Flexural check at high temperatures of reinforced concrete bridge decks strengthened with EBR-FRP

In this paper the thermo-mechanical behaviour of RC bridge decks strengthened with externally bonded FRP laminates is investigated by considering environmental conditions responsible of thermal states different from the normal ones. A parametric analysis is performed by varying the slab thickness, the FRP reinforcing percentage, the type of fibre and the thickness of the protection layer. The results are given in terms of ultimate bearing capacity of the slabs, which allows individuating the conditions responsible of premature FRP-to-concrete debonding or temperature levels greater than the glass transition temperature of the adhesive

HIGH Tg FRP & CEMENTITIOUS ADHESIVE, Potential benefits in fire for NSM FRP strengthened reinforced concrete beams

Near surface mounted FRP strengthening is potentially less prone to damage due tofire exposure than externally bonded FRP reinforcement (EBR), provided that: (a) an FRP strengthening material with high glass transition and decomposition temperatures (Tg andTd, respectively); and (b) a bonding agent with low thermal conductivity and good thermal stability, are used. This paper presents a project undertaken on a specific high Tg and cementitious adhesive bonded NSM FRP strengthening system. Dynamic Mechanic Analysis (DMA) and Thermogravimetric Analysis (TGA) performed on the novel high Tg commercial CFRP bar yielded a Tg value of 220°C (based on  peak) and Td of about 360°C. Thermal conductivity tests were also performed on the cementitious grout. The results were used to better explain the failure modes of NSM FRP strengthened reinforced concrete beams at elevated temperature. The paper highlights the importance of understandingthe thermo-mechanical properties of the various constituent materials for improving the performance of FRP strengthening systems in fire

INVESTIGATION ACTIVITY ABOUT A COLLAPSED STEEL STRUCTURE SUBJECTED TO A REAL FIRE, Fire scenarios and structural behaviour of a real steel structure

The paper describes the behaviour ofa real steel structure collapsed under a fire event. 3D structural analyses were performed with SAFIR program (J-M Franssen, 2005). Different modellingare implemented with some fire load models and analyses of thebehaviour of the whole structure. The main purpose of this work was to investigate the failure types of a warehouse structure under fire conditions. Different fire conditions were applied to the steel frame sections, with ISOcurve (ISO EN 834-8:2002) and zone model approach. The analyses show that with unprotected steel sections, horizontal structures are more critical than columns. Trough applying a performance basedapproach,structure has 30 minutes of fire resistance

Ambient temperature performance of cementitious matrices for fire-safe NSM FRP strengthening of concrete structures

The near surface mounted (NSM) fiber reinforced polymer (FRP) strengthening technique is a demonstrated, attractive and efficient alternative to externally bonded reinforcement (EBR) strengthening systems. NSM strengthening can be used to enhance the stiffness and the strength of deficient reinforced concrete members, with high utilisation of the FRP's mechanical properties (at ambient temperature) when epoxy is used as bonding agent. However, owing to epoxy adhesives’ sensitivity to elevated temperature exposure, recent research has focused on the use of cementitious adhesives, which are less sensitive to elevated temperature, in NSM FRP applications. This paper presents results from 22 bond pull-out tests at ambient temperature on concrete prisms with an embedded carbon FRP bar NSM strengthening system. Different bonding agents (i.e. epoxy resin or cementitious grout), positions of the bar in the groove (i.e. in the centre or at the top of the groove), bar surface treatments (smooth and ribbed) and bond length (300 and 400 mm) are investigated

NSM FRP STRENGTHENING RC BEAMS USING HIGH TG FRPS & CEMENTITIOUS ADHESIVES – RESPONSE AT AMBIENT AND ELEVATED TEMPERATURE

The aging of the built heritage and infrastructures throughout the civil and industrialized world, as well as their deterioration due to environmental effects, and/or changing in service demand, lead to increasing interest in novel techniques aimed to design, maintain and rehabilitate concrete structures. Among the available strengthening techniques for improving the performance of concrete structures, the strengthening with Fibre Reinforced Polymers (FRP) gained huge and fast popularity during the last twenty years in the field of civil engineering. In the field of external strengthening of RC members the strengthening technique that experienced a widespread in the recent years is the Near Surface Mounted (NSM) strengthening system, in which the FRP is placed into the groove, cut into the surface of structural members, and bonded through an adhesive (epoxy resin or cement mortar). Several studies demonstrated that NSM with epoxy adhesive exhibits a better bond behaviour than EBR. Moreover, NSM is less prone to external attack, since the FRP is completely embedded in the adhesive. Despite that, the effectiveness of epoxy adhesive may be affected by high temperature, whereas cementitious adhesive may offer better performance than epoxies, keeping low the temperature in the FRP. The available literature about the behaviour of NSM FRP strengthened RC members is still limited, if compared to that available for EBR strengthening technique. Even worse is the knowledge about NSM with cementitious adhesive in not ordinary condition (fire). For this reason, very limited indications are available in the current codes for designing and predicting the capacity of the NSM strengthened members. Keeping that in mind, the thesis shows a research project undertaken on a high T_g FRP strengthening system bonded in a groove (NSM) with cementitious adhesive, aimed to define its response at ambient and at elevated temperature. The Chapter 2 of the Thesis presents a review of available literature on the behaviour of both NSM and EBR flexural strengthening applications for reinforced concrete members. Firstly, an overview of the main thermal and mechanical properties of the commercial FRPs and adhesives is reported. Then, the most recent experimental activities conducted on EBR and NSM, both at ambient and high temperature are summarized. Finally, the available international and national codes and guidelines are cited. Chapter 3 provides information about the techniques commonly used to assess the main thermal and mechanical properties of structural materials. Particularly, the most common techniques for measuring the thermal properties, such as Tg and Td of polymeric materials are shown. Moreover, the results of the tests, e.g., Dynamic Mechanic Analysis (DMA), Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC), conducted on a novel commercial CFRP bar are shown. In addition, the results of thermal conductivity tests of CFRP and cementitious grout are discussed. Finally, the results of the tests conducted for the mechanical characterization of the concrete, the steel bars, the cementitious grout and the CFRP bar, used to manufacture the above-mentioned beams, are shown. The Chapter 4 summarizes the results of the bond pull-out tests conducted on the new commercial cementitious-bonded CFRP NSM strengthening system, by varying the bond affecting parameters. Moreover, a comparison between the performances of this strengthening system and the performances of a resin-bonded CFRP NSM strengthening system is also shown. The Chapter 5 shows the design of the un-strengthened and strengthened beams with a prediction of their flexural capacity by means of cross sectional analyses. Moreover, a photographic report of the manufacturing and strengthening stages, the test setup and of the instrumentation of the beams is also shown. Particular attention is dedicated to the Digital Image Correlation (DIC), nowadays considered as innovative and powerful technique for displacements and strains measuring. Finally, the results of the tests of NSM FRP strengthened reinforced concrete beams, performed at both ambient and elevated temperature, by varying also the heating configuration, are discussed. The Chapter 6 reports a detailed description of the finite element models implemented in SAFIR 2011 for the thermal analysis of the tested NSM FRP strengthened RC beams, subjected to both the non-standard heating history recorded during the experimental tests and to the standard fire curve ISO834. The results of the thermal analyses, compared with the experimental results, are also provided and discussed. Finally, useful fire safety design criteria of NSM FRP strengthened RC beams are provided, based on the results of thermo-mechanical analysis, conducted on this structural typology

Calibration of a simplified method for fire resistance assessment of partially encased composite beams

Purpose-This paper aims to deal with the evaluation of the bending moment resistance of partially encased composite beams, heated from below by the standard-time temperature curve (ISO 834). Design/methodology/approach-EN 1994-1-2 provides two calculation models for evaluating the sagging and hogging moment resistance: the "general simplifed rules" and the "simplifed models" proposed in the Annex F. Findings-In this paper, these simplifed calculation models were implemented on several partially encased composite beams, by means of a parametric analysis. Then, the results were compared to those obtained through an advanced calculation model, such as the Moment-Curvature model, by means of a comparative analysis. Originality/value-The aim of the "parametric-comparative" analysis is the evaluation of the reliability of the Annex F simplifed models. This analysis was conducted by means of both numerical-numerical and numerical-experimental comparisons. This paper provides an alternative simplifed calculation model, which is easy to implement and very reliable

Performances at high temperature of RC bridge decks strengthened with EBR-FRP

Fiber reinforced polymer (FRP) can be successfully used to externally strengthen reinforced concrete (RC) bridges where fire is not a primary concern. Nevertheless, common maintenance activity on a bridge deck, such as the laying of bituminous paving, can easily lead the FRP to temperatures higher than the glass transition temperature, Tg. Exceeding Tg does not necessarily imply a drastic reduction in strength and stiffness of the reinforcement. Nevertheless, the softening of the resin implies a drastic reduction in its adhesion properties. Therefore, the efficiency of the strengthening system for existing structures, which mainly depends on the effectiveness of the bond between FRP and concrete, is significantly affected by temperature. The relationships suggested by Italian and American codes in order to evaluate the limit strain for FRP debonding at normal temperature are modified to take into account the effect of high temperature. Then, performances at high temperature of RC bridge decks strengthened with externally bonded FRP plates (EBR) are investigated by considering thermal fields in the structural members which are different from the normal ones. Both fire and the laying of bituminous paving on the decks are considered. In addition, the thicknesses of the slabs and the protective layer are varied to assess their influence on the thermal field in the slabs. The results are discussed with reference to both ultimate and serviceability limit states