Examination of Rutting and Weathering Characteristics of various Danish Road Classes Danish Road Institute Report 111 2001 Ministry of Transport - Denmark Road Directorate Danish Road Institute Elisagaardsvej 5 P.O. Box 235 DK-4000 Roskilde Denmark Telephone: +45 46 30 70 00 Telefax: +45 46 30 71 05 e-mail: vd@vd.dk web: www.vd.dk Title: Author: Dated: Copyright: Published by: ISBN: ISSN: Examination of Rutting and Weathering Characteristics of various Danish Road Classes Jørn Raaberg, Jeanette Rosenberg September 2001 Road Directorate, All rights reserved Road Directorate, Danish Road Institute 87-90145-88-7 0909-1386 Examination of Rutting and Weathering Characteristics of various Danish Road Classes Jørn Raaberg Jeanne Rosenberg Danish Road Institute Report 111 2001 1 1 Contents Contents.........................................................................................................................3 Abstract ........................................................................................................................4 1. Introduction ..............................................................................................................5 2. The investigated road sections.................................................................................6 3. Material composition................................................................................................7 4. Stripping test.............................................................................................................9 5. Dynamic creep test .................................................................................................10 6. Discussion ................................................................................................................12 7. Conclusion...............................................................................................................14 8. Literature ................................................................................................................15 3 Abstract The Danish road network is divided into four classes: Motorways, main roads, rural roads and local roads. In order to optimise the lifetimes of the bituminous surfacings in these road classes economically, it is necessary to consider the rutting and weathering resistance of road pavements. Optimisation of rutting resistance often results in a lean, stone rich layer that is less weather resistant than a bitumen rich layer. For low trafficked roads it is therefore, important to optimise weathering resistance, whereas for heavily trafficked roads, optimising rutting resistance is more important. A three year project commenced in 1998 to research the functional performance of bituminous wearing courses and road base materials designed and produced by Danish asphalt contractors. The objective of the project was to ensure that bituminous materials tested in the Danish Asphalt Rut Tester can be related to other well known testing methods and to measurements of rutting actually occurring on roads so that a basis can be established for evaluating the rutting and weathering resistance of future bituminous materials. Ten pavements were selected that were due for overlaying and which suffered from rutting generated mainly in the bituminous layers. Cores were taken from these pavements in and between the wheeltracks. On the basis of their rutting and weathering resistance a number of laboratory tests were conducted to determine whether these different bituminous materials could be classified as suitable surfacing materials for three road classes. These laboratory tests included material composition, stripping and dynamic creep test on the samples from the upper 10 to 18 cm of the surfacing in the selected pavements. This paper describes the part of the project in which the observed in situ conditions were compared to the results from the laboratory tests carried out on the samples taken from the ten pavements. This comparison resulted in a classification of the bituminous mixes investigated in terms of their rutting resistance and weathering resistance. 4 1. Introduction This paper describes the properties of flow rutting and stripping of the bituminous layers of selected road pavements. Research regarding flow rutting and stripping of samples prepared in the laboratory have been performed previously at the Danish Road Institute (DRI) [1]. A great difference in the structural design has however been observed between laboratory prepared samples and samples taken in situ [2, 3, 4]. Some uncertainty must therefore be expected when test results of these two types of sample preparations are compered. In the light of the previously mentioned, it was decided in this project exclusively to use samples taken in situ for the laboratory testing and compare data from these tests with data from in situ measurements of selected pavements In Denmark, it is common practice to measure rutting of the pavements of the Danish road network regularly with the Profilograph. The criteria for the selection of the pavements for this investigation were thus road pavements where there had been or currently was rutting to a greater or lesser extent generated in the asphalt layers. In order to characterise the individual asphalt types and pavement designs of the selected road pavements, the following laboratory tests were performed on samples taken in situ: Material composition of the individual layers. Durability test, where the material's resistance to stripping was tested. Dynamic creep test, where the material's resistance to permanent deformation was tested. In the following sections, a characterisation of the pavements, the laboratory tests and the results obtained are described in detail. 5 2. The investigated road sections In 1998, DRI selected in a co-operation with four road administrations possible road pavements as investigation objects, based on the above criterion. Ten road sections were selected, based on the following road classes: · · · 2 motorways (M) with an ADT > 20,000 and with an EASELs >1400. 7 main roads (MR) with an ADT between 6,000 and 12,000 and with an EASELs between 300 and 1300. 1 rural road (RR) with an ADT < 4,000 and with an EASELs < 300. On the ten road sections, the following material types are included in the upper three to four asphalt layers, Table 1. Type Classification Air voids (%) ABS *) £ 8.0*) 60*) £ 9.0 60, 85, 180 GAB S Hot rolled asphalt with chippings Asphalt concrete, dense Asphalt concrete, gap graded Soft asphalt, dense Stone mastic asphalt Binder course GAB Basecourse Course Wearing course AB t AB aa PA t SMA Binder and Basecourse Bitumen content (%) Penetration (1/10 mm) 300 or softer £ 7.0 60 £ 8.0 ³ 4.8 60 £ 8.0 / £ 10.0 ³ 5.0 / ³ 4.5 60, 85 *) The requirements are based on a previous Road Standard [5], since hot rolled asphalt with chippings is not included in the present Road Standard [6]. Table 1: Grade of materials and some selected requirements from the Road Standard. 6 3. Material composition In order to substantiate the results from the dynamic creep tests and weathering resistance tests, the material composition was determined for each layer of the upper 10-18 cm, corresponding to the upper three to four asphalt layers of the pavements. Drilled cores from the pavements were used for the material examination with a diameter of 100 mm, taken in and between the wheeltracks. Table 2 shows some of the data of the material composition from the ten road sections. All data is determined according to Danish Test Methods [7,8]. It can be observed from Table 2 that the road sections 152/614 (layer 1 and 3), 332B (layer 3), 332C (layer 3), 363 (layer 1) and 608 (layer 1) contain one or several layers with a relatively low calculated air voids content. Furthermore, Table 2 shows that layer 3 of road sections 14 and 608 has a relatively high calculated air voids content. On several of the pavements there is a marked difference in the calculated air voids content in cores taken in and between the wheeltracks. Also it can be observed that road sections 125, 152/614 and 608 contain one or several asphalt layers with a relatively high penetration, corresponding to a soft binder. This can be considered acceptable for the rural road section 608, as a pavement of this type of road class according to the Road Standards does not require a hard type of binder. However, it is critical for main roads such as road sections 125 and 152/614, since the traffic intensity is higher. The third layer of road section 363 consist of an asphalt layer with relatively low penetration and low bitumen content. 7 Road classes Section 11 Motorway 14 125 152/614 332 A Main road 332 B 332 C 363 401 Rural road 608 Materials Types Air voids in wheelbetween track wheeltrack (volume-%) (volume-%) 4.1 3.4 4.6 4.5 5.2 5.9 2.6 3.5 4.0 3.2 10.9 11.3 3.9 5.2 3.4 5.4 2.1 2.7 0.3 1.4 3.6 5.8 -0.2 3.1 4.1 3.4 4.6 4.5 5.2 5.9 5.2 7.2 2.7 4.4 1.3 1.2 4.7 7.4 7.6 7.8 0.2 0.6 1.6 2.0 4.9 4.5 6.2 6.8 2.0 2.6 3.7 6.3 2.4 8.4 8.0 7.7 1.8 1.4 Bitumen data Bitumen Penetracontent tion (weight-%) (1/10 mm) 6.6 65 5.5 41 4.9 40 6.9 44 5.6 24 4.6 45 5.1 101 5.1 109 5.5 122 5.3 95 6.7 99 6.4 245 6.7 69 4.9 85 5.3 59 5.3 45 5.3 54 5.0 64 5.2 56 5.0 46 5.6 62 5.6 69 5.3 49 4.3 16 5.1 28 4.9 67 5.9 57 4.6 50 5.1 146 ABS GAB GAB ABS AB GAB AB t GAB GAB AB t AB PA t SMA GAB GAB AB aa GAB S GAB AB GAB GAB GAB GAB GAB AB t AB AB t GAB AB t Layer thickness (mm) 35 30 40 40 40 50 30 40 90 30 15 25 35 70 30 30 45 75 30 50 80 60 65 60 25 20 15 55 30 GAB 60 5.6 7.1 4.8 50 AB 30 10.9 9.1 4.8 25 Table 2: Selected data of the material composition of samples from the ten road sections. 8 4. Stripping test The test method used in Denmark to determine the adhesion between an aggregate and a bitumen is normally the "Rolling Bottle Method" [9]. This method tests only a certain fraction of the aggregate and does not give any information about the properties of the asphalt mix. In this particular project, the Danish Road Institute has used a method where the mix itself is tested. The weathering resistance test is performed by determining the Stiffness Modulus of the individual layer of the pavement samples (cores with a diameter of 100 mm) before and after the test samples have been exposed to an accelerated condition procedure according to Lottman, 1982 [10]. The Stiffness Modulus was determined by the Nottingham Asphalt Tester (NAT) according to British Standard DD 213:1993 [11]. It should be mentioned that only asphalt layers with a thickness greater than 30 mm have been tested, as required in the test method. The result of this test is shown in Figure 1 as the mean change of the Stiffness Modulus in percent. Difference in Stiffness Modulus (%) 10 0 -10 40 50 60 70 80 -20 90 100 Basecourse Binder course Hot rolled asphalt with chippings -30 -40 -50 -60 -70 Voids filled with bitumen (%) Figure 1: Results from the weathering resistance test shown as the difference in Stiffness Modulus as a function of the air voids filled with bitumen of samples taken from the road sections. Only the changes of more than 20% in the Stiffness Modulus after accelerated conditions are significant. Based on the results shown in Figure 1 the materials have been divided according to the amount of voids filled with bitumen with regard to their resistance to stripping, as follows: · Good resistance to stripping: voids filled with bitumen > 80%. · Medium resistance to stripping: 60%< voids filled with bitumen < 80%. · Poor resistance to stripping: voids filled with bitumen < 60%. 9 5. Dynamic creep test In order to test the pavements' resistance to rutting, samples were taken as drilled cores with a diameter of 150 mm of the entire pavement construction between the wheeltracks. By using samples taken between the wheeltracks it is possible to obtain a material structure and properties which are similar to the material which originally was used for paving, since the pavement in this area only has been exposed to low traffic load. There is however a hardening of the binder, which will influence on the rutting properties of the pavement. Dynamic creep tests were performed according to FAS method 468-97 [12]. The method prescribes that on samples with a layer thickness less than 30 mm the test should be performed on a stacked layer of the same material. This was done for two different types of asphalt mixes and compared to the dynamic creep test performed on the entire pavement construction of the two layers. The results showed that the two tests were comparable. It was therefore decided to perform dynamic creep tests of the entire pavement construction of the three to four upper asphalt layers corresponding to a layer thickness of 10-18 cm. According to the method described above the total strain should be stated after 3,600 pulses. Two dynamic creep test series reached a state of tertiary flow before the 3,600 pulses were reached. Therefore, the creep rate was additionally given. Based on the results of the dynamic creep tests, the road sections can be divided into the following categories of rutting properties: Rutting performance Good Medium Poor Section 332 B 332 C 14 363 401 608 11 332 A 152/614 125 Road classes M M M MR MR RR M MR MR MR Creep rate (me/pulse ) Total strain (me) Year of construction E10 1999 ADT 1999 1.0 1.0 1.1 1.2 1.2 1.5 3.0 5.5 6.0 7.5 11,652 13,814 12,112 11,133 12,738 12,614 24,695 33,510 + + 1994 1995 1983 1998 1987 1984 1987 1983 1988 1986 695 695 1916 760 680 213 1466 1223 900 399 6363 6363 32,834 6923 7688 3,531 24,432 11,376 11,193 6500 * Rutting has been repaired three times + Tertiary flow before 3,600 pulses Mean rutting in situ (mm) 3.0 1.6 8.5 5.8 4.7 3.4 11.3 6.9* 8.1 8.8 Table 3: Results from dynamic creep test of samples taken from the ten road sections. 10 As can be observed from Table 3, it could be considered appropriate to divide the road classes according to the creep rate with regard to their resistance to rutting, as follows: · Good resistance to permanent deformation: Creep rate ≤ 2.0 me/pulse for motorways and heavily trafficked main roads. · Medium resistance to permanent deformation: 2.0 me/pulse < creep rate ≤ 4.0 me/pulse for main roads. · Poor resistance to permanent deformation: Creep rate > 4.0 me/pulse for rural roads. Table 3 shows that the pavement of the rural road section 608 has good resistance to permanent deformation, which - seen from an economic point of view is not appropriate - since the traffic load is relatively low. On the motorway such as road section 11, the pavement has a medium resistance to permanent deformation, which again is not appropriate since the pavement has a relatively high traffic load. Also the materials on main road sections 332A, 152/614 and 125 are not sufficiently resistant to permanent deformation in relation to the traffic load on the pavements in question. It should be mentioned that the data measured by means of dynamic creep test do not give a particularly good correlation to the mean rutting measured in situ. This can be due to the fact that the permanent deformations measured in situ partly come from lower-lying asphalt layers and/or unbound layers. 11 6. Discussion Variations in compaction of asphalt pavements in situ lead to a variation in material properties. Variation in test results based on in situ samples is therefore larger than test results based on laboratory prepared samples. It was however, decided to perform laboratory tests using in situ samples to obtain a good basis for comparison to measurements performed in situ. A comparison of the material composition and the creep tests shows clearly that the pavements where a soft binder was used have the highest creep rates. This could lead to rutting problems, for pavements exposed to high traffic loads. Other mix design parameters, such as aggregate size distribution and the type of aggregates used, can also lead to less rut resistant roads. A comparison of the material composition and the weathering resistance test shows that asphalt materials with a high content of air voids have a poor weathering resistance in contrary to asphalt materials with a high content of bitumen. The testing of the pavement's resistance to stripping has made it possible to differentiate between the different materials from the content of voids filled with bitumen. Figure 2 below shows the correlation between weathering resistance of the asphalt layer with the highest difference in Stiffness Modulus after accelerated conditions and rutting properties for the full construction for the ten road sections investigated. 85 Voids filled with bitumen (%) 80 332 A 75 152/614 125 70 0 1 2 3 65 11 332 B 363 4 5 6 7 8 Creep rate (me/pulse) 60 332 C 401 55 Motorway 608 50 14 45 Main road Rural road 40 Figure 2: Characteristics of rutting properties and weathering resistance for the ten road sections shown as the correlation between creep rate and air voids filled with bitumen. 12 Figure 2 supports the conclusions reached in part 5: Dynamic creep test. The rural road section 608 has good rutting resistance, but poor durability, which is not optimal seen from an economic point of view in relation to the actual traffic load. The motorway section 11 has only medium rutting resistance despite the fact that the actual traffic load is considerably higher. The main road sections 125, 332 A and 152/614 has poor rutting resistance and the pavements are not economically viable. The material tests have shown that on some of the road sections, an inexpedient selection of pavement type for the actual traffic load has been made. This could be due to the fact that there in general has been a considerably larger increase in traffic load than expected. 13 7. Conclusion No simple correlation was found between in situ rutting and the laboratory tests of samples taken in situ. This may be due to the fact that there are too many variable factors which have an influence on the rutting properties of asphalt materials, such as the hardness of the binder, the amount of bitumen, the aggregate size distribution, air voids content, etc. The ten road sections have however been classified according to their rutting properties based on creep rate determined by dynamic creep tests and their stripping properties based on the voids filled with bitumen. It is demonstrated that the resistance to stripping and resistance to permanent deformation of the asphalt materials were not always optimised in relation to traffic load. By performing a few simple tests of the in situ asphalt materials, a more economically viable solution can be ensured. It should however be mentioned that the tests described in this paper regarding the rutting and stripping properties are not suitable for mix design, where laboratory samples usually are used. The tests described are relevant when an existing road pavement is due for repairing. The tests could improve the evaluation whether it would be economically profitable if one or several asphalt layers should be removed before overlaying. 14 8. Literature [1] Raaberg, J., et al., Fællesafprøvning af diverse asfalttyper (i forbindelse med revision af vejregel for varmblandet asfalt). Eksternt notat 8, Vejteknisk Institut, 1998. [Testing of various types of asphalt - in connection with a revision of the Road Standard for hot-mixed asphalt]. [2] Eriksen, K., Wegan, V., Krarup, J., Air void content and other air void characteristics of asphalt concrete by image analysis. Contract No.: SHRP-88-AIIR13, Phase 2. [3] Eriksen, K., Air void characteristics in asphalt-concrete samples from the compaction study, Contract No.: SHRP-88-AIIR-13, Phase 3a. [4] Raaberg, J., Investigation of gyratory compaction used for asphalt mix design. Proceedings of the 7th Euroseminar on Microscopy Applied to Building Materials, Delft, June 29-July 2, 1999. [5] Udbuds- og anlægsforskrifter, varmblandet asfalt, Almindelig arbejdsbeskrivelse. November 1994 [Danish Road Standard - hot-mixed asphalt]. [6] Udbuds- og anlægsforskrifter, varmblandet asfalt, Almindelig arbejdsbeskrivelse. November 1998. [Danish Road Standard - hot-mixed asphalt]. [7] Prøvningsmetoder - Vejmaterialer, 0-59. Vejdirektoratet. [Test methods - Road materials 0-59]. [8] Provisoriske prøvningsmetoder, 1-99.Vejteknisk Institut. [Provisional test methods 1-99]. [9] Bituminous mixtures - Test methods for hot mix asphalt - Part 11: Determination of the compatibility between aggregate and bitumen, prEN 12697-11:1999. [10] Lottman, R. P., Predicting Moisture-induced damage to asphalt concrete, Field evaluation. National Cooperative Highway Research Program, Report 246, Transportation Research Board, May 1982. [11] BSI, DD213:1993 Draft for development, Technical Committee B/510. [12] Bestämning av deformationsresistens med dynamisk kryptest. FAS Metod 46897. 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