Bahman Ghiassi

Accelerated hygrothermal aging of bond in FRP-masonry systems

AbstractThis paper addresses the results of accelerated hygrothermal (coupled temperature and moisture) tests on fiber-reinforced polymer (FRP) strengthened clay bricks, aimed at investigating bond degradation mechanisms. The exposures are selected to simulate different environmental conditions and the bond degradation is periodically investigated by visual inspection and by conventional single-lap shear bond tests. The changes in the properties of material constituents were also monitored; the results are presented and critically discussed. A decay model is adopted for simulating the observed degradation in the specimens. The model, once validated, is used for long-term performance prediction of FRP–masonry systems and the results are compared with the environmental reduction factors proposed by available design guidelines.

Seismic Evaluation of Masonry Structures Strengthened with Reinforced Concrete Layers

AbstractCoating the walls with reinforced concrete layers is a conventional method of strengthening masonry structures in Iran. However, because of the lack of analytical and experimental information about the behavior of strengthened masonry walls with this method, the design of these walls is generally conducted based on empirical relations and decisions that may result in uneconomical or underdesigned strengthening details. This paper aims to develop a rational method for design and seismic evaluation of unreinforced masonry walls strengthened with reinforced concrete layers. In the proposed method, four failure modes are considered for these walls, and the strength relations and acceptance criteria for each of them are provided in accordance with FEMA 356 and ASCE 41 relations for reinforced concrete and masonry walls. The accuracy of the proposed method in predicting the nonlinear behavior and governing failure modes of the strengthened walls is validated by comparing the results with available exper...

Debonding damage analysis in composite-masonry strengthening systems with polymer- and mortar-based matrix by means of the acoustic emission technique

Different types of strengthening systems, based on fiber reinforced materials, are under investigation for external strengthening of historic masonry structures. A full characterization of the bond behavior and of the short- and long-term failure mechanisms is crucial to ensure effective design, compatibility with the historic substrate and durability of the strengthening solution. Therein, non-destructive techniques are essential for bond characterization, durability assessment and on-site condition monitoring. In this paper, the acoustic emission (AE) technique is evaluated for debonding characterization and localization on fiber reinforced polymer (FRP) and steel reinforced grout-strengthened clay bricks. Both types of strengthening systems are subjected to accelerated ageing tests under thermal cycles and to single-lap shear bond tests. During the reported experimental campaign, AE data from the accelerated ageing tests demonstrated the thermal incompatibility between brick and epoxy-bonded FRP composites, and debonding damage was successfully detected, characterized and located. In addition, a qualitative comparison is made with digital image correlation and infrared thermography, in view of efficient on-site debonding detection.

Effect of Environmental Aging on the Numerical Response of FRP-Strengthened Masonry Walls

AbstractRecent durability studies have shown the susceptibility of bond in fiber-reinforced polymer (FRP) strengthened masonry components to hygrothermal exposures. However, it is not clear how this local material degradation affects the global behavior of FRP-strengthened masonry structures. This study addresses this issue by numerically investigating the nonlinear behavior of FRP-masonry walls after aging in two different environmental conditions. A numerical modeling strategy is adopted and validated with existing experimental tests on FRP-strengthened masonry panels. The model, once validated, is used for modeling of four hypothetical FRP-strengthened masonry walls with different boundary conditions, strengthening schemes, and reinforcement ratios. The nonlinear behavior of the walls is then simulated before and after aging in two different environmental conditions. The degradation data are taken from previous accelerated aging tests. The changes in the failure mode and nonlinear response of the walls...

On the identification of earlywood and latewood radial elastic modulus of Pinus pinaster by digital image correlation: a parametric analysis

This work addresses the reconstruction of strain gradient fields at the wood growth ring scale from full-field deformation measurements provided by digital image correlation. Moreover, the spatial distribution of the earlywood and latewood radial modulus of elasticity is assessed. Meso-scale tensile tests are carried out on Pinus pinaster Ait. wooden specimens oriented in the radial–tangential plane under quasi-static loading conditions. A parametric analysis of the two-dimensional digital image correlation extrinsic and intrinsic setting parameters is performed, in a balance between spatial resolution and resolution. It is shown that the parametric module is an effective way to quantitatively support the choice of digital image correlation parameters in the presence of the high deformation gradient fields generated by the structure–property relationships at the scale of observation. Under the assumption of a uniaxial tensile stress state, the spatial distribution of the radial elastic modulus across the growth rings is obtained. It is observed that the ratio of the radial modulus of elasticity between latewood and earlywood tissues can vary significantly as a function of the digital image correlation parameters. It is pointed out, however, that a convergence value can be systematically established. Effectively, earlywood and latewood stress–strain curves are obtained and elastic properties are determined assuming the converged digital image correlation setting parameters.

Micromechanical modeling of tension stiffening in FRP-strengthened concrete elements

This article presents a micromodeling computational framework for simulating the tensile response and tension-stiffening behavior of fiber reinforced polymer–strengthened reinforced concrete elements. The total response of strengthened elements is computed based on the local stress transfer mechanisms at the crack plane including concrete bridging stress, reinforcing bars stress, FRP stress, and the bond stresses at the bars-to-concrete and fiber reinforced polymer-to-concrete interfaces. The developed model provides the possibility of calculating the average response of fiber reinforced polymer, reinforcing bars, and concrete as well as the crack spacing and crack widths. The model, after validation with experimental results, is used for a systematic parameter study and development of micromechanics-based relations for calculating the crack spacing, fiber reinforced polymer critical ratio, debonding strength, and effective bond length. Constitutive models are also proposed for concrete tension stiffening and average response of steel reinforcing bars in fiber reinforced polymer–strengthened members as the main inputs of smeared crack modeling approaches.

Simple Numerical Homogenization Model for FRCM Reinforced Masonry Panels Subjected to Out-of-Plane Loads

The paper deals with the analysis of FRCM reinforced masonry panels in simple bending by means of a two-step homogenization approach. In the first step, homogenization is performed on unreinforced masonry through a straightforward procedure already validated for in-plane loaded FRCM reinforced panels, where the elementary cell is discretized by means of 24 CST elastic elements (bricks) and joints are reduced to interfaces with holonomic softening behavior. Out-of-plane homogenized quantities (moment-curvature relationships) are then obtained by simple on-thickness integration. At a structural level (step two), masonry is modeled with rigid elements and homogenized torsional and bending springs, whereas FRCM by two noded trusses with equivalent mechanical properties evaluated on the base of experimental data available. The implementation is handled within the commercial FE software Abaqus. As a matter of fact, the Concrete Damage Plasticity Model (CDP) already implemented into the code allows taking into account the progressive change of stiffness of brittle supports when subjected to alternate cracking-crushing mechanisms, as well as to properly deal with the cyclic behavior. The validation of the procedure is conducted against a series of full scale masonry walls tested by Nanni and co-workers at the University of Miami. A tri-dimensional heterogeneous micro-modeling technique is also used for both unreinforced and FRCM strengthened panels to further validate the homogenization model proposed. The discretization adopted is adjusted in order to allow the presence of two eight-noded FEs along the mortar joint thickness. FRCM is modeled by means of truss elements whose mechanical properties are calibrated to reproduce the behavior of the textile net embedded into the cementitious matrix. The elastic and inelastic mechanical parameters of all the materials involved into the numerical analyses have been calibrated in agreement with laboratory tests, when available. Pros and cons of the two numerical strategies are discussed mostly with respect to the accuracy of the outcomes in terms of global behavior of the panels and damage mechanisms, as well as with respect to the computational effort required by the two approaches to complete the analyses.

Modelling of the In-Plane and Out-of-Plane Performance of TRM-Strengthened Masonry Walls

Tensile Reinforced Mortars (TRMs) are promising composites that address the compatibility demands required for strengthening of masonry and historical constructions. Although many recent studies have been devoted to the use of these materials for strengthening purposes, several issues such as efficiency for improving the structural performance are clearly open. The aim of this paper is to numerically investigate the effectiveness of TRM systems on the in-plane and out-of-plane response of masonry walls. Numerical models are adopted to describe the nonlinear behaviour and the failure mechanisms of unreinforced and strengthened walls. It is shown that the implementation of TRM layers improve largely the performance of the masonry walls both in terms of strength and displacement capacity.

A Comparative Study on Deflection-Hardening Behavior of Ductile Alkali-Activated Composite

Alkali-activation technology as an environmental friendly approach to produce construction materials can lead to a great CO2 emission reduction and provide high potential for waste reutilization. Alkali activated materials (AAMs) generally exhibit better durability and comparable or superior mechanical performance in comparison to traditional cementitious materials. However, like cementitious binders, AAMs are inherently brittle (quasi brittle). When it comes to engineering practices, the size effect also contributes to a more severe brittleness, which limits the applications in large scale structures. Fiber reinforcement is one of the solutions, which can be used to control the brittleness of AAMs. This paper presents a comparative study on the deflection-hardening behavior of ductile alkali-activated slag/fly ash (DAASF) composites reinforced with three types of synthetic fibers. All the developed DAASFs exhibited a deflection hardening with multiple cracking behavior under four-point bending tests. The influence of different fiber types, different slag/fly ash ratio as well as liquid/solid (L/S) ratio were investigated. The obtained results are a first step contribution to understanding the feasibility of using different fiber types in AAMs and development of mixture design of DAASF composites.

Tensile and Bond Characterization of Natural Fibers Embeeded in Inorganic Matrices

Innovative composite materials made of continuous fibers embedded in mortar matrices have been recently received attention for externally bonded reinforcement of masonry structures. In this regards, application of natural fibers for strengthening of the repair mortars is attractive due to their low specific weight, sustainability and recyclability. This paper presents experimental characterization of tensile and pull-out behavior of natural fibers embedded in two different mortar-based matrices. A lime-based and a geopolymeric-based mortar are used as sustainable and innovative matrices. The obtained experimental results and observations are presented and discussed.