A Blowout Witness for Fresh-Concrete Formwork on Informal Construction Sites

Fresh-concrete lateral pressure on a 3.0 m wall computed with the ACI 347R equations over pour rates 0.5-4.0 m/h and temperatures 5-30 C, with resulting tie and panel loads. The 18 mm plywood panel spanning 400 mm governs at 40.5 kPa, below the pressure of an ordinary 2.0 m/h pour at 20 C (48.7 kPa). A low-cost mechanical blowout witness (reference line and 3 mm / 4 mm feeler shims reading panel deflection, USD 8.00 in site materials) warns at 30.6 kPa with a 24.4 percent pressure margin, 12.4 minutes of time margin at 2.0 m/h, an 18.54 percent false-alarm rate and a 0.17 percent missed-detection rate in a 20,000-case Monte Carlo. Defeat conditions, including self-compacting concrete and failures away from panel midspan, are stated in full. SIMULATED, NOT MEASURED; nothing was built or field-tested.