Cold recycling in modern road rehabilitation
For economical and ecological reasons, the bound bituminous material that accumulates during the rehabilitation and reconstruction of roads is being reused more and more frequently. This can be carried out either in cold-mixing plants, by adding the milled asphalt material to a new aggregate mix, or directly ‘in situ’, using suitable cold-recycling machinery. The use of foamed bitumen as a binding agent for this procedure has recently generated much lively interest within the industry.
While the principle of foamed bitumen is not new, the utilization of this process for binding cold material in road construction is relatively new and innovative. In Europe, for example, cold recycling is a fairly young discipline for which generally accepted rules do not yet exist. However, this does not apply in South Africa, Norway or the Baltic republics. Owing to positive experiences with the use of foamed bitumen in these countries, the rehabilitation of roads using this environmentally friendly technology is becoming increasingly important worldwide. In Norway and the Netherlands, especially, the ‘mix-in-place’ recycling method with foamed bitumen has become very popular. In Norway, for example, where cold recycling with foamed bitumen has been in use since 1983, approximately 1.8 million m2 of roads were rehabilitated using this process in 1997. Other successful examples are Brazil and Iron where, in total, more than 2 million m2 of pavement structure are treated with foamed bitumen each year.
What is foamed bitumen?
Foamed bitumen is produced by adding small amounts of water to hot bitumen (approximately 2-3% by weight of bitumen). The binder used for this process if ordinary penetration-grade bitumen, type B60 to B200, which is also used for asphalt production in new road construction.
When injected into the hot bitumen the water evaporates abruptly, causing foaming of the bitumen in the saturated steam. The water is the carrier of the atomized bitumen and within only a few seconds the bitumen expands by 20 to 30 times its original volume.
The intensity and effectiveness of the foaming process can be further improved by controlling basic physical conditions such as pressure and temperature. In road construction machines or laboratory plants, this process takes place in an expansion chamber where the water is injected into the hot bitumen, which has a temperature of approximately 180?C, a pressure of around 5 bar. The foamed bitumen thus produced escapes the expansion chamber through a nozzle and can immediately by mixed with the mineral aggregate to be treated.
Foaming of the bitumen results in a number of improvements with regard to its characteristics and its ‘handling’ as a binding agent:
- foam has a considerably larger surface area than the liquid bitumen and can therefore better coat the cold and humid mineral aggregate mix
- the foaming process reduces the bitumen viscosity and thus improves its dispersed properties; this, too, ultimately results in improved coating of the mineral aggregate mix
- unlike normal hot bitumen, foamed bitumen has a temperature of only 50-60?C.
The main parameters determining the quality of the final foamed bitumen product are ‘expansion’ and ‘half-life’. Expansion is defined as the ratio between the maximum achieved volume of the foamed bitumen and the original volume of the non-foamed bitumen. To obtain foamed bitumen of a high quality, expansion should be greater than 15.
Half-life is defined as the time it takes for the foamed bitumen to reduce by 50% from its maximum achieved volume. This is normally measured in seconds. As a rule, the larger the expansion and the longer the half-life, the better the quality of the foamed bitumen. For foamed bitumen of high quality, the half-life should be more than 10sec.
The parameters ‘half-life’ and ‘expansion’ – and thus the quality of the foamed bitumen – can be influenced by a number of factors, such as the temperature of the hot bitumen, the quantity of water added or the pressure applied during the foaming process.
Assessment of quality
The foaming process of various types of bitumen can be extensively tested using a facility such as the Wirtgen WLB10 mobile laboratory unit. In order to achieve the optimum foamed bitumen, a series of tests can be carried out in which bitumen temperature and the quantity of water and air added vary. The foamed bitumen can also be injected directly into a laboratory mixer for the production of test specimens which can then be used for mix-design tests.
Foamed bitumen as a binding agent for cold-treated material
Initially, the aggregate mix is tested, and subsequently the cold-treated material mixed with foamed bitumen is tested to determine the optimum bitumen content.
The fines content in the mineral aggregate (ie passing a 0.075mm sieve) is very important when using foamed bitumen as a binding agent. The foaming process leads to a surface expansion of the bitumen and results, at the same time, in a reduction in its viscosity. The dispersal properties thus improved ensure a better coating of the components in the mineral aggregate mix, which have a relatively small particle size and are thus bound like mortar by the foamed bitumen. If there is a deficiency in the required fines content of approximately 4-5% in the mineral aggregates to be treated, crushed sand or a hydraulic binding agent (lime or cement) can be added as an alternative.
Testing the properties of the cold-treated material
As mentioned above, a sufficient fines content in the material to be stabilized is of great importance. The sieve analysis of a well graded material with a satisfactory distribution from fine- to large-grained aggregates will help in drawing conclusions as to whether fines (crusher dust and/or cement) will have to be added. The determination of the optimum water content and the maximum dry density is also important. The compaction capability of a material mix, depending upon its water content, is usually determined by the Proctor test.
Determining the optimum foamed-bitumen content
Having determined the optimum foam properties, test briquettes can be produced from the cold-treated material. Here, too, the WLB10 unit can be of assistance. The bitumen foam can be injected directly on to the homogenized aggregate that has been placed into a laboratory mixer. In order to establish the optimum bitumen content, varying bitumen quantities with optimum water content are added to the aggregate mix.
The properties of cold-treated material
Cold-treated material produced with foamed bitumen (without the addition of cement) can be stored for very long periods of time. The storage properties depend upon the water content. If the water content in the cold-treated material is kept close to the optimum value by suitable measures, such as protection from the sun and wind, it can be stored almost indefinitely. Minor humidity losses can be compensated for by adding a respective quantity of water and by homogenizing the cold-treated material once again, for example in a mixing plant.
Material treated with foamed bitumen can, for a prolonged period of time, also be placed under unfavourable weather conditions, such as heavy rain. The bitumen is not washed out from the aggregate.
A particular advantage on road-rehabilitation job sites is the fact that the cold-treated material can be trafficked immediately after compaction. Traffic hindrances caused by the job site execution can therefore be kept to an absolute minimum.
The fact that foamed bitumen can be produced from ordinary penetration-grade bitumen simply by adding small quantities of water results in economic benefits due to reduced costs for binding agent and transport facilities.
Characteristic values for layers stabilized with foamed bitumen
The most important characteristic values for cold-treated material stabilized with foamed bitumen are listed below. In addition to foamed bitumen, 1-2% cement can be added, ensuring additional early high strength. Taking into consideration the quantity of cement added, the quantities of bitumen to be added are usually within the following ranges:
- Milled asphalt material (RAP/crushed stone 50/50 blend) – 1.5-3.0%
- Crushed stone – 2.5-4.0%
- Natural gravel – 3.0-4.5%
Indirect tensile strength
This test is carried out on Marshall-manufactured briquettes with a temperature of 25?C.
Typical values are:
- Milled asphalt material (RAP/crushed stone 50/50 blend) – 350-800kPa
- Crushed stone – 400-900kPa
- Natural gravel – 250-500kPa
Typical values of the resilient modulus of material stabilized with foamed bitumen and cement are usually:
- Milled asphalt material (RAP/crushed stone 50/50 blend) – 2,500-5,000MPa
- Crushed stone – 3,000-6,000MPa
- Natural gravel – 2,000-4,000MPa
Machine and process technology for placing cold-treated material produced with foamed bitumen
The processing of road construction materials (usually in form of a defective pavement on a rehabilitation job site) can be carried out in situ with suitable recycling machinery, such as the Wirtgen WR2500 recycler or the new Wirtgen 2200CR cold recycler. Foamed-bitumen technology allows the treatment and processing of cold or damp construction materials, produced during the milling process, with ordinary penetration-grade bitumen to form a bituminous, and thus flexible, base layer.
Should the required minimum fines content of 5% not be achieved during the milling process, crusher dust or a hydraulic binding agent can be introduced by spreading directly on to the pavement. Alternatively, an integrated spreader device can be used on the recycler. Contrary to this, if indicated by the grading curve, coarse aggregate can be added.
Similar procedures can be carried out if the layer of existing pavement to be rehabilitated is not thick enough. The spreading of fines and/or additional coarse material should be carried out just prior to milling the existing damaged pavement material. After the first machine pass, which mainly serves to homogenize the existing material, the binding agent can then, in a second machine pass, be blending into the soil. After profiling with a grader and subsequent compacting, the base layer can be trafficked immediately.
As an alternative to the in-situ production of new base layers, cold-treated material with foamed bitumen can also be produced ‘in plant’. Storable cold-treated material stabilized with foamed bitumen can, for example, be produced in the Wirtgen KMA150 mobile cold-recycling mixing plant, using either reclaimed asphalt material or new, graded aggregates. Then, like a normal hydraulically bound layer, the cold-treated material can be placed by an asphalt paver according to the specified profile. Here, too, the pavement can be trafficked immediately after compaction.
For further information on foamed-bitumen technology, contact: Wirtgen GmbH, Hohner Strasse 2, 53578 Windhagen, Germany; tel: +49 2645 1310; fax: +49 2645 131499; email: [email protected]