1. Senior Researcher, Council for Scientific and Industrial Research (CSIR), P.O. Box 395, Pretoria 0001, South Africa (corresponding author).
2. Candidate Researcher, Council for Scientific and Industrial Research (CSIR), P.O. Box 395, Pretoria 0001, South Africa.
3. Professor, Paul F. Kent Endowed Faculty Scholar, Univ. of Illinois at Urbana-Champaign, Dept. of Civil and Environmental Engineering, 205 N. Mathews Ave., Urbana, IL 61801.