Rod McClelland, senior technical services manager with the Warrington, U.K.-based contractor Alfred McAlpine PLC, also suspects recommended design pressures may be too conservative, following a limited number of tests he ran 10 years ago. They might as well be as old as the Bible, say some, who consider ACI's recommendation for a full liquid head too prudent but don't trust the formulas for regular concrete for use on superplasticizer concrete. ACI 347 doesn't put an upward limit on the pressure rating for designing formwork to resist a full liquid head. That hurts the most for continuous lifts 10 to 12 ft tall and greater. That can be an astronomical expense, says formwork engineer Jim Hughes, executive vice president of Conesco Industries Ltd., Little Ferry, N.J., a manufacturer and supplier of formwork systems.
People taking responsibility have to assume the worst case.
Johnston has no way to prove his suspicions about maximum pressure without conducting research. Each placement is different, so it is tricky to develop a design pressure factor applicable across the board. With enough field investigation, you can come up with dependable data. More hotly disputed is how critical is the need for research. He nevertheless sees a need for more research.