Importance of the zero point in DCP testing of structural capacity of flexible pavements

Abstract

Relatively large errors attributable to the use of an incorrect position of the zero point may occur in estimates of pavement structural capacity and other parameters using DCP methods. The standard South African method requires the zero reference point to be taken with the top of the vertical shoulder of the cone level with the top of the surfacing of the pavement. However, the Kleyn granular pavement model of analysis was developed from DCP testing in which the zero point was taken at the top of the base course. The De Beer lightly cemented pavement model was developed using DCP tests in which the zero point was taken at the top of the surfacing. In many cases these differences may not be significant. However, those which are likely to be include old pavements with thick, hard surfacings and new pavements with soft, bleeding seals. Those aspects which are most severely affected are the DCP structural number and estimates of the strength of the upper pavement, structural capacity and relative damage exponent, whilst the obtained structural balance of the pavement may also be affected. It is not the purpose of this paper to review or cast aspersions on previous work, but only to show the importance of selecting the correct zero point for the analysis model used. Two case histories of a total of 15 different, mostly granular pavements with old seals and one of 15 single point analyses on one new, cemented base pavement with a new, bleeding seal are presented. In the case of the old pavements analysed according to the granular model significantly higher estimates of structural number, structural capacity, strength of the “base” and relative damage exponent were obtained by including the seal. In the case of the new pavement analysed according to the cemented model significantly higher estimates of structural capacity were generally obtained when the seal was excluded. Until a more authoritative and detailed analysis is available it is recommended that, in cases of doubt, the DCP software program be run both with and without the surfacing and the more conservative results used.