International Concrete Abstracts Portal

This month’s Q&A discusses the reinforcement stiffness ratio ρne that can be used for comparing the performance of various reinforcement types in slabs-on-ground and the design approach for glass fiber-reinforced polymer reinforcement in ACI/NEx MNL-6(23) based on percent reduction in unrestrained shrinkage strain from the enhanced aggregate interlock design of ACI PRC-360-10.

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.

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.

The serviceability and ultimate limit states of a concrete member are reliant upon the bond of reinforcement. The performance of glass fiber reinforced polymer (GFRP) reinforced concrete structures is influenced by multiple parameters and one of these parameters is the bond length of GFRP rebars. The scope of the present research is to experimentally study the effects of fully and partially bonded rebars on the load-bearing capacity and cracking of GFRP-reinforced concrete beams. The beams with partially bonded reinforcement show reduced capacities compared with those with fully bonded reinforcement, and the former reveals localized cracks. The partially bonded beams fail as a result of concrete splitting, while their fully bonded counterparts fail by concrete crushing.

The use of fiber-reinforced polymer (FRP) composites for external bonding has become a popular and widely accepted technique for enhancing the strength of concrete structures due to its excellent mechanical performance, corrosion resistance, and ease of construction. However, premature debonding is a major challenge as it prevents the full capacity of FRP composites from being achieved, resulting in material waste. Recently, grooving the surface of concrete before bonding FRP has emerged as a potential solution to this problem. Several experimental studies have evaluated the bond strength of FRP-to-concrete joints with grooves. To facilitate the practical application of this technique, it is necessary to develop comprehensive reliability-based design guidelines that account for the uncertainty arising from various aspects such as materials, model errors, and loading. A critical factor of such analysis is the calibration of model uncertainty which significantly affects the accuracy of reliability-based design and analysis. The objective of this study was to measure the model uncertainty of the existing prediction model for FRP-to-concrete joint with a longitudinal groove by involving the model factor which is defined as the ratio of observed values from experimental test to calculated values from prediction models. To eliminate the potential correlation from critical parameters, the residual model factor was isolated from model factor by separating the systematic part. The lognormal distribution was found to be the most suitable distribution function to describe the residual model factor, and the mean and variance were determined. With this newfound knowledge, we are better equipped to account for uncertainties in the design and construction of FRP-to-concrete connections with grooves, which will ultimately result in more durable and reliable structural improvements.