With the specification of a new pavement system on parts of the Waikato Expressway, the NZTA is aiming for a stronger pavement with a longer life. Mary Searle Bell talks to the agency about Hi-Lab.
The High strength Low fines aggregate base, known as Hi-Lab, has been in use by the agency as part of maintenance rehabilitation work and in trial sections over the past eight years, but this is the first time it is being used over a significant project length.
Hi-Lab is a pavement system modelled on the original Macadam pavement design philosophy, pioneered by Scottish engineer John Loudon McAdam around 1820. McAdam developed a road pavement design that was comprised of clean broken stone, mixed and laid so it became bound by its angularity into a firm and compact mass.
Hi-Lab is constructed from coarse-graded large stone aggregate, combined with a slurry of cement and finer stones to fill remaining air voids. The structural strength of the pavement is derived by larger stones interlocking ensuring load transfer through direct stone-on-stone contact. Modern construction techniques are used to ensure the pavement layers are laid and compacted to the appropriate density with the same productivity as traditional construction methods.
Transport Agency national pavements manager John Donbavand says Hi-Lab is made up of a AP65 or AP40 maximum stone size with up to 12 percent fines (material passing through a 2.36mm sieve) mixed with cement. This is substantially less than the maximum amount of fines allowed in the traditional M4 basecourse aggregate, which is 33 percent. Consequently, the grading envelope curves for Hi-Lab are much tighter than for traditional basecourse.
“This is an important part of the design since the cement and fines forms weld points around the larger aggregate, and there are insufficient fines to form an uninterrupted matrix throughout the base course,” says John.
“This means typical reflective cracking, as a result of displacement within the matrix of cemented fines, is prohibited by the full stone interlock, restricting movement between rock particles.”
In other words, traditional basecourse will have larger stone particles floating in a matrix of fines, which can be susceptible to thermal cracking through the whole layer. Conversely, the lack of fines within the Hi-Lab layer as a result of stone interlock restricts any displacement of finer particles within the layer preventing cracking.
The Transport Agency has been constructing Hi-Lab over the past eight years as part of maintenance rehabilitation under the performance specified maintenance contract (PSMC001) and latterly through trials as part of new capital projects. This includes multiple roundabouts, and sections within the Cambridge, Ngaruawahia and Te Rapa sections of the Waikato Expressway.
John says the performance of these trial sections compares very favourably with the adjacent traditional project pavement designs, and Hi-Lab shows a lot of promise.
Further testing at the Canterbury Accelerated Pavement Testing Indoor Facility (CAPTIF) in Christchurch has exceeded expectations. The trial sections are performing well after four million load repetitions. When converted to an equivalent standard axle loading, this is similar to the 25-year design traffic loading assumed for the Ngaruawahia section of the Waikato Expressway.
Principal pavement engineer David Alabaster says testing in CAPTIF provides a controlled environment and the agency can test various pavement samples ranging from those specifically designed to fail through to those which shouldn’t fail at all.
“So far, those sections designed to fail lost stiffness early, as expected, but are still performing extremely well despite the cement bonds breaking,” he says.
“The section designed not to fail is only showing very minor signs of distress in rutting (most likely in the surfacing), with very minor changes in other pavement layers.”
The Transport Agency has specified the Hi-Lab pavement design for three major Waikato Expressway projects, namely the Huntly, Hamilton and Longswamp sections. The main alignment carriageways for these projects will total approximately 170 lane kilometres of Hi-Lab pavement construction.
With the specification of the Hi-Lab product on the Waikato Expressway, the agency has taken a step toward a more method-based design and construction model.
Agency principal technical adviser – pavements Gerhard van Blerk, who developed and pioneered the Hi-Lab design technique within New Zealand, says the NZTA acknowledges the shift away from the more traditional design and build contract model to a more method-based specification for pavements.
“The agency believes the shift will deliver improved long-term pavement performance, combined with a new innovative construction technique, delivering value for money to all road users,” he says.
As a result, the agency has accepted greater design responsibility. However, this has been offset by an increased focus on construction and material quality control, in line with the “quality right” principles being developed by the agency.
Production and monitoring of aggregate quality is well specified, with a renewed focus on aggregate shape control and weathering.
The flakiness value to control particle shape and ethylene glycol accelerated weathering are new to the roading industry, says John. Therefore, suppliers have requested time to complete further testing. As a result, the NZTA has worked with quarry suppliers to obtain a compliant product and ensure production and delivery of the Hi-Lab product for the Waikato Expressway projects.
The Hi-Lab product is produced through a batching plant with weigh bins, making use of specific stone size fractions. The production is similar to structural asphalt, which only uses the required stone fractions.
John says there is a perception that the Hi-Lab product produces more waste.
“However, the fraction scalped from the material passing the 19mm sieve can be used within the concrete industry and/or marketed as AP20,” he told Q&M.
“Also, smaller fractions can be further crushed to produce sealing chip.”
The agency has been working closely with the contracting industry to optimise the specification, as concerns were raised regarding the practical nature of some contractual requirements. The main concern was the aggregate grading compliance of the Hi-Lab behind the stabiliser, which John says has been resolved.
Hi-Lab looks promising for the future of our roads as a cost-effective alternative, in addition to its performance benefits.