Bond behavior in NSM-strengthened masonry

doi:10.1016/j.engstruct.2018.03.091

Engineering Structures

Volume 166, 1 July 2018, Pages 302-313

https://doi.org/10.1016/j.engstruct.2018.03.091 Get rights and content

Highlights

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The bond behavior in NSM system was investigated by testing 36 FRP-strengthened bricks.
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A new test setup was developed to avoid the pre-compression load before test.
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Majority of specimens failed at the adhesive-to-brick interface.
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Accuracy of existing bond prediction models was evaluated using a large database.
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A fracture-based model was proposed in this study to predictthe bond strength.

Abstract

Near Surface Mounted (NSM) reinforcement is an interesting technique for seismic strengthening of masonry and historical structures. Despite having several advantages compared to conventional techniques, little attention has been given to understanding the involved mechanisms (such as bond behavior) in the performance of masonry components strengthened with this reinforcement technique. This study presents an experimental and analytical investigation on the bond performance of NSM-strengthened masonry bricks aiming at filling the existing gaps in the available experimental results in the literature. The main focus is on the effect of test setup and bond length, but attention has also been given to the groove size and loading regime effect on the bond performance. The accuracy of the existing bond strength prediction models is also assessed and the required modifications are proposed.

Introduction

Fiber reinforced polymers (FRP) have been extensively used for strengthening of masonry and concrete structures due to advantages such as low weight, ease of application, corrosion resistance and high durability. These composites are generally used for Externally Bonded Reinforcement (EBR) or Near-Surface Mounted (NSM) strengthening techniques. The disadvantages related to the EBR strengthening techniques (such as special requirement for surface treatment and susceptibility to aggressive environmental conditions), promotes the use of NSM technique. Additionally, the NSM reinforcement does not change the aesthetics of the structure which is a great concern when dealing with restoration of historical structures [1], [2]. NSM systems are also more efficient due to their larger bonded area to cross section ratio in comparison with EBR systems. The available literature on strengthening of masonry structures using NSM technique have shown its notable effect on increasing the ductility and capacity of the structures [3]. Despite these advantages, the available literature on characterization and performance assessment of NSM-strengthened masonry is still limited, see e.g. [3], [4], [5], [6].

The NSM technique involves introducing FRP laminates or bars into slits prefabricated on the tensile face of structural elements using an epoxy adhesive [7]. In these systems, the stresses are transferred from the substrate to the reinforcing material through the adhesive and the interfacial stresses. The adhesive-to-substrate and the FRP-to-adhesive bond performance are therefore critical mechanisms. Although the bond performance has been subject of several studies in case of NSM-strengthened concrete elements, see e.g. [8], [9], [10], [11], [12], [13], [14], [15], little attention has been given to strengthened masonry components, see e.g. [2], [16], [17], [18]. The effect of different parameters (including the dimensions and shape, the adhesive type, the mechanical strength of substrate, the groove dimensions and the bonded length have been deeply investigated in NSM-strengthened concrete components [1], [2], [8], [19], [20]. The available studies on NSM-strengthened masonry, has not yet fully covered all these parameters and is mostly devoted to the effect of bond length on limited types of substrate. The majority of available studies are devoted to large bonded lengths and the bond performance in short bonded lengths still remains unexplored.

This paper presents an experimental assessment of the bond performance in CFRP NSM-strengthened bricks with special attention to short bonded lengths to fulfill the current gap in the literature. Attention has also been given to the effect of test setup, groove dimensions and loading regime. Based on the produced experimental results and the available data in the literature, a survey is also performed on accuracy of the existing bond strength analytical models and suitable modifications are proposed.

Section snippets

Specimens

The specimens were composed of solid clay bricks with dimensions of 200 mm × 100 mm × 50 mm strengthened with S&P® CFRP strips made of unidirectional carbon fibers. The strips had 10 mm width and 1.4 mm nominal thickness. A two-part epoxy adhesive (S&P resin 220) was used to bond the CFRP strips to the bricks following the near surface mounted (NSM) strengthening technique.

For preparation of the specimens, rectangular grooves were initially cut on the bricks’ surfaces by an electrical saw with

Typical failure modes

Three distinct failure modes were generally identified in the tested specimens as presented in Fig. 5: (A) flexural splitting of the bricks at the free end; (B) debonding of the laminate from the brick substrate (at the adhesive-to-brick interface) accompanied by diagonal cracks inside the brick (tensile cracking) and (C) failure at the adhesive-to-brick interface. The effect of different parameters and test conditions on the failure mode of the specimens are discussed in the following

Existing bond strength predictive models

Numerous models can be found in the literature for predicting the bond strength in NSM-strengthened concrete components [1], [10], [12], [32], [33]. Whereas, due to the lack of enough experimental data, few models exist for NSM-strengthened masonry elements [5], [18], [31]. These models, although not always clearly separable, can be generally categorized as (a) empirical models proposed based on regression analysis of experimental data or (b) fracture mechanics based models. Among the available

Assessment of accuracy of existing models

The accuracy of the existing models (presented in the last section) in predicting the maximum debonding force in NSM-strengthened masonry components is evaluated in this section. A database of experimental results on NSM-strengthened masonry components is initially formed. The database consisted of 70 experimental results available in the literature [2], [5], [16], [17], [18], [31] and the experimental results produced in the current study, leading to a total of 103 test records. As the

Proposed model

Most available bond strength models for NSM-strengthened masonry systems [18], [31], are developed based on modification of the model proposed by Seracino et al. [10]. The flexural tensile strength of the masonry is generally substituted for the cylindrical compressive strength and the other parameters are modified based on the best fit with the available experimental data. Although the predictions were in an acceptable range in the model proposed by Kashyap et al (NSM) [18], Fig. 11d, it seems

Conclusions

An experimental investigation on the bond performance of Near Surface Mounted (NSM)-strengthened bricks was presented in this paper. The effect of test setup and anchorage method on the experimental results were initially investigated and discussed. The focus was then given to the effect of different parameters on the bond behavior including the bond length, groove size and loading conditions. The following conclusions can be drawn from the experimental results:

(1)
The test setup was found to have

Acknowledgements

The second author acknowledges the financial support of the financial support of the European Union's Marie Curie Individual Fellowship program under REA grant agreement No. 701531.

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