The need to satisfy aerospace industry's demand not met by traditional materials motivated researchers and scientists to look for new solutions. The answer was found in developing new material systems by combining together two or more constituents. Composites, also known as fiber reinforced polymers (FRP) consisting of a reinforcing phase (fibers) embedded into a matrix (polymer), offered several advantages with respect to conventional materials. High specific modulus and strength together with other beneficial properties, corrosion resistance and transparency to electrical and magnetic fields above all, made FRP also suitable for use as construction materials in structural engineering. In the early years of the twenty-first century, the publication by the American Concrete Institute (ACI) of design guidelines for the use of FRP as internal reinforcement and for external strengthening of concrete members accelerated their implementation for structural engineering applications. To date, FRP have gained full acceptance as advanced materials for construction and their use is poised to become as routine as the use of conventional structural materials such as masonry, wood, steel, and concrete. However, new concrete columns internally reinforced with FRP bars and FRP confinement for existing prismatic reinforced concrete (RC) columns have currently important unsolved issues, some of which are addressed in this dissertation defense. The dissertation is articulated on three studies. The first study (Study 1) focuses on RC columns internally reinforced with glass FRP (GFRP) bars; the second (Study 2) on RC prismatic columns externally confined by means of FRP laminates using glass and glass/basalt fibers; and the third (Study 3) is a theoretical attempt to interpret and capture the mechanics of the external FRP confinement of square RC columns. Study 1 describes an experimental campaign on full-scale GFRP RC columns under pure axial load undertaken using specimens with a 24 by 24 in. (0.61 by 0.61 m) square cross section. The study was conducted to investigate whether the compressive behavior of longitudinal GFRP bars impacts the column performance, and to understand the contribution of GFRP ties to the confinement of the concrete core, and to prevent instability of the longitudinal reinforcement. The results showed that the GFRP RC specimens behaved similarly to the steel RC counterpart, while the spacing of the ties strongly influenced the failure mode. Study 2 presents a pilot research that includes laboratory testing of full-scale square and rectangular RC columns externally confined with glass and basalt-glass FRP laminates and subjected to pure axial load. Specimens that are representative of full-scale building columns were designed according to a dated ACI 318 code (i.e., prior to 1970) for gravity loads only. The study was conducted to investigate how the external confinement affects ultimate axial strength and deformation of a prismatic RC column. The results showed that the FRP confinement increases concrete axial strength, but it is more effective in enhancing concrete strain capacity. The discussion of the results includes a comparison with the values obtained using existing constitutive models. Study 3 proposes a new theoretical framework to interpret and capture the physics of the FRP confinement of square RC columns subjected to pure compressive loads. The geometrical, physical and mechanical parameters governing the problem are analyzed and discussed. A single-parameter methodology for predicting the axial stress - axial strain curve for FRP-confined square RC columns is described. Fundamentals, basic assumptions and limitations are discussed. A simple design example is also presented.

Ένα από τα κύρια προβλήματα που συναντώνται σε κτίρια ή γέφυρες που έχουν κατασκευασθεί πριν από το 1980, είναι η μειωμένη καμπτική αντοχή και πλαστιμότητα, το οποίο αρκετά συχνά οφείλεται στην έλλειψη περίσφιξης και στη παρουσία κοντών αναμονών που είχαν οι κατασκευές αυτές. Ο κύριος σκοπός της παρούσας διατριβής είναι να παρουσιάσει και να αξιολογήσει πέντε από τα διαθέσιμα στη βιβλιογραφία αναλυτικά μοντέλα προσδιορισμού του απαιτούμενου πάχους του εξωτερικά εφαρμοζόμενου μανδύα για την αποφυγή της αστοχίας των ματιζομένων οπλισμών των υποστυλωμάτων συμπεριλαμβανομένου και του αντίστοιχου μοντέλου το οποίο δίνεται στο Σχέδιο 1 και Σχέδιο 2 του ΚΑΝΕΠΕ. Τα αναλυτικά μοντέλα αξιολογούνται μέσω πειραματικών αποτελεσμάτων από τη βιβλιογραφία. Η αξιολόγηση γίνεται σε δύο επίπεδα. Στο πρώτο επίπεδο αξιολογείται η αξιοπιστία πρόβλεψης του απαιτούμενου πάχους του υλικού ενίσχυσης –χρησιμοποιώντας τις μέσες τιμές των υλικών- ενώ στο δεύτερο επίπεδο εξετάζεται η αντίστοιχη καταλληλότητα κάθε προσομοιώματος για το σχεδιασμό –χρησιμοποιώντας τις αντίστοιχες τιμές σχεδιασμού των υλικών-. Τροποποιημένες εξισώσεις βασιζόμενες στο προσομοίωμα του ΚΑΝΕΠΕ παρουσιάζονται. Η χρήση των τροποποιημένων εξισώσεων ελέγχεται μέσω διαθέσιμων πειραματικών αποτελεσμάτων και προκύπτει ικανοποιητική σύγκλιση με αυτά. / Reinforced concrete frames or bridges constructed in the early 80s or before, were usually designed and detailed to resist lower lateral forces than those required today. Building columns were commonly designed for compression only and as a result they do not have the adequate lateral strength to resist the imposed earthquake loads. One of the main deficiencies in these older structures is the limited flexural strength and ductility often due to short and lightly confined lap splices. The main aim of this thesis is to present and evaluate five of the proposed analytical models in order to rehabilitate reinforced concrete columns with short lap splices by external confinement, including and the confinement model given by the draft version of the Greek Retrofitting Code (GRECO). The above analytical models are validated against experimental results. The validation is performed in two levels. In the first level, the reliability of the prediction for the required jacket thickness given by the models, is examined, by using the average values of the materials. In the second level, the propriety for the design of each model is examined by using the design values of the materials. A modified equation based on the model given by GRECO is presented as well. By using the proposed modified equation a satisfactory agreement with the experimental results was accomplished.