Herman Mostert, Element Consulting Engineers, Namibia
Bitumen Rubber binders have been used in Chip seals in South Africa since the early 1980’s. Over the past 3 decades the use thereof has extended to southern Africa, including Namibia where it has been extensively incorporated into the recent reseal program of approximately 3,200km of roads, of which approximately 700km roads have been surfaced using Bitumen rubber between 2016 and 2019. A Total of 340km of these rubber seals were done using the S-R2 bitumen binder, instead of the conventional S-R1 bitumen rubber.
According to the TRH3 design manual of 2007, no data for the adjustment for the non-homogenous S-R2 binder to conventional binder is available. The common practice was to use the S-R1 conversion factors. This use of the S-R1 conversion factor led to extensive bleeding in some cases on Namibian roads. One of the key benefits of using rubber bitumen is that due to the stiffness of the binder which provides resistance to the orientation of the surfacing stone, higher bitumen application rates can be applied compared with conventional binders, normally in the range of 1.8 – 2.3 l/m² hot for a 14mm seal. It was observed that the stone orientations rates differed between the conventional S-R1 and S-R2 bitumen rubbers resulting in early bleeding, subsequently justifying different binder application rates and conversation factors.
The rate of stone orientation was observed using two different S-R2 products sprayed at different application rates in the same direction of travel on a trial section. The use of two SR-2 products from different suppliers was used to established if there is any significant difference in the behavior in the SR-2 binders itself and to established the rate of stone orientation. Application rates ranged from 1.8 l/m² to 2.1 l/m².
Texture depth measurements were taken directly after surfacing prior to traffic and rechecked every day for a week, thereafter weekly for a month and finally monthly over a period of four months. The location of the texture depth measurements was strategically placed in and between the wheel tracks to observe the sensitivity of the S-R2 bitumen rubber to early stone orientation due to traffic. No marked difference in rate of texture loss due to traffic compaction was observed between the two S-R2 products, but it was noted that S-R2 has a lower viscosity specification at a lower temperature than S-R1 which leads to a less stiff bitumen at the time of application. Higher application rates can thus be achieved with S-R1 than with S-R2 binder which reverts to a stable condition at around 5000 vehicle passes. The outcome of the S-R2 trials in Namibia lead to the provisional conversion factors for S-R2 binders, as published in the recently released SABITA Manual 40 / TRH3 2021.