IN-PLANE BEHAVIOR OF ALTERNATIVE MASONRY RETROFITTED WITH POLYUREA MEMBRANES

Transcription

1 IN-PLANE BEHAVIOR OF ALTERNATIVE MASONRY RETROFITTED WITH POLYUREA MEMBRANES J. J. Myers Y. Tanizawa Missouri University of Science and Technology Missouri University of Science and Technology Civil, Architectural and Environmental Engineering Dept Civil, Architectural and Environmental Engineering Dept 325 Butler-Carton Hall, 1870 Miner Circle, Rolla, MO Butler-Carton Hall, 1870 Miner Circle, Rolla, MO U.S.A. U.S.A. KEYWORDS: alternative construction materials, E-glass fiber, in-plane shear resistance, polyurea, unreinforced masonry infill wall. ABSTRACT This research program investigates the feasibility for an application of an alternative masonry unit in infill wall construction and the effects of an external Fiber Reinforced Polymer (FRP) strengthening system on the in-plane behavior of the alternative unreinforced masonry (URM) walls. The alternative masonry unit in part utilizes a high volume of recycled white oak wood-fibers and Class-C fly ash as its primary raw materials. The external strengthening system consists of an elastomeric polyurea surface coating and chopped E-glass fibers, directly sprayed on the URM wall surfaces without wall surface preparation. The system has rapid cure characteristics. The alternative URM wall system, in conjunction with the external FRP retrofit, is intended to provide economic and environmentally friendly solution for building infill wall systems with improved ductility for hazard applications. The experimental program includes manufacture of the alternative masonry units, URM wall construction, wall surface strengthening, and diagonal compression tests of the walls. The test program consists of two phases. In the first phase, three URM walls were constructed with the alternative masonry with different unit strength and the walls were tested without surface retrofit. The in-plane load capacity and the failure modes were observed in this phase. In the second phase, five URM walls with units having comparable strength were constructed and each wall was retrofitted by different strengthening scheme. The effects of surface strengthening were compared in terms of a load capacity, degree of fragmentation, and overall deformation. From the test results, it was observed that the failure mode of UMR wall is highly dependent upon the unit strength and retrofit condition. The retrofit technique was effective not only in preventing debris scatter at failure, but also in improving the load capacity and ductility of the URM walls. INTRODUCTION Masonry units are common building materials in the United States. Clay and concrete are widely used in masonry production, and some other materials such as natural stone are also used as alternatives. An alternative material for infill wall systems was developed by Joshi and Myers (2006). Class-C fly ash obtained from a local coal-fired power plant and wood fibers of white oak obtained from a local tree farm have been utilized as the raw materials for the alternative material. Based on this material development research Hrynyk and Myers (2007) studied the out-of plane behavior of the unreinforced masonry (URM) infill walls which utilize the alternative base masonry material. Concurrently with this study, Tinsley and Myers (2007) investigated the low velocity impact resistance and near field blast resistance of the material while Carey and Myers (2008) investigated the wind borne impact resistance of the material and application in barrier systems. The alternative URM infill wall system is intended to reduce the material cost and to recycle Class-C fly ash and waste wood. In conjunction with an external fiber reinforced polymer (FRP) retrofit, the wall system is intended to provide improved ductility and reduced scatter of debris at the wall failure.

2 The main objectives of the following research study were to investigate the feasibility of applying the alternative masonry unit to infill wall construction and to investigate the effects of an external FRP strengthening system on the in-plane response of the alternative URM walls. The external strengthening system consists of an elastomeric polyurea surface coating and chopped E-glass fibers. The strengthening system has a rapid cure characteristic and is directly sprayed on the wall surfaces. The experimental program of this research study includes manufacture of the alternative masonry units, alternative URM wall construction, wall surface strengthening, and in-plane load tests of the walls. EXPERIMENTAL STUDY Material Description During this experimental program, one polyurea was investigated and tested under tension with and without E-glass fiber using test method ACI 440.3R-04.. Coupon specimens were fabricated using each elastomeric polyurea and E-Glass fiber by varying fiber content and fiber length. Tables 1, 2 and 3 list the mechanical properties for the tested materials. Prism tests were performed in accordance with ASTM C , Standard Test Method for Compressive Strength of Masonry Prisms. Table 1. Mechanical properties of elastomeric polyureas Material Modulus of Tensile strength, f fu Elasticity, E f Polyurea A Polyurea A without E-glass with E-glass Table 2. Mechanical properties of E-Glass fiber Mechanical properties Dry Wet Tensile strength Flexural strength Table 3. Compressive Strength of Wood-Fiber Fly Ash Prism Material Mix (Wall ID) Compressive Strength f mt, P P P P P P P P Standard Deviation The compressive strength of masonry mortar was tested with Two-inch (50-mm) test cubes according to ASTM C109/C 109M-07. The average cube strengths of the mortar used in wall construction were: 3.0 MPa for Wall P1-1, 2.4 MPa for Wall P1-2, and 2.2 MPa for Wall P1-3 in Phase 1. In Phase 2, Wall P2-1, P2-3, P2-4, P2-5 were built using Series 2-1 to 2-5 with the average strength of 9.0 MPa. The Wall P2-2, masonry prisms, and triplets were built using Series 2-6 to 2-10 with the average strength of 880 psi (6.1 MPa)

3 Test Matrix Details and Fabrication In the first phase of the research study, three URM walls were constructed with the alternative masonry units. The wall specimens were built with 70 x 64 x 241 mm alternative masonry units. The nominal dimensions of the wall specimens in phase 1 were 914 x 914 x 70mm. In the second phase, the dimensions of the alternative masonry units were 102 x 67 x 241 mm. The nominal dimensions of the wall specimens were 1219 x 1219 x 102 mm (see Figures 1 and 2 for details and construction).. None of the wall specimens in phase 1 and phase 2 had internal reinforcement. All of the wall specimens were built in a running bond pattern by experienced masons to ensure uniformity and quality of the specimens. Conversions: 1 inch= mm Figure 1. Wood-fiber fly ash masonry wall in phases 1 and 2. (a) Sample Course and Bed Joint Fabrication (b) Completed Wall Fabrication Figure 2. Wood-fiber fly ash masonry wall in phase 1. The wall specimens in phase 1 were tested without the surface strengthening. In the second phase of the wall tests, five URM walls were constructed with the alternative masonry units. Wall P2-1 was tested without the surface retrofit as a control wall in phase 2. The other four walls were retrofitted by the external FRP strengthening system. Wall P2-2 was strengthened by the polyurea surface coating on the entire surface of the one side of the wall. Wall P2-3 was strengthened by the same strengthening material as the wall P2-2 on both sides of the wall. Walls P2-4 and P2-5 were strengthened on both sides by a polyurea surface coating and chopped E-glass fibers (see Figure 3 for details).

4 (a) Polyurea. (b) Polyurea with E-glass fiber. Figure 3. Strengthened wood-fiber fly ash masonry wall in phase 2. After the wall is prepared, the test fixture is set. The steel shoes were placed on the diagonally opposite corners of the wall. Two steel bars went through the steel shoes and tied the shoes together. A hydraulic jack and a load cell were set on the lower corner of the wall to provide an in-plane force and to measure the compressive load. Figure 4 shows the loading system at the lower corner of the wall specimen. Two LVDTs were set on the surface of the wall to measure the wall shortening and crack opening. One LVDT was set parallel to the line of compression. Another LVDT was set on the other side of the wall, perpendicular to the line of compression. (a) Steel shoe and hydraulic jack. (b) Steel shoes set-up. Figure 4. Lower corner of the wall and test-set-up. In-plane Load Tests Table 4 shows the summary of the test results. The compressive strength of masonry f mt was obtained from the prism tests according to ASTM C as noted previously. The maximum applied load was obtained according to the in-plane load test method by Yu et al. (2004). Figure 5 shows the in-plane load tests of the alternative walls. In this research, an in-plane compressive load was applied to the wall specimens by a hydraulic jack through steel shoes placed on the corners of the wall specimen. The steel bars went through the two steel shoes and connected the shoes together. After the test was ready, an in-plane load was applied by a hydraulic jack through the steel shoes. The load was increased continuously by a manual pump connected to the hydraulic jack, until the maximum applied load was reached and the wall specimen got unstable. During the load test, the diagonal compressive load was measured using a load cell. The diagonal compressive shortening and diagonal tensile extension of the wall specimen were measured by two linear variable differential transformers (LVDTs).

5 Table 4. Wall specimens and summary of test results. Phase 1 2 Wall Compressive Strength of Masonry f mt, Maximum Applied Load, (kn) Retrofit Scheme P P P P P One side, polyurea P Double sides, polyurea P Double sides, polyurea and E-glass fibers P Double sides, polyurea and E-glass fibers (a) Wall P2-1 (b) Wall P2-3 Figure 5. In-plane load tests of the alternative URM walls. RESULTS AND CONCLUSIONS Results and Discussion In the scope of the test results in phase 1, the maximum applied load tended to increase as the compressive strength of masonry f mt increased. Wall P1-1 failed with a stair-stepped diagonal crack. For wall P1-2, a horizontal sliding appeared along the bed joints at its failure. The cracking at the failure of the wall P1-3 involved both the joints and the units. The walls in phase 1 failed in a brittle manner. If the compressive strength of the alternative masonry units is sufficiently high enough to prevent splitting failure of the units, it is possible that the joint sliding failure dominates and the failure load depends primarily on bed-joint sliding shear strength. Figure 6 shows the normalized failure load of the wall in the phase 2. The effects of the external FRP strengthening systems on the maximum applied loads of the wall specimens were compared using the maximum applied load for wall P2-1 as a control for normalization. In the figure, the maximum applied load for each wall was divided by the maximum applied load of the wall P2-1. Wall P2-2 was strengthened by the polyurea surface coating on one side, and its normalized failure load was 2.52 compared to 1 for the wall P2-1. Wall P2-3 had the strengthening on both sides of the wall, and achieved a maximum applied load more than twice as much as that of the wall P2-2. The additional chopped E-glass fibers were used in the external FRP strengthening systems for walls P2-4 and P2-5 increased the maximum applied load of the wall specimens. The increase in the maximum applied load by the polyurea surface coating with E-glass fibers was larger than that by the polyurea coating without

6 fiber. This may legitimate the use of chopped E-glass fibers which is economical compared to the polyurea surface coating. A further investigation is needed to discuss the effects of the fiber type and fiber content level. Normalized Failure Load P2-1 P2-2 P2-3 P2-4 P2-5 Phase 2 Wall Specimens Figure 6. Normalized failure load of the wall specimens in phase 2. Figure 7 shows the shear stress-shear strain relationship for the wall specimens in phase 2. The shear stress and the shear strain were calculated by the method described in ASTM The maximum shear stress of the wall P2-1 was MPa. It was observed that the wall P2-2 experienced an out-ofplane deformation during the test and the diagonal extension of the wall P2-2 was not measured. The shear strain of the wall P2-2 was thus calculated by considering only the contribution of the diagonal shortening. In Figure 7, it may be observed that the shear strain at failure of wall P2-2 was smaller than that of P2-3. Wall P2-3, with two-sided polyurea surface coating, failed at MPa, which was about 6 times the maximum shear stress of the wall P2-1. Wall P2-4 was strengthened using two-sided polyurea surface coating with additional chopped E-glass fiber reinforcement, which resulted in the maximum shear stress of MPa. The maximum shear stress of the wall P2-4 was about 7 times that of the wall P2-1. The maximum shear stress of the wall P-2-5 was MPa. To measure the improvement in the ductility of the alternative URM walls provided by the surface strengthening, a ductility index can be defined as the area under the shear stress-shear strain curve. The ductility index of each wall can be normalized by dividing the index by that of the wall P2-1. The normalized ductility indices were 17 for the wall P2-3, 28 for P2-4, and 36 for P2-5. In the scope of the test results, the chopped E-glass fibers were effective in increasing the ductility index of the wall specimens Shear Stress (psi) (P2-2) P2-3 P2-4 P P Shear Strain (in./in.) Conversions: 1 psi = MPa, 1 in./in. =1 mm/mm Figure 7. Shear stress-shear strain relationship for the wall specimens in phase 2.

7 Conclusions The following conclusions are deduced from the experimental results: 1. The strength of the wood-fiber fly ash masonry units provides a contribution toward the in-plane load capacity of wood-fiber fly ash URM infill wall. The failure mode of the alternative URM infill wall varied as the compressive strength of masonry f mt changed. The in-plane load capacity of the wood-fiber fly ash masonry in Phase 1 deferred from kn to kn while the compressive strength of wood-fiber fly ash masonry deferred from MPa to MPa. As the unit strength increases, the failure mode changed from the combination of the joint sliding and the diagonal tension failure to the joint sliding failure. 2. The in-plane diagonal load capacity of the alternative URM infill wall can be strengthened using a polyurea retrofit. The alternative wall with polyurea surface coating on one side showed an in-plane diagonal load capacity 2.5 times greater than that of the control wall. Strengthening on both sides provided an in-plane diagonal load capacity 5.9 times greater than that of the control wall. 3. The additional discrete chopped E-glass fibers integrated within the polyurea surface coating showed a strengthening effect on the in-plane diagonal load capacity of the alternative URM infill walls. The load capacity of the alternative wall strengthened by the polyurea surface coating and the integrated discrete chopped E-glass fibers was about 7 times greater than that of the control wall. 4. The alternative URM infill walls strengthened using the polyurea retrofit showed improved ductility before failure. The brittle failure modes of the alternative URM infill walls were prevented by the external retrofit. In terms of the ductility index, defined as the area under the shear stress-shear strain diagram obtained from the in-plane load test, two-sided polyurea surface coating provided the wall with a ductility index 17 times as much as that of the control wall. The addition of integrated discrete chopped E-glass fibers within the polyurea surface coating improved the wall ductility. The ductility index of the walls strengthened by the polyurea and chopped E-glass fibers were 28 to 36 times as much as that of the control wall. Also, the strengthened walls endured further deformation without sudden loss of load carrying capacity or debris scatter. Acknowledgements The authors would like to acknowledge Encore Building Solutions, Inc. and BASF for material donations for this experimental investigation. In addition, the authors would like to thank staff support for experimental testing from the Department of Civil, Architectural, and Environmental Engineering, the Center for Infrastructure Engineering Studies (CIES) and the NUTC s Center for Transportation Infrastructure and Safety (CTIS-NUTC) at Missouri S&T. REFERENCES ASTM E , Standard Test Methods for Diagonal Tension (Shear) in Masonry, American Society of Testing and Materials, West Conshohocken, PA, ASTM C , Standard Test Methods for Compressive Strength of Masonry Prisms, American Society of Testing and Materials, West Conshohocken, PA, Carey, N. and Myers J.J., "Blast and Impact Resistance of Hybrid Systems for Barrier and Wall Panel Applications," Center for Infrastructure Engineering Studies Report, Missouri University of Science and Technology, Rolla, Missouri, Hrynyk T. D. and Myers J.J., "Static Evaluation of the Out-of plane Behavior of URM Infill Walls Utilizing Modern Blast Retrofit Systems," Center for Infrastructure Engineering Studies Report, University of Missouri-Rolla, Rolla, Missouri, Joshi N. and Myers J.J., "Investigation of an Alternative Wood Fiber-Fly Ash Material for Infill Wall Systems," Center for Infrastructure Engineering Studies Report 06-60, University of Missouri-Rolla, Rolla, Missouri, Tanizawa, Y., Myers, J.J., and Sinclair, R., In-plane Response of an Alternative URM Infill Wall System with and without a Polyurea Retrofit, FRPRCS-9 Proceedings, Sydney, Australia, July 2009.

8 Tinsley, M. and Myers J.J., "Investigation of a High-Volume Fly Ash-Wood Fiber Material Subjected to Low-Velocity Impact and Blast Loads," Center for Infrastructure Engineering Studies Report, University of Missouri-Rolla, Rolla, Missouri, Yu, P., Silva, P. F., and Nanni, A., Application of Bondo Polyurea in Structural Strengthening of RC Beams and URM Walls, Center for Infrastructure Engineering Studies Report 04-49, University of Missouri-Rolla, Rolla, Missouri, August 2004, 38 pp.