LEAB: sustainable low-temperature production of asphalt Maarten Jacobs and Rémy van den Beemt, BAM Wegen bv

Transcription

1 LEAB: sustainable low-temperature production of asphalt Maarten Jacobs and Rémy van den Beemt, BAM Wegen bv Introduction Climate and sustainability issues are increasingly being given priority by government bodies. Although asphalt is perhaps not the first product that comes to mind in connection with sustainability, low-temperature asphalt and sustainability can actually go very well together. With its LEAB (Dutch acronym for: Low Energy Asphalt Concrete), BAM Wegen has shown that a decision to opt for low-temperature asphalt does not necessarily have any effect at all on the durability of the resulting product. As a matter of fact, the stiffness characteristics as well as the fatigue resistance and susceptibility to rut formation of LEAB are equivalent to that of hot-mix asphalt. In recent years, experience has been gained with the production and processing of LEAB, which has been very positive. As a result, the authorities responsible for the national road network can now also demonstrate their concern for the environment within the framework of new road construction. This article deals with the specific characteristics of LEAB and describes the concept behind this product. The environmental benefits of using LEAB instead of asphalt mixtures produced in a more traditional fashion are also discussed. Finally, the mechanical characteristics of the product are reviewed. Introduction to LEAB A normal 70/100 penetration bitumen is used for the production of LEAB. The main difference, however, is that the temperature remains below 100 C during the production and processing of LEAB. In order to nevertheless realise a sufficiently low viscosity of the binding agent (which in turn makes it possible to effectively coat the mineral aggregate), the binding agent is added in the form of foamed bitumen. The principle of using foamed bitumen is not new (Csanyi, 1957) and has been used for some time in road construction in order to produce road bases and pavements. However, due to the expiry of several patents, the use of foamed bitumen technology for building asphalt roads has gained increasing popularity in the last two decades. The composition of LEAB is identical to normal asphalt concrete, produced at ±165 C (Dutch acronym: StAB). However, an additive is used which ensures that the quality of the foamed bitumen is maintained over a longer period of time. The additive results in a high expansion value and, in particular, a long half-life for the foamed bitumen. These characteristics are very important to realise an effective coating of the mixture and ensure that the mixture being produced can be worked with for a longer period of time. For years, it was assumed that it was necessary to introduce extra liquid into the mixture to ensure that the foam bitumen was effectively distributed throughout the mixture. However, based on recent experience, one may conclude that this is not necessary, and even not desirable, in relation to the present production process for LEAB.

2 LEAB is also different in so far as it enables the use of recycled materials. In the Netherlands, a recycling percentage of 50% is normal. At a 50% recycling percentage, the functional characteristics of LEAB remain the same as for hot-mix asphalt. The production of LEAB Since 1998, BAM Wegen has been making investments in order to master the issues associated with foamed bitumen and to modify it for practical use. A research project financed by the government was carried out which took almost 4 years and was realised in cooperation with the Technical University of Delft and the University of Stellenbosch in South Africa. The research project was a success, but initial attempts to upscale the results obtained in the laboratory to a practical scale proved to be very difficult. However, after several unsuccessful experiments, a procedure was finally developed which turned out to be very successful! As a result, almost all the theories previously developed became outdated and mixing procedures had to be rewritten. The point of departure for the research project was the production of high-quality asphalt in a conventional asphalt production facility according to the foamed bitumen concept, without having to make large-scale investments in or make changes to the production facility. In the end, this resulted in the construction of a foamed bitumen generator in the Zuid Nederlandse Asfalt Centrale (a Dutch asphalt production facility in the south of the Netherlands) in Breda, which could replace the present hot-mix bitumen dosage system. However, it is not the foam bitumen generator and dosage unit that are critical to the success of the project. The procedure followed is the secret behind its success and is therefore protected via a patent. BAM Wegen has now already fitted two asphalt production facilities with a foamed bitumen unit, making it possible to produce LEAB on a large scale. The two facilities include a batchprocess based facility and a continuous production facility, which justifies the conclusion that the production process developed by BAM Wegen can be applied on quite a large-scale. As mentioned previously, this opens up opportunities for developing more environmentally friendly asphalt mixtures incorporating the use of foamed bitumen, which in turn can lead to a more sustainable infrastructure. Working with LEAB Working with and compacting LEAB has actually never been a problem. The product can be applied using a standard spreader and roller, making it possible to realise a compaction level of 100% in the road without problems. According to asphalt roller operators, the asphalt rolling process is actually significantly easier and quicker when using LEAB than for a comparable hot Asphalt Concrete mix. LEAB can be compacted at temperatures between 55 C and 90 C. The mechanism behind the compaction of LEAB at these relatively low temperatures is not yet clear (Chip Seal Asphalt Concrete can be compacted at temperatures up to approx. 80 C, for example).

3 Density (%) 101 0/22 HMA STAB Density 89 Temperature ton roller 6 ton roller Roller passes Compaction of LEAB 101 0/22 HW STAB Density (%) Density 89 Temperature 87 6 ton roller 12 ton roller Roller passes Compaction of normal Asphalt Concrete Temperature (C) Temperature (C) Until now, the maximum period of time between the production of LEAB and its final application has been six hours. However, it is expected that it will be possible to work with and apply LEAB effectively without any problems even after a period of six hours has gone by. The functional characteristics of LEAB In developing LEAB, the aim has always been to produce a grade of asphalt concrete with functional characteristics which are equivalent to those of hot-mix asphalt concrete. The best way of demonstrating such equivalence is to do so in a fundamental fashion via the applicable European standards (section 5.4 of EN ). It must be emphasised here that equivalence on the basis of empirical characteristics (e.g. density, porosity, and Marshall characteristics) is a much less rigorous comparison and further removed from the practical application of asphalt for road-building purposes than equivalence on the basis of functional characteristics. This is also why some alternative low-temperature procedures (e.g. the LEA-CO approach in France, the Nynas LT-Asphalt and Shell s WAM procedure) have until now not been as successfully applied in the Netherlands. For a functional comparison, four mechanical properties are analysed: sensitivity to water, stiffness modulus, fatigue resistance and resistance to rut formation: 1. The sensitivity to water (a general requirement) is determined in accordance with EN and , whereby the sensitivity to water is determined by comparing the indirect tensile strength of artificially aged test samples with test samples that have not

4 been aged. During the aging procedure, a set of three test samples are placed in a water bath at 40 C (with a margin of error of 1 C) for a period of 70 (with a maximum margin of 2 hours). A second set of test samples of practically the same density is stored at room temperature during the same period. After the aging period, the indirect tensile strength is determined for both sets of test samples. The ratio of the average splitting strength of both sets is a measure of the sensitivity to water of the asphalt mixture. 2. In the Netherlands, the stiffness is determined with the four-point bending apparatus in accordance with Annex B of EN In the test, a prism-shaped test sample (450 x 50 x 50 mm) is clamped into the apparatus at four points, after which the two supporting points in the middle are moved vertically up and down. By measuring the force and the distortion during the test, it is possible to calculate the stiffness of the test sample. Four-point bending test The test is carried out on 18 bars of asphalt at a single temperature (20 C) and 10 different frequencies (between 0.1 and 30 Hz). The test results are used to compare different materials or as input for design calculations for paved surfaces. 3. Immediately after the stiffness of a test sample is determined, the fatigue resistance is determined in accordance with Annex D of EN During the fatigue test (at 20 C, 30 Hz, strain controlled), the test sample is forced to move repeatedly in a sinusoidal fashion. This leads to fatigue in the test material, causing the stiffness to decrease and in the end, it breaks. The fatigue-related lifespan is defined as the number of repeated deformations required to reduce the stiffness to 50% of the original value. The fatigue tests are carried out at various strain levels: the larger the distance over which the test object is forced to move, the shorter the lifespan will be. The fatigue analysis is carried out at three strain levels, which are chosen so that the lifespans determined for the test pieces range between 10 4 and deformations. For each strain level, the test is repeated 6 times. Based on the 18 results thus obtained, the fatigue line (N f = k 1 ε k2 ) is calculated, after which the deformation level can be calculated, ε 6, at which the lifespan is 10 6 load repetitions. In the Netherlands, the k 1 - and k 2 - values are used to calculate the required thickness of asphalt pavements, whereby the prevention of crack formation from the bottom is used as the design criterion. 4. In the Netherlands, the resistance to permanent deformation is determined in accordance with test method B in EN (tri-axial test). In this test, which is repeated four times, a cylinder-shaped test sample (diameter = 100 mm; height = 60 mm) is subjected to a load over two axes in a so-called tri-axial cell at 40 C or 50 C. In this cell, the load

5 exerted on the test sample is comparable to what happens in an asphalt road. A supporting load is provided in a horizontal direction, simulating a situation in which the asphalt test sample is part of a large section and is supported by the surrounding material. Vertically, a load is then applied which is comparable to the load applied when a lorry tyre passes over. Triaxial cell The test is carried out by repeatedly applying the load, thereby simulating the ongoing load applied to the asphalt by moving traffic. By measuring the deformation of the test sample during the test, it is possible to determine the resistance to permanent deformation of the asphalt mixture being tested. This resistance is expressed as the slope f cmax of the line which defines the relationship between the measured vertical deformation of the test sample and the number of repeated loads applied. The test results are used to compare materials with each other or as input for pavement design models used for roads. The comparison of LEAB to Chip Seal Asphalt Concrete, whereby the composition of both materials is identical but the bitumen in the former is added as foamed bitumen at 95 C and in the latter as a liquid binding agent at 165 C, was carried out several times. This was done using asphalt mixed and compacted in the laboratory as well as products mixed in an asphalt production unit and compacted on-site. The table below summarises the results of one of these studies. Characteristic Test Parameter Sensitivity to water EN ITS ITSR Stiffness EN , Annex B Smix Fatigue resistance Resistance to deformation EN , Annex D EN , method B LEAB 2.50 MPa 80-95% MPa ε μm/m f cmax μm/m/n Normal Asphalt Concrete 2.50 MPa 75-80% MPa μm/m μm/m/n

6 Based on these results, one may conclude that LEAB is identical to hot-mix Asphalt Concrete in terms of functional characteristics. The environmental characteristics of LEAB In the Netherlands, the government has implemented a project called Sustainable Purchasing. The aim of this project is to encourage government authorities to take environmental aspects into account when purchasing products. In this way, government authorities (national and provincial governments, municipalities and water boards) aim to ensure that 100% of products are purchased in accordance with sustainability guidelines by In cooperation with SenterNovem, which works for the Dutch Ministry of Economic Affairs in the area of sustainability and innovation, the independent research bureau INTRON carried out a study to estimate the environmental scores that can be assigned to LEAB. The conclusion was that LEAB scores significantly higher in the area of sustainability (so-called environmental profile ) in comparison to regular asphalt concrete. This was due in particular to the lower energy consumption (25% to 40% reduction) and lower CO 2 emissions (25% reduction). By making adjustments to the asphalt mixture and modifying the means of transport and transport distances, the environmental benefits in comparison to hot-mix asphalt can be further optimised. Based on these results, environmental points can be awarded to LEAB and other products. In this way, sustainability can be given an economic value and can be taken into account in establishing criteria for sustainable purchasing practices. Conclusions Based on the experience gained with LEAB, one can conclude that asphalt mixtures can be produced at temperatures below 100 C which have mechanical characteristics equivalent to those of hot-mix asphalt. This comparison is based on functional characteristics and is therefore more rigorous than equivalence on the basis of empirical characteristics. This method of comparing the performance of different asphalt mixtures makes it possible to conclude that the approach used by BAM Wegen in applying the foam bitumen procedure leads to a high-quality product, making it possible to realise substantial reductions in energy consumption and greenhouse gas emissions. Within the framework of a sustainable global future, this is a very interesting development.