Masoud Motavalli

Structural Strengthening with Prestressed CFRP Strips with Gradient Anchorage

AbstractThis paper presents the principle and the application of an innovative anchorage technique for prestressed carbon fiber–reinforced polymer (CFRP) strips in structural strengthening. Additionally, large-scale static loading tests of retrofitted concrete beams are shown. The gradient anchorage, based on the adhesive’s ability to undergo accelerated curing at high temperatures, consists of a purely concrete-adhesive strip connection without any mechanical devices, such as bolts or plates. In a first step, this study summarizes anchorage techniques presented in the literature and introduces the basic principles of the new method as well as the necessary components. In a second step, an application on a full-scale RC beam is explained in detail. A commercially-available CFRP strip is prestressed up to 0.6% prestrain and subsequently anchored by sequential epoxy-curing and force-releasing steps at both strip ends. Furthermore, uniaxial tensile tests on the epoxy adhesive and the CFRP strip are used for ...

Front and Side View Image Correlation Measurements on FRP to Concrete Pull-Off Bond Tests

Understanding the transfer of force by bond between externally bonded fiber-reinforced polymer (FRP) reinforcement and concrete is an important step in formulating good models for predicting debonding failures observed in externally bonded reinforcement strengthened systems. In this paper, a 3D optical displacement measurement system was used to capture the full-field displacements from the front and side view in pull-off bond specimens. The experiments were carried using six specimens with carbon FRP (CFRP) strips having different axial stiffnesses but a constant bond length to the concrete substrate. Using the optical measurements, it was possible to obtain the in-plane displacement or slip and the out-of-plane displacement or separation between the CFRP strip and the concrete. It was demonstrated, that the usual assumption of pure shear stresses in such pull-off tests is not true and that the bond behavior is a two-dimensional problem involving shear and peeling stresses. The bond behavior in CFRP stri...

Innovative CFRP-Prestressing System for Strengthening Metallic Structures

An innovative retrofit system to prestress carbon fiber reinforced polymer (CFRP) plates and attach them to existing metallic beams was developed (patent number CH 706 630 B1) and tested. The system does not require any glue between the CFRP plates and the beams; therefore, surface preparation is not necessary, which reduces the time and cost of retrofitting. The proposed prestressed unbonded reinforcement (PUR) system includes a pair of mechanical clamps that function based on friction. Each clamp can simultaneously hold and attach three CFRP plates to the beam. The design considerations of the clamps, which are the most important elements of the PUR system, were explained. The system has a trapezoidal configuration that offers an easy on-site installation and uninstallation procedures without residual damage on the metallic beam. Three 5-m-long steel beams were statically tested until failure, including one reference unstrengthened beam and two beams that were strengthened with 15% and 31% CFRP prestress levels. A considerable increase in the yielding and ultimate load capacities of the retrofitted beams was achieved. The ultimate load-carrying capacity of the strengthened beams with 15% and 31% CFRP prestress levels increased by more than 23% and 31%, respectively, compared to that of the reference specimen. A finite-element (FE) model was created to simulate the behavior of the retrofitted beams. Next, the FE results were compared with those from the experiments.

Long-term monitoring of cable stays with a wireless sensor network

Wireless sensor networks (WSNs) are a promising technology that could induce a significant innovation in the field of structural monitoring. The main advantages of WSNs are fast deployment, little interference and self-organisation. However, since WSN are battery powered, the power management of the sensor nodes significantly influences the operation method and the overall data management process. Since data communication is the most energy-consuming task, a significant data reduction has to be attained in the sensor nodes to achieve system lifetimes that are useful for real life applications. This paper discusses several basic aspects of data processing and data management for long-term monitoring with WSNs. It presents a specific monitoring system and illustrates a long-term field test performed with this system on a bridge. The test results demonstrate that in-network data reduction is a very promising but challenging approach, since it has to be implemented with very limited computational and memory resources.

A sandwich beam with electrostatically tunable bending stiffness

The tuning of the bending stiffness of structural elements is of interest for, among other things, the suppression of vibrations related to resonance phenomena. For a given cross-sectional area and geometry, the variation of the elastic properties of the material composing the structure provides a viable approach to this task. Only very limited options are available for such changes in material properties. The use of NiTi shape memory alloys has been proposed for this purpose. A new, energetically less expensive method for the modification of the bending stiffness of sandwich beams is presented. The proposed method makes use of electrostatic forces to modify the transfer of shear stresses at the interface between the faces and the core of the sandwich. Changes in bending stiffness of up to 18 times could be obtained for a prototype beam. A simple model for describing the behavior of the beam is presented.

Electrostatically tunable bending stiffness in a GFRP?CFRP composite beam

The suppression of vibrations in structures is commonly considered a useful measure for the extension of their lifetime, when high amplitude vibrations are observed. In the experiments presented in this work, the modification of the stiffness of a beam as a means to suppress vibrations due to resonance is proposed as an alternative to the introduction of discrete damping devices. The stiffness of a beam is modified by applying an electric field between the main element of the structure and additional stiffening elements applied to its surface, thus coupling the latter to the former by transfer of shear stresses. The effect of electrostatic tuning of the bending stiffness (and consequently of its eigenfrequencies) of a large size GFRP–CFRP beam is shown by the shift of the resonance peak for the first bending mode to higher frequencies. The discrete character of the stiffness increase in multi-layer beams (n≥3) is postulated.

Cycle energy control of magnetorheological dampers on cables

The dissipated cycle energy of magnetorheological (MR) dampers operated at constant current results from controllable hysteretic damping and from almost current independent, small viscous damping. Thus, the emulation of Coulomb friction and linear viscous damping necessitates current modulation during one vibration cycle and therefore current drivers. To avoid this drawback, a cycle energy control (CEC) approach is presented which controls the hysteretic MR damper part such that the total MR damper energy equals the energy of optimal linear viscous damping by constant current during one cycle. The excited higher modes due to the hysteretic damping part are partially damped by the MR damper viscous part. Simulations show that CEC copes better with damper force dynamics and constraints than emulated linear viscous damping due to the slow control force dynamics of CEC which are given by cable amplitude dynamics. It is demonstrated that CEC of MR dampers with viscosity of approximately 4.65% of the optimal modal viscosity performs better than optimal linear viscous damping. The reason is that this damper viscosity represents an optimal compromise between maximum energy spillover to higher modes due to the controllable hysteretic part which produces more cable damping and maximum viscous damping of these higher modes. Damping tests on a cable with an MR damper validate the CEC approach.

Stress recovery behaviour of an Fe-Mn-Si-Cr-Ni-VC shape memory alloy used for prestressing

This paper describes the stress recovery behaviour of an Fe‐17Mn‐5Si‐10Cr‐4Ni‐1(V, C) (mass%) shape memory alloy used for prestressing of civil structures. The prestressing due to the shape memory effect was simulated by a series of tests with pre-straining of the material followed by heating and cooling back at constant strain. Different pre-strain and heating conditions were examined. Moreover, the response due to additional mechanical and thermal cyclic loading has been investigated. These results were used to predict the partial prestress loss in a structure due to variable loading during operation. Finally, a heating test at constant strain was performed after the cyclic loading to check the possibility of reactivating the prestress lost during an exceptionally high load. (Some figures may appear in colour only in the online journal)

Mechanical performance of cold-curing epoxy adhesives after different mixing and curing procedures

Abstract This paper presents strength, stiffness, and porosity characteristics of commercially available cold-curing epoxy adhesives for structural engineering applications in the field of externally bonded and/or near-surface mounted composite strip reinforcements. Depending on specific requirements, accelerated curing of the adhesive under high temperatures might be necessary. Experimental investigations aimed at assessing the possible differences in strength and stiffness between samples cured at elevated temperatures for a defined time span and the ones cured at room temperature. It could be demonstrated that for the same specimen age, nominal tensile strength and stiffness are lower after an initial accelerated curing process at elevated temperatures. Furthermore, it could be shown that the specimens after an accelerated curing at elevated temperatures exhibited an increased porosity. The development of a numerical code for image analysis allowed a detailed inspection of several fracture surfaces and subsequently to assess the level of decrease in available cross-section due to an increased overall porosity. Cross-section area losses in the range of 10–15% compared to the reference specimens could be deduced. The subsequent derivation of the actual tensile strength exhibits smaller differences between the room and high temperature exposed specimens while curing. Regardless of the short-term material strength, the observed porosity might be subject of important durability issues on a long-term and needs further investigation.

Acoustic wood tomography on trees and the challenge of wood heterogeneity.

Abstract The assessment of tree stability requires information about the location and the geometry of fungal decay or of a cavity in the interior of the trunk. This work aims at specifying which size of decay or cavity can be detected non-destructively by acoustic wood tomography. In the present work, the elastic waves that propagate in a trunk during a tomographic measurement were visualized by numerical simulations. The numerical model enabled to systematically investigate the influence of fungal decay on tomographic measurements neglecting the heterogeneity of wood. The influence of wood heterogeneity was studied in laboratory experiments on trunks. The experiments indicated that the waveforms of the measured signals are by far more sensitive to the natural heterogeneity of trunk wood than the travel times, thereby making waveforms unsuitable for decay detection. Thus, it is recommended to further develop the travel time inversion algorithms for trunks and to neglect the information in waveforms or amplitudes. Fungal decay is detectable if the influence of the decay is distinguishable from the influence of the heterogeneity. It was found from the numerical analysis that the cross-section of a cavity, which is larger than 5% of the total cross-section of the trunk, can be detected by acoustic wood tomography.