The chapter delves into the significant problem of potholes in roadways and the need for more durable and cost-effective repair solutions. It introduces polymer concrete as a promising alternative to traditional cold-mix asphalt materials. The study focuses on the design and assessment of various polymer concrete mixes using recycled materials such as unrefined polypropylene shards, rubber crumbs, and crusher-sand and -stone. The mixes are tested for compressive strength, flexural strength, and abrasion resistance, with results indicating that polymer concrete significantly outperforms traditional materials in terms of durability and lifespan. The chapter concludes by emphasizing the potential cost savings and reduced frequency of repairs when using polymer concrete, making it a compelling option for rapid pothole repair.
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Abstract
The paper describes an investigating into the application of a polymer concrete using a recyclable material such as polypropylene and chopped tire rubber as aggregate material to provide a durable and cost-effective alternative to the existing cold mix asphalt (CMA) pothole repair material. Three different aggregate mix designs, including traditional crusher- sand and -stone, rubber crumbs, and plastic chips, in combination with Vinyl Ester, Polyester, Polyurethane and Furan resin were combined to create 12 possible polymer concrete mixes to be used as a rapid pothole repair material. These polymer concrete mix designs were tested for common properties and characteristics typically experienced on road surfaces. These include characteristics such as compressive strength, flexural strength, and abrasion resistance. All results obtained from the various tests performed indicated that the polymer concrete mix designs exhibit enhanced physical properties compared with CMA and would thereby increase the durability and lifespan of repaired potholes when using this material.
1 Introduction
Roadways are an extremely important and expensive part of a country’s infrastructure. In 2019, the South African budget estimate for national expenditure allowed for a total of 900 000 km2 of blacktop patching to repair potholes. In 2015, an estimated area of 1 497 281 km2 was done [3]. Potholes in roadways are a worldwide obstacle although the most common method of repair offers a short-term solution [1].
Potholes are predominantly caused by the infiltration and erosion of water through cracks in the asphalt layer. Cold-mix asphalt, a bituminous mixture containing medium to fine aggregates, is typically the most common material used for repairing potholes. Cold patches typically have a one-year lifespan before a more permanent method such as re-layering or resurfacing can be implemented. Polymer concrete, however, could serve as a viable replacement for cold-patch material [1, 2].
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The use of polymer concrete to repair potholes in asphalt surfaces was a concept developed to rapidly repair runways of bomb craters [4]. The need arose for a rapidly curing material that could easily be installed using a small crew and little to no specialized equipment. To date, advancements have been made to repair potholes using polymer concrete, although the concept has not widely been accepted [5].
This study deals with the design and assessment of various polymer concrete (PC) mixes using recycled material such as unrefined polypropylene shards, rubber crumbs refined from tires and crusher-sand and -stone. The three different aggregate mixes include; recycled plastic, rubber crumbs, and crusher-sand and -stone. The polymer binders used in conjunction with the three different aggregates mixes include: Vinyl Ester, Polyester, Polyurethane, and Furan resin. The various PC mix designs were all tested for the properties and characteristics typically required for use on road surfaces, and which may affect the road surface durability and service life of the repair. These include characteristics such as compressive strength, flexural strength, and abrasion resistance. Each of the PC mix designs were tested and compared to a cold mix asphalt material typically used for pothole repair to compare the suitability of the PC mix as a rapid pothole repair material.
2 Material and Methods
2.1 Materials
The selection of the polymer binder was primarily based on the expected physical characteristics, such as compressive and flexural strength, of the composite material created using the polymer as binder. The polymer’s UV resistance was the second criteria in the selection process because of the prolonged UV exposure that the repair material would experience. Polyester (PE), Vinyl Ester (VE), Polyurethane (PU), and Furan resin (FU) were selected for evaluation as binders. These resins were also considered suitable for polymer concrete pothole repair material due to the unit cost and availability of local suppliers in Johannesburg. The polyurethane resin obtained contained a bitumen emulsion used to improve flexibility and adhesion with in-situ bitumen material.
The aggregates were selected based on cost-effectiveness and individual physical properties that may aid in creating a more durable composite material. The following material was selected:
Rubber crumbs (0.2–2.00 mm) recycled from used motor vehicle tyres
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The more traditional aggregates (crusher-sand and -stone) were selected; to reduce the cost of the final repair material, wide availability, and its proven record of providing superior mechanical properties to the composite material. Recycled materials such as the plastic shards and rubber crumbs were used to potentially impart greater flexibility and a higher resistance to impact loading cracking onto the composite material.
2.2 Specimen Preparation Procedures
Various specimen sizes were required for the different testing procedures although each of test specimens manufactured were compacted in a similar method using an electrical rotary hammer as a compactor. This procedure was used to proportionally simulate the on-site installation of pothole repair material using either a plate or roller compactor. The rotary hammer drill, set to the hammer setting, was applied to each layer with a downward force of 0.6 kN for a duration of 30 s using a 45 × 45 × 10 mm square high tensile steel plate. The material was compacted twice for each layer. All moulds were coated with a release agent to decrease adhesion of the material to the mould.
2.3 Optimal Mix Design
The optimal mix design was derived from a trial-and-error method whereby small specimens were created and evaluated based on void content, surface texture, and proportions of aggregate to polymer ratios. Various factors such as workability, void content, cost-effectiveness, and ease of use when mixing and installing were taken into consideration when examining the small specimens created. From the observations made, the mix design was adjusted accordingly, and different aggregates were tested to observe the behaviour of the final composite material. All tests conducted were compared to a CMA baseline specimen that was prepared similar to the polymer concrete specimens.
2.4 Compressive Strength
Compressive strength tests were conducted following the SANS 5863: 2006 standard for compressive strength testing of hardened concrete using 100 mm cubes. The cube moulds were filled in three layers and compacted as per the SANS 5863: 2006 standard. The specimens were removed from the moulds 24 h after casting and were then stored in a climate-controlled room at a temperature of 25 ℃ for 5 days prior to testing. The compressive strength of a 15 MPa Portland Cement concrete mix design and CMA specimens were also determined to be used as a baseline for comparison. The concrete specimens were prepared in accordance with SANS 5861–3:2006 and cubes were also cured at 25 ℃ and submerged in water, every day for a period of two hours, to simulate precipitation or dew conditions. The compressive strength testing apparatus applied a constant load of 0.25 kN/second and was set to cease further loading once the applied load had decreased more than 15% of the peak applied load. The maximum applied loading was recorded, and photographic evidence of the tested cubes was taken to evaluate how the specimen failed.
2.5 Flexural Strength
The flexural strength of the polymer concrete was determined following the SANS 5864 code on flexural strength testing for hardened concrete. The beams were cast using 40x40x160 mm moulds that were placed and compacted in three separate layers. A simple three-point, centre loading test was conducted to determine the flexural strength following the standard mentioned. The machine increased the load applied from the centre point onto the beam at a constant rate of 3.03 kN/min and recorded the maximum load once the beam had failed either in excessive yielding or total beam failure as per the SANS 5864 code requirement.
2.6 Abrasion Resistance
As no applicable asphalt abrasion resistance standards or procedures were found in the literature, a comparative test was developed to investigate the abrasion resistance of the polymer repair material. A towable sledge was developed to facilitate the exposure of several 100 mm cube specimens to a constant wearing process by being dragged over an asphalt surface of 0.61 mm texture depth. A constant load of 3kg were applied vertically to each of the specimens during the process. The sledge with attached specimens were dragged at a constant speed of 5–7 km/h over 400 m intervals whereafter each specimen was cleaned of any loose debris and the mass loss due to the abrasive wear was recorded.
3 Results
3.1 Optimal Mix Design
Using the crusher stone and crusher sand as common base aggregate, three different PC mix designs were selected via a trial-and-error method based on its unique physical properties and proportions. These three PC mix designs are shown in Table 1. The three different aggregate mix designs (N - Normal, P - Plastic and R - Rubber) were tested in more detail using the four different polymers as binders selected to evaluate each specific mix design for the suitability for use as a rapid road repair material. As can be seen from Table 1, the trial-and-error method led to a final, optimal mix design for each specific aggregate. Using only the plastic and rubber aggregates, respectively, a mix design with acceptable mechanical properties was not achieved. Therefore, crusher-stone and -sand were added to the plastic and rubber aggregate mix designs to provide additional mechanical strength. It was hypothesized that both the recycled aggregates will impart characteristics such as flexibility to the final composite material. Unrefined polypropylene shards, as aggregate, were selected due to its increased adhesion properties and its reduced cost compared to refined polypropylene pellets. It should be noted that the increased polymer binder content of the “R” design was required due to the rubber crumb aggregate consisting of small particles thus increasing the surface area compared to the larger plastic particles. Table 2 provides illustrates the properties of the polymers used as binders.
Table 1:
Optimal aggregate mix designs selected by a trial-and-error method.
*Gel time dependant on type and volume of catalyst used.
The optimal mix designs were determined by systematically increasing the fines content of the mixes to decrease the overall void content resulting in minimizing the polymer binder content requirement to decrease the overall costs of the mix designs.
3.2 Compressive Strength
Five 100 mm cube specimens of each mix design were randomly selected from the specimen batch for testing 5 days after casting and curing in a climate-controlled room at 25 ℃. Figure 1 indicates the average compressive strengths obtained for each of the various mix designs. Although the South African road authorities, such as SANRAL, do not recommend the use of Portland Cement concrete to repair potholes on asphalt pavements, it is common for communities to fill potholes using Portland Cement concrete in urban roads. As such, the compressive strength determined of a standard 15 MPa Portland Cement concrete mix and standard CMA specimen under the trial testing and curing regime were used as a baseline for comparison.
Fig. 1:
Compressive strength of various mix designs.
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All the PC mixes outperformed the traditional CMA baseline significantly. The increased compressive strength indicates that the PC mix designs have higher compressive resistance to heavy loads experienced on roadways and using the PC mixes could thus benefit the lifespan of pothole repairs compared to the traditional cold patch material.
The difference in compressive strengths between the N and R mix designs decrease by 91.9% for VE and 80.93% for PE respectively. The general decline in compressive strength from the N to P to R mixes, with the sole exception of PU binder, indicate a reduction of compressive resistance caused by the more compressible plastic shard and rubber crumb aggregates. Typically, when a compressive load is applied to a material, the particles within the material distribute the load uniformly throughout the material to minimise the stress-stain on the individual particles. As a result of the lower stiffness of the plastic and rubber aggregates, these particles deform in the direction of the applied load resulting in the surrounding natural aggregate and binder matrix experiencing higher stress-strain loadings. This condition causes a decrease in compressive resistance of the material and hence a lower recorder strength.
A substantial decrease in compressive strength is observed when comparing the PU and FU mixes with VE and PE, due to the rigid 3D structure formed after polymerization of both the VE and PE polymers. The addition of the bitumen emulsion to the Polyurethane leads to an increase in flexibility that resulted in a decrease in rigidity and therefore a reduced compressive strength. It is noteworthy that all the PU resin mix designs, although failing under the compressive load in accordance with standard SANS 5863: 2006, only showed signs of deformation whereas the VEN and PEN specimens failed completely in an hourglass shape.
3.3 Flexural Strength
The flexural strength of a pothole repair material, specifically in terms of a flexible asphalt pavement, is a crucial element as the material needs to withstand cyclic vehicular induced loading without developing flexural failure. An increase in the flexural strength of the repair material will allow the repaired area to withstand loading more effectively, especially if deterioration occurs in the base layer. Figure 2 represent the results obtained of the flexural strength tests performed on the various mix designs.
Fig. 2:
Flexural strength of various mix designs.
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The CMA mix exhibited an average of 0.81MPa with the singular highest value reached being 1.2 MPa. All PC mix designs outperformed the CMA test group significantly with the VE, PE, and FU based mixes exhibiting increased flexural strength results of up to 4950% than that of the CMA mix.
Although the PU specimens recorded substantially lower results compared to other binder, the PU specimens did not break when failing under the applied load but deformed significantly with the largest deformation of 19 mm recorded at the centre of the test beam. In addition, the PU specimens, were re-tested after being rotated 90 degrees. All PU specimens achieved a minimum of four rotations before small surface cracks were observed. When considering the different aggregate mix designs, a decrease in flexural strength is observed for the R mixes compared to that of the P and N mixes, as shown in Fig. 2.
It is notable that specimens containing the plastic aggregate, did not fail by breaking into two sections as the elongated plastic shards served as tensile reinforcement within the aggregate matrix. When studying Fig. 2, the PE and VE mixes outperformed the other binder mixes. The PEN maximum flexural strength could not be measured as the specimen did not fail at the testing machine’s maximum load capacity of 40 MPa. The FU specimens outperformed the CMA baseline mix, and the FUR mix exhibited the largest flexural strength when compared with all the other binder mixes containing rubber as aggregate. In addition, it is noteworthy that the FUR mix exhibited the most uniform flexural strength independent of the aggregate type used in the mix.
3.4 Abrasion Resistance
The abrasion resistance of the various PC mix designs were tested to compare the wearability and possible lifespan of the various mix designs. Material used for road surfacing requires an acceptable level of abrasion resistance caused by regular traffic interaction. Abrasive force is applied when the tire surface exerts forces (horizontal, vertical, acceleration and de-acceleration) upon the road surface during vehicular motion. On roads with inclines or declines, the need for increased traction also increases the abrasive force exerted onto the road. Figure 3 presents the results of the comparative abrasion resistance.
Fig. 3:
Abrasion resistance illustrated by mass loss vs distance dragged over asphalt surface.
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When observing Fig. 3, the difference in abrasion resistance between the various PC mix designs are significant. The mass loss of each specimen was recorded until the 100 mm cube specimens were abraded down to a height of 40 mm.
When observing Fig. 3, the CMA specimen exhibited the lowest abrasion resistance and failed at a dragging distance of 0.8 km at which point the specimens weight decreased by 45.7%. All PC mixes outperformed the traditional CMA baseline mix and thus exhibited higher abrasion resistance. After the specimens were dragged for 4.8 km, all of the specimens excluding the PEN, VEN, PEP and VEP specimens, reached the height of 40 mm and were thus considered failed. When considering the rate of mass loss of the PEN, VEN, PEP, and VEP specimens during the 16.8 km dragging exposure, it is estimated that in order for the respective PC mix designs to reach a similar 45.7% decrease in mass loss, as compared with the CMA mix which reached this value after 0.8 km of dragging, a dragging distance of 224, 93, 70, and 64 km, respectively, needed to be applied. Therefore, an increase of 280, 116, 87 and 80 times the dragging distance when comparing the CMA mix with those of the PEN, VEN, PEP, and VEP mixes respectively. In addition to these findings, no cracks were visible on the exposed surfaces of the PC mix specimens after the extended abrasion exposure.
It was observed that the VER and PER specimens underperformed when compared with their plastic and normal aggregate alternatives. When physically handling the PER specimens, an increase in specimen temperature was observed when compared to the PEN and PEP specimens which were all simultaneously tested. At the 13.6 km dragging interval, the PER specimen had a recorded temperature of 37.4 ℃ on the exposed surface, whereas 32.4 ℃ and 33.1 ℃ were recorded for PEN and PEP respectively. The increase in surface temperature caused by friction forces, softened the rubber aggregates resulting in decreased adhesion between aggregates and the binder in the PER specimen.
Similarly, to the PE and VE specimens, FUN and FUP specimens outperformed their rubber aggregate counterpart. Whilst FUR failed after 2.4 km travelled, FUP and FUN failed after 5.6 and 7.2 km, respectively. The PU mix designs exhibited increased mass loss rates compared to the other specimens and, when examining the exposed surface, was evident that the PU specimens exhibited lower surface rigidity than that of other polymer binder mixes. The exposed surfaces of all PU mixes exhibited clear signs of small material particles being ripped from the cube. All other polymer mixes illustrated a smooth and intact surface during testing.
In terms of abrasion resistance, PEN, VEN, PEP, and VEP outperformed all other mix designs including CMA by significant margins, followed by PER, VER, FUN, FUP, and PUP, respectively.
4 Conclusion
The results obtained during this investigation indicated that polymer concretes manufactured using specific polymeric binders and specific aggregates, can be used as a rapid pothole repair material. The compressive and flexural strength, in addition to the abrasion resistance of these selected polymer concretes, exceeded that of the traditionally used CMA material. Given the mechanical strength tests conducted, the PEN, VEN, PEP, and VEP mix designs outperformed the alternative mix designs. The use of the polyurethane with a bitumen emulsion (PU mixes) as a pothole repair material exhibited a lower durability compared to alternative polymers when exposed to abrasion forces. Since both the traditional CMA mixes and the proposed PC mixes require similar preparation, placement, and compaction methods, only an increased initial material cost is applicable when using the polymer concrete mixes. Using the known typical CMA lifespan and strength characteristics as basis for comparison, the increased durability and lifespan of the PEN, VEN, PEP, and VEP polymer concrete mixes tested, it can be considered that these mixes incur a lower Equivalent Uniform Annual Cost (EUAC) as compared to the CMA material resulting in a more cost-effective pothole repair material. As per Van Zyl et al. (2021) it was shown that the PEN, VEN, PEP, and VEP, may have an EUAC of as low as 8% that of the CMA material [6]. In addition to the cost effectiveness of the repair material, the use of polymer concrete with increased durability will allow road construction workers to repair potholes less frequently before re-layering or resurfacing of the road is required compared to using the traditional CMA material.
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