Strengthening: masonry walls

Masonry Construction, April, 2003 by Gustavo Tumialan, Antonio Nanni

Structural weakness or overloading, dynamic vibrations, settlements, and in-plane and out-of-plane deformations can cause the failure of unreinforced masonry (URM) structures. In the case of overstressing, these buildings have features, such as unbraced parapets, inadequate connections to the roof, floor, and slabs, and the brittle nature of the URM elements, that can threaten human lives.

Organizations such as The Masonry Society (TMS) and the Federal Emergency Management Agency (FEMA) 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. Post-earthquake observations in Northridge, Calif. (1994) and Izmit, Turkey (1999) proved this point.

Researchers worldwide have started experimental and analytical investigations on this topical area in order to understand performance, optimize reinforcement, and produce design guidelines. In addition, a variety of educational and government institutions in North America, Europe, and more recently South America have been fully involved in the investigation of strengthening masonry with FRP composites.

Under the URM Building Law of California passed in 1986, about 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 year 2000 building evaluation showed that 96% of the buildings needed to be retrofitted, which would result in approximately $4 billion in expenditures. To date, only about half of the owners have taken remedial actions, which is probably attributed to high retrofitting costs.

Obviously, there is an urgent need for the development of effective and affordable retrofitting techniques for masonry elements.

FRP composites

Externally bonded fiber reinforced polymer (FRP) laminates and bars have been successfully used to increase the flexural and/or the shear capacity of reinforced concrete for retrofitting of the civil infrastructure. FRP composites also can provide solutions for the strengthening of URM walls that are subjected to in-plane and out-of-plane overstresses caused by high wind pressures or earthquake loads. Available test results show that each potential failure cause of URM walls is prevented and/or lessened by using FRP composites.

The most important characteristics of a strengthening work are the predominance of labor and shutdown costs, as opposed to material costs, time, site constraints, and long-term durability. The advantages of FRP versus conventional steel for use as a reinforcement include higher strength, lower installation costs, improved corrosion resistance, on-site flexibility of use, and minimum changes in the member size after repair.

FRP material systems are composed of fibers embedded in a polymeric matrix and exhibit several properties which make them suitable for use as structural reinforcing elements. FRP composites are characterized by excellent tensile strength in the direction of the fibers and by negligible strength in the direction transverse to the fibers. Composites do not exhibit yielding, but instead are elastic up to failure. They are also characterized by a range of low to high modulus of elasticity in tension and low compressive properties. FRP composites are corrosion resistant and perform better than other construction materials in terms of weathering behavior.

The FRP matrix consists of a polymer (or resin) used as a binder for the reinforcing fibers. The matrix has two main functions: enable the load to be transferred among fibers and protect the fibers from environmental effects. In a composite material, fibers have the role of the load-bearing constituent. Fibers give the composite high tensile strength and rigidity along their longitudinal direction.

Several types of fibers have been developed for use in FRP composites. For structural applications, research and development have been conducted using carbon, aramid, and glass fibers. In the order listed, these fibers exhibit an ultimate strain range of 1 to 4% with no yielding occurring prior to failure. The ultimate strength range is about 830 to 480 ksi and the elastic moduli range is from 39,000 ksi to 10,000 ksi.

FRP composites are available in the form of pre-cured laminates or fiber sheets installed by hand lay-up. The application of the latter offers several advantages, such as ease of bonding to curved or irregular surfaces, lightweight, and the fibers can be oriented along any direction. As a point of reference, the thickness of an installed ply (which includes fibers and adhesive) is in the range of 0.04 to 0.12 inch.

FRP composites are also found in bar form. In certain cases, FRP bars are more convenient than using FRP laminates because of anchoring or aesthetic requirements. The bars are manufactured using a pultrusion process. Typical fiber content is 75% by weight. Sizes between #2 and #10 are commercially available.