Conclusions 152

6.1.1 Experimental Research

After the release of prestress force applied to the NSM CFRP laminate, the distribution of tensile strain in the extremity region of the CFRP bonded length varies from zero (at its end section) to its effective tensile strain during the prestress transmission zone, and reaches a plateau beyond this zone, while the bond shear stress starts from high value at the end of the CFRP bonded length and continues decreasing along the prestress transmission zone, and becomes approximately null after that. Moreover, although this bond shear stress increases with the prestress level applied to the NSM CFRP reinforcement, this prestress level has a negligible influence on the distribution of CFRP normalized tensile strain (divided by the applied prestrain).

After releasing the prestress force a negative camber is generated due to the negative bending moment caused by the eccentricity of this force in relation to the centroidal axis of the beam’s cross section. This negative camber led to a decrease of tensile strain in the prestressed CFRP laminate, which represents the short-term prestress loss immediately after release. Furthermore, this prestress force created an initial compressive strain in the tensile steel reinforcement and surrounding concrete, which led to an increase of the load carrying capacity at concrete cracking and steel yielding initiations, as well as an enhancement in terms of load carrying capacity at serviceability limit state conditions. However, the ultimate deflection of the beams strengthened with NSM prestressing technique decreased with the increase of applied prestress level. These results, which imply a decrease of the ductility index with the prestress level, suggest the adoption of an upper limit of the prestress level to be applied to the CFRP laminates in order do not compromise the ductility performance of the RC beams strengthened according to the NSM prestressing technique. On the other side, by increasing the prestress level, the cracked zone length and average crack width decrease compared to the passive strengthened beam.

Chapter 6: Conclusions and future works 153 The higher strengthening ratio adopted in the NSM hybrid technique led to an increase of load carrying capacity at concrete cracking, SLS conditions, steel yield initiation, and ultimate stage, as well as a decrease of average crack width when compared to the use of NSM prestressing technique for the flexural strengthening of RC beams. Moreover, the results showed that both prestressing and hybrid techniques decrease the possibility of the concrete crushing as prevailing failure mode of the strengthened beams. On the other hand, reduction of the ultimate deflection capacity of the beams strengthened with NSM prestressing technique was observed with the increase of the prestress level, resulting a decrease in terms of energy absorption and deformability indexes, while the aforementioned indicators for the hybrid NSM CFRP beams were influenced by the type of prevailing failure mode at the maximum capacity. The hybrid strengthened beam, failed by the rupture of the prestressed CFRP at the maximum capacity, showed a higher energy absorption and deformability indexes compared to the beams strengthened with NSM prestressing technique with similar prestress level.

6.1.2 Numerical Analysis

A 3D nonlinear FE approach simulating both concrete-epoxy adhesive and laminate-epoxy adhesive interfaces, as well as, the relevant nonlinear behavior of the intervening materials, and the prestress process adopted in the test setup, was developed, and its good predictive performance was demonstrated. This modelling strategy can be used to design the type of structures investigated in the present work.

The numerical analysis evidenced that the possibility of occurring the concrete cover delamination (as a premature failure model before the rupture of the CFRP) is highly dependent to the NSM CFRP development length in shear span of the strengthened beam. Accordingly, to decrease the susceptibility to the concrete cover delamination failure, the CFRP bonded length should be extended as much as possible close to the support.

Moreover, this analysis showed that by increasing the passive CFRP strengthening ratio in the hybrid system, a higher energy absorption and deformability indexes can be achieved for the hybrid strengthened beams when the rupture of the prestressed CFRPs occurs before the premature failure modes at the maximum capacity.

6.1.3 Analytical Approaches

An analytical approach was developed based on the strain compatibility and principles of static equilibrium to predict the moment-curvature and load-deflection relationships of RC beams

strengthened with prestressed CFRP reinforcement that can be applied according to the EBR or NSM techniques. This approach considers two types of failure modes, comprising yielding of the steel bars in tension followed by either concrete crushing or rupture of the CFRP reinforcement. The developed formulation assumes that the moment-curvature response of a beam’s cross section can be simulated by a trilinear diagram defining precracking, postcracking, and postyielding stages. Two further stages are proposed, namely: concrete and steel decompression stages, in order to assess the initial effects of the prestress force applied by the CFRP reinforcement. A good predictive performance of the model was demonstrated by simulating the tests executed in the experimental programs. The predictive performance was also assessed by comparing the moment-curvature and force-deflection relationships obtained with the developed model and using a computer package based on a cross section layer model. Moreover, a methodology to obtain an upper limit of the prestress level that can be applied to the CFRP reinforcement is proposed in order to maintain a sufficient degree of ductility for the prestressed strengthened beams, and a limit of 60% was determined.

In the case of the NSM technique, a design framework methodology is proposed to obtain the ultimate flexural capacity of the NSM CFRP strengthened beams when failing by concrete cover delamination, and its good predictive performance was determined. Concrete cover delamination is predicted by assessing the possibility of occurring the concrete tensile fracture at the extremities of the CFRP reinforcement in comparison with debonding and rupture of the CFRP failure modes. Finally, the concrete cover delamination is adopted as prevailing failure of the strengthened beams when its ultimate flexural capacity is less than the one corresponding to the occurrence of the conventional flexural failure modes (concrete crushing or rupture of the CFRP reinforcement). According to this proposed methodology, by decreasing the bonded length of the CFRP reinforcement, the resistance to the occurrence of concrete cover delamination can decrease, while a higher concrete tensile strength, and also a higher concrete cover depth below the tensile steel bars can increase this resistance. On the other side, this resistance is influenced by the distance between the two adjacent CFRPs, as well as the distance between the lateral face of the beam’s cross section and the nearest CFRP. Accordingly, by adopting a strengthening configuration for consecutive NSM CFRPs according to minimizing the interaction of the concrete tensile fracture of the adjacent CFRPs, can provide the maximum resistance to the occurrence of concrete cover delamination.

Closed form analytical formulations were developed to predict the distribution of CFRP tensile stress and bond shear stress along the NSM CFRP bonded length, as well as the prestress transfer length when the prestress force is released, and a good predictive performance of these analytical formulations was determined. The analytical parametric studies showed that by increasing

Chapter 6: Conclusions and future works 155 the elasticity modulus of the concrete and epoxy adhesive, the prestress transfer length decreases and the bond shear stress increases, while the opposite occurs with the increase of the CFRP elasticity modulus. Moreover, by increasing the thickness of epoxy adhesive layer, the prestress transfer length increases. On the other side, the CFRP reinforcement with rectangular cross section requires a lower transfer length compared to the use of square cross section with equal cross sectional area. In other words, a higher width/height ratio of the CFRP cross section increases the prestress transfer length. Besides, the analytical model offers a formula to predict the evolution of the prestress transfer length during the curing time of epoxy adhesive.