Experimental programme

5.1.1 FBM technologies and RAP material

The same bitumen foaming methods for the production of FBMs in Chapter 4 were used. These methods include: 1) a mechanical process that consists of the direct application of cold water into hot bitumen using a laboratory foaming plant and, 2) a regular foamed WMA technology that includes the direct addition of a foaming additive (i.e. zeolite) into the bitumen prior to mixing with the aggregates.

The RAP material used in this study was produced in the laboratory with the aim of controlling the characteristics of its components (i.e. RAP aggregate gradation, bitumen grade, etc.), while reducing issues related with the variability of their properties that could affect the reliability of the results. The laboratory procedure for FAM-RAP production consisted of subjecting a loose sample of FAM mixture to a controlled artificial long-term ageing protocol in the laboratory. Initially, the FAM mixture was manufactured with the same composition (i.e. bitumen content and aggregate gradation)

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as the virgin HMA presented in Section 4.1.3, at the same mixing temperature (i.e. 160^oC). Then, the FAM loose mixture was subjected to an ageing protocol which consisted of exposing the FAM loose mixture to heat in the oven for 32 hours (non-continuous ageing: 8h periods over 4 days) at 105^oC, as suggested by Oke (2010). Once the RAP material was produced, the bitumen was recovered by distillation following the BS EN 12697-4:2005. In order to characterise this bitumen, penetration at 25°C (BS 49:2007), softening point (BS 58:2007), rotational viscosity at 150°C (BS 2000-505:2010 ) and Fraass breaking point (BS 2000-80:2007 ) tests were performed. The same tests were performed on the virgin bitumen, which is the same as described in Section 4.1 – Chapter 4 used to manufacture the Virgin HMA. The results of both binders are presented in Table 5-1.

Table 5-1 Properties of the 70/100 pen virgin bitumen and recovered bitumen from RAP

Bitumen Penetration at 25^oC (dmm)

Softening point (^oC)

Rotational viscosity

@150^oC (mPa s)

Fraass breaking point (^oC)

Virgin bitumen 73 46 140 -12

Recovered bitumen 38 53 217 -5

5.1.2 Characteristics of the FAM mixtures

Six FAM mixtures manufactured with the two foaming technologies described previously in combination with RAP material were manufactured in the laboratory using three different production temperatures (i.e. 90, 120 and 160^oC). These mixtures include three FBMs produced through a laboratory foaming plant plus RAP material (Mechanical Foamed-RAP), and three FBMs containing zeolites plus RAP material (Zeolite-RAP). For the Mechanical Foamed-RAP mixtures, the bitumen was heated at 160^oC in all cases, and the aggregates were heated 20^oC above the mixing temperature in order to achieve the targeted mixing temperatures (i.e. 90, 120 160^oC). For the Zeolite-RAP mixtures, both bitumen and aggregates were heated to 90, 120 and 160^oC to achieve these target final temperatures of the mixtures.

Furthermore, three control mixtures were manufactured for comparison purposes, which include: a FAM mixture containing 50% RAP material, with no foaming technology labelled as ‘HMA-RAP’, a 100% RAP mixture labelled as ‘100%-RAP’, and a virgin FAM mixture - no RAP material- with no foaming technology, labelled as ‘Virgin-HMA’, which corresponds also to the materials used to produce the RAP material. The three control mixtures were manufactured at 160^oC, and both RAP and virgin materials were manufactured using the same FAM mixture design (i.e. gradation and bitumen content) described in Section 3.1.

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Based on the information presented previously, Table 5-2 summarizes the characteristics of the FAM mixtures evaluated. The amount of RAP material required for the 50%RAP mixtures was heated up to the corresponding temperature for 6 hours prior its incorporation with the virgin material. Also, in order to account for the stiffened RAP bitumen present in the mixtures containing RAP, a soft virgin bitumen was used in these mixtures, whose properties were determined using a bitumen blending design as explained in the next section.

Table 5-2 Characteristics of the FAM mixtures with RAP material

Mixture Type

Cylindrical FAM specimens of 50 mm in height by 12.5 mm in diameter were prepared for each FAM mixture following the methodology described in Section 3.2. Frequency and temperature sweep tests were conducted on the cylindrical FAM samples for all the mixtures by means of DMA testing to determine the linear viscoelastic material properties using the testing conditions described in Section 3.2.1. Two specimens were tested after observing low variability in their rheological properties (i.e.

differences between │G*│values below 10%). Since the SGC process results in different air void distribution along the sample, the resultant cylindrical samples contain different air void contents as well. Therefore, the specimens to be tested were carefully selected to have similar air void contents to be able to compare the impact of the components of the mixtures in their mechanical properties. Table 5-3 presents the volumetric characteristics of the tested specimens for each mixture including the measured air void content determined by volumetric calculations.

91 Table 5-3 Volumetric properties of the tested specimens

Mixture

5.1.3 Bitumen blend design

For the mixtures containing 50% RAP, equations (1) and (2) were used to determine the properties of the expected final bitumen blend to predict the required properties of the virgin bitumen to obtain a mixture of 70/100 pen (i.e. equal to the virgin bitumen in the HMA mixture for comparison). Table 5-4 shows the properties of each component of the final bitumen blend.

Table 5-4 Blend design for the RAP-FBMs

Bitumen Penetration at 25^oC (dmm) Softening point (^oC)

Estimated virgin bitumen 141 39

Recovered bitumen from RAP 38 53

Final expected bitumen blend 73 46

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Following these results, a virgin bitumen of 145 dmm penetration at 25^oC, softening point of 41^oC, and Fraass breaking point of -13^oC, was used for the soft virgin bitumen in the HMA-50% RAP mixture and as the foamed bitumen present in the RAP- FBM materials. Based on the previous calculations, it is expected that this virgin bitumen when blended with the RAP bitumen, will produce a final binder with an equivalent 70/100 penetration. Therefore, for the production of these mixtures, 50% of the amount of virgin materials (i.e. virgin bitumen and virgin aggregates) were replaced by RAP material, targeting the same gradation and bitumen content described in Section 3.1.

5.1.4 FAM mixing

For the mechanical foamed-RAP mixtures, mixing was carried out using a Hobart type mixer with a flat type agitator as explained in Section 4.1.4. First, the amount of RAP material and virgin aggregates, previously heated at the correspondent temperature as shown in Table 5-2, were added to the Hobart mixer bowl and mixed for approximately 30 seconds. Then, while the aggregate mixing was still in progress, foamed bitumen was sprayed out from the nozzle into the bowl and directly mixed with the pre-heated aggregates for up to 3 minutes until all the aggregates were visually fully coated. The bitumen foaming temperature was 160^oC in all cases, and the same foaming water content used for the manufacture of the mechanical foamed mixtures in Section 4.1 was used (i.e. 3% by mass of bitumen). The mixtures were then placed in the SGC mould for further compaction, without reheating the material.

For the preparation of the Zeolite-RAP mixtures, hand mixing was carried out in a heated mixer, which was heated at the corresponding mixing temperature of the mixture. First, as in the Mechanical foamed-RAP mixtures, the amount of RAP material and virgin aggregates, previously heated at the correspondent temperature as shown in Table 5-2, were added to the mixer and mixed manually for approximately 30 seconds. Then the same process as explained in Section 4.5.2 was followed. In summary, the process consisted of pre-blending Advera^® (at room temperature) with the corresponding pre-heated bitumen at 90, 120 and 160^oC, for further incorporation to the RAP-virgin aggregates. Then, hand mixing was performed until the aggregates were fully coated. The mixtures were then placed in the SGC mould for further compaction, without reheating the material. The dosage rate of Advera® was the same used for the preparation of the Zeolite mixtures in Section 4.5.2, which corresponds to 5% by weight of the bitumen.

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For the preparation of the Virgin-HMA and the HMA-50%RAP mixtures, hand mixing was performed as well until the fine aggregates were fully coated. These mixtures were placed in the oven at 140^oC for 1 hour before compaction following the BS EN 12697-35:2016.