Modulus is a microcomputer-based backcalculation system that can be used on 2-, 3-, or 4-layer pavement systems with or without rigid bedrock layers. It uses a linear elastic program to generate a data base of deflection bowls. Once generated, a pattern search routine is used to fit measured and calculated bowls; error minimization is rapid, less than 5 sec per bowl on a 386 type microcomputer. The system is general purpose and can process data from any nondestructive testing device. The user has several options when performing backcalculations, including specifying the depth to bedrock or using existing default data bases for common pavement structures. Outputs include a summary listing showing the mean and variances of moduli values and also a graphical output that plots moduli values along a project and automatically performs subsectioning according to the recommended aashto procedure. The modulus system is described together with discussion on continuing efforts to validate the moduli values. These validations include (a) coparison of laboratory and field moduli values, and (b) the use of multidepth deflectometers to monitor deflections within the pavement system. The results of monthly deflection measurements on experimental pavements around the state of texas are also described. Finally, current efforts to improve the modulus system are described. These attempts include automatically estimating the depth to bedrock using either the error minimization or zero deflection approach. This paper appears in transportation research record no. 1260, Measurement of pavement surface condition 1990.
Abstract