Allen Browne from the Hiway Group spoke on the company’s experience with foamed bitumen at the 18th annual NZTA/NZIHT conference held late last year. This is a precis of his presentation.
Despite some pretty good results on several Rodney road trial sections, we realised pretty quickly that you need the right gear for the foamed bitumen (FB) process and, in 2005, the company invested in a purpose-built FB stabiliser which didn’t leave much change from $1 million.
After a few small Auckland region projects, we learnt a lot quickly on a large 20,000 square metre rehabilitation project with a lengthy three-year maintenance period on the Coronet Peak Road up to the ski field. It also showed how it was more difficult to heat bitumen to 180ºC at altitude.
We learnt a lot during those early years, undertaking lots of research and extensive mix design/QA testing for a large number of projects.
We were very active in getting on top of the optimum methodology for FB in New Zealand, which at the time was a fairly straightforward set of design parameters with no specific construction specification.
We learnt how important it was to ensure the material grading and plasticity were suitable to ensure improved performance, and we developed means to mitigate poor properties with materials evaluation, supplementary aggregate/binders and/or pre-treatment and a designated quality engineer on-site for all FB stabilisation projects.
Since 2005 we’ve been doing FB on a lot of state highway and council projects, involving many awkward treatment sites and challenging materials. We also participated in the development of the stabilisation construction specification for NZTA (released 2008) with the Roading New Zealand Stabilisation Working Group.
Over the next couple of years we had hundreds of projects that performed well, and across the industry several projects that were challenging. Amongst these was motorway work that some years after construction, developed localised areas of cracking – which surprisingly was environmental or shrinkage cracking, rather than fatigue cracking, and the client, NZTA, wanted to get to the bottom of this.
The agency gathered a large group of industry contracting and consulting practitioners around the table in 2015 to work through all elements of FB and this was the basis for developing a more rigorous FB methodology, which has been incorporated recently in the newly released NZTA Guide to Pavement Evaluation & Treatment Design (July 2017).
There are a lot of changes to the FB process in the new guide, some of them minor and more for clarification, and some of them more meaningful in terms of determining where it’s appropriate to use FB.
One of the more significant changes to design philosophy is that the designer is now looking more closely at the material properties that are achievable in context of where the FB layer sits in the pavement system rather than just focusing on the material suitability of the layer itself.
The new sub-layering protocol means that we have to determine what the design modulus is of the material underlying the FB layer.
This is because the new guide constrains the modulus of the FB layer to the minimum value of either 800 MPa or five times the underlying sublayer design modulus. We want to avoid the FB sitting on poor underlying material.
We are also required to have at least 100mm of aggregate immediately under the FB layer and that has to achieve a minimum design modulus of 100 MPa (calculated using Austroads sublayering rules), but it’s nice to have a lot more than that for robustness. Another requirement is to try to keep the active filler (cement) to one percent, with a maximum of 1.25 percent (where bitumen is typically around three percent).
If we need to sublayer the FB layer to meet the modular ratio requirements, the minimum FB stabilised base thickness allowable is nominated in the guide as 220mm. NZTA doesn’t want to have pavements constructed with thick asphalt on top of FB, so if you are intending to have more than 60mm of asphalt surfacing, there is a significant reduction to the underlying design FB modulus regardless of the laboratory.
The rationalisation for that is based around the stress-dependency of materials classified as unbound aggregates. There has been some pretty robust industry discussion about this.
Another requirement now is that the asphalt surfacing should be modelled mechanistically if it’s at least 40mm thick.
This is a good thing because our pavement designers in New Zealand will now (as well as checking the subgrade fatigue in the traditional fashion) check the strain development for the entire pavement system, which requires performance criteria for the asphalt and a check that we’re getting a satisfactory design life out of the surfacing. Ten years is a good minimum target with 15 years modelled life ideally.
Foamed bitumen basecourse thickness design
Another thing we are happy to see formalised is the minimum layer thickness for FB, which has to be at least the Austroads nominated thickness of premium basecourse, which for typical highway loading needs to be somewhere from 160mm to 200mm minimum. This avoids the selection of inappropriately thin layers of FB in an effort to save dollars.
FB should fall in the stiffness range where it is too hard to rut, yet not so high that it becomes brittle and is susceptible to fatigue or shrinkage cracking. There has been much discussion over the appropriate strength band and what lab properties mean with respect to field performance. Laboratory testing typically measures performance via interpolated tensile properties and there is some debate over how this reflects the in-situ performance, which is characterised by compressive shear properties similar to an unbound aggregate.
Once the FB layer is constructed it is extremely moisture insensitive, however, it is important to have somebody present who understands how to determine moisture conditions and can let the stabilising crew know if an adjustment is required.
Cue to the “lubricating” effect of the bitumen the compaction moisture is only required to be around 70 percent of the natural aggregate for compaction. Checking moisture levels is typically done by the squeeze test, which is just taking a handful of stabilised material and squeezing to check it lightly holds together and is not too crumbly (dry) or clumpy (wet). This is also useful to confirm that the FB is dispersing appropriately by noting that discrete spots of bitumen are left on your hand after squeezing.
If the moisture content is jumping around a bit during the stabilising operation, this monitoring gives the ability to assess how close to optimum condition the material is and the crew can react quickly to make the required adjustments.
Take-up in New Zealand
There have been many millions of square metres of FB done here since 2005, and with the early indications of strong performance in a number of settings and materials, it started being used as a bit of a ‘silver bullet’ in some instances with some inappropriate applications and corresponding failures. It may be that as an industry we were a little slow to react and clarify guidelines.
There has been much more rigour in New Zealand application in recent years but currently there’s some caution over using FB for larger capital works projects.
There’s general acceptance for FB stabilisation on a case-by-case basis for parts of the country around state highways/high volume roads and we have seen a strong take-up from local authorities, especially around Auckland where it has a very good performance record for the past 12 years. In level constrained urban settings it can permit recycling to get 30-year design life without requiring full reconstruction from subgrade level, saving a great deal of time, material and disruption. Older aggregates tend to suit the FB process better because of their (typically) finer grading which the foaming process prefers.
As an industry, we’re still learning about optimising methodologies and design. Some things we still need to consider include appropriate minimum and maximum modulus, lab-to-field relationship and research on 10-plus year-old sites to calibrate performance against design.
There’s a lot of protection against weaker layers underlying the FB in the new guide, but it’s worth reminding ourselves that some of the main issues that we’ve had on the motorway network have been where we’ve had very strong lower pavement that has permitted the foam material to develop an extremely high modulus (results of >10,000 MPa), which has facilitated the development of shrinkage cracks. Further to this, a weaker lower pavement that flexes a little bit more will not permit the FB base to develop such strong bonds, so it has a lower/less risky stiffness.
Provided it is durable and strong enough where you’re not going to get rutting, it’s actually fulfilling its design purpose even if not as strong as what it could achieve on a rigid support.
The Indirect Tensile Strength test is the New Zealand standard for mix design and construction quality assurance and it is a difficult test for a non-continuously bound material such as FB.
We do have some sites where some of the quality assurance tensile strength (ie, splitting the compacted and cured sample) testing is problematic, on occasion giving us an inferred modulus that may be below the 800MPa long-term target.
However, almost inevitably the strength determined from Falling Weight Deflectometer testing of the finished treatment gives us a good result, suggesting that in-situ stiffness testing is optimal rather than taking ex-situ samples from behind the stabiliser, undertaking indirect tensile splitting tests and interpolating what the modulus is.
Foamed bitumen in Australia
In Australia FB is going gangbusters with huge quantities of in-situ and ex-situ FB coming to market in the past few years.
Conversely to New Zealand much of this is on strategic motorways and expressways. The interesting thing is that some States were resistant to use FB stabilisation up until recently, however a number of trials and research projects have been undertaken with positive results alongside good performance of FB projects. In one research project, ARRB looked at 100 kilometres of foamed bitumen stabilised state highway with high loading and more than 10-years’ service life and evaluated that over 95 percent of this area was performing above expectations.
So that’s a pretty good performance indicator. A recent national harmonisation process to bring together all the different specifications from each State has made the process more consistent and certainly easier for practitioners working interstate.
The Australians have taken a completely different design philosophy to New Zealand, where they design FB as a structural asphalt.
They use similar binder quantities as New Zealand, but instead of designing as a “super modified” granular material they take the volume of bitumen and calculate what their performance criteria are (using the Shell equation) and then model the FB exactly the same as an asphalt. For the higher loadings, rut resistance testing is incorporated as part of the design process.
Indirect Tensile Stiffness Modulus testing is standard which uses a pulsed load rather than the ITS splitting test used for New Zealand. While there are still some subtle differences, all the States are designing FB in fundamentally the same manner now as prescribed recently by Austroads for the Australian market.
Foamed bitumen in Fiji
We’ve been working in Fiji for more than 15 years and undertaking FB basecourse there for about five years using the New Zealand modelling and construction protocols. The performance has been exemplary.
The key challenge in Fiji is providing upper pavement layers than can accommodate the stresses imparted by the heavy vehicle overloads. The Fiji legal truck rear twin axle loading is 16 tonnes but there are a number of “big bin” mining trucks using the network that we’ve put over a weighbridge and they measured up to 40 tonnes on the rear twin axles when loaded to the waterline.
A little bit of analysis suggests that a 25-year design life then drops to one or two months. You can’t even really do Austroads related modelling because the loadings are just so completely outside the box in terms of stresses and materials response, which is not linear with increasing loading.
The LTA / Fiji Road Authority has been working hard to combat overloading with rigorous policing and fines, but unless these trucks are removed from the network the temptation is always going to be to maximise cartage efficiency via full loading.
Other challenging pavement issues include lots of shallow services, very regular rainfall (especially in the Suva region), irregular gradings with big chucks of coral within treatment depth and high innate plasticity of the basalt aggregates forming the bulk of basecourse supply. Only structural asphalt or FB basecourse has been proven to handle these extreme loadings and other challenges as outlined above.
There is an issue of cost as you cannot just apply structural asphalt and FB to the entire network, so some rationalisation of treatment is required. However, Fiji is another country where FB is seen as being a really good robust solution for heavily loaded arterials and is widely specified.
To wrap up
The new NZTA rehabilitation and pavement design guides will not likely significantly change the current approach and utilisation of FB in the near future, but the changes from our perspective are virtually all an improvement putting more cross-industry consistency and rigour into the design process and, collectively, we are moving forward.
The difference in treatment selection for key strategic expressways and modelling philosophy in different countries is interesting and time/monitoring will provide more clarity over which means of modelling is the most appropriate.
General consensus of industry (and particularly the FB suppliers) in New Zealand is that the process is robust and provides a high performance basecourse where designed and constructed appropriately.