Gravel roads play an important role in the transport infrastructure. However, their maintenance (especially the control of dustiness using chemical dust suppressants) is expensive. Besides, the condition of gravel roads results in low driving comfort, longer travelling time, faster vehicle amortization, and so forth. Typically, these problems are solved by paving gravel roads with asphalt wearing layer. However, north countries practice had shown pavement structure high susceptibility to frost due to insufficient thickness of frost resistance layer. The construction of thicker frost resistant layer increases road construction cost by 25% and, in most cases, there is no need to increase bearing capacity by increasing total thickness of pavement structure. In 2012 19 gravel roads were constructed using cost effective rehabilitation technologies—soft asphalt and double Otta Seal in Lithuania. This paper focuses on those two technologies’ performance within first three years of constructed roads exploitation. The implemented experimental research consisted of three parts by evaluating constructed roads base layers bearing capacity; pavement roughness; and pavement distresses and defects. As a result, the acceptable performance indicators were determined for both technologies—soft asphalt and double Otta Seal. Also recommendations for construction and exploitation improvement were defined.
Gravel roads play an important role in the transport infrastructure. They connect rural areas, recreation zones, and forestry into a single transport system. However, the maintenance of these kinds of roads requires a high part of the budget especially if the subgrade soils are weak. Praticò et al. [
Laurinavičius and Žilionienė [
The dustiness is caused by unbound aggregate particles, which are thrown up by passing vehicles. Jones [
Consequently, the control of dustiness is one of the most important activities of the maintenance. Typically, dustiness is restricted applying chemical dust suppressants, for example, calcium or magnesium chloride and calcium lignosulphonate. The analysis of the literature revealed that chemical dust suppressants might reduce the dustiness up to 80% and the total aggregate loss up to 42–61% [
The permanent solution for control of dustiness is the paving of gravel roads. However, naturally, it is impossible due to the lack of funds. For instance, according to Lithuanian Roads Administration under the Ministry of Transport and Communication data (2015), in Lithuania, more than third (33.9%) of state roads are gravel roads. The paving of all these roads needs millions of Euros. Also, the volume of traffic in many gravel roads is so low that paving is not economically feasible. According to Lithuanian requirements, gravel roads are paved if AADT ≥ 150 vpd in nonresidential area and if AADT ≥ 80 vpd in residential area.
As a rule, in Lithuania, gravel roads are paved with a single-layer asphalt pavement. The thickness depends on the number of equivalent single axle loads (ESALs) and usually varies from 6 cm to 10 cm. However, practice has shown that this kind of pavement is susceptible to frost because there is too thin frost resistance layer. It is solved by laying more than 30 cm thick layer of less frost-susceptible soils or improving subgrade. Unfortunately, those additional works increase roads construction cost up to 25% and usually are not fulfilled [
In foreign countries, gravel roads are effectively rehabilitated using soft asphalt or Otta Seal technologies [
Soft asphalt consists of aggregates, aggregates filler, and soft (low viscosity) bituminous binder. The requirements for soft asphalt are specified in Standard EN 13108-3. There are several types of soft asphalt (type A; type B; type C; and type S). The type is chosen according to desirable mechanical properties of asphalt mixture under specific climatic conditions.
The practice has shown that soft asphalt requires low temperature mixing and is elastic, durable, self-healing, well workable, recyclable, and so forth. However, it has low light reflection and limited resistance to abrasion. Sometimes it is unstable or it can form a water film on a new constructed pavement. Also, the flux used for binder is environmentally unfriendly [
Based on the Swedish General Technical Construction Specifications for Roads [
Otta Seal consists of soft (low viscosity) binder spraying on the surface followed by the spreading and rolling graded aggregates. The layer thickness is about 16–32 mm depending on the number of layers (single or double). Rolling and trafficking push aggregate particles into binder layer and the binder rises upwards through the aggregate particles and mechanically interlocks them. It results in a dense, durable matrix [
The practice has shown that Otta Seal is a durable and flexible pavement surface. It is tolerant of relatively high deflections, which are expected on low-volume roads. Also, the dense and closed texture slows down the rate of oxidation of the surfacing; that is, it slows binder ageing and hardening. Furthermore, there is a scope for utilizing labour-based methods in many aspects of Otta Seal construction. The main advantage is the ability to use relatively inferior aggregates, for example, screened gravel instead of crushed rock. It results in low construction cost [
The main disadvantage of the Otta Seal is its initial, inconsistent, and somewhat patchy appearance during the first 4–6 months of service life and a need for postconstruction care (especially, 3-4 weeks after construction). During this stage, the surface may become rich in bitumen or even “bleed.” On this kind of areas, sand or crushed dust is spread. It absorbs the excess of bitumen. Also, a newly constructed Otta Seal can be dusty and produce “flying stones” [
The application of Otta Seal is based on the traffic level. It is recommended to use the single Otta Seal with sand cover seal when the AADT ≤ 500 vpd and the double Otta Seal when the AADT > 500 vpd [
In Lithuania 19 roads sections of gravel were constructed using cost effective rehabilitation technologies. In 5 roads sections (total length 7.39 km) the wearing layer was soft asphalt; in 14 roads sections (total length 12.57 km) it was double Otta Seal (Table
Experimental roads sections of soft asphalt and double Otta Seal.
Road number | Road name | Road section (km) | Length (km) | Type of wearing layer |
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Klaipėda-Tauragė | ||||
1708 | Padubysys-Vosbutai-Butkiškė-Putriai | 9.70–10.35 | 0.65 | Double Otta Seal |
1716 | Pilis-Raudonėnai | 3.80–5.20 | 1.40 | Soft asphalt |
1717 | Armeniškiai-Aukštvilkiai | 9.19–9.30 | 0.11 | Double Otta Seal |
4516 ( |
Skaudvilė-Bijotai-Kelias Kaltinėnai-Kražiai | 9.95−10.90 | 0.95 | Double Otta Seal |
4516 ( |
Skaudvilė-Bijotai-kelias Kaltinėnai-Kražiai | 10.90−11.53 | 0.63 | Double Otta Seal |
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Šiauliai-Telšiai | ||||
2735 | Tirkšliai-Lėteniai | 2.00–2.62 | 0.62 | Double Otta Seal |
3208 | Rietavas-Lioliai-Mažieji Mostaičiai | 17.75–19.48 | 1.73 | Double Otta Seal |
4028 | Kairiai-Vėgeliai | 1.11–2.12 | 1.01 | Soft asphalt |
4118 | Laukuva-Vaitkaičiai-Kelias Rietavas-Tverai-Varniai | 9.30–10.00 | 0.70 | Double Otta Seal |
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Panevėžys-Utena | ||||
1235 | Kurtiniai-Aušra | 0.80–2.12 | 1.32 | Double Otta Seal |
2427 | Salamiestis-Stuburiai | 1.70–3.10 | 1.40 | Double Otta Seal |
2430 | Subačius-Čečeliai | 0.65–1.75 | 1.10 | Soft asphalt |
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Vilnius-Alytus | ||||
2518 | Veisiejai-Vainiūnai-Barzdžiūnai | 0.75–3.30 | 2.55 | Soft asphalt |
3918 | Jašiūnai-Keidžiai | 3.43–4.03 | 0.60 | Double Otta Seal |
4726 | Grendavė-Gruožninkai | 0.00−1.05 | 1.05 | Double Otta Seal |
5017 | Noškūnai-Panočiai | 7.52–8.38 | 0.86 | Double Otta Seal |
5235 | Šumskas-Vindžiūnai-Medininkai | 4.23–5.56 | 1.33 | Soft asphalt |
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Kaunas-Marijampolė | ||||
2642 | Mikoliškiai-Jakimavičiai | 0.00–0.95 | 0.95 | Double Otta Seal |
5123 | Gižai-Balsupiai | 2.50–3.50 | 1.00 | Double Otta Seal |
AADT and heavy vehicles AADT at experimental roads sections.
Soft asphalt was laid on the newly constructed unbound base layer of crushed stones that was on the existing gravel pavement. The thickness of the wearing layer and base layer was 4.5 cm and 15 cm, respectively. If the thickness of the existing gravel pavement was lower than 35 cm, it was increased using unbound materials.
The type of soft asphalt was selected according to the laboratory tests results assessed by the SAW method [
Characteristics of soft asphalt (SA 16-d-V6000 type C).
Characteristic | Road number | ||||
---|---|---|---|---|---|
1716 | 2430 | 2518 | 4028 | 5235 | |
Binder V6000 content (%) | 4.7 | 5.0 | 4.7 | 4.8 | 4.6 |
Air voids content (%) | 5.5 | 6.5 | 5.5 | 6.9 | 6.3 |
Ratio of indirect tensile strengths (ITS) | 60.0 | 99.8 | 66.0 | 68.0 | 71.0 |
Double Otta Seal was laid on the newly constructed unbound base layer of crushed stones that was on the existing gravel pavement. The thickness of the base layer was between 7 cm and 10 cm. If the thickness of the existing gravel pavement was lower than 30 cm, it was increased using unbound materials.
Double Otta Seal was used that consisted of 0/16-fraction aggregate and bitumen emulsion. The aggregate gradation of 0/16 fractions is shown in Figure
Characteristics of bitumen emulsions.
Bitumen emulsion type | Characteristics | ||
---|---|---|---|
Bitumen content (%) | Kinematic viscosity (mm2/s) | Adhesion (%) | |
C60B1PA-V6000 | 60.3–62.3 | 5305–10384 | 80–100 |
C60BF1-PA | 57.0–61.5 | 4481–8713 | 30–80 |
Aggregate gradation for double Otta Seal.
The bearing capacity of the base layer had to be not less than 120 MPa in both rehabilitation cases.
The research on soft asphalt and double Otta Seal performance consisted of three parts: evaluation of bearing capacity of the base layer; evaluation of the pavement roughness; qualitative visual assessment of defects.
Bearing capacity of the base layer was evaluated after its construction by static plate load test according to Lithuanian Standard LST 1360.10.
Pavement roughness was evaluated at the beginning of the service (autumn 2012) and after 1 year of the operation (summer 2013) by International Roughness Index (IRI). The Roads Surface Profilometer Dynatest 5051 RSP and the Mobile Roads Survey Laboratory RST 28 were used.
The qualitative visual assessment of defects was carried out annually. Roads sections of soft asphalt were assessed in spring and summer; roads sections of double Otta Seal were assessed in spring and autumn. Longitudinal cracking, transverse cracking, potholes, raveling, seals, and bleeding were the main factors representing soft asphalt performance. The performance of double Otta Seal was evaluated by three groups of defects and longitudinal and transverse cracks. The first group (P1) comprised fatting-up, bleeding, and tracking. The second group (P2) consisted of scabbing, tearing, and longitudinal joint crack. The third group (P3) was streaking. All defects which are the criteria for assessing the pavement performance are shown in Figure
Defects that are the criteria for assessing the pavement performance.
The amount of defects (except streaking) was expressed in percentage. It characterized the area or length (if there were analysed longitudinal and transverse cracks) of distressed pavement. The streaking was expressed in meters. It presented the length of the line or lines in which were lost chippings from a completed surface dressing.
The results of the qualitative visual assessment of defects strongly depend on researcher’s experience. Besides, the complexity of the assessment of fatting-up, bleeding, and tracking was noticed after first inspection of double Otta Seal performance. That is why in the report it was suggested to write 5%, 10%, 30%, 50%, and more than 50% if pavement suffers from fatting-up, bleeding, and tracking, from 0% to 5%, from 5% to 10%, from 10% to 30%, from 30% to 50%, and more than 50%, respectively.
The bearing capacity of the base layer after its construction is presented in Figure
Bearing capacity of the base layer (horizontal hyphenated line shows the required minimum value).
The pavement roughness at the beginning of the service (autumn 2012) and after 1 year of the operation (summer 2013) expressed by IRI is shown in Figure
IRI (horizontal hyphenated line shows the maximum value).
On roads sections with soft asphalt within three years of exploitation, longitudinal cracks, transverse cracks, bleeding, potholes, ravelling, and seals have been observed. The degree of pavement suffering from longitudinal cracks on each road section is shown in Figure
Longitudinal cracks on roads sections covered with soft asphalt.
In spring 2015 less transverse cracks were observed than in 2014 summer. These data might be determined because cracks healing could continue after conducted qualitative visual assessment of defects (typically the qualitative visual assessment of defects is carried out during the hottest period of summer) and the winter 2014/2015 was not severe. Similar tendencies of longitudinal cracks development were observed on road number 2518, which is located in Vilnius-Alytus region. There, the maximum and the minimum values were 4.15% and 0.39%, respectively. On road number 1716, which is located in Klaipėda-Tauragė region, longitudinal cracks (0.07%) completely healed in summer 2013 and have not formed again. In both roads (road number 4028, which is located in Šiauliai-Telšiai region, and road number 5235, which is located Vilnius-Alytus region) longitudinal cracks did not form from the beginning of exploitation.
The degree of pavement suffering from transverse cracks on each road section is shown in Figure
Transverse cracks on roads sections covered with soft asphalt.
The degree of pavement suffering from bleeding on each road section is shown in Figure
Bleeding on roads sections covered with soft asphalt.
Potholes appeared only on road number 2518 after half a year of operation. It was less than 0.01%. Because of pavement deterioration it has increased 2.25 times but still remained less than 0.01%.
The degree of pavement suffering from ravelling on each road section is shown in Figure
Ravelling on roads sections covered with soft asphalt.
There, the average amount of ravelling within three years of operation is only 0.02% and the maximum value of 0.03% was determined in spring 2015. The reason of ravelling is a reduction in adhesion, that is, the loss of bond between aggregate particles and the binder. It results from water on the pavement, which causes the loss of the binder, and vehicular traffic, which gradually ravels away the aggregate particles.
The degree of pavement suffering from seals on each road section is shown in Figure
Seals on roads sections covered with soft asphalt.
On roads sections with double Otta Seal within three years of operation, longitudinal cracks, transverse cracks, fatting-up, bleeding, tracking, scabbing, tearing, longitudinal joint crack, and streaking have been observed. The degree of pavement suffering from longitudinal cracks on each road section is shown in Figure
Longitudinal cracks on roads sections covered with double Otta Seal.
The degree of pavement suffering from transverse cracks on each road section is shown in Figure
Transverse cracks on roads sections covered with double Otta Seal.
Similar tendency of transverse cracks decrease was also observed on other roads. Hence, on the roads covered with double Otta Seal, transverse cracks self-heal under high pavement temperatures. Five roads (road number 1708, road number 1717, road number 3918, road number 2642, and road number 5123) are resistant to development of transverse cracks. On other roads the average amount of transverse cracks within three years of operation was less than 1%.
The degree of pavement suffering from P1 (fatting-up, bleeding, and tracking) on each road section is shown in Table
Region | Road number |
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2012 | 2013 | 2014 | 2015 | |||||
Autumn | Spring | Autumn | Spring | Autumn | Spring | Autumn | ||
Klaipėda-Tauragė | 1708 | 5 | 5 | 5 | 5 | 5 | 5 | 5 |
1717 | 30 | 30 | 30 | 30 | 30 | 30 | 30 | |
4516 ( |
10 | 10 | 30 | 30 | 30 | 30 | 50 | |
4516 ( |
5 | 5 | 10 | 10 | 5 | 5 | 10 | |
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Šiauliai-Telšiai | 2735 | 5 | 5 | 5 | 5 | 5 | 5 | 5 |
3208 | 30 | 30 | 30 | 30 | 30 | 30 | 30 | |
4118 | 5 | 5 | 10 | 10 | 10 | 5 | 10 | |
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Panevėžys-Utena | 2427 | 50 | 50 | 50 | 30 | 50 | 30 | 50 |
1235 | 30 | 30 | 50 | 50 | >50 | 50 | >50 | |
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Vilnius-Alytus | 3918 | 5 | 5 | 10 | 10 | 30 | 50 | 50 |
4726 | 5 | 5 | 5 | 5 | 5 | 5 | 5 | |
5017 | 10 | 10 | 30 | 30 | 30 | 10 | 10 | |
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Kaunas-Marijampolė | 5123 | 5 | 5 | 5 | 5 | 5 | 5 | 5 |
2642 | 5 | 5 | 5 | 5 | 5 | 5 | 5 |
The number of roads sections covered with double Otta Seal according to severity of failure P1.
The degree of pavement suffering from P2 (scabbing, tearing, and longitudinal joint crack) on each road section is shown in Figure
P2 (scabbing, tearing, and longitudinal joint crack) on roads sections covered with double Otta Seal.
The degree of pavement suffering from P3 (streaking) on each road section is shown in Figure
P3 (streaking) on roads sections covered with double Otta Seal.
The implemented research on soft asphalt and double Otta Seal performance within first three years revealed the following: The construction of soft asphalt and double Otta Seal is a reasonable solution for dustiness control on gravel roads with AADT and ESALs less than 500 vpd and 0.1 mln, respectively. As the bearing capacity of constructed roads base met technical regulation requirements it had no negative impact on soft asphalt and double Otta Seal performance. However, the deformation module All tested road sections met IRI requirement for roads of regional significance (≤3.5 m/km). It can be stated that both technologies have no negative impact on pavement roughness after three years of exploitation. Both technologies showed self-healing effect after warm seasons. Dependently on the road peculiarities and technology it was observed that from 13% to 100% distresses healed after each summer. Longitudinal cracks, transverse cracks, and bleeding can be defined as dominant distresses of road sections with soft asphalt. Fatting-up, bleeding, and tracking (P1) and, on some roads, scabbing, tearing, and longitudinal joint crack (P2) can be defined as dominant distresses of road sections covered with double Otta Seal. There is not any evidential relationship between the degree of distresses and AADT, ESALs, or road section location. Critical factor for road sections covered with double Otta Seal is incomplete construction and maintenance of constructed road sections. During construction phase light moistening should be applied on the base before bitumen emulsion spreading. Two-stage construction of first and second layer of double Otta Seal is strictly recommended; the second one should be applied not less than after four weeks of road exploitation and maintenance of emerged defects in first layer. The trial emulsion and aggregate spreading and compaction rolling are strictly recommended for Otta Seal technology in order to verify the emulsion and aggregate application rates and a sufficient number of roller passes.
The authors declare that they have no competing interests.