Rebuilding a scenic route in South Africa
First publishedin World Highways
The KMA unit was important to the processing operation
Equipment from Wirtgen has played a key role in the rehabilitation of a major route in Cape Town. The scenic Camps Bay Drive provides access to Camps Bay and Hout Bay
Based on the project brief and geometric analysis, Camps Bay Drive required widening to cope with high volumes of tourist traffic and MyCiti buses. In order to minimise the impact on traffic, the design brief targeted construction outside the summer peak tourist season and also required investigation into shortened construction periods. The key role was played by Wirtgen cold recycling technologies, using the KMA 200 mobile cold recycling mixing plant.
The South African metropolis has large volumes of reclaimed asphalt (RA) stockpiled from numerous maintenance works undertaken around the city. This material has generally been used for hard standing areas and shoulder construction. In order to reuse this material, WorleyParsons was asked to research more efficient and sustainable methods. For this reason, the use of a foamed bitumen technology was investigated and the base layer on Camps Bay Drive was constructed using 100% RA as a foamed bitumen stabilised base (BSM) base layer.
Originally, the project consisted of localised road widening to accommodate the proposed MyCiti buses, however due to the large volume and sizes of the proposed buses a geometric analysis resulted in the entire portion of the road being widened by 1.4m. The proposed pavement design involved light rehabilitation with the widening area requiring full depth construction. During construction, numerous weak horizons were discovered along the road.
The project was divided into three construction sections. Section 1 from Geneva Drive to Prima Avenue, Section 2 from Prima Avenue to Rontree Avenue and Section 3 from Rontree Avenue to Houghton Road. In order to ensure more uniformity and better quality of the mix, a static mobile mixing plant was specified for production of the BSM.
Power Construction was contracted to carry out construction, with Milling Techniks carrying out the production of the BSM using its Wirtgen KMA 200.
During construction, the stockpiled RA was sent for technical testing at BSM Laboratories in Durban. The design was carried out on three design options using the two dedicated stockpiles of materials from two sources which were created for the project by the City.
Based on the results obtained, the most cost-effective option to yield a BSM 1 was the 100% RA mix design using 2.1% foamed bitumen with 1% cement.
The batching plant was set up at the Ndabeni Roads and Stormwater depot in Maitland. The depot provided sufficient space to establish the KMA 200 as well as allowing for the stockpiling of the unprocessed RA, screened RA and processed BSM. The specification called for crushing of the RA to remove the oversized fractions. An alternate proposal in the form of screening the material to minus 19 mm in place of crushing the material by the contractor was accepted provided the fines fraction (0.075mm) met the specification of 4%. This was monitored for the duration of the project with a fines fraction of between 3% and 4% recorded. The screened RA was processed using the KMA to form a BSM 1 which was stockpiled at the depot.
The scenic Camps Bay route has now been repaved using Wirtgen machinery
In order to ensure an adequate quality of the BSM and consistency within the mixing process, Power Construction, Milling Techniks and WorleyParsons created a quality system tailored for the BSM production using the KMA. This was implemented to ensure the mix met the BSM 1 specification throughout the project. Once the BSM was produced, the material was either placed on site immediately or protected and stockpiled at the Ndabeni Depot.
The BSM was paved using a heavy-duty tracked paver from Power Construction in two 100mm-thick layers in the same process. Before starting with paving and compaction, a testing of the BSM was undertaken by Soillab who established a testing rig in accordance with the TG2, 2009 specification. Acceptance control was carried out on the Indirect Tensile Strength (ITS) and MDD taken from the batching plant and site samples.
The three sections identified were constructed using similar techniques, however in Sections 1 and 3, the BSM was opened to traffic immediately after construction and in some cases 24 hours after construction.
The Camps Bay Drive rehabilitation project provided key information for dealing with BSMs and more particularly, 100% RA BSMs. Important findings included that static mobile mixing of BSMs should be undertaken in the summer months in the Western Cape region due to the weather conditions. Low temperatures and high moisture contents result in a poorer quality product. They also found that an impact crusher should be used to crush the oversized RA. This may create more fines in the mix as well as reduce the need for stockpiling of the oversized RA. In addition, planning and training of the operators prior to the trial section is crucial as this product is not an asphalt material and different placement and compaction methods apply. And finally, the 100 RA BSM has voids within the mix so when poor weather is imminent, the BSM should be sealed to prevent the ingress of water.
Using cold recycling with foamed bitumen delivered high quality at a reduced cost. Using the 100% BSM showed that the design is very versatile and minor changes to the RA and mix design do not have a significant effect on the quality of the BSM. It also showed that the material behaves extremely well if the road is opened early to traffic. This also showed that there is a large cost saving when using the 100% TA which amounted to around €2.95/m². This cost only includes the material cost saving and not additional traffic accommodation and time cost savings. The cost could be further reduced by producing the material closer to the site or mass producing the material as the greatest cost component of the BSM production is the haul of the RA and BSM to the site.
Overall, the project was a success. During the project, approximately 8,150tonnes of RA (4,200m³) was processed using 165tonnes of bitumen and 78tonnes of cement. Using the RA within the pavement structure provides a much more cost-effective and sustainable solution for the future with depleting aggregate resources.