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.

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.

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.

A literature review of the effects of high strain rates on the properties of steel reinforcing bars (rebars) was conducted. Static and dynamic properties were gathered for bars satisfying ASTM A615, A15, A432, A431, and A706, with yield stresses ranging from 42 to 103 ksi (290 to 710 MPa). Strength enhancement with strain rate was expressed in the form of a dynamic increase factor (DIF) defined as the ratio of the dynamic to static yield (or ultimate) stress. It was observed that the DIF would increase for lower rebar yield stress. A simple relationship is proposed which gives the DIF (for both yield and ultimate stress) as a function of strain rate and yield stress. This relationship is of importance for the analysis of reinforced concrete structures subjected to blast or highly dynamic loads.

The use of methylcellulose (0.4 to 0.8 percent by weight of cement) as an admixture in cement paste or concrete was found to increase the shear bond strength with steel rebar, steel fiber or carbon fiber to values attained by using latex (20 percent by weight of cement) as an admixture, even though latex was used in a much larger quantity than methylcellulose. The bond strength increased with increasing methylcellulose amount. The contact electrical resistivity between cement and fiber or between concrete and rebar was increased by latex addition, but not changed by methylcellulose addition. The combined use of silica fume (15 percent by weight of cement) and methylcellulose (0.4 percent by weight of cement) as admixtures was found to give concrete that exhibited high bond strength to steel rebar, in addition to previously reported high tensile modulus, tensile ductility, flexural strength and flexural toughness; the bond strength attained was higher than that attained by using either silica fume or methylcellulose as admixture. Latex in combination with silica fume did not work due to low workability. Methylcellulose in combination with silica fume was effective due to silica fume promoting densification and methylcellulose promoting adhesion.