Morris De Beer

A study of crumb rubber modified bitumen used in South Africa

In South Africa, an empirical characterisation of crumb rubber modified (CRM) bituminous binders has historically been the only means of predicting their performance in pavement layers, short of constructing pavement test sections. An improved characterisation is provided by means of rheological analysis using a dynamic shear rheometer (DSR). However, the heterogeneous morphology of CRM bitumen makes it a challenge to test using current methods and equipment. DSR testing of CRM bitumen requires a plate gap adjustment to avoid any influence by the rubber particles. This has been done by monitoring the effect of changing the DSR plate gap setting on the measured linear visco-elastic properties of the binder. An adjusted gap was adopted for rheological measurements so as to characterise CRM bitumen properties with ageing. But, the incomplete recovery of CRM binder from asphalt/seals makes it impossible to monitor the rheological properties of the in situ binder within a pavement layer. This has led to indirect methods of investigating relationships between tested properties of the pure CRM bitumen to those of the in situ binder.

Impact of Tire Loading and Tire Pressure on Measured 3D Contact Stresses

Three-dimensional (3D) tire–pavement contact stresses for two types of tires used by the truck industry (new generation wide-base tire [WBT] and dual-tire assembly [DTA]) were measured and compared. The testing matrix was composed of five loads () (26.6, 35.5, 44.4, 62.1, and 79.9 kN) and four tire inflation pressures (ߪ ௢ ) (552, 690, 758, and 862 kPa). The equipment used for measuring the 3D- contact stresses is described along with the testing procedure and the methodology followed during data processing. The effect of applied load and tire-inflation pressure on the variation of longitudinal, transverse, and vertical CONTACT STRESSES along the contact length of each tire type was analyzed. Differences in the distribution and magnitude of the aforementioned stresses were observed between WBT and DTA; these differences are an important factor linked to pavement damage caused by each tire configuration. This experimental effort is part of a national study to evaluate the effect of WBT on pavement damage and compare it to that of DTA.

Analytical Approach for Predicting Three-Dimensional Tire–Pavement Contact Load

Three-dimensional tire–pavement contact loads of two truck tires–-a new-generation wide-base tire (WBT) and a dual tire assembly (DTA)–-were measured and analyzed. Extreme and typical values of tire inflation pressure (552, 690, 758, and 862 kPa) and tire loading (26, 35, 44, 62, and 79 kN) were considered in the experimental program. The measurements were performed with the stress-in-motion Mk IV system at the Council for Scientific and Industrial Research in South Africa. Peak values in three directions were compared, and the importance of tangential contact stresses was highlighted. In addition, characteristic variations of the measurements in the longitudinal, transverse, and vertical directions were identified. A function depending on two regression parameters, applied load, and distance along the contact length was proposed to represent the contact load in the vertical and transverse directions. An analysis was performed on the measurements to obtain the regression parameters, and a simplified procedure was proposed to determine tire–pavement contact loads. The contact area and contact length of the WBTs and the DTA were also compared.

Long-term crushing performance of lightly cementitious pavement materials – update to the South African procedure

Evaluation of the crushing (or compression) failure and associated surface deformation of lightly cementitious (stabilised) materials used for base/sub-base course in pavements has been well established in the South African pavement design practice since the 1990s. This paper presents a re-evaluation of this earlier work, focusing on expanding the empirically derived crushing performance relationships (i.e. transfer functions or damage laws) for lightly cementitious materials at in situ moisture conditions, i.e. without the adverse effect of additional or artificial water infiltration into the lightly cementitious base, which limited the practical application of the original transfer functions for significant rutting. The re-evaluation was undertaken to determine whether the original crush initiation relationship was reasonable for as-built moisture conditions and also to develop practical advanced crushing damage relationships for various rut depths developing within the base, based on the measured response of the original test sections prior to water ingress. The newly derived crush initiation relationship differs little from the original and either can be used with similar confidence. However, the new 10 mm rut relationship differs significantly from the original relationships (which were derived from the adverse statistical bound of the crush initiation data), the previous relationship should now be abandoned, and the new relationships adopted for well-protected pavements. The newly developed advanced crushing damage relationships for 5, 10, 15 and 20 mm level of deformation (“rut”) within the lightly cementitious base layers are proposed for practical application to in situ as-constructed (as built) moisture conditions.