Laura De Lorenzis

Bond of fiber-reinforced polymer laminates to concrete

Fiber-reinforced polymer (FRP) laminates are being successfully used for strengthening existing reinforced concrete structures. The bond of FRP reinforcement to the concrete substrate is of great importance for the effectiveness of this technique. In this research, flexural test specimens were prepared to address some of the factors expected to affect bond such as bonded length, concrete strength, number of plies (stiffness), ply width, and, to a lesser extent, surface preparation. Experimental results are presented and discussed. A linear bond stress-slip relationship, along with a simple shear model for the evaluation of the slip modulus, is used to predict the strain distribution at moderate load levels. Lastly, expressions of the peeling load and the effective bond length are provided. A design equation is proposed for calculating the effective FRP ultimate strain to be used in design to account for bond-controlled failure.

Shear Strengthening of Reinforced Concrete Beams with Near-Surface Mounted Fiber-Reinforced Polymer Rods

The use of near-surface-mounted (NSM) fiber-reinforced polymer (FRP) rods is a promising technology for increasing flexural and shear strength of deficient reinforced concrete (RC) members. The structural behavior of RC elements strengthened with NSM FRP rods needs to be fully characterized. In this research, 8 full-size beams were tested. Carbon FRP deformed rods were used for shear strengthening. The variables examined in the shear tests were spacing of the rods, strengthening pattern, end anchorage of the rods, and presence of internal steel shear reinforcement. In this paper, performance of the tested beams and modes of failure are presented and discussed. Test results confirm that NSM FRP rods can be used to greatly increase the shear capacity of RC elements, with efficiency that varies depending on the tested variables. Results of the experimental tests are compared with the predictions of a simple model, showing reasonable agreement.

Bond between Near-Surface Mounted Fiber-Reinforced Polymer Rods and Concrete in Structural Strengthening

Use of near-surface mounted (NSM) fiber-reinforced polymer (FRP) rods is a promising technology for increasing flexural and shear strength of reinforced concrete (RC) members. As this technology emerges, the structural behavior of RC elements strengthened with NSM FRP rods should be fully characterized, and bond is the first issue to be addressed. Bond is of primary importance as it is the means for the transfer of stress between the concrete and the FRP reinforcement to develop composite action. This research program aimed to investigate bond between NSM FRP rods and concrete. Some of the factors expected to affect bond performance are addressed here, namely: bonded length, diameter and surface configuration of the rod, type of FRP material, and size of the groove in which the rod is embedded. Results are presented and discussed.

A modified pull-out test for bond of near-surface mounted FRP rods in concrete

Among the strengthening techniques based on fiber-reinforced polymer (FRP) composites, the use of near-surface mounted (NSM) FRP rods is emerging as a promising technology for increasing flexural and shear strength of deficient concrete, masonry and timber members. In order for this technique to perform effectively, bond between the NSM reinforcement and the substrate material is a critical issue. Aim of this project was to investigate the mechanics of bond between NSM FRP rods and concrete, and to analyze the influence of the most critical parameters on the bond performance. Following up to previous investigations, a different type of specimen was designed in order to obtain a test procedure as efficient and reliable as possible. Among the investigated variables were: type of FRP rod (material and surface pattern), groove-filling material, bonded length, and groove size. Results of the first phase of the project are presented and discussed in this paper.

Anchorage Length of Near-Surface Mounted Fiber-Reinforced Polymer Bars for Concrete Strengthening—Experimental Investigation and Numerical Modeling

Near-surface mounted (NSM) fiber-reinforced polymer (FRP) bars are being increasingly recognized as a valid alternative to externally bonded FRP laminates for enhancing flexural and shear strength of deficient concrete, masonry, and timber members. The ultimate capacity and service performance of strengthened members are deeply impacted by bond characteristics of the strengthening system on which, in the case of NSM bars, limited data is available. This paper follows up on prior investigations into the mechanics of bond of NSM bars to concrete. Experimental results completing a previous test series are reported and discussed, and a global evaluation of results of 3 different test series is attempted. A 3-D finite element model for bond of NSM reinforcement is proposed and calibrated on the basis of some experimental findings.

Rocking Stability of Masonry Arches in Seismic Regions

This study evaluates the susceptibility of masonry arches to earthquake loading through experimental testing and progresses toward a specific criterion by which arches can be quickly assessed. Five different earthquake time histories, as well as harmonic base excitations of increasing amplitude, were applied to model arches, and the magnitude of the base motion resulting in collapse was determined repeatedly. Results are compared with failure predictions of an analytical model which describes the rocking motion of masonry arches under base excitation. The primary impulse of the base excitation is found to be of critical importance in causing collapse of the masonry arch. Accordingly, a suite of failure curves are presented which can be used to determine the rocking stability of masonry arches under a primary base acceleration impulse which has been extracted from an expected earthquake motion.

Anchorage Length of Near-Surface MountedFiber-Reinforced Polymer Rods for Concrete Strengthening—Analytical Modeling

Among strengthening techniques based on fiber-reinforced polymer (FRP) composites, near-surface mounted (NSM) FRP rods are one of the most recent and promising acquisitions. Because bond performance is a critical aspect of the technology, experimental and theoretical investigations on this issue were performed, with this paper focusing on the latter. Analytical modeling is carried out in 2 phases. First, the experimental local bond stress-slip curves are modeled analytically and the numerical solution of the differential equation of bond allows computing the bond failure load as a function of the bonded length and the anchorage length of NSM FRP rods required in design. In the 2nd phase, an approximate bi-dimensional analysis in the elastic range and a limit analysis assuming plastic behavior of concrete are conducted to compute upper and lower bounds to the local bond strength. In both phases, analytical predictions are compared with experimental results, showing good agreement.

Cooperativity in the Enhanced Piezoelectric Response of Polymer Nanowires

Multilayered, aligned arrays of organic nanowires show unique advantages in their piezoelectric response. Here, the cooperative, electromechanical mechanism at the base of the enhanced response of aligned arrays of piezoelectric nanostructures in mutual contact is unveiled. An enhancement of the piezoelectric voltage by two orders of magnitude compared with individual nanofibers is demonstrated in the arrays.

Effect of Construction Details on the Bond Performance of NSM FRP Bars in Concrete

Near-surface mounted (NSM) fiber-reinforced polymer (FRP) reinforcement has proven effective for strengthening reinforced concrete structures and is gaining increasing attention. The NSM reinforcement is installed by grooving the surface of the member to be strengthened and embedding FRP bars or strips in the grooves with an appropriate binder. This paper reports the results of an investigation designed to evaluate the influence of some construction parameters on the bond performance of NSM FRP round bars. These parameters include the groove depth and width-to-depth ratio, the groove distance from the edge of the member, the mechanical properties of the groove-filling epoxy, and the use of transverse FRP sheets for confinement of the joint. During the tests, conducted with a direct shear setup, the loaded-end and free-end slips and the strain distributions along the reinforcement and in the transverse plane were monitored. Tests were conducted on both short and long NSM bar anchorages. Results are here reported and analyzed to determine the effect of the test variables on local bond-slip behavior and development capacity. The local bond-slip curves obtained from the tests with short bond length are modelled analytically and used to predict the capacity of the specimens having long bond length and the same other test variables.

Computational homogenization of fibrous piezoelectric materials

Flexible piezoelectric devices made of polymeric materials are widely used for micro- and nano-electro-mechanical systems. In particular, numerous recent applications concern energy harvesting. Due to the importance of computational modeling to understand the influence that microscale geometry and constitutive variables exert on the macroscopic behavior, a numerical approach is developed here for multiscale and multiphysics modeling of thin piezoelectric sheets made of aligned arrays of polymeric nanofibers, manufactured by electrospinning. At the microscale, the representative volume element consists in piezoelectric polymeric nanofibers, assumed to feature a piezoelastic behavior and subjected to electromechanical contact constraints. The latter are incorporated into the virtual work equations by formulating suitable electric, mechanical and coupling potentials and the constraints are enforced by using the penalty method. From the solution of the micro-scale boundary value problem, a suitable scale transition procedure leads to identifying the performance of a macroscopic thin piezoelectric shell element.