1. Postdoctoral Associate, Dept. of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139 (corresponding author). E-mail address:
2. Senior Research Assistant, Eidgenössische Technische Hochschule Zürich, Institute for Building Materials, Schafmattstrasse 6, 8093 Zurich, Switzerland. E-mail address:
3. Professor, Eidgenössische Technische Hochschule Zürich, Institute for Building Materials, Schafmattstrasse 6, 8093 Zurich, Switzerland. E-mail address:
4. Professor, Politecnico di Torino, Dept. of Structural and Geotechnical Engineering, Corso Duca degli Abruzzi 24, 10129 Torino, Italy. E-mail address: