The road upgrade between Vaal River and the Kroonvaal Plaza reports a high-percentage use of reclaimed asphalt and warm mix asphalt technology.
THE SOUTH AFRICAN National Roads Agency (Sanral) initiated this road upgrade as part of its routine maintenance programme. This particular section of road last received a light rehabilitation around the early 2000s, which comprised isolated base corrections and an ultra-thin friction course (UTFC). At the time, U-LM was applied for the first time in South Africa.
V&V Consulting Engineers was appointed to design the rehabilitation of the section with the aim of extending the life cycle of the road by another 15 years. The structural capacity analysis of the pavement prior to the rehabilitation indicated that the remaining life of the pavement was inadequate to handle the required traffic load.
Visual inspection of the northbound carriageway showed surfacing defects such as bleeding and deformation over the entire length and structural patches over the last 3 km. Surfacing defects on the southbound carriageway were mainly dryness and brittleness of the surface with aggregate loss and surface cracks. Some sections were badly cracked and pumping was visible in certain areas.
A first for South Africa
One of the specifications and design parameters Sanral insisted on was to reclaim and reuse the asphalt millings from the project. The new replacement asphalt mix therefore had to contain a minimum of 40% reclaimed asphalt (RA). The concept is based on the very successful warm mix asphalt trials, where 40% RA was used in a number of the trial mixes. The main reason for using RA is to ensure the sustainability of our natural resources by saving on the use of aggregates. The fact that RA materials still contain aggregates of a very good quality means that with the correct handling and processing, it can confidently be reused.
The project duration is 10 months; site handover took place at the end of last year and the project was due for completion at the end of August. However, contractual changes have resulted in a completion date of end September.
To satisfy the design requirements while ensuring the extended life performance of the road, the old asphalt layers had to be milled to a depth of between 85 and 100 mm, and replaced with a new asphalt containing 40% RA. The binder that was selected was an A-P1, which was achieved by using EVA blended with 70/100 penetration grade bitumen supplied by the SAPREF refinery. Not only is this the first project in South Africa where 40% RA was specified on a full-scale project, but it was also the first where such a base is overlaid with a UTFC. In this instance, the UTFC proposed by the contractor was the National Asphalt licensed U-LM, which is also certified by Agrément South Africa.
Ahead of the game
Due to National Asphalt’s extensive involvement in the WMA and RA trials that have been ongoing since 2008, it was fortuitous that they had placed an order at Comar Plant Design and Manufacturing to build a mobile plant capable of handling at least 40% RA, well in advance of the project even being advertised. When contractors asked for asphalt for the project, it was much to the delight of National Asphalt that they had a home for the plant much sooner than they had anticipated. This allowed the company to give prices to the market with confidence. When the project was awarded to Roadmac Surfacing, and it became apparent that the plant would be positioned at Vanderbijlpark, certain adjustments had to be made to ensure that some of the infrastructure on the site could be incorporated with the new plant, including hot storage silos and cold feed bins.
The plant is a 100 tph twin drum with the drying drum incorporating a counter flow double barrel section for heating of the RA to a temperature of only 100°C, while virgin aggregates are still heated to normal requirements. The RA and virgin aggregate are then brought together at the end of the drum and transferred into the second “mixing” drum where the bitumen and fillers are added and thoroughly mixed. At the beginning of the project, there were challenges with handling materials into the existing hardware of the old plant. Ultimately, these were overcome and the plant is currently working very well and efficiently.
Finding the right mix
The final asphalt mix design was done at National Asphalt’s Bon Accord facility under the guidance of Wynand Nortje. As technical manager at National Asphalt, he has had immense exposure and experience with RA and WMA mixes. All the design parameters specified were met, including the high RA content specified in both TRH 21 and SABITA manual 32 that optimise binder quantity. As mentioned earlier, the base binder selected is a 70/100, modified to conform to the TG1 A-P1 specifications. The high RA content not only required a base bitumen that was one grade softer to be used, but a rejuvenator was also required to achieve the correct binder properties after mixing. Sasol Wax’s SW 1665 rejuvenator was selected for this purpose. Modern rejuvenators not only rejuvenate the old binder in the recycled asphalt, but also have the additional benefit of transforming the asphalt into a WMA. Some of the advantages of WMA are:
• longer haul distances can be achieved
• it allows lower application temperatures, i.e.
winter paving, night paving, etc.
• lower mixing temperatures
• fewer emissions, which improve worker
comfort and safety
• lower energy consumption, resulting in
• various benefits to the environment.
To date, the project has been a success for all the parties involved in this contract. The project commenced during the summer months and has maintained the same mixing and paving temperatures and rolling techniques well into the winter months to no ill effect. As to the time of writing, almost all of the total 43 000 t of base asphalt and 47% of the 13 000 t of UTFC had been placed.