Sealed deal

By Kim Kemp

Rehabilitating one of Gauteng’s busiest routes, during peak-hour traffic and in all types of weather, takes some doing. KPMM and National Asphalt were up to the task.

sealingThe truck moves swiftly over the milled section, applying the tack coat.

Stretching 16 kilometres along the N4, from Proefplaas to the Solomon Mahlangu Interchange, a road project is presently underway. It forms part of South African National Roads Agency’s (SANRAL’s) preventative maintenance programme and is being undertaken by KPMM, a construction company specialising in township infrastructure, road construction, and road rehabilitation.

Key supplier to KPMM, is National Asphalt, a specialist in asphalt products to the construction, mining, and building industries throughout southern Africa. National Asphalt is tasked with the manufacture and supply of the asphalt used on the project.

While the road is not new, there have been interventions during its life to ensure that it is maintained as part of SANRAL’s maintenance code. “Every five to 10 years, an intervention generally must take place to prevent roads from falling into disrepair,” comments Dave Bennett, general manager at National Asphalt.

Scope of work

The roadworks entail the resurfacing and resealing of a section of one of the busiest highways in Gauteng, as well as the rehabilitation of three over-passes and four underpasses along the 13-kilometre route. Included in the scope of work is road-widening along a section to provide access to a proposed SANRAL/CSIR/UP Transportation Research Centre.

The national route N4-01 generally consists of a six-lane dual carriageway, which becomes a four-lane dual carriageway near the Simon Vermooten Interchange.

The total area to be resurfaced is 451 100m2 and the project is estimated to take 13 months, having started in August last year.

The major component in this project is the asphalt, which is manufactured at National Asphalt’s Bonn Accord branch in Pretoria.

IRIThe international roughness index (IRI) is the roughness index most commonly obtained from measured longitudinal road profiles, which determines both smoothness of road surface and ride.

The asphalt ‘recipe’

Neels Smith, operations manager at National Asphalt, explains what goes into making the fit-for-purpose asphalt.

The plant produces the planned daily quantity, says Smith and points out that the company has both static and mobile plants, supplying the industry across the country.

“While we manufacture asphalt for a variety of clients, each with their own requirement,” says Smith, “for this project, we produce 300-tons of A-E2 SMA per day.”

Aggregate and bitumen are the ingredients for asphalt, with the aggregate coming from a quarry within a 30km radius, Smith explains. “Different aggregate fractions are required for the mix; it depends on the ‘recipe’/specifications of the project. Both ingredients are mixed in a drum mixer at the asphalt plant. It is heated to about 165°C and then loaded onto the haulage trucks. The lab at National Asphalt takes samples of each truckload, which are subjected to a battery of Marshall tests to ensure that it has been manufactured to the approved mix design. It is then taken to the site and loaded into the paving unit.”

entire rigLined up along the stretch, the entire rig comprises a Vögele Super 1803-2 paver and the MT 3000-2 offset Power Feeder, with a truck emptying its load into the paver hopper.

National Asphalt has a process laboratory where the final product is tested to ensure that it complies with and is within acceptable design limits. Included in this process is checking that the grading of the aggregate complies with the desired specifications of the mix for the specific project. “The SMA for the Proefplaas project consists of 10mm, 7.1mm aggregate and crusher dust with binder content specified at 6.5%,” assures Breyten Dirks, regional laboratory manager. Also included is the cellulose fibres that prevent drain down of the binder. Full Marshall testing on the final product ensures adherence to specifications.

Further to National Asphalt implementing process control to produce a product that meets the specifications, an independent SANAS-accredited laboratory was appointed on this project and is responsible for acceptance control on the A-E2 and SMA. Statistical acceptance procedures are used to decide whether work should be accepted, rejected, or accepted at a reduced payment.

The maximum voidless density (SANS 3001—AS11), often referred to as the Rice test method, is used to calculate air voids in the compacted sample and the amount of bitumen absorbed aggregate. The bulk density (SANS 3001-AS10) is also determined, whereby the asphalt is compacted (at 142°C) to determine the unit weight of the compacted asphalt briquettes and to obtain the percentage air voids. “The target on this particular mix is 4.7%,” Dirks adds.

Binder extraction involves extracting the binder from the mix with a solvent. The binder content is calculated as the difference of the mass of the mix and that of the extracted aggregate. SMA is a stone skeleton mix and for this project is manufactured with an AE2 binder. This binder is also subjected to various testing to ensure it provides sufficient deformation resistance. It should not be too rigid and prone to cracking otherwise the mix can start to deform, creating ruts in the road, for example in designated truck lanes.

“Before offloading the AE2 into the working tanks, on every load of AE2, the softening point is checked,” Dirks says, “to ensure it is within its range of 65 to 85°C and complies with its relevant specification.”

“With conventional bitumen, the softening point is 46 to 56°C,” Smith adds. “Over and above the Marshall tests, the asphalt is also subjected to performance testing, such as wheel tracking, gyratory, and Modified Lottman tests.”

After the contractor has completed a base production, cores are drilled from the road to determine density. If the density has not been achieved, the section is pulled up. “Paving 1 000 tons a day means nothing if the density is wrong,” says Smith and adds, “If the mix does not meet specification, it is rejected and needs to come out.”

feederThe versatile Vögele feeder with pivoting conveyor ensures that the asphalt can be precisely fed to the desired place.

Resealing process

Riaan van Rensburg, construction manager at KPMM, describes the resealing process: “We need to prepare the road, concrete side drains and any joins. The old asphalt is initially milled, or scoured, to create a roughened surface to which ultimately, the asphalt will adhere. This surface is then sprayed with SS60, a low viscosity anionic slow set bitumen emulsion, which relies on the evaporation of the water component to cure, acting as a tack coat for an asphalt overlay. This is diluted with water and reduced to SS30.”

The initially brown in colour tack coat is left to dry, turning black within a brief period — depending on ambient conditions — undergoing a process called ‘breaking’. It is now ready to receive the asphalt.

The asphalt-loaded trucks tip the asphalt into an asphalt transfer vehicle, which transfers the asphalt to the asphalt paver.

A paver is used to distribute, shape, and partially compact the layer of asphalt on the surface of the road. As the paver passes over the prepared road area, the feeder conveyors move the asphalt to the rear of the paver, while the distribution augers push the asphalt outward to the desired width. The screed then levels the layer of asphalt and partially compacts it to the desired shape, after which a heavy, steel-wheeled roller follows the paver to further compact the asphalt to the desired thickness.

“Hand labour also ensures that the joins are even, and the depth correct before rolling,” Van Rensburg adds.

Using a manual depth measure for spot checks and relying on pre-set electronic levelling beams on the paver, the desired asphalt thickness is achieved. Smith adds, “There is also an international roughness index (IRI) specification, which determines the texture of the road surface to ensure vehicle traction and good rideability, measured with a laser, which picks up any irregularities on the road surface. The specifications are so strict that should the IRI not meet spec, the layer is removed, at significant cost.”

“The results of this are heavily dependent on the feeder and the equipment we use,” Van Rensburg states, adding, “So far, we have had excellent results as we use only the best machines.”

Once the asphalt is laid, compacted, and cooled down, it is only a matter of a couple of hours before the section can be reopened to the traffic, as the surface sets within a short time.

Wirtgen Group’s Vögele range was chosen for the project, comprising the Super 1803-2 paver and the MT 3000-2 offset Power Feeder, with a whopping R14-million-plus price tag.

In response to asking whether removed asphalt can be recycled, Smith says, “National Asphalt does recycling mixes comprising between 20 and 40%; it’s just that it isn’t popular in South Africa yet, whereas overseas they use up to 100% recycled mix.” Smith maintains that the company has laid some test sections that will take a couple of years to assess quality.

milledIn preparation for the new asphalt layer, the road is milled to create a better, rougher bonding surface.

Challenges

Executed along a high traffic volume section of the freeway brings its own challenges, the biggest being that the traffic must continue to flow, despite the roadworks. This vehicle stream must be diverted when a lane is resurfaced, with workers cordoning off the specific lane, in the face of oncoming traffic.

The restricted working hours have also impacted the project, which is only permitted to work between 09:00 and 15:00 on weekdays and 07:00 to 14:00 on Saturdays, with absolutely no work undertaken during public holidays, including all foreseeable statutory declared election days. “We also have to be off the road the day before and after the Easter weekend, as well as the days of school-term closure,” adds Francois van Wyk, KPMM site agent.

“The heavy rain we have had over the past few weeks has also impacted productivity,” Van Wyk adds, saying that to date, 18 days have been lost owing to weather.

With bitumen a main component of asphalt, Smith points out that supply can be problematic, more so when refineries are undergoing maintenance “or shut down unexpectedly”, he says. The commodity is expensive as it is directly related to the current oil price. However, Smith explains, the project caters for this fluctuation in its commercial clauses.

Van Rensburg adds, “This project is a reseal, that is why it is 25mm and this specific stone mastic asphalt is intended to carry heavy traffic, made specifically for this project by National Asphalt.” He refers to the project’s documentation, which shows that at the Proefplaas intersection where it joins the N1 for example, the traffic volumes reach 60 000–70 000 vehicles a day, of which 7.4% are trucks.

Understandably, diverting this amount of traffic around the roadworks requires detailed, planned operations, Van Rensburg stresses.

This traffic challenge is exacerbated by the average South African driver’s mentality, where a shortcut is the norm, jumping lanes a given, and intolerance the standard, Van Wyk points out.

The project is proceeding on time and both KPMM and National Asphalt are confident that it will be completed within its time frame.

Table 1: Traffic count data
Table 1 Traffic count data

*Hans Strijdom Drive is the old name for Solomon Mahlangu Drive, but still the name of the CTO Station.


 

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