FRP-masonry Interface Research Articles

A Simple and Low-Cost Numerical Model for FRP-Masonry Interface Behavior

In recent years, strengthening with Fiber Reinforced Polymers (FRPs) has emerged as an effective way for the structural upgrading of masonry elements. In such typology of external reinforcement, the bond quality is crucial for the increase of the load bearing capacity. The bond efficacy beyond the elastic limit can be studied analytically or numerically via several different models, where the most important issue to tackle is the reproduction of the typical brittle behavior of the substrate. In this paper, a simple numerical approach which models FRP as elastic and lumps all non-linearity on the FRP/masonry interface is proposed. The non-linear behavior of such interface is modeled in a simplified but effective way integrating numerically the differential equations deduced from equilibrium and compatibility (once that a non-linear constitutive relationship between tangential stress and slip is assumed at the interface). Such integration is carried out by means of a particularly simple forward scheme that requires the estimation of the slip value and its derivatives on specific knot points. A comparison against existing literature indicated that the proposed numerical procedure can adequately reproduce global load-displacement curves in standard single lap shear tests, as well predict the local slip behavior.

Approximate Evaluation of Maximum Force Transferable at FRP-Masonry Interface

AbstractThe debonding phenomenon between fiber-reinforced polymer (FRP) composites and masonry is influenced by the presence of mortar joints that could reduce the bond capacity of the FRP-brick in...

The effect of the shape of the cohesive material law on the stress transfer at the FRP-masonry interface

Debonding of the fiber-reinforced polymer (FRP) from the substrate that occurs before its tensile strength is reached typically controls the effectiveness of the stress transfer mechanism between FRP composites and a quasi-brittle substrate, such as masonry. The FRP-substrate interface is usually modelled as a zero-thickness interface whose fracture Mode-II cohesive material law (CML) is defined in terms of shear stress and slip at the interface. In this paper, the effect of the shape of the CMLs of the FRP-brick and FRP-mortar interfaces on the stress transfer process will be presented and discussed. Several multi-linear CMLs, obtained from experimental data for both mortar and brick interfaces, will be adopted to obtain the load response of the FRP-masonry interface. It will be shown that the shape of the CMLs adopted does not strongly affect the load response if the fracture parameters are the same or similar among the CMLs. Finally, certain geometric relationships of brick and mortar joints imply more pronounced differences in terms of load response between the CMLs adopted.

A simple model for performing nonlinear static and dynamic analyses of unreinforced and FRP-strengthened masonry arches

Abstract A numerical model for analysing unreinforced and fiber-reinforced masonry arches is presented. It is based on a constitutive equation formulated for one-dimensional masonry elements, assumed to be made of a nonlinear material with no resistance to tension and limited compressive strength. A softening behaviour under compression is accounted for, and a procedure is provided for irreversible damage occurring to the masonry. The reinforcement, which is instead assumed to have no resistance to compression, is applied on the inner and/or outer side of the arch. In order to capture any possible FRP debonding, the values of the shear and normal stresses at the FRP-masonry interface are evaluated. The model, implemented in the non-commercial computer code MADY, allows for short computational times in studying reinforced masonry arches with any geometry, restraints and loading conditions. Comparisons with some experimental results are carried out to verify the effectiveness and accuracy of the model's predictions under static loads. Some dynamic analysis are also performed for a case study, and preliminary results are reported on the effectiveness of some FRP strengthening arrangements against earthquake actions.

FRP-masonry interfacial debonding: An energy balance approach to determine the influence of the mortar joints

Abstract In this paper a close-form relationship between the transferable load at the FRP-masonry interface and the characteristics of the FRP-brick and FRP-mortar interfaces, which are expressed in terms of cohesive material laws, is proposed. Prior to obtaining the aforementioned close-form relationship, experimental evidence is used to highlight that the fracture process at the FRP-masonry interface depends on the characteristics of the constituent materials and the geometry of the masonry. In addition, an analytical procedure previously proposed by the authors, which employs two simplified cohesive material laws for the FRP-brick and FRP-mortar interfaces, is revisited to show that the length of the stress-transfer zone of, and the transferable load at, the FRP-masonry interface vary periodically in accordance with the periodic pattern of bricks and mortar joints.

Periodic variation of the transferable load at the FRP-masonry interface

This work sheds light into the effect of the periodic pattern of bricks and mortar joints on the stress transfer at the interface between fiber-reinforced polymer (FRP) composites and masonry. Experimental evidence is used to highlight that the fracture process at the FRP-masonry interface depends on the characteristics of the constituent materials and the geometry of the masonry. Two simplified cohesive material laws are proposed for the FRP-brick and FRP-mortar interfaces, which are associated with finite effective bond lengths of the two interfaces. The aforementioned simplified interfacial laws are employed to compute the load response of the FRP-masonry interface, which is compared with the experimental one. The results indicate that length of the stress-transfer zone of, and the transferable load at, the FRP-masonry interface vary periodically in accordance with the periodic pattern of bricks and mortar joints.

A Study of the Fracture Process at the FRP-Masonry Interface: The Role of the Periodic Pattern of Bricks and Mortar Joints

This paper sheds light into the effect of the periodic pattern of bricks and mortar joints on the load-carrying capacity of the interface between fiber-reinforced polymer (FRP) composites and masonry. Two simplified cohesive material laws are proposed for the FRP-mortar and FRP-brick interfaces, which allow for the computation in closed form of a finite effective bond length Leff of the interfaces. The aforementioned simplified interfacial laws are employed to compute the load response of the FRP-masonry interface, and to obtain the interfacial shear stress, the FRP axial strain, and the slip profiles along the bonded length. The results indicate that length of the stress-transfer zone (LSTZ) of the FRP-masonry interface varies periodically as its location shifts with respect to the position of the mortar joints. Furthermore LSTZ can be different from the effective length of the FRP-brick interface and is influenced by the size of the bricks and mortar joints.

FRP-Masonry Debonding: Numerical and Experimental Study of the Role of Mortar Joints

Fiber-reinforced polymers (FRP) composites are used as supplementary reinforcement to increase the in-plane shear capacity or to provide out-of-plane load-carrying capability of masonry walls and to modify the collapse mechanism in arches and vaults. In these applications, the efficiency of load transfer is limited by the debonding of FRP from the masonry substrate. In this paper, the debonding mechanism of FRP-masonry is experimentally studied. Experimental procedures and test specimens are designed to investigate the progressive debonding of FRP from brick and mortar substrates and relate it to the response obtained from the FRP-masonry interface. Surface displacements during debonding are obtained using digital image correlation. A one-dimensional numerical model is developed for predicting the fracture behavior along the FRP-masonry interface using the cohesive fracture parameters from the brick and mortar interfaces obtained from the computed strains.

Prediction of Load-Slip Behavior of FRP Retrofitted Masonry

The out-of-plane bending resistance of a fiber-reinforced polymer (FRP)-strengthened masonry wall is generally governed by debonding failure mechanisms. The behavior at the FRP-to-masonry interface is a key factor affecting the flexural capacity as it is the means for transfer of stress between the FRP and the substrate to develop composite action. Although the beneficial effects of strengthening masonry structures with FRP materials have been demonstrated in the literature, the knowledge of the bond at the FRP-to-masonry interface is still relatively limited. To improve this understanding, this paper presents the results of a numerical investigation into the local behavior at the FRP-masonry interface. Two analytical procedures (i.e., a new generic numerical procedure and a closed-form mathematical solution) were developed to predict the global load-slip response of FRP-to-masonry pull tests by using various local bond-slip relationships. This study compares the effects of homogeneous and heterogeneous representations of the masonry substrate on the prediction of this global response and the influence of different bond-slip models and bond-slip parameters. The analytical results were validated against test data with both methods predicting the experimental behavior well. The paper concludes with recommendations regarding the influence of the heterogeneous nature of masonry and the effect of the bond-slip parameters on the load-slip response of FRP-to-masonry pull tests.

Experimental Bond Behavior of FRP Sheets Glued on Brick Masonry

This paper deals with the experimental characterization of the mechanical tensile and shear bond behavior of fiber-reinforced polymer (FRP) sheets externally glued on masonry prisms, in terms of load capacity and stress distribution along the bonded length. The brick masonry adopted tries to replicate ancient brick masonry, by using handmade low-strength solids bricks and low-strength lime-based mortar. Key parameters relative to the FRP-masonry interface response, particularly bonded length, FRP materials, anchor scheme adopted, and shape of masonry substrate, were studied. Finally, an analytical bond stress-slip formulation was developed, allowing deducing local bond stress-slip curves directly from the experiments.

Stress Analysis of Reinforced Masonry Arches

The paper deals with the modeling and the analysis of masonry arches reinforced with FRP materials. A nonlinear elastic model for the masonry material, characterized by no tensile capacity and limited strength in compression, is proposed; the FRP is modeled as a linear elastic material with brittle failure, considering a perfect adhesion between the masonry and the FRP reinforcement. A novel numerical procedure based on the stress approach of the structural problem, i.e. on the minimization of the complementary energy, is developed within a dual formulation of the arc-length continuation method. The proposed model and the developed numerical procedure are implemented in a computer code. Moreover, a new post-computation technique of the stresses at the FRP-masonry interface, based on a micromechanical analysis that takes into account the heterogeneity of the masonry material, is proposed. Numerical applications are developed to assess the model effectiveness and the efficiency of the numerical procedure. The results obtained using the proposed model and implemented procedure are put in comparison with the ones carried out considering an elasto-plastic masonry model implemented in a finite element procedure; finally, a comparison between numerical and experimental results is provided.

Load carrying capacity of 2D FRP/strengthened masonry structures

An adaptive discontinuous finite element model is formulated for limit analysis of masonry structures strengthened by fiber composites. The model is able to predict the effects of fracture damage and delamination on the load carrying capacity of the reinforced structures. A numerical investigation on the collapse mechanisms of masonry structures under plain strain/stress is presented, accounting for different mechanical properties of FRP–masonry interface and different placements of the reinforcement in the masonry structures.