E. Nigro

Steel and concrete composite beams with flexible shear connection: “exact” analytical expression of the stiffness matrix and applications

Abstract A displacement-based finite element model for the analysis of steel and concrete composite beams with flexible shear connection is presented in this paper. No approximations are introduced in the displacement field, because the stiffness matrix and the fixed-end nodal force vector are directly derived from the “exact” solution of Newmarks differential equation. Therefore, the present finite element may employ only one element per member, because internal nodes are only needed for variations of geometrical and mechanical properties and in the presence of concentrated forces. A simple criterion to establish if partial interaction effects may be neglected in the analysis of composite beams is also suggested.

Bond Efficiency of EBR and NSM FRP Systems for Strengthening Concrete Members

This paper reports the results of an experimental program to investigate the bonding behavior of two different types of fiber-reinforced polymer (FRP) systems for strengthening RC members: externally bonded carbon (EBR) plates and bars or strips externally applied with the near-surface-mounted (NSM) technique. The overall experimental program consisted of 18 bond tests on concrete specimens strengthened with EBR carbon plates and 24 bond tests on concrete specimens strengthened with NSM systems (carbon, basalt, and glass bars, and carbon strips). Single shear tests (SST) were carried out on concrete prisms with low compressive strengths to investigate the bonding behavior of existing RC structures strengthened with different types of FRP systems. The performance of each reinforcement system is presented, discussed, and compared in terms of failure mode, debonding load, load-slip relationship, and strain distribution. The findings indicate that the NSM technique could represent a sound alternative to EBR systems because it allows debonding to be delayed, and hence FRP tensile strength to be better exploited.

Experimental Investigation of FRP-Concrete Debonding under Cyclic Actions

The effectiveness of externally bonded fiber-reinforced polymer (FRP) laminates for strengthening existing reinforced concrete (RC) structures is strictly related to the bond of the FRP reinforcement to the concrete substrate. Although numerous experimental studies have investigated this bond, experimental data concerning cyclic tests on both FRP sheets and plates applied on concrete specimens are still lacking. Thus, a series of single shear tests (SSTs) under both monotonic and cyclic actions, without inversion of sign, were performed on concrete prismatic specimens reinforced with carbon FRP (CFRP) sheets or plates. To evaluate and compare the influence of cyclic external actions on the bonding behavior of sheets and plate reinforcements, the results provided by monotonic and cyclic tests are reported and discussed in this paper. In particular, force-displacement relationships, axial strains, and shear stresses recorded along the FRP reinforcement are reported; the influence of the load path on the FRP debonding behavior is also examined.

Indirect Identification Method of Bilinear Interface Laws for FRP Bonded on a Concrete Substrate

The mechanical response of Reinforced Concrete (RC) beams strengthened by an Externally Bonded Reinforcement (EBR) made out of Fiber-Reinforced Polymers (FRPs) is deeply influenced by the interaction between the concrete substrate and the FRP system, either cured-in-place (sheets) or preformed (plates). In particular, the strength of FRP-EBR RC beams is often controlled by debonding phenomena to develop at the adhesive-to-concrete interface. The most recent theoretical formulations and some experimental results obtained in the last years pointed out the differences that characterize the debonding strength of FRP sheets and plates. According to the findings of those studies, the fracture energy is a fundamental parameter governing the debonding phenomenon. However, determining its value is not sufficient for simulating the behavior of the FRP-to-concrete interface and modeling relevant problems such as intermediate debonding in RC beams externally strengthened by FRP. Consequently, formulating and calibrating local bond-slip models, which take into account the different behavior of sheets and plates, is of fundamental importance for modeling FRP-strengthened RC members. This paper is aimed at identifying bond-laws for sheets and plates through an Indirect Identification Method (IndIM), recently implemented and validated by the authors. A wide collection of experimental results obtained by pull-out tests on FRP sheets and plates is first reported and then employed for identifying the previously noted bond-slip laws. Finally, the results of the identification procedure demonstrate that the debonding phenomenon, described as a fracture process in mode II, should be modeled by assuming different bond-slip relationships for FRP plates and sheets.

On the Behavior of FRP-to-concrete Adhesive Interface: Theoretical Models and Experimental Results

Fiber-reinforced polymers (FRP) are more and more commonly employed for strengthening existing structures of both reinforced concrete (RC) and masonry. Since FRP sheets (cured in situ) or plates (preformed) are externally bonded on a concrete or masonry substrate, the issue of adhesion on those materials generally controls the effectiveness of strengthening in members stressed either in bending or shear (Motavalli & Czaderski, 2007). The use of composite materials for structural strengthening of civil structures and infrastructures began with some pioneering application at the middle of the ‘80s (Meier, 1987) of the past century. Plenty of experimental work and theoretical investigations have been carried out in the following years with the aim of demonstrating the feasibility of strengthening civil structures by means of composite materials (Swamy et al., 1987; Meier, 1995). However, composite materials were already widely used in other fields of structural engineering, such as aerospace (Hart-Smith, 1973), aeronautics and, later, automotive. Thus, the initial research activities about the possible use of composites in civil structures were not mainly focused on the behavior of composites themselves. They were rather intended at addressing two main issues regarding, on the one hand, the different behavior of composites with respect to more traditional materials (basically, steel) commonly used as a reinforcement in civil structures (Arduini & Nanni, 1997; Naaman et al., 2001; Triantafillou et al., 2001) and, on the other hand, the aspects related to the adhesive connection of the FRP laminates to the concrete (or masonry) substrate (Taljsten, 1997; Neubauer & Rostasy, 1997). The main findings of the research activities carried out in the ‘90s contributed to guidelines (fib, 2001; CNR-DT200, 2004; ACI 440-2R-08) for designing FRP-based strengthening intervention of RC and masonry members. Bonding between FRP laminates (sheets or plates) and concrete emerged as a cutting-edge issue from the first decade of research activities on composite materials for civil structures. In particular, several failure modes due to loss of adhesion between the externally bonded FRP element and the concrete substrate have been observed experimentally and recognized as specific features of this kind of members (Meier, 1995; Bonacci, 1996).