New Zealand is an island nation in the South Pacific with a traditional export-based, rural economy. Most produce is transported by road, which has resulted in a large network of roads carrying less than 5,000 vehicles per day. In the post-war years the rural economy flourished and most of this network was sealed with a layer of hot-sprayed bitumen and chip. New Zealand is now faced with maintaining these sealed road networks in the face of diminishing resources and rising costs, hence new contractual and construction techniques are developing.
Nasty buildup
The climate varies from frost and snow in the winter to summer temperatures in the low 30°C range. However, New Zealand’s proximity to the sea and mountains means its weather can be very variable, localized and rapidly changing.
This variability imposes an added stress to the asphalt chip seal designs, which can experience 50°C surface temperature during the day and 5°C at night. As a result of these thermal extremes, the traditional binders used have been soft grades of 180/200 penetration with the addition of 2% diesel and 3-5% of kerosene. Over the past 50 years the buildup of these seal layers has resulted in many chip seals becoming unstable multiple layers of soft binder and chip. Flushing then occurs due to chip embedment, leading to a dramatic reduction in skid performance.
As well as allowing for colder winter temperatures, binders in New Zealand must be pliable enough in winter to withstand the flexing of thin-depth pavements, while still maintaining waterproofing capability.
By understanding this problem, pavers are developing an integrated approach to designing the right emulsion product and sealing technique and offering improvements in the road network. Merely attempting to substitute an emulsified binder into an existing hot-sprayed seal technique will not grow and develop the emulsion market.
Looking flushed
As the layers of bitumen and chip have built up over the years, the ratio of binder to chip grows to a point where the mixture becomes unstable. Often layers of 50 mm to 100 mm thick of soft bitumen and uniform-size chip have built up. As traffic loading increases and the binder softens in summer weather, the chip embeds into the mobile binder, resulting in flushing and a loss of surface texture and skid resistance. Traditionally, engineers have looked at a reseal as the cheapest option to cover up the flushing and improve the skid resistance. However, this is only a temporary option, and in the long term, where traditional hot-sprayed sealing techniques are used, it only aggravates the already poor chip-to-binder ratio.
Once instability has been reached, the only reliable current cure is to mill the binder-rich seal layers into the base course and reconstruct the seal coats from new. This, of course, is not a cheap option, at approximately $13.90 (U.S.) a sq meter. If this solution can be delayed for several years then there is a significant economic gain to the client’s network.
By analyzing a typical multilayer seal core, a paving contractor can develop a better understanding of the seal structure. The core was taken from an area where a significant portion of the network is affected by pavement instability. Thirty years ago the seals would have been designed for much lower traffic volumes, which also has aggravated the stone-binder ratio problems of today.
Comparing traditional hot-sprayed binder application rates to proven emulsion sealing designs, utilizing the same stone ratios, revealed a total of 8.8 liter/sq meter of hot-sprayed binder applied compared to a probable 5.96 liter/sq meter of residual binder from an emulsion-based application technique. This is a savings of 2.84 liter/sq meter of binder for the same buildup of seal depth.
At the time of application, most of the binder would have had a softening point in the low 300°C range. Due to traffic density increases, probably only the first two layers had an opportunity to age and harden to a level that would have helped the seal’s stability.
If we make some assumptions of seal layer depths and binder application rates, we can calculate a chip-to-binder ratio of 17% by volume for the hot-sprayed sealing options. This was verified by laboratory analysis of the core. By comparison, the emulsified binder designs gave a chip-to-binder ratio of 10%. A chip-to-binder ratio of 12% to 15% by volume is the maximum recommended. Above that, the mix becomes unstable.
The recovered core showed evidence that the hot-sprayed seals were effectively unstable at 28 years, whereas the emulsion sealing options could extend to 50 years of good service life in terms of seal stability.
The seal types and application rates listed for Lagoon Creek came from a seal design program that has been in use for 12 years. The designs are based on a rural state highway with a traffic count of 2,000 ADT at 10% heavy commercial vehicles. All application rates are residual binder rates, and the emulsion options (CRS68MV) and polymer emulsion options (Emograb) offer significant binder reductions compared with the hot-sprayed (180/200 plus three parts kerosene) application rates.
The client for the project was asking for a single-coat, hot-sprayed seal, utilizing a Grade 2 (13-mm) chip. The existing surface was very flushed and binder rich, and the site was prone to repeat flushing. The client’s hope was that the void in the large chip would help absorb the excess binder already in the seal. Sites done in this manner have not shown good long-term results.
The consultant chose to convert the seal type to G2 (13-mm) and G5 (6-mm) Sandwich seal utilizing a high-viscosity emulsion binder. This mix used 0.69 liter/sq meter less residual binder intergraded with an interlocking seal structure, which offers superior seal strength.
Emulsified solution
In the New Zealand market it currently costs 15 to 20% more ($3.11 U.S./sq meter hot-sprayed seal versus $3.66 U.S./sq meter for an emulsion seal) to construct an emulsion seal compared to a hot-sprayed seal. Hence in a cash-strapped industry, it is difficult to explain to clients the cost benefits of using emulsified binders.
On the other hand, emulsions offer the opportunity for full chip embedment immediately after the chip is applied. Embedment is especially useful in lower-traffic-density areas where there is often insufficient time for good chip embedment under traffic prior to winter with conventional hot-sprayed seals. Emulsions greatly reduce the risk of chip loss as the binder becomes stiffer in cold temperatures. Combined with a very accurate application and spacing of the chip, an emulsified binder can normally withstand cold temperatures the night following construction and can withstand the hot and cold extremes experienced within one day.
If the chip is applied correctly to an emulsified binder immediately after spraying, the binder is displaced by the volume of the chip and raises high up into the seal structure. As it breaks, the moisture is drawn up the chip by capillary action and draws the emulsion with it. As the water is expelled from the emulsion, the residual binder left is less than that needed for a hot-sprayed seal, and yet a significantly greater chip surface area is obtained in contact with the binder.
To demonstrate this effect, an experiment was undertaken to compare the rate of chip embedment into hot cutback binder as compared with the embedment into an emulsified binder with 10% less residual bitumen than the hot cutback. Once fully cured, the binder rises significantly further up the chip than the original embedment level, demonstrating a significant improvement in chip-coating activity.
Introducing a range of viscosity-based emulsion formulations helps operational crews construct better emulsion seals. With many options, the crew can order a product precisely customized to their job requirements.
The high-viscosity product is typically between 800 cps and 1,500 cps to give a thick on-road emulsion, which will not flow and will sit up high in a seal structure or stay in place on steep inclines. A medium-viscosity product is typically between 400 cps and 800 cps to give a medium on-road emulsion, which will flow a bit to get down into the void of a multicoat seal and is easier to spray without streaking, but will still not flow off the road at moderate gradients. The low-viscosity product is typically between 200 cps and 400 cps to give a thin on-road emulsion, which will flow into voids and spray well at very light application rates.
The other important feature of an emulsion is its break rate once spray and chip have been applied. The faster the seal becomes traffic-tolerant, the sooner the sealing crew can leave the site and move to the next job. A lot of effort has been made to create emulsions with approximately two weeks storage ability that will break and be rain-resistant within 15 minutes on a 150°C ground temperature day. (Break time is very dependent on application rates and atmospheric conditions). There is a very fine balance between particle distribution, emulsifier levels and application temperatures needed to achieve this, but once it is right, the chances of obtaining a successful emulsion seal increase greatly, and the often-reported “cheesy” stage of emulsion curing can be eliminated.
The recognized benefits of emulsion sprayed seals are:
- In contracts where the binder of choice has been emulsions, there has been a significant reduction in rework issues;
- The reduced risks and longer life expectancy of emulsion seals has made them the binder of choice on most of our South Island multiyear, bulk-funded network contracts; and
- The gains in operational efficiencies that can be achieved can offset the additional costs of applying an emulsion seal. Contractors and clients have an additional tool available to them in their road surfacing kit.
