Experimental analysis on tensile and bond properties of PBO and aramid fabric reinforced cementitious matrix for strengthening masonry structures
Introduction
Composite materials constituted by textiles and inorganic matrices represent a relatively new technique for strengthening structures. The application of these materials to reinforce the masonry structural elements seems to be an optimum solution to carry out a compatible, minimal and reversible intervention. The textiles are usually made of continuous carbon, glass, polyparaphenylene benzobisoxazole (PBO), aramid, steel or basalt fibers, while the matrix is frequently made of cement or lime based mortar, or a geopolymer. In accordance with US standards [1] and [2], the acronym that identifies these composite materials is FRCM (Fabric Reinforced Cementitious Matrix). However, different acronyms have been used in the technical literature, such as TRM (Textile Reinforced Mortar) or TRC (Textile Reinforced Concrete) [3]. In this paper the US acronym will be used.
The mechanical compatibility between textile and mortar is crucial to ensure an effective shear stress transfer between the fibers and the substrate. The shear stress transfer phenomenon is influenced by the capacity of the mortar to impregnate the single fibers of the yarn, the fibers-matrix and fiber-fiber bond properties and the arrangement of the fibers in the textile [4]. When textiles made of dry fibers are adopted, the failure modes of the FRCM systems submitted to tensile or shear bond tests are frequently characterized by a slippage between the external and internal fibers of the yarns. This produces a different stress of the fibers in a single yarn and the successive fiber failure layer by layer from the outer to the inner ones (telescopic failure) [3], [5]. The fabric of some FRCM strengthening systems is submitted to a coating treatment to impregnate and cover the fibers; this process affects the bond between fibers and matrix, avoiding the telescopic failure [6].
The use of FRCM materials for the structural upgrade and repair of existing concrete and masonry structures needs a deep mechanical characterization, both in terms of tensile response and bond-to-substrate performances. Experimental and theoretical studies have been carried out in the last years to characterize the mechanical behavior of FRCM reinforcements and to identify proper parameters for the strengthening design. The results of tensile tests performed on several FRCM composites are presented and discussed in Refs. [5] and [7]. Specifically, Carozzi et al. [5] studied the performances of two types of FRCM materials characterized by different textiles of PBO fibers. Ascione et al. [7] presented the results of tensile tests performed on FRCM materials with basalt, carbon, glass, aramid, and steel fiber textiles. Different clamping methods were adopted by the researchers to perform the tensile tests [5], [8], [9], [10]. Specifically, De Santis et al. [8] and Arboleda et al. [9] discussed the influence of the clamping method on the obtained mechanical response. The mentioned experimental works show that three stages characterize the tensile response of FRCM systems [7]: the un-cracked stage, the crack development stage, and the cracked stage. The black line in Fig. 1 shows an ideal stress-strain response recorded during a tensile test of an FRCM material.
The stress is conventionally evaluated as the ratio of the applied force F to the cross-sectional area Af of the fibers [7]. In the un-cracked stage (stage I, branch OA in Fig. 1), the matrix is un-cracked and the tensile behavior is mainly governed by the mechanical properties of the matrix; in the crack development stage (stage II, branch AB in Fig. 1), the tensile strength of the matrix is overcome, cracks develop and the tensile load is gradually transferred to the textile; in the cracked stage (stage III, branch BC in Fig. 1) the tensile load is almost completely transferred to the textile and the tensile behavior is mainly governed by the mechanical properties of the fibers. The idealized tensile response shown in Fig. 1 is frequently regarded as constituted by three branches with constant slopes (red lines in Fig. 1), named E1, E2, and E3. Furthermore, on the basis of the tensile tests presented in Refs. [5], [6], [7], [8], [9], the typical failure modes of FRCM materials tested in tension can be identified [7]. These failure modes are summarized in Fig. 2a–c: failure at the clamps (mode A, Fig. 2a); fibers tensile failure after the cracking of the matrix (mode B, Fig. 2b); cracking of the matrix and fibers slippage (mode C, Fig. 2c). The failure mode represented in Fig. 2d occurred during the experimental work presented in this paper and will be discussed in the following sections. The occurrence of a certain failure mode among the ones represented in Fig. 2 strongly depends on the transversal stress applied to the ends of the specimen by the clamping devices and the fibers-matrix and fiber-fiber bond performances.
Experimental analysis have been carried out to characterize the bond behavior of FRCM systems applied to a masonry [5], [7], [11], [12], or concrete [13], [14], [15], substrate. The mentioned experimental works allowed the typical failure modes of an FRCM-substrate joint (Fig. 3) to be identified [7].
The influence of the test setup on the results of shear bond tests has been studied by Sneed et al. [13]. They proved that if during the double-lap shear tests the debonding occurs simultaneously in both composite strips, the load responses of single-lap and double-lap shear tests are similar. Since frequently in the double-lap shear test configuration the debonding does not occurs symmetrically in both the composite strips, the single-lap shear test seems to be most reliable than the double-lap shear test configuration [13].
The static and seismic capacity of masonry structural elements strengthened with FRCM systems has been investigated by means of flexural tests on masonry walls [16], load tests on arches and vaults [17], [18], [19], and shaking table tests on masonry vaults [20], [21], [22], and walls [23], [24]. These experimental works show the significant gain of static and seismic capacity provided by the externally bonded FRCM reinforcements [25]. Nonetheless, the use of FRCM materials for strengthening masonry and concrete structures needs a detailed mechanical characterization of the strengthening material in order to define proper design parameters and procedures. For this purpose, reliable testing methods and criteria for the post processing of the experimental results need to be established. The technical community agrees on the necessity to perform tensile tests and shear bond tests. Nonetheless, important details of the testing procedures, such as geometry of the specimens, clamping devices, load and displacement rates, type of bond test (single-lap shear test, double-lap shear test, beam test) and type of transducer to be used need to be established. Furthermore, the criteria for the post processing of the experimental results to obtain the design parameters still need to be established. Ascione et al. [7] proposed to qualify and characterize the FRCM systems combining the results of direct tensile and shear bond tests. This approach involves the experimental evaluation of the bond capacity of the strengthening material by means of single- or double-lap shear tests and the evaluation of its tensile stiffness on the base of the results of the tensile tests. A different approach, based on the results of single- or double-lap shear tests, is applied in Refs. [3], [12], [13], [14], [15]. Recent experimental works shows that the failure of concrete [26], [27], [28], and masonry [18] structural elements strengthened with FRCM materials is often caused by the loss of bond. The debonding of the strengthening material usually occurs at the fibers-matrix interface (modes C, D, and E in Fig. 3). Consequently, the approach suggested in Refs. [3], [12], [13], [14], [15], is based on the adoption of the tensile properties of the dry textile and the calibration of a shear stress-slip relation characterizing the fibers-matrix interface surface, on the base of the experimental results of single- or double-lap shear tests.
The increasing use of FRCM systems for the strengthening of concrete [29] and masonry [30] structures and the lack of European guidelines for the evaluation of the mechanical properties of the strengthening materials and the identification of design procedures justify the attention of the scientific community to the field of composites with inorganic matrix. In this framework, the RILEM Technical Committee “Composites for Sustainable Strengthening of Masonry” (TC 250 CSM) has been constituted. This group is composed by more than 30 institutions (universities, public bodies, research agencies, industrial partners). The work of the TC 250 CSM aims to evaluate the mechanical properties of different FRCM materials applied to masonry substrates and to develop reliable testing procedures. To achieve these purposes, a round robin test has been carried out on FRCM materials characterized by different textiles (carbon, glass, steel, basalt, aramid, PBO) [31], [32], [33], [34]. The experimental study included direct tensile tests on composite specimens and shear bond tests on the FRCM-masonry interface. This paper collects the experimental results of the round robin tests on FRCM strengthening systems characterized by PBO (PBO-FRCM) and Aramid (A-FRCM) textiles. The tests on PBO-FRCM systems were performed by five research institutions: Cracow University of Technology (CUT), Politecnico di Milano (POLI_Mi), University Claude Bernard Lyon-1 (UNI_Ly1), University eCampus (UNI_eC), University Roma Tre (UNI_Ro3). The tests carried out by UNI_eC were performed in the laboratory of the Department of Structures for Engineering and Architecture of the Federico II University of Naples. The institution involved in the experimental investigation on A-FRCM systems was the University of Chieti (UNI_Ch). Specifically, the paper concerns three types of strengthening system: a PBO-FRCM system characterized by a bidirectional balanced textile and two types of A-FRCM systems characterized by a unidirectional and a quadriaxial textile. The principal aims of the present work are the following: 1) characterize the tensile behavior of the three strengthening systems; 2) define the FRCM-masonry bond capacity of the three systems; 3) analyze the different failure modes; and 4) define mechanical parameters useful for the strengthening design. The rest of the paper is organized as follows. In section 2 the experimental results are presented. In particular, section 2.1 deals with the material characterization; section 2.2 deals with the tensile tests; section 2.3 deals with the bond shear tests. Section 3 is devoted to the general discussion on the experimental results and comparison of these results with previous literature results. The last section deals with the concluding remarks.
Section snippets
Experimental program
The study includes direct tensile tests (TT) and shear bond tests (SBT) of three FRCM strengthening systems, named PBO-FRCM, A1-FRCM, and A2-FRCM. For each system the mortar provided by the manufacturer of the textile was used to make the FRCM material.
The PBO-FRCM system is characterized by a bidirectional balanced textile made of polyparaphenylene benzobisoxazole (PBO) fibers (Fig. 4a).
The yarn width of PBO textile is about 3 mm and the clear distance between yarns is about 12 mm (in weft and
General discussion
The displacement rate of the crosshead used by UNI_eC during the TTs and SBTs is considerably higher than that used by other universities. Consistent with the recent studies on loading rate effect [43], the peak stresses recorded by UNI_eC (Fig. 28) are lower than those recorded by UNI_Ro3 and POLI_Mi.
The procedure proposed in Ref. [7] for the qualification of the FRCM strengthening systems and the determination of their mechanical parameters is based on the results of TTs and SBTs. The
Conclusions
The lack of European guidelines for the determination of the mechanical properties of cement based composite materials for strengthening masonry structures induced the RILEM Technical Committee “Composites for Sustainable Strengthening of Masonry” (TC 250 CSM) to organize a round robin test involving tensile tests and shear bond tests on several FRCM materials. This paper collects the results of tensile tests and shear bond tests performed on a PBO-FRCM (PBO fibers textile) and two A-FRCM
Acknowledgements
This research has been carried out thanks to the collaboration of San Marco Terreal, RUREDIL SPA, SACEN S.r.l.
The authors would like to thank the researches that cooperated to the experimental campaign: Piotr Krajewski (Cracow University of Technology), Arkadiusz Kwiecień (Cracow University of Technology), Francesca Roscini (Roma Tre University), Binh Nguyen Duc (University Claude Bernard Lyon1), Alberto Balsamo (University of Naples), Ivano Iovinella (University of Naples), Gian Piero Lignola
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