NEW DEVELOPMENTS WITH HALF WARM FOAMED BITUMEN ASPHALT MIXTURES FOR SUSTAINABLE AND DURABLE PAVEMENT SOLUTIONS
M.F.C. van de Ven, B.W. Sluer, K.J. Jenkins, C.M.A. van den Beemt
Contact person: Delft University of Technology P.O. Box 5048 2600 GA Delft, the Netherlands
[email protected]
Abstract Internationally, a growing health, safety and environmental awareness of the public can be observed. In this context significant efforts are underway to develop advanced technologies to reduce the use of non-renewable fossil fuels, in order to reduce emissions and human exposure. Also, in the road industry the search for new sustainable solutions is eminent. The introduction of asphalt mixtures which can be produced at lower temperatures is an important development in that direction. Lower temperature means in general lower than 140ºC. In this paper developments with so-called half warm foamed bitumen mixtures produced at temperatures below 100 ºC and even as low as 90 ºC will be discussed. Mixtures used in binder and base layers are considered. The role of Life Cycle Assessment (LCA) as used in the Netherlands will be given for half warm foamed bitumen mixtures. The performance properties according to the CE marking will also be discussed at the same time. Finally the workability of these mixtures are studied, showing how compaction of this type of mixtures can be managed. In all cases a reference hot base course mixture is used for comparison.
KEY WORDS Half warm foamed bitumen mixture, base course, LCA, performance, compaction
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CONTENTS Abstract Introduction The Dutch situation LCA: Environmental issues LCA and Half Warm Foamed Bitumen Mixtures Performance properties: CE marking and field performance Compactability (workability) of half warm foamed bitumen mixtures Conclusions References
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INTRODUCTION The goal of sustainable practices is to sustain economic prosperity and a high quality of life for everybody. At the same time the natural systems of planet earth need to be protected. So sustainability includes economical, environmental, and social components. The road building industry in the Netherlands is using large quantities of bulk materials in asphalt concrete. Each year approximately 10 million tons of asphalt mixtures are produced, so it is important to study how these quantities can be produced in a sustainable way. A very positive aspect in this is that the Dutch Roads Agency (RWS) is following a sustainability approach in all its activities. RWS is focusing on sustainable procurement of all kind of services and products, also roads. RWS is officially stimulating the market since 2010 in sustainable practices. As a consequence RWS has officially decided to apply sustainable procurement for all tenders on projects from 2010 on. In this way RWS will have a strong impact via their procurement policy on the development of a sustainable operating market in the civil sector. For this purpose a computer program called Dubocalc has been developed by RWS (Schweitzer and Duijsens, 2010). Dubocalc is a computer program in which the costs of environmental effects can be quantified. It calculates with a live cycle analysis (LCA) all relevant environmental effects of the material- and energy consumption during the total life of a project. These environmental effects are converted into Euros with a so-called ECI (Environmental Cost Indicator). This approach really makes the process of sustainable procurement much more transparent, because solutions in a tender, can be judged objectively on sustainability both by RWS and the candidate contractor. Dubocalc is an important tool to shape sustainable procurement by the Roads Agency. It uses harmonized methods to determine the environmental impact of civil projects like road construction, bridges, etcetera. Essentially in this case is the data that is used as input in Dubocalc. In the Netherlands a national environmental database is used for civil works. Both the method and the database are managed by a foundation called “Bouwkwaliteit”. The policy used by RWS stimulates significant efforts by the industry to develop advanced and innovative technologies to reduce the use of non-renewable fossil fuels, in order to reduce emissions and exposures. Also in the road industry the search for new solutions for sustainable production is eminent. An important development into that direction is the introduction of asphalt mixtures that can be produced at lower temperatures (Doh J.S., 2010). Lower temperatures means in general lower than 140°C. The production of asphalt mixtures at lower temperatures is successful if the final product in the pavement can compete with the normally hot produced mixtures, and also when the environmental effect of additives is low enough to get good results in Dubocalc. As a consequence aspects like mechanical properties, quality, workability and durability of the product play an important role, which need to be included into the LCA. At several places in the world extensive experience is available with the production and application of half-warm asphalt mixtures. The Dutch trends for half warm foamed bitumen mixtures will be described and supported with results in which the role of Life Cycle Assessment is shown. Especially the role of reclaimed asphalt is essential in this case.
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THE DUTCH SITUATION Usually the production (and construction) temperatures are low for cold mix plants (emulsion, foam mix plants) and above 160°C for hot mix plants. In the Netherlands the market is dominated by the hot mix asphalt industry. More than 40 hot mix plants are spread over the country, making hot mix asphalt a strong contender for a number of reasons like, relatively short transport distances to the construction site, product ready after cooling (important in a rainy climate) and in general excellent performance of these hot produced mixtures. Dependent on the binder used and mix type the production temperatures are mostly above 160°C with highest temperatures around 200°C for some mixtures with polymer modified bitumen. The high temperatures ensure complete drying of the mineral aggregates and sufficiently low viscosity of the bitumen, resulting in excellent coating, workability and compaction. An important aspect for the Dutch asphalt industry is the high amount of reclaimed aggregate.. Most hot mix plants are of the batch plant type with a parallel drum, because in the Netherlands practically all the milled asphalt is recycled again in hot mixtures. The Dutch roads agency has stimulated maximum use of reclaimed asphalt in the eighties of the last century. As a consequence almost always percentages of 50% recycled asphalt (RA) are used thanks to the parallel drum system. In 2009 approximately 3 million tons of RA has been recycled in a total hot mix asphalt production of 10 million tons. The RA is heated in the parallel drum to a maximum temperature of 130°C to prevent the RA to stick together in large clumps. As a consequence it is necessary to overheat the virgin aggregate above 200°C at an RA percentage of 50% to get a final mixture temperature above 160°C. It is clear that both hot mix production with 100% virgin material and hot mix production with high RA (50%) addition as is usual in the Netherlands requires quite a lot of energy. As explained in the introduction the roads agency is forcing the asphalt industry to explore all possible ways to reduce the environmental impact of the asphalt production. With hot mix asphalt production reduction of the production temperature offers great possibilities in reducing consumption of fossil fuels, emissions of greenhouse gases, emissions of fumes for workers in the plant and at pavers, safety and working conditions for workers at the plant and work site. From the point of view of environmental benefits the focus is on lower temperatures, less energy, less CO2 and less odor. In the Netherlands this has to be combined with a maximum possible reuse of RA to reduce the use of nonrenewable resources. The Netherlands is an international transportation hub, so at the same time the roads agency has to push for maximum availability of the road to traffic to prevent delays, resulting in a focus on high durability (long service life), high stability, high skid resistance, good noise reduction of surface layers and faster opening of the road after maintenance. In a balanced approach all these aspects need to be taken into account and the use of mixtures with decrease in production-, transport-, laying- and compaction temperatures are no exception to this. Reducing production temperatures and good performance have to be linked to one another in a sustainable approach. So this means that for a successful introduction of low temperature asphalt a number of aspects need to be ensured, like: − excellent mechanical performance immediately after laying,
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− − − −
comfort, low noise production, good skid resistance during the service life, use of available equipment (hot mix plant, paver, compaction equipment), costs: contribution of recycling, durability, energy consumption, sustainability: reduction of heating, gases, emissions.
Foaming bitumen is known to increase the volume of bitumen and in this way creates a lubricating effect. Originally it was used for cold mix production, but (Jenkins, 2000) showed that at aggregate temperatures around 100°C excellent wetting could be obtained on practically all aggregate sizes. Essential for application in the Netherlands of lower temperature mixtures is that it is possible to also use large quantities of RA (50%) in these mixtures. Only in that case lower temperature mixtures will be able to compete with hot produced mixtures with high amounts of RA, which is the largest production market in the Netherlands. This was also shown in (Jenkins, 2000) with a laboratory study on the fatigue properties of half-warm foamed bitumen mixtures with 50 % RA. LCA: ENVIRONMENTAL ISSUES LCA is based on the life cycle, it is an inventory of the exchange between the environment and economy (energy and materials input, emissions and waste output) and it assumes that the function needed will be delivered. A life cycle assessment is of huge importance in a sustainable approach. For example, the use of recycled materials in a pavement construction requires a good assessment of all the associated environmental impacts including energy consumption, emissions, leaching. The hart of the LCA methodology is the inventory part. This is also the most reliable part of the LCA methodology. Life cycle inventory (LCI) is based on standard LCI methodologies and follows the internationally accepted standards presented in the ISO 14040 series. In this way the LCA is a versatile tool to investigate the environmental aspects of a product, a process or an activity by identifying and quantifying energy and material flows for the system under investigation (Huang et al, 2009). Every single industrial activity is actually a complex network of activities that involves many different parts of society. Therefore, the need for a system approach rather than a single object approach has become vital in environmental studies. It is not good enough to consider just a single step in the production. The entire system has to be considered. The LCI methodology has been developed in order to handle this system approach. An LCI covers the entire cycle from “cradle to grave” including raw material extraction, manufacturing, transport and distribution, use of the product, service and maintenance, recycling, final waste handling like incineration of landfill. In a life cycle assessment a mathematical model of the system is designed. This model is a representation of the real system including various approximations and assumptions. With the LCI methodology it is possible to study complex systems (including existing interactions between different parts of the system) to provide the best possible picture of the environmental impacts from an activity like the realization of a product.
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[%]
It will be clear that for complete assessments it is necessary to input a lot of information. This is also the case when using half warm foamed bitumen mixtures. The outcome of the assessment should be accepted by the whole road building industry. For the LCI a range of impact categories can be used. The impact categories are described in the standards and contain categories like: global warming, ozone layer depletion, eutrophication, aquatic ecotoxicity, human ecotoxicity, non-hazardous waste, hazardous waste, energy, terrestrial ecotoxicity, summer smog, acidifaction, depletion of non-renewables, etc. Every impact category has reference limits, list of impacts and impact factors. The outcome of a LCI is a so-called environmental profile.
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2
3
1. Abiotic depletion 2. Global warming 3. Ozone layer depletion 4. Human toxicity
4
5
6
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5. Ecotoxicity, fresh water 6. Ecotoxicity, terrestric 7. Photochemical oxidation 8. Acidification
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9
10
11
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9. Eutrohpication 10. Energy 11. Waste, hazardous 12.Waste, non-hazardous
Figure 1: Example of the impact of transport systems: relative comparison of transport by truck versus transport by boat of aggregates ( Meijer, 2009) In Figure 1 an example is given of the environmental profile based on an impact assessment of the transport of aggregates over a certain distance by a 28 tons truck over the road against the transport over water by boat. It is clear that the environmental profile of the boat is much better than the profile of the truck. In the Netherlands both possibilities can be used, so this is an important consideration for contractors if they want to score in an LCA. LCA AND HALF WARM FOAMED BITUMEN MIXTURES (LEAB) To show how the LCA system works an example will be given of a product that has gone through the process of LCA. In this case a special foamed bitumen product is used for the comparison, called LEAB (lower energy asphalt concrete). In fact this is a foamed bitumen mixture completely produced in a hot mix asphalt plant at approximately 100 °C (van de Ven et al. 2007). A relative comparison has been made with a hot asphalt mixture. Naturally, assumptions had to be made, because half-warm foamed bitumen mixtures are only in production for a
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few years now in the Netherlands. For an asphalt mixture the LCA has to cover the aspects as given in Table 1. Also the differences used in the LCA calculation between hot mix asphalt and LEAB are given in Table 1. Table 1: Differences in the LCA between hot mix asphalt and LEAB System Materials
Energy Emissions Logistics Durability
Aspects Fully recyclable Closed loop Fossil fuel based binder Mineral aggregates Mineral filler Engineered chemicals Hot asphalt mixture Different fuel types Different emission patterns Soot, VOC, PAH during application Different distances Maintenance and replacements
Difference LEAB Equivalent Equivalent Equivalent Equivalent Equivalent Not necessary Lower temperature Natural gas/bio-oil Less and different Less Equivalent Equivalent
As can be seen from Table 1 the large differences can be found in the energy and emissions (in italic). In this paper an environmentally oriented life cycle inventory (LCI) has been reported for the functional unit: 1 ton of LEAB used for application as a binder or base layer in asphalt conform to the Dutch Standards (CROW, 2010). The product described in the functional unit has been produced in a gas fueled asphalt plant. The mixture that is leaving the asphalt plant has a lower temperature, which does not give logistic or workability problems according to the contractor. The mixture has a higher stiffness and the workability by hand is a bit more heavy than for hot mix. Advantages are that compaction can be done faster (see later in this paper) and that emission (carbon-hydrogens, dust, etc) is lower during production and construction. The functional performance has to be at least identical to the standard mixtures for these applications in binder and base layers. This is discussed in the mechanical properties part of the paper, where the functional properties of this half warm foamed bitumen mixture are given. In Table 2 the composition of 1 ton of LEAB is given together with the reference mixture as given in Dubocalc. Table 2: Composition of one ton of LEAB mixture and the reference hot mixture LEAB mixture Reference STAB mixture Component Ton/ton component Ton/ton Bestone 0.303 Bestone 0.254 Natural sand 0.159 Crusher sand 0.190 Dust 0.009 Filler (weak) 0.008 Filler (weak) 0.032 Bitumen (70/100 Pen) 0.017 Bitumen (70/100) 0.024 Reclaimed asphalt (RA) 0.504 Reclaimed asphalt (RA) 0.500
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The life cycle used in this LCA is restricted to certain system boundaries. The system boundaries determine which phases and processes of a life cycle are taken into account in the LCA. In this case production of basic components, production of the LEAB mixture, transport to the construction place, re-use at the end of the life cycle (recycling) are taken into account. Because of the large amount of information it is not possible to describe the collection of all input-and output data, quality of the data, validation of economic streams, validation of environmental calculations, calculation procedures in this paper. All this information can be found in (Meijer J., 2009, TAUW, 2008). The environmental profile and the environmental measures are given in Table 3. Table 3: Environmental profile and measures for production, delivery, removal of 1 ton LEAB Environmental effects Unit Production Delivery Removal Abiotic depletion kg Sb 6.4E-01 3.0E-02 1.4E-02 Global warming kg CO2 3.7E+01 5.3E+00 2.2E+00 Ozone layer depletion kg CFK11 9.3E-06 6.6E-07 3.0E-07 Human toxicity kg 1.4DB 8.5E+00 1.5E+00 &.3E-01 Aquatic ecotoxicity 9.4E-01 2.8E-01 6.5E-01 kg 1.4 DB Terrestrial ecotoxicity kg 1.4DB 4.4E-02 5.9E-03 1.9E-03 Photochemical oxidation kg ethyl 7.3E-03 1.2E-03 3.0E-04 Acidification kg SO2 2.0E-01 3.5E-02 3.0E-03 Eutrophication kg PO432.5E-02 7.3E-03 1.2E-02 Environmental measure Unit Production Delivery Removal Basic components jr-1 3.8E-11 1.9E-12 8.2E-13 Energy MJ 5.4E+02 6.4E+01 3.0E+01 Emissions Jr1 8.0E-10 1.6E-10 9.3E-11 Waste(hazardous) kg 4.7E-01 4.1E-03 1.7E-03 Waste (non hazardous) kg 5.7E-02 2.5E-02 1.0E-02
The environmental performance of LEAB is predominantly determined by: production, transport to the construction site and recycling. The numbers of Table 3 are presented graphically in Figure 2. In Figure 3 the dominant points in the environmental profile during production are given. Looking at the environmental profile for the production in Figure 3, it can be seen clearly that the important materials and processes are: asphalt plant (dark blue), bitumen (fuchsia), foreign aggregates (green), transport of the RA to the plant (turquoise).
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1
2
3
4
5
6
7
8
9
10
[%]
Figure 2: Dominant points in the environmental profile for production, delivery, removal of LEAB
1
2
3
1. Abiotic depletion 2. Global warming 3. Ozone layer depletion 4. Human toxicity
4
5
6
7
5. Ecotoxicity, fresh water 6. Ecotoxicity, terrestric 7. Photochemical oxidation 8. Acidification
8
9
10
11
12
9. Eutrophication 10. Energy 11. Waste, hazardous 12.Waste, non-hazardous
Figure 3: Dominating aspects in the environmental profile for the production of LEAB, gas fueled To get a better feeling of the relative importance of the several effect scores, a so-called normalization step can be introduced (Meijer J., 2009). Each score is compared to the total known effect of this category in a certain region and a certain year. In this study, Europe in the year 2000 has been chosen as the reference (Eurobitume, 1999). The result
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is a fraction, and the contribution of the variant to the total environmental loading for the functional unit under consideration. Of course the economy is many times larger than the production of 1 ton LEAB, so the absolute number is small. The results of the normalization are given in Figure 4. No information is available on the factors for the scores of energy and waste, so these are not given in Figure 4.
Figure 4: Normalized profile for the production, delivery and removal of LEAB Figure 4 shows that the contribution of the abiotic depletion is the largest, followed by acidification, eutrophication and global warming. This can be explained by the fact that gas is used in the asphalt plant, diesel for transport and bitumen as binder from oil. Table 4: ECI indicator for the production, fueled asphalt plant) in Euro’s/ton Environmental effects Unit Total Euro/ton Global warming Euro/ton Ozone layer depletion Euro/ton Humane toxicity Euro/ton Aquatic ecotoxicity Euro/ton Terrestic ecotoxicity Euro/ton Photochemical oxidation Euro/ton Acidification Euro/ton Eutrophication Euro/ton
delivery and removal of LEAB (gas score 4.6E+00 2.2E+00 3.1E-04 9.6E-01 5.6E-02 3.1E-03 1.8E-02 9.5E-01 4.0E-01
Percentage 48% 0% 21% 1% 0% 0% 21% 9%
RWS has developed the so-called Environmental Cost Indicator (ECI) for the ten used environmental impact categories as they were required in Dubocalc at the moment of writing this paper. All these environmental impact categories are rated in one score. For the ECI the so-called shadow prize method is used (Meijer, J., 2009). The shadow prize is 10
the highest acceptable cost level per unit emission prevention. These are general environmental costs, no project specific costs. The scores are rated in Euros as can be seen in Table 4. Global warming, human toxicity and acidification are rated highest in the ECI system as can be seen in the table. The use of a certain percentage of RA and a comparison with conventional STAB as defined in Dubocalc (the reference) are discussed below. Percentage of recycled material In case more RA is used in the mixture the energy use of the asphalt plant increases slightly. In Table 5 the result is given for three RA percentages. Table 5: Influence of amount of RA on energy use in the asphalt plant Amount of RA in LEAB Use of energy 0% RA 205 MJ/ton 25% RA 210 MJ/ton 50% RA 215 MJ/ton The influence of the change in the mixture and the energy efficiency in the asphalt plant on the performance of the environmental profile of LEAB is given in Figure 5.
Figure 5: Environmental profile for the production, delivery and removal of LEAB at 0% RA (red), 25% RA (green) and 50% RA (yellow). LEAB without RA is rated at 100% From Figure 5 it becomes clear that the environmental performance of mixtures that contain RA improves significantly, despite the increase of fuel use in the asphalt plant
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with increasing RA. The only environmental effect that significantly worsens is the ozone layer depletion because of the increase in transport movements by trucks going from the project site to the asphalt plant. In Dubocalc this parameter does not make a contribution in the final result. Comparison with conventional base course ( STAB) A comparison is made with a standard base course mixture STAB with 50% RA as is normally produced in the Netherlands. The composition of the standard mixture is given in Table 2. In both mixtures foreign aggregates (Bestone) are used.
Figure 6: Relative comparison of the environmental profile for the production, delivery and removal of LEAB (gas fueled asphalt plant) and STAB (green), both with 50%RA Figure 6 shows that LEAB scores much better than the hot mix reference STAB, with both mixtures containing 50% RA. Also the ECI indicator confirms this result. The indicator for LEAB is significantly better. As can be seen in Table 6 the ECI indicator for LEAB is in total 1.5 Euro/ton lower than STAB, which is about 25%. Considering a market price for 1 ton of these asphalt mixtures with 50% RA of 30 Euro/ton, the ECI of 4.6-6.1 Euro/ton and the sustainability benefit of 1.5 Euro/ton for LEAB is significant. So from the LCA analysis it becomes clear that the half warm foamed bitumen production with LEAB considerably contributes to a better environmental profile compared to a hot mix asphalt mixture for the same application, in this case binder and base layers. Of course each application should be considered separately, but from the results it can be concluded that the production at low temperature, in this case lower than 110°C, together with the possibility to add up to 50% RA will always score better than the alternative hot mixture.
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In a complete LCA the service life is also of extreme importance for the estimation of the total environmental impact. Some information on the performance will be given in the next section. Table 6: ECI indicator (euro/ton) for the production, delivery and removal of LEAB and STAB , both with 50% RA Environmental effects Unit LEAB STAB Total Euro/ton 4.6+00 6.1+00 global warming Euro/ton 2.2E+00 3.1E+00 ozone layer depletion Euro/ton 3.1E-04 5.6E-04 humane toxicity Euro/ton 9.6E-01 1.2E+00 aquatic ecotoxicity Euro/ton 5.6E-02 7.4E-02 terrestic ecotoxicity Euro/ton 3.1E-03 9.9E-03 photochemical oxidation Euro/ton 1.8E-02 2.4E-02 acidification Euro/ton 9.5E-01 1.2E+00 rutrophication Euro/ton 4.0E-01 4.6E-01
PERFORMANCE PROPERTIES: CE marking and field performance As has been made clear before, it is very important that a lower temperature mixture has a better environmental profile, but this is not enough. The service life of the low temperature asphalt mixture needs to be sufficient to compete with hot asphalt mixture. On the one hand road managers lack budget to finance sustainability at a higher price than conventional products, the main questions at the introduction of a new product concern the price and the performance or durability. On the other hand it has already been explained in the previous paragraphs that besides the environmental profile of products also information is needed about energy consumption, transport distances, project amounts of materials and durability and service life to complete a life cycle analysis. In the case of LEAB this leads to a strong focus on the mechanical properties in the development of the warm mix. The success in the development of LEAB started with a product which was produced in an asphalt plant and compacted in the road. Samples were taken from the road and the mechanical properties of the asphalt were determined in the laboratory. In the Netherlands the performance of base course mixes has to be examined by four functional properties according to the European Standards for asphalt concrete. These mechanical properties are sensitivity to water, stiffness modulus, resistance to fatigue and resistance to rutting: 1. The water sensitivity is determined in accordance with EN 12697-12 and 12697-23. In this test the indirect tensile strength (determined at 15°C using a deformation speed of 50 mm/min) of a set of three retained specimens (70 hours in a water bath of 40°C) is compared to a set of three not retained specimens. The ratio in percentage between both mean tensile strength values is a measure for the water sensitivity of the mix; 2. The stiffness modulus of the mix is determined with the four point bending test (4PB). This test is described in Annex B of EN 12697-26. To determine the stiffness modulus
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of a mix, 18 prismatic specimens (450*50*50 mm) have to be tested at 20°C and various frequencies (between 0.1 and 30 Hz). The stiffness at 8 Hz is used as the reference stiffness value. The test results are used to compare different materials or as input in pavement design models; 3. Directly after determination of the stiffness, the fatigue properties of the specimen are determined according to Annex D of EN 12697-24. This displacement controlled test is carried out at 20°C and 30 Hz. To determine the fatigue properties of a mix, 18 specimens are tested. Three deformation levels are chosen in the tests, aiming at a life span of the specimen of 104, 105 and 2·106 load repetitions. Based on the 18 test results, the fatigue line is calculated using the following equation: Nf = k1 εk2 Finally the characteristic strain value ε6 is calculated where the life span of the mix is 106 load repetitions; 4. The resistance to permanent deformation is determined by means of the triaxial test according to Annex B of EN 12697-25. In this test four cylindrical specimens are tested at 40°C, using a constant confining pressure of 0.05 MPa and a vertical dynamic stress varying between 0.05 MPa and 0.45 MPa. The vertical load pulse is a haversine with a loading time of 0.4 s, followed by a rest period of 0.6 s. The total test period is 10,000 load repetitions. Finally, the slope of the secondary phase of the relation between the permanent deformation and the number of load repetitions determines the permanent deformation parameter fcmax. Table 7 shows the results of the mechanical properties of the first official LEAB in 2005, produced in an asphalt plant and compacted in the road. Table 7:Various functional properties of LEAB compared to the reference properties of the hot mix StAB Year 2005 2007 2008 2008 2009 2010 2011 Mixing in Plant Plant Lab Plant Plant Plant Lab Compaction in Road Lab Lab Lab Lab Road Lab RAP [%m/m] 50 50 50 50 50 50 50 ITS [MPa] 2076 2696 2230 2.042 ITSR [%] 93 96 75 87 83 83 Smix [MPa] 7700 7679 8000 7029 7954 9200 9685 ε6 [µm/m] 94 97 83 103 110 101 92 fcmax [µm/m/N] 0.43 0.43 0.53 0.54 0.38 0.10 After the first success in 2005 the research carried on with the ultimate goal to really understand the production and compaction of the warm asphalt mix before starting the development of other types of asphalt, such as porous asphalt, stone mastic asphalt and asphalt concrete for surface layers. One of the most important aspects in the research was to find a way to produce and compact the asphalt in the laboratory and find mechanical properties on these samples which equal the properties of the product which is produced in an asphalt plant and compacted in the road. The results of this research are also shown 14
(1)
in Table 7 and based on these results and the Dutch requirements for base layers (CROW, RAW 2010) as shown in Table 8, it can be concluded that both in the laboratory and practice a steady and reliable product can be delivered for all types of roads. Moreover there is proof of the performance and durability of LEAB being equal to the hot mix in the initial stage. Table 8: Dutch requirements for base course asphalt concrete (CROW, RAW 2010) Requirements primary road
Requirements low traffic road
StAB hot mix (OL-C)
StAB hot mix (OL-A)
ITSR [%]
≥ 70
≥ 70
Smix [MPa]
≥ 7000
≥ 4500
ε6 [µm/m]
≥ 90
≥ 100
fcmax [µm/m/N]
≤ 0.4
≤ 1.4
Figure 7 and Figure 8 show the properties of the official LEAB for base courses, which is delivered with CE-marking in the Netherlands. These results of type testing are also direct input for pavement design calculations. Frequency [Hz] 0,1 0,2 0,5 1 2 5 8 10 20 30
Stiffness [MPa] 1579 2146 3156 4111 5246 6982 7954 8410 9956 10943
Figure 7: Results of the stiffness tests on LEAB for CE marking
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Parameter k1 k2 r2 ε6 [μm/m]
Value 16,22 -5,01 0,93 110
log( N ) = k1 + k 2 ⋅ log(ε )
Figure 8: Results of the fatigue tests on LEAB for CE-marking During the development of low temperature porous asphalt it was found [Salil, 2010] that there is a difference in aging behavior of asphalt if the virgin bitumen is just heated (hot mix) or heated and foamed (LEAB) during the asphalt production. At high frequencies or low temperatures the stiffness of LEAB is higher and the phase angle is lower than for the hot mix as shown in Figure 9. This raises the question if this difference in aging affects the long term mechanical behavior of LEAB in a negative way.
Figure 9: Master curves of bitumen taken from LEAB and two hot mixes
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Stiffness [MPa]
In 2009 an asphalt slab was taken from a pavement with LEAB of a demonstration project on Highway A2 in the Netherlands. This slab was located near a construction joint and afterwards it showed that this had some effect on the compaction rate of one of the two LEAB layers in the pavement. The density of the samples from the one layer met the requirements (‘good samples’) and the density of the samples from the other layer did not meet the requirements (‘bad samples’). The density of a poor sample is more than 30 kg/m2 (1.3-1.5%) below target density. Ten good and ten bad samples were prepared for four point bending tests to determine the development of stiffness and resistance to fatigue of LEAB in time. The samples are stored in a climate chamber at a constant temperature of 15°C, which in fact is about the mean annual temperature for a base course mix in the Netherlands. Tests are carried out 4 weeks, 4 months, one year and two years after compaction on a set of two good and two bad samples. Figure 10 and Figure 11 show the results of these tests.
LEAB, Development stiffness modulus in time
12000 11500 11000 10500 10000 9500 9000 8500 8000 7500 7000 6500 6000 5500 5000 0
20
40
60
80
Age [weeks]
100
mean
120
smix
Figure 10: Development of stiffness in time for LEAB Figure 10 shows an increase in time of the stiffness modulus. In this figure all the results below the mean value correspond with the bad samples. Based on these results it is clear that the stiffness modulus of LEAB increases in time mainly due to hardening of the binder, regardless of the quality of compaction. Apparently, however, the stiffness modulus of the good samples is about 20-30% higher than the modulus of the bad samples. Figure 11 shows almost identical results for the resistance of fatigue, but the fatigue behavior of the bad samples obviously does not improve in time. Both figures proof the importance of the quality of compaction of LEAB, since insufficient compaction can lead to a tremendous reduction in service life of the pavement based on the measured difference in the resistance to fatigue of the good and
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the bad samples. It should be noted that this is in general also the case for almost all other types of asphalt.
LEAB, development resistance to fatigue in time
ε6 -value [µm/m]
120
110
100
90
80 0
20
40
60
80
Age [weeks]
100
mean
120
measured
Figure 11: Development of resistance to fatigue in time for LEAB
COMPACTABILITY OF BITUMEN MIXTURES
HALF
WARM
FOAMED
Because of the fact that the quality of compaction is very important, it is interesting to compare the results of compactability tests in the laboratory and compaction tests in field trials between the hot mixture base course and the LEAB base course. The workability and compaction of LEAB has never been a problem (Jacobs, 2010]. The product can be applied using standard pavers and rollers. One of the differences between the hot and the warm mix is that during compaction, the rollers can not use vibrating compaction: all compaction must be carried out with static rollers. Despite this difference a compaction degree of 100% can be achieved for the warm mix with less roller passes than for the hot mix. The asphalt roller operators even state that the warm mix is much easier and quicker to compact than the hot mix. In Figure 12 the compaction process of LEAB and a hot mix is given schematically.
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2450
140
2400
Nuclear density [kg/m3]
100
2300 2250
80
2200 60
2150 compaction LEAB compaction STAB Target density Cooling curve STAB Cooling curve LEAB
2100 2050 2000 1950 0,0
20,0
40,0
60,0
40
Temperature [ºC]
120
2350
20 0 100,0
80,0
Time after laying the asphalt [min] Figure 12: Compaction process of LEAB and equivalent hot mix
Resistance [A]
At this moment research is underway to find out if it is possible to determine the compactability characteristics of an asphalt mix in the laboratory. Preliminary results of this research are shown in Figure 13 (Beunckens, 2011).
Temperature [°C]
Figure 13: Compactability of LEAB and hot mix Based on the results in Figure 13, which of course still have to be validated, the conclusion can be drawn that a suitable compaction window for LEAB could be 110°C19
70°C and for the hot mix 150°C-80°C. Plotting these compaction windows in an example of measured cooling curves gives some very important information for the compaction process of LEAB in practice. Cooling curves 60 mm STAB and 60 mm LEAB 160
33 min
140
120
150º - 80º 0; 110
Asphalt temperature
100
80 110º - 70º
STAB LEAB
60
35; 59,8
21 min
40
20
0
0
10
20
30
40
50
60
70
Minutes after laying the asphalt
Figure 14: Cooling curves and compaction windows for LEAB and hot mix For instance, for the specific weather conditions of the cooling curves in Figure 14 the compaction window for LEAB leads to the conclusion that the compaction process should be completed within 20-25 minutes. This information provides the road paving practice with a powerful tool to manage the product quality of every specific type of asphalt. Compaction studies on warm mixes in South Africa for continuously graded surfacing mixes (Mbaraga, 2010) produced at 125ºC and compacted at 110ºC, show that a range of different technologies provide compactability of mixes at is comparable or better than the equivalent HMA, see Table 9. In particular, the foamed bitumen technology yielded similar results to the reference mix of HMA. The Stellenbosch University Slab Compaction method in this study used a consistent energy of compaction from a pedestrian roller for all comparative mixes.
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Table 9: Slab Compaction Results for WMA Technologies including 10% RAP at 60/70 Pen binder (Mbaraga, 2010)
CONCLUSIONS A sustainable approach requires new inputs from the asphalt industry. In this paper the effect of half warm foamed bitumen mixtures is shown to be relevant. An LCA will become an important tool in the coming years to show that sustainability is a key aspect in the Dutch industry. Lower temperature asphalt production makes an important contribution to sustainable industry practices. In this paper an example is given for binder and base course asphalt mixtures for a foamed bitumen mixture produced around 100°C. A relative LCA done with the Dubocalc program shows that the environmental profile of lower temperature asphalt is considerably better than for hot mix asphalt. For the Netherlands the low temperature asphalt mixtures need to include high percentages of reclaimed asphalt to be competitive with hot mixtures with 50% reclaimed asphalt. Lower temperature asphalt mixtures need to satisfy the same performance properties as hot mixtures. In this paper it is proven that the functional requirements for CE marking are as good as for hot mix asphalt. Compaction of foamed bitumen mixtures produced in an asphalt plant is very good possible in the time window available. The lower starting temperature is not a problem.
REFERENCES Schweitzer G., Duijsens J., 2010. Duurzaam inkopen met behulp van DuboCalc. Infradagen 2010, CROW, Ede. Huang Y., Bird R., Bell M., Allen B., 2009. Life Cycle Assessment of Asphalt Pavements. Advanced Testing and characterization of Bituminous Materials, Rhodes, Greece.
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Jenkins, K.J., 2000. Mix design considerations for cold and half-warm bituminous mixes with emphasis on foamed bitumen, PhD dissertation, University of Stellenbosch. Doh J.S., Kim J.C., Ryu M.Y., Kim K.N., 2010. Evaluation of selected warm-mix additives for asphalt recycle. Paper 10-1063, TRB, Washington USA. Olard F., Beduneau E., Seignez N., Dupriet S., Bonneau D., 2009. Laboratory performance based assessment of half-warm mix asphalt with high recycling rate by means of the factorial experiment design approach. Advanced Testing and characterization of Bituminous Materials, Rhodes, Greece. Van de Ven M.F.C., Jenkins K.J., Voskuilen J.L.M., van den Beemt R., 2007. Development of (half-) warm foamed bitumen mixes: state of the art. Special Issue: foamed asphalt for Pavements, International Journal of Pavement Engineering, ISSN 1029-8436. Meijer J., 2009. Life cycle analysis (LCA) of LEAB produced in a gas fueled asphalt plant. Intron report (in Dutch). Eurobitume, 1999. “Partial life cycle inventory of ‘eco-profile’ for paving grade bitumen”, eurobitume report 99/07.. TAUW, 2008. Asphalt Plant BAM Amsterdam. Results of emission measurements. TAUW report. R001-4553711DBS-nja-V04-NL (in Dutch). NEN 8006/A1, Environmental information of building materials, building products and building elements for in environmental declarations. –Determination method according to the life cycle analysis method (LCA). NEN 8006: 2004/A1: 2008 Addendum. (in Dutch) Mohan S., 2010. Winter Damage of Porous Asphalt – Case study using a mesomechanics based Tool for Lifetime Optimization of Porous Asphalt. Master’s Thesis, Delft University of Technology, the Netherlands Jacobs M.M.J., Van den Beemt C.M.A., Sluer, B.W., 2010. Successful Dutch Experiences with Low Energy Asphalt Concrete, ISAP paper, Ngoya, Japan.
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Beunckens B., 2010. Lage temperatuur asphalt, De weg naar de toekomst? Master’s Thesis, XIOS Hogeschool Limburg, Belgium (in Dutch). CROW, 2010, Standaard RAW Bepalingen 2010 (Dutch Standard for Asphalt Pavements), CROW, Ede, the Netherlands (in Dutch). Mbaraga A.N. 2011. Performance of warm mix asphalt: Flexural modulus and fatigue resistance. MEng thesis. University of Stellenbosch
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