Experimental investigation of substandard RC columns confined with SRG jackets under compression

This paper aims to explore the behaviour of substandard reinforced concrete (RC) columns confined with Steel-Reinforced Grout (SRG) jackets under monotonically increasing uniaxial compression. A total of 24 specimens of short RC columns of square cross section were designed to fail due to longitudinal reinforcement buckling. Single-layered SRG jackets were applied to 18 of these specimens, whereas the rest served for control without SRG jackets. Parameters of this investigation were the type and density of the steel fabric as well as the corner radius of the cross section. The employed SRG jacketing managed to increase the strength and strain capacity and postpone the buckling of the longitudinal steel bars to occur at higher compressive strain level. Confinement effectiveness with respect to the lateral confining pressure exerted by the used SRG jacketing is discussed along with the observed mode of failure

Concrete confinement with steel-reinforced grout jackets

The potential of steel fiber reinforced jackets combined with inorganic matrix (cementitious grout matrix) as an alternative strengthening system to fiber-reinforced polymer (FRP) jackets was investigated experimentally in the current study. For this purpose, the novel jacketing device was applied on cylindrical specimens subjected to monotonic concentric uniaxial compression load. Parameters of investigation were the type of the steel fiber reinforced fabric, its density, the overlap length, and the concrete compressive strength. The 3X2 and 12X types of steel fabric were used with three alternative densities, characterized as high, medium and low density. Experimental evidence has shown that a single layer of SRG jacket has increased substantially both axial strength and deformation capacity. From the response of the SRG confined cylinders the degree of penetration of the grout matrix through the fabric as well as the overlap length are considered rather critical parameters for the effectiveness of the method. The experimental data were used for the derivation of a simple empirical confinement model which correlated well with other well-established FRP confinement models. The knowledge gained from this experimental study renders SRG jacketing a remarkably promising retrofit solution for reinforced concrete confinement

Shear upgrade of R/C beams utilizing CFRP or SFRP strips with or without anchorage under monotonic or cyclic loadings: experimental and numerical study

The present PhD thesis aims to investigate the overall behavior of externally strengthened R/C beams using open hoop strips of either CFRP or SFRP with or without the combination of an anchoring scheme. The strengthened beams are subjected to either monotonic or cyclic loadings. The aforementioned investigation focuses on the use of a novel anchoring device developed at the Laboratory of Strength of Materials and Structures of Aristotle University of Thessaloniki and patented at Europe, USA and Canada.The basic investigated material is divided to four sections:1.From the synthetic evaluation of the extended experimental study the following summarised results have been arisen:SFRP strips can be used as an alternative material for strengthening applications.As long as the density of SFRP strips is increased the shear capacity of the strengthened beam is further increased. When an effective anchoring scheme is used the aforementioned remark lacks of importance.The use of an anchoring scheme removes the mode of failure from debonding to either FRP rapture or failure of the anchoring device itself.The influence of type of the imposed loading (monotonic vs. cyclic) is important especially when no anchoring device is utilized (QFRP,monotonic =1.7QFRP,cyclic). When an anchoring device is used the influence becomes less important (QFRP,monotonic =1.37QFRP,cyclic).The behaviour of pre-cracked shear strengthened R/C beams is similar to the behaviour of non-cracked initial beams.2.The proposed analytical model can predict the contribution of FRP strips (Vf,cal) of a shear strengthened R/C beam with accuracy. The ratio QFRP / Vf,cal varies between 0.92 and 1.173.The present PhD thesis is furthermore focusing on the development of a numerical 3D FE model utilizing the software ABAQUS. All non-linear behaviors are simulated (materials, modes of failure, anchoring device…). The comparison between numerical results and experimental results proved to be very good. As a result a tool has been developed to accurately predict the overall shear behavior of a strengthened R/C beam.4.The fourth and last section of the present PhD thesis is focusing on the prediction of the shear behavior of strengthened R/C beams utilizing three international design codes (ACI440-R2, Eurocode8-3, KAN.EPE). The experimental results are compared with the codes’ prediction. When a strengthened R/C beam is subjected to cyclic loadings, a reduction factor should be adopted. Finally all codes should adopt a procedure to safely design an anchoring device. At least the basic equation Qdesign ≤ QFRP,strip ≤ Qancchoring should be proposed.Γενικό στόχο της παρούσας διατριβής αποτελεί η διερεύνηση της συμπεριφοράς συστήματος διατμητικής ενίσχυσης με ανοιχτές λωρίδες ινοπλισμένων πολυμερών τόσο από ίνες άνθρακα (ΑΙΟΠ) όσο και από ίνες χάλυβα υψηλής αντοχής (ΧΙΟΠ), με ή χωρίς αγκυρώσεις όταν η ενίσχυση αυτή εφαρμόζεται σε δοκούς Ο/Σ που υποβάλλονται είτε σε μονότονες είτε σε ανακυκλιζόμενες φορτίσεις. Στα πλαίσια του ανωτέρω γενικού στόχου διερευνήθηκε σε βάθος η εν λόγω συμπεριφορά σε συνδυασμό με μια καινοτόμο διάταξη αγκύρωσης που αναπτύχθηκε για αυτόν τον σκοπό και κατοχυρώθηκε σε Ευρώπη, Αμερική και Καναδά στο Εργαστήριο Πειραματικής Αντοχής των Υλικών και των Κατασκευών υπό τη διεύθυνση του Καθηγητή Γεωργίου Χ. Μάνου.Το βασικό υλικό της διερεύνησης χωρίζεται σε 4 ενότητες: 1. Από την συνθετική επεξεργασία όλων των πειραματικών αποτελεσμάτων προκύπτουν τα εξής βασικά συνοπτικά συμπεράσματα:•Οι λωρίδες ΧΙΟΠ αποτελούν αξιόπιστη εναλλακτική λύση χρήσης υλικού διατμητικής ενίσχυσης.•Όσο πιο πυκνές είναι οι λωρίδες ΙΟΠ τόσο πιο αυξημένη προκύπτει η φέρουσα ικανότητα σε τέμνουσα. Στην περίπτωση χρήσης αποτελεσματικής αγκύρωσης η πυκνότητα των λωρίδων παύει να έχει κυρίαρχη σημασία. •Η χρήση διατάξεων αγκύρωσης σε συνδυασμό με λωρίδες ΙΟΠ μεταφέρει τη μορφή αστοχίας από την αποκόλληση των λωρίδων ΙΟΠ (χωρίς αγκύρωση) στην θραύση των λωρίδων ή στην αστοχία της διάταξης αγκύρωσης. •Η επιρροή του τύπου της φόρτισης είναι σημαντική ιδιαίτερα στην περίπτωση απουσίας αγκύρωσης (QΙΟΠ,μονότονης =1.7QΙΟΠ,ανακύκλ.). Στην περίπτωση χρήσης αγκύρωσης η επιρροή αυτή περιορίζεται (QΙΟΠ,μονότονης =1.37QΙΟΠ,ανακύκλ.) •Η συμπεριφορά των προ-ρηγματωμένων δομικών στοιχείων δοκών Ο/Σ δεν επηρεάζεται από την ύπαρξη των προϋφιστάμενων διαγώνιων ρηγματώσεων λόγω προηγούμενης φόρτισης των εν λόγω δομικών στοιχείων.2. Οι εκτιμήσεις της φέρουσας διατμητικής ικανότητας που συνεισφέρουν οι λωρίδες ΙΟΠ μέσω του αναλυτικού υπολογισμού (Vf,cal) έχουν αρκετά ικανοποιητική σύγκλιση με τις αντίστοιχες πειραματικές τιμές (QIOΠ), με τον λόγο QIOΠ / Vf,cal να παίρνει τιμές από 0.92 έως 1.17.3. Στα πλαίσια της παρούσας διατριβής γίνεται προσπάθεια πιστοποίησης της αριθμητικής προσομοίωσης της παρατηρηθείσης πειραματικά μετρημένης συμπεριφοράς με τη βοήθεια του προγράμματος ABAQUS. Στα πλαίσια της παρούσας διατριβής επιβεβαιώνεται η καταλληλότητα χρήσης ενός αξιόπιστου αριθμητικού εργαλείου πρόβλεψης της διατμητικής συμπεριφοράς δοκών Ο/Σ ενισχυμένων διατμητικά με ανοιχτές λωρίδες ΙΟΠ, είτε απλά επικολλημένες είτε αγκυρωμένες, ως ένα σύστημα διατμητικής ενίσχυσης των εν λόγω δοκών.4. Τέλος διερευνάται η ισχύς των προβλέψεων της συμπεριφοράς της εν λόγω διατμητικού τύπου ενίσχυσης μέσω τριών διεθνών κανονισμών: α) ACI440R2, β) EC8-3 και γ) ΚΑΝ.ΕΠΕ. Από την διεξοδική διερεύνηση αυτή τεκμαίρεται ότι η χρήση των κανονιστικών διατάξεων προβλέπουν εν γένει με ασφάλεια την διατμητική φέρουσα ικανότητα των λωρίδων ΙΟΠ (cVf,cal). Κρίνεται απαραίτητη η υιοθέτηση επιπρόσθετου συντελεστή ασφαλείας στην περίπτωση που ένα δομικό στοιχείο υποβάλλεται σε ανακυκλιζόμενη σεισμικού τύπου φόρτιση. Τέλος οι κανονιστικές διατάξεις που προβλέπουν την εκτίμηση της φέρουσας διατμητικής ικανότητας συστήματος διατμητικής ενίσχυσης με την χρήση ανοιχτών λωρίδων ΙΟΠ σε συνδυασμό με αγκυρώσεις προτείνεται να απαιτούν τον κατάλληλο σχεδιασμό και την πιστοποίηση της αποτελεσματικότητας του συστήματος αγκύρωσης για το φορτίο που καλείται να παραλάβει. Προτείνεται τουλάχιστον η υιοθέτηση της γενικής αρχής σχεδιασμού: QΙΟΠ ≤ Qλωριδων ≤ Qαγκύρωση

The Role of Scaffolds in Tendon Tissue Engineering

Tendons are unique forms of connective tissue aiming to transmit the mechanical force of muscle contraction to the bones. Tendon injury may be due to direct trauma or might be secondary to overuse injury and age-related degeneration, leading to inflammation, weakening and subsequent rupture. Current traditional treatment strategies focus on pain relief, reduction of the inflammation and functional restoration. Tendon repair surgery can be performed in people with tendon injuries to restore the tendon’s function, with re-rupture being the main potential complication. Novel therapeutic approaches that address the underlying pathology of the disease is warranted. Scaffolds represent a promising solution to the challenges associated with tendon tissue engineering. The ideal scaffold for tendon tissue engineering needs to exhibit physiologically relevant mechanical properties and to facilitate functional graft integration by promoting the regeneration of the native tissue

Seismic Retrofit of R/C T-Beams with Steel Fiber Polymers under Cyclic Loading Conditions

This paper presents results of an experimental study on seismic response of reinforced concrete (RC) T-beams with shear deficiencies strengthened with externally bonded steel fiber reinforced polymer (SFRP) strips. Seven cantilever RC beams were strengthened with externally bonded uniaxial SFRP strips in a U-shape configuration and were tested under cyclic loading conditions. The two main variables examined were the strip spacing and the use of anchoring system. Among the investigated anchoring systems, one was patented, and it is studied in the present manuscript. The examination of the results leads to the conclusion that the anchoring system has a significantly more pronounced effect on the performance of the beams and the mode of failure than the type or spacing of the strips. Furthermore, SFRP strips seem to have a great potential to be used for shear strengthening, especially since the use of mechanical anchoring systems drastically improves their performance. On the contrary, the lack of mechanical anchoring results in premature delamination of the strengthening system, and thus an undesirable SFRP material performance

Reinforced Concrete Beams Retrofitted with External CFRP Strips towards Enhancing the Shear Capacity

The practical difficulties in upgrading the structural performance of existing reinforced concrete (RC) structures is discussed, when retrofitting structural members by conventional RC jacketing. The use of retrofitting schemes employing externally applied fiber reinforcing polymer (FRP) strips attracted considerable research attention as a preferable alternative. Such retrofitting FRP schemes aiming to upgrade the shear capacity of existing RC beams have been examined in many published works employing such externally applied FRP shear reinforcing schemes without confronting the practical difficulties arising from the presence of the RC slab. Anchoring external CFRP strips aiming to shear upgrade, which is the focus here, overrides this difficulty. It is shown that effective anchoring, using either mechanical anchors such as the ones devised by the authors or CFRP anchor ropes produced by the industry, can effectively upgrade the shear capacity of an RC T-beam under-designed in shear to the desired level. A novel laboratory test set-up, devised by the authors, can be utilized to quantify the tensile capacity of CFRP stirrups with or without anchors, that can be of practical use. The predicted, according to design guidelines, upgraded shear capacity of the tested prototype RC T-beam, employing the used shear retrofitting schemes, under-estimates the measured shear capacity by 58%. This conservatism can counter-balance uncertainties arising from in situ conditions in constructing the various parts of such a shear retrofitting scheme

From Three-Dimensional (3D)- to 6D-Printing Technology in Orthopedics: Science Fiction or Scientific Reality?

Over the past three decades, additive manufacturing has changed from an innovative technology to an increasingly accessible tool in all aspects of different medical practices, including orthopedics. Although 3D-printing technology offers a relatively inexpensive, rapid and less risky route of manufacturing, it is still quite limited for the fabrication of more complex objects. Over the last few years, stable 3D-printed objects have been converted to smart objects or implants using novel 4D-printing systems. Four-dimensional printing is an advanced process that creates the final object by adding smart materials. Human bones are curved along their axes, a morphological characteristic that augments the mechanical strain caused by external forces. Instead of the three axes used in 4D printing, 5D-printing technology uses five axes, creating curved and more complex objects. Nowadays, 6D-printing technology marries the concepts of 4D- and 5D-printing technology to produce objects that change shape over time in response to external stimuli. In future research, it is obvious that printing technology will include a combination of multi-dimensional printing technology and smart materials. Multi-dimensional additive manufacturing technology will drive the printing dimension to higher levels of structural freedom and printing efficacy, offering promising properties for various orthopedic applications

Influence of the cross section shape on the behaviour of SRG-confined prismatic concrete specimens

The effectiveness of the steel-reinforced grout (SRG) jacketing technique in increasing both strength and deformation capacity has been substantiated by experimental evidence [1-3]. What it has not been addressed yet is the influence of the cross section shape on the behaviour of SRG-confined prismatic unreinforced concrete specimens. An experimental study was carried out where 18 concrete small scale specimens were tested to failure under concentric uniaxial compression load. A single layer of the steel-reinforced fabric was applied to circular, square and square specimens with rounded edges. Test results demonstrated that both strength and deformation capacity increased as the shape of the cross section changed from square to circular. In case of the square specimens, the gain in compressive strength was satisfying, whereas the ductility increase was more significant compared to that of the square specimens with rounded edges. An analytical expression for the lateral confining pressure exerted by the composite system was derived based on the assumption that the steel cords were treated as well-anchored steel stirrups placed at a distance equal to the spacing between the steel cords. The ability of the various steel and fiber-reinforced polymer confinement models from literature to predict the normalized compressive strength of non-circular specimens confined with SRG jackets was also explored

Three-Dimensional-Printed Scaffolds for Meniscus Tissue Engineering: Opportunity for the Future in the Orthopaedic World

The meniscus is a critical component of a healthy knee joint. It is a complex and vital fibrocartilaginous tissue that maintains appropriate biomechanics. Injuries of the meniscus, particularly in the inner region, rarely heal and usually progress into structural breakdown, followed by meniscus deterioration and initiation of osteoarthritis. Conventional therapies range from conservative treatment, to partial meniscectomy and even meniscus transplantation. All the above have high long-term failure rates, with recurrence of symptoms. This communication presents a brief account of in vitro and in vivo studies and describes recent developments in the field of 3D-printed scaffolds for meniscus tissue engineering. Current research in meniscal tissue engineering tries to combine polymeric biomaterials, cell-based therapy, growth factors, and 3D-printed scaffolds to promote the healing of meniscal defects. Today, 3D-printing technology represents a big opportunity in the orthopaedic world to create more specific implants, enabling the rapid production of meniscal scaffolds and changing the way that orthopaedic surgeons plan procedures. In the future, 3D-printed meniscal scaffolds are likely to be available and will also be suitable substitutes in clinical applications, in an attempt to imitate the complexity of the native meniscus