Francis Walley (NCE 9 September) expresses surprise that WSP could not use aircraft pavement branch data on pavement deflections under aircraft. I am unaware of measurements of deflections under actual aircraft, but from 1948 to 1987 the standard pavement evaluation method was a plate bearing test, usually with the 70t Plate Bearing Test rig.
Originally the rigid pavement test did not measure deflection under the plate. In 1982 we introduced a new test, linked to A Guide to Airfield Pavement Design and Evaluation (PSA, 1989), that did.
The deflection measurements were compromised by use of a rubber mat, and the stiffness of the plate altered deflections compared to a tyre. Both tests measured failure loads, and not the pavement parameters needed to correlate calculated deflection to load.
Plate Bearing Test data therefore gave limited information on real deflections under actual aircraft. No testing was carried out on slabs cracked into crack and seat patterns.
I operated the rig from 1982 to 1987, and still mourn the passing of a historic piece of equipment, and the records lost on privatisation of PSA, but I don't miss nights working under an old tank transporter in sub-zero temperatures and gale force winds.
Graham Woodman, senior technical director, WSP, graham. woodman@wspgroup.