International Concrete Abstracts Portal

Carbon Fibre Reinforced Polymers (CFRPs) are a material of choice in the aerospace and automotive industry, but despite decades of research into their application in structural engineering applications, and in particular in new-build construction of buildings and bridges, CFRP elements remain regarded as somewhat exotic in structural engineering and their widespread take-up is mostly limited to the non-prestressed strengthening of conventional structural members. The study presented in this paper assessed the performance of CFRP bridge tendons, prestressed for 18 years at 45% of their design ultimate tensile capacity in a non-conditioned outdoor environment, over water, in Lucerne, Switzerland. The performance of the tendons is considered alongside pristine samples of the same tendons never used and stored, unstressed, indoors since 1997. Thermal characterization (matrix digestion, thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC)) was used to determine the fibre volume fraction and glass transition temperature, and tensile tests were performed and compared against available original baseline results from 1997. This comparisons show that the in-service tendons do not appear to have been adversely affected by 18 years service under sustained loading, and have retained the vast majority of their original, unstressed material properties. The in-service tendons only lost about 10.5% of their ultimate tensile capacity over time, while the pristine (unstressed) tendons also lost 7.9% of their capacity; this suggests that sustained loading and an external, unconditioned service environment do not significantly adversely affect the mechanical properties of the tendons after 18 years in service.

The release of ACI 440.11-22 building design code for concrete structures reinforced with GFRP bars comes with several challenges at various fronts. One such challenge is tackled in this paper which is the development of limit interaction diagrams for elliptical bridge columns reinforced with GFRP bars under biaxial bending plus axial compression/tension. This type of columns requires special considerations at all levels. This paper depicts the various formulations encountered herein in a detailed treatment highlighting the critical steps to build an efficient analysis algorithm. The formulation is implemented into a user-friendly software developed using object-oriented programming, namely the C# programming language. The robustness of the formulation is tested by comparing interaction diagrams of elliptical sections to those of corresponding rectangular sections. The significance of an ACI code comment requiring bar orientation being considered for circular sections with less than 8 bars is also examined in this paper. This paper also tests the ACI recommendation to neglect GFRP action in compression. Results indicate reasonable similarity among interaction diagrams of elliptical and rectangular sections leading to the conclusion that the formulation presented herein provides an accurate tool to analyze elliptical sections.

A systematic literature review was conducted on pure tension strengthening of concrete structures using fiber-reinforced polymer (FRP), specifically for larger FRP tie applications. This work yielded a dataset of 1,627 direct tension tests, and highlighted the limitation of existing studies on studying thick and long FRP ties, which are typical in real construction scenarios. To overcome this shortcoming, 51 single lap shear tests were conducted on thicker and longer FRP ties, with the dimensions being 0.5 to 6 mm [0.02 to 0.24 in.] thickness, and 300 to 1,524 mm [12 to 60 in.] long. The critical parameters under consideration were concrete compressive strength, FRP thickness, and bond length. The findings demonstrate that thicker and therefore stiffer FRP ties have higher debond force capacity, while longer ties exhibit greater post-elastic deformation capacity but do not affect the debond force capacity. Concrete had a limited effect on either debond force or deformation capacity. A strength model is proposed for FRP systems under axial pure tension, which aligns well with both the published and tested results. This paper focuses on the development of design guidelines and codes to predict the debond strain for EB-FRP systems incorporating thicker and longer FRP ties, aiming to enhance the applicability of FRP to real-world construction scenarios.

The American Concrete Institute (ACI) 440.1R-15 Guide for the Design and Construction of Structural Concrete Reinforced with Fiber-Reinforced Polymer (FRP) Bars linearly reduces the bar stress and thereby pull-out capacity of FRP bars to zero from an embedment length at 20 bar diameters (db) or less. Most experimental research and data examine the development length of various FRP bars at longer, more traditional, embedment lengths. A database was created from select available data in literature to compare to empirical standards. This investigation examines the bond performance of short embedded FRP bars into concrete considering a pull-out failure mode to expand the understanding of short embedded FRP bars into concrete. Based upon the database collected, for the glass fiber-reinforced polymer (GFRP) rebars, the current 440.1R appear quite conservative. For the basalt fiber-reinforced polymer (BFRP) rebar database collected, the current ACI 440.1R-15 provisions appear unconservative for a statistically significant number of the specimen test results within the database. In the case of the carbon fiber-reinforced polymer (CFRP) database, which is quite limited, the data appears to develop considerably less bond strength than the current 440.1R provisions might suggest which requires deeper investigation for the case of short embedment length bonded CFRP bars.

This study evaluates the bond performance of concrete epoxy bonds using an image segmentation-based image processing technique. The Concrete Epoxy Interface (CEI) plays a crucial role in the structural performance of FRP-repaired concrete as it transfers stresses from the concrete to the epoxy. By employing the image segmentation technique, the performance of the CEI is assessed through the ratio of Interfacial Failure (IF) to other failure types, namely cohesive failure in Epoxy (CE) and Cohesive cracks in Concrete (CC). The effects of sustained loading duration on CEI bond performance are quantitatively analyzed using 21 single-lap shear (SLS) specimens and 28 notched 3-Point Bending (3PB) specimens. The findings highlight vital conclusions: CE is the least failure mode in SLS and 3PB specimens. In contrast, CC is the predominant failure mode, indicating the susceptibility of the concrete substrate in FRP-repaired concrete. Moreover, IF generally increases with longer sustained loading durations in 3PB specimens but decreases with increased loading duration in SLS specimens. The study also demonstrates the effectiveness of the image segmentation approach in evaluating CEI performance in 3PB specimens, where color distinguishes epoxy, FRP, and concrete substrate.