Theory 053/Engineering & Materials /Structural materials
Creep Follows the Load Ratio
Creep is estimated from stress. The material may respond to how close it is to its limit.
Concrete creep is set by the ratio of sustained load to strength rather than by load alone, so a high-strength mix under heavy load can creep less than a weak mix under light load.
01The evidence
Creep coefficients vary widely between mixes and are not explained by stress alone, and higher-strength concretes show lower creep at equal stress.
02The tension
Creep models use stress and age, with a mix-specific coefficient fitted from tests. The fitting absorbs the strength dependence rather than modelling it.
03The mechanism
Creep is the time-dependent response of the cement paste, driven by the same bonds that carry strength. The closer the load is to the strength, the more of those bonds are near their limit and the faster they rearrange.
That makes the load ratio the controlling variable, and predicts that creep should collapse across mixes when plotted against the ratio rather than against stress.
04What this is not
Stress models fit a coefficient per mix. This predicts collapse on the ratio, which removes the fitting.
05Predictions
- P1Creep should collapse onto the load ratio across mixes.
- P2High-strength mixes should creep less at equal stress.
- P3The ratio should predict better than stress.
06Refutation
Creep tracking stress at controlled strength would restore the fitted model.
07What it changes
It would simplify creep prediction for design.