Review of the development and uses of hard grade asphalts in France

1. Introduction

The concept of “enrobés à module élevé” (EME) or “high modulus asphalt concrete” originated in France in the 1980s to meet the maintenance and reinforcement needs of pavements subjected to heavy traffic in urban areas and for the slow lanes of motorways (Caroff and Corté, 1994; SETRA, 1988; Verhée and Delorme, 1991). High modulus asphalt concrete is characterized by the use of hard paving grade asphalt and a higher binder content than in traditional base mixes. It has better mechanical performance (higher modulus, improved resistance to rutting and fatigue cracking) (Corté, 2001; Geng et al., 2013; Hernández, 2015; SETRA, 1997). Originally characterized by a minimum modulus of 11,000 MPa (at 15 °C and 10 Hz), the minimum value was increased to 14,000 MPa in the first French standard on high modulus asphalt mixes, NF P98-140, published in 1999 (AFNOR Editions, 1999; Brosseaud and Saint-Jacques, 2015). This standard also sets a high fatigue resistance ε6 ≥ 130 × 10−6 (two-point bending test with imposed deformation on trapezoidal specimens (CEN, 2013)) for class 2 EMEs which were subsequently the only ones used in pavements for binder or base layers.

Due to the performance of EME, many European countries have shown interest in the use of high modulus asphalt concrete including Switzerland (Graf et al., 2002), Spain (Cuenca et al., 2003; Potti and Rubio, 1993), United Kingdom (Sanders and Nunn, 2005), Portugal (Capitão and Picados-Santos, 2006), Belgium (De Backer, 2007; De Backer et al., 2008), etc.

Interest in high modulus asphalts has also spread far beyond the European borders to South Africa (Denneman and Nkgapele, 2011; South African Bitumen Association, 2013), Australia (Denneman et al., 2015), the United States (Geng et al., 2013), Canada (Bitume Québec, 2014; Moghaddam, 2019) or China (Chen et al., 2019; Zhou and Lu, 2005).

A high value of the modulus of the asphaltic binder is necessary to achieve the required performance of high modulus asphalt concrete. This can be obtained using hard-grade paving asphalt having low penetration and a high softening point, by modification with a polymer, by addition of lake asphalt or with the use of certain additives. In France, since the 1980s, the approach taken by the oil industry has been to develop hard grade paving asphalts, i.e., having a penetration at 25 °C lower than 25 mm/10.

The production of hard grade asphalt in France was 39,000 tons in 1990, 77,000 tons in 1995 and reached 100,000 tons in 2000. At the turn of the 21st century, this placed France in a leading position for the use of this type of binder according to the survey carried out by the World Road Association, PIARC (PIARC, 1998). Currently it is around 100,000 tons in total for the grades 10/20 and 15/25 and it represents about 4.5% of the total production of asphalt in France (data from Eurobitume France for 2019). If the general objectives guiding the use of hard asphalts were identified a long time ago, it took several years before processing methods led to binders which exhibit the desired hardness together with good field long-term performance.

Given the current interest in China in high modulus asphalt concrete as demonstrated by numerous papers recently published on this topic (Chen et al., 2019; Wang et al., 2017; Xu et al., 2020; Yan et al., 2020), this paper intends to provide to practitioners a short history of the evolution of paving asphalts in France, followed by a presentation of data on rheological characteristics, results of ageing tests for the hard asphalts produced in France. Current European specifications for hard grade asphalts for paving applications are presented together with a comparison with requirements proposed recently for China (Li et al., 2020).

2. Evolution of asphalts in France

2.1. The sixties-early stages and first changes

Until the beginning of the sixties in France, almost all paving asphalts were 80/100 and 180/220 penetration grade. They were produced by direct distillation from heavy crudes imported from Central America. With a strong increase in heavy vehicle traffic at that time, the search for a higher rigidity and a better resistance of the asphalt mixes to plastic deformations (rutting) led to the use of harder asphalts. Production of 40/50 and 60/70 pen grade asphalts started in 1966 and the first 20/30 pen grade appeared in 1968 (Sauterey, 1973). The publication by the French Road Directorate in September 1969, of a directive for the construction of the surface courses in asphalt mixes (SETRA, 1969), had a decisive role in this evolution. This directive introduced a climatic criterion for the choice of the asphalt, with the division of the French territory in three zones.

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Zone 1: having a Mediterranean type of climate characterized by mild winters and hot summers.
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Zone 2: having an oceanic type of climate, characterized by both mild winters and summers.
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Zone 3: having a continental type of climate, characterized by cold winters and hot summers.

Recommendations made at the time for the choice of the asphalt are summarized in Table 1.

Table 1. Recommendations for the choice of asphalt grades in surface layers (SETRA, 1969).

Elevation (m)	Zone 1	Zone 2	Zone 3
≤500	40/50	40/50 or 60/70	60/70 or 80/100
>500	60/70 or 80/100∗	60/70 or 80/100	80/100

Note: “∗” is applied if elevation >1000 m.

In order to further increase the resistance to rutting of surface layers, the technique of air-blowing was used to reduce the thermal susceptibility of the asphalt, at the end of the sixties, for the production of 40/50 pen grade asphalt with a high penetration index (Simoncelli and Chanut, 1970). If this proved to be effective with respect to rutting, on the other hand it led very quickly to extensive cracking at the pavement surface, which brought to the abandonment of this solution.

The explanation of this poor performance is to be linked to the effect of the manufacturing process on the structure of the asphalt. Air-blowing at high temperature produces a dehydrogenation of certain molecules followed by their reticulation. It results in an increase of the size of the naphteno-aromatic resins. If one refers to the separation of the components of the asphalt between saturated, aromatics, resins and asphaltenes, the proportion of asphaltenes increases, that of the resins and aromatics decreases, and that of saturated compounds remains about constant. This results in an increase of the colloidal instability index $IC = \frac{asphaltenes + saturated}{aromatics + resins}$ (Gaestel et al., 1971). This evolution of the structure of the asphalt, in which the asphaltene micelles are not any more completely peptized but are more or less flocculated, leads to a change in behavior from a “sol” type to a “gel” type. The penetration index PI (Pfeiffer and Van Doormaal, 1936) which reflects the more or less great susceptibility to temperature, then reached values of 2 and more.

2.2. The seventies-the consequences of the oil crises

The 1973 and 1979 oil crises deeply modified the French oil market as well with regard to the volume of refined oil, as to the origin of the imported crudes. These crises involved the need for an adaptation of industry with the closing of several refineries but the construction of new and more powerful units of distillation. The larger share of the crudes imported from the Middle-East, less heavy than those from Central America, and the demand for 40/50 and 60/70 pen grade asphalts led to the generalization of the process of partial air-blowing for the production of these grades. This process was applied to rather soft bases which led to chemical changes in the asphalts.

2.3. The eighties-impact of specifications on the evolution of asphalts processing methods

The revision of the system of specification for paving asphalts, initiated by the French Road Administration at the beginning of the eighties, with participation of the oil industry and road contractors, will have an important impact on the processing methods. Whereas earlier French specifications were primarily based on the penetration value (the other physical characteristics were only supplementing this information), the first change was the introduction in 1986, of the ring and ball (R&B) softening point, as a cornerstone of the new system of grading. The second step in this evolution was related to the concern of the impact of ageing of asphalts on the field performance of the asphalt mixes. A large experimental program involving a hundred job-sites made it possible to collect relevant data to assess representativeness of the rolling thin film oven test (RTFOT) (AFNOR Editions, 2014) for simulation of ageing during coating and laying (Groupe National Qualité des Bitumes, 1992). According to field performance of 35/50 and 50/70 pen grade asphalts used in wearing courses, specifications were fixed which limit hardening of the binder after RTFOT, with on the one hand a maximum increase in 9 °C of the R&B softening point and on the other hand a minimal value of residual penetration (function of the paving grade). These requirements were adopted in 1990 by the asphalt producers and were introduced in 1992 in the French AFNOR standard T65-001 for paving asphalts (AFNOR Editions, 1992).

The 6 °C interval for the R&B softening point, for the definition of the width of each grade, as well as the requirements on the limitation of ageing after RTFOT led the asphalt producers to adapt the methods of production.

The possibility of obtaining a high point of cut was made possible by the use of structured packing internals in the vacuum distillation column. It became thus possible to eliminate fractions with a point of distillation under atmospheric pressure up to 550 °C. Contrary to the process of air-blowing which results, by reticulation, in an increase of the components which already have a large mass, direct distillation involves elimination of the saturated and naphteno-aromatic components of lower masses. This process is applicable only to the heavy crudes which already contain enough asphaltenes. The resulting hard asphalts have a penetration index PI which is this time less than 2 and are less susceptible to ageing. It is recalled PI provides an indication on the temperature susceptibility of the asphalt, the value ranges from −3 for highly temperature susceptible asphalts to around 7 for highly blown low temperature susceptible asphalts (Read and Whiteoak, 2003). Partial air-blowing remains used, in France, by one producer for the hardest grades.

Production of propane precipitated asphalt, which is a process well adapted to the production of very hard grades, is only used in France in two refineries specialized in the production of lubricants. All in all, asphalts resulting from this process represent only approximately 5% of the French production of asphalts (Bitume Actualités, 1990).

2.4. Recommendations for the selection of paving asphalts

Recommendations for the choice of plain paving grades were redefined in 1994 in a technical guide issued by SETRA and LCPC (SETRA, 1994). As in the above-mentioned 1969 directive, three climatic zones are specified. This time they are based on average values of maximum daily temperatures in July and August and minimum daily temperatures in January and February observed over the past thirty years.

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Zone 1: with dominant oceanic climate (Tmax ≤ 27 °C and Tmin ≥ 0 °C).
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Zone 2: with dominant southerner climate (Tmax > 27 °C and Tmin ≥ 0 °C).
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Zone 3: with dominant continental or mountainous climate (Tmin < 0 °C).

In order to relate France climatic conditions to the Superpave approach (ASTM International, 1999), Fig. 1 shows two maps of France for low and high temperatures with indication of the performance grades (PG) for the asphalt binder.

For base asphalt concrete, the grade most generally selected is 35/50 if not 50/70. For wearing courses, in the case of heavy traffic, the advised grades are indicated in Table 2 (in the case of less severe traffic conditions, one would accept a softer grade 50/70 instead of 35/50 and 70/100 above 1000 m).

Table 2. Plain asphalt paving grades for wearing courses in cases of heavy traffic (1994 recommendations for the French national network).

Altitude (m)	Zone 1	Zone 2	Zone 3
<500	35/50	35/50	35/50
From 500 to 1000	50/70	50/70	50/70
>1000	Of no concern	50/70	70/100

With respect to the 1969 directive, one can note a certain hardening of the asphalts with in particular the generalization of the use of 35/50 pen grade, the process of vacuum distillation giving good long-term performance.

The use of a harder grade is to be considered in the case of very demanding situations (important slow or channeled heavy traffic, high temperatures) or when altimetric constraints limit the thickness of the pavement.

3. Rheological characteristics of hard asphalts

Hard asphalts are defined here as having a penetration less than 25 mm/10 at 25 °C. In the beginning of the 21st century there are three grades produced in France: 15/25, 10/20 and 5/10. Grade 5/10 was at a trial stage, whereas the others have been marketed for several years. Typical characteristics of these hard asphalts are given in Table 3.

Table 3. Typical hard asphalt characteristics (before ageing).

Characteristic	Grade
Characteristic	15/25	10/20	5/10
Ring and ball softening point (°C)	66	62–72	87
Pfeiffer penetration index	0.2	0.5	1.0
Dynamic viscosity at 170 °C (mm2/s)	420	700	980
Complex modulus at 7.8 Hz, \|E∗\| (MPa)
At 0 °C	425.0	700.0	980.0
At 10 °C	180.0	300.0	570.0
At 20 °C	70.0	110.0	300.0
At 60 °C	0.4	0.7	7.0

Penetration at 25 °C does not determine of course the rheological characteristics. Performances, for asphalts of the same penetration grade, vary depending on the crude and the manufacturing process. Table 4, extracted from the study of Glita and Conan (1996), compares the rheological characteristics of seven 10/20 pen grade asphalts with, as a reference, those of a plain 35/50 asphalt. One notes the modulus can vary, for the same test conditions, in a ratio from 1 to 2, and the sensitivity to permanent deformation, as indicated by G∗/sin δ, is quite different from one asphalt to another.

Table 4. Rheological characteristics of seven 10/20 asphalts and a 35/50 asphalt (Glita and Conan, 1996).

Characteristic	35/50 pen grade	10/20 pen grade
Characteristic	35/50 pen grade	A	B	C	D	E	F	G
\|G∗\| (15 °C, 10 Hz) (MPa)	34.5	53.7	88.0	88.0	83.7	71.1	43.7	47.3
\|G∗\| sin δ (60 °C, 5 Hz) (MPa)	0.016	0.131	0.184	0.247	0.122	0.165	0.184	0.103
SR	3.55	4.30	3.64	3.94	3.30	3.58	4.53	4.10
$T (G^{'} = G^{″}) (° C)$	17	28	29	31	24	26	34	25
$T (G_{\max}^{″}) (° C)$	−10	−10	0	−5	0	−5	−15	−10
δ (−10 °C, 5 Hz) (°)	12.2	11.6	7.0	9.1	6.5	8.3	12.7	11.0

Note: SR means standard deviation of the relaxation spectrum.

One can see from this table that the temperature for which the viscous and elastic components of the modulus are equal (phase angle equal to 45°) is definitely lower for the 35/50 asphalt than for 10/20 asphalts. The hard asphalts thus should have a lower capacity of healing than the softer 35/50 asphalt.

Brittleness at low temperature can be estimated from the temperature corresponding to the maximum of the viscous component G'' of the complex modulus, or by the value of the phase angle at low temperature, here at −10 °C. Table 4 shows, according to these two criteria, that asphalts B, D and E can be regarded as most fragile, whereas asphalts A, F and G show characteristics similar to those of the 35/50 asphalt.

Up to 2006, hard grade asphalts were not included in European standards. Hence for the information of users, French asphalt producers used the approach of “Avis techniques” (note: “Avis techniques” are French technical assessments for innovative products issued by the “Comité Français des Techniques Routières”, French Committee for Road Techniques). Table 5 gathers information on hard asphalt characteristics, produced in France for use in high modulus asphalt concrete included in these “Avis techniques”. These are not guaranteed values; those are likely to vary with the origin of the crude oil.

Table 5. Data on hard asphalts produced in France for high modulus asphalt concrete.

Characteristic	Issue number of the “Avis technique”
Characteristic	26	96	76	22	72	42	40
Asphalt before ageing
Penetration at 25 °C (0.1 mm)	16	21	21	12	13	13	16
R&B softening point (°C)	63.5	66.0	68.0	72.0	66.0	65.0	71.0
Penetration index (LCPC)	0.7	1.0	1.3	0.4	0.4	−0.2	0.5
Fraass temperature (°C)	−6	−8	−6	−5	−6	3	−3
Modulus E (7.8 Hz, 25 °C) (MPa)	54	40	34	60	56	61	66
Phase angle (7.8 Hz, 25 °C) (°)	37	39	38	35	29	34	–
Modulus E (7.8 Hz, 60 °C) (MPa)	0.6	0.6	0.5	0.9	0.9	0.6	1.0
Phase angle (7.8 Hz, 60 °C) (°)	64	62	63	62	64	64	59
Modulus E (250 Hz, 60 °C) (MPa)	6	6	5	8	9	7	10
Phase angle (250 Hz, 60 °C) (°)	63	56	57	59	60	67	61
Asphalt after RTFOT
Penetration at 25 °C (0.1 mm)	11	17	18		7/13
Residual penetration (%)	69	83	86
R&B softening point (°C)	75	72	74		62/76
Increase in R&B softening point (°C)	11.5	6.0	6.0
Fraass temperature (°C)	−4	−6	−6		0/4
Increase in Frass temperature (°C)	2	2	0
Modulus E (7.8 Hz, 25 °C) (MPa)	71	39	39
Phase angle (7.8 Hz, 25 °C) (°)	28	35	36
Modulus E (7.8 Hz, 60 °C) (MPa)	1.20	0.72	0.70
Phase angle (7.8 Hz, 60 °C) (°)	60	58	58
Modulus E (250 Hz, 60 °C) (MPa)	10	6	6
Phase angle (250 Hz, 60 °C) (°)	53	54	54		47

4. Influence of ageing

The incidence of ageing, as simulated by the rolling thin film oven test for the phases of coating and laying of the asphalt mix, then with the PAV for field evolution in the pavement, was studied on various 10/20, 35/50 and 50/70 paving asphalts, by the Regional Laboratory of Bridges and Roads of Aix en Provence. It considered composition, the traditional empirical characteristics (penetrability, R&B softening point, Fraass temperature) and the rheological behavior from bending beam rheometer tests.

From the point of view of the change in the composition of the asphalt, if one considers the n-heptane asphaltenes content, the increase in asphaltenes after ageing tests is all larger since the grade of the asphalt is soft. After RTFOT+20 h of PAV, the ranges are: from 50% to 80% for 50/70 asphalts, from 40% to 65% for 35/50, and from 15% to 35% for 10/20 grade. Iatroscan chromatography which provides a global analysis without preliminary separation of asphaltenes, gives a more complete image of the evolution of the generic components of the asphalt. The following changes are qualitatively comparable for the various grades.

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The proportion of saturated remains about constant.
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That of aromatics varies little after RTFOT, on the other hand the decrease is important after RTFOT+PAV and result in a transformation into resins.
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That of asphaltenes increases slightly, much less than the change observed by the n-heptane precipitation method (the difference comes from the fact that certain resins are dragged by the solvent in one case while in the other case they are precipitated).

With respect to the rheological behavior at low temperature, the bending beam rheometer test shows little influence of RTFOT on the temperatures of iso-modulus 300 MPa and m = 0.300. On the other hand, the effect of RTFOT+PAV is important. For the hard asphalts tested, the magnitude of the changes is comparable with that observed on softer grades 35/50 and 50/70, namely a rise in 2 °C–3 °C for the temperature of iso-modulus 300 MPa and 4 °C–7 °C for the temperature of iso-slope m = 0.300.

Tests carried out on two 10/20 asphalts with an initial R&B softening point of 64 °C showed, after RTFOT+PAV, a hardening slightly less than for the softer asphalts but nevertheless important: the increase in R&B softening point is 11 °C–13 °C, residual penetrability is 45%–53%. The R&B temperatures on aged asphalt exceed here the threshold value of 71 °C which had been correlated with the observation of surface cracking of wearing courses attributed to thermal fatigue. These observations were however made on asphalt mixes with 35/50 and 50/70 asphalts (LCPC, 1999) and can't be extrapolated directly to the harder grades.

5. Practical indications for use of hard asphalts

The higher viscosity of hard grade asphalts necessitates to raise the coating temperature; for 10/20 asphalts it will be around 170 °C–180 °C. Laying of the mix must be carried out between 150 °C and 170 °C approximately and compaction must in general be carried out at a temperature higher than 140 °C. Hence, to achieve correct compaction thin lifts should not be laid when the ambient temperature is low.

In France, hard paving grade asphalts are used in binder and base layers always with a surface layer in order to mitigate the effect of thermal shocks. As shown above by data of Fraass temperature in Table 5, hard paving grade asphalts exhibit a low resistance to thermal cracking, which might be a limitation of their use in regions with severe winter temperatures, when asphalt is subject to thermal shrinkage stresses that can lead to cracking. This has been analyzed and confirmed by different studies, some of which are mentioned here.

In Canada, susceptibility to cracking at low temperatures is assessed by the thermal stress restrained specimen test (TSRST) (AASHTO, 1993). The criterion used is a minimum failure temperature of −28 °C. This cannot be achieved with plain hard paving grade asphalts, but with polymer modification to obtain a PG 68-28 grade asphalt (Bitume Québec, 2014). According to Geng et al. (2013), hard paving grade asphalts should be used with caution when the design low temperature is below −16 °C. In Latvia, the specification for low temperature cracking resistance for base and binder courses is a minimum value of −20 °C at TSRST. To meet this criterion, plain 20/30 grade asphalt must be modified with an elastomeric polymer (Izaks et al., 2020). Rys et al. (2017) presented the results of field investigations on 80 sections of roads in service in Poland. The conclusion is that high modulus pure bitumen pavements are 2.45 times more likely to be cracked than traditional asphalt concrete pavements.

6. European standard for hard asphalts

The European standard for paving grade asphalts (EN 1259 “Bitumen and Bituminous Binders-Specifications for Paving Grade Bitumens”) was developed in the 1990s (British Standards Institution, 2009). It included a wide range of grades (designated by the nominal penetration range at 25 °C; eight classes in total from 20/30 to 180/220) suitable for the manufacture of the materials for road construction and maintenance used, and the climatic and traffic conditions encountered, in all the member states.

Following the development of hard asphalts and the wider use of materials for road construction and maintenance having very high modulus values, standard EN 12591 was complemented in 2006 by a new standard EN 13924 (“Bitumen and Bituminous Binders-Specifications for Hard Paving Grade Bitumens”) (British Standards Institution, 2006). Hard paving grade asphalts are designated by the penetration range at 25 °C, e.g., 10/20 pen or 15/25 pen.

The properties specified in this standard cover the following three essential characteristics.

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Consistency at intermediate service temperature.
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Consistency at elevated service temperature.
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Durability of the above.

Consistency at intermediate service temperature shall comply with the requirements for penetration at 25 °C. The grades are designated by the nominal penetration range at 25 °C.

Consistency at elevated service temperature shall comply with the requirements for ring and ball softening point and, if selected, viscosity. Hard paving grade bitumens are supplied for a variety of end uses, and thus a restricted softening point range, of ±5 °C about a mid-point, shall be declared by the supplier; the overall range shall be within the range in the tables of the standard.

Durability shall be demonstrated by consideration of the resistance to hardening tested according to the rolling thin film oven test (RTFOT). The required surrogate characteristics after RTFOT are as follows.

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The change of mass.
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The retained penetration.
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The softening point after hardening.
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The increase in softening point.

A few other characteristics which are not mandatory and indicated in the standard as they can be useful in some cases.

Standard EN 13924 was later revised, a new version, prEN 13924-1 was sent for public enquiry but is not yet published by CEN (National Standards Authority of Ireland, 2015). It is intended to replace EN 13924. The changes introduced in prEN 13924-1 are mainly related to the presentation of the properties split into two tables. The properties in Table 6 must be specified for all hard grade road asphalts. They are associated with regulatory or health, safety and environmental requirements. The properties in Table 7 are required to comply with particular local conditions. They are associated with regulatory or other local requirements.

Table 6. Properties specified for the three classes of hard asphalts (prEN 13924-1).