International Concrete Abstracts Portal

This study focuses on the characterization of the fracture properties of the concrete-epoxy interface (CEI) under FRP sheets and FRP Uwraps. In particular, the Mode I and Mode II fracture energies are obtained with and without the effect of the FRP Uwrap anchor. Results indicate that fracture energy of the CEI increases due to the confining effect of Uwrap. These material properties, along with a proposed bond-slip model, are used in numerical simulations of concrete elements strengthened with externally bonded FRP sheets and anchored with FRP Uwraps. Results show that the characterization of the fracture properties of the CEI is needed to accurately predict the complex behavior of the CEI under the FRP Uwrap.

During the last two decades, externally bonded uni-directional fiber-reinforced polymer (FRP) composites have been widely used for strengthening, repairing, and rehabilitation of reinforced concrete (RC) structural members. The bond characteristics contribute to the effectiveness of the stress transfer achieved between the FRP composite and the concrete substrate. Debonding of the FRP composite reinforcement is the most critical concern in this type of application. Under monotonic and fatigue loading conditions, FRP-concrete shear debonding has been idealized as a Mode-II fracture problem along the bi-material interface. A cohesive material law is used to describe the interfacial stress transfer at the macroscopic level. The area under the entire curve represents the fracture energy and is related to the load-carrying capacity of the interface. In this paper, previous experimental results and literature are discussed to show how the fracture energy can be considered a true fracture parameter. In addition, a simplistic one-dimensional numerical analysis of the direct shear test is presented with the intent of pointing out the effect of the fracture parameters related to the cohesive material law on the load carrying capacity. The results are instrumental to discuss the strain limits provided in the ACI 440.2R-08 document.

Externally-bonded fiber reinforced polymer (FRP) laminates are commonly used to strengthen or repair reinforced concrete members. The behavior of strengthened members is influenced not only by the properties of reinforced concrete and FRP laminates but also by their interface properties such as bond strength. Conventional strength based approaches lack the accuracy required to predict the interface behavior because they do not account for energy release during debonding. A fracture mechanics approach can provide a better alternative because it can account for post-cracking behavior, debonding propagation, and flaws and defects at the interface and within the materials. This paper presents a review on recent fracture mechanics approaches used to understand the debonding behavior of reinforced concrete members with externally-bonded FRP laminates. With a brief description of failure modes of FRP-strengthened beams, the methodology and limitations of strength based models of debonding are summarized and discussed. As the debonding of FRP from beams can be categorized as Mode-I, Mode-II, or Mixed Mode, test methods to attain the critical energy release rate of the FRP-concrete interfaces in specific modes are also presented. Analytical and numerical fracture mechanics based models are reviewed with respect to fracture energy and energy calculation methods.

The paper deals with the application of composite materials named Steel Reinforced Grout (SRG) for strengthening reinforced concrete (RC) elements. They differ from the well-known Fiber Reinforced Polymer (FRP) for the use of small unidirectional steel cords, combined to create a metallic fabric drowned in a matrix of cement mortar. In particular, this work develops an experimental program composed by two consequential phases. The first phase is aimed to find cement mortar matrixes with the best bond properties through pull-off tests in the case of cementitious substrate. The second part deals with flexural tests on RC beams strengthened with two SRG composites. On the basis of pull-off results two inorganic matrixes were selected according to their bonding and impregnation properties. The two SRGs were applied at the bottom of RC beams which were preliminary repaired with polymer-modified mortars in order to simulate a real on-site application of a strengthening layer on degraded RC elements. Flexural test results underline the high potentiality of the SRG strengthening technique also in the case of a double interface, concrete/ repair layer and repair layer/SRG. This technique needs a low level of specialization of workers and it is less expensive than FRP.

Structural behavior of Reinforced Concrete (RC) beams strengthened in shear by means of Fiber Reinforced Polymer (FRP) sheets is a very complex subject actually under discussion. A number of experimental programs have shown the importance of the FRP debonding/peeling failure and the mutual interaction between the existing steel web reinforcement and the external FRP sheets/laminates for the evaluation of the whole shear capacity of the structural element. In this work a three dimensional numerical Finite Element procedure, accounting for Mazars’ damage law, included in a contact algorithm, to model the mechanisms at the FRP-concrete interface, was implemented to catch the global failure mechanisms that characterize the ultimate shear capacity of RC members with transverse steel reinforcement and FRP strengthening. The study is based on the experimental tests, described in Pellegrino and Modena (2002), carried out on RC beams with transverse steel reinforcement with and without FRP shear strengthening. It has been shown that the numerical approach is able to describe the experimental behavior of the structural member taking into account the interaction between concrete, steel and FRP contributions to shear capacity and, in particular, how the presence of external FRP sheets can modify steel contribution to the ultimate shear strength of the beams when FRP debonding/peeling failure occurs.