BITUMEN & ASPHALT
Implementation of PG binder specification in South Africa The publication of the SATS 3208 technical specification for a performance-grade (PG) binder specification in South Africa coincided with CAPSA 2019. This milestone has its roots in CAPSA ’15, following a master class sponsored by Sabita, whereby John D’Angelo introduced the principles of a performance-based binder specification to a wider audience of practitioners. By Steph Bredenhann*
T
he purpose of the SATS 3208 was to provide a path towards complete acceptance of the PG specification and a path to a full SANS 4001 specification. The objective is to use SATS 3208 in parallel with the SANS 4001 specifications. As only one specification can be in force at a time, it is necessary to specify which one is applicable on a contract. Since CAPSA, some authorities decided to implement SATS 3208 as the specification on a per-project basis and test according to SANS 4001 simultaneously. In this article, the PG implementation so far will be discussed and pertinent issues that have arisen will be highlighted.
PG specification in a nutshell The purpose of this article is not to describe the PG specification in detail; it is to address implementation aspects that require attention. A summary of the most important points contained in the SATS 3208 specification is shown in Table 1. The PG specification was discussed in detail by Bredenhann et al. in the September 2019 issue of the SAICE Journal 1. A PG specification is a purchase specification that allows for the selection of a binder that can be expected to perform well
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IMIESA August 2021
in an asphalt mix or spray seal application. The real test for performance will always be the asphalt mix itself; the mere choice of a binder will not guarantee performance. The PG specification is an improvement on the current penetration-grade specification in that environment, traffic and ageing are considered. Definitions for Tmax and Tmin are given in Table 1 and the question is often raised why the UTI (useful temperature interval) of 80°C must be maintained. Minimum temperatures of -22°C are not experienced in South Africa. The lowest temperature is in the region of -6°C in a remote area where traffic is light at best. The choice of UTI = 80°C was introduced to bring the intermediate temperature, TIT, in line with the temperatures where fatigue is experienced. Researchers had noted that the pavement temperature associated with fatigue was somewhat higher than the MAAT (mean annual air temperature) and +4°C was seen as a reasonable estimate. TIT is calculated as (Tmax + Tmin)/2 + 4°C = 22°C for a PG58-22 binder. Selecting specification criteria to ensure satisfactory resistance to fatigue cracking is a difficult challenge. The larger part of South Africa falls in the PG58 high-temperature zone, including the highly trafficked roads
of southern Gauteng, up the southern coastline from Cape Town to Durban. The 22°C intermediate temperature is, compared to experience in the USA and Europe, a very reasonable temperature for fatigue criteria in this area. The ΔTc ≥ -5°C requirement addresses the fatigue to a certain extent, but no specific criteria are included in SATS 3208. However, it is required that an isotherm at TIT is reported to allow for further analysis and research into fatigue and durability. This is perhaps the research area that is the most important and hopefully the universities will respond. The first step in specifying an appropriate binder is to define the environment and traffic level for the operating region where the binder will be used. The environment is defined by the maximum and minimum pavement temperatures and the traffic level by the equivalent number of 80 kN axles and traffic speed. Test methods in the SATS 3208 specification are performed according to the ASTM standard specification series. An example: traffic load is 40 million E80axles, traffic speed on the design section is 80 km/h in Cape Town, thus the PG grade is PG58V-22 and PG58E-22 on inclines where truck speeds will drop below 80 km/h.
