Seismic Strengthening of Reinforced Concrete Railway Viaducts with Fiber Plastic Sheets

AN EFFECTIVE

EARTHQUAKE-RESISTANT

REINFORCEMENT METHOD

*Sheets with fibers laid out in two directions are also available.

The Hyogoken-Nanbu (Kobe) Earthquake in January 1995 caused severe damage to a large number of railway structures, and particularly to moment-resisting reinforced concrete viaducts. Although conventional construction methods such as steel plate lining have previously been adopted for the restoration and earthquake-resistant reinforcement of damaged structures, there is now increasing demand for easier and more effective reinforcement methods. At the same time, excellent materials suitable for earthquake-resistant reinforcement have recently been developed.

From a variety of proposed earthquake-resistant reinforcement methods, a method using fiber sheets made from these new materials is introduced below and its application in viaducts is discussed.

FIBER SHEETS MADE OF

NEW MATERIALS AND

REINFORCEMENT STEPS

New materials such as carbon, aramid, glass, and vinylon fibers have performed outstandingly in terms of weight, strength and long-term durability. These materials are now finding wide use in aerospace, the sports and leisure industries, telecommunications, as well as in civil engineering fields, where they are used as reinforcing materials, rods and grids in PC stressing for concrete structures.

A thin sheet formed by laying out fibers made of these new materials in a single direction* to flexibly follow the shape of existing concrete members, as shown in Photo 1, is called a fiber sheet. As earthquake-resistant reinforcing materials, particular attention is being given to carbon and aramid fiber sheets. Table 1 lists the primary physical properties of these sheets. The earthquake-resistant reinforcement method for concrete structures is to apply these sheets to concrete surfaces using epoxy resin.

Two types of aramid fibers with different physical properties are available depending on chemical

compositions

As shown in Fig. 1, the reinforcing steps consist of the following: surface preparation to make existing concrete surfaces smooth by removing pits and projections; primer coating to enhance adhesion between resin and concrete; and application of fiber sheets to the concrete with resin. In addition, to protect sheet surfaces from scratches, enhance durability, improve appearance, and assure fire resistance, appropriate finishing and protective work is performed as required.

Characteristic features of the reinforcement method using fiber sheets include: small increase in dead loads after reinforcement because of the light weight of fiber sheets, about one-fifth of the specific gravity of steel; less change in the shape of members after reinforcement because of high strength, about 10 times greater than that of steel; stabiilty against corrosion and deterioration in almost all environments because of long-term durability. In addition, simplified reinforcing steps allow work in a limited space, and specific work experience and heavy duty construction equipment are unnecessary.

There is also the flexibility in varying the amount of reinforcement according to job requirements. For instance, flexural reinforcement by applying fiber sheets to members longitudinally to exert the same reinforcement effects as longitudinal steel reinforcement; shear reinforcement by applying fiber sheets perpendicular to the axis of members to exert the same reinforcement effects as perpendicular steel reinforcement; and ductility reinforcement by wrapping fiber sheets around members to provide containment effects and improve ductility. As compared with conventional construction methods, depending on the number of fiber sheets applied, this reinforcement method can cut down subsidiary work costs and shorten construction periods.

EFFECTS OF FIBER SHEETS

ON EARTHQUAKE

RESISTANCE

Photo 2 Typical results of shear tests on specimens reinforced with carbon fiber sheets

Research into the effects of carbon and aramid sheets on the earthquake resistance of reinforced concrete members dates from the late 1980s and the early 1990s, respectively. Since then, basic data about improvements in strength and ductility relative to the amount of reinforcement has been accumulated through cyclic loading tests. Before preparing guidelines, full-scale tests were conducted to verify performances.

(1) Flexural reinforcement effects

Fig. 2 Effects of fiber sheet reinforcement on shear strength obtained from an analysis by a finite element method

Fig. 3 Typical results of ductility tests on specimens reinforced with carbon fiber sheets

To verify flexural reinforcement effects, simply-supported beams with fiber sheets applied to the bottom were subjected to static loading tests. Flexural reinforcement using fiber sheets is dependent on the degree of anchorage and adhesion, and the type of fiber sheets. It works best when stresses are transmitted through cover concrete. Basically, the effects can be estimated by calculating the flexural strength of reinforced concrete members, with fiber sheets regarded as tensile reinforcing materials and the area of rupture strength of fiber sheets used based on Navier's hypothesis.

(2) Shear reinforcement effects

For research into shear reinforcement effects, simply-supported and cantilever beams wrapped with fiber sheets were subjected to static and dynamic load testing for analysis. Photo 2 shows typical results of shear tests on specimens reinforced with carbon fiber sheets. Some test results indicate that the shear strength of members is determined by the rupture of fiber sheets when the amount of reinforcement is relatively small, as shown in the photo, but by the rupture, not of fiber sheets, but of concrete in a compressive zone when the amount is large.

Fig. 2 shows the relationship between the amount and effects of reinforcement obtained from an analysis by a non-linear finite element method. As is clear from the figure, in the domain where the shear strength of members is determined by the rupture of fiber sheets, shear strength increases with the amount of reinforcement, and shear reinforcement effects measured are slightly lower than those calculated from a truss theory. Further, when the amount of reinforcement exceeds a certain value, the shear strength of members is determined by the rupture of concrete, showing little reinforcement effect of fiber sheets.

(3) Ductility reinforcement effects

Ductility reinforcement effects were verified through dynamic tests using cantilever beams wrapped with fiber sheets. Fig. 3 shows typical results of ductility tests on specimens reinforced with carbon fiber sheets, and the relationship between horizontal displacement and load. As is obvious from the figure, a stable hysteresis loop with a large energy absorbing capacity can be obtained to minimize earthquake damage by the use of sufficient ductility reinforcement.

Fig. 4 shows the relationship, obtained from tests, between shear strength ratio and joint translation angle in an ultimate state, where it is assumed that the shear strength of fiber sheets is in proportion to the amount of reinforcement. As shown in the figure, ductility increases with the amount of reinforcement up to a joint translation angle of about 1/15, indicating that the ductility reinforcement steel bears comparison with steel plate lining.

GUIDELINES FOR

EARTHQUAKE-RESISTANT

REINFORCEMENT METHODS

USING FIBER SHEETS

In order to adopt this method using fiber sheets for the reinforcement of railway reinforced concrete structures, it is necessary to check safety in the same manner as specified in "Design Standard for Railway Structures (Reinforced Concrete Structures)" which is based on a limit state design method. To do so, equations of the strength (flexural and shear) and ductility ratio of reinforcing materials were formulated and safety factors suitable for the dimensions of actual structures established. The basic concept of earthquake-resistant reinforcement for railway reinforced concrete structures since the earthquake has been not to cause shear failure, or brittle failure, but to reinforce the structures sufficiently to provide ductility against assumed earthquakes. Based on these concepts, rules for reinforcement design were established.

EARTHQUAKE-RESISTANT

REINFORCEMENT OF

RAILWAY VIADUCTS

A diagnosis was made on the earthquake-resistance of existing railway reinforced concrete viaducts after the earthquake. As a result, about 45,000 viaduct piers were judged to be of pre-shear failure type and thus needed to be reinforced. The earthquake-resistant reinforcement work is now being carried out by the railway companies, and is scheduled for completion within three years for new trunk (Shinkansen) lines and five years for local lines.