1632Properties of FRP Strengthening Systems

Carbon fiber-reinforced epoxy laminates (or strips) are the most commonly used in the adhesively bonded type of products. Depending on the type of carbon fiber used in the strip, different longitudinal strengths and stiffnesses are produced. Strips are typically thin (less than 0.100 in.) and are available in a variety of widths (typically 2 to 4 in.). Since the strips are reinforced with unidirectional fibers they are highly orthotropic with very low properties in the transverse and through the thickness directions. Manufacturer's typically only report properties in the longitudinal directions and report very little data on physical properties. The strips are bonded to the concrete with a compatible adhesive that is supplied by the strip manufacturer. Typical properties of strips are shown in Table 16.5. It is important to note that the properties shown for the strips are properties of the FRP composite and not the properties of the fibers alone.

In the type of products for FRP strengthening that consist of dry fibers in sheet or fabric form with compatible polymer saturating resins there is a greater array of available products that depend on fiber type and sheet or fabric architecture. In this group of products a unidirectional, highly orthotropic carbon fiber tow-sheet is produced by a number of manufacturers and is often used in strengthening applications. The individual carbon tows in the sheet are held together by a very light polymeric binder (or a light stitching) and it is supplied on a wax paper backing. Sheets are typically 10 to 40 in. wide and are often applied in multiple layers with different orientations. Other common fabric materials in this group are woven glass fiber materials typically consisting of 12 to 32 Oz/yd2 materials with a variety of weaves, which can give the fabric properties from highly orthotropic to square symmetric. Carbon fiber tow fabrics and hybrid fabrics (with more than one fiber type) are also available. Fabrics are typically much thicker than tow-sheets and are also used in multiple layers. Because of the wide variety of products available and their different thicknesses it is not easy to compare their properties directly. In addition, the fibers must be used with a compatible resin system applied with a controlled volume fraction to achieve an FRP composite with measurable properties. In the case of sheet and fabric materials, manufacturers typically report the mechanical properties of the dry fibers and the thickness (or area) of the fibers. It is important to note that when reported in this fashion the properties are not the properties of the FRP composite. Properties of some commonly available fiber sheet materials are listed in Table 16.6.

The performance of the FRP strengthening system is highly influenced by the properties of the adhesive layer in the case of the bonded strip and by the properties of the saturating polymeric resin in the case of the sheets and fabrics. The interface between the FRP composite and the concrete substrate transfers the loads from the concrete to the FRP composite. In the case of flexural, shear, or axial tensile strengthening this load transfer is primarily in shear and the properties and the quality of the interface bond between the FRP composite and the concrete effects load transfer into the FRP strengthening

TABLE 16.5 Properties of Typical Commercially Produced FRP Strengthening Strips

Standard modulus High modulus carbon-reinforced carbon-reinforced Glass-reinforced Carbon-reinforced

TABLE 16.5 Properties of Typical Commercially Produced FRP Strengthening Strips

Standard modulus High modulus carbon-reinforced carbon-reinforced Glass-reinforced Carbon-reinforced

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