Load test recovery criterion for flexural members A structure subjected to an imposed load after 72 hours for a period of 24 hours is deemed unacceptable if, on removal of the load, it fails to recover at least what minimum percentage of the net deflection?

Introduction / Context:
Load testing of reinforced concrete floors and beams assesses serviceability by checking residual deflection after unloading. Codes specify recovery criteria to ensure that time-dependent effects and microcracking have not impaired stiffness beyond acceptable limits.

Given Data / Assumptions:

• Load is applied after 72 hours and maintained for 24 hours.
• Net deflection refers to the deflection due to the imposed load only (after subtracting dead-load effects).
• Acceptability depends on the percentage recovery within 24 hours after unloading.

Concept / Approach:
Two checks are typical: (a) immediate recovery after 24 hours should not be less than a codal percentage (often 75%), else the test may be repeated; (b) if, on repeat or overall assessment, the recovery is less than 80%, the structure is considered unacceptable. The 80% threshold protects service performance against creep/damage.

Step-by-Step Solution:
Step 1: Determine the acceptance threshold for recovery of net deflection.Step 2: Apply the specified limit: if recovery < 80% → unacceptable.Step 3: Choose 80% as the minimum acceptable recovery; anything less fails.

Verification / Alternative check:
Several solved-code examples summarize: first check 75% within 24 h; if recovery remains < 80% even after 72 h cycle, declare unacceptable. The decisive acceptability threshold is 80%.

Why Other Options Are Wrong:
90%, 85%: Too conservative relative to the prescribed acceptability limit.
75%: Used for the preliminary 24 h check, not the final acceptability criterion.
70%: Too low; would permit excessive residual deflection.

Common Pitfalls:

• Confusing the interim 75% recovery check with the final 80% criterion.
• Using total deflection instead of net (superimposed load) deflection.

Final Answer:
80%