GB2472995A

GB2472995A - Half-warm foamed asphalt process

Info

Publication number: GB2472995A
Application number: GB0914829A
Authority: GB
Inventors: Ignacio Artamendi; Paul Phillips; Bob Allen
Original assignee: Aggregate Industries UK Ltd
Current assignee: Aggregate Industries UK Ltd
Priority date: 2009-08-26
Filing date: 2009-08-26
Publication date: 2011-03-02
Anticipated expiration: 2029-08-26
Also published as: GB0914829D0; GB2472995B

Abstract

A process for the preparation of a half-warm foamed asphalt mixture comprises mixing aggregates pre-heated to a temperature in a range from 70°C-110°C with foamed bitumen which is formed prior to addition to the aggregates. The aggregates have a moisture content of from 0.5% to 1.5% by weight of aggregates. Typically, the foamed bitumen is produced by injecting water into hot bitumen at a temperature in the range from 120°C to 180°C. The amount of water added to foam the bitumen can be from 1.5% to 3.5% by weight of bitumen. Preferably, the mixture comprises 3% to 8% bitumen by weight of total mixture. A polymer modified binder, such as styrene-butadiene-styrene (SBS) or styrene-butadiene rubber (SBR), may be used to produce the half-warm foamed mixture. An additive may be added to the half-warm foamed asphalt mixture, wherein the additive can be a plant derived oil.

Description

I

Half-Warm Foamed Asphalt Process

FIELD OF THE INVENTION

The invention relates to the development of half-warm foamed asphalt (HWFA) mixtures at temperatures below 100°C.

BACKGROUND OF THE INVENTION

Increase in fuel prices and environmental concerns regarding global warming has led the asphalt industry to consider seriously developing new processes and products in order to reduce mixing and compaction temperatures of asphalt mixtures. Hot mix asphalt (HMA) is typically produced at temperatures between 150°C and 190°C, depending upon the grade of the binder used. Lower plant mixing temperatures can lead to a considerable decrease in fuel energy consumption and, consequently, to lower fuel costs and a reduction in carbon dioxide emissions.

Cold mix asphalts (CMAs), such as emulsions and foamed asphalt were first proposed as an alternative to HMA. These materials can in principle be produced and laid at ambient temperatures and therefore have considerable environmental benefits (Leeche, D., 1994. "Cold mix bituminous materials for used in the structural layers of roads". TRL Project Report 75, Department of Transport). Conventional cold foamed asphalt is produced by mixing foamed bitumen with aggregates, typically reclaimed asphalt planings (RAP),. at ambient temperatures to produce foamed bitumen-stabilised mixtures. The mixtures are often modified with a hydraulic binder to enhance their early life performance (Khweir, K., 2007. "Performance of foamed bitumen stabilised mixtures".

Transport 160, Issue TR2, Proc. of the Institution of Civil Engineers, pps 67 - 72). However, concerns still persist regarding the durability and performance of cold asphalts in use, especially in heavy traffic conditions.

Over the years, various products have become available to reduce the mixing and compaction temperature of asphalt. The products include: Asphamin� (zeolite), Sasobit� (wax) and Evotherm� (chemical additive) (Hurley, G.C. and Prowell, B.D., 2006. "Evaluation of potential processes for use in warm mix asphalt". Journal of the Association of Asphalt Paving Technologists, Vol. 75, pps 41 -90). Also, specially designed binders such as Shell "S" grades are used to manufacture asphalt at lower temperatures. These materials are typically produced at temperatures ranging from 120°C to 140°C and are generically known as warm mix asp halts (WMAs).

In the late 1990s Shell developed the Warm Asphalt Mix (WAM)-Foam� process for the production of asphalt at temperatures of 110°C-I 20° C. In this process, a soft binder is first used to pre-coat the aggregates heated at 120°C. A foamed hard binder is then added into the mixture. It is claimed that material performance is at least equivalent to HMA (Poncelet, K. 2008. "WAM-Foam presentation to Aggregate Industries". Research and Development Department, Hulland Ward).

Also in the late I 990s, half-warm foamed asphalt (HWFA) mixtures were first introduced (Jenkins, K.J., 2000. "Mix design considerations for cold and half-warm bituminous mixtures with emphasis on foamed bitumen". PhD Dissertation, University of Stellenbosch, South Africa). The main difference between this process and the cold foamed process is that the aggregates are pre-heated at temperatures just below 100°C. Jenkins showed that by heating the aggregates at temperatures close to 100°C, binder dispersion and particle coating was significantly improved. Similar findings have also been reported by others (Gaudefroy, V., Olard, F., Cazacliu, B., de La Roche, C., Beduneau, E. and Antoine, J.P., 2007. "Laboratory investigations on the mechanical performances of foamed bitumen mixes using half-warm aggregates".

Transportation Research Board TRB, Annual Meeting, Washington D.C.).

Furthermore, performance related properties for HWFA mixtures have been found comparable to those for HMA5 (Jenkins, K.J., Molenaar, A.A.A., de Groot, J.L.A. and Van de Ven M.F.C., 2002. "Foamed asphalt produced using warmed aggregates". Journal of the Association of Asphalt Paving Technologists, Vol. 71, pps 468 -492).

In Europe, various commercial organisations have developed half-warmed asphalt processes. For example, in the Netherlands, LT-Asphalt Group and Nynas have developed Low Temperature Asphalt Process using foamed bitumen and heated aggregates at approximately 95°C (Landa, P.A., Kneepkens, T. and Zwan, J.Th.v.d., 2004. "Low temperature asphalt: A production process with the possibility to produce and pave hot mix asphalt at temperatures below 100 00 or 212 °F". Eurasphalt & Eurobitume, Vienna, Austria). In this process the foamed bitumen is sprayed onto the warm aggregates and a hydrophilic filler (0.5-1.0%) and special binder is also used.

In France, EIFFAGE Travaus Publics and FIARCO have also developed the Low Energy Asphalt (LEA�) process to produce asphalt at around 95°C. The LEA� process is based on the ability of the hot bitumen to foam when in contact with the residual moisture in the aggregates heated at temperatures close to 100°C. However, specific additives are generally used to improve the foaming and coating ability of the binder (Olard, F., Antoine, J.P., Héritier, B., Romier, A. and Martineau, Y., 2007. LEA� (Low Energy Asphalt): A new generation of half-warm asphalt mixtures". Proc. of the mt. Conf. on Advanced Characterisation of Pavement and Soil Engineering Materials, Athens, Greece, ppsl37l-1381).

With regard to energy consumption, the energy required to produce asphalt depends on both the mixing temperature and the initial moisture content of aggregates, as shown in Figure 1.

At mixing temperatures above 100°C, energy is required to overcome the latent heat of water evaporation (L 2250 kJ/kg). This amount represents approximately 500 times the energy required to raise the same amount of water by 1°C (specific heat of water, C = 4.2 kJ/kg/°C). Thus, the advantages of mixing temperatures below the boiling point of water are evident.

Energy savings reported in the literature vary from about 15% for warm mix asphalt produced at temperatures of about 140°C up to 40-50% for half-warm mix asphalts produced at 90°C (Olard, F., Antoine, J.P., Héritier, B., Romier, A. and Martineau, Y., 2007. "LEA� (Low Energy Asphalt): A new generation of half-warm asphalt mixtures". Proc. of the mt. Conf. on Advanced Characterisation of Pavement and Soil Engineering Materials, Athens, Greece, pps 1371 -1381). Energy savings between 30-40% have also been reported for the WAM-Foam process at temperatures around 110°C (Poncelet, K. 2008.

"WAM-Foam presentation to Aggregate Industries". Research and Development Department, Hulland Ward). Reduction in carbon dioxide emissions has also been found to be in the same order of the energy savings.

Foamed Bitumen Foamed bitumen was developed in the I 960s to improve the quality of the asphalt mix. Heating the bitumen to approximately 130°C -180°C and injecting 2-7% cold water into it, causes the bitumen to expand and to reduce its viscosity. Foamed bitumen is produced by injecting under high pressure, air and water, into hot bitumen. As water turns into steam, bitumen changes from liquid state into foam with no change in its chemical composition. When the bitumen is foamed, its volume increases significantly and its viscosity decreases. After a short period of time (approximately a few seconds), the foam collapses and the bitumen recovers its original viscous condition,

SUMMARY OF THE INVENTION

There is a need for an improved process for the production of half-warm foamed asphalt mixtures that is relatively easy to perform by mixing heated aggregates having a higher moisture content than previously described in the art with half-warm foamed asphalt mixtures to give a product having excellent workability, high densities and low voids after compaction and mechanical properties similar to those of conventional HMA mixtures.

In a first aspect of the invention there is provided a process for the preparation of a half-warm asphalt mixture, comprising mixing aggregates pre-heated to a temperature in a range from 70°C -110°C with foamed bitumen, characterised in that said aggregates have a moisture content of from 0.5% to 1.5% by weight of aggregate and that said foamed bitumen is formed prior to addition to the aggregates.

Preferably, the average size of the aggregate is in a range from 2 mm to 30 mm, more preferably in a range from 4 mm to 20 mm. Typical aggregates suitable for use in the invention include limestone aggregate and hardstone aggregate.

Preferably, the aggregates are pre-heated to a temperature in a range from 70°C-95°C.

Preferably, the aggregates and foamed bitumen are mixed at a temperature in a range from 75°C -95°C.

The foamed bitumen is formed prior to addition to the aggregates. Typically, the foamed bitumen is produced by injecting water into hot bitumen at a temperature in a range from 120°C and 190°C. Preferably the temperature of the bitumen is in a range from 130°C and 180°C, and most preferably the temperature of the bitumen is in a range from 140°C and 170°C.

Typically the mixture of the present invention comprises 3% -8% bitumen by weight of the total mixture, and more preferably 4% -7% bitumen by weight of the total mixture.

Preferably, the amount of water added to foam the bitumen is 1.5 to 3.5 % by weight of bitumen, more preferably 2% to 3%. This amount is controlled by a flow meter system.

Water is injected at a pressure in a range from 0.5 and 6.0 bar to produce finely dispersed water bubbles within the bitumen phase. Preferably the pressure is in the range from 1.0 and 6.0 bar. The resulting foamed bitumen is referred to as umicro4oamu This process differs to other prior art systems (such as LEA�) wherein the bitumen is foamed on contact with wet aggregates.

Preferably the moisture content by weight of the aggregate is in a range from 0.5% to 1.5%, and more preferably it is in a range from 0.5% -1.0%. This is a considerably higher aggregate moisture content than has been disclosed in the prior art (such as EP 1767581A). The present inventors have found that the aggregate moisture content has a fundamental effect on particle coating. At a lower moisture content (1%) the foamed mixtures appear fully coated. As the moisture content is increased more particles appear uncoated.

In one aspect of the invention, penetration grade bitumen with penetration in the range from 10 to 220 dmm can be used in this process.

In another aspect of the invention polymer modified binders can be used to produce half-warm foamed bitumen mixtures. These include elastomeric and plastorneric polymer modified binders. Elastomeric polymers typically used in bitumen modification include Styrene-Butadiene-Styrene (SBS), Styrene-Butadiene Rubber (SBR) and polybutadiene. Thermoplastic polymers such as polystyrene and Ethylene Vinyl Acetate (EVA) are also used. Preferably Styrene-Butadiene-Styrene (SBS) polymer modified binders are used.

In a further preferred aspect of the invention, additives can be included in the mixture. Preferably, the additives for use in the first aspect of the invention are plant derived oils. More preferably, they are vegetable oils. Yet more preferably, they are waste plant oils and most preferably they are waste

vegetable oils.

The present invention can be understood further by consideration of the following examples with reference to Figures 1 and 2.

Figure 1 discloses the energy consumption for different mixing temperatures and moisture contents.

Figure 2 discloses evolution of voids with number of gyrations for mixtures I and 2.

Example I

Limestone aggregates, reclaimed limestone filler and 100/1 50 pen bitumen were used to produce a half-warm foamed asphalt mixture equivalent to Asphalt Concrete with a maximum aggregate nominal size of 20 mm (AC 20). The composition of the mixture is given in Table I below: Table I: Composition of HWFA (AC 20 equivalent) Components Composition (%) 10/20 mm 37.2 6.3/14 mm 7.6 4/10 mm 8.6 2/6.3 5.7 0/4 mm 34.2 Rec'aimed LS Filler 2 Binder (100/1 50 pen) 4.7 The aggregates were first heated at a temperature of 100 � 5°C. Foamed bitumen was then produced by injecting water into hot bitumen at 150 °C. The amount of foaming water to be obtained was 2% by weight of binder. To foam the bitumen, the water pump was run for 10 seconds giving 2 litres per batch at a flow rate of 1 litre every 5 seconds. For a batch size of 1750 kg containing 82.25 kg of binder (4.7% bitumen by weight of total mixture), this gave 2.4% water by weight of bitumen.

As a control experiment, a hot mix asphalt (HMA) comprising the same aggregates and binder as the half-warm asphalt was produced at a temperature in a range from 140-150 °C. Both the HMA and HWFA materials were produced in 1750 kg batches. Mixing times were 30 and 32 seconds for the hot and half-warm mixtures respectively. A total of four loads of HWFA and one load of HMA were produced. Each load weighed approximately 16 tonnes.

Temperatures recorded on the truck just after discharge from the mixer for the HWFA are presented in Table 2: Table 2: Temperatures of HWFA Load Temperature on truck Temperature on paver Temperature after paver (°C) (°C) (°C) 1 101 -- 2 103 101 90 3 -90 86 4 -89 80 Average 102 93 85 The HWFA was laid using a conventional paver. Workability of the HWFA was considered as good as that of the HMA. Results of the test carried out on 100 mm diameter cores are presented in Table 3:

Table 3: Summary of test results (average values)

Property Material HW-FAM (AC2O) HMA (AC2O) Maximum density, kg/rn3 2506 2494 Bulk density, kg/rn3 (Proc. A-dry) 2401 2324 Air void content, % 4.2 6.8 Bulk density, kg/rn3 (Proc. C-sealed) 2378 2302 Air void content, % 5.1 7.7 Stiffness, MPa 1265 1301 Water sensitivity (Stiffness ratio) 0.87 0.86 Bulk densities and void values of the HWFA were considered appropriate which indicates good workability and compactability of the material produced at lower temperatures (80-90 °C).

Table 3 above, also shows that the stiffness values of the two materials were very similar. These values can be considered typical of this type of mixture (AC with a 100/1 50 pen binder).

Water sensitivity tests showed that there were no major differences on the stiffness ratio after water immersion. The high stiffness ratios achieved (>0.8) indicated good resistance to water damage for both mixtures.

Compositional analysis of the two materials was carried out, and are presented in Table 4. The penetration and softening point of the binder recovered from the HWFA mixture were also determined and are shown in Table 5.

Table 4: Compositional analysis

Sieve size % Passing Specification

mm HW-FAM (AC2O) HMA (AC2O) AC 20 Dense Bin 31.5 100 100 98-100 100 100 90-100 14 76 81 - 61 64 52-70 6.3 50 52 38-56 2 30 30 23-37 0.25 11 11 6-16 0.063 6.8 7.4 3.0 -9.0 Binder content, % 5.0 4.5 4.0 -5.2 Table 5: Penetration and softening point values Property Material HW-FAM (AC2O) HMA (AC2O) Penetration, dmm 108 79* Softening point temperature, °C 43.8 504* It can be seen that the binder recovered from the half-warm mixture showed less hardening (oxidation) than those typically recovered from the hot mixtures.

This is due to lower mixing temperatures. A low degree of binder oxidation and hardening during mixing is expected to enhance the durability and long term performance of the material. Complete aggregate coating at a temperature of 100 °C by half-warm foamed asphalt is achievable. The workability and compactability of the HWFA at temperatures in the range from 80°C to 90°C were considered good. Furthermore, adequate levels of voids and densities were achieved.

The amount of fumes produced at both the plant and during paving operations was considerably reduced as a result of lower working temperatures.

Furthermore, there is a substantial reduction in energy consumption as a result of lower heating temperatures, typically in the region of 30% or more compared to the energy required for the production of HMA.

Example 2

In this trial, two half-warm foamed asphalt mixtures, a 10 mm close graded surface and 14 mm close graded surface course, were produced and laid.

Greywacky aggregates and 100/1 50 pen bitumen was used. Compositional analysis of the mixtures is presented in Table 6. *11

Table 6: Material composition Sieve size (mm) Composition (%)

HWFA HWFA

(10 mm CGSC) (14 mm CGSC) 20mm 100(98-100) 14mm 100(98-100) 98(92-100) 10mm 99(92-100) 80(73-87) 6.3 63(57-71) 58(48-62) 2 mm 25 (23 -35) 26 (22 -34) 1mm 18 19 0.250 11 12 (7-15) 0.063mm 6.1(4-8) 6.5(4-8) Binder content (%) 5.8 (4.7 -5.7) 5.2 (4.6 -5.6) The aggregates were first heated at a temperature of 100 � 5 00. Foamed bitumen was produced by injecting water into hot bitumen at 150 00. The amount of foaming water to be obtained was 2% by weight of binder. The materials were mixed in 2 tonne batch sizes. The temperatures recorded on the truck just after discharge from the mixer are presented in Table 7 below.

Table 7: Temperature of HWFA mixtures after discharge from mixer Load Temperature ( °C)

HWFA HWFA

(10 mm CGSC) (14 mm CGSC) 1 98 93 2 105 94 3 89 93 4 108 92 96 90 6 96 98 7 96 -Average 98 93 It was observed that no fumes were produced from the mixture due to the reduced temperature of production. Furthermore, a reduction in carbon dioxide emissions during asphalt production at the plant was recorded. This can be attributed to plant modifications and the use of bio-fuels together with the half-warm asphalt production process.

Example 3

1. Design of half-warm foamed asphalt mixtures: Step 1: Graded aggregates (Mixtures I and 2 in Table 8 below) were preheated in an oven at 100°C for a minimum of 8 hours and were then transferred to a mixing bowl, also at 100°C.

Step 2: Water was added onto the aggregates in three different proportions, 1, 3 and 5% by weight of dry aggregate and mixed for approximately 30 seconds.

Step 3: Foamed bitumen at 18000 and 2% foaming water content was added to the wet and heated aggregates at a content of 4.3% (mixture 1) and 4.7% (mixture 2) by weight of the dry mix. An extra 0.5% of foamed bitumen by weight of dry mix was added to compensate for losses during foaming/mixing processes. (Table 8 below indicates the quantities of aggregates, added water and foamed bitumen used).

Step 4: The foamed mixture was transferred to an orbital, more powerful, temperature controlled mixer and mixed for approximately 6 minutes.

Step 5: The mixture was then poured into gyratory compaction mould (100 mm diameter) preheated at 95°C in quantities of 1.2 kg per mould, The first sample was compacted immediately the second and third samples were returned to the oven at 95°C. The second sample was compacted as soon as the first finished and the third sample was compacted thereafter. Standard gyratory compactor settings were used (load =600 kPa, angle=1.25°, 200 gyrations).

The materiat not used for gyratory samples was allowed to cool down, spread out and visually examined for binder coverage.

Gyratory samples were kept in the mould for 24h at ambient temperature and then extruded.

Gyratory specimens were tested for stiffness modulus.

Table 8: Batch quantities of the two foamed asphalt mixtures MIXTURE COMPOSITION, g

MIXTURE COMPONENTS

Batch 1 Batch I Batch 1 a) Plant mixture (Mixture 1) Aggregates 5000 5000 5000 Added water 50 (1%) 150 (3%) 250(5%) Foamed bitumen 250 (4.3+0.5%) 250 (4.3+0.5%) 250 (4.3+0.5%) b) Lab mixture (Mixture 2) Aggregates 5000 5000 5000 Added water 50 (1%) 150 (3%) 250(5%) Foamed bitumen 275 (4.7+0.5%) 275 (4.7+0.5%) 275 (4.7+0.5%) Visual examination of the loose mixtures showed that the higher the water content, the less coated the aggregates. In HWFA with 1% added water, the aggregates appeared fully coated. Mixture 2 (containing 1% added water) was found to give the best aggregate coating, most likely as a result of higher residual binder content. As the amount of added water is increased, more particles appeared uncoated In conventional cold foamed asphalt, the temperature gradient between aggregates at ambient temperature and bitumen at approximately 110 °C is high. The viscosity of the foamed bitumen, therefore, increases rapidly upon contact with the cold aggregate and the time available to disperse the bitumen is relatively short. Consequently, the smaller aggregate temperatures are predominantly coated. In the case of half-warm asphalt, the equilibrium temperature is considerably higher than the cold process and more time is available for binder dispersion during mixing. Half-warmed asphalt, therefore, favours full coating of the particle.

The gyratory compactor was used to assess the workability of conventional (hot) and half-warm foamed asphalt mixtures. The conventional hot mixtures (mixtures I and 2) were mixed at 160°C and compacted at temperatures between 135 and 145°C. The evolution of void content with number of gyrations is shown in Figure 2 (average values from four specimens). Good workability was observed for both mixtures. Furthermore, low voids and high densities were achieved at 200 gyrations.

Foamed mixtures were mixed at approximately 95°C and compacted at temperatures between 71°C and 89°C. The workability of the foamed mixtures was considered acceptable as shown in Figure 2 (average values from four specimens). It can also be seen that the foamed mixtures were not as workable as the conventional mixtures. However, it was observed that workability increased with increasing aggregate moisture content. This was attributed to the lubricating effect of the water. Foamed Mixture 2 was found to be more workable than foamed Mixture 1 as the former has a higher residual binder content than the latter.

Compaction effort was assessed by determining the number of gyrations needed to compact the mixtures to the same level achieved for roller compacted specimens of conventional mixtures. Results indicate that a higher compaction effort (i.e. a higher number of gyrations) is required to compact the foamed mixtures to the same level of voids as conventional mixtures. This is shown in Table 9 below.

Table 9: Number of gyrations to achieve same compaction as roller compacted specimens

CONVENTIONAL FOAMED

ROLLER COMPACTED MIXTURES MIXTURES

SPECIMENS AT VOIDS, °" Water Added 1 5 % 0 3 a) Number of Gyrations at 52 > 200 105 NA 4.6 % voids (Mixture 1) a) Number of Gyrations at 37 81 70 46 5.0 % voids (Mixture 2) The Indirect Tensile Stiffness Modulus (ITSM) test as per BD DD213 (British Standard Institution, Method for the determination of the indirect tensile stiffness modulus of bituminous mixtures, Draft for Development 213, BSI, London, 1993) was used to determine the stiffness of the materials used in this study.

Roller compacted specimens cored from slabs and gyratory specimens were tested for conventional and foamed mixtures, respectively. Average values are presented in Table 10 be'ow. Results indicate that Mixture I has a higher stiffness than Mixture 2 due to bother lower binder content and lower air void content. For the foamed mixtures, stiffness values were found similar to those for the conventional mixtures. The stiffness was seen to decrease as the moisture content of the foamed mixtures was increased. This could be due to the detrimental effect of excess water on the aggregate coating.

Table 10: Stiffness modulus of conventional and foamed mixtures

COMPACTION DENSITY VOIDS ITSM

MATERIAL

METHOD kg/me % MPa Mixture I Roller 2392 4.6 3616 Foamed Mixture 1 Gyratory 2335 6.9 3954 1% added water Foamed Mixture 1 Gyratory 2443 2.5 3865 3% added water Mixture 2 Roller 2369 5.0 2989 Foamed Mixture 2 Gyratory 2411 3.3 3894 1% added water Foamed Mixture 2 Gyratory 2466 1.0 2909 3% added water Foamed Mixture 2 Gyratory 2510 -0.7 2790 5% added water

Claims

CLAIMS: 1. A process for the preparation of a half-warm foamed asphalt mixture, comprising mixing aggregates pre-heated to a temperature in a range from 7000 -110°C with foamed bitumen, wherein said aggregates have a moisture content of from 0.5% to 1.5% by weight of aggregates and that said foamed bitumen is formed prior to addition to the aggregates.
2. A process according to claim 1, wherein the aggregates are pre-heated to a temperature in a range from 70°C -95°C.
3. A process according to claim 1 or claim 2, wherein the aggregate and foamed bitumen are mixed at a temperature in a range from 75°C -95°C.
4. A process according to any of claims I to 3, wherein the foamed bitumen is produced by injecting water into hot bitumen at a temperature in the range from 120°C to 190°C.
5. A process according to any of claims 1 to 4, wherein the foamed bitumen is produced by injecting water into hot bitumen at a temperature in the range from 130°C to 180°C.
6. A process according to any of claims 1 to 4, wherein the foamed bitumen is produced by injecting water into hot bitumen at a temperature in the range from 140°C to 170°C.
7. A process according to any of claims I to 6, wherein the mixture comprises 3% -8% bitumen by weight of the total mixture.
8. A process according to any of claims I to 6, wherein the mixture comprises 4% -7% bitumen by weight of the total mixture.
9. A process according to any of claims I to 8, wherein the average size of the aggregate is in a range from 2 mm to 30 mm.
10. A process according to any of claims I to 8, wherein the average size of the aggregate is in a range from 4 mm to 20 mm.
II. A process according to any of claims I to 10, wherein aggregates are selected from the group consisting of limestone aggregate and hardstone aggregate.
12. A process according to any of claims I to 11, wherein the amount of water added to foam the bitumen is from 1.5% to 3.5% by weight of bitumen.
13. A process according to any of claims I to 11, wherein the amount of water added to foam the bitumen is 2% to 3% by weight of bitumen.
14. A process according to any of claims Ito 13, wherein the water is injected into hot bitumen at a pressure in a range from 0.5 to 6.0 bar to produce half-warm foamed bitumen.
15. A process according to any of claims I to 13, wherein the water is injected into hot bitumen at a pressure in a range from 1.0 to 6.0 bar to produce half-warm foamed bitumen.
16. A process according to any of claims Ito 15, wherein the moisture content by weight of the aggregate is in a range from 0.5% to 1.5%.
17. A process according to any of claims Ito 15, wherein the moisture content by weight of the aggregate is in a range from 0.5% -1.0%.
18. A process according to any of claims I to 17, wherein the penetration grade bitumen with penetration in the range from 10 to 220 dmm is used to produce the foamed bitumen.
19. A process according to any of claims Ito 18, wherein a polymer modified binder is used to produce a half-warm foamed mixture.
20. A process according to claim 19, wherein the polymer modified binder is an elastomeric or a plastomeric polymer modified binder.
21. A process according to claim 19, wherein the polymer modified binder is selected from the group consisting of Styrene-Butadiene-Styrene (SBS), Styrene-Butadiene Rubber (SBR), polybutadiene, polystyrene and Ethylene Vinyl Acetate (EVA).
22. A process according to claim 19, wherein the polymer modified binder is Styrene-Butadiene-Styrene (SBS).
23. A process according to any of claims I to 22, wherein an additive is added to the half-warm foamed asphalt mixture.
24. A process according to claim 23, wherein said additive comprises a plant derived oil.
25. A process according to claim 24, wherein said plant derived oil is a virgin plant oil or a waste plant oil.
26. A process according to claim 25, wherein said waste plant oil is a wastevegetable oil.
27. A half-warm foamed asphalt mixture obtainable by a process according to any one of claims I to 26.
28. A process for the preparation of a half-warm foamed asphalt mixture as described herein with reference to the examples and figures.
29. A half-warm foamed asphalt mixture obtainable by a process according to claim 28.