Permanent deformation behaviour of flexible pavements

Abstract

Permanent deformation (rutting) is a pavement distress condition visible in the surfacing layer of a pavement. It occurs along the wheel path and results from the accumulation of load-induced permanent deformation developed from all individual pavement layers, including the subgrade. It is one of the major distress conditions in flexible pavements. Plenty of research regarding permanent deformation in flexible pavements exists, but it is mainly focused on asphalt surface layers and granular base, subbase, and subgrade layers. The South African National Roads Agency Ltd (SANRAL) completed the construction of seven flexible pavement sections on the R104, between the east of Pretoria and Bronkhorstspruit, during 2013. In-situ pavement response and environmental related data have been collected from these test sections ever since on a number of occasions. The seven flexible pavement structures include a natural gravel (G4) base, a high-quality graded crushed stone (G1) base, a Foam Treated Base (FTB), an Emulsion Treated Base (ETB), a Cement Treated Base (CTB), a Bitumen Treated Base (BTB), and a High Modulus Asphalt (HiMA)/Enrobés à Module Elevé (EME) base. The permanent deformation behaviour of different flexible pavements relative to each other was investigated by processing, validating, and analysing the relevant in-situ pavement response and environmental related data collected from each of the SANRAL test sections. With the focus on total and base layer deformation, it was found that in terms of a short-term loading response and under normal operating conditions, bituminous pavements show superior performance to cement/bitumen stabilised pavements, while the latter performs better than granular pavements. CTB and ETB pavements are very similar with FTB pavements closely behind. The only granular exceptions are inverted crushed stone pavements, which should closely follow bituminous pavements at the top end of the performance range. For permanent deformation behaviour in terms of a longer-term recovering response, it was found that bituminous pavements tend to recover a larger amount of the permanent deformation attained after load application than granular pavements, probably due to the delayed elasticity (visco-elastic properties) of bituminous materials. The possibility of a transfer function for linking the permanent deformation behaviour of a pavement to its structural integrity was also investigated by determining a representative pavement number for each of the SANRAL test sections. It was found that the permanent deformation behaviour of flexible pavements relates relatively well to their structural integrity as a general decrease in permanent deformation (rut rate) was observed with an increase in pavement number. A negative power function for linking permanent deformation behaviour to structural integrity was proposed (y = 76.657x-0.752, R2 = 0.77). Additionally, it was found that post-compaction trafficking has a significant effect on the permanent deformation behaviour of flexible pavements during the initial stages of their life cycle; temperature variations can have a major influence on the in-situ performance and behaviour of bituminous layers, and the permanent deformation behaviour of flexible pavements correlates positively with the corresponding dynamic response as an increase in permanent deformation (rut rate) was observed with an increase in maximum dynamic deflection (positive linear function, y = 0.0361x - 2.5687, R2 = 0.92).