Potholes (Figure 1) and how to repair them are not just a UK problem but one faced by all European countries. The immense economic loss due to the damage, the repair of potholes with materials that are only good on a short–term basis and, most importantly, the increasing numbers of crashes with resulting injuries and even deaths caused by potholes requires an improvement in the methods and techniques and road agencies need methodologies to deal with these problems. Many approaches just deal with repair methods which are durable only on a short–term basis and are, therefore, not cost–effective. Road agencies need durable construction and maintenance methods for the repair of damage which occurs after hard winters due to repeated frost–thaw cycles and other mechanisms.
A European research project, POTHOLE, was set up to address the issues. The project is part of the Joint Research Programme “ERA- Net Road” funded by Belgium, Germany, Denmark, Finland, France, Netherlands, Norway, Sweden, Slovenia and United Kingdom represented by the Austrian Research Promotion Agency (FFG). The study was completed in September 2013 having been undertaken by a consortium of the Karlsruhe Institute of Technology from Germany as coordinator, FEHRL from Belgium, Danish Road Institute from Denmark, University of Twente from the Netherlands, University of Žilina from Slovakia, Slovenian National Building and Civil Engineering Institute from Slovenia and TRL from the UK.
A questionnaire was circulated asking about the definition(s) of “pothole” used around Europe (Nicholls, 2011). The spread of results was extensive but inconclusive (Figure 2). Nevertheless, a definition was developed from the best ideas which did not conflict with other existing definitions. This definition is:
“a local deterioration of the pavement surface in which the material breaks down in a relatively short time and is lost, causing a steep depression”
To give more detail, the following notes were added:
The questionnaire (Nicholls, 2011) also covered the tests and evaluation methods used to select pothole repair materials and techniques. From these results, it was found that pothole repair materials and techniques need to be assessed by a certification procedure prior to use because the size of works makes compliance checking impractical. The principal requirements of the materials that need to be assessed in order to ensure durability, together with the obvious test methods to be used to assess the principal requirements, are given in Table 1.
Almost no requirements for material properties were found in the gathered documents. There are some test methods listed in a few standards or technical specifications but no values are given for the requirements needed (only some broad limits for particle size distribution of the aggregate grading).
The size of the aggregate used for repair material depends on the depth of the pothole to be repaired. In most cases, repair materials contain aggregates which have a maximum aggregate particle size not more than 10 mm or 11 mm. The aggregate grading has a great effect on the performance of an asphalt mixture, with dense-graded asphalt mixtures supposedly performing well at warm and hot temperatures whilst an open-graded asphalt mixture is required for satisfactory workability at freezing temperatures (CSIR, 2010).
The main types of material used for pothole repair (Ipavec, 2012) are:
Cold-mix asphalt is mostly used as temporary repair but it can be more durable, with proper installation. The major limitation for these materials is that they cannot normally be compacted to the same level as hot-mix asphalts. The advantage is short application time and applicability in harsh winter conditions. The binder can be either cutback bitumen or bitumen emulsion. Cutback bitumen can be difficult to work at low temperatures and often requires some warm-up time in the sun before use whilst bitumen emulsion can have a relatively short time to break and cure, so relatively fast-setting emulsions are required. Hot-mix asphalt presents a more durable solution which is easy to install and to compact and provides more effective bonding with the existing asphalt pavement. Attention must be paid to the required mixture temperature for compaction, with hot-box equipment being needed to maintain the temperature for multiple small repairs. There are two generic types of hot mix asphalt:
Cement-based materials are fast-setting or rapid-hardening cement-based materials that are intended for rapid pavement repair. However, because any repaired patch deflection under the traffic needs to be similar to that of the surrounding pavement, repairs using strongly cement-based materials are not recommended.
Pothole repair techniques include temporary repairs that are used in emergency circumstances when a pothole represents a potential hazard for safety and rideability or in harsh winter conditions when there is no alternative solution and when a defect should be repaired immediately or in a short time. The methods include throw- and-go (no preparation or cleaning of the pothole and compaction by traffic only; usable in harsh winter conditions and with a high rate of application, but the worst durability; normally cold-mix-asphalt), throw-and-roll (no preparation or cleaning of the pothole and compaction by the tyres of the maintenance crew truck; usable in harsh winter conditions and with a high rate of application; normally cold-mix asphalt), edge seal method (similar to throw-and- roll but with a ribbon of bituminous tack material on top of the patch edge) and spray-injection patching (placing heated bitumen emulsion and virgin aggregate simultaneously into a pothole with no compaction).
Semi-permanent procedure (using hot or cold-mix asphalt) involve removing water and debris from the pothole, forming the vertical edges to the pothole, placing the mixture into the hole and then compacting it using vibratory plate compactors, drum vibratory rollers or tamper. An option for smaller potholes is to leave out the edge straightening, but this omission could have the effect of shorter durability.
Permanent or more durable repairs involve preparation including edge formation (by saw cutting), cleaning the excavation with the removal of all debris, loose material and water (drying), the application of bond coat to base (bottom) and sides, infilling the pothole with asphalt material (mostly hot- mix, also cold-mix asphalt or cement-based material is used) and then compaction with vibrating plates, drum vibratory rollers or tamper.
The proper preparation of potholes is essential for a good repair. No matter how good quality and durable the material that is used for pothole infilling is, it will not perform well and not last long enough if it is applied in inappropriate circumstances. The prepared patch area (normally rectangular shape) must include the whole area affected by the pothole and any associated distress in the surrounding area. The cut edges should be clean and neat. All unsound and debonded material should be removed. A good bond is needed, usually with a cationic emulsion, that must be evenly applied. Every type of infill material should be fully compacted. Finally, blinding with some coarse sand over the second layer of emulsion, if applied, ensures that the bitumen does not stick to vehicle tyres. For deeper potholes (more than 40 mm), the asphalt should be installed in multiple layers (each compacted separately).
The issued questionnaire was later supplemented with a secondary one which focused on experience gained by dedicated trial sections for the durability of various pothole repair materials. Three countries reported systematic trial sections.
In 2012 a total of 25 different repair materials had been surveyed for those responsible for the trials for periods ranging from one to 12 years covering four generic types of materials:
Detailed description of conditions at the trial sites, selection criteria and other accessible material data can be found in (Rosenberg, 2012). From the various trial sections, the generic types could be evaluated according to their estimated durability and divided into four age groups (Table 2).
From the relatively limited number of assessments based on one trial site from Denmark with 19 repair materials and several separate trial sites from the United Kingdom with four repair materials, from which information from only three were available, a summary of the results is given in Table 3.
The proper preparation of potholes is essential for a good repair. The results from this evaluation were as follow:
Three types of material (hot asphalts, cold asphalts and synthetic-binder mixtures) were tested (Koma ka and Remišová, 2012). The hot asphalts (AC 11 and SA 8) were chosen as a benchmark for comparison with the cold asphalts and the synthetic-binder materials.
Fourteen cold asphalts that are currently available in the European market were tested (Figures 4 and 5).
Tests of particle size distribution, binder content and air voids content were used to select one cold asphalt from each country involved in the testing (Germany, Slovakia, Slovenia and the UK) for further investigation in terms of compactibility, indirect tensile strength test, water sensitivity (and sensitivity to freeze-thaw cycle) on specimens prepared and tested at two temperatures (5°C and 20°C). The results of the tests demonstrated the following:
The following findings emerged from the comparison of the test results for the cold and hot asphalts:
Taking into account the findings mentioned above, it seems useful to determine some requirements for the components of cold asphalts and the parameters of the final mixture. These could include:
Different approaches could be used to apply the requirements above. All of the requirements could be accepted, or only some of them could be used. Moreover, various formulations of the requirements for each parameter are possible. Numbers, limits, and descriptive requirements are suitable. One of the possible sets of requirements was recommended by the consortium as follows:
New knowledge about synthetic-binder materials has been gained. Two materials with different compositions and synthetic binders were tested. From experience and the test results, the following can be concluded:
Potholes can be repaired with different materials and different techniques. In addition, materials and techniques can be combined. It would not be feasible to address all of them in LCCBA. The combinations that were considered are depicted in Table 4 (Hartmann, 2013). A special case is milling and resurfacing which is not used to repair single potholes but to renew the entire section where potholes occur. It will allow the determination of the moment when patching is no longer cost- effective and renewing the entire section is the better alternative.
In order to schedule pothole repair, an agency needs to know when and how many potholes are likely to occur within the remaining life time of the asphalt section. Predicting the occurrence and progression of potholes is, however, a challenging task because a number of factors need to be considered, such as road design, asphalt age, traffic intensity, and weather conditions.
Due to the difficulties of addressing every possible contextual situation, it is proposed that four scenarios varying on four factors are selected: thickness of top asphalt layer, remaining service life, traffic intensity, and amount of precipitation (Table 5). It is believed that these scenarios cover typical but also contrasting repair situations.
For each scenario, it is assumed that, on the length of a road section (1 km), one pothole appeared at the start of the first analysis year. As a result, scenario 1 and 2 are characterised by a high number of additional potholes per year which even increases over the remaining service life of the asphalt section. Scenarios 3 and 4 show only one additional pothole per year. The total number of potholes to be repaired depends on the patching survival which, in turn, is influenced by repair material and technique. It becomes apparent that the total number of potholes to be repaired in one year can increase if the survival rate of the already repaired potholes is less than the remaining service life. The four scenarios suggest that an agency has to repair potholes every year of the remaining service life. However, the scenarios do not show when the repair will take place or, in other words, how long a pothole exists from its occurrence until its repair. For an agency, it can be cost-effective to wait before a pothole is repaired (except for emergency repairs). From a user perspective, a period with unpatched potholes means a period of higher risks of accidents, longer travel time and higher vehicle operation costs; the more potholes there are, the higher the impact is on users and the higher the user costs are. Two response times are distinguished: immediate repair and deferred repair. Twelve repair strategies were analysed which combine repair alternative and response time.
After running LCCBA, the following conclusions can be drawn:
As a result of the POTHOLE project, a final report has been written (Kubanek, 2013a) as well as the more detailed reports of the different work packages. Furthermore, the findings of the project have been summarised into a “Guideline for pothole repairs” (Kubanek, 2013b). This guideline can be used by stakeholders to choose a suitable repair method based upon the choice of generic material and the desired durability.
Council for Scientific and Industrial Research (2010). Potholes – Technical guide to their causes, identification and repair. South Africa: CSIR. Downloadable from www.csir.co.za/pothole_guides/ docs/Pothole_CSIR_tech_guide.pdf
Hartmann, A (2013). Life-cycle cost-benefit analysis. Brussels: FEHRL.
Komacˇka, J and E Remišová (2012). Comparison of the performance of common and new materials for repairs of potholes. Brussels: FEHRL.
Kubanek, K (2013a). Final report. Brussels: FEHRL.
Kubanek, K (2013b). Guidelines for pothole repairs (Annex of final report of the project “POTHOLE”). Brussels: FEHRL.
Ipavec, A (2012). Study of existing standards, techniques, materials and experience with them on the European market. Brussels: FEHRL.
Nicholls, J C (2011). Definition of potholes and test methods for materials used in their repair. POTHOLE WP1 Report. Brussels: FEHRL.
Rosenberg, J (2012). Evaluation of techniques and materials from existing trial sites in Europe. POTHOLE WP4 Report. Brussels: FEHRL.