Shear behavior of concrete structures strengthened with ultra-high performance fiber-reinforced concrete (UHPFRC)

Abstract

Due to aging, increase in service loads, and/or upgrade of design codes, many existing concrete structures do not, or soon will not satisfy the required load-bearing capacity. Therefore, measures such as reconstruction or repair are necessary. Among various strengthening methods, ultra-high performance fiber reinforced concrete (UHPFRC), due to its high mechanical properties and superior durability, has emerged as a promising solution for strengthening concrete structures by enhancing load and deformation capacity, and improving durability. While extensive research has been conducted on the use of UHPFRC for flexural strengthening, studies focused on shear strengthening remain limited. After strengthening, the interface between UHPFRC and traditional reinforced concrete (RC) structures is formed. However, interface behavior has not been thoroughly investigated, further posing a gap in understanding the optimal strengthening design of UHPFRC in hybrid UHPFRC-concrete systems.

This dissertation focuses on the shear performance of concrete structures strengthened with UHPFRC, with a focus on the interface behavior between UHPFRC and normal concrete, and the mechanical properties of UHPFRC.

To assess the shear strengthening efficiency of UHPFRC, this dissertation starts with a literature review (Chapter 2) that addresses three major aspects. The first aspect focuses on the shear performance of hybrid UHPFRC-RC structures. Strengthening applications of UHPFRC in shear, and current analytical and numerical methods to predict the shear capacity of RC structures strengthened with UHPFRC are critically analyzed. The second aspect is focused on the UHPFRC-concrete interface behavior which is governing the response of the hybrid beams. From the review, the role of governing parameters on the interface behavior, including the effects of bonding technique, moisture exchange between the two materials, differential shrinkage and the role of coupled environmental and mechanical loads, are discussed. The final aspect deals with the application of non-destructive techniques (NDTs) to assess the strengthening efficiency of UHPFRC, focusing on evaluation of (i) UHPFRC material properties and (ii) UHPFRC-concrete interface performance in hybrid structures.

Experimental design is presented in Chapter 3, where the material and structural tests are systematically introduced. This chapter provides a series of material tests to, among others, characterize the workability, shrinkage and mechanical properties of both UHPFRC and normal concrete (NC). It also introduces the design and setup of a comprehensive structural test to evaluate the shear performance of UHPFRC-strengthened RC beams. Following the experimental methodology from Chapter 3, Chapter 4 presents the results of the material and structural tests. Through comparative analysis, this chapter examines the material and structural behavior of UHPFRC-strengthened beams and its constituents by varying different parameters, thereby setting a basis for evaluating the shear improvement in strengthened RC beams. In order to provide a deeper analysis on the UHPFRC-concrete interface quality in strengthened beams, Chapter 5 focuses on the assessment of possible delamination between UHPFRC and existing concrete by applying active infrared thermography. Combined with both experimental and numerical analysis, a systematic procedure is developed to detect subsurface delamination in hybrid UHPFRC-NC specimens. Besides the interface properties, another governing parameter, namely the material properties of UHPFRC, is examined in Chapter 6. This chapter investigates the fiber distribution and orientation within UHPFRC elements, important factors influencing material properties of UHPFRC, and therefore further affecting its shear strengthening efficiency. Using a calibrated electromagnetic method and validated through x-ray computed tomography (CT scanning), the effect of governing parameters, including casting direction and vibration time, on fiber distribution and orientation in UHPFRC elements is investigated. This chapter helps to clarify the connection between the material properties of UHPFRC and its structural performance.

Based on the structural test results (Chapter 4), evaluation of interface properties (Chapter 5) and material properties of UHPFRC (Chapter 6), in Chapter 7, a numerical model is developed to simulate shear performance of UHPFRC strengthened concrete structures, validated by the experimental results. A parametric study is further conducted to investigate key factors including interface properties, UHPFRC tensile behavior and non-uniform fiber distribution in UHPFRC. The numerical analysis offers insights on the influence of these parameters on shear strengthening efficiency.

In the final Chapter 8, the findings regarding the overall shear strengthening performance of UHPFRC in RC beams are given, providing practical insights for optimizing UHPFRC applications in concrete structures. Finally, suggestions for future research are given.