The present paper deals with numerical analysisof post cracking behaviour of Steel FiberReinforced Concrete (SFRC) beams in bending.Starting from experimental results of four pointbending tests (4BPT), made on notched beams,the scope of the work was the determination ofmaterial tensile constitutive relationship by meansof numerical modelling ("inverse analysis"). Thenumerical analyses were carried out by using thesoftware TNO Diana 9.4.2.Two different modelswere developed to analyse the post crackingbehaviour of SFRC notched beam: the first wasbased on the discrete crack approach and thesecond on the smeared crack approach. Theresults of the two analysis were than compared.The constitutive material relationship proposed byCNR DT 204-2006 was also analysed using boththe models afore mentioned. Results of thenumerical analysis are presented and discussed

It is well known that fibers are effective in modifying the cracking pattern development of concrete structural element, causing an higher number of cracks and, consequently, lower crack spacing values and narrower crack widths compared to the matrix alone. This effect could be exploited to improve durability of Reinforced Concrete (RC) structures, especially of those exposed to aggressive environments. The analytical prediction of crack width and spacing in Fiber Reinforced Concrete (FRC) structural elements in bending is still an open issue. A crack width relationship for RC elements with fibers similar to those developed for classical RC structural members would be desirable for designers. The recent development of important technical design codes, such as RILEM TC 162 TDF and the new MC2010, embrace this idea. However further validation of these models by experimental results are still needed. On the other hand, the study of the influence of a sustained load on crack width in presence of the fiber reinforcement is a topic almost unexplored and important at the same time. In the present work, the cracking behaviour of full-scale concrete beams reinforced with both traditional steel bars and short fibers has been analyzed under short and long term flexural loading. A theoretical prediction of crack width and crack spacing was carried out according to different international design provisions. The analytical results are discussed and compared in order to highlight the differences between the models and to check the reliability of the theoretical predictions on the basis of the experimental data.

The use of short fibers inside concrete matrix is an effective method for reducing the vulnerability of concrete constructions subjected to harsh environment. The action of the short fibers in reducing the crack opening is the main issue that needs a research effort in order to optimize the expected results. At the moment the analytical prediction of the crack width and spacing in fiber reinforced concrete (FRC) structural elements under bending loads is still an open problem. A crack width relationship for FRC/RC elements similar to those developed for plain concrete structural members would be desirable for designers and engineers involved in the design of FRC structural elements. The recent development of important technical design codes, such as RILEM TC 162 TDF and the new Model Code (MC) 2010, embrace this idea. However further validation of these models by experimental results is still needed. On the other hand the study of the influence of a sustained load on crack width in presence of a short fibers reinforcement is a topic almost unexplored and important at the same time. In this research the cracking behaviour of full-scale concrete beams reinforced with both traditional steel bars and short fibers has been analyzed under short and long term bending condition. A theoretical prediction of crack width and crack spacing was carried out according to international design provisions based on different analytical models. The theoretical results are discussed and compared in order to highlight the differences between the available models and to check the reliability of the theoretical predictions on the basis of the experimental data. A modified relationship to take into account of the presence of stirrups has been proposed on the basis of experimental results; furthermore, some critical aspects, such as the influence of the type of fibers and the effect of loading-time, have been underlined that should be addressed in future research work.

Corrosion of steel reinforcement is one of the principal causes of deterioration in concrete structures. Corrosion mechanisms are accelerated by the presence of cracks which represent a preferable way for ingress of water and aggressive agents. Fibers embedded into concrete matrix generally reduce the crack widths of ordinary reinforced concrete structural elements and, hence, may improve durability of reinforced concrete structures, especially of those exposed to aggressive environments. To date, quantitative results on the effect of fibers on durability of cracked RC members under service loads is still lacking in the literature. The present research aims at studying the influence of fibers on durability of RC beams exposed in a coastal area under sustained load. Durability tests were performed in order to assess the influence of the presence of fibers on carbonation depth and chloride content.

Il tema della durabilità delle opere in calcestruzzo armato è oggi di grande attualità: il costo elevato connesso alla manutenzione e riparazione delle strutture esistenti è diventato un problema non solo economico e sociale ma anche di sostenibilità ambientale per l'intero settore delle costruzioni. La durabilità strutturale migliora nella misura in cui si impedisce alle sostanze aggressive l'ingresso nella matrice di calcestruzzo. Ciò è possibile agendo su due fronti: il primo è rappresentato dalla porosità interna del calcestruzzo che può essere ridotta abbassando il rapporto acqua/cemento; il secondo è rappresentato dalle fessure che rappresentano una via preferenziale per raggiungere le parti più interne della struttura.Una possibile soluzione potrebbe essere quella di investire nell'impiego di nuovi materiali progettati ad hoc per migliorare la durabilità degli elementi strutturali e dare maggiori prospettive di vita utile, soprattutto a quelle opere esposte in ambienti particolarmente aggressivi.In quest'ambito, l'impiego di calcestruzzi fibrorinforzati (FRC- Fiber Reinforced Concrete) può risultare di grande utilità. Infatti, l'aggiunta di fibre all'impasto di calcestruzzo consente di migliorarne le caratteristiche di tenacità: le fibre, agendo come "crack arrestors", limitano l'ampiezza delle fessure, ingresso preferenziale di agenti aggressivi che causano la corrosione delle armature.I codici normativi vigenti prevedono una limitazione dell'ampiezza delle fessure che, soprattutto per classi di esposizione ambientale severe, costringono il progettista a variare parametri come la percentuale di armatura e le dimensioni delle sezioni, al fine di soddisfare i requisiti imposti. L'impiego delle fibre consentirebbe una riduzione dell'ampiezza delle fessure senza variare parametri progettuali significativi, che comporterebbero un aggravio dei costi della costruzione ed un condizionamento architettonico maggiore.Sul mercato sono oggi disponibili diversi tipi di fibre: variando materiali e dimensioni, è possibile avere un controllo della fessurazione a diversi livelli. Fibre flessibili e corte consentono una riduzione della micro-fessurazione causata dal ritiro, mentre fibre lunghe e ad elevata rigidezza consentono di limitare la macro-fessurazione. Entrambi i fenomeni possono essere modulati utilizzando FRC di tipo ibrido, cioè che impiegano cioè più tipi di fibre contemporaneamente. Il documento CNR DT 204-2006 [1] rappresenta oggi un valido rifermento normativo in Italia per l'impiego degli FRC per usi strutturali.L'Università del Salento e di Brescia hanno condotto sinergicamente, insieme al Laboratorio Italcementi di Mesagne (BR), un'attività sperimentale rivolta allo studio dell'impatto delle fibre sulla durabilità di travi armate esposte in ambiente marino. Per fare questo sono state realizzate 10 travi armate e sottoposte ad un carico di lunga durata, allo scopo di simulare le condizioni di esercizio dell'elemento all'interno d

The positive effect of fibers on the bond of reinforcing bars in concrete is widely recognized and supported in literature; on the contrary information are not available on recycled steel fiber reinforced concrete (RSFRC). The experimental work discussed in this paper represents a part of a wider analysis, performed by the authors, on the mechanical performance of RSFRC. In particular the main objective is to investigate on the bond behaviour between steel bar and recycled steel fiber (from waste tires) reinforced concrete. To this aim eccentric pull-out test on prismatic samples were designed varying the type of fiber (recycled and new industrial steel fibers) and the concrete cover-bar diameter ratio; in addition similar tests were carried out on plain concrete for comparison purpose. The experimental data in terms of peak bond stress, mode of failure and bond stress-slip curves are analysed and discussed. The good bond performance of specimens realized with recycled steel fiber reinforced polymer is evidenced in comparison with those realized with both plain and industrial fiber reinforced concrete.

Mechanical behaviour of fibre-reinforced concrete is influenced both by the properties of the fibres (geometry, aspectratio, dosage) and the properties of the matrix (concrete grade, curing time, water-to-cement ratio). Many researchstudies have been published that focus on the influence of different kinds of fibres on the final properties of concrete.Against this the influence of the matrix grade on the mechanical properties of fibre-reinforced concrete is a topic thatis almost unexplored. A mechanical characterisation of fibre-reinforced concrete in the hardened and fresh states iscarried out, varying the concrete grade, the fibre dosage and the fibre type (steel and polyester). The results of theexperimental research indicate the importance of the matrix grade on the bond between the steel fibre and the matrixand, consequently, on the mechanical performances of the composite material. Furthermore the concrete grade also; influences the minimum volume fraction of polyester fibres affecting the matrix toughness.

The positive effect of fibers on the bond of reinforcing bars in concrete is widely recognized and supported in literature; on the contrary information are not available on recycled steel fiber reinforced concrete. The experimental work discussed in this paper represents a part of a wider analysis, performed by the authors, on the mechanical performance of RSFRC. In particular the main objective is to investigate the bond behavior between steel bar and recycled steel fiber (from waste tires) reinforced concrete. To this aim eccentric pull-out test on prismatic samples were designed varying the type of fiber (recycled and industrial steel fibers) and the concrete cover-bar diameter ratio; in addition similar tests were carried out on plain concrete for comparison purpose. The experimental data in terms of peak bond stress, mode of failure and bond stress-slip curves are analyzed and discussed evidencing the good bond performance of specimens realized with recycled steel fiber reinforced concrete compared with both those realized with plain and industrial fiber reinforced concrete.

Many of the properties of Fiber Reinforced Concrete (FRC) may be used to improve performances of concrete flexural members reinforced with conventional steel bars (RC members). It is well known that fibers, embedded into a concrete matrix, increase its tensile performances and enhance its ductility and toughness, acting as crack arrestors. In particular, fibers are effective in modifying crack propagation causing an higher number of cracks and, consequently, lower crack spacing and a smaller crack widths, as compared to the matrix alone. This effect could be exploited to improve durability of reinforced concrete structures, especially of those exposed to aggressive environments. Few results are present in the literature on durability of full scale FRC structural element. A quantitative correlation between the effect of fibers on crack width and deterioration of RC members exposed to aggressive agents, represents an urgent need for designers. Furthermore, the effect of fibers on cracking has generally been studied considering short term loading test; thus, few data are available on cracking behavior of FRC members under long term loading.In the present research, the influence of fibers on durability of ordinary reinforced concrete beams has been investigated. In particular, beams with and without fibers have been exposed for seventeen months to natural weathering, in a coastal zone, under a sustained load. The influence of long term loading on the cracking behavior and, consequently, on structural durability, has been analyzed. An analytical prediction of the crack width according to Eurocode 2, Model Code 2010 and RILEM TC 162, is proposed. A comparison between experimental and theoretical values of crack width is shown.

Early applications of Fiber Reinforced Concrete (FRC) mainly concerned structural elements where the convenience of using short fibers was found in the possibility of substituting conventional reinforcement. However, FRC toughness can be conveniently introduced into the engineering practice with another perspective, to take advantage of the crack control for enhancing structural durability.

In this paper, the use of ultrasonic pulse velocity (UPV) testing as a reliable technique to determine the compressive strength of a calcarenitic stone typical of Salento (South of Italy), known as Lecce Stone (LS) has been investigated. The scope of the experimental research is to establish correlations between the results obtained by non-destructive and destructive tests, in order to reduce the use of destructive methods within the diagnostic procedures for the mechanical analysis and qualification of ancient masonries. Furthermore, the presence of water as a variable affecting the test was investigated. The results of the tests show that the UPV values are well correlated with the compressive strengths and this method showed to be efficient in predicting the strength of LS.

One of the major goals in the field of rehabilitation and renovation of existing structures is to determine mechanical properties of materials as well as their level of damage, namely the presence of defects, cracks, weathering effects, etc., by means of non-destructive (NDT) techniques. NDT tests, in fact, are easier and more economics than destructive ones because they do not necessitate sample extraction and preparation; furthermore they are often the unique way to assess the material properties in case of historic and architectural buildings, where the possibility of extracting core samples is limited or not possible. The ultrasonic pulse velocity testing has been proved to be a useful and reliable non-destructive test for assessing the compressive strength and the elastic modulus of concrete in existing structures. Furthermore, the use of both ultrasonic tests and Schmidt hammer tests allow to have a good estimation of concrete compressive strength (SONREB method) by reducing the influence of the variables affecting the two technique when used alone. Both the technics have also been suggested to investigate mechanical and physical properties of rocks, but further experimental data are needed to confirm the reliability of the method. The present work is a part of a wider research aimed at set up non-invasive diagnostic procedures for the mechanical analysis and qualification of the ancient masonries; it is specifically devoted to verify the effectiveness and/or to point out critical aspects and limits of the above mentioned non-destructive tests - already applied in the field of concrete and compact stones -with reference to the characterization of soft stones.

A deep knowledge of the physical and mechanical properties of the constituent materials of ancient masonries is of crucial importance in the choice of the proper intervention techniques. In case of historical buildings sustainable diagnostic procedures responding to the conservation constraints, should have the lowest degree of intrusion and the fullest respect for their physical integrity. The extraction of samples from existing structures for laboratory tests is one of the major problems in the field of the diagnosis of ancient buildings and this has moved the scientific community to propose alternative non-destructive techniques to evaluate the mechanical and physical properties of building stones. In the present work, non-destructive (NDT) and destructive (DT) tests have been investigated as tools for assessing the compressive strength of "Lecce stone", a soft calcarenite used as traditional building material in the Southern Italy. Ultrasonic pulse velocity and Schmidt hammer test have been compared with standardized mechanical destructive tests on cubes in order to research correlations between the results. The final aim is to assess the reliability of the non-destructive investigated methods in describing the mechanical performance of the stone, limiting the use of destructive analyses on masonries. Compressive strength on microcores, reducing at least the intrusion of the sampling for mechanical test in laboratory conditions, was also determined and good correlation was found with the strength results obtained by the standard compressive test.

In the research work presented herein a study of the cracking behavior of fiberreinforced concrete (FRC) beams is illustrated. The beams were tested in bendingand were designed according to the guidelines of the National Research Council DT-204/2006, without stirrups in the constant moment region. This choice is due to thefact that consolidated experiences, reported in the literature, show how the presenceof transverse steel reinforcement generates a localization of cracks. In the presentstudy FRC with short steel and polyester fibers were investigated. The study wasdivided into three phases: 1) design of concrete-mix, 2) destructive tests on beams,and 3) analysis of results and analytical comparisons.Scientific investigations in the field of FRC provide information in order to develop anaffordable design approach for FRC beams in bending, therefore this research workaims to provide an in-depth knowledge in the field. A comparison with the recentanalytical models provided by the National Research Council and 2010 Model Codeof CEB-FIB is shown and commented.The case study presented here shows the great importance of the beneficial role exercisedby short fibers dispersed in the cement matrix in reducing cracking of reinforced concretebeams. In particular results obtained in absence of transverse reinforcement provide neworiginal data. Stirrups are known to act as a region of discontinuity in the cement hardenedpaste, creating a cracking opening zone at the interface.Bending tests were performed on 10 beams (2 per concrete mix), deflection atquarter and mid-span were measured, the deformations in the compressed concreteand in the tensioned steel rebars were also measured in correspondence of theincreasing applied load. In correspondence of five load levels the distance, the heightand the width of the cracks was also measured over the whole length of the beams,with particular attention to the constant moment region. The steel fibers demonstrated tobe more effective than respect to polyester fibers in terms of crack arrestors. Theexperimental data were also compared to the theoretical prediction obtained by applyingthe analytical models of CNR DT-204/2006 and Model Code 2010.

UPV as non-destructive technique can effectively contribute to the low invasive in situ analysis and diagnosis of masonry elements related to the conservation, rehabilitation and strengthening of the built heritage. The use of non-destructive and non- invasive techniques brings all the times many advantages in diagnostic activities on pre-existing buildings in terms of sustainability; moreover, it is a strong necessity with respect to the conservation constraints when dealing with the historical-architectural heritage.In this work laboratory experiments were carried out to investigate the effectiveness of ultrasonic pulse velocity (UPV) in evaluating physical and mechanical properties of Lecce stone, a soft and porous building limestone. UPV and selected physical-mechanical parameters such as density and uniaxial compressive strength (UCS) were determined. Factors such as anisotropy and water presence that induce variations on the ultrasonic velocity were also assessed. Correlations between the analysed parameters are presented and discussed.The presence of water greatly affected the values of the analysed parameters, leading to a decrease of UPV and to a strong reduction of the compressive strength. A discussion of the role of the water on these results is provided.Regression analysis showed a reliable linear correlation between UPV and compressive strength, which allows a reasonable estimation of the strength of Lecce stone by means of non-destructive testing methods such as the ultrasonic wave velocity. Low correlation between UPV and density was found, suggesting that other factors than density, related to the fabric and composition, also influence the response of the selected stone to the UPV. They have no influence on the UCS, that instead showed to be highly correlated with the packing density.

UPV as non-destructive technique can effectively contribute to the low invasive in situ analysis and diagnosis of masonry elements related to the conservation, rehabilitation and strengthening of the built heritage. The use of non-destructive and non-invasive techniques brings all the times many advantages in diagnostic activities on pre-existing buildings in terms of sustainability; moreover, it is a strong necessity with respect to the conservation constraints when dealing with the historical-architectural heritage. In this work laboratory experiments were carried out to investigate the effectiveness of ultrasonic pulse velocity (UPV) in evaluating physical and mechanical properties of Lecce stone, a soft and porous building limestone. UPV and selected physical-mechanical parameters such as density and uniaxial compressive strength (UCS) were determined. Factors such as anisotropy and water presence that induce variations on the ultrasonic velocity were also assessed. Correlations between the analysed parameters are presented and discussed. The presence of water greatly affected the values of the analysed parameters, leading to a decrease of UPV and to a strong reduction of the compressive strength. A discussion of the role of the water on these results is provided. Regression analysis showed a reliable linear correlation between UPV and compressive strength, which allows a reasonable estimation of the strength of Lecce stone by means of non-destructive testing methods such as the ultrasonic wave velocity. Low correlation between UPV and density was found, suggesting that other factors than density, related to the fabric and composition, also influence the response of the selected stone to the UPV. They have no influence on the UCS, that instead showed to be highly correlated with the packing density.

Many of the properties of Fiber Reinforced Concrete (FRC) may be used to improve performances of reinforced concrete (RC) flexural beams. It is well known that fibers embedded into concrete matrix, enhance its ductility and toughness, increase its tensile performances, acting as crack arrestors. In particular, fibers are effective in modifying crack propagation, causing an higher number of cracks and consequently lower crack spacing values and narrower crack widths compared to the matrix alone. This effect could be exploited to improve durability of reinforced concrete structures, especially into aggressive environments..In this work, an experimental research on the influence of short fibers on global and cracking behavior of conventionally reinforced concrete beams in bending, is reported. In particular, beams reinforced with steel and polyester fibers, together with ordinary steel reinforcement (bars and stirrups), has been tested in bending. The scope of this research is to investigate the role of the presence of fiber and the fiber type on the cracking behavior of the beams in terms of crack widths, crack length and crack spacing.