SCOPE OF THE RESEARCH

An experimental study was formulated to evaluate the performance of RM walls strengthened with EB-FRP. A series of twelve strengthened specimen tests were conducted, eight of which were strengthened in flexure with either unidirectional E-glass fiber (impregnated with an epoxy resin) or with CFRP/epoxy composite laminate system.

The RM walls were tested under out-of-plane cyclic load to study the effect of different parameters such as the type of FRP composite, the fiber reinforcement ratio, the masonry bond pattern, the steel reinforcement ratio (ρ), and the effect of surface preparation. Table 1 provides details of the all masonry walls considered in this study. A simple and user-friendly model was developed to predict the FRP debonding strain. Also, the moment curvature relation was proposed to predict the full behavior of strengthened specimens.

Supplementary material tests were conducted to determine the masonry components’

properties (masonry unit, mortar, grout, and steel reinforcement) in addition to the EB-FRP system components (fiber and epoxy adhesive). This paper describes the experimental steps and presents the experimental and theoretical results in addition to conclusions from this research.

2.1. MATERIAL PROPERTIES

2.1.1. Masonry Wall Components and Steel. A series of tests was performed to determine each material’s mechanical properties. Compressive strength test was conducted for masonry prisms constructed with two masonry concrete units and cured with the same lab condition of the walls. Also, the 28-day average compressive strength of the grout and type S mortar was evaluated. An experimental tensile test on three

specimens of mild steel was conducted. The results of all these tests based on ASTM standards associated with each test are summarized in Table 2.

2.1.2. Fibers and Bonding Materials. The mechanical properties of FRP are dependent on the fiber and resin properties, the manufacturing technique, and the quality control of the production process. The SEH fabrics are composed of glass fibers, while the Tyfo S epoxy matrix is an ambient cure adhesive composed of two components.

According to the ASTM D3039-14, the minimum ultimate tensile strength and tensile modulus for the glass fiber composite in primary direction of Tyfo SHE-51 composite were 575 Mpa (83 ksi) and 26.1 GPa (3785 ksi) respectively. One layer of glass fiber composite with 1.3 mm (0.05 in.) thickness has an elongation at break of 2.1%. The pre-cured CFRP laminate used in this study was made of fibers embedded into epoxy resin under a pultrusion process with a typical 60% fiber content by volume. Based on ASTM D3039-14, the guaranteed tensile strength of CFRP is reported by the manufacturer to be 2400 MPa (350 ksi), with a tensile modulus of elasticity of 131 GPa (19000 ksi). The CFRP laminate with 1.4 mm (0.055 in.) thickness has an ultimate strain of 1.7% at failure.

Two types of structural bonding adhesive were selected for this study. Tyfo S epoxy matrix was used to bind SEH glass fiber. Components A and B of the matrix must be mixed at a volume ratio of 100:42 (A: B). SikaDur 30, an adhesive bonding material as a mixture of two parts, resin (A) and hardener (B), was used to bind CFRP laminate.

The properties of the adhesive are as presented in Table 3. Bond strength between FRP and masonry substrate is critical to composite design systems. The bond strength was measured for GFRP and CFRP by pullout test based on ASTM D7913-14. Two

specimens for each type of fiber were used to validate test results since the mode of failure was bond failure in masonry substrate without rupture in tension, slip at anchoring section, or split at the concrete masonry unit as shown in Fig. 1. The bond strength of GFRP and CFRP systems were 5.2 MPa (765 psi) and 4 MPa (589 psi), respectively, which exceeds the minimum bond strength of 2.5√𝑓_𝑚́ . The adhesive strength generally

exceeds the masonry strength in order to prevent the adhesive failure.

2.2. MASONRY WALL SPECIMENS AND IDENTIFICATION

The experimental program consists of twelve steel reinforced masonry walls with dimensions of 1220 x 610 x 152 mm (48 x 24 x 6 in.), as shown in Fig. 2. Each specimen was constructed using 152.5 mm (6 in.) standard masonry concrete blocks in running or stack masonry bond pattern. Four specimens served as an unstrengthened control to represent specimens in running or stack bond pattern with different steel reinforcement ratio, while the other specimens were strengthened with GFRP sheet or CFRP laminate for the EB-FRP system. Different steel reinforcement amounts of 2#3, 2#4, and 1#5 were used in fully grouted specimens of this study. These reinforcement levels comply with specification in MSJC-13 design code. These reinforcements satisfied the reinforcement size limitations, the minimum reinforcement ratio, and the maximum area of flexural tensile reinforcement.

The specimens are designated with two parts. The first part consisted of two characters represented the strengthening system information. The first character identified the number of FRP sheets or laminates: “S” for single sheet and “D” for double sheets.

The second character represented the type of FRP, namely “C” for carbon and “G” for

glass. The second part identifies the internal steel reinforcement (number and size of steel rebar). For specimens with stack bond pattern, additional character “S” added between two parts.

2.3. TEST SETUP AND INSTRUMENTATION

All reinforced masonry walls were tested under four-point bending with simply supported boundaries, as shown in Fig. 3. An MTS double-acting hydraulic jack with a push-pull capacity of 620 kN (140 kips) was used to apply a vertical load on the specimen. The load was transferred to the masonry specimen by means of continuous steel plates and bars along the full width of specimens providing two equal line loads. A piece of thick rubber sheet was placed at all interfaces between the steel plate and specimen. The rubber distributed the load evenly and minimized any stress concentration due to unevenness of the wall surface. The distance between these two lines was 200 mm (8 in.). The FRP was 1118 mm (44 in.) long in order to ensure that the ends were not clamped by the supports. The load was applied in cycles of loading and unloading as a displacement control at a rate of 1.27 mm/min (0.05 in./min). The displacement amplitude increment was 6.35 mm (0.25 in.); double half loading cycle was applied for each amplitude level as illustrated in Fig. 4. Displacements at the mid and third spans were measured using three linear variable displacement transducers (LVDTs) at each side. In addition, strain gauges were installed on the steel reinforcement and fiber to measure their strains during loading. It may be noted that in previous testing of FRP strengthened URM walls, an airbag was used to apply uniform load to the test walls adjacent to a vertical strong wall as the boundary element. However, because this testing

program focused on FRP strengthened RM walls; airbag loading was not an option due to the wall capacity with the added internally fully grouted steel reinforcing.