Tensile properties of wire and fibre reinforced cementitious matrix composites for strengthening of masonry

Structures 23 (2020) 164–179

Contents lists available at ScienceDirect

Structures journal homepage: www.elsevier.com/locate/structures

Tensile properties of wire and fibre reinforced cementitious matrix composites for strengthening of masonry

T

Pravin Kumar Venkat Rao Padalua, Yogendra Singha, , Sreekanta Dasb ⁎

a b

Dept. of Earthquake Engineering, IIT Roorkee, India Civil and Environmental Engineering, University of Windsor, Canada

ARTICLE INFO

ABSTRACT

Keywords: Mortar-based composites Tensile test Wire mesh Basalt fibre mesh ACK model Simplified tri-linear model

Currently, wire and fibre reinforced inorganic matrices are receiving a lot of attention amongst the structural engineers, especially for strengthening of existing masonry structures. The motivation behind this development stems from the drawbacks of organic (epoxy resin) matrices of fibre reinforced polymers (FRP). A number of cement-based composites have been developed in the recent years in form of fibre reinforced cementitious matrix (FRCM) and wire reinforced cementitious matrix (WRCM) composites. Since the development of composites made of cementitious matrix is still under development, the evaluation of mechanical properties of such systems is a crucial step in their application. The tensile strength of the composite is an important parameter for the design of the strengthening system. This article presents an experimental study on 75 welded wire mesh (WWM) and basalt fibre mesh (BFM) reinforcement and corresponding composite specimens. The specimens are tested following the relevant ASTM and ACI standards. The mechanical behaviour of the composite is compared with the constituent reinforcement. The failure modes and tensile behaviour of the composites before and after cracking are also discussed. The test results are analyzed and compared with ‘Aveston-Cooper-Kelly (ACK)’ theory and the ‘simplified tri-linear (STL) model’.

1. Introduction Nowadays, inorganic (mortar-based) composites are gradually becoming popular for strengthening of heritage masonry structures, as they significantly improve the strength without varying the geometry, rigidity and mass of the structure [1]. The use of fibre reinforced polymer (FRP) with organic (epoxy resin-based) composites for applications in masonry structures is discouraged, because of their drawbacks pertaining to diminished performance at elevated temperatures, requirement of protective coatings, degradation of mechanical properties after continuing exposure to certain environmental surroundings, relatively higher level of care and supervision required in application [2–6]. Innovative systems for strengthening such as fibre-reinforced cementitious matrices (FRCM) have recently been proposed to address the drawbacks of FRP, and also to ensure better compatibility with masonry substrate in terms of chemical, physical, and mechanical properties, durability (especially in cases of historical structures), better performances at high temperatures, ease in application on irregular surfaces, lower susceptibility to debonding phenomenon, and the

⁎

ability to undergo the repair without altering the structural characteristics [6–10]. Similar to FRCM, wire-reinforced cementitious matrix (WRCM) composites have also been used for repair and retrofitting of masonry structures, however, the research on such systems is lacking. The development of mortar-based composites resulted in an alternative solution to FRP. Unlike polymeric binders, cementitious matrices cannot fully impregnate individual fibres. Therefore, the fibre sheet typically used in FRP by manual wet-layup, is replaced by a reinforcing mesh, in case of wire/fibre reinforced cementious matrix (WRCM or FRCM) composite. The rovings of the reinforcement in a grid mesh are made of wires or fibre yarns. Composite action in such a system is mainly attained by mechanical interlocking between the cementitious matrix and the reinforcing mesh, which enables the mortar to penetrate through openings of the grid [11,12]. The stresses in wire/fibre are transferred to the cementitious mortar matrix through this interlocking mechanism resulting in development of shear stress within the mortar [2]. The stiffness of the WRCM or FRCM is contributed by both cementitious matrix and wires/fibres depending on the respective crosssectional area and elastic modulus.

Corresponding author. E-mail addresses: [email protected] (P.K.V.R. Padalu), [email protected] (Y. Singh), [email protected] (S. Das).

https://doi.org/10.1016/j.istruc.2019.10.006 Received 23 July 2019; Received in revised form 11 October 2019; Accepted 14 October 2019 2352-0124/ © 2019 Institution of Structural Engineers. Published by Elsevier Ltd. All rights reserved.

Structures 23 (2020) 164–179

P.K.V.R. Padalu, et al.

In such systems, fibre/wires provides significant tensile strength and stiffness; whereas, the inorganic matrix transfers the load (while in an uncracked state) among the reinforcement and safeguards the fibres/ wires from the weathering agencies. However, the mortar cracks at an early stage and the post cracking stiffness and strength is contributed by the reinforcement alone. Moreover, the mortar matrix should be suitable for use and should be consistent enough for the method of application [2]. The main difference between the epoxy-based (FRP) and mortar-based composites (FRCM/WRCM) is the debonding phenomenon at the reinforcement matrix interface [13]. In case of FRP, the matrix to reinforcement bond is perfect, whereas FRCM or WRCM does not behave as a linear-elastic material up to failure. Depending on the combination of matrix and reinforcement, FRCM or WRCM presents a strain hardening behaviour, which relies heavily on matrix bonding and reinforcement characteristics [2]. To ensure the effectiveness of FRCM/WRCM, adhesive bonding between the reinforcement and matrix is important [14]. Past study by Carozzi and Poggi (2015) [15] shows that the debonding at the matrixreinforcement interface is the most common failure of FRCM systems under loading. This mode of failure differs from the debonding failure (substrate-reinforcement interface) observed in FRP strengthening systems [13]. Since, the inorganic matrix cannot fully penetrate the fiber yarns, the bond between the mortar and reinforcement is weak compared to FRP systems. The internal fibers are not in direct contact with the mortar matrix during cohesive failure at the matrix-reinforcement interface and may slip [15]. Past studies have shown that a polymer coating treatment on fiber roving can increase the bond strength between textile and mortar [16] and thus resulting in higher capacity of the FRCM systems. To design the composite reinforcement system for various structural applications, the composites tensile properties and bond strength to the masonry surface are required. The tensile test provides basic parameters for design, such as the strength, stiffness, and strain; whereas the bond strength test at reinforcement-mortar interface is of equal importance to understand the complex behaviour (stress transfer between the filaments within a yarn and between the yarn and the matrix) of the FRCM or WRCM system. In the past, researchers have conducted tensile tests on the composites made with carbon [16–20], hemp [21], sisal [21], cotton [21], glass [15,22], polyparaphenylene benzobisoxazole (PBO) [14,15], and steel fibres/cords [23] with various inorganic matrices. Recently, environment-friendly, natural fibers to strengthen structures have gained tremendous attention. Basalt fiber, an emerging green material has been found to be very promising for seismic retrofit of masonry structures [24]. Welded wire mesh (WWM) in cement mortar matrix can also prove to be a good material for seismic strengthening, because of its good mechanical characteristics and low cost [25]. According to the past studies, it has been observed that the mechanical properties like tensile strength, ultimate strain and modulus of elasticity of a single fibre/wire and those of the fibre/wire reinforced composites, are different. Therefore, investigations are required before inorganic matrices using wires/fibres can be used confidently to safeguard the masonry structures. However, very few experimental tests have been carried out, so far, on basalt fibre reinforced cementitious matrix (BFRCM) composites [2,26–28] and only one study [1] is available on cement-based composites consisting of steel reinforcement in the form of thin fibres/cords, whereas no such study is available on wire reinforced cement matrix (WRCM) composite. For this reason, the mechanical properties of the dry wire/fibre mesh, and the wire/fibremortar composites have been investigated experimentally and compared. The test results provide stress–strain curves, strength, stiffness, and failure modes to evaluate the performance of constituent reinforcement and the composites. The obtained mechanical properties in uni-axial tensile behaviour constitute the background information required for design of strengthening using WRCM or BFRCM.

2. Mechanical characterization 2.1. Reinforcement In the present study, two types of reinforcements have been studied: (a) A bi-directional welded wire mesh (WWM) with two different grid spacing, 25 and 50 mm. The diameter of wires in both the mesh are measured as 2.965 and 2.956 mm, respectively (Fig. 1a). (b) A bi-directional basalt fibre mesh (BFM) made of basalt yarns in ‘warp’ and ‘weft’ directions, with two different grid spacings of 25 and 50 mm. It consists of 4 number and 2 number of fibre yarns in each fibre roving of 50 mm and 25 mm grid spaced mesh, respectively (Fig. 1b). The nylon fibres have been used for weaving the fibre rovings at the intersection of warp and weft directions. The tensile behaviour of the wires in a WWM and fibre rovings of a BFM used as reinforcement has been studied according to the guidelines of ASTM A370-17a [29] and ASTM D5034-09 [30]. A number of tensile tests on a single wire and single fibre roving have been carried out to determine the mechanical parameters of reinforcement. Total 12 specimens of wires in a WWM and 24 specimens of fibre rovings in a BFM have been tested in different directions. Specimens are designated by the alphanumeric strings, ABC-i, where A represents the grid spacing (25 or 50 mm), B represents the type of reinforcement (W- welded wire mesh, and F- basalt fibre mesh), C represents the warp direction (P) or weft direction (T), and i represents the specimen number (refer Tables 1 and 2). 2.2. Composite The tensile properties of the composite specimens (wire- or basalt fibre-reinforced cementitious matrix) have been evaluated using direct tension test on coupons. The coupon dimensions have been decided using the AC 434–2013 [31] guidelines. The length, width, and thickness of the coupons ranged from 420 to 437 mm, 78 to 88 mm, and 11 to 12 mm, respectively. The width of the coupon has been chosen such that it includes a minimum of 2 wires/fibre rovings (Fig. 1) and it is not less than 4 times the thickness of the coupon [31]. The construction procedure consists of the following steps (refer Fig. 2): (a) preparation of plywood moulds; (b) placing the primary layer of cement-sand mortar (1:4 proportion) of 5 mm thickness in the mould; c) placing of wire/fibre mesh on freshly laid mortar; (d) application of the second layer of mortar to attain the required thickness of 11 mm; and (e) giving the desired finish to form a rectangular coupon. The coupons have been initially cured for 7 days in a saturated atmosphere, and then the mould has been removed and stored further for next 21 days at room temperature (approximately 20–25 °C and relative humidity of 50–60%). According to the AC 434–2013 [31] test guidelines, metal tabs of length and thickness of 75 mm and 2 mm, respectively, and the width same as the composite specimen, have been bonded on both sides of the specimen in the gripping zone using an epoxy glue (refer Fig. 2e). This arrangement makes it easier to grip the coupon in the test machine without being damaged by the hydraulic grips. Total 12 number of WRCM and 27 number of BFRCM coupons have been tested in between 28 and 35 days after the construction. Specimens have been tagged as ABC-D-i, where A, B, and C denote the grid size, type of reinforcement, and direction of test (warp and weft, in case of BFRCM), respectively, D denotes the composite type (i.e. WRCM or BFRCM), and i represents the specimen number (refer Tables 3 and 4). 2.3. Test setup Direct tensile test has been used to characterize the tensile behaviour of reinforcement and composites. The direct tension test setup for reinforcement and composite specimens has been shown in Fig. 3. The 165

Structures 23 (2020) 164–179

P.K.V.R. Padalu, et al.

Fig. 1. Details of the reinforcement used in the study: (a) welded wire mesh; and (b) basalt fibre mesh.

tests on different specimens have been executed on a 100 kN tensile testing machine. The load has been applied at a rate of 0.2 mm per minute under displacement control [10,30,31]. Prior to the test, a preload of 15 to 20 N has been applied. The heads of the testing machine contain hydraulic grips for holding the test specimens. Clamp- or clevis- type grips have been used in the past studies, to transfer the tensile force to the coupon specimens [1]. For clevis-type grips, sliding of the reinforcement takes place inside the matrix and the failure is governed by the sliding, without reaching the reinforcement rupture. This clamping method appears to be unsuitable for characterizing the tensile behaviour of mortar-based composites up to failure [1]. On the other side, clamp-type grips have been reported [1] to limit sliding, allowing the reinforcement to reach its tensile capacity and fail in tension. Therefore, in the present study, clamp-type gripping has been used, where the composite specimens (coupons) have been clamped directly on the mortar matrix after having applied metal tabs on both

the sides in the gripping area, to prevent crushing of mortar due to stress concentration and to improve the stress-transfer mechanism at the reinforcement-mortar interface. The details of the gripping arrangement have been shown in Fig. 4. The strain along the length has been found by measuring the total relative deformation of the machine heads. This decision was based on the understanding that the machine heads are quite rigid and no slippage was observed between the machine heads and the metal tabs mounted on the ends of the specimens. This was ensured by marking the position of the machine jaws on the tabs and noting that the position did not shift during the test. However, as it will be discussed later in the article, this could not rule out the slippage of the reinforcement inside the mortar matrix. The tensile force has been measured using a load-cell and all the measurements have been recorded using a data acquisition system.

166

Structures 23 (2020) 164–179

P.K.V.R. Padalu, et al.

Table 1 Geometrical and mechanical properties of wire mesh (WWM) reinforcement. Specimen type

Identifier

Aw (mm2)

lg (mm)

Pmax (kN)

fmax_w (MPa)

fy (MPa)

Ew (GPa)

εy (%)

εu (%)

25 mm grid spaced wire mesh

25 25 25 25 25 25

W-1 W-2 W-3 W-4 W-5 W-6

50 50 50 50 50 50

W-1 W-2 W-3 W-4 W-5 W-6

6.90 6.91 6.90 6.91 6.91 6.93 6.90 0.002 6.85 6.86 6.86 6.86 6.88 6.85 6.86 0.001

115 115 115 115 115 115 115 – 215 215 215 215 215 215 215 –

6.08 6.48 6.52 6.27 6.15 6.39 6.32 0.03 5.56 5.48 5.48 5.49 5.43 5.30 5.45 0.02

880 938 944 908 890 925 914 0.03 810 798 798 799 791 772 795 0.02

831 865 877 847 841 855 853 0.02 763 763 769 779 755 742 762 0.02

66.01 65.11 69.07 65.67 63.29 70.67 66.63 0.04 101.83 99.19 89.51 87.60 93.29 95.20 94.44 0.05

1.26 1.33 1.27 1.29 1.33 1.21 1.28 0.03 0.75 0.77 0.86 0.89 0.81 0.78 0.81 0.06

1.66 1.74 2.07 1.86 1.72 1.99 1.84 0.08 2.73 2.35 2.32 1.50 2.10 1.68 2.11 0.19

Mean value Coefficient of variation 50 mm grid spaced wire mesh

Mean value Coefficient of variation

Note: Aw = Area of a wire; lg = gauge length; Pmax = maximum tensile load; fmax_w = tensile strength of wire; fy = yield stress; Ew = tensile modulus of wire; εy = yield strain; εu = ultimate strain.

3. Test results and discussion

3.1. Wire/textile specimens

The uniaxial tensile tests have been conducted on wire/fibre rovings of a mesh and the composites. Tables 1–4 show the average values and coefficient of variation of the mechanical properties for each type of specimens, while the stress–strain response of all the specimens are shown in Figs. 5, 7, and 8.

To characterize the wire/fibre mesh under tension, 12 specimens of wires (6 no. of 25 mm spaced grid, and 6 no. of 50 mm spaced grid), and 24 specimens of basalt fibre rovings (6 no. in warp direction and 6 no. in weft direction, each, for 25 mm and 50 mm spaced grids) have been tested. In the present study, the diameter of a wire in WWM has

Table 2 Geometrical and mechanical properties of basalt fibre mesh (BFM) reinforcement. Specimen type

Direction

Identifier

Af (mm2)

nf

lg (mm)

Pmax (kN)

fmax_f (MPa)

Ef (GPa)

εmax (%)

25 mm grid spaced basalt fibre mesh

Warp

25FP-1 25FP-2 25FP-3 25FP-4 25FP-5 25FP-6 Mean COV 25FT-1 25FT-2 25FT-3 25FT-4 25FT-5 25FT-6 Mean COV 50FP-1 50FP-2 50FP-3 50FP-4 50FP-5 50FP-6 Mean COV 50FT-1 50FT-2 50FT-3 50FT-4 50FT-5 50FT-6 Mean COV

0.75 0.75 0.75 0.75 0.75 0.75 0.75 – 1.0 1.0 1.0 1.0 1.0 1.0 1.0 – 0.75 0.75 0.75 0.75 0.75 0.75 0.75 – 1.0 1.0 1.0 1.0 1.0 1.0 1.0 –

2 2 2 2 2 2 2 – 2 2 2 2 2 2 2 – 4 4 4 4 4 4 4 – 4 4 4 4 4 4 4 –

135 135 135 135 135 135 135 – 135 135 135 135 135 135 135 – 165 165 165 165 165 165 165 – 155 155 155 155 155 155 155 –

1.56 1.83 1.73 1.17 2.05 1.59 1.66 0.16 1.13 2.36 1.89 1.78 1.48 1.63 1.71 0.22 5.17 4.62 4.76 4.00 4.14 3.80 4.41 0.11 3.37 3.76 2.77 4.20 2.93 4.40 3.57 0.17

1040 1220 1153 780 1366 1060 1103 0.16 565 1180 945 890 740 815 856 0.22 1723 1540 1587 1333 1380 1267 1472 0.11 843 940 693 1050 733 1100 893 0.17

69.33 64.10 63.37 56.25 64.36 55.21 62.10 0.08 49.85 49.16 50.26 52.66 51.75 51.69 50.90 0.03 63.13 63.63 67.04 65.57 62.72 60.41 63.75 0.03 48.32 50.18 53.13 48.98 50.63 46.48 49.62 0.04

1.64 2.01 1.93 1.43 2.22 2.02 1.87 0.14 2.12 2.54 1.97 1.98 1.51 1.67 1.96 0.17 2.87 2.52 2.65 2.08 2.46 2.25 2.47 0.10 2.45 1.94 1.45 2.25 1.49 2.48 2.01 0.21

Weft

50 mm grid spaced basalt fibre mesh

Warp

Weft

Note: Af = area of a fibre yarn; nf = number of fibre yarns in the loading direction; lg = gauge length. Pmax = maximum total tensile load; fmax_f = tensile strength of fibre yarn; Ef = modulus of elasticity of yarn; εmax = strain corresponding to maximum tensile stress; and COV = coefficient of variation.

167

Structures 23 (2020) 164–179

P.K.V.R. Padalu, et al.

Fig. 2. Sequential process of preparation of composite specimens: (a) preparation of mould of the required size of coupons; (b) application of first layer of cement-sand mortar; (c) laying of wire mesh/basalt fibre mesh on the fresh mortar layer; (d) application of second layer of cement-sand mortar; and (e) finished coupon specimens.

been measured using a micrometer screw gauge, whereas in the case of BFM, the value provided by the manufacturer has been used due to the inherent difficulty and sophisticated instrumentation required for determining the cross- sectional area of the fibre yarn. The maximum strain has been found by dividing the recorded displacement by the gauge length (Tables 1 and 2). The gauge length has been considered as

the clear distance between the machine heads. The modulus of elasticity of the reinforcement has been determined as chord modulus between 30% and 60% of the tensile strength of wire/fibre yarn [1]. It has been observed that the stress–strain curves for wires (Fig. 5), are quite linear before yield, and are repetitive. The mean modulus of elasticity (Ew) of 25 and 50 mm spaced grid wires has been found as 67

Table 3 Geometrical and mechanical properties of wire reinforced cementitious matrix (wire-mortar composite) coupons. Specimen type

Composite acronym

L (mm)

b (mm)

t (mm)

Aw (mm2)

nw

Pmax (kN)

fmax_c (MPa)

Ec (GPa)

εmax (%)

Composite(wire mesh of 25 mm grid spacing + 1:4 cementsand mortar)

25 25 25 25 25 25

W-C-1 W-C-2 W-C-3 W-C-4 W-C-5 W-C-6

50 50 50 50 50 50

W-C-1 W-C-2 W-C-3 W-C-4 W-C-5 W-C-6

425 420 425 430 427 430 426 – 430 432 435 427 433 435 432 –

90 89 86 89 86 90 88 – 81 77 81 80 79 81 80 –

11.0 11.0 10.5 11.0 10.5 12.0 11 – 11.5 12.0 11.0 10.0 12.0 11.5 11 –

6.90 6.91 6.90 6.88 6.91 6.91 6.90 – 6.85 6.86 6.86 6.86 6.88 6.85 6.86 –

3 3 3 3 3 3 3 – 2 2 2 2 2 2 2 –

14.5 13.5 13.6 13.2 13.8 15.9 14.08 0.06 9.1 10.0 9.8 9.3 9.2 10.7 9.68 0.06

701 651 657 639 665 766 680 0.06 664 728 714 677 668 780 705 0.06

94.28 101.91 85.92 94.88 92.81 84.76 92.42 0.06 94.32 99.15 88.72 96.79 120.12 69.32 94.74 0.16

1.45 0.94 2.00 1.48 1.19 1.47 1.42 0.23 1.52 2.07 1.24 1.14 1.02 1.53 1.42 0.24

Mean value Coefficient of variation Composite (wire mesh of 50 mm grid spacing + 1:4 cementsand mortar)

Mean value Coefficient of variation

Note: L, b, and t denotes the length, width and thickness of coupon respectively; Aw = Area of a wire; nw = number of wires in the loading direction; Pmax = maximum total tensile load; fmax_c = maximum tensile strength of wire-cement composite, computed on the reinforcement area only, ignoring the mortar area; Ec = tensile modulus of elasticity of composite specimen computed on the reinforcement area only; εmax = strain corresponding to maximum tensile stress.

168

Structures 23 (2020) 164–179

P.K.V.R. Padalu, et al.

Table 4 Geometrical and mechanical properties of basalt fibre reinforced cementitious matrix (fibre-mortar composite) coupons (According to AC 434, Annex – A [31]). Specimen type

Composite (basalt fibre mesh of 25 mm grid spacing + 1:4 cement-sand mortar)

Direction

Warp

Weft

Composite (basalt fibre mesh of 50 mm grid spacing + 1:4 cement-sand mortar)

Warp

Weft

Composite acronym

L (mm)

25FP-C-1 25FP-C-2 25FP-C-3 25FP-C-4 25FP-C-5 25FP-C-6 25FP-C-7 Mean COV 25FT-C-1 25FT-C-2 25FT-C-3 25FT-C-4 25FT-C-5 25FT-C-6 Mean COV 50FP-C-1 50FP-C-2 50FP-C-3 50FP-C-4 50FP-C-5 50FP-C-6 50FP-C-7 Mean COV 50FT-C-1 50FT-C-2 50FT-C-3 50FT-C-4 50FT-C-5 50FT-C-6 50FT-C-7 Mean COV

420 420 422 430 431 427 420 424 – 420 420 419 421 419 422 420 – 421 417 420 422 422 420 422 421 – 435 437 435 440 440 438 437 437 –

b (mm)

77 80 80 79 75 76 79 78 – 77 77 81 78 80 79 79 – 86 89 90 83 86 90 88 87 – 79 86 88 90 88 88 87 87 –

t (mm)

12 12 14 11 12 10 11 12 – 11 11 13 10 11 12 11 – 11 12 11 12 11 12 12 11 – 10 11 11 10 10 12 11 11 –

nr

3 3 3 3 3 3 3 3 – 3 3 3 3 3 3 3 – 2 2 2 2 2 2 2 2 – 2 2 2 2 2 2 2 2 –

nf

6 6 6 6 6 6 6 6 – 6 6 6 6 6 6 6 – 8 8 8 8 8 8 8 8 – 8 8 8 8 8 8 8 8 –

Pmax (kN)

2.04 1.82 2.19 1.66 1.15 1.08 2.90 1.83 0.32 2.36 2.61 2.19 2.18 2.45 2.32 2.35 0.06 2.13 3.11 2.79 3.30 3.22 2.05 2.99 2.80 0.17 2.64 1.62 1.81 2.83 1.96 2.52 1.78 2.16 0.20

Uncracked part

Cracked part

fft* (MPa)

εft (%)

Efu (GPa)

ffu* (MPa)

εfu (%)

Efc (GPa)

270 207 245 192 140 185 370 230 0.30 200 195 155 165 190 243 191 0.15 128 300 235 225 130 205 250 210 0.27 185 140 165 145 100 120 130 141 0.18

0.16 0.27 0.15 0.13 0.10 0.10 0.31 0.17 0.44 0.10 0.18 0.17 0.16 0.11 0.23 0.16 0.27 0.10 0.25 0.21 0.15 0.12 0.17 0.21 0.17 0.30 0.45 0.29 0.21 0.15 0.10 0.15 0.10 0.21 0.56

168.75 76.67 163.33 147.69 140.00 185.00 119.35 142.97 0.23 200.00 108.33 91.17 103.12 172.72 105.65 130.17 0.32 142.22 120.00 111.90 150.00 108.33 120.58 119.04 124.58 0.12 41.11 48.27 78.57 96.66 100.00 80.00 130.00 82.08 0.34

453 404 487 369 256 240 644 407 0.32 393 435 365 363 408 387 392 0.06 355 518 465 550 537 342 498 466 0.17 330 203 226 354 245 315 223 270 0.20

0.99 1.90 1.36 0.73 0.78 0.42 1.82 1.14 0.46 1.50 1.85 1.85 1.31 1.47 1.36 1.56 0.14 1.01 1.33 1.36 1.45 1.33 1.64 1.26 1.34 0.13 1.32 2.46 0.44 1.17 0.71 0.74 0.58 1.06 0.60

25.80 13.96 27.08 32.94 20.01 28.68 18.24 23.82 0.26 16.30 15.63 12.40 17.16 16.20 16.18 15.65 0.10 25.91 28.17 27.35 25.54 37.09 10.98 27.84 26.13 0.27 17.28 3.96 34.45 20.29 24.41 37.05 25.28 23.25 0.44

Note: L, b, and t denotes the length, width and thickness of coupon respectively; nr = number of fibre rovings; nf = number of fibre yarns in the loading direction; Pmax = maximum total tensile load; fft and εft = tensile stress and strain corresponding to the transition point, respectively; Efu = tensile modulus of elasticity of the uncracked specimen; ffu and εfu = ultimate tensile strength and strain, respectively; and Efc = tensile modulus of elasticity of the cracked specimen. *The stresses have been computed on reinforcement area only, ignoring the mortar area.

Fig. 3. Test-setup for specimens in direct tension.

169

Structures 23 (2020) 164–179

P.K.V.R. Padalu, et al.

(εy) recorded in wire specimens has been found as 1.28 and 0.81% for 25 mm and 50 mm spaced grid wires, respectively. The mean ultimate strain (εu) recorded in the 25 mm grid spaced wires is 1.84%, whereas, for 50 mm grid spaced wires, it is 2.11%. The difference in the mechanical properties of the WWM may be due to different sources of supply from the local market, which may have resulted in different chemical compositions and manufacturing methods. The fibre yarns of a roving in BFM has shown linear stress–strain curves and a brittle failure (Figs. 7–8). For fibre mesh of 25 mm and 50 mm spaced grid, the mean elastic modulus (Ef) of a fibre yarn has been computed as about 63 GPa in the warp direction and about 50 GPa in weft direction. The sequential load drops and partial regains in the stress–strain curves of the BFM near the failure, can be attributed to the progressive breaking of individual fibres or yarns of a roving (Fig. 6b–e). The mean tensile strength (fmax_f) of 25 mm and 50 mm spaced grid fibre yarn of a mesh has been observed as 1103 and 1472 MPa, respectively, in warp direction, whereas in weft direction, it has been observed as 856 and 893 MPa, respectively. This variation in tensile strength of BFM is similar to the studies carried out by Lignola et al. (2017) [2] and Caggegi et al. (2017) [26]. This is due to the woven structure of the grid in both the warp and weft directions, and the effect is mostly caused by the unevenness (ecentricity), and the possible accidental non-uniformity in the load [2]. In case of BFM, the mean ultimate strain (εmax) of 25 mm and 50 mm spaced grid in warp direction has been observed as 1.87 and 2.47%, respectively, whereas in weft direction, it has been observed close to 2.0% in both the grid spacing. This difference in properties in warp and weft directions has been observed due to the influence of the weaving process [34]. All the experimental results of the tests on wire/fibre yarns show fairly low scatter (coefficient of variation ≤ 0.22) as reported in Tables 1 and 2.

Fig. 4. Schematic details of composite specimen in direct tensile test-setup.

3.2. WRCM/BFRCM coupons

and 94 GPa, respectively. The WWM reinforcement has shown some ductility (Fig. 5b), especially in case of the 50 mm spaced grid mesh before rupture of the wires (Fig. 6a). The mean yield strength (fy) of 25 mm and 50 mm spaced grid wires has been obtained as 853 and 762 MPa, respectively. The variation in tensile strength of wires has been observed to be similar to the past test results by Kadam et al. (2014) [32] and Shermi and Dubey (2017) [33]. The mean yield strain

The mechanical properties of a reinforcement-mortar coupon, primarily depends on the tensile strength of the reinforcement and the bond strength between the reinforcement and mortar interface. Fig. 9 shows the three most common failure patterns observed during tensile test of composite specimens [20,22]. Total 12 wire-mortar composite specimens (6 of 25 mm spaced grid, and 6 of 50 mm spaced grid), and 27 specimens of basalt fibre-mortar

Fig. 5. Stress–strain response curves of the wires of a mesh (shown in blue colour) and wire mesh-mortar composite specimens (shown in red colour) with: (a) 25 mm spaced wire grid; and (b) 50 mm spaced wire grid. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

170

Structures 23 (2020) 164–179

P.K.V.R. Padalu, et al.

Fig. 6. Failure patterns of reinforcement: (a) wire of a mesh; (b, c) rupture of yarns in 25 mm spaced grid fibre mesh along warp and weft directions, respectively; (d, e) failure of fibre yarns in 50 mm spaced grid mesh along warp and weft directions, respectively.

coupons (7 in warp direction for 25 mm and 50 mm spaced grid each, 6 in weft direction for 25 mm spaced grid, and 7 in weft direction for 50 mm spaced grid) have been tested. In case of WRCM composite specimens, the cracks occurred along the width and were distributed evenly in the full length of the specimen (Fig. 10a). The final failure has been caused because of the sequential breaking of wires (Fig. 10b) inside the cementitious matrix (failure mode ‘A’ in Fig. 9) for majority of the specimens, but for a few specimens showed tensile failure of wires near the gripping area (failure mode ‘B’ in Fig. 9). Majority of the specimens of BFRCM coupons have shown slippage (failure Mode ‘C’ in Fig. 9) between the mortar and the fibres, except one specimen, in which sequential rupture of fibre yarns has been observed (Fig. 11). The sequential breaking of wires/fibres corresponds to the lack of ecentricity. The slippage has typically been observed in the gripping areas, without any tensile rupture of fibres. The slippage has been preceded by

numerous cracks along the width, distributed homogeneously over the entire specimen length. The slippage of fibres can be mainly attributed to the low bonding strength at the interface between the mortar and the fibre-mesh. The stress–strain curves of WRCM coupons are shown in Fig. 5. It can be seen that the coupons showed a behaviour, reasonably close to the strength and stiffness of the WWM reinforcement. For comparison purposes, the tensile stress in the coupons has also been determined by dividing the load applied in the coupons by the cross-sectional area of the reinforcing wires only, ignoring the mortar area. As shown in Table 3, there has been a reduction of 25% and 11% in strength, and 22% and 33% in total strain of the 25 mm and 50 mm spaced grid wiremortar matrix, respectively, in comparison with the wire reinforcement, whereas, the initial stiffness is in a close match. This reduction in the strength can be attributed to the sequential failure of the multiple

Fig. 7. Stress–strain curves of 25 mm spaced fibre yarns of a grid (shown in blue colour) and fibre grid-mortar composite coupons (shown in red colour), tested in: (a) warp direction; and (b) weft direction. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

171

Structures 23 (2020) 164–179

P.K.V.R. Padalu, et al.

Fig. 8. Stress–strain curves of 50 mm spaced fibre yarns of a grid (shown in blue colour) and fibre grid-mortar composite coupons (shown in red colour), tested in: (a) warp direction; and (b) weft direction. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 9. Failure modes of composite specimens in direct tension: (a) Mode ‘A’ – cracking within the length of specimen followed by rupture of reinforcement; (b) Mode ‘B’ – tensile failure near the gripping area; and (c) Mode ‘C’ – cracking of specimen followed by slippage of reinforcement.

wires present in the coupons. This effect is caused due to the ecentricity and the stress concentration in a single wire, which is divided by the larger cross-sectional area, when tensile tress is calculated. Two separate phases (Figs. 7 and 8), i.e. cracked and uncracked behaviour in stress–strain response, have been identified for BFRCM coupons. The AC 434–2013 [31] proposes a bi-linear idealization (Fig. 12) of the stress–strain curves of BFRCM coupons. The initial portion of the curve is considered a straight line representing the behaviour up to cracking of cementitious matrix. Up to this stage, the tensile force is resisted primarily by the composite action of the basalt fibre mesh and cementious matrix. After the crack formation, it deviates from the initial curve, and follows another line at a reduced slope (stiffness), until the start of slippage and progressive breaking of fibre yarns inside the rovings, as illustrated in Fig. 12. A bi-linear curve can represent the complete stress–strain plot with a bend-over point (transition point as defined in AC 434–2013 [31]) corresponding to the intersection of the initial and secondary linear segments. The initial slope

of the linear segment of the curve, represents the tensile modulus of elasticity (Efu) of uncracked section, wheras the slope of the second segment, corresponds to the linear behaviour of the basalt FRCM post cracking, and is represented by the cracked tensile modulus of elasticity (Efc). The tensile stress corresponding to the ultimate point has been calculated as

f fu =

Pmax Af ws

(1)

where Pmax denotes the maximum load before failure, Af is the crosssectional area of grid-mesh reinforcement per unit width; and ws denotes the specimen’s width. Fig. 12 shows the typical original and idealized stress–strain curves for the 6 specimens tested in weft direction of the 25 mm spaced grid mesh. The cracked modulus (Efc) has been obtained from the slope of the chord between two points on the experimental curve at stress levels equal to 90% and 60% of the tensile strength (i.e. 0.9 ffu and 0.6 ffu), 172

Structures 23 (2020) 164–179

P.K.V.R. Padalu, et al.

Fig. 12. Bi-linear idealization of stress–strain curves of FRCM composite specimen in tension (According to AC434, Annex – A [31]).

To characterize the complete behaviour of BFRCM coupon in tension, some more parameters such as tensile strength and strain at transition point (fft, εft) have also been determined from the experimental curves. Table 4 shows the tensile properties of BFRCM coupons along the warp and weft directions for different grid spacing. It can be seen from the comparison of Tables 2 and 4 that the average ultimate tensile strength (calculated based on the area of fibre yarns) of fibremortar matrix coupon shows a reduction of 65% and 62% for the warp and the weft directions, respectively, compared to the tensile strength of fibre yarn. There is also a marked reduction in average maximum strain, equal to 42% and 33% in the warp and the weft directions, respectively, compared to the dry fibre yarn. The reduction in stiffness (average cracked modulus of elasticity) of the BFRCM coupon is even larger (60% and 61%, respectively in the warp and the weft directions). These results indicate that the basalt FRCM coupons did not fully utilize the tensile capacity of fibres. This reduction in ultimate strength and modulus of elasticity of the BFRCM coupons in comparison with the embedded fibre mesh has also been observed by other researchers [8,28], which can be attributed to the gradual slippage of fibres (Fig. 11) in matrix and flat branch of the curves (Figs. 7 and 8) is caused by friction between fibres and matrix, and sequential failure of the fibre

Fig. 10. Failure patterns of wire reinforced cementitious matrix (WRCM) composites subjected to direct tension: (a) multiple cracks in mortar; and (b) sequential rupture of wires.

given as [31]

Efc =

0.9f fu f @0.9f fu

0.6f fu f @0.6f fu

(2)

The tensile strain corresponding to the ultimate point has been determined as the y-intercept of the line used to calculate Efc (i.e. yintercept = 0.6 ffu - Efcε[email protected]) and given as fu

=

f fu

yintercept Efc

(3)

Fig. 11. Failure modes of basalt fibre reinforced cementitious matrix (BFRCM) coupons in tension: (a) initial cracking of cement mortar; (b) expulsion of mortar fragments and slippage of fibre yarns in the mortar matrix; and (c) sequential failure of fibre yarns. 173

Structures 23 (2020) 164–179

P.K.V.R. Padalu, et al.

yarns in a roving within the cementitious matrix as seen in Fig. 11. All the experimental results of the tests on composite specimens (Table 4) show higher scatter (coefficient of variation up to 0.60) in comparison with the dry mesh (Table 2).

d) The frictional interface shear stress is constant along the debonded interface. e) Global load sharing is assumed for the reinforcement. f) Load sharing is uniform between the longitudinal reinforcement (no post-breakage redistribution).

4. Analytical modelling

According to the ACK theory, the slope of the first branch (Stage-I) has been defined by the constitutive law of composite:

Two models are available in literature for modelling of fibre reinforced cementious matrix (FRCM) composites. In the Aveston–Cooper–Kelly (ACK) model, three different stages in axial tension response of FRCM have been identified. In the simplified trilinear (STL) model simplified relationships for estimating the response in the three stages have been provided. In the present study the two models have been applied to the WRCM and BFRCM composites and the predicted response has been compared with the experimental results.

(4)

Et , I , ACK = Em Vm + Er Vr

where, Et,I represents the stiffness of the composite, Em and Er are the tensile elastic moduli (stiffness) of mortar and reinforcement (wire/ fibre), respectively, and Vm and Vr are the corresponding volume fractions. The volumetric fractions of mortar and reinforcement can be calculated as

4.1. ACK theory

Am

Vm =

The Aveston-Cooper-Kelly theory (ACK) was developed by Aveston et al. (1971) [35] and Aveston and Kelly (1973) [36], to theoretically estimate the stress–strain behaviour of a composite with a brittle matrix, in which the reinforcement-matrix bond remains intact after the matrix has cracked. Later, a few researchers [21,27,37,38] modelled the multiple cracking in brittle matrix composites using the ACK theory. The ACK theory consists of an idealized model of the experimental stress–strain curve of composite system (Fig. 13). In this approach, the tensile constitutive law of composite system is characterized by means of a tri-linear behaviour. The model represents three different behaviour stages in the tensile behaviour of the composite [27,35] (Fig. 13). Stage-I (Pre-cracking), represents the linear elastic stage without cracks, where mortar and wire/fibre mesh deform together. Stage-II (Crack formation), represents multi-cracking of brittle mortar, and in Stage-III (Post-cracking), the stresses are borne by the wire or fibre mesh, after cracking of the mortar. The nature of the response in StageIII is governed by the reinforcement, which is expected to be linear elastic in case of BFRCM and non-linear due to yielding of WWM wires in case of WRCM. The basic assumptions used in the development of the ACK theory are [21,27,37]:

Vr =

Ar

(5)

Am

Ar Am

(6)

where, Ar is the cross-sectional area of the reinforcement, and Am is the sectional area (=b × t) of composite specimen. Stage-I ends at a tensile stress (σmc,ACK), when the cracking initiates in the mortar. At this stage, the stress in the composite is governed by the tensile failure stress of mortar (σmu). The tensile stress at the end of Stage-I, has been calculated as mc, ACK

=

Et , I , ACK Em

mu

(7)

where, σmu represents the tensile strength of mortar. The strain corresponding to σmc,ACK at Stage-I has been calculated as

=

t , I , ACK

mc, ACK

(8)

Et , I , ACK

The second branch (Stage-II), corresponding to multiple crack formation along the specimen, has been defined by a constant load plateau, until the following axial strain is developed:

a) The reinforcement is capable of carrying the load only along its longitudinal axis. b) The matrix-reinforcement bond is weak. c) Once the matrix and the reinforcement debond, a pure frictional shear stress governs the matrix-reinforcement interface behaviour.

t , II , ACK

= (1 + 0.666 e)

mu

(9)

Em

where, αe denotes the weighted homogenization coefficient, defined as [27] e

=

Em Vm Er Vr

(10)

In the Stage-III, only reinforcement (wire/fibre) contribute to the stresses associated with the applied deformation. The stress in the matrix (mortar) remains constant despite the increase in the tensile load (linearly in case of BFRCM) up to the ultimate capacity of the reinforcement. The composite stiffness in this stage has been defined as (11)

Et , III , ACK = Er Vr

The tensile strength and elastic modulus of the reinforcement is obtained from the experimental results of the wire/fibre yarn testing. Thus, the strain at the end of Stage-III has been determined as [21] t , III , ACK

=

Ar fmax Am Et , III , ACK

mc, ACK

Et , III , ACK Et , III , ACK

t , II , ACK

(12)

where, fmax is the tensile strength of the reinforcement. The ultimate tensile strength of composite at the end of Stage-III has been determined as Fig. 13. Typical stress–strain curve and ACK model of FRCM in tension [27]

mf , ACK

174

= Et , III , ACK

t , III , ACK

(13)

Structures 23 (2020) 164–179

P.K.V.R. Padalu, et al.

Fig. 14. Comparison between analytical and experimental tensile response of wire reinforced cementitious matrix (WRCM) composites for: (a) 25 mm spaced wire grid; and (b) 50 mm spaced wire grid.

Fig. 15. Comparison between analytical and experimental tensile response of basalt fibre reinforced cementitious matrix (BFRCM) composites for: (a) 25 mm grid spaced mesh along warp direction; (b) 25 mm grid spaced mesh along weft direction; (c) 50 mm grid spaced mesh along warp direction; and (d) 50 mm grid spaced mesh along weft direction.

175

Structures 23 (2020) 164–179

Composite (wire mesh of 50 mm grid spacing + 1:4 cement-sand mortar)

Note: Vm = volumetric fraction of mortar in composite matrix; Vr = volumetric fraction of reinforcement (wire) in composite matrix; Et,I = stiffness of the composite in Stage-I; σmc = first cracking tensile stress at the end of Stage-I. εt,I = strain corresponding to σmc at Stage-I; εt,II = strain at the end of Stage-II; Et,III = stiffness of the composite in Stage-III; σmf = ultimate tensile stress of the composite; εt,II = strain at the end of Stage-III; EXP = experimental value; ACK = Aveston-Cooper-Kelly model; and STL = simplified tri-linear model.

1.37 1.37 1.37 1.37 1.37 1.37 1.37 0.84 0.84 0.84 0.84 0.84 0.84 0.84 1.34 1.34 1.34 1.34 1.34 1.33 1.34 0.81 0.81 0.81 0.81 0.81 0.81 0.81 1.45 0.94 2.00 1.48 1.19 1.47 1.42 1.52 2.07 1.24 1.14 1.02 1.53 1.42 14.65 13.78 15.05 13.47 15.26 14.71 14.49 9.76 10.81 11.00 11.61 9.70 11.48 10.73 1.39 1.41 1.53 1.40 1.53 1.28 1.42 1.39 1.40 1.45 1.62 1.37 1.39 1.44 0.07 0.07 0.07 0.07 0.07 0.08 0.07 0.07 0.07 0.07 0.06 0.08 0.07 0.07 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 1.51 1.51 1.52 1.51 1.52 1.51 1.51 1.52 1.52 1.53 1.53 1.52 1.52 1.52 0.79 1.02 0.80 0.63 0.85 0.58 0.78 0.44 0.56 1.39 0.43 0.58 1.32 0.79 25 W-C-1 25 W-C-2 25 W-C-3 25 W-C-4 25 W-C-5 25 W-C-6 Mean 50 W-C-1 50 W-C-2 50 W-C-3 50 W-C-4 50 W-C-5 50 W-C-6 Mean Composite (wire mesh of 25 mm grid spacing + 1:4 cement-sand mortar)

0.9791 0.9788 0.9771 0.9789 0.9770 0.9808 0.9786 0.9853 0.9851 0.9846 0.9828 0.9855 0.9853 0.9848

0.0209 0.0212 0.0229 0.0211 0.0230 0.0192 0.0214 0.0147 0.0149 0.0154 0.0172 0.0145 0.0147 0.0152

1.44 1.69 1.60 1.27 2.12 1.15 1.55 0.74 1.13 1.39 0.54 1.16 1.66 1.10

29.67 29.68 29.74 29.68 29.75 29.60 29.69 29.84 29.85 29.89 30.00 29.83 29.84 29.88

29.65 29.66 29.73 29.66 29.73 29.59 29.67 29.83 29.84 29.87 29.99 29.82 29.83 29.86

1.52 1.52 1.52 1.52 1.52 1.51 1.52 1.52 1.52 1.53 1.53 1.52 1.52 1.52

0.06 0.06 0.05 0.05 0.03 0.05 0.05 0.06 0.05 0.10 0.08 0.05 0.08 0.07

0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005

0.08 0.09 0.10 0.12 0.08 0.10 0.10 0.10 0.10 0.15 0.12 0.12 0.24 0.14

1.97 2.16 1.97 2.00 2.13 1.63 1.98 1.39 1.47 1.37 1.66 1.74 1.02 1.44

1.36 1.37 1.49 1.37 1.49 1.24 1.39 1.33 1.34 1.39 1.56 1.31 1.33 1.38

18.61 18.87 20.46 18.79 20.50 17.07 19.05 11.20 11.32 11.76 13.16 11.06 11.21 11.62

19.10 19.35 20.94 19.27 20.98 17.55 19.53 11.69 11.81 12.25 13.64 11.54 11.70 12.11

ACK EXP STL ACK EXP STL ACK EXP ACK/STL EXP STL ACK EXP STL ACK EXP STL ACK EXP

σmf (MPa) Et,III (GPa) εt,II (%) εt,I (%) σmc (MPa) Et,I (GPa) Vr Vm

Composite acronym Specimen type

Table 5 Comparison of the experimental results for the wire reinforced cementitious matrix (wire-mortar composite) coupons with the estimates from the ACK and simplified tri-linear models.

εt,III (%)

STL

P.K.V.R. Padalu, et al.

4.2. Simplified tri-linear model The simplified tri-linear model proposed by Minafo and Mendola (2018) [37] also considers three stages in the tensile response of FRCM composites, similar to ACK model. The first cracking stress in Stage-I has been calculated as

mc, STL

(A

m

=

Er A Em r

+

)

mu

Am + Ar

(14)

Similarly, The strain corresponding to σmc,STL at Stage-I has been calculated as t , I , STL

mu

=

(15)

Em

The strain (εt,II,STL) has been calculated similar to the ACK model, and given by Eq. (9). Finally, the stress at the end of Stage-III is calculated as mf , STL

= fmax Vr +

mu Vm

(16)

The strain at the end of Stage-III has been calculated as t , III , STL

mf , STL

=

(17)

Er Vr

The tensile modulus of elasticity of composite at the end of Stage-III has been calculated as

Et , III , STL =

mf , STL

mc, STL

t , III , STL

t , II , STL

(18)

In the present study, the tensile strength of mortar has been considered as 1.48 MPa (one tenth of compressive strength of the mortar), whereas the tensile modulus of mortar has been obtained as 28.88 GPa, using Eq. (19) [39].

Em = 4.3

1

() w c

+

21T T + 1.5

(19)

where, w/c is the water-cement ratio (=0.48), and T is the curing period in days (=28). The results of the ACK and simplified tri-linear (STL) models have been compared with the experimental results, in Figs. 14 and 15 for WRCM and BFRCM, respectively. It has been observed that both the models predict the response quite close to each other, and the first cracking load is predicted reasonably close to the experimental results for the WRCM as well as BFRCM composites (Figs. 14 and 15). However, the initial stiffness has been substantially overestimated. This error can be due to the fact that the tensile elastic modulus (Em) of the mortar has been estimated analytically using empirical expression, as the experimental evaluation of Em is a quite difficult task. The slope of the third stage is well predicted in case of the WRCM, for both 25 W-C (Fig. 14a) and 50 W-C (Fig. 14b) specimens. However, it has been overestimated in case of all the BFRCM composite specimens (Fig. 15). This discrepancy is caused due to the progressive rupture of filaments inside the fibre yarns and slippage (debonding) of the fibres in the mortar between the metal tabs in the gripping area. As discussed earlier, caution was taken to monitor the slippage of the specimens between the machine grips, by marking the position of grip jaws on the metal tabs. Use of hydraulic grips avoided the slippage of the metal tabs, but the slippage of the fibres could not be avoided. Tables 5 and 6 compare the major model parameters with the experimental results for the WRCM and BFRCM respectively. It is observed that the ultimate stress at Stage-III in WRCM composites has been over-predicted by 17% by both the models compared to the average experimental results. This difference has occurred due to non-uniform distribution of stress and sequential failure of wires (Fig. 10) in the mesh. In case of BFRCM composite specimens, both the models overestimate the ultimate stress at Stage-III, significantly. As discussed earlier, the possible reason for 176

177

Weft

Warp

25FP-C-1 25FP-C-2 25FP-C-3 25FP-C-4 25FP-C-5 25FP-C-6 25FP-C-7 Mean 25FT-C-1 25FT-C-2 25FT-C-3 25FT-C-4 25FT-C-5 25FT-C-6 Mean 50FP-C-1 50FP-C-2 50FP-C-3 50FP-C-4 50FP-C-5 50FP-C-6 50FP-C-7 Mean 50FT-C-1 50FT-C-2 50FT-C-3 50FT-C-4 50FT-C-5 50FT-C-6 50FT-C-7 Mean

Composite acronym 0.9951 0.9953 0.9960 0.9948 0.9950 0.9941 0.9948 0.9950 0.9929 0.9929 0.9943 0.9923 0.9932 0.9937 0.9932 0.9937 0.9944 0.9939 0.9937 0.9937 0.9944 0.9941 0.9940 0.9899 0.9915 0.9917 0.9911 0.9909 0.9924 0.9916 0.9913

Vm

0.0049 0.0047 0.0040 0.0052 0.0050 0.0059 0.0052 0.0050 0.0071 0.0071 0.0057 0.0077 0.0068 0.0063 0.0068 0.0063 0.0056 0.0061 0.0063 0.0063 0.0056 0.0059 0.0060 0.0101 0.0085 0.0083 0.0089 0.0091 0.0076 0.0084 0.0087

Vr

0.82 0.36 0.66 0.76 0.70 1.10 0.62 0.72 1.42 0.77 0.52 0.79 1.18 0.67 0.89 0.90 0.67 0.68 0.94 0.69 0.67 0.71 0.75 0.42 0.41 0.65 0.86 0.91 0.61 1.09 0.70

29.04 29.04 29.01 29.05 29.05 29.08 29.05 29.05 29.04 29.04 29.01 29.05 29.03 29.02 29.03 29.10 29.08 29.09 29.10 29.10 29.07 29.09 29.09 29.09 29.06 29.05 29.06 29.07 29.04 29.05 29.06

29.04 29.03 29.01 29.05 29.05 29.08 29.05 29.04 29.03 29.03 29.00 29.05 29.03 29.02 29.03 29.10 29.07 29.09 29.10 29.10 29.07 29.09 29.09 29.09 29.05 29.05 29.06 29.07 29.04 29.05 29.06

1.31 0.97 0.98 0.99 0.70 1.10 1.92 1.14 1.42 1.38 0.88 1.27 1.30 1.54 1.30 0.81 1.69 1.42 1.41 0.82 1.14 1.48 1.25 1.87 1.18 1.36 1.29 0.91 0.91 1.09 1.23

1.48 1.48 1.48 1.48 1.48 1.49 1.48 1.48 1.48 1.48 1.48 1.48 1.48 1.48 1.48 1.49 1.49 1.49 1.49 1.49 1.48 1.49 1.49 1.49 1.48 1.48 1.48 1.48 1.48 1.48 1.48

ACK

EXP

STL

EXP

ACK

σmc (MPa)

Et,I (GPa)

1.48 1.48 1.48 1.48 1.48 1.48 1.48 1.48 1.48 1.48 1.48 1.48 1.48 1.48 1.48 1.49 1.48 1.49 1.49 1.49 1.48 1.49 1.49 1.49 1.48 1.48 1.48 1.48 1.48 1.48 1.48

STL 0.16 0.27 0.15 0.13 0.10 0.10 0.31 0.17 0.10 0.18 0.17 0.16 0.11 0.23 0.16 0.10 0.25 0.21 0.15 0.12 0.17 0.21 0.17 0.45 0.29 0.21 0.15 0.10 0.15 0.10 0.21

EXP

εt,I (%)

0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005

ACK 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005

STL 0.33 0.34 0.40 0.31 0.32 0.27 0.31 0.33 0.28 0.28 0.34 0.25 0.29 0.31 0.29 0.25 0.28 0.26 0.25 0.25 0.28 0.26 0.26 0.20 0.24 0.24 0.23 0.22 0.26 0.24 0.23

ACK/STL

εt,II (%)

0.13 0.07 0.11 0.17 0.10 0.17 0.09 0.12 0.12 0.11 0.07 0.13 0.11 0.10 0.11 0.16 0.16 0.17 0.16 0.24 0.06 0.17 0.16 0.17 0.03 0.28 0.18 0.22 0.28 0.21 0.20

EXP 0.30 0.29 0.25 0.32 0.31 0.37 0.32 0.31 0.36 0.36 0.29 0.39 0.35 0.32 0.35 0.40 0.36 0.39 0.40 0.40 0.35 0.38 0.38 0.50 0.42 0.41 0.44 0.45 0.38 0.41 0.43

ACK

Et,III (GPa)

0.27 0.26 0.21 0.29 0.28 0.34 0.29 0.28 0.33 0.33 0.25 0.36 0.31 0.29 0.31 0.38 0.33 0.36 0.38 0.38 0.33 0.35 0.36 0.47 0.39 0.38 0.41 0.42 0.34 0.38 0.40

STL 2.21 1.89 1.96 1.91 1.28 1.42 3.33 2.00 2.78 3.08 2.08 2.79 2.78 2.45 2.66 2.25 2.91 2.82 3.46 3.41 1.90 2.95 2.81 3.34 1.72 1.87 3.15 2.23 2.39 1.86 2.36

EXP

4.88 4.68 3.94 5.22 5.02 6.04 5.22 5.00 5.57 5.57 4.39 6.10 5.35 4.93 5.32 8.85 7.78 8.43 8.76 8.85 7.69 8.24 8.37 8.56 7.06 6.89 7.45 7.63 6.28 6.98 7.26

ACK

σmf (MPa)

5.37 5.17 4.43 5.71 5.52 6.53 5.71 5.49 6.06 6.06 4.88 6.58 5.84 5.42 5.81 9.34 8.27 8.92 9.25 9.34 8.18 8.73 8.86 9.04 7.55 7.38 7.94 8.12 6.77 7.46 7.75

STL

0.99 1.90 1.36 0.73 0.78 0.42 1.82 1.14 1.50 1.85 1.85 1.31 1.47 1.36 1.56 1.01 1.33 1.36 1.45 1.33 1.64 1.26 1.34 1.32 2.46 0.44 1.17 0.71 0.74 0.58 1.06

EXP

1.61 1.61 1.58 1.62 1.62 1.64 1.62 1.62 1.55 1.55 1.51 1.56 1.54 1.53 1.54 2.19 2.17 2.18 2.19 2.19 2.17 2.18 2.18 1.70 1.68 1.68 1.69 1.69 1.67 1.68 1.69

ACK

εt,III (%)

1.78 1.78 1.78 1.78 1.78 1.78 1.78 1.78 1.68 1.68 1.68 1.68 1.68 1.68 1.68 2.31 2.31 2.31 2.31 2.31 2.31 2.31 2.31 1.80 1.80 1.80 1.80 1.80 1.80 1.80 1.80

STL

Note: Vm = volumetric fraction of mortar in composite matrix; Vr = volumetric fraction of reinforcement (fibre) in composite matrix; Et,I = stiffness of the composite in Stage-I; σmc = first cracking tensile stress at the end of Stage-I. εt,I = strain corresponding to σmc at Stage-I; εt,II = strain at the end of Stage-II; Et,III = stiffness of the composite in Stage-III; σmf = ultimate tensile stress of the composite; εt,II = strain at the end of Stage-III; EXP = experimental value. ACK = Aveston-Cooper-Kelly model; and STL = simplified tri-linear model.

Composite (basalt fibre mesh of 50 mm grid spacing + 1:4 cement-sand mortar)

Warp

Composite (basalt fibre mesh of 25 mm grid spacing + 1:4 cement-sand mortar)

Weft

Direction

Specimen type

Table 6 Comparison of the experimental results for the basalt fibre reinforced cementitious matrix (fibre-mortar composite) coupons with the estimates from the ACK and simplified tri-linear models.

P.K.V.R. Padalu, et al.

Structures 23 (2020) 164–179

Structures 23 (2020) 164–179

P.K.V.R. Padalu, et al.

this is the gradual slippage of fibres within the mortar in the gripping area (Fig. 11). As a result, the fibre yarns could not reach the ultimate stress.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

5. Conclusion

Acknowledgement

A series of tensile tests has been carried out on dry mesh reinforcement and cement-mortar based composites comprising of wiremesh and basalt fibre-mesh to gather the basic information required for design of strengthening system for masonry structures. The main conclusions of the study are as follows:

The authors declare that they have no conflict of interest. The first author has received a fellowship from the Ministry of Human Resource Development (MHRD), Government of India (Grant No: OH-31-23-200428), for conducting this research work. The support received from the funding agency is gratefully acknowledged.

• The stress–strain curves for wire and fibre reinforcement are fairly • • •

•

•

repeatable with a COV of mechanical properties being less than 22%. However, the composite specimens have shown relatively larger scattering with the COV up to 60%. WRCM and BFRCM have shown a non-linear behaviour comprising of three distinct stages. The first stage is controlled by the stiffness of the mortar. The second stage is characterized by multiple cracking of mortar, and the third stage is governed by the tensile behaviour of the reinforcement. The uncracked and cracked phases are found to be more pronounced in the basalt fibre-mortar composite, as the contribution of the cement-mortar matrix is significant in the initial stage (pre-cracking), while it is almost negligible in case of the wire-mortar composites. The tensile strength, breaking strain, and elastic modulus of composites is invariably lower than the reinforcement. The difference is particularly large in case of BFRCM (nearly 63%, 37%, and 60%, in the tensile strength, breaking strain, and elastic modulus, respectively), because of the slippage of fibres within the mortar matrix, particularly in the gripping area. In case of the WRCM coupons, only 18% and 27% of reduction in strength and strain, respectively has been observed compared to the wire reinforcement, since the failure mode involved rupture of wires, however, in a sequential pattern. On the other hand, the fibres in the BFRCM specimens could not reach their ultimate stress due to slippage. Both ACK and the STL models based on linear stress–strain behaviour of mortar and reinforcement, yield a good prediction of the first cracking load, whereas the initial stiffness could not be estimated accurately due to non-availability of the reliable estimates of the tensile modulus of elasticity of the mortar. The composite stiffness in Stage-III has been predicted with reasonable accuracy in case of WRCM, whereas for BFRCM composites, the experimental stiffness was much lower than the predicted value, mainly due to slippage of fibres inside the mortar matrix in the gripping zone. This highlights the need for further evaluation and standardization of the testing procedure, especially the gripping of the specimen in the tension test machine. Both the wire and basalt fibre mesh reinforced cementitious matrix (WRCM and BFRCM) composites have demonstrated a significant potential and may be used for strengthening of masonry structures. The experimentally obtained mechanical properties can be used to design the strengthening scheme.

References [1] De Santis S, De Felice G. Tensile behaviour of mortar-based composites for externally bonded reinforcement systems. Compos B Eng 2015;68:401–13. [2] Lignola GP, Caggegi C, Ceroni F, De Santis S, Krajewski P, Lourenco PB, et al. Zuccarino L. Performance assessment of basalt FRCM for retrofit applications on masonry. Compos B Eng 2017;128:1–18. [3] Wu Z, Wang X, Zhao X, Noori M. State–of–the–art review of FRP composites for major constructions with high performance and longevity. Int J Sustain Mater Struct Syst 2014;1(3):201–31. [4] Cao S, Wu Z, Li F. Effects of temperature on tensile strength of carbon fiber and carbon/epoxy composite sheets. Adv Mater Res 2012;476-478:778–84. [5] American Concrete Institute, ACI 440.7R-10. Guide for the design and construction of externally bonded fiber-reinforced polymer systems for strengthening unreinforced masonry structures. 2010. [6] Padalu PKVR, Singh Y, Das S. Out-of-plane flexural strengthening of URM wallettes using basalt fibre reinforced polymer composite. Constr Build Mater 2019;216:272–95. [7] Papanicolaou CG, Triantafillou TC, Karlos K, Papathanasiou M. Textile reinforced mortar (TRM) versus FRP as strengthening material of URM walls: in-plane cyclic loading. Mater Struct 2007;40(10):1081–97. [8] De Felice G, De Santis S, Garmendia L, Ghiassi B, Larrinaga P, Lourenço PB, et al. Mortar-based systems for externally bonded strengthening of masonry. Mater Struct 2014;47(12):2021–37. [9] Valluzzi MR, Modena C, De Felice G. Current practice and open issues in strengthening historical buildings with composites. Mater Struct 2014;47(12):1971–85. [10] American Concrete Institute, ACI 549.4R-13. Design and construction guide of externally bonded FRCM systems for concrete and masonry repair and strengthening. 2013. [11] Triantafillou TC, Papanicolaou CG. Textile reinforced mortars (TRM) versus fiber reinforced polymers (FRP) as strengthening materials of concrete structures. Int Concr Abstr Portal Spec Publ 2005;230:99–118. [12] Ombres L. Analysis of the bond between fabric reinforced cementitious mortar (FRCM) strengthening systems and concrete. Compos B Eng 2015;69:418–26. [13] Padalu PKVR, Singh Y, Das S. Out-of-plane flexural behaviour of masonry wallettes strengthened using FRP composites and externally bonded grids: Comparative study. Compos B Eng 2019;176:107302. [14] D’Ambrisi A, Feo L, Focacci F. Bond-slip relations for PBO-FRCM materials externally bonded to concrete. Compos B Eng 2012;43:2938–2949. [15] Carozzi FG, Poggi C. Mechanical properties and debonding strength of fabric reinforced cementitious matrix (FRCM) systems for masonry strengthening. Compos B Eng 2015;70:215–30. [16] Donnini J, Corinaldesi V, Nanni A. Mechanical properties of FRCM using carbon fabrics with different coating treatments. Compos B Eng 2016;88:220–8. [17] Carozzi FG, Milani G, Poggi C. Mechanical properties and numerical modeling of fabric reinforced cementitious matrix (FRCM) systems for strengthening of masonry structures. Compos Struct 2014;107:711–25. [18] Carozzi FG, Bellini A, D’Antino T, De Felice G, Focacci F, Hojdys L, et al. Experimental investigation of tensile and bond properties of carbon–FRCM composites for strengthening masonry elements. Compos B Eng 2017;128:100–19. [19] Donnini J, Corinaldesi V. Mechanical characterization of different FRCM systems for structural reinforcement. Constr Build Mater 2017;145:565–75. [20] Kim HS, Truong GT, Park SH, Choi KK. Tensile properties of carbon fiber-textile reinforced mortar (TRM) characterized by different anchorage methods. Int J Concr Struct Mater 2018;12(73):1–13. [21] Mercedes L, Gil L, Bernet-Maso E. Mechanical performance of vegetal fabric reinforced cementitious matrix (FRCM) composites. Constr Build Mater 2018;175:61–173. [22] Leone M, Aiello MA, Balsamo A, Carozzi FG, Ceroni F, Corradi M, et al. Glass fabric reinforced cementitious matrix: Tensile properties and bond performance on masonry substrate. Compos B Eng 2017;127:196–214. [23] D’Antino T, Papanicolaou C. Mechanical characterization of textile reinforced inorganic–matrix composites. Compos B Eng 2017;127:78–91. [24] Padalu PKVR, Singh Y, Das S. Efficacy of basalt fibre reinforced cement mortar composite for out-of-plane strengthening of unreinforced masonry. Constr Build Mater 2018;191:1172–90.

For practical application of FRCM in strengthening of masonry structures, further study needs to be conducted for other strengthening systems, utilizing different reinforcements and cementitious matrices. In addition to tensile behaviour, bond performance between the reinforcement-mortar composite and the substrate is also required. The comparison of the test results using different clamping systems also needs to be explored by studying the effect on the tested properties of the composites. The behaviour of the multi-layer composites is another issue which needs to be investigated.

178

Structures 23 (2020) 164–179

P.K.V.R. Padalu, et al. [25] Padalu PKVR, Singh Y, Das S. Experimental investigation of out-of-plane behaviour of URM wallettes strengthened using welded wire mesh. Constr Build Mater 2018;190:1133–53. [26] Caggegi C, Laoye E, Djama K, Bassil A, Gabor A. Tensile behaviour of a basalt TRM strengthening system: Influence of mortar and reinforcing textile ratios. Compos B Eng 2017;130:90–102. [27] Larrinaga P, Chastre C, Biscaia HC, San-Jose JT. Experimental and numerical modeling of basalt textile reinforced mortar behavior under tensile stress. Mater Des 2014;55:66–74. [28] Larrinaga P, Chastre C, San-Jose JT, Garmendia L. Non–linear analytical model of composites based on basalt textile reinforced mortar under uniaxial tension. Compos B Eng 2013;55:518–27. [29] American Society for Testing and Materials, ASTM A370-17a. Standard test methods and definitions for mechanical testing of steel products. 2017. [30] American Society for Testing and Materials, ASTM D5034-09. Standard test method for breaking strength and elongation of textile fabrics. 2017. [31] ICC Evaluation Service Inc., AC434. Acceptance criteria for masonry and concrete strengthening using fiber-reinforced cementitious matrix (FRCM) composite systems. 2013. [32] Kadam SB, Singh Y, Li B. Out-of-plane behaviour of unreinforced masonry

strengthened using ferrocement overlay. Mater Struct 2014;48(10):3187–203. [33] Shermi C, Dubey RN. Study on out-of-plane behaviour of unreinforced masonry strengthened with welded wire mesh and mortar. Constr Build Mater 2017;143:104–20. [34] Gabrijelcic H, Cernosa E, Dimitrovski K. Influence of weave and weft characteristics on tensile properties of fabrics. Fibres Text East Eur 2008;16(2):45–51. [35] Aveston J, Cooper GA, Kelly A. Single and multiple fracture, the properties of fibre composites. In: Proceedings of the conference national physical laboratories, IPC Science and Technology Press Ltf. London 1971;15–24. [36] Aveston J, Kelly A. Theory of multiple fracture of fibrous composites. J Mater Sci 1973;8:411–61. [37] Minafo G, Mendola LL. Experimental investigation on the effect of mortar grade on the compressive behaviour of FRCM confined masonry columns. Compos B Eng 2018;146:1–12. [38] Monaco A, D’Anna J, Oddo MC, Minafo G, La Mendola L. Numerical Modelling of the tensile behaviour of BFRCM strips. Key Eng Mater 2019;817:15–22. [39] Yoshitake I, Rajabipour F, Mimura Y, Scanlon A. A prediction method of tensile young’s modulus of concrete at early age. Adv Civil Eng 2012. https://doi.org/10. 1155/2012/391214.

179