On-site manufacture of hot mix asphalt using pellets

Powder Technology 344 (2019) 58–67

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On-site manufacture of hot mix asphalt using pellets that can be melted by induction energy Hadel Obaidi, Breixo Gomez-Meijide ⁎, Alvaro Garcia Nottingham Transportation Engineering Centre [NTEC], Department of Civil Engineering, University of Nottingham, Nottingham NG7 2RD, UK

a r t i c l e

i n f o

Article history: Received 8 August 2018 Received in revised form 30 October 2018 Accepted 20 November 2018 Available online 22 November 2018 Keywords: Binder pellets Induction heating Steel wool Potholes repair

a b s t r a c t Despite extensive research into different pothole repair method, up to date, there is no research using the induction heating technology to address the pothole repair has been investigated. The utilization of induction heating with the innovative pothole repair materials is proposed in the current research, evaluated and compared to current patching materials. Binder pellets are composed of bitumen, steel wool and a protective shell to avoid them sticking to each other. Such pellets can be packed, stored, and transported like a simple aggregate. Once on-site, the required amount of pellets is heated and mixed with cold aggregates in a portable mixer with an electromagnetic induction generator. In this research, heating, volumetric and mechanical properties (indirect tensile, tensile and shear strength, as well as resistance to permanent deformation) of the resulting material were assessed and compared to conventional road repairing materials. Results of the use of the innovative materials showed excellent durability and performance; higher than a road repaired by cold mix asphalt and even similar to the performance of a road recently repaved. © 2018 Published by Elsevier B.V.

1. Introduction Flexible asphalt pavement is the most common material used around the world to build surface layers of pavement [1,2]. Its service life on road pavements is assumed to be between 10 and 30 years [3,4], however the surface layers are commonly repaired more often, approximately every eight years [5], due to the use of poor quality materials [6] and the overload produced by the increasing traffic and the number of heavy vehicles [7]. When the surface damage develops enough, potholes may appear on the roads. If they are not repaired at early formation stages, the costs of eventual major maintenance can be significantly higher [8]. Moreover, traffic delays due to damaged roads or maintenance works can involve great costs when they are produced in delicate areas [9]. For instance, in 2012, the congestion costs of roadworks in London were estimated at around £2000 per roadwork hour at the busiest places on the TRLN (Transport for London Road Network) [10]. Furthermore, the total cost spent in England and Wales to fill potholes since ALARM (Annual Local Authority Road Maintenance) 2017 is estimated at £102.3 million [11]. The selection of appropriate maintenance plays an important role in the duration of pavement life and, as a consequence, on these total costs [12]. Hot mix asphalt (HMA) and cold mix asphalt (CMA) have been commonly applied as pothole patching materials. HMA has high quality among the patching materials which are obtainable in the market [13]. ⁎ Corresponding author. E-mail address: [email protected] (B. Gomez-Meijide).

https://doi.org/10.1016/j.powtec.2018.11.089 0032-5910/© 2018 Published by Elsevier B.V.

HMA needs a higher temperature of mixing to be more workable. However, the high temperature means that relatively large amounts of fuel are needed to heat up the aggregates and the bitumen [14–16]. On the other hand, CMA is a mix of asphalt emulsion, water and aggregates that can be applied at ambient temperature and that drastically reduces the economic and ecological impact [17,18]. But the mechanical performance of CMA is significantly lower especially at the early curing stages, when the interstitial water has not yet been evaporated [19]. The throw-and-roll method is considered as a temporary repair method, having a low cost and being usable in adverse weather because it can be implemented quickly. The semi-permanent method needs less time and effort to repair potholes but it requires more labour and equipment [17–20]. Contemporary maintenance techniques such as inlay/ overlay or hot-on-hot paving processes have also been developed but are often expensive, involve high consumption of energy and pose significant health risks for operators due to high work temperatures and hazardous fumes and conditions [21]. Over the last years, many studies have been carried out on practical methods of road maintenance such as self-healing technology [22–24] by addition of metal particles to the asphalt mix [25–27]. When the road is damaged, the simple application of an electromagnetic field induces micro-currents through these particles that increase their temperature and, as a consequence, the temperature of the embedding bitumen [26–28]. This produces a reduction in binder viscosity and its thermal expansion making it flow through cracks, which remain sealed once the temperature decreases after the treatment [29]. In order to reduce the costs of introducing metal particles, previous

H. Obaidi et al. / Powder Technology 344 (2019) 58–67

investigations have also tried particles from metal waste, obtaining encouraging results [30]. Another study [31] proposed a new patching process consisting of cutting the pothole into standard dimensions and filling it with a prefabricated asphalt tile wrapped in a modified bitumen membrane (bonding layer) containing metal particles that can be heated by electromagnetic induction until the old and new material are firmly bonded. Another significant approach was the use of asphalt pellets [32], an energy-efficient alternative to conventional road materials (e.g. hot or cold asphalt mixes), designed to simplify and reduce the costs of pothole reparations. According to the mentioned publication, the main disadvantage of this technique is its reduced versatility, as they are manufactured in an asphalt plant with a predetermined recipe. To overcome this problem, the goal of the present investigation is to develop the production of asphalt pellet materials and produce them as an innovative way to repair potholes that (1) is suitable for use in any environmental condition, (2) creates a patch of comparable quality and durability to the original road material, (3) reduces the production of debris materials, (4) improves the work conditions for operators, as heating elements and noxious fumes are not necessary, (5) minimises traffic disruption thanks to a higher time/cost efficiency, and (6) has comparable life-cost to current asphalt patching methods.

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induction coil was multi-turn helical, connected to a 7.5 kW induction heating generator (EASYHEAT LI 5060) with a working frequency of 272 kHz. The current was set at 300 A, and the power at 1625 W during the experiments. The coil had 6-turns with external diameter of 12 mm, and distance between consecutive turns of 11.6 mm (total length of coil about 130 mm). Inside the crucible, an aluminium (non-conductive) paddle with dimensions about 100x90x0.15 mm3 was placed, connected to an electric mixer drill (ERBAUER ERB 728) that operates with 1600 W of power output, 220–240 V of voltage supply and at a speed of 0–700 rpm. When a pothole needs to be repaired, loose particles and water must first be cleaned from it. Then the required amount of pellets and aggregates are placed into the mixer and heated by induction while the paddle spins at a speed of 140 rpm. The pellets are melted and blended with aggregates at around 120–140 °C, the temperature reached in 5 min (depending on the steel content). After that, the mix can be easily poured into the pothole and compacted to produce a road material that can resist traffic loads. 3. Materials and methods 3.1. Materials

2. Description of the proposed technology The present investigation proposes an innovative material for pothole reparation, which consists of the following two stages (Fig. 1): 1. Prefabrication stage: in this stage binder pellets containing bitumen and electrically conductive particles (steel wool) are manufactured. In order to make them suitable for storage, packing and transportation at ambient temperature, it is necessary to add a protective shell which prevents them sticking to each other. In the present investigation, pellets covered in mineral filler, in a shell of alginate and calcium chloride, and in a combination of both were compared with pellets without any protective measure. 2. On-site stage: A concrete cylindrical crucible was manufactured surrounded by an induction coil which will generate an alternating electromagnetic field inside. This varying magnetic field can induce eddy currents in the metallic fibres contained in the pellets previously placed into the crucible. By the Joule effect and due to their electrical resistance, the fibres and embedding bitumen will increase in temperature. The length of the crucible was 20 mm and the inner and external diameter 100 mm and 150 mm respectively. The

Binder pellets were composed of bitumen 40/60 pen with a density of 1.03 g/cm3 and steel wool with a diameter between 16 and 72 μm and a length of 0.15–5 mm. This type of fibre, with small size, produces uniform and homogeneous distribution within the mix, avoiding the formation of clusters. In order to produce a protective shell, the following components were used: (a) sodium alginate (C6H7O6Na), which is an anionic polysaccharide, widely distributed in the cell walls of brown algae; (b) a calcium source, provided by Sigma-Aldrich as anhydrous, granular pellets of calcium chloride (CaCl2), of 7 mm diameter, and 93% purity; and (c) limestone filler. The aggregate used was limestone with continuous gradation and maximum size of 2 mm. In order to optimise the asphalt mixture made with pellets, three variables were investigated (see Table 1 below for a summary): 1. Type of protective shell. As explained before, four types of pellets were produced: (a) with no protective shell, (b) covered in mineral filler, (c) encapsulated into an alginate-calcium chloride shell and (d) encapsulated into an alginate-calcium chloride shell and then covered in mineral filler. All these pellets were manufactured with 50% bitumen and 50% steel content.

Fig. 1. Schematic pothole repairing process by induction-heating binder pellets.

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Table 1 Composition of asphalt mixtures made with binder pellets. Mix number

Pellets Content

Aggregates

Bitumen/Steel Wool Content in Pellets

Type of Shell

1 2 3 4 6 7 8 9 10 11 12 13

5% 10% 15% 20% 15%

95% 90% 85% 80% 85%

50%/50%

Alginate + Calcium Chloride + Limestone Filler

Alginate + Calcium Chloride + Limestone Filler

15%

85%

35%/65% 50%/50% 65%/35% 80%/20% 50%/50%

2. Amount of steel in the pellets. Four asphalt mixtures were manufactured by fixing the pellets content at 15% but changing the steel content in the pellets (20%, 35%, 50% and 65%). 3. Amount of pellets in the mix. Four asphalt samples were manufactured by fixing the type of pellets (encapsulated into an alginate-calcium chloride shell and covered in mineral filler) and their steel content (50%), while changing the amount of pellets in the mix (5%, 10%, 15% and 20%). Results were compared to different types of control mixes, see Table 2: 1. The mix named HMA2 consisted of HMA made with the same materials as the pellets (limestone aggregates with maximum size 2 mm and bitumen pen 40/60) but without the pelletising process. Hence, the effect of pelletising the asphalt could be assessed. The bitumen content was fixed at 8% of total weight, which corresponds with the mix (No.3) made with 15% of pellets (50% bitumen and 50% steel content). 2. The mix named HMA10 was designed as an HMA with maximum size 10 mm that could be commonly found in roads. The binder content was fixed in this case at 5% by weight. This mix allows the comparison of the proposed material with the original material of a road recently repaved. 3. A commercial CMA, composed of granite aggregates, with dense gradation and maximum size 6 mm and bitumen emulsion with 5% of residual bitumen was also compared. This material is widely used to repair potholes in UK and represents a low-cost and lowenvironmental impact alternative.

No Shell Filler Alginate + Calcium Chloride Alginate + Calcium Chloride + Limestone Filler

3.2. Manufacturing of binder pellets A summary of the steps carried out to manufacture the pellets can be seen in Fig. 2. First, the core of binder pellets was manufactured by mixing bitumen 40/60 pen and steel wool in four different contents (65%, 50%, 35% and 20% by weight of total mix). The blending was carried out at 170 °C for 2 min, time enough to ensure homogeneous mixing. Then, the loose mix was spread in a tray and cooled down at −5 °C to facilitate its removal from the tray. The non-compacted layer was then broken by hand and by using a hammer into small irregular pieces with different sizes, which formed the core of the pellets. These were stored in plastic containers at 3 °C to maintain the properties of bitumen and avoid them sticking to each other. In order to reduce the adhesion between pellets at ambient temperature, the first modification was to sieve the cores with limestone filler, which produced a cover of dry material. A second method consisted of encapsulating the pellets into a protective shell. For that, 500 ml of tap water and 15 g of alginate were introduced into a 1-l glass container. Both components were mixed by using a laboratory gear drive mixer at 400 rpm for 10 min, which created a homogenous emulsion, where the alginate acts as the emulsifier. Simultaneously, a calcium chloride solution was prepared by mixing 500 ml of tap water with 15 g of calcium chloride (CaCl2) in another 1-l glass container. The cores were introduced first into the alginate emulsion and then into the calcium chloride solution, producing the ionotropic gelation of the alginate over all the surface of the pellets, as follows [33]: ww

To simulate the interior surface of the pothole, an asphalt mixture for base courses, AC 20 base 40/60 (EN 13108-1), was selected for this study. In tests, such as tensile strength or shear strength, blocks of the material made with pellets were adhered to blocks made with this. Limestone aggregates (Tunstead, UK) with maximum size 20 mm and bitumen 40/60 pen grade were used to fabricate the asphalt blocks and slabs with 5% bitumen content.

Table 2 Composition of different asphalt mixtures. Sample

Binder pellets

HMA2

HMA10

0/0.063 mm 0.063/0.125 mm 0.125/0.5 mm 0.5/2 mm 2/4 mm 4/6.3 mm 6.3/10 mm Binder content a

27.1 33.5 52.2 100 100 100 100 7.50%

27.1 33.5 52.2 100 100 100 100 8.00%

– – – – 50 80 100 5.00%

a

By mass of total mix.

COO‐þ Na þ

ww

Ca2þ

COO‐þ Na → ww COO– ‐Ca2þ ‐‐‐ OOCww þ 2Naþ

Finally, they were introduced into an electrical drier at 35 °C for 6 h to remove the moisture and produce the hardening of the polymeric protective shell. One last type of pellet was produced by a combination of previous methods. Hence, the cores were first encapsulated into an alginatecalcium chloride polymeric shell and then sieved with limestone filler to cover the shell with an extra layer of mineral powder before drying in the electrical drier. This type of pellet can be seen in Fig. 3.

3.3. Preparation of test specimens For the tests of tensile strength, shear strength and resistance to permanent deformations, the samples were composed of two layers. The top layer was made of asphalt with the repairing material (binder pellets, HMA or CMA). The bottom layer represents the interior surface of the pothole and was manufactured with HMA, type AC 20 base 40/60 (EN 13108-1).


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Fig. 2. Steps for pellets manufacturing.

3.3.1. Manufacture of bottom layer with HMA The asphalt blocks with dimensions of 100x100x50 mm3 were obtained by cutting, with a radial saw blade, 305x305x50 mm3 asphalt slabs compacted at 140 °C by roller compactor with target air

void content of 5%, in compliance with the Standard BSI 13108– 1:2006. The test samples of asphalt made with pellets were prepared as explained above, by mixing pellets with aggregates into the induction-

Fig. 3. Images of pellets: (a) pellets with alginate-calcium chloride and filler shell; (b) internal structure of pellets obtain by CT-Scan.

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terms of heating, mixing and compaction, with the exception that, this time, cylindrical samples of 100 mm diameter and 200 mm height were used.

Fig. 4. Basic equipment used to heat and mix binder pellets with aggregates at ambient temperature.

3.3.3. Manufacture of top layer with HMA2, HMA10 and CMA For comparison of the performance of asphalt pellets with conventional repairing materials, test samples with the same dimensions as those in the previous sections were manufactured with HMA2, HMA10 and CMA layers. Test samples containing HMA2 and HMA10 layers were produced by pre-heating bitumen and aggregates with maximum size 2 mm and 10 mm at 170 °C and mixing them for 3 min at 170 °C. Then, the materials were placed on top of an asphalt block or slab and vibratorycompacted for 1 min, following the same procedure described before for test samples containing pellets. To produce CMA layers, 955 g of this material were added on top of an asphalt block or slab and compacted using the vibratory method for 1 min obtaining samples with the same dimensions as in the previous cases at ambient temperature. Finally, test samples containing CMA were cured for 7 and 28 days at room temperature (20 ± 2 °C), respectively. 3.4. Description of experimental tests

heating crucible especially designed for this investigation (Fig. 4). Pellets and aggregates were introduced in consecutive pairs of layers. Every time a new layer of pellets and a new layer of aggregates was added, 1 min of induction heating and mixing was applied. Hence, the total duration of the mixing was approximately 5 min. Then, the mixture was introduced into a prismatic steel mould with inner dimensions 100x100x100 mm3. The material was compacted with a vibrating hammer Humboldt H-4115.3 for 1 min until it reached a thickness of 35 mm. Finally, the specimen was left to cool down at room temperature to simulate road conditions. These test samples were used for tensile and shear strength tests.

2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0

3.4.2. Volumetric properties The air voids of the studied materials were evaluated by using the methodology based on the calculation of the maximum and bulk densities proposed by BS EN12697-5:2009 and BS EN12697-6:2012

7 6 Air Voids (%)

Indirect tensile strength (MPa)

3.3.2. Preparation of top layer with binder pellets For the Hamburg wheel-tracking test, 300 × 300 × 65 mm3 asphalt slabs were introduced into a concrete prismatic mould with inner dimensions 310 × 310 × 100 mm3. Then the mould was filled with the mixture of binder pellets and aggregates until the height of the slabs was 100 mm, and the same procedure, detailed above, in terms of heating, mixing and compacting was followed. In order to assess the cohesion of a layer made of pellets with different shells, cylindrical samples were tested under indirect tensile stress conditions. These samples, with a diameter of 100 mm and a thickness of 40 mm were produced in the same way as described above in

3.4.1. Storability test This test was performed to study the type of cover for the pellets which produces the best storage-stability by reducing the adhesion between them for long storage periods at ambient temperature. Four types of pellet were tested, all of them containing 50% bitumen and 50% steel wool: (1) pellet cores without cover; (2) pellets covered in limestone filler; (3) pellets encapsulated into alginate-calcium chloride polymer shell and (4) pellets encapsulated into alginate-calcium chloride polymer shell and then covered in limestone filler. For the test, hollow plastic cylinders of 150 mm diameter and 120 mm height were vertically filled with pellets and a steel plate of 7.5 kg was placed on top. This weight represents approximately the weight of a column of 1 m of pellets on top of the tested samples. Then the containers were placed into a cabinet at 20 °C for 7 days. After this, the steel plate and the cylinder were removed. The distance between the original height and the height of the sample after removing the cylinder was measured and compared.

5 4 3 2 1 0

No shell

Filler

(a)

Alginate Alginate + Calcium + Calcium Chloride Chloride + Filler

No shell

Filler

Alginate Alginate + Calcium + Calcium Chloride Chloride + Filler

(b)

Fig. 5. The relation of different shells for pelletised asphalt with (a) indirect tensile strength and (b) air voids.


63

Fig. 6. Results of storage-stability test obtained by pellets with different protective shells.

respectively. Hence, the air voids content according to BS EN12697– 8:2003 was calculated as: Va ¼

ρ 1− b 100 ρm

3.4.4. Tensile bond test (TBT) The adhesion between pellet layers and the bottom asphalt layers was measured according to Standard BS EN 12697-48:2013 by using an INSTRON hydraulic press. The top and bottom surfaces of the specimens were bonded with epoxy glue (Araldite) to steel plates and the set was introduced into a temperature-controlled cabinet for 24 h at 20 °C. During the pull-off test, a tensile load was applied from the plates at a deformation rate of 20 mm/min until the bonding between both layers failed. The tensile strength was calculated as the ultimate tensile load resisted by the sample (N) divided by its cross-sectional area.

ð1Þ

where ρb is the bulk density in the mixture measured in kg/m3, ρm is the theoretical maximum density of the mixture without voids measured in kg/m3, and Va is the air voids content in the asphalt pellets mixture measured in %. 3.4.3. Indirect tensile test (ITS) Three cylindrical asphalt specimens made of each type of pellets were conditioned at 20 °C before tested. The samples had a diameter of 100 mm and a thickness of 40 mm. The ITS test involved applying diametric compression with a constant deformation rate of 50 ± 2 mm/min to the samples between two loading strips, which creates tensile stresses along the vertical diametric plane causing a splitting failure. The test was conducted using an INSTRON hydraulic press in accordance with Standard BS EN 12697-23:2017. After testing them, the indirect tensile strength of each sample could be calculated by using the following equation: ITS ¼

2P πDH

3.4.5. Shear bond test (SBT) The shear strength of the interface between layers was measured by the same INSTRON hydraulic press and a shearing rig specially designed for the dimension of the test samples used in the present research. The samples were also pre-conditioned at 20 °C for 24 h. During the test, a shear load was applied by moving top and bottom layers in opposite directions at a constant rate of 20 mm/min until the sample failed. The shear strength was calculated as the ratio between the maximum load (N) and the cross-sectional area of the interface. 3.4.6. Simulation of traffic loads Traffic was simulated by using a Hamburg wheel tracking device, according to Standard AASHTO T 324-04, with the samples immersed in water at 25 °C. The dynamic load was applied by passing 20,000 cycles of a 47 mm width wheel with vertical load of 705 N. Every case was repeated on two specimens, obtaining the final results as their average value. The parameters analysed in this research were the accumulated deformation of the centre of the specimens at the end of the test.

ð2Þ

180

180

160

160

Heating Temperature (ºC)

Heating Temperature (ºC)

where ITS is the indirect tensile strength (MPa), P is the total applied vertical load at failure in Newton, D is the diameter of specimen in millimetres (mm) and H is the height of specimen in millimetres (mm). The tests were repeated on three samples per case, obtaining the final result as the average of them.

140 120 100 80 60

65%Steel 50%Steel 35%Steel 20%Steel

40 20

140 120 100 80 60

20% pellets 15% pellets 10% pellets 5% pellets

40 20

0

0 0

1

2

3

4

Heating Time (min) (a)

5

6

0

1

2

3

4

5

6

Heating Time (min) (b)

Fig. 7. Maximum temperatures of pelletised asphalt mix depending on (a) the content of steel wool and (b) the content of pellets.

64


25

35

20

25

Air Voids (%)

Air Voids (%)

30

20 15 10

15 10 5

5

0

0 0%

0%

5% 10% 15% 20% 25% Pellets Content (a)

3% 6% 9% 12% 15% Bitumen Content in Mix (b)

Fig. 8. Illustration of air void contents of different (a) pellets content and (b) different bitumen content in binder pellets mixtures.

4. Results 4.1. Type of shell

0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00

4.2. Heating potential of binder pellets Fig. 7 shows the heating temperature reached by the mix of pellets and aggregates during the blending process in the crucible and under the action of the electromagnetic induction. As explained above, the test was carried out on mixes: (1) containing a fixed amount of pellets but these containing different amounts of steel (Fig. 7 (a)), and (2) mixes containing the same type of pellets but in different amounts (Fig. 7 (b)). In both cases, it can be seen that the correlation between heating time and temperature is quasi-exponential where the curvature reduces when the steel or pellets contents increase. Thus, the curvature is more noticeable in the samples, whose heating capacity is lower (mixes with a low amount of pellets or containing pellets with a low steel content). For the mixes with the greatest heating potential, the resulting curves were practically linear. In addition, it can be also seen that, for any given time, the temperature of the mix clearly increases with the metal content. Hence, samples made with pellets containing 20% steel, reached 41 °C and 145 °C

Tensile Strength (MPa)

Tensile Strength (MPa)

Fig. 5 (a) shows the indirect tensile strength of cylindrical samples of asphalt made of pellets (50% bitumen/50% steel wool) without shell and with the different covers explained above. From the results of this figure, it can be observed that the type of shell does not affect significantly the mechanical properties of the compacted material. Hence, although the maximum strength (1.615 MPa) was reached by pellets covered with alginate-calcium chloride polymer and filler, the truth is that the strength of pellets without any cover was 1.582 MPa, only 2% lower. On the other hand, Fig. 5 (b) shows that the use of alginate-calcium chloride shells increases the air voids content, from values of 3.35% and 3.32% to 6.26% and 5.87%, which is a significant difference. The evaporation of water from the alginate-calcium chloride shell, leaving empty voids, might be the main reason behind this behaviour. Although as seen before, the type of shell does not affect significantly the strength of the resulting material, in Fig. 6 it can be seen that it is a key factor directly related to the storability of the pellets. Pellets without any protection and pellets just covered in limestone filler formed a consolidated block during the 7 days of the test at 20 °C inside the cylinder. Hence, when this was removed, the shape of the set remained practically unaltered. On the contrary, when an alginate-calcium chloride protective shell was added, a significant amount of pellets crawled down, leaving the top surface about 30 mm lower than before removing

the cylinder (25% of total height). Finally, when both alginate-calcium chloride shell and filler were used, the collapse of the sample was practically complete, with a height loss of 100 mm. This indicates that this is the most suitable method by far to ensure the store-stability of the material at ambient temperature, although as seen above, it will tend to increase the air voids content and likely the costs.

0%

5%

10% 15% 20% 25% Pellets Content (a)

0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 0%

3%

6%

9%

12%

15%

Bitumen Content in Mix (b)

Fig. 9. Tensile strength of samples with (a) different pellets content, and (b) with pellets containing different bitumen contents.


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of mix, produced very dense mixes with 0.22% air voids content. On the other hand, mixes containing 5.25% bitumen and 9.75% steel produced porous mixes with 18.37% air voids. For these reasons, it can be concluded that air voids content can be decreased by increasing the amount of pellets and, as a consequence, the mixing and compaction temperature. However, more important than that, is to increase the amount of bitumen in the mix, which can be produced by either increasing the amount of pellets or by increasing the bitumen content in these. 4.4. Tensile strength Fig. 10. Pictures of samples were made with different pellets.

after 1 min and 5 min respectively. On the other hand, mixes made with pellets containing 65% steel, reached, for the same times, temperatures of 62 °C and 168 °C. In average terms, the fact of increasing the steel content, from 20% to 35%, 50% and 65% produced increases of the temperature of 8.1%, 19.2% and 32.4%. In an analogous analysis, it can be seen how the fact of increasing the amount of pellets from 5% to 10%, 15% and 20%, produced increases in the average temperature of the mix of 13.5%, 28.3% and 40.4%. Both experiments show that in order to increase the heating potential of the mix, it is fundamental to increase the steel content. This can be obtained through any of the previous methods; that is, either increasing the steel content within the pellets to be used, or by adding more pellets to the mix. 4.3. Volumetric properties of binder pellets samples

4.5. Shear strength Analogously to the previous case, Fig. 11 (a) shows the results of shear strength of asphalt samples, which were manufactured either with a fixed type of pellet (50% bitumen/50% steel) but different

1.4

1.4

1.2

1.2 Shear Strength (MPa)

Shear Strength (MPa)

Fig. 8 (a) shows the air void content of asphalt samples depending on their pellets content. In general, the air void content reduced with increasing amounts of the pellets, from 26.8% with 5% of pellets content to 3.4% with 20% of pellets. This is due to: (1) an increase in the bitumen content which can fill the pores, and (2) a higher mixing and compacting temperature (explained in Fig. 7b), which reduces the viscosity of the binder allowing it to flow better and reach higher compaction levels. In addition, Fig. 8 (b) shows the results of samples, all containing the same amount of pellets, but changing the bitumen content of these. It can be seen that by using pellets richer in bitumen, the air voids content also decreases very sharply despite, in this case, lower steel contents being used and, as a consequence, also lower mixing temperatures. Hence, mixes made with 12% of bitumen and 3% of steel by total weight

Fig. 9 (a) shows the results of the tensile test obtained by samples containing different amounts of pellets. For this test, all the pellets used were composed of 50% bitumen and 50% steel wool. It can be observed that the tensile strength increased sharply when the amount of pellets was higher than 15%, which can be translated into 7.5% of bitumen and 7.5% of steel in total mix. With lower contents, the material did not have the consistency necessary to actually produce significant values. However, after these critical contents, the strength reached values around 0.35–0.40 MPa. Fig. 9 (b) shows the results of samples, in which the content of pellets was fixed to 15% of the total mix but pellets with different bitumen/steel contents were used. Again, it can be seen that very similar tensile strength values, around 0.35–0.40 MPa, were obtained once the critical value of 7.55% bitumen content was reached. Fig. 10 shows the picture of two samples made with pellets 50% bitumen–50% steel, and 35% bitumen–65% steel. In the first case, (a), the total bitumen content in the mix was 7.5%, while in the second, (b), the bitumen content was 5.25%. As can be seen, the cohesion of the second sample, whose bitumen content is lower than the critical value of 7.5%, is clearly poor, the low test results (0.006 MPa against 0.39 MPa) not being surprising. Taking previous considerations into account, it can be concluded that in order to optimise the asphalt mix from a mechanical and economic point of view, the content of asphalt pellets, as well as the bitumen content of these, must be the minimum possible, as long as the critical bitumen content, which ensures the cohesion of the material, is exceeded (in this case 7.5% bitumen content).

1 0.8 0.6 0.4

1 0.8 0.6 0.4 0.2

0.2

0

0 0%

5%

10% 15% 20% 25% Pellets Content (a)

0%

3%

6%

9%

12% 15%

Bitumen Content in Mix (b)

Fig. 11. Shear strength of asphalt mixes containing (a) different amounts of pellets and (b) different bitumen content in binder pellets.

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H. Obaidi et al. / Powder Technology 344 (2019) 58–67 1.2

Normalised Strength

1.0

Pellets 0.85

Tensile Shear

0.8 0.6 0.4

0.35 0.22

0.2

0.12

0.07

0.01

0.09

0.03

0.0 HMA2

HMA10

Commercial Commercial CMA (7 days) CMA (28 days)

Fig. 12. Normalised strength ratio of the samples made of different materials.

amounts of pellets, or with the same amount of pellets (15% of total mix) but containing these different amounts of bitumen. As can be seen, the results are completely in line with those extracted from tensile test. Hence, the maximum strength (1.16 MPa) was obtained again for a pellets content in the mix of 15%, which corresponds to a bitumen content of 7.5% and a steel wool content of 7.5% by weight of total mix. In Fig. 11 (b), it is confirmed again that mixes with bitumen content lower than 7.5% did not have the cohesion necessary to produce any significant results. In addition, after this point, increases in bitumen content and, consequently, reductions in steel content and mixing temperature, produced reductions in strength. For these reasons, the asphalt mix can be optimised by minimising the amount of pellets and bitumen in the mix, as long as a minimum value, which ensures the cohesion of the material is exceeded (in this case 7.5% bitumen content).

4.6. Comparison between the mechanical performance of pelletised asphalt mixes and current technologies Fig. 12 shows a comparative analysis between the mechanical performance of asphalt made with pellets and the control mixes of HMA2 (8% binder content, 2 mm aggregates maximum size), HMA10 (5% binder content, 10 mm aggregate maximum size), and a commercial CMA (5% residual bitumen content and 6 mm aggregate maximum size) after 7, and after 28, curing days. In the figure, the results of tensile and shear stress obtained by these mixes were divided into the results obtained by pellets samples. Hence, if the result is higher than one, it means that the material is stronger than asphalt made of pellets, while if it is lower than one, then the performance of the material is worse. In general, it can be seen from this figure that all control mixes had lower strength (tensile and shear) than the strength of binder pellets materials. The tensile and shear strengths of HMA2 were 7 and 1.6 times stronger than HMA10, and 9.4 and 11.6 times stronger than CMA even cured for 28 days. This was expected, as HMA2 had the lowest air voids content (6.1% against 19.46% of HMA10). Although the

Normalised Rutting Deformation

50

46.89

45

40.62

40 35 30 25 20 15 10 5

1.08

1.22

Pellets

HMA2

HMA10

Commercial Commercial CMA (1 days) CMA (7 days)

0

Fig. 13. Normalised rutting ratio of the samples made of different materials.

strength of CMA increased over the curing time, results were never comparable to those obtained by binder pellets. Finally, the wheel tracking test was carried out on slabs composed of a bottom layer of conventional HMA and a top layer of the previous repairing materials. The objective of this experiment was simulating the performance of a repaired pothole, which was filled by different materials under the action of traffic loads. For the case of HMA10, results could be also understood as the performance of a new road, right after its construction. Fig. 13 shows the rutting behaviour registered at the midpoint of the slabs after 20,000 loading cycles. Again, in order to facilitate the comparison, the results were divided by the results obtained by pellets. Unlike the previous case, now when the values are higher than 1, it means that the rutting was higher and, as a consequence, the mechanical behaviour was poorer. The final deformation obtained by asphalt samples made of pellets was 0.37 mm. As can be seen in Fig. 13, the deformation obtained by CMA after 7 curing days (17.35 mm) and even after 28 curing days (15.03 mm) was significantly higher. In addition, the results obtained by HMA2 and HMA10 were practically the same, or even slightly higher, which means that pelletised asphalt mixes can provide as good, or even better, rutting resistance, not only than other conventional repairing materials but also than a recently repaved road. 5. Conclusions In the present experimental investigation, a pelletised bitumen containing steel wool particles that can be stored and transported at ambient temperature as a granular material, and heated and mixed on-site by electromagnetic induction, was proposed as a new technology for repairing potholes in asphalt roads. The mechanical performance of this material was studied through different tests and compared to conventional materials commonly used in pothole reparation. From this study, the following conclusions could be extracted: 1. Pellets encapsulated within an alginate-calcium chloride polymer shell and covered in limestone filler were the best option to prevent their agglomeration and, consequently, increase their storability at ambient temperature, without reducing their mechanical performance. However, they also tended to produce higher air voids and costs. 2. The heating potential of the material being mixed in the crucible (pellets and aggregates) is directly correlated with the amount of added steel. Hence, in order to increase it, it is necessary either to increase the steel content within the pellets or increase the added pellets to the mix. 3. The air voids content of asphalt can be reduced by either increasing the bitumen content of the pellets or by increasing the amount of pellets in the mix. In the second case, not only the amount of bitumen is increased, but also the amount of steel, which produces higher mixing temperatures and, consequently, lower binder viscosity and higher compaction levels. By changing these parameters, asphalt samples with air voids content from 0.22% to 26.8% were obtained. 4. The tensile and shear strength of the binder pellets increased sharply, from values close to zero to values around 0.35–0.40 MPa, when a critical bitumen content (7.5%) was exceeded. For higher contents, the fact of using pellets with higher bitumen content tends to reduce the strength, as less steel is added to the mix. Consequently, also the mixing temperature is reduced. 5. Samples manufactured with 15% of pellets content (pellets containing 50% bitumen and 50% steel wool) produced a good mixing temperature (higher than 135 °C after 4 min), and low air voids content in the mix after compaction (18%). In addition, this mixture produced better mechanical performance, not only than other conventional repairing materials (HMA and CMA) but also than a road, which was recently repaved.


6. These samples also performed practically at the same level as samples made with the same bitumen and aggregates but heated and mixed by conventional methods (conventional HMA). Hence, the induction heating approach proposed in this investigation does not produce any detriment on the performance of the resulting material, compared to traditional methods. Based on previous conclusions, it can be summarised that the binder pellets are a highly-versatile alternative to conventional road repairing materials, as they allow the production of a different mix in every section they are used. In addition, they showed excellent durability and performance, higher than a road repaired by cold mix asphalt and even similar to the performance of a road recently repaved. Moreover, they reduce the production of waste materials and health and safety issues, by avoiding the handling hot materials and emission of noxious fumes. The main disadvantage is an expected cost, higher than conventional materials, due to the addition of metal particles and other constituents of the protective shell. Finally, the need of a mobile mixer with induction heating, involves further investment and training for the using companies. Acknowledgements The authors would like to acknowledge the financial support of the Higher Committee of Education Development in Iraq for the PhD scholarship of the first author and the EPSRC project EP/M014134/1, Induction heating for closing cracks in asphalt concrete. References [1] T. Willway, L. Baldachin, S. Reeves, M. Harding, The Effects of Climate Change on Highway Pavements and How to Minimise Them: Technical Report, vol. 1, 2008 1–111. [2] B. Sutradhar, Evaluation of Bond between Bituminous Pavement Layers, Doctoral Dissertation 2012. [3] L. Poulikakos, R. Gubler, M. Partl, M. Pittet, L. Arnaud, A. Junod, A. Dumont, E. Simond, Mechanical Properties of Porous Asphalt, Recommendations for Standardization: Technical Report, 2006 (No. LAVOC-REPORT-2008-020). [4] Z. Li, S. Madanu, Highway project level life-cycle benefit/cost analysis under certainty, risk, and uncertainty: methodology with case study, J. Transp. Eng. 135 (2009) 516–526. [5] S. Burningham, N. Stankevich, Why Road Maintenance is Important and How to Get it Done, The World Bank, Washington DC, 2005 Transport Note. (No. TRN-4). [6] G. Airey, Y.-K. Choi, State of the art report on moisture sensitivity test methods for bituminous pavement materials, Road Mater. Pavement Des. 3 (2002) 355–372. [7] U. Sharmaa, A. Kanoungob, Study of causes of potholes on bituminous roads – a case study, J. Civ. Eng. Environ. Technol. (JCEET) 2 (4) (2015) 345–349 New Delhi, (AprilJune 2015). [8] J. Santos, G. Flintsch, A. Ferreira, Environmental and economic assessment of pavement construction and management practices for enhancing pavement sustainability. Resour. Conserv. Recycl., 116, 15–31. Washington DC (2017) 10–16.

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