International Concrete Abstracts Portal

The bond property between deformed bars and concrete plays a significant role in the safety of construction. Numerous database-dependent empirical models are proposed to evaluate the bond behavior without considering the effect of additional confinement, whose application range is quite limited as a result of unstable accuracy. In this paper, a new model was established based on the thick-walled cylinder model and fictitious crack theory, which can predict bond strength and bond-slip response with fiber-reinforced polymer (FRP)-steel confinement. The effects of various factors on the bond behavior, such as concrete strength, concrete cover, rebar diameter, bar surface geometry, and FRP/steel confinement, were comprehensively discussed. According to the radial crack radius, the radial stress and displacement induced on the bond interface can be calculated, and thus the analytical formulae of bond strength and slip were respectively developed in conjunction with deformed bar surface geometry. Finally, a new analytical model was proposed, which can simulate the bond-slip curves of the specimens with different confinement levels, covering unstrengthened, FRP-strengthened, stirrup-strengthened, and FRP-stirrup dually strengthened specimens. Compared with existing models, the proposed model can provide better agreement with existing test results.

An approach to improve the progressive collapse resistance of conventional RC frame structure was put forth by using unbonded post-tensioning strand (UPS). Two UPSs with a straight profile are mounted at the bottom of the beam section. A static loading test was conducted on an unbonded prestressed RC (UPRC) beam-column sub-assemblage under a column removal scenario. The structural behaviors of the test specimen, such as the load-carrying capacity, failure mode, post-tensioning force of the UPSs, and rebar strain, were captured. By analyzing the results of the tested substructure, it was found that the compressive arch action (CAA) and catenary action (CTA) were sequentially mobilized in the UPRC sub-assemblage to avert its progressive collapse. The presence of UPSs could significantly improve the load-carrying capacity of conventional RC structures to defend against progressive collapse. Moreover, a high-fidelity finite element (FE) model of the test specimen was built by using the software ABAQUS. The FE model was validated by the experimental results in terms of the variation of vertical load, horizontal reaction force, and post-tensioning force of the UPSs against middle joint displacement (MJD). Finally, a theoretical model was proposed to evaluate the anti-progressive collapse capacities of UPRC sub-assemblages. It was validated by the test result as well as by the FE Models of the UPRC sub-assemblages which were calibrated using the available experimental data.

a The complex frequency-dependent impedance of the interface between concrete and reinforcing steel bars (rebar) can be indirectly measured using a four-electrode array on the surface of the concrete. Impedances were measured at several corrosion rates, which that were simulated by anodic polarization at specific currents, and at different corrosion extents, which were simulated by the accumulation of corrosion products after the application of fixed currents for various time periods. The measured impedances changed consistently with the change in the corrosion states in the 0.01 to 1000 Hz band. The results are in agreement with those obtained using standard electrochemical impedance spectroscopy (EIS), but the present method has three significant advantages: i1) no contact need be made with the reinforcing bar; ii2) the measurement is specific to the small portion of the rebarreinforcing bar beneath the measurement array, ; and iii3) because the time constant of the impedance relaxation spectrum depends on the area of rebarreinforcing bar involved, the frequencies are higher than those needed when direct contact methods are used (thus reducing measurement time). It appears that the surface measurement is a powerful method to study the corrosion process, both in the laboratory and in the field.

This paper addresses the need for a ductile or pseudo-ductile fiber reinforced plastic reinforcement for concrete structures. The criteria to be met by the FRP, which are based on the properties of the steel rebar it is to replace, are threefold: high initial modulus, a definite yield point, and a high ultimate strain. It is shown that the use of a fiber architecture based design methodology facilitates the optimization of the performance of FRP through material and geometric hybrid. Consequently, the advantages of FRP such as high strength, low weight and chemical inertness or noncorrosiveness can be fully exploited. Using the material hybrid and geometric hybrid, it is demonstrated that the pseudo-ductility characteristic can be generated in FRP rebar. Critical material and geometric parameters such as elastic modulus, fiber volume fraction, twisting, crimp, and helical effect in the specimen components were investigated and parametric studies are reported. Ductile hybrid FRP bars were successfully fabricated at 3 mm and 5 mm nominal diameters using an inline braiding and pultrusion process. Tensile specimens from these bars were tested and found to have consistent pseudo-ductile behavior and very good agreement with the analytical predictions.

The bond between concrete and steel reinforcing bar was evaluated by electromechanical pull-out testing, which involved measuring the shear bond strength and contact electrical resistivity of each sample. The bond strength was increased by steel rebar surface treatment (acetone, water, ozone or sand blasting, with ozone being most effective and acetone being least effective), silica fume and polymer addition to concrete, increase in water/cement ratio of concrete (particularly from 0.45 to 0.50), and decrease in curing age (particularly from 14 to 7 days). The origins of these effects are rebar cleansing for acetone treatment (accompanied by contact resistivity decrease), rebar surface oxide film formation for water and ozone treatments (accompanied by contact resistivity increases), rebar surface roughening for sand blasting, polymer interface layer formation for polymer addition (accompanied by contact resistivity increase for latex addition, but not for methylcellulose addition), decreased interfacial void content (accompanied by contact resistivity decrease) for water/cement ratio increase (due to fluidity increase) and for curing age decrease (due to shrinkage decrease), and increased matrix modulus for silica fume addition. Corrosion initially caused the bond strength to increase while the contact resistivity increased, but further corrosion caused the bond strength to decrease while the contact resistivity continued to increase.