1. Ph.D. Candidate, Dept. of Engineering, Univ. of Cambridge, Trumpington St., Cambridge, CB2 1PZ, U.K. E-mail: pmma2@eng.cam.ac.uk
2. Reader in Concrete Structures, Dept. of Engineering, Univ. of Cambridge, Trumpington St., Cambridge, CB2 1PZ, U.K. (corresponding author). E-mail: cjb@eng.cam.ac.uk