Retrofitting techniques for seismic upgrades of unreinforced masonry structures

Structural weakness or overloading, dynamic vibrations, settlements, and in-plane and out-ofplane deformations can cause failure of unreinforced masonry (URM) structures. URM buildings have features that can threaten human lives in overstressing situations. These include unbraced parapets; inadequate connections to the roof, floor and slabs; and brittle URM elements. The Masonry Society, the Federal Emergency Management Agency, and other organizations have determined that failures of URM walls result in more material damage and loss of human life during earthquakes than any other type of structural element.This was evident from the post-earthquake observations in Northridge, Calif., in 1994 and in Izmit,Turkey, in 1999.

The development of effective and affordable retrofitting techniques for masonry elements is an urgent need.

Under the URM Building Law of California, passed in 1986, approximately 25,500 URM buildings were inventoried throughout the state. Even though this number is a relatively small percentage of the building inventory in California, it includes many cultural icons and historical resources. The building evaluation showed that 96 percent of the buildings needed to be retrofitted, which would result in approximately $4 billion in retrofit expenditures. To date, it has been estimated that only half of the owners have taken remedial actions, which may be attributed to high retrofitting costs.

FRP solutions

For retrofitting of the civil infrastructure, externally bonded fiber reinforced polymer (FRP) laminates have been used successfully to increase the flexural and shear capacity of reinforced concrete and masonry members. An alternative to FRP laminates is the use of near-surface mounted (NSM) FRP bars. This technique consists of placing a bar in a groove cut into the surface of the member being strengthened. The FRP bar can be embedded in an epoxy-based or cementitious-based paste, which transfers stresses between the substrate and the FRP bar. The successful use of NSM FRP bars in the strengthening of concrete members has been extended to URM walls, which are one of the components of a building most prone to failure during a seismic event.

The use of NSM FRP bars for increasing the flexural and shear strength of deficient masonry walls, in certain cases, can be more convenient than using FRP laminates because of anchoring requirements or aesthetic considerations.

Application of NSM FRP bars does not require any surface preparation work and requires minimal installation time compared with FRP laminates. Another advantage is the feasibility of anchoring these bars into members adjacent to the one being strengthened. For instance, in the case of the strengthening of a masonry infill with FRP bars, they can be anchored easily to columns and beams.

NSM techniques

This article describes two applications of FRP bars for strengthening URM walls. In the first application, NSM FRP bars are used as flexural reinforcement to strengthen URM walls to resist out-ofplane forces. In the second application (a structural repointing retrofitting technique), FRP bars are placed into horizontal masonry joints as shear reinforcement to resist in-plane loads.

In both applications, glass FRP (GFRP) bars are used to increase either the flexural or shear capacity.The GFRP bars are deformed by a helical wrap with a sand coating to improve the bond between the bar and the embedding paste. The bars are produced using a variation of the pultrusion process using 100 percent vinyl ester resin and e-glass fibers.Typical fiber content is 75 percent by weight. The bars are commercially available in high volumes with stocking locations in several points throughout North America and Europe.

Flexural strengthening

FRP bars can be used as a strengthening material to increase the flexural capacity of URM walls. The successful use of NSM bars for improving the flexural capacity of reinforced concrete members led to extending their application for strengthening URM walls. The use of NSM FRP bars is attractive since their application does not require any surface preparation work and requires minimal installation time.

Strengthening procedure—With the NSM technique, FRP reinforcing bars are installed in slots grooved into the masonry surface. An advantage of this method is that it does not require sandblasting and puttying. The strengthening procedure involves grooving of slots having a width of approximately 1.5 times the bar diameter and then cleaning the surface, applying epoxy- or cementitious-based embedding paste, encapsulating the bars in the joint, and finishing (refer to the sidebar on page 23 for additional details).

For hollow masonry units, special care must be taken to ensure that the groove depth does not exceed the thickness of the masonry unit shell and that local fracture of the masonry does not occur.

If an epoxy-based paste is used, strips of masking tape or other similar adhesive tape should be attached at each edge of the groove to avoid staining of the masonry surface.

Depending upon the kind of embedding material (cementitious-based or epoxy-based), a mortar gun used for tuck pointing or an epoxy gun can be used.

The guns can be hand, air, or electric powered, with the latter two being the most efficient (see photo above).

Flexural test results—Three masonry walls constructed with 3.75- x 24- x 48-inch concrete blocks were strengthened with No. 3 GFRP bars having a tensile strength of 110 kips per square inch (ksi) and a modulus of elasticity of 5,900 ksi. The strengthening layout was intended to represent URM wall strips with GFRP bars at different spacing.Wall R1 was strengthened with one GFRP bar at 24 inches-on-center and wall R2 with two GFRP bars at 12 inches-on-center. To compare the performance of FRP bars and laminates, wall L1 was strengthened with one, 3- inch-wide GFRP laminate. The amount of strengthening reinforcement was equivalent to that of wall R1 in terms of axial stiffness.Load capacity of an URM wall was estimated to be equal to 800 pounds. The walls were tested under simply supported conditions.

Wall R1 failed due to debonding of the embedding material from the masonry. Initial flexural cracks were located primarily at the mortar joints. A cracking noise revealed a progressive cracking of the embedding material.

Since the tensile stresses at the level of the mortar joints were being taken by the FRP reinforcement, a redistribution of stresses occurred. As a consequence, cracks developed in the masonry units oriented at 45 degrees (see Figure 1) or in the head mortar joints. Some of these cracks followed the epoxy paste and masonry interface, causing their debonding and subsequent wall failure.Wall R2 failed due to shear. Similarly to wall R1, cracking started in the mortar joints at the maximum bending region. In general, initial cracking was delayed and the crack widths were thinner as the amount of FRP reinforcement increased.

The flexural strength and stiffness of the FRP-strengthened walls increased as the amount of reinforcement increased.

Increments of 4 and 14 times the original masonry capacity were achieved for walls R1 and R2, respectively.

Shear strengthening

The FRP structural repointing technique, which is basically a variant of the NSM technique, consists of placing FRP bars in the mortar joints.

Repointing is a traditional retrofitting technique commonly used in the masonry industry. The term "structural" is added because this method does not merely consist of replacing missing mortar in the joints, but allows for restoring the integrity or upgrading the shear and flexural capacity of walls.

Strengthening procedure—FRP structural repointing has a number of advantages over FRP laminates. The method itself is simpler since surface preparation is reduced (sandblasting and puttying are not required) and the aesthetics of the masonry can be preserved. In this technique, the diameter size of the FRP bars is limited by the thickness of the mortar joint, which usually is not larger than 3/8 inch. The strengthening procedure consists of the following steps: 1) cutting out part of the mortar using a grinder, 2) filling the bed joints with an epoxy-based or cementitious- based paste, 3) embedding the bars in the joint, and 4) retooling.

Dust must be removed from the grooves with an air blower prior to filling the bed joints to ensure a proper bonding between the epoxy-based paste and masonry. Masking tape or another suitable adhesive tape can be used to avoid staining. Stack bond masonry allows installing FRP bars in the vertical joints, if required. In this case, since the face shell thickness of the masonry units does not limit the groove depth, this can be deeper.

Shear test results—Three masonry walls built with 6- x 8- x 16-inch concrete blocks were strengthened with No. 2 GFRP bars having a diameter of 1/4 inch, a tensile strength of 120 ksi, and a modulus of elasticity of 5,900 ksi. One URM wall (wall R0) was selected as the control specimen.Wall R2 was strengthened with GFRP bars at every horizontal joint. Wall L2 was strengthened with GFRP laminates; the amount of FRP was equivalent to that of wall R2 in terms of axial stiffness. Thus, wall L2 was strengthened with four horizontal, 4- inch-wide GFRP strips. The specimens, tested in a close loop fashion, were loaded along one diagonal of the specimen.

In the control wall R1, the failure was brittle, controlled by bonding between the masonry units and mortar. When the tensile strength of masonry is overcome, the wall cracks along the diagonal, following the mortar joints (stepped crack vertical/horizontal). In the strengthened wall R2, failure occurred when the shear cracks widen and GFRP bars are not able to carry tensile stresses because of debonding at the top and bottom epoxy/block interface (see Figure 2). The shear capacity was increased by about 80 percent. The strengthened walls showed stability after failure (such that no loose material was observed). This fact can reduce risk of injuries caused by partial or total collapse of walls also subjected to out-of-plane loads. In addition, as a result of the reinforcement eccentricity, which caused the crack growth on the unstrengthened side to increase at a greater rate than the strengthened side, wall R2 tilted to the direction of the strengthened face.

Failure in wall L2 was caused by sliding shear along an unstrengthened joint.

The walls strengthened with FRP bars (wall R2) and FRP laminates (wall L2) had similar shear capacity; however, the pseudo-ductility was less in wall L2, which can be attributed to the occurrence of the sliding shear failure.

Grandview High School

Two URM walls at Grandview High School in Kansas City, Mo., cracked along the bed joints at the mid-height region. The inward movement of an unstable exterior concrete wall, which displaced the open-web steel joists of the floor framing that were supported by both concrete and masonry walls, caused the cracking. Because of a change of floor elevations within the building, the joists were connected to the masonry wall at approximately mid-height. The consequent out-of-plane loading caused overstressing in the wall. The use of NSM FRP bars was proposed to reinstate the integrity of the cracked masonry walls.

The design approach consisted of restoring the flexural capacity of the cracked walls to that of the original uncracked walls with No. 2 GFRP bars. Before proceeding with the strengthening, the exterior wall was stabilized by steel tiebacks and the cracks were injected.

The strengthening of the walls was completed with minimal disruption to the building occupants. In addition, since the walls had a stacked-bond pattern, the aesthetics of the classrooms were not changed because the reinforcement was placed in the vertical mortar joints.

Guidelines and future developments

Many government, university, and industrial organizations are actively involved in research and advancement of FRP composites in concrete and masonry applications. With more than 1,000 member organizations, the American Composites Manufacturers Association (ACMA) is a recognized leader in FRP composites education, training, and applications development. Each year, the ACMA holds a technical conference and trade show where experts provide in-depth educational programs on composite applications and current research. Technical papers can be accessed through the web site www.compositesresearch.org.

The American Concrete Institute (ACI) also has published several important documents on FRP composites used in concrete structures. ACI Committee 440 provides a forum to discuss research and industry issues, develop design guidelines and standards, and share best practices on composite applications in concrete and masonry. The committee currently is reviewing modifications to ACI 440.2R-02 to document design methodologies for FRP/NSM strengthening for concrete structures. In addition, a subcommittee of 440 is drafting a design guide specification for externally bonded FRP systems for strengthening unreinforced masonry structures.

The ACI and Committee 440 hosted the 7th International Symposium on FRP Reinforcement for Concrete Structures (FRPRCS-7) on Nov. 6-9, 2005.

FRPRCS-7 is a biannual international conference focusing on research, development, testing, and field application of FRP composites in concrete and masonry structures. Held in conjunction with the ACI Fall Convention, the symposium showcased the latest developments in the use of FRP composites in concrete in 12 sessions and more than 95 technical papers. One session dedicated to masonry focused on design methodology of retrofitting masonry structures for both inplane and out-of-plane applications. For more information on the symposium, visit http://frprcs7.ce.umn.edu. A copy of the symposium proceedings (publication SP 230) can be obtained through the ACI website at www.concrete.org.

Conclusion

Strength and pseudo-ductility can be increased substantially by strengthening masonry walls with NSM FRP bars.

Masonry walls reinforced with NSM FRP bars exhibit similar performance to walls strengthened with FRP laminates.

For flexural strengthening, increments ranging between four and 14 times of the original masonry capacity can be achieved. Strengthening URM walls by FRP structural repointing provides remarkable increases in shear capacity.

The support of the National Science Foundation Industry/University Cooperative Research Center at the University of Missouri—Rolla is greatly acknowledged.

J. Gustavo Tumialan, Ph.D., P.E., is staff engineer at Simpson Gumpertz & Heger Inc. (SGH) in Boston, an ENR 500 consulting engineering firm that applies advanced engineering to buildings, infrastructure, and special structures. Prior to joining SGH, Tumialan served as research engineer at the Center for Infrastructure Engineering Studies at the University of Missouri-Rolla. He can be reached at gtumialan@sgh.com or at 1-781- 907-9291. Antonio Nanni, Ph.D., P.E., is the Vernon and Maralee Jones Professor of Civil Engineering and director of the Center for Infrastructure Engineering Studies at the University of Missouri-Rolla. He can be reached at nanni@umr.edu or at 1-573-341-4497. John Busel is director of the Composites Growth Initiative for the American Composites Manufacturers Association. He can be reached at jbusel@acmanet.org or at 1-914-961-8007.

Sidebar: American Composites Manufacturers Association Installation Guidelines for near surface mounted fiber reinforced polymer*

After assessment of the condition of the existing structure and design by a competent professional, installation of the near surface mounted (NSM) fiber reinforced polymer (FRP) strengthening is performed according to the following general guide:

1. Cut groove—Using a diamond blade saw or grinder, a groove 1.5 times the bar diameter (in the case of a rectangular FRP shape, 1.5 times the depth and 3 times the thickness) is cut as prescribed. The use of two diamond blades on the saw arbor may be necessary.
2. Prepare groove—The groove is prepared with masking tape or similar product to prevent excess adhesive from marring the surface. The groove is thoroughly cleaned using a vacuum or compressed air.
3. Apply adhesive—Structural adhesive gel or grout is filled in the groove.Take care to avoid entrapped air voids.
4. Place FRP rod or shape into groove—After the adhesive has been applied into the groove, the rod is placed and pressed into the groove to ensure proper location of the rod or shape.
5. Finish—After the FRP rod or shape is seated into the groove, the adhesive is smoothed and any additional adhesive is added. General clean up and removal of the masking tape is necessary.

*These guidelines are according to FRP Product Gateway: External Reinforcement Systems, "Near Surface Mounted (NSM) Applications," American Composites Manufacturing Association, Arlington, Va., www.mdacomposites.org.