This section describes the materials used in this research, as well as the tests carried out to evaluate the bituminous mixtures made with reclaimed asphalt pavement (RAP) and biomass bottom ash (BBA).
2.1. Materials
The materials used in this research correspond mainly to industrial wastes or by-products. In turn, commercial materials such as limestone aggregate were used to evaluate the quality of the incorporation of the waste. In this way, the advantages that can be achieved in the bituminous mixtures conformed with the use of BBA can be verified.
The materials used, waste or commercial materials, were used in an unaltered form. That is, the products were taken directly from the producing industry to evaluate their physical and chemical characteristics without any type of treatment.
It should be noted that all the materials were dried at a temperature of 105 ± 2 °C for 24 h to eliminate humidity. This simple process to be carried out at industrial level was executed at laboratory level with the sole purpose of reducing the variables that could influence the test results. However, the existence of humidity in the materials would not be detrimental to the final product; it would simply have to be taken into account in order to carry out a correct dosing of the materials.
In turn, the BBA used, which is the main by-product of this research, was analyzed over time. The chemical and physical analyses carried out in different production batches reflected the unalterability of the mentioned properties, so that the characteristics were maintained in different production batches. This fact is essential, as it is essential for the use of a residue that its properties be unalterable. Otherwise, a product of higher or lower quality would be created depending on the production batch, as is the case, for example, with sewage sludge.
The following sections describe the materials used, describing their origin and general characteristics.
2.1.1. Biomass Bottom Ash (BBA)
BBA, as previously mentioned, corresponds to the residue of the energy generation industry from the combustion of almond shells and alpeorujo. These BBAs are produced in industries in the region of Andalucía, Spain. In these industries, the combustion of different types of biomass is produced, mainly related to the agricultural sector. In this particular case, the biomass used derives from the combustion of almond shells and alpeorujo.
The sample was taken directly from the producing industry without alteration, i.e., the sample contained all the particle sizes produced in the industry. This sample was dried according to the procedure described above and used for the different tests detailed in the methodology. The preparation of BBA for the different tests is detailed in the methodology.
2.1.2. Reclaimed Asphalt Pavement (RAP)
The reclaimed pavement belongs to the road that joins the towns of Linares and Bailen, in Spanish territory. This pavement showed clear signs of ageing, as there were cracks in irregular patterns on the surface. This type of defect represents the loss of the volatile and elastic part of the bitumen owing to the passage of time and, consequently, the loss of resistance to repeated tensile loads.
This pavement was milled with machinery similar to the one that would be used in the execution of the cold in-place recycling with bitumen emulsion in order to perform the laboratory tests as reliably as possible. The sample of the milled pavement was taken to the laboratory to carry out the subsequent tests detailed in the methodology.
2.1.3. Bitumen Emulsion
The bitumen emulsion used in this research is a cationic slow breaking emulsion, named after the European standard C60B5 REC. This type of slow breaking emulsion is very appropriate for RAP, as its longer breaking time makes it possible to coating the smallest aggregates. It should be noted that, depending on the aggregate size, the emulsion used will be different, i.e., bituminous mixtures with a higher proportion of coarse aggregate will use faster breaking bituminous emulsions than bituminous mixtures with a high proportion of fine aggregate. At the same time, it is necessary to mention that a bituminous emulsion is a suspension of bitumen in water. The emulsion of these two immiscible materials is achieved through a suitable manufacturing process and the use of emulsifiers. Depending on the nature of the emulsifier, the emulsion will have greater or lesser compatibility with the aggregate. Therefore, the emulsion in contact with the aggregate must break, i.e., produce the separation of bitumen and water. The bitumen remains adhered to the aggregate and the water evaporates by natural processes. Therefore, the additional and main advantage of this technique is that it can be performed at ambient temperature without the need for high temperature conformation. In this particular case, a slow cationic emulsion compatible with the RAP was used, as well as with the BBA used.
For further details,
Table 1 shows the technical characteristics of the bitumen emulsion used.
2.1.4. Limestone Aggregate
Limestone aggregate is a material usually used in cold in-place recycling with bitumen emulsion for the correction of the grading curve. This aggregate belongs to the area of Andalucía, Spain, as do the other materials.
Limestone aggregate is derived from calcareous rocks. Calcareous is a sedimentary rock composed mainly of calcium carbonate (CaCO3), generally calcite, although it frequently presents traces of magnesite (MgCO3) and other carbonates.
The limestone aggregate has an adequate adhesiveness with bitumen emulsion, as well as with bitumen. The particle density of the limestone aggregate used was 2.62 Tn/m3, with a bulk density in kerosene of 0.70 Tn/m3. The plasticity index was very low, reflecting a value of less than 5. The resistance of the limestone aggregate is lower than that of a siliceous aggregate. However, it is adequate for the type of bituminous mixture developed in this research.
2.2. Methodology
The methodology detailed in this research shows the tests carried out to evaluate the suitability of the use of BBA as filler material for reclaimed asphalt pavements and the cold in-place recycling with bitumen emulsion conformation.
The methodology followed is objective and sequential, being collected in the Circular Order 8/2001 [
31]. This Spanish, European standard describes the procedure to be followed for the formulation of cold in-place recycling with bitumen emulsion. It should be noted that, although this regulation has been repealed, it was used in this research because it has been used in hundreds of successful cases in Spain. In addition, it allows the physical and mechanical properties to be adequately characterised, the optimum combination of materials to be determined with a high degree of accuracy, and the differences between mixtures made with biomass bottom ash and those made with limestone aggregate to be correctly evaluated.
First, BBA and RAP were chemically and physically characterized. Then, an optimum combination of RAP and BBA was defined according to the grading envelope established by the aforementioned regulations. It should be remembered that BBAs have two different functions in the bituminous mix: on the one hand, they correct the RAP particle size to provide fine aggregate, and on the other hand, they provide the cementitious characteristics of the BBA to increase mechanical strength.
Subsequently, the compatibility of the aggregates with the bitumen emulsion used was evaluated and the maximum density of the mixture was obtained. This higher density will correspond to a higher mechanical strength. At the same time, different bituminous mixtures were conformed with different percentages of emulsion and precoating water, evaluating which were the optimum percentages that develop the highest mechanical resistance of the bituminous mixture. These results were compared with those obtained with bituminous mixtures made with RAP and limestone aggregate.
In the following sections, each of the phases developed in this research is described in detail for further clarification and to be able to reproduce the results objectively.
2.2.1. Characterization of Initial Materials
First, the materials used in this research must be characterized chemically and physically. These materials or by-products are BBA and RAP.
The first of the chemical tests performed on BBA was elemental analysis. This test quantifies the percentage of carbon, hydrogen, nitrogen, and sulfur present in the sample. For this purpose, the BBA sample was calcined at a temperature of 950 degrees, analyzing the gases produced in this combustion.
Subsequently, and because of the fact that BBA is a mainly inorganic material, the X-ray fluorescence test was performed. This test allows the quantification of the chemical elements with the highest atomic weight in the sample.
However, it should be noted that the chemical elements have greater or lesser activity, even contaminating power, depending on the chemical compound in which they are combined. Therefore, it is essential to determine the chemical compounds present in BBA. For this purpose, the X-ray diffraction test was performed.
Finally, the BBA leachate test was carried out according to the UNE-EN 12457-3 standard. The leachate obtained from this test was analyzed to determine that the concentrations of potentially contaminating elements were lower than the limits set by the standard [
32]. This ensures that the use of BBA in bituminous mixtures for roads will not produce subsequent negative effects on the environment.
Once the chemical composition of BBAs was analyzed, their physical properties were determined. The first of the tests performed was the particle density test, UNE-EN 1097-7. This test is essential to assess whether volumetric corrections are necessary, owing to a density different from that of a conventional aggregate. In turn, the bulk density test in kerosene, UNE-EN 1097-3, evaluates the specific surface of the material, as well as whether or not the material is powdery. A powdery material will present problems in the proportioning of the material in the factory and will absorb excessive percentages of binder, because the bulk density must be within the established limits. The plasticity index determines the existence of a clayey particle. These particles can damage the final material due to expansivity problems, so it is desirable that the plasticity index be as low as possible.
It should be remembered that BBAs have two different functions in the bituminous mix under study. One of them is the correction of the granulometric curve of the milled pavement. Therefore, it is essential to determine the particle size curve of BBA, UNE-EN 933-1. In this way, the percentage of combination of BBA with RAP can be calculated to obtain a grading curve formed with both materials that is within the grading envelope defined by the aforementioned standard.
RAP was also evaluated through different chemical, physical, and mechanical tests. First, the grading curve of the milled pavement was analyzed according to the UNE-EN 933-1 standard. Subsequently, the binder was separated from the coarse aggregates, fine aggregates, and filler, according to standard UNE-EN 12697-1. Once the different materials making up the RAP were separated, the bitumen was analyzed. This binder was evaluated by means of penetration tests, standard UNE-EN 1426, and the softening point test, standard UNE-EN 1427. On the other hand, the coarse aggregate, fine aggregate, and filler of the aged bituminous mix were evaluated through different physical and mechanical tests. The test of determination of resistance to fragmentation, standard UNE-EN 1097-2, was carried out on the coarse aggregate of RAP in order to evaluate its resistance. The determination of the percentage of crushed and broken surfaces in coarse aggregate particles, according to UNE-EN 933-5, and the flakiness index, according to UNE-EN 933-3, were also determined. The RAP fine aggregate was characterized with the sand equivalent test, UNE-EN 933-8 standard, and the plasticity index, UNE-EN ISO 17892-12, to determine the presence of colloidal or clayey particles that could damage the new asphalt mixture.
With the tests carried out, the feasibility of using RAP for the conformation of new bituminous mixtures was assessed, as well as the usefulness of BBA as an additive.
2.2.2. Bituminous Mixtures’ Manufacturing and Testing
Once the initial materials were analyzed, different bituminous mixtures were formed. First, and according to the grading curves of BBA and RAP, the optimum combination of both materials was obtained. This optimum combination should produce a grading curve within the grading envelope defined by the standards detailed above for this type of bituminous mix.
With this optimum combination of materials, the modified Proctor test was carried out, according to the UNE 103501 standard. The modified Proctor test provides, for a given material, the humidity necessary to obtain the maximum compaction density. This humidity is called optimum humidity and coincides with the theoretical fluid content (TFC) for this research. The theoretical fluid content would be the percentage of fluids to be added to the combination of RAP and BBA, these fluids being the precoating water plus the bitumen emulsion.
However, the theoretical fluid content must be corrected for the properties of the emulsion; this new percentage is called the optimum fluid content (OFC). The optimum fluid content is determined by means of the coating test, NLT-389/00 standard. This test is performed with a fixed percentage of emulsion, 3% on aggregate, and variable percentages of precoating water, from CTF to CTF–2%. The optimum percentage of fluids is determined according to the achieved coating, with the optimum percentage of fluids being the one subsequently used in the different families of bituminous mixtures formed.
Spanish regulations establish that, for this type of bituminous mixture, the percentage of emulsion over aggregate must be between 2.5% and 4% over aggregate. Therefore, different families of bituminous mixtures were made with RAP and BBA with emulsion percentages between 2.5% and 4%, with increments of 0.25% and with varying percentages of precoating water. The precoating water was equal to the difference between the optimum fluid content and the emulsion percentage.
The families of bituminous mixtures were made according to NLT-161/98. For this purpose, RAP was mixed with BBA in the determined proportion, subsequently adding the corresponding percentages of water and emulsion for each group of test samples. The resulting mixture was poured into a standard mold to apply the compaction load. This load consisted of an initial pressure of 1 MPa and then the application of a load of 21 MPa in a time of 2 to 3 min. Finally, the test specimens were stripped and subjected to the curing process at a temperature of 50 ± 2 °C for at least 3 days and up to constant mass. This process is carried out to eliminate water from the emulsion and to obtain the final properties of the mixture.
Subsequently, the physical properties of the bituminous mixtures made from the different families were determined. These properties are the maximum density according to standard UNE-EN 12697-5, the bulk density according to standard UNE-EN 12697-6, and the voids index according to standard UNE-EN 12697-8. The test samples from each family or group of samples were then separated into two groups to study the effect of water on the bituminous mix. This test is called the immersion-compression test, according to standard NLT-162/00. One of the subgroups of test samples from each family is immersed in water at a temperature of 49 ± 2 °C for 4 days. The other subgroup of specimens is subjected to ambient conditions (20 ± 2 °C). Finally, the test samples of the two subgroups of samples, for each family, are subjected to the simple compressive strength test according to the NLT-161/98 standard, determining the influence of water on the strength of the bituminous mixture.
It should be noted that, in order to determine the quality of the incorporation of BBA for the formation of recycled pavements in situ with bitumen emulsion, families of bituminous mixtures were also made with RAP and limestone aggregate, in the same proportion of combination with RAP as BBA and with the same procedure described above. These test samples were also physically and mechanically evaluated through the tests mentioned above.
2.2.3. Determination of the Optimum Combination of Materials
The results obtained from the different families of bituminous mixtures manufactured, with varying percentages of emulsion and precoating water, were analyzed to determine which percentage of emulsion and water produced the highest mechanical strength and, in turn, obtained acceptable physical properties according to the aforementioned standards. For this purpose, the values of resistance to simple compression without immersion were graphically represented. In this way, it was easy to evaluate graphically which was the optimum emulsion percentage, as long as this percentage obtained an adequate simple compressive strength with immersion and acceptable physical properties. This process carried out for bituminous mixes incorporating BBA was also performed for bituminous mixes made with limestone aggregate.
Once the theoretical percentage of bitumen emulsion was determined, test samples were again formed according to the described procedure and the physical and mechanical tests mentioned above were performed again. In this way, the properties of the bituminous mix were corroborated with the percentage of emulsion obtained graphically.
Finally, the results of the tests performed on the bituminous mixes with BBA and limestone aggregate were compared, showing objectively the advantages of incorporating BBA.