Per la versione in Italiano: https://www.stradeeautostrade.it/pavimentazioni-e-manti/asfalti-tappabuche-e-green-deal/
Compared to 1990 values, the main EU sustainability targets are to reduce greenhouse gas emissions by at least 55 per cent by 2030 and to achieve “zero emissions” by 2050. With reference to road superstructures, when talking about sustainability, one normally thinks about how to build new asphalt and how to produce new asphalt mixes.
Since safety and service life are key elements of sustainability, it is actually crucial to preserve the existing assets first. Asphalt pavement distresses are becoming increasingly common due to the ageing of pavements and, in particular, the severity of potholes is also increasing due to the environmental conditions related to climate change, especially in the case of heavy rainfalls.
Considering the aggravation of pavement deterioration, potholes are one of the most critical factors, also representing the greatest danger to road users (especially those with two-wheeled vehicles) and the cause of potential damage to vehicles.
The maintenance of potholes is one of the challenges that Managing Authorities have been trying to face for several years, through strategies that include the detection of the state of deterioration and its repair.
Road maintenance works are related to important issues concerning traffic congestion and safety of both road users and site workers. These aspects are the subject of the InfraROB project, funded by the European Commission’s Horizon 2020 research programme, which aims to reduce the exposure of workers to vehicle traffic and the action of construction machinery, including pothole maintenance, and to reduce the cost of repetitive tasks while increasing the safety of road users.
Flexible pavement potholes and traditional rehabilitation methods
The term “pothole” generally indicates a localised deterioration of the road asphalt, in which material breaks down and is lost, causing a large depression of a more or less circular shape.
Depending on the prevailing causes that produce them or determine their evolution (speed and intensity of traffic, type of passing vehicles and environmental conditions), potholes are typically at least 30 mm deep and between 100 mm and 1 m in diameter. The size may also increase abruptly and/or others may emerge in the vicinity soon after the first one has appeared .
In addition to accelerating the degenerative process that leads to the end of the service life of the pavement, potholes can cause wheel damage, bumps and damage to the underside of vehicles, sudden braking and swerving, which can consequently lead to collisions and serious accidents.
Maintenance of potholes generally requires temporary and emergency repairs, and then, in theory, permanent and/or definitive repairs (including resurfacing) in the warmer months. Basically, if a repair is done correctly, one can wait even longer before resurfacing.
The detection and assessment of deterioration is one of the most important actions in determining the correct maintenance strategies for a road infrastructure. Manual detection is time-consuming and expensive from the point of view of the technicians involved.
Therefore, in recent years, a number of systems have been developed to automatically detect and recognise road disruptions (e.g. based on in-vehicle acceleration, 3D reconstruction and photographic imaging), contributing to the improvement of detection efficiency and asphalt quality through preliminary investigation .
There are many manuals indicating the steps to be taken during repairs, for example the one by the Ministry of Infrastructure of Cambodia: Guidelines for Repairing Defects of Roads  is very interesting.
The duration of pothole repair depends substantially on the materials used and the compaction carried out. The most commonly used methods worldwide are as follows :
- Throw-and-Go: the hole is filled with the repair material, if necessary, slightly thickened with a shovel, and then left to be compacted by traffic. Unless special materials are used, the repair is very short-lived and must be resurfaced within a few days;
- Throw-and-Roll: the pothole is filled with repair material and then compacted by the action of a heavy vehicle (e.g. truck). The duration tends to be short;
- Spray injection: special machines are used to spray the material inside the pit. The duration can be quite long or even high. The main problem is the encumbrance of the machine that interferes with traffic;
- Semi-permanent: (Figure 1) debris and water are removed from the hole, a clean cut is made along the sides, bitumen emulsion is sprinkled, the material is filled in and compacted with special tools (vibrating plates, compactors, rollers, etc.). This is the method that allows the repair to last longer;
- Partial reconstruction: a part of the pavement is demolished, down to the damaged layers, and the entire road package is restored. It is the best method, but also the most expensive one.
In return, pothole formation can also be avoided or slowed down through the study of preventive maintenance during the project phase, beacuse it is the most suitable way to extend the service life of an asphalt pavement and to minimize the need for major and expensive repairs. An international example is the State of Washington, which has adopted an integrated approach to preservation, examining the overall lifecycle and processes of deterioration formation and planning for pavement preservation and maintenance .
The main causes of pothole formation are [6, 7, 8 e 9]:
- insufficient thickness of the pavement (including the foundation) compared to the loads it must withstand, which lead to a bending greater than tolerable, generating cracks and permanent deformations;
- use of improper and/or poor-quality materials depending on traffic and environmental conditions (sun, heat, cold, temperature ranges, rain, etc.) and/or not properly designed/packaged (mix design);
- inappropriate water management that accelerates the process of formation and evolution of the pothole (e.g. insufficient gradients due to design or construction errors, depressions due to the settling of the asphalt, poor maintenance of drains, etc.);
- construction of trenches and patches, normally due to interventions on subservices, or failure of previous maintenance interventions;
- failure to manage cracks and deformations that are not immediately sealed and/or treated, through which water infiltrates.
The progress of the pothole depends on the simultaneous action of water-traffic-temperatures:
- fatigue failure that begins with cracks and then turns into a hole. Pressure water due to loads accelerates the process. Thin pavements (less than 15 cm) are those most affected by this phenomenon;
- stripping of aggregates from the wearing course (main causes are the poor chemical-mechanical adhesion between aggregates-bitumen and less binder content than optimal) with the formation of raveling that, within two-five years, turns into the formation of potholes;
- water-temperature: in winter the water freezes and its expansion is similar to the action of a wedge that widens the cracks, until crumbling the pavement forming potholes.
Potholes patching materials must be specially formulated and studied and the two main production methods are:
- hot bituminous mix or HMA from the English term Hot Mix Asphalt: in theory it is the best material, but it should be considered that in winter most asphalt plants are closed and that a typical load of truck (1-8 tons) is used for repairing many potholes before exhausting the mixture. Therefore, considering at most 3 hours of transport before its cooling and the impossibility of being properly compacted, it is evident that its use is very complicated;
- cold bituminous mix or CMA from the English term Cold Mix Asphalt: it is the most used material because it is versatile, storable and easily workable even at very low temperatures. It is produced hot, warm or cold, through the use of additives, fluidizing substances and bituminous emulsions.
The old method called “cutback”, with solvents or diesel, is now prohibited for health and environmental reasons. An important factor to mention is also the effect of vehicular traffic during the pothole rehabilitation phases, both from the point of view of safety and operability.
In such a “simply complicated” context, if we could use an innovative pothole patching technology, the major objectives would be: sustainability, durability, ease of installation, automation, low interference with traffic, high safety and low costs. Below we present the essential aspects of the ongoing InfraROB research, financed with European funds, which pursues precisely these qualifying objectives.
The InfraROB project and sustainable, durable and safe repair methods
InfraROB is a project funded by the European Commission’s Horizon 2020 research programme with the Grant Agreement No. 955337. The project is shared by the Italian Association of Road Safety Professionals (AIPSS) and its members: Research Centre for Transport and Logistics (CTL – Sapienza University of Rome) and Engineering Department of Roma Tre University (Figure 2 above).
The aim is to reduce workers’ exposure to vehicular traffic and the action of construction machinery, reduce the cost of repetitive activities and increase the safety of road users, while increasing the availability of the transport network.
The prospect of the introduction of autonomous machinery and equipment in the construction sector is attracting great interest among manufacturers (Komatsu, Caterpillar, Hitachi Construction Machinery, Volvo Construction Equipment and others), although today their level of penetration is low and limited to off-road vehicles, machinery and equipment used mainly or exclusively in closed areas and construction site.
In particular, the project aims to maintain the integrity, performance and safety of the road infrastructure through autonomous and modular robotic solutions, also for the repair of small potholes of the road surface.
One of the various activities of the project is the development of a “self-propelled 3D printer” capable of extruding a specific mixture for the filling of cracks and small potholes. The research conducted by the Italian working group, described above, is structured as follows:
- step 1: definition of the optimal parameters of the 3D printer/mixture system;
- step 2: study of different materials (in parallel to the previous one), commonly used for the repair of road potholes, possibly with characteristics of environmental sustainability. The aim is to balance different contrasting performances: consistency, homogeneity, smoothness and internal structure that have repercussions on extrudability, fluidity, grip times and on the final performance;
- step 3: on site tests to verify the effectiveness of the compaction of the mixture under traffic loads.
Attracting much interest, the details of the 3D machine study have already been presented at several conferences and seminars, such as during the AIIT 3rd International Conference on Transport Infrastructure and Systems of TIS Roma 2022 .
The first operation of the proposed new system consists in the survey of the hole with a laser scanner or photogrammetric techniques for the detection of the point cloud to be provided to the 3D printer for the construction of the object to be extruded.
The next step involves putting the robot on the road whose printer will be able to extrude the filling material according to the characteristics of the hole being processed. Here are some details of phases 2 and 3.
Innovative pothole patching mixture
The mixtures analyzed for the InfraROB project were different, each with its own peculiarities. The one that today is the most functional and performing is a bituminous mix composed of 100% reclaimed material resulting from the demolition of old asphalt pavements (RAP – Reclaimed Asphalt Pavement) rejuvenated through a special formulated product.
The rejuvenator is the result of several years of research and it is already used in the field of sustainable solutions, but has been specially recalibrated for the research in question. The use of RAP instead of non-renewable raw materials (aggregates and bitumen) allows to pursue the objectives of environmental sustainability and therefore the reduction of equivalent CO2 emissions.
The correct treatment of RAP includes the demolition of the pavement (separation of the surface courses from the deeper ones), the initial control (presence of foreign bodies, size of the elements and environmental compatibility), the treatment by crushing and/or sieving (chemical and dimensional control of the elements and mixing if they come from different sources), storage and stock management for possible recycling.
It is known that bitumen undergoes a chemical transformation both during the storage and production of bituminous mixes and during the use of the pavement. The chemical process involves bitumen oxidation, loss of volatile parts and deterioration of performance.
For the recycling of RAP, it is possible to use two different categories of products: flux oils (they improve the workability of the mixture containing RAP to facilitate its use) and rejuvenators (they partially or totally reintegrate the chemical components of the aged bitumen contained in the RAP, giving the mixture adequate workability for construction, and restoring the performance of bitumen, ensuring a new service life).
The category to which a product belongs can be easily detected by laboratory testing . The rejuvenator used (2.0÷3,5% of Iterlene ACF 1000 HP on the weight of RAP) in this case is composed of several chemical components, including rejuvenating agents, anti-aging, plasticizers, moisturizers, dispersant additives, and it is added to the RAP at ambient temperature to completely cover the granules and form the new mixture.
The laboratory tests were carried out with the aim of obtaining the correct mix design with an adequate balance between extrudability (for the 3D printer) and performance related to the durability of the restoration. From a performance point, the following characteristics were evaluated: particle size, Marshall stability, void content, Indirect Tensile Strength and particle loss (Cantabro test).
At the same RAP particle size curve, different mixtures were studied by varying the additive content in the range of 1,5-3,5% and the water content in the range of 3,1-5%, for a total of eight mixtures. Depending on the size of the potholes that will be subjected to the action of the robot, the maximum aggregate size was chosen equal to 8 mm. Mixing and compaction (50 blows of Marshall hammer) took place at ambient temperature.
Marshall stability and indirect tensile strength tests were carried out at 25 °C after seven-day conditioning period at the same temperature in a conditioning chamber. The particle loss susceptibility of the mixtures was studied with the Cantabro test according to EN 12697-17, after a conditioning period of 28 days at 25 °C. The extrudability was verified with a horizontal screw (Figure 3) that will be installed on the 3D printer.
Figure 4 shows the particle size curve of the final mixture and it also shows the results of the analyzed mixtures, compared to the minimum required for this technology by a specification taken as a reference .
It is evident that the percentage of residual voids is high, both for the low compaction energy and for the lack of fine material that is enclosed within the crushed RAP granules. With the exception of mixture G, the other mixtures show much higher Marshall Stability and Indirect Tensile Strength than the minimum value. The optimal mixture has a rejuvenator content of 3,0% and water content of 3,1%.
Interestingly, the mixture D reaches the maximum value of Indirect Tensile Strength and Marshall Stability, but the particle loss is slightly higher than the requirements. A technical peculiarity of the proposed technology is that it is not necessary to resize, clean and/or dry the pothole to ensure the duration of the restoration. In addition, the pothole is immediately driveable after filling. As for particle loss, the best mixtures are the E and F ones.
Since the Cantabro test does not faithfully reproduce the possible damage caused by traffic and in order to verify the effectiveness of the repairs, the material was laid in some potholes, specially made with appropriate shape and size, near the headquarters of the Faculty of Civil and Industrial Engineering of the Sapienza-University of Rome.
The choice of site derives from two reasons:
- the site is private, so the tests were carried out without disruption to traffic and the samples were monitored over time with controlled traffic;
- the pavement is made of soil and it was therefore easy to dig the potholes with depth and diameter of 10 cm.
The potholes used for the test were filled with the best two mixtures selected among those indicated in Figure 5: mixtures E and F. The repairs of the four potholes were loaded with a road vehicle (FIAT Doblò) immediately after the laying phase and they were initially monitored after 50 and 100 steps.
The surveys were carried out visually to verify that there was no loss of material, especially at the beginning when the mixtures had not taken hold yet and the material was only “self-contained”.
In-situ tests showed substantial and good thickening, with no material disintegration or loss of cohesion, even immediately after installation (Figure 6). So far, after six months of traffic, the backfill material shows no detachment or raveling.
In addition to continuing to monitor the potholes highlighted above, pothole repair work on asphalt pavements and related inspection activities are currently ongoing.
This article presents the first results of a study conducted as part of the european research project InfraROB, whose aim is to increase the safety of workers and road users at road maintenance sites.
In particular, as part of the project, a system for repairing small potholes by means of an autonomous device is currently being studied. A 3D printer will be installed on this device in order to extrude a mixture suitable for the pothole to be repaired, without the prior cleaning and compaction of the backfill. In particular, this article presents the laboratory and in situ study aimed at defining the optimal characteristics for the mixture to be used to power the 3D printer.
Grain size, Marshall stability, void content, indirect tensile strength and material loss were studied in the laboratory using the Cantabro test. These characteristics were evaluated by varying the additive content in the range of 1.5-3.5 per cent and the water content in the range of 3.1-5%.
The maximum aggregate size was limited to 8 mm, because the InfraROB project only deals with the repair of small potholes, with a maximum size of 5 cm depth and 20 cm diameter.
From the point of view of sustainability, the technology involves the use of 100% rejuvenated RAP (Iterlene ACF 1000 HP Green) with blending at room temperature. A total of eight mixtures were tested. Laboratory tests gave very good results in terms of indirect tensile strength, Marshall stability and particle loss; the mixture proved to be very stable during in-situ tests.
Other cold asphalt mixtures are still being studied within the InfraROB project, but at the moment, the mixture presented in this article seems to meet all the requirements of consistency, homogeneity, fluidity and internal structure necessary to be used with a 3D printer.
References – Asphalt for pothole patching and Green Deal
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Asphalt for pothole patching and Green Deal – Per la versione in Italiano: https://www.stradeeautostrade.it/pavimentazioni-e-manti/asfalti-tappabuche-e-green-deal/