1. Department of Civil Engineering, Kitami Institute of Technology, 165 Koen-cho, Kitami, Hokkaido, 090-8507 Japan
2. Maintenance and Management Section, Civil Engineering Research Institute, Hokkaido Development Bureau, Hiragishi 1-3, Toyohira-ku, Sapporo, 062 Japan
3. Research Institute, Nippon Hodo Co., Ltd., 32-34, 3-chome, Higashi-Shinagawa, Shinagawa-ku, Tokyo 140-0002 Japan