Francesco Micelli

Strengthening of masonry structures with FRP composites

Unreinforced masonry (URM) walls are prone to failure when subjected to out-of-plane and in-plane loads. The development of effective and affordable strengthening strategies is a need. In this context fiber reinforced polymer (FRP) materials offer viable solutions to solve or lessen the effects of overloading. This paper describes FRP systems proposed for use in the strengthening of masonry elements as well as the impact of such systems on the building being retrofitted. Also, field applications, some potential and others already implemented applications, are described. Field applications include strengthening for natural hazards (i.e. earthquakes and high wind pressures) as well as man-caused hazards (i.e. blasting). The potential of FRP systems for retrofitting of historic structures is also illustrated. Finally, research needs in this area are discussed.

Tensile Characterization of FRP Rods for Reinforced Concrete Structures

The application of FRP rods as an internal or external reinforcement in new or damaged concrete structures is based on the development of design equations that take into account the mechanical properties of FRP material systems.The measurement of mechanical characteristics of FRP requires a special anchoring and protocol, since it is well known that these characteristics depend on the direction and content of fibers. In this study, an effective tensile test method is described for the mechanical characterization of FRP rods. Twelve types of glass and carbon FRP specimens with different sizes and surface characteristics were tested to validate the procedure proposed. In all, 79 tensile tests were performed, and the results obtained are discussed in this paper. Recommendations are given for specimen preparation and test setup in order to facilitate the further investigation and standardization of the FRP rods used in civil engineering.

Review Study on the Durability of FRP-Confined Concrete

AbstractThe use of fiber-reinforced polymer (FRP) composites has recently experienced a steep increase in civil engineering applications because of the high mechanical and low-density properties of such materials. Over the last few decades, concrete columns externally confined with FRP sheets have been widely investigated for their use in structural rehabilitation and the seismic strengthening of civil constructions. There is much scientific literature based on experimental results and analytical or empirical theoretical models. In fact, several numerical models and analytical procedures are able to predict the behavior of FRP-confined structural elements subjected to axial or seismic loads, and researchers worldwide have experimentally studied and analytically calibrated a wide range of significant variables. Nevertheless, there are still few results concerning the durability of FRP-confined concrete exposed to severe environmental conditions, despite being an important issue in design and safety assessm...

Experimental Study on Bond Behavior in Fiber-Reinforced Concrete with Low Content of Recycled Steel Fiber

AbstractThe use of recycled steel fibers from waste tires as reinforcement in concrete matrix appears to be a promising solution, thanks to the performance of the material in terms of toughness and postcracking behavior. The main objective of this paper is to analyze the bond behavior of recycled steel fiber–reinforced concrete (RSFRC) and steel bars, and compare the results with those of industrial steel fiber–reinforced concrete (ISFRC). The paper focuses on the characterization of the mechanical properties of concrete reinforced with short steel fibers from waste tires and on the results of pull-out tests executed both on RSFRC and ISFRC. The experimental results, in terms of failure mode, maximum bond stress, and bond stress versus slip behavior are analyzed and discussed. Finally, a theoretical analysis of the bond-slip behavior was performed. The experimental results show that most of the known performance of the ISFRC can be extended to RSFRC. Referring to the bond performance, an improved behavior...

Fiber-reinforced polymer for structural strengthening: Post-tensioning of steel silos

This paper illustrates a case study in which FRP pultruded laminates were used for strengthening circular steel silos. FRP laminate hoops were applied as external “belts” resting on discrete supports on the perimeter of the silos. Using a simple mechanical device, the laminates were manually post-tensioned to the desired load level, measured indirectly using electric strain gages. Active strengthening allowed the laminates to compensate for part of the existing deformation of the walls, and to optimise the state of stress in the steel structure by tailoring the post-tensioning level. This paper examines in detail the design process, the experimental tests conducted for proper design of the construction details and the implementation of the strengthening system.

Confinement of Full-Scale Masonry Columns with FRCM Systems

The effectiveness of FRP systems as a confinement technique to strengthen masonry columns has been widely investigated in the last decades. Recently, a new technique, Fabric Reinforced Cementitious Matrix (FRCM), based on the use of fibrous nets embedded in inorganic matrix, has been developed and investigated as a strengthening solution in masonry buildings. Actually, the number of experimental tests on masonry columns confined by using FRCM systems is very limited, especially for real scale specimens. To fill such gap an experimental program aimed at investigating the behaviour of full scale columns made of limestone masonry blocks confined with different FRCM systems has been carried out. The results of four uniaxial compression tests are illustrated and discussed. The used FRCM systems are made with glass and basalt dry nets embedded in a lime-based mortar. The influence of transverse confinement by using internal reinforcement in forms of pultruded GFRP bars has been also investigated. The mechanical properties of the confined specimens resulted increased in terms of load-carrying capacity and ultimate axial strain.

Concrete Reinforced with Recycled Steel Fibers from End of Life Tires: Mix-Design and Application

The manufacturing technology of reinforced concrete with the use of steel fibers to improve its mechanical properties is well-known and commonly used in civil engineering. Generally, steel fibers as discontinuous reinforcement of the concrete matrix are used to limit the cracking growth following the load application. Thus, the obtained concrete is characterized by an improvement of the typically brittle behavior of the ordinary matrix, mainly referring to toughness and post-cracking behavior. In this paper the results of a recent experimental campaign carried out at the University of Salento will be discussed. It was designed to study the optimization of concrete mixtures reinforced with recycled steel fibers from end of life tires (ELTs) to be used for the realization of precast panels. This experimental campaign is part of a wider research project aimed to validate the idea that the constituent elements of the ELTs, especially rubber and steel, can be effectively reused in concrete mixtures. Taking into account the high annual amount of ELTs generated around the world and their negative impact on the global environmental sustainability, the recovery of their constituent materials and their reuse as raw materials in different technologies, is certainly an excellent way for a sustainable development.

Cracking Behaviour of FRC Beams under Long-Term Loading

Abstract The extensive research activity carried out over the last decades on fibre-reinforced concrete (FRC) has shown that such material has enhanced mechanical and durability properties compared to plain concretes. The presence of short fibres in the concrete mass allows to control cracking and have moderate time-dependent effects. Compared to plain concrete, FRC flexural members show a higher number of cracks with reduced mean width. The experimental study presented herein discusses the mechanical behavior of FRC flexural beams subjected to sustained service load and environmental exposure for 72 months. The effects of different short fibers (polyester and steel), sustained loading and aging were investigated. A comparison with the results of a previous research is shown, with reference to the same kind of beams exposed for 17 months under the same conditions. The results show the beneficial effects of the fibers in terms of reduced crack width and increased flexural stiffness. The mechanical tests also highlighted how the presence of short structural fibers could play an effective role in mitiga ting creep effects in the concrete elements.

Ageing and fatigue combined effects on GFRP Grids

Many areas of Europe, especially Italy, Greece, Slovenia and other Balkan States, are generally associated with earthquakes. In the last two decades Fiber Reinforced Polymers (FRP) have gained an increasing interest, mostly for upgrading, retrofitting and repair of masonry and timber structures belonging to the architectural heritage. Recent researches demonstrated that masonry constructions or single structural elements are likely to be effectively repaired or enhanced in their mechanical properties using FRPs. The objective of the research presented in this paper is to study the long-term behavior of composite grids, made of E-CR glass fibers and epoxy-vinylester resin, subjected to harsh environmental factors including fatigue loading and ageing in aqueous solution. The paper presents new original test results on the relationship between the durability and the governing material properties of GFRP (Glass Fiber Reinforced Polymers) grids in terms of tensile strength and axial strains, using specimens cut off from GFRP grids before and after ageing in aqueous solution. The tensile strength of a GFRP grid was measured after conditioning in alkaline bath made by deionized water and Ca(OH)2, 0.16% in weight, solution. The reduction in terms of tensile strength and Young’s modulus of elasticity compared to unconditioned specimens is illustrated and discussed. This degradation indicated that extended service in alkaline environment under fatigue loads may produce reductions in the GFRP mechanical properties which should be considered in design, where cyclic loads and aggressive conditions are prevented in service life.

The Interface Behavior between External FRP Reinforcement and Masonry

The need to guarantee higher safety levels of masonry structures under both short and long term conditions, have led to the use of new materials and technologies, in conjunction or in place of traditional ones. In this context, fiber-reinforced composite materials have gained an increasing success, mostly for strengthening, retrofitting and repair existing structures. As well known, the analysis of the interface performance of FRP (Fiber Reinforced Polymer) composites and masonry substrate is a critical problem as it influences the effectiveness of the technique. The present paper reports part of a large research project, still in progress, focused on the analysis of the bond performance between FRP sheet and different type of masonry substrates. The obtained experimental data were analysed in terms of bond strength and the kind of failure. The influence of the deformability of the strengthening material as well as the mechanical performance of the substrates are also discussed.