INTRODUCTION - MATERIALS AND PROCESSES

There are three distinct technologies used in Japan for the external strengthening/rehabilitation of functionally or structurally deficient infrastructure elements using composites. The first class of solutions, which uses grids and external linear type reinforcement elements, was covered in Chapter 3. The second class of solutions uses fiber tow through the wet-winding process and is restricted to use on chimneys and some columns. Its use is decreasing in favor of a third process, which consists of the external placement of fabric sheet forms onto the surface of the structural element to be repaired/retrofitted. This process has been used extensively, primarily with carbon fiber, and to a lesser extent with aramid fibers. Applications range from use on columns (Fig. 4.1), floors, beams and slabs in buildings (Fig. 4.2), girders and deck soffits of bridges (Fig. 4.3), to use on chimneys, retaining walls (Fig. 4.4), tunnel linings (Fig. 4.5) and other concrete structural elements. In addition, the fiber sheet material has often been used as a protective coating for aging and deteriorating material as a preventive measure.

The application method itself is fairly simple. Because of the light weight of the fabric/sheet material, it can be used in closed areas or areas where there are pre-existing utility ducts, pipes, etc. that cannot be removed. Furthermore, the material conforms to most geometrical changes and can be applied very rapidly. This decreases the inconvenience caused to inhabitants of a building, or to commuters due to traffic delays and disruptions resulting from repair or retrofit of bridge elements.

(a) viaduct column

(b) internal column in a building

(c) external column
Fig. 4.1. Use of fabric/sheet material on columns.

(a) repair of cracked floor

(b) repair of slabs
Fig. 4.2. Use of fabric/sheet material in buildings.

(a)

Fig. 4.3. Use of fabric/sheet material for the strengthening of bridge deck soffits.

Fig. 4.4. Use of fabric/sheet material on retaining walls.

Fig. 4.5. Use of fabric/sheet material on tunnel lining.

Prior to the application of the composite, the surface of the element to be repaired/retrofitted is thoroughly cleaned and loose or cracked material or exfoliating concrete surface layers are removed. The entire surface is made uniform by filling of depressions with mortar/epoxy grout. The surface is then prepared by sanding (Fig. 4.6).

Fig. 4.6. Grinding of concrete surface in preparation.

Next, a primer coat is applied to fill small cracks and to facilitate the formation of a good bond (Fig. 4.7). Mortar/epoxy putty is used to remove irregularities.

Fig. 4.7. Application of primer on the prepared concrete substrate.

The surface is then covered with a layer of resin on top of which the fabric/sheet material is placed in a manner similar to the application of wallpaper (Fig. 4.8). Air pockets and wrinkles are removed through the application of pressure and more resin if needed. Further layers are added as appropriate with a new layer of resin applied in between adjacent layers of fabric (Fig. 4.9). Once the required number of layers is applied, with the direction of fibers varying between layers, the composite is allowed to cure. Then a top coat of paint is applied. This layer is generically made up of a urethane, acrylic, acrylic-urethane, or fluorine-based resin to facilitate UV and environmental protection.

Fig. 4.8. Placement of fabric/sheet material (fibers aligned vertically in this case).

Fig. 4.9. Application of resin top coat (on top of the final layer of fabric/sheet material).

Both carbon and aramid fibers are used in these applications, although carbon fiber predominates. Within the carbon fiber sheet/fabric segment, three different varieties are available:

1. FORCA tow sheet (Tonen Corp.)
2. Replark sheet (Mitsubishi Chemical Corp.)
3. Torayca cloth (Toray)

The FORCA tow sheet consists of unidirectional fibers stabilized through the use of 2 to 3% resin as a binder and held together by a thin netting of glass fibers. These sheets are available using carbon, glass and aramid reinforcing fibers. Mainly carbon fiber is used. Characteristics are given in Table 4.1. The Replark sheets are formed using the Dialead carbon fiber, which is impregnated with a low content of epoxy and then stretched to ensure unidirectionality. These sheets are very similar to the tow sheet form. Characteristic properties are given in Table 4.2.

A wide variety of epoxy resins are available for use with the reinforcing fabric/sheet. Formulations vary to provide resins specifically for use in normal weather, summer (hot), winter (cold) conditions, on damp surfaces, and where penetration through concrete (to fill cracks) is required. Based on the formulation, pot life varies from 20 to 120 minutes with viscosities between 90 cps (for penetration) to 45,000 cps (for use on damp surfaces). These resins generally are sold in Japan for about ¥3,200/kg.

An overall timeline for the development of this technology in Japan is given in Table 4.4.