Permanent Pothole Repairs


J C Nicholls


Technical Paper Asphalt Professional 60

This article is reproduced having been originally published in Asphalt Professional issue 60, April 2014. It is worth bearing in mind while the content would have been correct at the time of publication, standards may have changed since.

1. Introduction

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.

Figure 1. Examples of potholes
Figure 1. Examples of potholes

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.

Figure 2. Responses to the questionnaire
Figure 2. Responses to the questionnaire

2. Definition of the term “pothole”

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:

  • Generally, potholes require rapid remedial action to maintain the safety of road users.
  • Potholes will also need to be reinstated to maintain the functional requirements and comfort, but the time-constraints on rectification for these requirements will not be as immediate.
  • Potholes will typically have a depth of at least 30 mm and an area equivalent to a diameter between 100 mm and 1 m with the values for a specific situation depending on several factors including the traffic speed and intensity, the type of vehicle (particularly the presence of bicycles and pedestrians) and the climate.
  • Potholes can grow once they have emerged, but generally stop growing after a certain time. However, other potholes can appear close to an existing one.
  • Potholes can occur due to several mechanisms (such as fracture, attrition and seasonal effects).

Table 1. Principal requirements and associated test methods
Table 1. Principal requirements and associated test methods

3. Laboratory and in situ tests and evaluation methods

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.

4. Existing standards, techniques, materials and experience with them on the European market

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:

  • bitumen-based cold-mix materials (cold-mix asphalt CMA)
  • bitumen-based hot-mix materials (hot-mix asphalt HMA)
  • cement-based materials
  • synthetic binders, but not widely used.

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:

  • Mortar (or matrix) dominated mixtures (such as hot rolled asphalt and mastic asphalt) with higher bitumen content and lower permeability that are easy to compact and have good durability but the surface is often quite smooth, requiring chippings to provide better skid resistance.
  • Aggregate dominated mixtures (such as asphalt concrete and stone mastic asphalt) with lower bitumen content and higher permeability that require higher compaction energy and have shorter durability.

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).

5. Road Trials

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.

Figure 3. Trial of pothole repair options in Tuelsø
Figure 3. Trial of pothole repair options in Tuelsø

  • 19 materials on two sites at Tuelsø, Denmark, installed in 2008 and 2009 (Figure 3).
  • Six materials on one site at Ljubljana, Slovenia, installed in 1999.
  • Ten materials on two sites at Novo Mesto and Nova Gorica, Slovenia, installed in 2012.
  • Four materials on individual sites in the United Kingdom for the HAPAS scheme.

Table 2. Estimated durability of 25 generic pothole repair materials
Table 2. Estimated durability of 25 generic pothole repair materials

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:

  • Hot applied bituminous materials (four materials).
  • Cold applied bituminous materials (ten materials).
  • Cementitious materials (four materials).
  • Synthetic binders (seven materials; resins of acrylate, epoxy or polyurethane and two 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:

  • The same conditions (traffic and climate) and evaluation methods are not used for the different trial sites and, therefore, it is difficult to determine the actual durability of the various repair materials. It is, therefore, important to develop a common evaluation system that uniquely determines when damage is being assessed to have a negative effect on the functional properties of the repair.
  • If the assessment system is to be applicable throughout Europe, it will be necessary to define categories of durability with reference to the different climate zones and different traffic classes.
  • It has not been possible to classify the different generic types in durability categories based on the evaluation of the trial sites because, within each generic type of material, great variation exists in the estimated durability for each repair material.
  • It appears that each generic type of repair materials has an associated set of typical damage. It may, therefore, be necessary to differentiate the laboratory tests relative to each generic type of repair material.
  • It appears that some of the generic types of repair materials have limitations on their application, such as size of the pothole and the temperature during application.
  • It is suggested that the durability of pothole repair materials are categories as follows:
    − Category I: Durability less than 1 year (short-term durability)
    − Category II: Durability between 1-3 years (medium-term durability
    − Category III: Durability more than 3 years (long-term durability)

Table 3. An overview of repair materials for potholes classified by generic material types
Table 3. An overview of repair materials for potholes classified by generic material types

6. Laboratory tests

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:

  • Only a few of the cold asphalts have more than one aggregate fraction and continuous particle size distribution; one or two fractions of aggregate predominate in most of the tested cold asphalts.
  • The cold asphalts with unsuitable aggregate gradations have high air voids contents that may negatively influence their performance.
  • No cold asphalt with an air voids content lower than 10% was found.
  • Some of the cold asphalts contain binders that do not become hard after application of the material; the mixtures stay soft, and they have little or no resistance to loading (and some of them even disintegrated spontaneously).
  • Intensity of compaction is critical to cold asphalt performance; low compaction decreases the values of the performance parameters and shortens the service life.
  • Cold asphalts differ with regard to compactibility; the differences between the compaction curves prove that mixture composition (aggregate gradation and binder) is more important than compaction temperature. (It appears that the viscosity of the binders in the tested cold asphalts did not change significantly in the temperature range from 5°C to 20°C.)
  • The type of cold asphalt, the temperature conditions and the cure time influence the indirect tensile strength of the cold asphalts.
  • The highest indirect tensile strength values of cold asphalts were found for the compaction and conditioning temperatures of (20 ± 1) °C, and the lowest for (5 ± 1) °C.
  • Differences in the indirect tensile strength among the cold asphalts can be high (double or more) and depend on the temperature conditions before and after application; better results are achieved at elevated compaction and curing temperatures (20°C).
  • Cure time is a positive factor because the values for indirect tensile strength increase over time; the amount of change depends on the temperature conditions and can be significant (up to 50 %).
  • Only one of the tested cold asphalts was water resistant in all the tested scenarios of compacting and conditioning temperatures; the others had little or no resistance to the influence of water. (The test samples were soft or disintegrated before or during testing.)
  • Only one of the tested cold asphalts proved resistant to freeze-thaw cycles; the others have little or no resistance. (The test samples were soft or disintegrated before or during testing.)
  • Taking into account all the ways of comparing compactibility and indirect tensile strength (dry, wet, freeze-thaw), it appears that compactibility should be removed from the list of relevant properties that should be tested for cold asphalts.

The following findings emerged from the comparison of the test results for the cold and hot asphalts:

  • A large difference exists only in the first stage of compaction; thereafter, the ratio of the changes in height increases slowly. The increase means that the change in height of the compacted cold-asphalt specimens is faster.
  • The total changes in height of all the tested cold asphalts were higher than for SA 8; two of the cold asphalts had a greater change in height and two had lower when compared with AC 11.
  • Only one cold asphalt had values from the indirect tensile strength, water sensitivity and sensitivity to freeze-thaw cycle tests that were close to the hot asphalt values; even this material is comparable only with SA 8 with the soft 250/330 bitumen. The other tested cold asphalts were significantly weaker than the 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:

  • A minimum number of aggregate fractions that should be used in a mixture.
  • Limitations on the particle size distribution (minimums and maximums passing through defined sieves).
  • Required air voids content.
  • Binder properties.
  • Required values for the results of the selected tests.

Figure 4. Slovakian cold asphalts specimens
Figure 4. Slovakian cold asphalts specimens

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:

  • The maximum nominal size of the aggregate used in a mixture should be in the range of 4 mm to 10 mm.
  • The air voids content of a cold asphalt should be as low as practicable so that it is as similar to the original surrounding material as possible.
  • The indirect tensile strength (ITS) of a cold asphalt, as determined according to BS EN 12697-23, should be at least 20% of that of a hot asphalt’s ITS. The ITS requirement should be evaluated for cold-asphalt specimens under the following conditions: − temperature of the cold asphalt before compaction of +5ºC; − compaction of the specimens by an impact compactor according to BS EN 12697-30; − storage and conditioning of the specimens before testing at +5ºC; and − test temperature of +5ºC.

Figure 5. German cold asphalt specimens after ITS testing
Figure 5. German cold asphalt specimens after ITS testing

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:

  • The workability time of the synthetic- binder mixtures is very short, so the mixture must be prepared and compacted within a few minutes.
  • It is recommended to prepare at one time only the quantity of a mixture that is needed for one pothole.
  • The workability of the mixtures is lower and shorter at elevated temperatures (20°C).
  • Both the tested materials had comparable strength values regardless of differences in the material composition.
  • The temperature conditions before and after compaction of the specimens had only a small influence on the indirect tensile strength.
  • Both materials are resistant to water and frost.
  • Approximately the same values of indirect tensile strength were found at test temperatures of 5°C and 25°C; it seems that the stiffness of the tested synthetic-binder mixtures is independent of temperature.
  • The tested synthetic-binder materials had higher indirect tensile strength than the tested hot asphalts.

7. Whole-life cycle costs and benefits

Typically, potholes appear towards the end of the service life of an asphalt pavement and pothole repair aims at achieving the end of the planned service life. However, potholes can be repaired with different materials and different techniques which influence the survival rate of the pothole patching. For road agencies, the challenge is to decide on the most cost-effective set of repair activities for the remaining service life of the asphalt pavement. A tool that can facilitate road agencies in their decision making is life-cycle cost-benefit analysis (LCCBA).

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.

Table 4. Repair alternatives
Table 4. Repair alternatives

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.

Table 5. Pothole scenarios
Table 5. Pothole scenarios

After running LCCBA, the following conclusions can be drawn:

  • In all four scenarios, the agency costs for the immediate repair are higher than for the deferred repair, because traffic management costs can be reduced through the bundling of pothole repairs. On the other hand, user costs for deferred repair are higher than for immediate repair, because the existence of potholes for a longer period increases vehicle operation costs, travel time and accident risk.
  • In all four scenarios immediate repair strategies are preferable compared to deferred repair strategies. Although deferred repair strategies have lower agency costs, the user costs increase drastically and, thus, the total costs. Even for scenarios 3 and 4 with lower traffic intensity, the user costs are considerably higher compared to the agency costs.
  • In all four scenarios, the unprepared patching of potholes with cold-mix asphalt (alternative 1a) incurs the highest costs compared to other patching strategies. The low patching survival of this strategy increases the total number of potholes to be repaired.
  • In all four scenarios, the repair alternatives 1b, 2a, 3a and 3b show very similar costs. Although these strategies have different patching survival rates and repair costs, the longer patching survival and higher costs of one strategy is outweighed by the shorter patching survival and lower costs of another strategy.
  • In scenario 1 and 2, a deferred resurfacing of the road section is more cost-effective than a deferred patching of potholes. The high traffic intensity of both scenarios leads to high user costs which favour the resurfacing option.
  • In scenario 3 and 4, deferred patching of potholes is more cost-effective than a deferred resurfacing of the road section. The low traffic intensity of both scenarios reduces the user costs which favour the patching option.

8. Conclusions

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.

9. References

Council for Scientific and Industrial Research (2010). Potholes – Technical guide to their causes, identification and repair. South Africa: CSIR. Downloadable from 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.

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