International Concrete Abstracts Portal

This CD-ROM contains 10 papers sponsored by ACI committees 440 & 446. The papers provide information on recent developments on the use of the framework of fracture mechanics to evaluate the performance of reinforced concrete (RC) structures strengthened with FRP composites. The information provided is useful to researcher and practicing engineer by presenting experimental and analytical tools based on a fracture approach that can assess the shear and flexural capacity of strengthened RC members. Note: The individual papers are also available. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP-286

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.

The size effect in shear in reinforced concrete (RC) one-way members without shear reinforcement becomes more of concern when using glass fiber reinforced polymer (GFRP) reinforcement. In fact, the lower axial stiffness of GFRP reinforcement typically results in wider flexural cracks with respect to steel RC counterparts. This issue is especially relevant for the case of flexural members without stirrups, such as retaining walls and slab bridge superstructures. Little evidence has documented the extent of such effect. Cognizant of this knowledge gap, ACI Committee 440 (FRP Reinforcement) introduced the current nominal shear strength algorithm, which was calibrated in a conservative fashion based on test results from small beams. This algorithm assumes that the shear strength at the critical section is resisted predominantly through the uncracked concrete above the tip of the shear crack. Based on the same fundamental assumption, a fracture mechanics algorithm for steel RC beams was recently proposed by ACI Committee 446 (Fracture Mechanics of Concrete). In this paper, the ACI 440 and 446 algorithms are verified and discussed based on experimental evidence from tests on scaled GFRP RC beams without stirrups. The latter algorithm is modified to account for the smaller elastic modulus of GFRP, under the hypothesis that its relevant parameters and the shear failure mechanism are similar irrespective of the reinforcement material.