U.S. Department of Transportation
Federal Highway Administration
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Washington, DC 20590
This chapter discusses how different agencies specify or incorporate WMA into their specifications and the applications that WMA has been used for. Across the board, the expectation is that WMA should provide the same performance as or better performance than HMA (i.e., no compromises in performance are acceptable when using WMA). One factor that may affect the willingness of European agencies to allow WMA is the fact that short, 2- to 5-year workmanship warranties are included in most European paving contracts.
WMA has been used in all types of asphalt concrete, including dense-graded, stone matrix, porous, and mastic asphalt. It has also been used in a range of layer thicknesses. WMA sections have been constructed on roadways with a wide variety of traffic levels, from low to high. In Norway, the highest trafficked WAM-Foam section carried about 25,000 average daily traffic (ADT) (two directions) with 2,500 heavy vehicles per day (one direction). ADT values were not reported in Germany, but sections were placed with truck traffic levels of 1,600 heavy vehicles per day. The A81 toll road in France received 21,000 ADT (two directions), with 1,500 heavy vehicles per day (one direction). Legal axle loads vary in the countries visited from 10 metric tons (9.1 tons) in Norway to 13 metric tons (11.8 tons) in France. The ECOMAC process, a warmed cold mix, is recommended for low traffic only.
The Norwegian Public Roads Administration has allowed WMA to be used in lieu of HMA. The WMA (WAMFoam) must meet all the applicable specifications for HMA. There are no variances. The blending of a hard and soft binder to meet the specified binder grade, used in the WAM-Foam process, is already allowed. The Norwegian Public Roads Administration does require a 5-year materials and workmanship warranty. Items monitored under the warranty include potholes, longitudinal joints, delamination, voids, and ride.
The European standards on hot mix asphalt include requirements for constituent materials, which state: "Only constituent materials with established suitability shall be used. The establishment of suitability shall result from one or more of the following: 1) European Standard, 2) European Technical Approval, or 3) specifications for materials based on a demonstrable history of satisfactory use in asphalt. Evidence shall be provided on their suitability. This evidence may be based on research combined with evidence from practice."(29) In the case of WMA, BASt has been gathering data on the performance of Sasobit, Romontan-B, Licomont BS 100 (Sübit), and Aspha-min to meet option 3 using the research and test sites described in Chapter 4. This includes monitoring of field test sections placed under heavy traffic for a minimum of 5 years. Part of the data was gathered from test sections constructed by contractors on commercial projects. A compilation of WMA experiences has been prepared.(25) Germany also requires a materials and workmanship warranty, typically for 4 years after construction.
Based on these experiences, in August 2006, a "Merkblatt," or bulletin, on the use of WMA was issued. The bulletin provides general remarks and references for WMA, and serves as a step toward a standard construction method. The guidelines include information on five additives or modifiers for HMA, including Fischer- Tropsch wax, Montan wax, fatty acid amides, a blend of Montan wax and fatty acid amides, and zeolite. Zeolite is to be used only in HMA, not mastic asphalt. Table 7 presents the recommended mixing and compaction temperatures for WMA presented in the Merkblatt.(30)
|Type of Asphalt||Binder Grade (Pen)||Reference Production Temperature, °C||Reference Temperature Behind Screed, °C|
|Rolled Asphalt (HMA)||70/100||130 to 150||Minimum 120|
|50/70||130 to 150|
|30/45||140 to 160||Minimum 130|
|PmB 45 A||140 to 160|
|Pmb 25 A||150 to 170||Minimum 140|
The temperatures in table 7 represent a reduction of 36 to 54 F° (20 to 30 C°) from conventional HMA paving temperatures. A 70/100 pen binder is about equivalent to a PG 64-22. PmB represents polymer-modified binders. The temperatures in table 7 tend to be somewhat higher than production temperatures used for WMA in the United States.
The Merkblatt provides other notes on the use of WMA. Good tack coats and joints are important with WMA. A homogeneous blend of organic additives must be achieved. The content of organic additives can be determined using differential scanning calorimetry. Longer solvent extraction times are required to measure asphalt content for mixes with organic additives. The ring and ball softening point is required to be determined with organic additives.
In France, new innovations for road authorities are evaluated by a partnership between the developer (often a contractor) and the road directorate. SETRA, Service of Technical Studies of the Roads and Expressways, represents the road directorate. Both partners must fund the evaluation. The evaluation process starts with a laboratory evaluation; if successful, field trials are undertaken and then guidance papers are prepared for use of the product. The final step is to incorporate the product into existing standards or develop new ones if no applicable standard exists. Trial sections are constructed along with a control section, each a minimum of 500 meters long. Trial sections are monitored for a minimum of 3 years. Typically, at least three trial sections are used to evaluate a single product, and a certificate is awarded at the end of a successful evaluation. The certificate technically validates the product and provides guidelines for the product's use. Certificates are often used by contractors for marketing. In 2007, SERTA awarded a certificate to Aspha-min zeolite (figure 8). SERTA can recommend, but not mandate, that contractors use certified products. However, a representative from a private toll road company, Cofiroute, said that the company would not use an uncertified product unless it had conducted its own experiments. SERTA is not involved with urban roads. Therefore, municipalities, such as Paris, conduct their own experiments.
Figure 8: SETRA certificate for Aspha-min.
LCPC considers WMA to be in conformity with the existing French (soon to be European) performance-related design procedures. Mix designs would be expected to follow the same steps, with type testing of performance parameters to follow the optimization of the job mix formula. Only the field production temperatures would change.(11)
|Challenge 2: Address initial product approval. How do we sort out good innovative products from poor products? Accepted performance tests are needed to separate the good from the bad. The traditional practice of products and technologies being approved on a state-by-state basis needs to be changed. Products and technologies should be approved on a national, or at least a regional, basis.|
WMA has been used with all types of asphalt mixtures, including dense-graded asphalt, SMA, and porous asphalt. WMA has been used with polymer-modified binders and in mixes containing RAP. WMA has been placed on pavements with high truck traffic, up to 2,500 heavy vehicles per day, which over a 20-year design period would be expected to exceed 30 million 18-kip-equivalent single-axle loads. WMA has also been placed at bus stops, on airfields, and on port facilities.
As noted in Chapter 4, the European agencies visited during the scan expect the same performance from WMA as HMA. One factor that may affect the willingness of European agencies to allow WMA is the fact that short, 2- to 5-year workmanship warranties are included in most European paving contracts. Further, evaluation systems are in place in France and Germany to assess and approve new products. A similar process combining laboratory performance tests and controlled field trials should be developed in the United States for WMA as well as other innovative processes. The evaluation process should be implemented on a national level.
Overall, the use of WMA in Europe was not as high as the scan team expected. It should be emphasized that in most cases, WMA is allowed but not routinely used. Discussions with contractors and agencies suggested two reasons for this. First, many of the oldest WMA sections are just exceeding the period of the workmanship warranty. The contractors who developed these processes want to develop confidence in their long-term performance before using them widely. Second, in most cases WMA still costs more than HMA, even when fuel savings are considered. In discussing its environmentally friendly, energy-efficient (3E) asphalt mix in its annual report, Colas noted, "Customers who wish to act in support of sustainable development can now opt for this asphalt mix."(14) The French Department of Eureet- Loir noted that it was willing to pay more for WMA because it believed it would last longer.
|Challenge 3: Address issues with existing specifications that may prohibit the use of WMA. Examples include allowing blending of binders, minimum production and placement temperatures, and minimum ambient temperatures or cutoff dates.|
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