INNOVATIVE MATERIALS FOR THE VULNERABILITY MITIGATION

Transcription

1 INNOVATIVE MATERIALS FOR THE VULNERABILITY MITIGATION OF EXISTING STRUCTURES Gaetano Manfredi a and Luigi Ascione b a Department of Structural Engineering (DIST), University of Napoli, Italy, gamanfre@unina.it b Department of Civil Engineering, University of Salerno, Fisciano (SA), Italy, l.ascione@unisa.it 1 INTRODUCTION The use of fiber reinforced polymer (FRP) materials for the strengthening of masonry and concrete structures, represents a valid alternative to traditional techniques. There are, in fact, many advantages of using FRPs: lightweight, good mechanical properties, corrosionresistant, etc. In Italy, the use of FRP materials for reducing the seismic vulnerability of existing structures has been allowed for the first time through O.P.C.M and more recently by the D.M , that refer to the Italian National Research Council Design Guidelines (CNR-DT 200/2004) for the external strengthening of existing structures with FRP materials. These guidelines provide, within the framework of the Italian regulations, a document for the design and construction of externally bonded FRP systems for the strengthening of existing structures. In particular, several issues of seismic analyses have already been dealt but a further investigation is still required. Within this context, the main aim of this research task has been the experimental validation of design indications provided by the CNR-DT 200/2004 guidelines. The main topics investigated in this research task can be summarized as follow: - the mechanical behaviour of FRP materials; - the cyclic behaviour of RC elements strengthened by means of FRP; - the mechanical and chemical anchorage devices for FRP systems; - the ductility increasing of RC columns confined with FRP; - the RC joint strengthening with FRP; - the masonry strengthening with FRP; - the historical structure strengthening with FRP; - the quality control and monitoring of FRP applications; - the innovative fibers (steel fabrics, natural fibers, FRP grids, etc) and matrices (organic and inorganic); - the mechanical behaviour of concrete structures reinforced with fiber reinforced polymer bars; - innovative strengthening techniques (near-surface mounted (NSM) technique, FRP prestressed systems). 2 BACKGROUND AND MOTIVATION The research activity has been performed through experimental tests and theoretical studies, mainly devoted to the development of simple methods of analysis and design rules mainly to

2 2 G. Manfredi, L. Ascione improve the indications contained in the Document CNR-DT200/2006 concerning the design and construction of externally bonded FRP systems for strengthening existing structures. The durability of the FRP system is one of the main drawbacks to the use these materials in Civil Engineering. In particular structural adhesives usually represent the weakest point of the reinforced system and their mechanical behaviour and durability performance need to be investigated. The first problem that an engineer runs into when considering the advisability of using composite materials for structural reinforcement is the determination of their mechanical properties. Besides, no standard test method to perform short-term experimental evaluation of the tensile strength of FRP rebars and tendons is at present available while the codes JSCE (1997) and ISO (2003) simply state that the anchorage shall be suited to the geometry of the test pieces and to the test type. The bond between FRP and concrete is an ulterior very important issue because the debonding is a very brittle failure mechanism and must be avoided. In the literature [1], different experimental set-ups can be found dealing with FRP-concrete bond tests and it has been observed that different test methodologies may give different values of the debonding force. According to performance-based design or seismic evaluation of RC buildings, it is crucial to provide a reliable evaluation of the strength and ductility capacity of the RC columns and beam and their joints: experimental and direct observation of damages occurred during recent earthquakes highlighted this. In this direction a valid contribution can be offered by the employing of FRP systems. The effectiveness of FRP systems for seismic vulnerability mitigation of masonry structures is still in debate, despite it has moved a huge interest, becoming the outstanding system in the market for this type of applications. In fact DT 200/2004 has been the first guideline document trying to give full coverage of the different aspects of a real FRP layout design for masonry buildings. Many of its topics however, were derived by a number of very simple experimental analyses and a body of general rules holding for reinforced concrete structures. Two principal facts render the retrofit design of masonry structures an outstanding problem: first of all, the real masonry structure is load dependent and thus the FRP could be placed in an inactive zone of the resisting scheme. Secondly, masonry can activate a large number of local mechanisms which interact with global behaviour of masonry buildings. The non linear seismic assessment of FRP reinforced masonry structures is included also in the OPCM 3274 rule. The non linear analysis requires the precise statement of the constitutive law for the masonry material both in the unreinforced or strengthened situations. The definition of these laws is however function of the bonding forces in the FRP interface. In the recent years, huge scientific research has been conducted by national and international engineering community for the safeguard of historical buildings. These have led to several research projects; in particular, CNR DT 200/2004 technical recommendations [1] for the design and construction of strengthening techniques with FRP systems have been published in Italy to provide an aid to designers interested in the field of composite materials and to avoid their incorrect application. The Guidelines deal with different types of FRP applications to masonry and reinforced concrete structures and take into account the important phases of quality control and monitoring that should follow a strengthening intervention. In fact, several aspects affect the effectiveness of FRP systems: above all, surface preparation and FRP installation are the most important ones. Moreover, once FRP strengthening intervention has been carried out, monitoring by non-destructive or semi-destructive tests should be performed to assure the intervention quality and effectiveness [2 3 4]. It is worth noting that due to the increased number of composite material applications and in order to get a better

3 Innovative materials for the vulnerability mitigation of existing structures 3 understanding of the mechanical behavior and the interaction between FRP materials and the masonry substrate, experimental tests are essential to avoid premature loss of strength and collapses of the intervention. In the present research, semi-destructive and non-destructive techniques have been conducted for the quality control and monitoring of FRP applications to masonry structures, according to CNR DT 200/2004 Guidelines. 3 RESEARCH STRUCTURE In order to guarantee an optimal organization of the research, the Research Units have been grouped into the following ten Tasks, each one with a specific topic: - Task 8.1: the mechanical characterization of FRP systems at fixed environmental conditions under cyclic actions; - Task 8.2: the delamination under cyclic actions and design of anchorage mechanical devices for FRP systems; - Task 8.3: the confinement of RC and masonry columns subject to combined flexure; - Task 8.4: the strengthening in flexure and in shear of RC structural elements with FRP fabrics and near surface mounted (NSM) rods; - Task 8.5: the beam-column and beam-foundation joint reinforcement with FRP; - Task 8.6: the design criteria for the seismic retrofit of RC and RC-masonry composite structures with FRP; - Task 8.7: the design criteria for the seismic retrofit of masonry structures with FRP; - Task 8.8: the strengthening of masonry structural elements with FRP systems; - Task 8.9: the strengthening of masonry vaulted elements with FRP systems; - Task 8.10: the quality control and monitoring of FRP applications to existing masonry and RC structures. 4 MAIN RESULTS 4.1 Task 8.1: the mechanical characterization of FRP systems at fixed environmental conditions under cyclic actions Carbon fibre reinforced polymers (CFRP) are ideal products for structural retrofitting and seismic upgrading. Nonetheless the small knowledge on the durability of the system is one of the main drawbacks to the use of CFRP reinforcement in Civil Engineering. In particular structural adhesives usually represent the weakest point of the reinforced system and their mechanical behaviour and durability performance need to be investigated. The first problem that an engineer runs into when considering the advisability of using composite materials for structural reinforcement is the determination of their mechanical properties. The determination of the mechanical properties is a very hard task due to various factors: a) differences in the materials adopted to manufacture the bar and in the factory system governing parallelism and fiber distribution; b) difficulties in verifying in a laboratory the values declared by the manufacturers make. Besides, no standard test method to perform short-term experimental evaluation of the tensile strength of FRP rebars and tendons is at present available while the codes JSCE (1997) and ISO (2003) simply state that the anchorage shall be suited to the geometry of the test pieces and to the test type.

4 4 G. Manfredi, L. Ascione At the aim of characterize these materials the research has studied in dept the durability and mechanical behaviour of structural adhesives and FRPs, the mechanical characterization of FRP bars and strips and the values of the safety factors proposed in Design Recommendation. Durability and mechanical behaviour of structural adhesives and FRPs Several tests to determine the mechanical properties of composite materials and structural adhesives have been performed. Conforming to the ASTM requirements, the glass transition temperature (ASTM D3418), porosimetry and the coefficient of thermal expansion (ASTM D360) were determined. Adhesives were also tested under tensile (ASTM D360), compressive (ASTM D695) and flexural loading (ASTM D790). Adhesive shear strength was determined by punch tool tests (ASTM D732). Finally adhesive cylinder specimens were tested under pure torsion load. a) b) Figure 1 a) Execution of the punch-tool test and specimen after collapse, b) execution of the torsion test and specimen after collapse. Adhesive dumb-bell specimens were prepared for tensile testing and then artificially aged in an environmental chamber in order to analyze possible detrimental effects on the adhesive mechanical properties. Exposition to deicing salts, freeze-thaw cycles and moisture may in fact deteriorate the mechanical properties with consequences on the durability performances of strengthened structures. Tensile tests were performed conforming to the requirements of ASTM D360. In all the conditioning treatments, significant losses in adhesive stiffness and tensile strength were measured. The stiffness and tensile strength reductions after exposure to salt spray fog solution may be approximated by straight parallel lines as described in the Arrhenius life-temperature relationship. Fatigue tests on adhesive dumb-bell specimens were finally performed to attain the fatigue failure curves for the adhesive joint. Mechanical characterization of FRP bars and strips Tensile and relaxation tests were performed at the Politecnico di Milano on FRP bars with particular attention to the gripping system. Then, experimental tests and numerical simulations were performed to develop simple, economical and effective systems for the characterization of composite materials and adhesives. In particular an anchor system for tension testing of unidirectional fiber reinforced plastic (FRP) bars of large diameter was developed. In the system suggested each end of the bar is embedded in a conical polymeric head that fits a conical hole inside the anchoring device. In the anchor system, the anchor

5 Innovative materials for the vulnerability mitigation of existing structures 5 body shape came from experiences for testing steel ropes and prestressing steel tendons and the shape of the resin head from test investigation. Numerical analyses were also performed to investigate the effects of anchor parameters such as cone slope angle, thickness of resin head and friction coefficient between the anchor body and the resin head. Pull-out and beam tests were also executed. a) b) Figure 2 a) Anchor system for large diameter GFRP bars, b) numerical analysis. Experimental studies and numerical analyses were developed to define practical tests for the characterization of FRPs and adhesives mechanical properties. The main aim of this action was to provide the Composites Kit Test - COKIT ; a practical tool for professionals and engineers operating in the field of FRPs applications and dealing with FRP materials for structural retrofitting and rehabilitation. The technical document Istruzioni per la caratterizzazione ed il controllo di accettazione di materiali fibrorinforzati per il rinforzo strutturale COKIT was thus published and represents in every respect a annexe of the CNR-DT Recommendations. Refinement of the safety factors proposed in Design Recommendations The environmental conversion factors provided in the guidelines of the Italian National Research Council (DT200) were analyzed on the basis of the results of artificially aged adhesive specimens tested under tension. Exposition to deicing salts, freeze-thaw cycles and moisture leads to the deterioration of the mechanical properties of composite materials and in particular structural adhesives. On the basis of the experimental results, the safety factors suggested in the CNR DT200 recommendations may be considered as appropriate, but in aggressive environments the use of a slightly lower conversion factor seems to be more suitable.

6 6 G. Manfredi, L. Ascione Property Retention [%] Figure 3 Stiffness and tensile strength retention for the structural adhesive subject to freeze-thaw cycles of five hours each between 18 and +4 C for a total duration of about 2 months (FT), to salt spray fog for one month or three months (SF) and to one month humidity (HU) Tests were performed to refine the safety factor of FRP-steel systems: the fatigue behaviour of steel structures retrofitted by using FRP materials was investigated, S N curves were defined and the fatigue resistance of the steel-cfrp bond was compared to the one of welded detail categories described in the Eurocode 3. a) Stiffness Reduction [%] E E E E E+08 Number of Cycles N b) c) Figure 4 a) Steel-CFRP specimen b) Reduction in stiffness of retrofitted specimens during fatigue tests; c) S-N curve and comparison between the fatigue resistance of the steel-cfrp bond for a stiffness reduction of 5% (blue circles) and of 15% (red squares) and that of EC3 welded detail categories. After performing pull-pull delamination tests on FRP-concrete specimens, cylinders were obtained from each concrete prism. Based on Eurocode 2 compressive and splitting tests were carried out to determine the conditioning effects on concrete degradation. As a consequence of the environmental conditioning, concrete characteristic strength is assumed to increase by 16% for salt spray fog conditioned specimens and to decrease by 3% for specimens subject to freeze-thaw cycles.

7 Innovative materials for the vulnerability mitigation of existing structures 7 Salt Spray Fog Freeze-thaw cycles 8 6 observations kg, exp observations kg, exp kg DT200 = 0,064 kg DT200 = 0,064 Med. kg=0,059 5 % perc= 0,037 kg DT200 = 0, Fck Med.kg=0,052 kg DT 200 = 0,03 5% perc = 0, Fck a) b) Figure 5 Statistical distribution of the coefficient kg for specimens subject to a) salt spray fog and b)freeze-thaw cycles. Effects of elevated temperatures and freeze-thaw cycling on FRP laminates behavior The performances at elevated temperatures and/or at freeze-thaw cycling exposure of structural members strengthened by using externally bonded FRP laminates are mainly related to two aspects: the bond behaviour between FRP and the member substrate; the mechanical properties of laminates themselves. The latter aspect has been very limited experimentally investigated; only few tests have been performed to evaluate the residual tension strength of FRP coupons after exposure to elevated temperatures [1] or freeze-thaw cycling. Thus, the three series of experimental tension tests on carbon FRP (CFRP) laminates both under controlled temperature and relative humidity conditions or after freeze-thaw cycles exposure. In particular, due to reduced capacity that commercially available resins have to transfer loads over fibres around glass transition temperature, T g, two new systems based on epoxy resin have been formulated and characterized by dynamic mechanical analysis (DMA). The main goal of the new formulated systems was to increase T g, the elastic modulus in the rubber region of the resin and to improve their performances under freeze-thaw cycles. Two different approaches were investigated. First a new epoxy system (namely neat epoxy) was formulated and cured at 60 C after an hour at room temperature. Secondly, in order to improve the mechanical properties of epoxy matrix by curing at room temperature, a nanocomposite system was obtained by direct dispersion of preformed nanodimensioned silica particles to the neat epoxy resin.

8 8 G. Manfredi, L. Ascione L=360mm B=25mm Ltabs = 80mm T ( C); RH (%) Temperature ( C) RH (%) 1 hour at fixed environmental conditions 10 Time (s) Axial load P = 30%Pf,th P up to failure (a) (b) (c) Fig. 1- Specimen geometry; Temperature and relative humidity exposure profiles; test setup. The experimental results point out that the developed formulations of epoxy resins provide a significant increase of ultimate strength and strain of CFRP coupons both at room and elevated temperatures with respect to commercial systems, without significant change of the elastic modulus. Negligible influence of a low number of freeze-thaw cycles was observed on the mechanical properties of coupons independently of matrices. Experimental outcomes strongly confirmed that the use of matrices characterized by higher values of T g and elastic modulus in the rubber region with respect to those traditionally available on the market, could allow to overcome one of the main limit of FRP laminates related to their poor performances under elevated temperatures. 4.2 Task 8.2: the delamination under cyclic actions and design of anchorage mechanical devices for FRP systems The bond between FRP and concrete is very important issue because the debonding is a very brittle failure mechanism and must be avoided. In the literature [1], different experimental setups can be found dealing with FRP-concrete bond tests and it has been observed that different test methodologies may give different values of the debonding force. This task intends to define a standard FRP-concrete bond test to be used to evaluate the maximum transmissible force by an FRP anchorage, to be included in the new version of the Italian code for design of strengthening interventions with FRP [4]. Experimental round robin test on FRP concrete bonding An extensive experimental campaign on FRP-concrete debonding has been carried out by five different Italian Laboratories (University of Bologna, Federico II University of Naples, University of Sannio, Polytechnic of Milan and University of Calabria). The tests were devoted to the definition of a standard test procedure for the bond strength evaluation. According to the Round Robin procedure, 50 concrete prisms (same batch) strengthened with CFRP plates and sheets have been prepared by the same operator and subject to bonds test in five different Laboratories. The sets of homogeneous specimens have then been subject to bond test by five laboratories of the University partners using different test set-ups (see Fig. 1).

9 Innovative materials for the vulnerability mitigation of existing structures 9 Strain gauges Load cell mm 0 x m x i Reaction elements LAB1 LAB2 LAB3 Display Mechanical Jack LAB4 Pull Machine LAB5 Fig. 1 Experimental set-ups adopted by the five different Laboratories. Twelve specimens (6 strengthened with sheets, 6 strengthened with plates), with two different bonded lengths (100 mm and 400 mm), have been tested by each laboratory, repeating three times the same type of test. As for the test set-ups (Fig. 1), all the Laboratories adopted a single shear push-pull test. All the tests have been performed under displacement control of the FRP free end. In order to evaluate the variability of the results when different set-ups are adopted, the coefficient of variation (COV) for each set of homogeneous experimental tests has been calculated. The scatter of the results is in general small (COV about 10%), lower than that of the tension strength of the concrete, usually equal to 20-30%. For the plates, the scatter of the results is similar for the different Labs, whilst for the sheets the dispersion is usually higher. The results obtained by Lab 3 are very stable in both cases and close to the mean values. This study allowed to define a set of rules for the standardization of bond tests to be used to evaluate the maximum transmissible force by an FRP concrete anchorage.

10 10 G. Manfredi, L. Ascione Debonding Force [kn] SHEETS L=400 mm Debonding Force [kn] SHEETS L=100 mm LAB 1 LAB 2 LAB 3 LAB 4 LAB 5 (a) 5 0 LAB 1 LAB 2 LAB 3 LAB 4 LAB 5 (b) Debonding Force [kn] PLATES L=400 mm Debonding Force [kn] PLATES L=100 mm LAB 1 LAB 2 LAB 3 LAB 4 LAB 5 (c) 5 0 LAB 1 LAB 2 LAB 3 LAB 4 LAB 5 (d) Fig. 2 Debonding force for sheets with anchorage length (a) L=400 mm, (b) L=100 mm, and for plates with anchorage length (c) L=400 mm, (d) L=100 mm Somma di COV Somma di COV 0.18 COV SHEETS COV PLATES 0.16 LAB 1 LAB 2 LAB 3 LAB 4 LAB 5 L[mm] (a) LAB 1 LAB 2 LAB 3 LAB 4 LAB 5 Fig. 3 Coefficient of variation (COV) of the debonding force for (a) sheets and (b) plates. L[mm] (b) 4.3 Task 8.3: the confinement of RC and masonry columns subject to combined flexure The main goal of this task has been to validate the design equations provided by CNR DT200 for the confinement of masonry columns. The confinement has been performed employing an innovative typology of FRP system made of basalt natural fiber. An experimental campaign has been carried out also on cylindrical specimens strengthened by means of basalt fibers and inorganic matrix in order to validate the effectiveness of the proposed technique compared with different confinement schemes. Confinement of RC cylindrical specimens strengthened by means of basalt fibers and inorganic matrix The effectiveness of such system as a confinement technique is analyzed by means of an experimental campaign on concrete cylindrical specimens. The effectiveness of the proposed technique is assessed by comparing different confinement schemes: 1) uniaxial Glass Fibre Reinforced Polymer laminates; 2) alkali-resistant fibreglass grid bonded with a cement based mortar; 3) bidirectional basalt laminates pre-impregnated with epoxy resin or latex and then

11 Innovative materials for the vulnerability mitigation of existing structures 11 bonded with a cement based mortar; 4) cement based mortar jacket. The main objectives of the experimental program were: a) to investigate on the effectiveness of confinement based on basalt fibres pre-impregnated in epoxy resin or latex and then bonded with a cement based mortar (BRM); and b) to compare the performance (in terms of peak strength and ultimate axial strain gains) of different confinement techniques using advanced materials with respect to GFRP laminates jacketing. The investigation was carried out on 23 concrete cylindrical specimens with a diameter of D = 150 mm and a height of H = 300 mm. 1 st layer of mortar application (4mm) Pre-impregnation of laminates Basalt laminates installation Figure 1 BRM wrapping installation procedure 2 nd layer of mortar application (4mm) Experimental outcomes showed that: BRM confining system could provide a substantial gain both in compressive strength and ductility of concrete members inducing a failure mode less brittle than that achieved in the GFRP wrapped members; lower performance were observed by concrete confinement provided by a primed glass fibre grid bonded with cement based mortar with respect to BRM and almost no influence was generated by the jacketing with mortar only. maximum ultimate axial strain increases were provided by GFRP laminates wrapping. Unconfined GFRP GFRP grid (mortar) BRM (resin) BRM (latex) Cement mortar FRONT B FRONT A Figure 2 Failure modes.

12 12 G. Manfredi, L. Ascione Confinement of rectangular masonry columns subject to axial load An experimental campaign dealing with 18 square cross-section both listed faced tuff and clay brick masonry scaled columns subjected to uniaxial compression load. In particular, three different confinement schemes were experimentally analyzed in order to evaluate and compare the effectiveness of the proposed strengthening techniques: 1) uniaxial glass FRP laminates (GFRP) wrapping; 2) uniaxial carbon FRP (CFRP) laminates wrapping; and 3) uniaxial basalt FRP (BFRP) laminates wrapping. In particular 9 tests, were performed on square tuff masonry (external tuff blocks and inner core filled with tuff chips and mortar) scaled columns (mass density equal to about 1530 kg/m3): side average dimension equal to 220mm; and average height of about 500 mm corresponding to 8 courses of tuff bricks (height-width ratio equal to 2.27). Masonry was made by scaled yellow Neapolitan tuff bricks (50x50x100mm) and a pozzolan (local volcanic ash) based mortar (thickness of 12mm). Further 9 tests were performed on square clay brick masonry scaled columns (mass density equal to about 1700 kg/m3): side average dimension equal to 260mm, and average height of about 560 mm corresponding to 8 courses of clay bricks (height-width ratio of 2.20). Masonry was made by clay bricks (55x115x255mm) and a pozzolan (local volcanic ash) based mortar (thickness of 13 mm). A-A A A (a) (b) Figure 3. Specimen details (dimensions in mm): (a) tuff masonry; (b) clay brick masonry. Masonry columns were tested through monotonically applied axial compressive loading under displacements control mode with a rate of mm/s.

13 Innovative materials for the vulnerability mitigation of existing structures 13 Figure 4. Stress-axial strain relationships and specimens failure mode: (a),(c) and (e) tuff masonry; (b),(d) and (f) clay brick masonry. The experimental outcomes showed that: GFRP and CFRP jackets led to similar of compressive strength gains on tuff masonry columns under axial loads. GFRP and BFRP confining system led to similar compressive strength gains of brick masonry columns under axial loads. BFRP wrapping was more effective in terms of global ductility increase (i.e. ultimate strain gain equal to 413% and 259% for BFRP and GFRP wrapping, respectively) even if the mechanical external reinforcement ratio of FRP laminates was lower then GFRP ones; such result could be explained by the higher values of ratios ε fl /ε fu recorded on BFRP laminates. The use of high values of laminates unit height may significant reduce the effectiveness of FRP wrapping systems since it could be detrimental to the quality of confinement execution. The presence of voids and protrusions on masonry members reduces the ultimate transverse strain on FRP reinforcement with respect to that typically achieved on concrete members. An extensive experimental campaign has been carried out in order to show the behavior of columns built with clay or with calcareous blocks, commonly found in southern Italy, especially in historical buildings. Rectangular masonry columns were tested for a total of 33 specimens; uniaxial compression tests were conducted on columns taking into account the

14 14 G. Manfredi, L. Ascione influence of several variables: different strengthening schemes (internal and/or external confinement), curvature radius of the corners, amount of fiber-reinforced polymer (FRP) reinforcement, cross-section aspect ratio, and material of masonry blocks. Materials characterization was preliminarily carried out including a mechanical test on plain masonry. For all cases the experimental results evidenced a significant increase in load carrying capacity and ductility after FRP strengthening, which identified the columns as ductile elements despite the brittle nature of the unconfined masonry. Differences in mechanical behavior, due to the geometry of the columns, to the nature of different materials, to different strengthening schemes, and to the amount of reinforcement, have been taken into account. The calibration of design equations recently developed by Italian National Research Council, CNR was conducted to compare analytical prediction and experimental results. Fig. 1 Campioni in pietra leccese e laterizio The results obtained from the experimental campaign confirmed that innovative strengthening techniques, using FRP sheets and bars, are effective when confinement of masonry compressed elements is needed. Two types of masonry were investigated: the first made with clay bricks, the second made with limestone blocks. Even if the properties of the constituent materials were different, in both cases a significant increase was measured in terms of peak load and ultimate axial deformation. Two construction schemes were considered: fullcore and hollow-core columns; the last type reproduces the patterns often found in historical buildings. External and internal FRP confinement were tested, separately and combined. The proposed techniques are strongly recommended when a seismic retrofit is needed, since the external

15 Innovative materials for the vulnerability mitigation of existing structures 15 confinement introduces a plastic behavior of the compressed masonry which indicates a large capacity in storing elastic energy which is taken by the fibers placed in the transverse direction. The presence of internal bars used as an internal confinement system is recommended in addition to external FRP layers if ductility constitutes a main issue, since in columns strengthened only with bars the ultimate load was increased but brittle behavior of unconfined masonry remained. Columns with hollow core also showed a significant increase of mechanical properties when confinement was applied, especially in the cases of GFRP external sheets combined with internal bars. Confinement of circular masonry columns subject to axial load An extended experimental investigation has been performed in order to show the mechanical behavior of circular masonry columns built with calcareous blocks that may be commonly found in Italy and all over Europe in historical buildings. Different stacking schemes were used to build the columns, aiming to simulate the most common situations in existing masonry structures. Carbon FRP sheets were applied as external reinforcement; different amounts and different schemes of confining reinforcement were studied. The experiments have included a new reinforcement technique made by using injected FRP bars through the columns cross section. The structural behavior of masonry columns damaged under different levels of load and strengthened by using FRP reinforcements has been also investigated. Figura 11 Geometria e dimensioni in mm delle colonne Important remarks follow: High increase in ultimate strength and strain were evident after strengthening; Complete FRP jacketing was much more effective than discontinuous wraps; Displacement capacity resulted increased in all cases; strengthened columns tested showed an extended postpeak plastic branch in the load versus displacement curves; Columns confined with three 100 mm wide sheets showed higher mechanical properties with respect to the same columns confined with two 150 mm wide sheets; Damage caused by overloads applied in the precracking stage before strengthening did not reduce the mechanical properties of FRP-confined columns;

16 16 G. Manfredi, L. Ascione Presence of internal FRP rebars acted as an effective confining system for cross sections composed by four blocks; Application of design equations by Italian CNR furnished conservative results for complete FRP wrapping, whereas predic-tion of strength for masonry confined with CFRP strips showed a reduced scatter with respect to experimental results. Fig. 4. Specimens after failure: 4.4 Task 8.4: the strengthening in flexure and in shear of RC structural elements with FRP fabrics and near surface mounted (NSM) rods. In this task has been investigated a new technique for the shear and flexure strengthening of RC structural elements by means of near surface mounted rods (NSM). Starting from an analytical and experimental investigation of the bond between NSM bars and concrete, an analytical and experimental studied has been carried out in order to evaluate the shear strength contribution provided by a system of NSM FRP bars. An experimental campaign has been also performed on full scale prestressed concrete beams strengthened by means of CFRP laminate in order to investigate on ultimate flexural capacity, taking particular attention to the laminates anchoring systems. A calculation procedure suitable for practitioners has been developed by simplifying a more sophisticated predictive model recently developed (Bianco 2008; Bianco et al. 2009a-b). That procedure briefly consists of: a) evaluating the average structural system composed of the average-available-bond-length NSM strip confined to the corresponding concrete prism whose transversal dimensions are limited by the spacing between adjacent strips and the beam cross section width (Figure 1); b) determining the comprehensive constitutive law of the average system above (Figure 2); c) determining the maximum effective capacity that the average system can attain during the loading process of the strengthened RC beam by imposing a kinematic mechanism and d) determining the NSM shear strength contribution by summing the contribution provided by each strip. The constitutive law (Figure 2) and in turn the equations to determine the maximum effective capacity assume different features depending on the main phenomenon characterizing the ultimate behaviour of the average structural system of the specific case at hand. Hereinafter, for the sake of brevity, the main features of that computational procedure are shown only for the case of shallow concrete fracture ( u = 4) and a resulting resisting bond length whose value is equal to the effective bond length (Figure 3). Further details can be found elsewhere (Bianco 2008).

17 Innovative materials for the vulnerability mitigation of existing structures 17 The predictions obtained by that calculation procedure were also appraised on the basis of experimental results (e.g. Dias et al. 2007). τ τ 0 Beam flange a) L δ ( t d Li n) Z O θ β ξ 0.0 CDC plane x fi softening friction b) L fi γ ( t n ) s f X free slipping δ 1 δ s t h w b w CDC plane 2 FRP strip concrete fracture surface α α θ β s f FRP strip B A A l X i α B s f L f l i θ O β δ Li L tr, fi c) d) view A-A FRP strip b w CDC plane 2 α view B-B Figure 1. Main features of the calculation procedure: a) average-length NSM strip and concrete prism of influence, b) adopted local bond stress slip relationship, c) NSM strip confined to the corresponding concrete prism of influence and semi-pyramidal fracture surface, d) sections of the concrete prism. ( ; ) V L δ fi Rfi Li ( ; ) V L δ fi Rfi Li ( ; ) V L δ fi Rfi Li u = 1oru = 2 δ Lu u L Rfi u = 4 = Rfu tr1 LRfu L = 3 tr1 < L tr1 u = 5 L > L u 6 δ = ; L Lu Rfu δ Lu δ 1 a) δ Li δ 1 b) δ Li c) δ 1 δ Li Figure 2. Possible comprehensive constitutive law of an NSM CFRP strip confined within a prism of concrete: (a) concrete that reaches the free extremity ( u = 1 ) or strip tensile rupture ( u = 2 ), (b) superficial and/or absent concrete fracture and ultimate resisting bond length smaller ( u = 3 ), equal ( u = 4 ) or larger ( u = 5 ) than the effective bond length and (c) deep concrete fracture ( u = 6 ). ( ; ) V L δ fi Rfu Li L = L Rfu tr1 V ( γ ξ ) fi, CDC ; γ = 1.5 γ 4 3 γ 4 γ 3 γ 2 γ γ 1 δ Li ( γξ ; ) γ = 1.5 γ 4 3 V fi, eff ( γ ) ( ) max fi, eff = fi, eff f,max V V γ δ L3 γ 3 δ L1 γ = γ 1 2 δ L2 δ L1 δ L 3 a) δ Li ξ = 0 b) ξ = L d ξ = 0 Figure 3. Maximum effective capacity along the CDC for the case u = 4 : a) comprehensive constitutive law ; b) capacity Vfi, CDC ( γ ; ξ ) and c) imposed end slip δ Li, CDC ( γξ ; ) distribution along the CDC for different values of the CDC opening angle γ and d) effective capacity as function of the angle γ. c) ξ = L d 0.5 γ 1 γ 1 γ f,max γ 3 d) γ

18 18 G. Manfredi, L. Ascione The maximum effective capacity for the case of shallow concrete fracture and a resulting resisting bond length whose value is equal ( u = 4) to the effective bond length can be evaluated by: sf max 1 sf 2 AC 2 2 ( ) ( ) ( ) 2 π Vfi, eff = A1 C1 Ld γ f,max arcsin 1 A3 γ f,max Ld 1 A3 γ f,max Ld 1 1 A3 γ f,max L + + d Ld 2 A3 γ f,max 2 (1) where: 3 Lp J3 λ sin( θ+ β 2 ) λ sin A1 = ; A2= Lp J3 λ ; ( θ+ β) sf τ0 J1 sf τ0 J1 A3 = ; C1 = δ1 ; C 2 2 = 2 (2) 2 τ J λ λ 4 τ J δ1 γ f,max = γ1 = (3) L d sin ( θ + β ) Actual V f and design value V fd of the NSM shear strength contribution can be obtained as follows: 0 1 V 1 1 l ( max fd = V f 2 N f,int V fi, eff s inβ ) γ = γ (4) Rd Rd where γ Rd is the partial safety factor divisor of the capacity that can be assumed equal to according to the indeterminateness of the input parameters. Bond between NSM bars and surrounding concrete: experimental and analytical investigation Pull-out test were carried out to investigate both the qualitative and quantitative influence of some of the involved parameters on the bond performance (De Lorenzis and Galati 2006, Galati and De Lorenzis 2006). Those parameters encompass: ratio between depth and width of the slit, kind of epoxy-based adhesive used as binding agent, distance of the NSM bar from the edge of the concrete prism, distance between adjacent bars and employment of external FRP strips used to confine the joint. Tests were carried out by means of a tangential-pull device to apply the load, LVDT transducers to measure the slip at both the loaded and unloaded extremity and strain-gauges throughout the adhered length of the bar to measure the deformations along the joint. The measured quantities were processed to obtain the local bond stress-slip relationship for the different values of the test parameters. Cyclic tests were also carried out subjecting the joint at a limited number of cycles whose maximum load was assumed equal to different percentages of the peak static load. The cyclic tests were useful to evaluate the joint residual strength such as the one following a seismic action. An analytical investigation has followed the experimental program above (Rizzo and De Lorenzis b). In fact, the local bond stress-slip relationship obtained in the pull-out tests has been modelled by suitable analytical functions whose unknowns were calibrated for the different values of the test parameters. The local bond stress-slip relationship obtained by the cyclic tests was also modelled by analytical functions. Then, the numerical solution of the governing differential equation has allowed the peak pull-out load be determined as function of the available bond length. The pull-out tests were also simulated by a FE model, both in the Linear and Non Linear range. The Linear FE model was adopted to evaluate the bond-induced stresses on a plane transversal to the bar, evaluating the maximum stresses for different values of the geometrical and mechanical parameters of the joint and estimating so, local tangential stress inducing the first-cracking in both resin and concrete. After that, a Non Linear model was developed by modelling: a) the several materials according to the fracture mechanics and b) concrete/adhesive and adhesive/frp interfaces by employing interface elements.

19 Innovative materials for the vulnerability mitigation of existing structures 19 Experimental and analytical investigation on the shear strengthening contribution provided to a RC beam by a system of NSM FRP bars Four points bending tests were carried out on RC beams strengthened in shear by NSM FRP bars (De Lorenzis and Rizzo 2006, Rizzo and De Lorenzis a). Those beams were designed in such a way that the theoretical failure mode, for both the strengthened and unstrengthened beams was due to shear-tension. Parameters investigated were: spacing, type and inclination of the NSM bars and the shear-span-to-depth ratio. Some beams strengthened by NSM strips were also tested in order to asses the relative effectiveness of the two techniques. The system of FRPs was extensively equipped to measure the deformations in the bars crossing the CDC. Tests have highlighted the possibility of a global failure modes consisting in the detachment of the strengthened cover from the underlying beam core. Such mechanisms had not been pointed out by previous investigations. Two models were developed to predict the NSM shear strength contribution: a) one more simplified and b) a more sophisticated one. The former was based on the Mörsch truss and the employment of a perfectly plastic local bond stress-slip relationship. The latter takes into account a more realistic local bond stress-slip relationship and the interaction between existing steel stirrups and NSM bars. The different local bond stress-slip relationships obtained in the former phase of the investigation were employed to carry out some comparison. From those comparisons it was possible to point out the great importance of the fracture energy as opposed to the shape of the local bond stress-slip relationship. This phase of the investigation has led to the development of useful formulae for the evaluation of the NSM FRP shear strengthening contribution to a RC beam. Experimental investigation on full-scale prestressed concrete beams strengthened by means of CFRP Every year, several prestressed concrete (PC) bridge girders are accidentally damaged by over-height vehicles or construction equipment impact. Although complete replacement is sometimes deemed necessary, repair and rehabilitation can be far more economical, especially when the time and the social cost of the method are drastically reduced. The numerous advantages provided by the use of FRP laminates are leading in a sharp increase on their use for bridge construction strengthening. Experimental investigations were conducted in order to validate such strengthening technique on PC damaged members and accurately assess the upper limit of damage amount beyond which FRP laminates are no longer adoptable as repair solution. Starting from such purposes, an experimental campaign was conducted on five full-scale (13.0m long, 1.05m high) PC double T-beams with a reinforced concrete slab, designed according to ANAS (Italian Transportation Institute) standard specifications. One beam was used as control, and the other four were intentionally damaged in order to simulate a vehicle impact by removing the concrete cover and by cutting a different percentage of tendons (17% on two specimens and 33% on the remaining two). The repair, by using externally bonded carbon FRP (CFRP) laminates, aimed at restoring the ultimate flexural capacity of the member, taking particular attention to the laminates anchoring system. In particular, one test was performed on the control beam (referenced as S1), two tests were carried out on intentionally pre-damaged, to simulate an over-height vehicle collision, beams (named S2 and S3, respectively) and the remaining two on pre-damaged specimens upgraded by using two and three plies of CFRP laminates anchored by using U-wraps (named S4 and S5, respectively). In Figure xxx and Figure xxx the test setup and experimental load deflection curves are reported.

20 20 G. Manfredi, L. Ascione Steel Reaction Frame PC Beam Support Strong Floor 0,5 m 5,4 m 0,6 m 0,5 m 6 m Figure xxxx- Test setup P [kn] 1000 S5 S4 S S3 S f [mm] Figure xxxx Experimental load deflection curves. The experimental study has shown that: 1) a loss of strands equal to 17% and 33% caused a flexural capacity decrease equal to 20% and 26%, respectively; 2) to restore the ultimate flexural capacity of the undamaged PC specimen by using CFRP laminates it is necessary to prevent fibers debonding; 3) U-wraps (width w f = 100mm spaced at p f =150mm) were able to significantly delay debonding but if damaged existing concrete is patched by cementitious mortar, a perfect bond has to be guaranteed during the cross section restoration to prevent localized debonding of longitudinal reinforcement and thus fully exploit the potential effective FRP strain increase; 4) CFRP laminates increased both stiffness and flexural moment capacity of PC damaged beams (maximum moment recover equal to about 12% and 20% for specimens with 17% and 33% of strands loss, respectively; 5) the strengthening intervention led to weak failure mode with a global ductility loss. The experimental outcomes qualify the application of FRP technique, already adopted in several cases of impacted PC bridges, as an effective tool to restore the flexural capacity of PC girders; however the calibration of theoretical expressions for the computation of the