Antonio Bilotta

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.

Numerical sensitivity analysis of corrosion detection

The aim of the present work is the detection of corrosion damage along the inaccessible part of the boundary of a body under investigation. The data of the problem, besides all the information relative to the domain such as the geometry and the conductivity of the body, are the prescribed current fluxes and voltage measurements on the accessible part of the boundary. This constitutes, in general, a nonlinear inverse problem whose ill-posed feature requires a suitable solution procedure. The strategy proposed here is based on a linearization of the Robin boundary condition on the inaccessible part of the boundary and on the identification of a resistivity parameter related to the corroded surface. Besides giving a strategy to evaluate the corrosion damage parameter, this paper tries to sketch a sensitivity analysis of the computed solution with respect to all factors affecting the available information relative to the accessible boundary, such as the quantity and quality of data and the unavoidable errors corrupting the compatibility of the measured data.

Direct Evaluation of the Post-Buckling Behavior of Slender Structures Through a Numerical Asymptotic Formulation

The analysis of slender structures, characterized by complex buckling and postbuckling phenomena and by a strong imperfection sensitivity, is heavily penalized by the lack of adequate computational tools. Standard incremental iterative approaches are computationally expensive and unaffordable, while FEM implementation of the Koiter method is a convenient alternative. The analysis is very fast, its computational burden is of the same order as a linearized buckling load evaluation and the simulation of different imperfections costs only a fraction of that needed to characterize the perfect structure. In this respect it can be considered as a direct method for the evaluation of the critical and post-critical behaviour of geometrically nonlinear elastic structures. The main objective of the present work is to show that finite element implementations of the Koiter method can be both accurate and reliable and to highlight the aspects that require further investigation.

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.

Multiscale 3D mixed FEM analysis of historical masonry constructions

A multiscale approach to analyse historical masonry buildings is presented and the numerical results deriving from its implementation are discussed. The modelling of this kind of heterogeneous material composed by irregular, stones and mortar joints is performed at the level of the microstructure by also describing the nonlinear behaviour of stones and mortar joints through an elasto–plastic constitutive law. At the macro-level, a finite element description based on a generic anisotropic material is implemented. This micro–macro model aims to assess the structural behaviour and the safety condition of historic masonries.

Behavior of FRP Reinforced Concrete Slabs in Case of Fire: Theoretical Models and Experimental Tests

Several technical codes allow concrete structures reinforced with FRP to be designed, but few calculation models taking account of fire condition are available. Assuming that the anchoring of the reinforcement is ensured in cooler zones of the structure, a calculation procedure developed by authors allows the flexural capacity of the one-way FRP reinforced concrete slabs under fire conditions to be assessed. The procedure was used for the design in the fire situation of six concrete slabs reinforced with Glass Fiber Reinforced Polymer (GFRP) bars. Such slabs were tested in case of fire by exposing them to heat in a furnace according to ISO834 standard time-temperature curve: four slabs have been tested under typical design loads in fire situation and two unloaded slabs have been tested after the cooling phase in order to evaluate their residual resistance. The experimental results confirmed that the effects of the high temperatures on both the deterioration of the material mechanical properties and the decrease of bond between FRP reinforcement and concrete are key aspects of the structural behavior of concrete members reinforced with FRP bars. Nevertheless the anchoring length at the end of the members not directly exposed to fire could ensure a fire resistance time higher than 180 minutes.

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).

Bond Between EBR FRP and Concrete

This chapter provides an overview of the debonding process between the FRP reinforcement and the concrete substrate. The main aspects of the debonding phenomenon are described and discussed, showing also mechanical interpretation of different processes. Experimental techniques to study the bond behavior between FRP and concrete are also described and corresponding available experimental results are shown to compare performances of different set-ups. Finally, an extensive description of the existing bond capacity predicting models is reported, together with the main international Codes provisions, allowing the designer for operating in common practice.

Adhesion at High Temperature of FRP Bars Straight or Bent at the end of Concrete Slabs

Confidence in the use of Fiber Reinforced Polymer (FRP) for Reinforced Concrete (RC) members in multi-story buildings, parking garages, and industrial structures is poor due to lack of provisions and calculation models taking account of fire condition. In the past, to contribute to refining existing codes for the design of FRP-RC structures, authors tested six concrete slabs reinforced with Glass FRP (GFRP) bars characterized by different values of concrete cover and anchoring length in fire condition. Recently, further three fire tests were carried out on concrete slabs reinforced with GFRP bars bent at the ends. The anchoring of the FRP bars in the zone of slab not directly exposed to fire at the end of the members revealed essential to ensures slab resistance, once in the fire exposed zone of slab the glass transition temperature was attained and the resin softening reduced the adhesion at the FRP-concrete interface.