied. Also, the effect of different crack widths and the rib profile degradation for various degrees of corrosion on the bond strength were evaluated. In order to establish different levels of corrosion, a calibration curve establishing a relationship between the duration of the impressed current and the corresponding degree of corrosion was prepared. The magnitude of corrosion was measured as gravimetric loss in weight of the reinforcing bars. The results indicate that in the precracking stage (0–4% corrosion) the ultimate bond strength increases, whereas the slip at the ultimate bond strength decreases with an increase in the degree of corrosion. The degradation of bond results from the crushing of concrete keys near the bar lugs. When reinforcement corrosion is in the range of 4 to 6%, the bond failure occurs suddenly at a very low free-end slip. At this level of reinforcement corrosion, a large slip was noted as the ultimate failure of the bond occurred due to the splitting of the specimens. Beyond 6% rebar corrosion, the bond failure resulted from a continuous slippage of the rebars. The ultimate bond strength initially increased with an increase in the degree of corrosion, until it attained a maximum value of 4% rebar corrosion after which there was a sharp reduction in the ultimate bond strength up to 6% rebar corrosion. Beyond the 6% rebar corrosion level the ultimate bond strength did not vary much even up to 80% corrosion. In terms of the effect of rib profile, a sharp reduction in the bond strength was initiated when its degradation exceeded 25%. This decrease in bond strength continued up to 45%. Thereafter, there was no significant effect of the rib profile degradation on the bond strength.

Bond Stress/Deformation in Pull‐Out Masonry Specimens

An experimental investigation to determine bond stress, free-end slip, and relative deformation between a reference plane in the masonry and an originally coplanar point on the steel (slip) is conducted. Stack pull-out specimens constructed of grouted nominal 6-in concrete or clay masonry units, reinforced with #4 or #7 bars, are subjected to monotonic or cyclic loadings. The method for evaluating slip incorporated slip-wires connected to linear variable differential transformers (LVDTs). Strain gages provide strain in the steel bars from which stresses in the reinforcing bar and bond stresses can be calculated. Free-end slip information is also collected. These data are useful in developing an understanding of the physical mechanism associated with bond, slip, and bond deterioration. The bond stress, which develops between the reinforcement and the surrounding grout, is a complex phenomenon dominated by the interlocking action of the reinforcing bar ribs embedded in the grout mass. To determine bond stresses and slip, six specimens are constructed and tested. Three specimens of 16 in length are reinforced with a #4 bar, and three, of 32 in length are reinforced with a #7 bar. The bond stress and slip distribution curves are evaluated from experimental data. In addition, short embedment specimens, consisting of prisms of 6-in concrete masonry units (nominal size 6×8×8) are tested. The embedment length is varied from 1 to 6 in. Ten specimen configurations was replicated three times. The nominal bond stress and free-end slip are obtained.

A theory of transfer bond resistance of deformed reinforcing bars in concrete under lateral pressure

Synopsis The basic bond mechanism for reinforcing bars in concrete under external pressure is considered and an existing partly cracked elastic theory for bond resistance is extended to cover both uniform and non-uniform distributions of external pressure. Results obtained from this theory are compared with experimental results at various stages of bond failure. The prediction of pull-out force at 0·1 mm free-end slip is found to be satisfactory.

Bond of Coated Reinforcing Bars in Concrete

The bond strengths were determined for 23 epoxy-coated (10 different epoxy coatings), six polyvinyl chloride-coated (three different coating materials), and five uncoated reinforcing bars. The length of embedment of the reinforcing bar in the concrete was 12 in. (305 mm). The loads or bond stresses corresponding to a loaded-end slip of 0.01 in. (0.25 mm) or a free-end slip of 0.002 in. (0.051 mm) were considered as critical values in determining the bond strength, depending on which of these values of slip developed first. For coating film thicknesses ranging from 0.001 in. to 0.011 in., the epoxy-coated bars developed bond strengths essentially equal to the bond strengths for uncoated bars. Experimental values were higher than minimum acceptable values in building code and highway bridge requirements. Bond strengths of the polyvinyl chloride-coated bars and bars with thick epoxy coatings were unacceptable.