International Concrete Abstracts Portal

This paper presents a detailed investigation into the load-deformation response of reinforced concrete beams strengthened with mechanically fastened fiber-reinforced polymers (MF-FRP). A bearing-slip model was developed for MF-FRP connections fastened with concrete expansion anchors. The model was used to predict the interfacial change in strain between the concrete and the FRP. When combined with load deformation constitutive equations, the bearing-slip model better predicted the load-deformation behavior in MF-FRP strengthened concrete beams. Comparisons to experimental data showed that the developed method reasonably predicts (typically within 8%) actual beam response at ultimate load. Moment-curvature diagrams were also developed and used to predict midspan deflection, typically within 30% of experimental results.

Composite concrete bridges are widely used because they combine the advantages of precast concrete with those of cast-in-place concrete. However, because of the difference in shrinkage properties between the girder and the deck and because of the sequence of construction, the deck is subject to differential shrinkage tensile stresses. These tensile stresses may lead to excessive cracking. This paper demonstrates how the likelihood of deck cracking due to differential shrinkage can be reduced and how consequently the resistance of composite concrete bridges against time dependent effects can be enhanced by choosing a deck mix with low shrinkage and high creep. An experimental study on the long term properties of seven deck mixes is presented to identify a deck mix with the aforementioned properties. A comparison of three composite concrete bridge systems used for short-to-medium-span bridges is performed to identify the bridge system that is most resistant against time dependent effects. The mix with saturated lightweight fine aggregates appears to best alleviate tensile stresses due to differential shrinkage and the bridge system with precast inverted T-beams and tapered webs appears to be the most resistant.

A nonlinear 3D Finite Element (FE) analysis was performed to predict the post-exposure load-deflection responses of Reinforced Concrete (RC) beams strengthened in flexure using prestressed Near-Surface Mounted (NSM) Carbon Fibre Reinforced Polymer (CFRP) strips. Five RC beams (5.15 m [16.90 ft] long) were modeled including one un-strengthened control beam and four beams strengthened using NSM-CFRP strips prestressed to 0, 20, 40, and 60% of the ultimate CFRP tensile strain. The beams were severely deteriorated due to applying accelerated environmental conditions consisting of 500 freeze-thaw cycles: three cycles per day between +34°C [93°F] to -34°C [-29°F] with fresh water spray for 10 minutes at a rate of 18 L/min [4.8 gallon/min] at temperature +20°C. The accelerated environmental conditions used in this study equivalent to 0.46 year of exposure inside a chamber, corresponds to a minimum lifetime of 12.8 years in Canada. Simultaneously, each beam was subjected to a sustained load equal to 62 kN [13.9 kip] representing 47% of analytical ultimate load of the non-prestressed NSM-CFRP strengthened RC beam. The degradation which occurred in the concrete properties, concrete-epoxy interface, and steel reinforcement was considered in the FE model. Also, debonding at the concrete-epoxy interface was simulated by assigning shear and normal fracture energies and the prestressing was applied to the CFRP strip using an equivalent temperature. The FE model was validated with the experimental test results. However, the experimental and numerical load-deflection curves were comparable up to yielding but after yielding, the predicted curves were not in good agreement with the experimental ones.

When prestressed concrete (PC) bridge members such as girders and piles are exposed to chloride-laden environments, bonded prestressing strands can corrode prematurely. If corrosion is not detected timely, the strands may be damaged due to the effects of concurrent corrosion and tensile (prestressing) stresses, thus reducing the nominal strength of the member and increasing the risk of collapse. Accurate methods for detecting early corrosion in PC are needed to inform efficient decision-making for maintenance. Polarization resistance (R_p) is directly related to the charge transfer rate on the metal surface, making it a physically meaningful parameter to assess corrosion. Related measurements can be used to estimate corrosion rates in reinforced concrete, but their applicability to PC has not been studied. This paper discusses the feasibility of using R_p-based criteria for early corrosion detection in PC. Five PC pile specimens were exposed for over two years to salt water wet/dry cycles. Open-circuit potential and R_p measurements were routinely performed. After detecting sustained drops of open-circuit potential and R_p and keeping the specimens under exposure for different periods, the strands were removed and inspected to assess corrosion vis-à-vis electrochemical measurements. The results were used for the preliminary definition of open-circuit potential and R_p thresholds associated with different corrosion patterns on prestressing strands in PC structures.

Early-age cracking in cast-in-place reinforced concrete bridge decks is occurring more frequently now than three decades ago and principle factors that lead to early-age deck cracking are not fully understood. A finite element (FE) simulation methodology for assessing the role of shrinkage-induced strains in generating early-age bridge deck cracking is described. The simulations conducted indicate that drying shrinkage appears to be capable of causing transverse (and possibly longitudinal) bridge deck cracks as early as 9 to 11 days after bridge deck placement. The drying-shrinkage induced stresses would result in transverse cracking over interior pier supports in a typical bridge superstructure considered in the finite element simulations conducted.