Babalwa Nzuza, Naidu Consulting (Pty) Ltd
Environmental considerations and increased traffic demands make the use of rubber-modified bitumen an attractive alternative to polymer-modified bitumen. Bitumen-rubber as a binder in both seals and asphalt is used extensively in South Africa with a track record of excellent performance. It was decided to use a bitumen-rubber open graded asphalt (BRAOG) as a wearing course for the N3 corridor upgrade project. Bredenhann et al (2019) reported on the appropriate application of the new performance grade (PG) specifications with regards to bitumen-rubber modified bitumen to the 2019 CAPSA Conference. This paper will report on further work done with bitumen-rubber characterisation to be used in the BRAOG design for the N3 corridor project.
Currently there are two types of bitumen-rubber binders available in South Africa namely A-R1 (traditional and A-R2. The longer shelf life and lower emissions during manufacture are the primary benefits of A-R2 over A-R1. However, past experiences together with desktop level studies highlighted the varied performance of bitumen-rubber binders from various suppliers.
In order to ensure that the performance of the rubber modified binders supplied for the N3 corridor upgrades are within compliance limits, testing was carried out on the rubber modified binders with the objective of tightening up the existing binder specifications. Binders for testing were provided by participating suppliers. Each binder was tested for its empirical, PG and chemical properties. This paper will focus on the PG testing conducted to understand the rheological behaviour of the binders in relation to loading and temperature variations.
The rheological properties were assessed at three ageing conditions i.e. unaged, short term aged (rolling thin film oven test (RTFOT)) and long term aged (pressure ageing vessel (PAV)). The low service temperature behaviour of the rubber modified binders were evaluated using the bending beam rheometer (BBR) and high service temperature behaviour using the dynamic shear rheometer (DSR). The multiple stress creep-recovery (MSCR) test was also conducted on the RTFOT aged binders to gain an understanding of the stress / loading resistance of the binders at high service temperatures.
PG test results were stored in comma separated value (csv) format for ease of incorporation during assessment and analysis. Analysis of the data involved various modelling techniques utilised locally as well as internationally. With the simplest of digitisation modelling tools, the metadata collated from the tests conducted were simulated into graphical relationships, relationships predetermined and prescribed through the current ASTM standards and research drives in the binder testing industry.
Prior to data analysis, quality and acceptance control of the data was carried out using the validation and precision estimate requirements in accordance with the relevant ASTM standards. In addition, black space diagrams were utilised for the identification and removal of bad data points. Rheological index trend lines were overlaid onto the black space diagrams to aid in the assessment of the data points in relation to known bitumen trends.
Analysis of the results currently include the construction of master curves. Various shape defining functions, including the Christensen-Anderson (CA) and Christensen-Anderson-Marasteanu (CAM), were utilised depending on the suitability to the data points. In order to assess the data at the various temperatures, a reference temperature was set, and the various isotherms shifted to the reference utilising shift functions. The shift functions include the Arrhenius function and Williams Landel Ferry (WLF) function. The primary objective of this research is to assess the performance of various industry available rubber modified binders with the intention of tightening up the current specifications in preparation for the use thereof on the upcoming road upgrades.
Babalwa is a hardworking, enthusiastic, young technician who values understanding and knowledge transferal. She started her journey in the Civil Engineer working industry as a Trainee Technician at Naidu Consulting, and later got appointed as Civil Technician in the Pavement Engineering unit, having worked on a few projects as an Assistant Technician. She completed her National Diploma in Civil engineering at the Durban University of Technology, in 2018 and is currently progressing towards completing her Bachelor of Technology studies, specializing in Transportation.