Synopsis
Concrete cylinders 77 mm in diameter and 270 mm long were subjected to constant strains, and the changes in load (stress relaxation) were monitored for approximately 15 min from the time of loading. The variables examined were: humidity (on dry and saturated specimens); design 28 day strengths (of 13·8, 20·7, 27·6 and 34·5 N/mm2); initial stress levels (20, 40, 60 and 80 % of compressive strength); number of applications (initial and two subsequent loadings); age at loading (40 days and 9 months). Over 100 relaxation tests were performed in which constant strain was maintained by a closed-loop electro-hydraulic testing machine. The principal findings were that short-term stress relaxation may be approximated by a hyperbolic equation, of the type suggested by Ross during studies of creep, and that the behaviour of concrete under constant strain exhibits anomalous features with respect to humidity and water/cement ratio.
