EXPERIMENTAL PROGRAM SUMMARY

The experimental work presented in this paper is part of a large research program conducted on strengthening RM walls using different strengthening techniques. Table 1 provides an overview of the strengthened walls, materials and systems, wall bond pattern, and size of bars for NSM or width of fabric sheet for the FRCM system. This study considers tests and comparisons of twelve RM wall specimens, ten of which were

strengthened in out-of-plane with either FRP NSM bars (glass or carbon) or with FRCM (PBO or carbon). The reinforced walls were tested under cyclic load up to failure considering the overall effects of flexural strengthening systems, the effect of type and fiber axial stiffness, and the effect of type of masonry wall bond pattern.

2.1. MATERIAL CHARACTERIZATION

2.1.1. Masonry Wall Components and Steel. Concrete masonry units with

nominal dimensions of 152 x 203 x 406 mm (6 x 8 x 16 in.) and type S mortar were used in the walls construction. A series of experimental tests was performed to determine mechanical properties of each component. Masonry prisms were constructed with two masonry concrete units and cured under the same lab conditions as the walls. A compressive strength test was conducted according to ASTM C1314-12, and the average compressive strength of the prisms was 22.4 MPa (3,250 psi) based on three prisms with a coefficient of variation (COV) of 3.54%. Standard mortar specimens were tested according to ASTM C109-13 to determine the average compressive strength of type S mortar. An average 28-day value of 16.7 MPa (2,420 psi) was obtained with a COV of 7.24%. Figure 1 illustrates the constitutive relationship curves for masonry prism and mortar. The 28-day average compressive strength of the grout according to ASTM C1019-13 was 28.95 MPa (4,200 psi) with a COV of 6.63%. An experimental tensile test for mild steel rebar according to ASTM A370-13 was conducted on three replicate specimens. Uniaxial load was applied gradually until failure, and then the average yield stress of the steel reinforcement bar at 0.5% offset was obtained 463.63 MPa (67.25 ksi) with a COV of 3.9% along with the average modulus of elasticity was 200.3 GPa (29,051 ksi) with a COV of 3.07%.

2.1.2. Fibers and Adhesive Agents. The properties of composite materials are

dependent on the individual component properties, the manufacturing technique, and the quality control of the production process (Al-Salloum, Siddiqui, Elsanadedy, Abadel, &

Aqel, 2011). The open mesh fabric of PBO consists of fiber toes disposed along orthogonal directions, with the main direction tensile strength greater than tensile strength of the secondary direction, while symmetric open mesh for carbon fabric. The FRP bars used in this study were made of fibers embedded in to vinylester matrix under the pultrusion process. Based on AC434 protocol (AC 434, 2011) the FRCM coupon test results are presented in Table 2, while the FRP bars mechanical properties with results are summarized in Table 2 and 3, respectively. Compressive strength tests according to ASTM C109-13 were performed on the cementitious-based embedding materials used with NSM and the adhesive agents (mortar x750 and x25) used with an FRCM system.

The average compressive strength for the cementitious paste material was found to be 59.1 MPa (8,570 psi) with a COV of 4.7% at an age of 28 days. The average compressive strength for a matrix x750 used with PBO fabric was found to be 35 MPa (5 ksi) at an age of 28 days while it was 15 MPa (2,175 psi) with a COV of 5.13% for a matrix x25 used with carbon fabric.

2.2. CONSTRUCTION OF THE MASONRY WALLS

Twelve RM walls with dimensions of 1220 x 610 x 152 mm (48 x 24 x 6 in.) were constructed by a professional mason. Each specimen was constructed using 152.5 mm (6 in.) standard masonry concrete blocks in running and stack bond patterns. The steel reinforcement was constant for all specimens (2#4 bars) and the walls were fully grouted four days after construction to preclude damage to the mortar joints during the vibration

process. The steel reinforcement levels comply with specifications in design code. These reinforcements satisfied the reinforcement size limitation and were between the minimum and maximum reinforcement specified by MSJC-13. Two reinforced walls were used as control specimens in running and stack wall bond patterns. For both systems, the specimens were strengthened so that the fiber reinforcement ratio was less that the balance ratio and also to ensure there was no shear failure. Five walls were strengthened with NSM system and the remaining five walls were strengthened using FRCM system.

The specimens strengthened with carbon FRCM completely covered the tension face of the wall, while PBO fiber covered only 380 mm (15 in.) of the wall width. Figure 2 illustrates the dimensions of control and strengthened specimens.

2.3. SPECIMENS DETAILS

The specimens’ designation consisted of two parts. The first part consisted of two characters representing strengthening system information (fiber, thickness, or diameter).

The first character identified the fiber type: “C” for carbon, “G” for glass, and “PBO” for polyparaphenylene benzobisoxazole. The second character represented the FRP diameter for NSM, or fiber thickness for FRCM system. The second part of the designation identified the amount of fiber in the tension face and the wall bond pattern. The first character represented the number of FRP bars for NSM, or number of layers for FRCM.

The second character referred to the wall bond pattern applied: “R” for running and “S”

for stack.

2.4. TEST SETUP AND INSTRUMENTATION

The strengthened reinforced masonry specimens were tested under four-point bending with simply supported boundaries, as shown in Figure 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 specimen by means of continuous steel plates and bars along the full width of the external face of the reinforced walls to provide two equal line loads. A piece of thick rubber sheet was placed at all interfaces between the steel plate and specimen. The rubber sheet 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 load was applied in cycles of loading and unloading as a displacement control at a rate of 1.25 mm/min (0.05 in./min) through an MTS computer control station up to the load peak value. The displacement amplitude increment was 6.35 mm (0.25 in.); double half loading cycle was applied for each amplitude level, as illustrated in Figure 4. Deflections 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 and fiber to measure their strains during loading.

3. STRENGTHENING PROCEDURE 3.1. FRCM STRENGTHENING SYSTEM

The fabric with dimensions 1067 x 380 mm (42 x15 in.) for PBO and 1067 x 610 mm (42 x 24 in.) for carbon were prepared. The matrix was mixed as per the manufacturer specifications. The procedure of strengthening consisted of applying a first layer of cementitious matrix with a nominal thickness of approximately 5 mm (0.2 in.) on the tension surface of the specimen. A single ply of precut fabric was laid on the cementitious matrix and pressed gently into the first matrix layer. The second layer of cementitious matrix with a nominal thickness of 5 mm (0.2 in.) was then applied and

covered the fabric mesh. The procedure was repeated in the case of multi-ply strengthening.

3.2. NSM STRENGTHENING SYSTEM

No surface preparation was needed for the NSM system, and the strengthening procedure involved inserting FRP bar into a groove cut at the tension surface of the specimen. A special concrete saw was used to cut the grooves with dimensions double the diameter of the bar to avoid splitting failure of the epoxy cover (De Lorenzis &

Nanni, 2002). Deformed FRP bars with a sand coating were used to improve the bond between the FRP bars and cementitious material. The cementitious material was placed into the grooves to cover 2/3 of the groove depth. The FRP bar was installed to mid-groove depth as it was pressed into the bonding agent which flowed around the bar to ensure a complete bond between the bar and the sides of the groove. The groove was then filled with more cementitious material, and the surface was leveled.