Compression-Force Under-Reading in Student-Built Engineering Education Concrete-Cube Presses: A Proving-Ring Calibration Evaluation Linking Platen Bedding to Structural Safety-Margin Reliability

Student-built concrete-cube presses used in engineering education are typically calibrated against a proving ring at a single reference load, yet the bedding between platen and specimen governs how much of the applied force reaches the cube. The model treats the indicated force as the true force attenuated by a bedding-dependent under-reading factor, and derives the probability that a mix accepted by the press exceeds its design safety margin. A design-based evaluation of the before-after-control-impact estimator was conducted under a declared variance structure (between-cluster SD = 6.0, residual SD = 4.5, intraclass correlation = 0.64), with power estimated across 2000 design cells for cluster counts of 8, 16, 24 and 32 and effects of 3, 5 and 8 percentage points. The analysis indicates that 56 clusters are required to detect a 5-percentage-point effect at 80% power, and that the intraclass correlation of 0.64 dominates the detectable-effect curve. The contribution is a formal link between a workshop-level calibration defect and the structural reliability of the safety margin, which reframes proving-ring calibration as a reliability instrument rather than a single-point check.