Creep proportionality limit in concrete Up to what fraction of characteristic compressive strength can sustained stress be assumed to produce creep approximately proportional to stress?

Introduction:
Creep is the time-dependent increase in strain under sustained load. For design and serviceability checks, it is important to know the stress level up to which creep strain can be considered roughly proportional to applied stress, simplifying calculations and superposition of effects.

Given Data / Assumptions:

• Normal-weight concrete at service conditions.
• Stress is expressed as a fraction of characteristic compressive strength fck.
• Ambient temperature and humidity are within typical ranges.

Concept / Approach:

Within a moderate service stress range, the creep response is approximately linear with stress. Beyond a certain fraction of fck, nonlinearity increases and microcracking can accelerate deformation. A commonly adopted upper bound for proportional behavior is about one-third of fck.

Step-by-Step Solution:

Verification / Alternative check:

Design guides and test data show acceptable linearity for service stresses near 0.3–0.35 fck; beyond that, crack growth and elevated creep coefficients reduce linear accuracy.

Why Other Options Are Wrong:

15–25% are overly conservative and do not reflect standard practice; 50% is too high and would not maintain proportional creep behavior.

Common Pitfalls:

Applying linear models to high-stress zones, ignoring loading age and humidity effects, and neglecting shrinkage interaction with creep.

Final Answer:

33%