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Interlayer bonding of binder, base and subbase layers of asphalt pavements: Long-term performance
Construction and Building Materials 23 (2009) 2926–2931
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Interlayer bonding of binder, base and subbase layers of asphalt pavements: Long-term performance Christiane Raab *, Manfred N. Partl 1 Swiss Federal Laboratories for Materials Testing and Research EMPA, Ueberlandstr. 129, 8600 Duebendorf, Switzerland
a r t i c l e
i n f o
Article history: Received 7 September 2008 Received in revised form 10 December 2008 Accepted 8 February 2009 Available online 17 March 2009 Keywords: Interlayer bond Shear testing Asphalt pavements Binder Base and subbase layers Long-term performance
a b s t r a c t With bond testing of asphalt layers becoming a more important topic as well as in research as in every day pavement construction, the question of long term behaviour of bonding properties has come up. In recent research it has been shown that the bonding properties between surface and binder courses seem to improve with time considering that the surface course does not show distress phenomena and that the traffic volume does not exceed the traffic considered for pavement design. That bond failure is not only a question of the top layers is a well known fact; therefore the long term properties of the lower layers are also of interest. The paper focuses on the effect of long term behaviour of bonding properties of the lower pavement layers (binder or base and subbase courses) presenting the results of an extensive Swiss study. The study compares the bonding properties determined with the layer parallel shear test (LPDS) according to Leutner of high volume roads after a 9–13 years period. It was found that for rehabilitations or old constructions an improvement cannot be expected and especially in case of old constructions seems to depend on their service life. For the bond between lower layers decreases of shear force values down to 50% are possible. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction and objective During recent years testing of the bond between pavement layers has gained more and more importance throughout Europe [1–4]. In many projects the bonding properties are determined directly after construction but the question of how the originally determined properties develop over time was only investigated in a very limited way. Although some researchers [5–7] showed that the bonding improves with time, this finding was never investigated on a larger scale in a systematic way. In some cases [7] the re-evaluation of the shear strength took place just one year after construction and therefore cannot consider long term effects and influences. Furthermore, the improvement of bonding properties is stated as a general fact, but no reference to traffic data is provided. Only recently, the long-term performance of the bonding properties between surface and binder courses was investigated on a larger scale [8–9]. Fig. 1 summarises the results from the above mentioned research project. The figure depicts the shear force values determined with the shear test according to Leutner [12] directly after construction and 10–13 years later. Apart from sections 18 and
* Corresponding author. Tel.: +41 44 8234483; fax: +41 1 8216244. E-mail addresses: [email protected] (C. Raab), [email protected] (M.N. Partl). 1 Tel.: +41 44 8234113; fax: +41 1 8216244. 0950-0618/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2009.02.025
19 where the underlying course was old, all binder or base courses had been constructed at the same time as the surface courses. Since the coring took place before trafficking, there was no distinction between inside and outside the wheel track. In that research which compared the bonding properties of new surfaces courses and new or existing binder or base courses directly after construction and after a 10 years period it could be shown that the bonding properties generally improve by 10–30%. The percentage of improvement of the shear force depends on the value directly after construction. Here, when the values directly after construction are very high, only relatively low improvements can be observed (e.g. section 12 with MA), whereas in case of low values directly after construction a greater potential for improvement could be seen (e.g. section 13 with PA). It is important to note, that the bonding values only improve over time when the surface course is intact and does not show distress phenomena such as rutting or cracking. In case of ruts and cracks often the bonding properties in the wheel track as the most stressed parts of the pavement showed a decline, while the values outside the wheel track increased. Furthermore, the traffic volume plays an important role and a very high traffic volume or a high percentage of heavy vehicles could also lead to a deterioration of the bond. The aim of this paper is the investigation of the long term bonding properties for binder, upper and lower base as well as subbase layers and to compare these results to the results for surface courses. Since in the previous research project the shear force values had been determined for all the pavement, these data was now
2927
C. Raab, M.N. Partl / Construction and Building Materials 23 (2009) 2926–2931
Shear Force [kN]
50
SMA 11
AC 11
40
AC 16
Special: HRA, MA, PA
before after
30 20 10 0
2
3
8
5
7
14
6
10
15 Site
18
19
11
12
13
Fig. 1. Comparison of maximum shear forces at 20 °C between surface and binder courses for pavements with different surface mixes (SMA 11, etc.) directly after construction (before) and 10 years later (after) according to [8–9]. The error bars represent the standard deviation.
used to evaluate the long-term performance of the lower pavement layers. 2. Situation and material In 1999, a research project [10] on the interlayer bonding of new pavement layers was completed. It was found that the layer parallel direct shear test (LPDS) according to Leutner [12] was a useful tool for interlayer shear bonding assessment. The question of long-term performance of the interlayer shear bonding, in terms of the relationship between the bonding properties directly after construction and the bonding properties after some years of trafficking was only touched on in a limited way in this former project. Therefore, a follow-up project was conducted [8] where all 14 remaining sections were investigated again, after nine or more years of trafficking, in order to provide information on the long-term performance of the bonding properties. Table 1 summarises information on the pavement structure, the mixture types of the different layers and the available traffic data of the different sites. Since the construction took place before the European Standards became effective, all terms follow the old Swiss Standard [14]. Seven pavements had stone mastic asphalt (SMA) and four road sections had asphalt concrete (AC) surface courses. In addition, three coring sites with special surface courses, i.e. gussasphalt (mastic asphalt) (MA), hot-rolled asphalt (HRA) and porous asphalt (PA) were included. The surface courses were placed either on hot mix binder (AC) and mastic asphalt (MA) courses with a nominal maximum aggregate size of 16 mm or on hot mix base courses (AC) with a nominal maximum aggregate size of 22 or 32 mm. In two cases, pavements with new surface courses on unknown old AC courses were investigated (coring sites 18 and 19). According to an analysis the mixture type of these unknown layers could be evaluated to be most probably AC 10 (AC 10 old) according to the Swiss standard from 1976. For these two sections, again, the base course material, which was unknown and was determined to be AC 16 (AC 16 old) according to the Swiss standard from 1976 [15]. The second base courses normally consisted of a hot mix base course with maximum aggregate sizes of 22 or 32 mm. In one case the base course was even constructed according to the previous version of the Swiss Standard [15] when the maximum aggregate size had been 30 mm instead of 32 mm. For section 7 the special base course CSHM 22, a high modulus base course, had been used. For one motorway pavement (section 7) a third base course consisting of a hot mix base course (AC) with maximum aggregate size of 32 mm could be found. The subbase
courses, here called hot subbase mixes AC F (fundation) to distinguish them from the base courses, had a maximum aggregate size of 22 mm. In one case the subbase material was again according the old Swiss standard from 1976 and therefore had a maximum aggregate size of 40 mm (AC F 40 old). In the original investigation, all surface and binder or upper base courses apart from the binder courses of sections 18 and 19 were totally new. As for the investigated first and second base courses, six were totally new, while another six consisted of existing layers containing mixtures according to the Swiss standard from 1976 [15]. For nearly all road sections, traffic data, e.g. the average daily traffic in vehicles (ADT) and the percentage of heavy vehicles (vehicles >3.5 t) are given in Table 1 [11]. The coring for the investigation of the long-term performance was conducted a few meters away from the original coring site. For every road section 5 cores were taken inside and another five outside the wheel track.
3. Testing The layer parallel direct shear (LPDS) test device (Fig. 2), [10,13] is a modified version of equipment developed in Germany by Leutner [12] being more versatile in the geometry and more defined in the clamping mechanism. The modified LPDS test device fits into an ordinary servo-hydraulic Marshall testing machine and allows the testing of cores with a diameter of about 150 mm and rectangular specimens; no normal force can be applied. One part of the core (up to the shear plane to be tested) is placed on a circular u-bearing and held with a well defined pressure by a semicircular pneumatic clamp. The other part, the core head in Fig. 2, remains unsuspended. Shear load is induced to the core head by a semicircular shear yoke with a deformation rate of 50.8 mm/min, thus producing fracture within the pre-defined shear plane of 2 mm width [10,13]. The specimens were conditioned in a climate chamber for 8 h and all tests were conducted at a temperature of 20 °C.
Table 1 Sites with pavement characteristics and traffic data. Site
Mixture types Surface
2 3 5 6 7 8 10 11 12 13 14 15 18 19
SMA 11 SMA 11 SMA 11 SMA 11 SMA 11 SMA 11 AC 11 HRA 11 MA 11 PA 11 SMA 11 AC 11 AC 16 AC 16
Traffic Binder
AC 16
MA 16 AC 16
AC 10 old AC 10 old
1. Base
2. Base
AC AC AC AC AC AC AC AC AC AC AC AC AC AC
unknown
32 32 22 22 22 32 32 22 22 32 old 22 32 16 old 16 old
3. Base
AC 32 CSHM 22 AC 32 AC 32 AC 30 old AC 22 AC 30 old AC 32
Subbase
AC F 22 AC 32
AC F 22 ACF 40 old
ADT (veh/d)
>3.5 t (%)
– 18,350 32,700 94,990 19,800 31,450 77,890 6800 32,700 31,500 28,050 64,230 – –
Buses 9.8 7.6 4.4 4.4 5.5 11.1 2.6 7.6 4.6 5.8 8.2
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C. Raab, M.N. Partl / Construction and Building Materials 23 (2009) 2926–2931
Fig. 2. LPDS test device, schematic drawing.
From the LPDS test the shear force F as a function of shear deformation w is obtained. Additional to the shear force the maximum slope from the diagram of shear force F versus shear deformation w can be used to define the LPDS maximum shear ‘‘stiffness” value SLPDS as follows:
SLPDS ¼
DF Dw
ð1Þ
Note, that a comparison of shear loads is only valid in case of similar diameters. If diameters change, the nominate shear stress F/A can be used. 4. Test results 4.1. Shear force Fig. 3 shows the LPDS test results at 20 °C for the different test sections and top layer materials. The figure represents the comparison of the maximum shear force values immediately after construction (before) and the maximum shear force values after about 10 years later (after). In case of the values immediately after construction (before) all points are mean values of at least seven specimens. In case of the values after several years of trafficking (after) in the upper part of the figure the mean value of all five cores taken in the wheel track is given, while the lower part depicts the mean value of all five cores taken outside the wheel track. The standard deviation is also presented. In comparison with Figs. 1 and 3 shows that the long term behaviour of the bond between binder on upper base and lower base on subbase courses is different to the one between surface and binder or upper base courses. When in the latter case an improvement can be generally stated (see Fig. 1), for the lower layers the trend towards an improvement is less obvious. Comparing the upper and lower part of Fig. 3 also reveals that there are considerable differences between the shear force values inside and outside the wheel track. As visible in the figure, from 14 tested cases overall only six show a clear improvement of shear forces over time and therefore their values can always be found above the 45° line of equality. While in two cases, AC 22 upper base (sections 12) and AC 32 lower base (section 13), the values before and after (in and outside the wheel track) more or less remain the same (values on or near the 45° line), in one case a distinct deterioration of the shear force value of more than 50% can be found (AC 10 binder, sections 18, wheel track). For sections 2, 7, 10, 11, 14 and 19 the behaviour changes with respect to the values inside or outside the wheel track. Here, a dramatic decrease in the wheel track, such as for AC 32 upper base (sections 10) and AC 22 upper base (section 7), or outside the wheel track, AC 22 upper base (section 14), is observed.
Fig. 3. Comparison of maximum shear forces at 20 °C between lower pavement layers directly after construction (before) and about 10 years later in (after, wheel track) and outside the wheel track (after, outside).
The differences between the values in and outside the wheel track appear to be higher for the binder on upper base courses, which are more affected by the traffic loads than the lower base on subbase courses. Nevertheless, it is necessary to mention that the number of tested binder and upper base courses is considerably higher than the one for lower base on subbase courses. When looking at the different mixture types and aggregate sizes, regarding an increase or decrease of the shear force with time, no difference can be found between them. This fact is not surprising since all investigated layers consist of dense graded AC mixtures. In order to see if a difference in long-term performance regarding the age of the structure can be found, Fig. 4 was composed. In this figure the same results are depicted as in Fig. 3. But this time the focus is on whether the construction was totally new (new/
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C. Raab, M.N. Partl / Construction and Building Materials 23 (2009) 2926–2931
Shear Force [kN]
before
new/new
50
after, wheel track
old/old
new/old
Base/Subbase new/new old/old
after, outside
40 30 20 10 0
5
7
8
12
14
15
2
10
11 14 Site
13
18
19
5
12
13
Fig. 4. Comparison of maximum shear forces at 20 °C between lower pavement layers directly after construction (before) and about 10 years later in outside the wheel track, grouped according to the age of the pavement structure.
new), or a rehabilitation on an existing construction (new/old) or whether the bond between existing layers had been tested (old/ old). Again, the results inside and outside the wheel track are shown. Fig. 4 illustrates that, similar to the behaviour of surface courses (which were always new), new constructions of binder and upper base courses seem to have the greatest potential for an improvement of the shear force values. According to Fig. 4, this finding also appears true for the bond between lower base and subbase courses. Since in this investigation only lower layers were evaluated, it was not possible to detect surface distress phenomena. Sections 3 and 15 which showed distresses such as cracking and rutting, had either no further layers (section 3) or their lower layers appeared to be in order (section 15). Regarding the traffic data of section 2, which had a high number of bus traffic, but relatively high shear force values for the bond between surface and upper base layer, obviously had more problems with the bond between upper and lower base layer. The same proves to be the case for section 10, where the daily traffic volume (ADT) of 77,000 veh/d and the percentage of heavy vehicles (11%) were extremely high and a considerable decrease in shear force be-
Fig. 5. Comparison of maximum shear forces at 20 °C between lower pavement layers in and outside the wheel track.
Fig. 6. Comparison of maximum shear stiffness at 20 °C between lower pavement layers directly after construction (before) and 10 years later in (after, wheel track) and outside the wheel track (after, outside).
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C. Raab, M.N. Partl / Construction and Building Materials 23 (2009) 2926–2931
Shear Stiffness [kN/mm]
50
new/new
Base/Subbase old/old
old/old
new/old
new/new
40
before after
30 20 10 0
5
7
8
12
14
15
2
10
11 14 Site
13
18
19
5
12
13
Fig. 7. Comparison of maximum shear stiffness at 20 °C between lower pavement layers directly after construction (before) and about 10 years later (after), grouped according to the age of the pavement structure.
tween first and second base course in the wheel track could be found. The observation, that section 5 (AC 22) had three times higher shear force values after 10 years of trafficking can be attributed to the fact that this section had severe bonding problems directly after construction. When taking cores, already many of them broke before testing. This finding demonstrated that also in case of newly constructed lower layers the bond may improve over time due to traffic induced compaction and settlement. Fig. 5 shows the differences in shear forces inside and outside the wheel track for binder on upper base courses and upper on lower base courses. The figure distinguishes between combinations which had been new at the time of construction (new/new) and others where binder or upper base course had been put over an existing lower base course layer (new/old). It is not surprising that for the lower layers no trend regarding the range of the values inside or outside the wheel track can be established. As seen earlier, the values vary for the single sites and seem only to depend on the condition and situation of the investigated layers. Here, it even does not matter if the structure had been totally new at the time of surface layer construction or if a new layer had been put on an existing one. 4.2. Shear stiffness Figs. 6 and 7 depict the shear stiffness at 20 °C directly after construction (before) and about 10 years later (after). In Fig. 6, again the evaluated data are depicted according to the top layer material. Fig. 7 assembles the different data according to the age of the construction (new/new, new/old or old/old). In Fig. 6 the upper part displays the values in the wheel track, while the lower part shows those outside. Since for some sites no stiffness data had been determined, these diagrams have less data than the shear force diagrams. Contrary to the measured shear forces, the shear stiffness seems to improve for nearly all investigated sites and layers over the regarded 10 years period independently if the shear forces were determined inside or outside the wheel track. As visible in Fig. 6, from eleven tested sections, nine values can be found above the 45° line of equality and therefore show an improvement of shear stiffness over time. As depicted in Fig. 7, the two sections (sections 13 and 18) with decreasing stiffness are both constructions older than the time of the first investigation (before). This finding leads to the conclusion that the potential of improvement of the shear stiffness is greater in case of new constructions. Regarding the shear force and shear stiffness results it is interesting to see, that section 18, with decreasing stiffness over the years, also showed a decrease in shear force over the years by nearly 50%. Whereas, section 5, with 30% increase of shear stiffness, also revealed a strong increase in shear forces over time.
Fig. 8. Comparison maximum shear stiffness at 20 °C between lower pavement layers in and outside the wheel track.
In Fig. 8 the differences in shear stiffness inside and outside the wheel track for binder on upper base courses and upper on lower base courses are depicted. This time, the values again show sites, where binder and base courses had been constructed either at the same time (new/new) or where AC courses had been put on existing base layers (new/old). Again, for the lower layers no trend regarding the range of the stiffness values inside or outside can be observed. The stiffness values vary for the single sites and seem only to depend on the condition and situation of the investigated layers. 5. Conclusions From the results of this extensive, but still limited Swiss longterm performance study of bonding properties of binder, base and subbase asphalt concrete layers the following main conclusions can be drawn: 1. Traffic induced compaction and settlement after construction has a positive effect on bonding properties of carefully designed and manufactured roads. For new constructed binder, base and subbase courses an increase in shear resistance in terms of shear force between these layers after 10 years can be assumed, but is not always the case.
C. Raab, M.N. Partl / Construction and Building Materials 23 (2009) 2926–2931
2. For rehabilitations or old constructions an improvement cannot be expected and especially in case of old constructions seems to depend on their service life. For the bond between lower layers decreases of shear force values down to 50% are possible, a situation which for surfaces courses would most probably lead to complete damage. 3. Shear force and shear stiffness values inside and outside the wheel track in the lower pavement layers vary for the single sites and seem only to depend on the condition and situation of the investigated layers, but no trend can be derived. 4. In terms of shear stiffness, a general increase can be observed for nearly all tested combinations (binder/upper base, upper base/lower base and lower base/subbase). This finding proves to be valid, particularly for new constructions due to aging. 5. A high percentage of heavy vehicles over a long period of time can cause bonding problems. It is interesting to note, that even in case of increasing shear force values between surface and binder courses, for the same site, a decrease in shear forces may occur in the lower layers. 6. This investigation provides valuable quantitative practical input, e.g. for standardization purposes. It shows that interlayer shear properties are clearly subjected to change over the lifetime of a pavement. Hence, further investigations will also have to focus on the continuous monitoring of the whole process of change, i.e. the development of interlayer properties over time as a function of traffic and climatic history as well as of the evolution of volumetric and chemo-physical properties. This future task will certainly not only be a challenge in modelling but also in sensoring and monitoring techniques.
References [1] Choi Y, Collop A, Airey G, Elliot RA. Comparison between interface properties measured using the Leutner test and the torque test. J Ass Asphalt Paving Technol 2005;74E [ISSN: 1553–5576, CD-ROM].
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[2] Diakhate M, Phelipot A, Millien A, Petit C. Shear fatigue behaviour of tack coats in pavements. Road Mater Pavement Des 2006;7:201–22. [3] Kruntcheva MR, Collop AC, Thom NH. Properties of asphalt concrete layer interfaces. J Mater Civil Eng 2006;18(3):467–71. [4] Tschegg EK, Macht J, Jamek M, Steigenberger J. Mechanical and fracturemechanical properties of asphalt–concrete interfaces. ACI Mater J 2007;104:474–80. [5] Canestrari F, Ferrotti G, Partl MN, Santagata F. Advanced testing and characterization of interlayer shear resistance. Transport Res Record 1929; 2005: p. 68–78. [6] Rabiot D, Morizur M. Polymer-modified bitumen emulsions an advantage for various road applications. In: Proceedings of the Euroasphalt and Eurobitumen congress, Strasbourg, France; 7–10 May 1996. [7] Stöckert U. Schichtenverbund – Prüfung und Bewertungshintergrund [Interlayer bonding – testing and assessment background], Straße + Autobahn [Road and Motorway], 2001;11:324–631. [8] Raab C, Partl MN. Langzeiterfassung des Schichtenverbunds – Relation zwischen Prüfwert nach Einbau und Langzeitverhalten [Long-term performance of the adhesion properties – relation between test value after construction and long-term performance]. ASTRA-Project FA No. 1195; 2007. 56p. [9] Raab C, Partl MN. Investigation on long-term interlayer bonding of asphalt pavements. Baltic J Road Bridge Eng 2008;3(2):65–70. [10] Raab C, Partl MN. Methoden zur Beurteilung des Schichtenverbunds von Asphaltbelägen [Methods to determine the bond of asphalt pavements]. ASTRA-Project FA 12/94, Report No. 442; 1999. 118p. [11] Bundesamt für Straßen ASTRA; Bundesamt für Statistik BFS [Switzerland Federal Road Office (FEDRO), Switzerland Federal Office for Statistics (FSO)] 2006. Schweizerische Straßenverkehrszählung 2005/Comtage Suisse de la circulation routière 2005 [Swiss Traffic Survey 2005], Bern, 56p. [12] Leutner R. Untersuchungen des Schichtenverbunds beim bituminösen Oberbau [Investigation of the adhesion of bituminous pavements]. Bitumen 1979:84–91. [13] Partl MN, Raab C. Shear adhesion between top layers of fresh asphalt pavements in Switzerland. In: Proceedings of the 7th conference on asphalt pavements for Southern Africa, Victoria Falls, Zimbabwe; 29th August–2nd September 1999. p. 5.130–7 [CAPSA’99]. [14] Swiss Standard SN 640431a, Asphaltbetonbeläge. Konzeption, Anforderung, Ausführung/Revêtement en béton bituminiex. Conception, exigences, exécution; 1988. 32p. [in German and French, invalidated 2001]. [15] Swiss Standard SN 640431, Asphaltbetonbeläge. Konzeption, Anforderung, Ausführung/Revêtement en béton bituminiex. Conception, exigences, exécution; 1976. 32p. [in German and French, invalidated 1988].
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Interlayer bonding of binder, base and subbase layers of asphalt pavements: Long-term performance Christiane Raab *, Manfred N. Partl 1 Swiss Federal Laboratories for Materials Testing and Research EMPA, Ueberlandstr. 129, 8600 Duebendorf, Switzerland
a r t i c l e
i n f o
Article history: Received 7 September 2008 Received in revised form 10 December 2008 Accepted 8 February 2009 Available online 17 March 2009 Keywords: Interlayer bond Shear testing Asphalt pavements Binder Base and subbase layers Long-term performance
a b s t r a c t With bond testing of asphalt layers becoming a more important topic as well as in research as in every day pavement construction, the question of long term behaviour of bonding properties has come up. In recent research it has been shown that the bonding properties between surface and binder courses seem to improve with time considering that the surface course does not show distress phenomena and that the traffic volume does not exceed the traffic considered for pavement design. That bond failure is not only a question of the top layers is a well known fact; therefore the long term properties of the lower layers are also of interest. The paper focuses on the effect of long term behaviour of bonding properties of the lower pavement layers (binder or base and subbase courses) presenting the results of an extensive Swiss study. The study compares the bonding properties determined with the layer parallel shear test (LPDS) according to Leutner of high volume roads after a 9–13 years period. It was found that for rehabilitations or old constructions an improvement cannot be expected and especially in case of old constructions seems to depend on their service life. For the bond between lower layers decreases of shear force values down to 50% are possible. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction and objective During recent years testing of the bond between pavement layers has gained more and more importance throughout Europe [1–4]. In many projects the bonding properties are determined directly after construction but the question of how the originally determined properties develop over time was only investigated in a very limited way. Although some researchers [5–7] showed that the bonding improves with time, this finding was never investigated on a larger scale in a systematic way. In some cases [7] the re-evaluation of the shear strength took place just one year after construction and therefore cannot consider long term effects and influences. Furthermore, the improvement of bonding properties is stated as a general fact, but no reference to traffic data is provided. Only recently, the long-term performance of the bonding properties between surface and binder courses was investigated on a larger scale [8–9]. Fig. 1 summarises the results from the above mentioned research project. The figure depicts the shear force values determined with the shear test according to Leutner [12] directly after construction and 10–13 years later. Apart from sections 18 and
* Corresponding author. Tel.: +41 44 8234483; fax: +41 1 8216244. E-mail addresses: [email protected] (C. Raab), [email protected] (M.N. Partl). 1 Tel.: +41 44 8234113; fax: +41 1 8216244. 0950-0618/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2009.02.025
19 where the underlying course was old, all binder or base courses had been constructed at the same time as the surface courses. Since the coring took place before trafficking, there was no distinction between inside and outside the wheel track. In that research which compared the bonding properties of new surfaces courses and new or existing binder or base courses directly after construction and after a 10 years period it could be shown that the bonding properties generally improve by 10–30%. The percentage of improvement of the shear force depends on the value directly after construction. Here, when the values directly after construction are very high, only relatively low improvements can be observed (e.g. section 12 with MA), whereas in case of low values directly after construction a greater potential for improvement could be seen (e.g. section 13 with PA). It is important to note, that the bonding values only improve over time when the surface course is intact and does not show distress phenomena such as rutting or cracking. In case of ruts and cracks often the bonding properties in the wheel track as the most stressed parts of the pavement showed a decline, while the values outside the wheel track increased. Furthermore, the traffic volume plays an important role and a very high traffic volume or a high percentage of heavy vehicles could also lead to a deterioration of the bond. The aim of this paper is the investigation of the long term bonding properties for binder, upper and lower base as well as subbase layers and to compare these results to the results for surface courses. Since in the previous research project the shear force values had been determined for all the pavement, these data was now
2927
C. Raab, M.N. Partl / Construction and Building Materials 23 (2009) 2926–2931
Shear Force [kN]
50
SMA 11
AC 11
40
AC 16
Special: HRA, MA, PA
before after
30 20 10 0
2
3
8
5
7
14
6
10
15 Site
18
19
11
12
13
Fig. 1. Comparison of maximum shear forces at 20 °C between surface and binder courses for pavements with different surface mixes (SMA 11, etc.) directly after construction (before) and 10 years later (after) according to [8–9]. The error bars represent the standard deviation.
used to evaluate the long-term performance of the lower pavement layers. 2. Situation and material In 1999, a research project [10] on the interlayer bonding of new pavement layers was completed. It was found that the layer parallel direct shear test (LPDS) according to Leutner [12] was a useful tool for interlayer shear bonding assessment. The question of long-term performance of the interlayer shear bonding, in terms of the relationship between the bonding properties directly after construction and the bonding properties after some years of trafficking was only touched on in a limited way in this former project. Therefore, a follow-up project was conducted [8] where all 14 remaining sections were investigated again, after nine or more years of trafficking, in order to provide information on the long-term performance of the bonding properties. Table 1 summarises information on the pavement structure, the mixture types of the different layers and the available traffic data of the different sites. Since the construction took place before the European Standards became effective, all terms follow the old Swiss Standard [14]. Seven pavements had stone mastic asphalt (SMA) and four road sections had asphalt concrete (AC) surface courses. In addition, three coring sites with special surface courses, i.e. gussasphalt (mastic asphalt) (MA), hot-rolled asphalt (HRA) and porous asphalt (PA) were included. The surface courses were placed either on hot mix binder (AC) and mastic asphalt (MA) courses with a nominal maximum aggregate size of 16 mm or on hot mix base courses (AC) with a nominal maximum aggregate size of 22 or 32 mm. In two cases, pavements with new surface courses on unknown old AC courses were investigated (coring sites 18 and 19). According to an analysis the mixture type of these unknown layers could be evaluated to be most probably AC 10 (AC 10 old) according to the Swiss standard from 1976. For these two sections, again, the base course material, which was unknown and was determined to be AC 16 (AC 16 old) according to the Swiss standard from 1976 [15]. The second base courses normally consisted of a hot mix base course with maximum aggregate sizes of 22 or 32 mm. In one case the base course was even constructed according to the previous version of the Swiss Standard [15] when the maximum aggregate size had been 30 mm instead of 32 mm. For section 7 the special base course CSHM 22, a high modulus base course, had been used. For one motorway pavement (section 7) a third base course consisting of a hot mix base course (AC) with maximum aggregate size of 32 mm could be found. The subbase
courses, here called hot subbase mixes AC F (fundation) to distinguish them from the base courses, had a maximum aggregate size of 22 mm. In one case the subbase material was again according the old Swiss standard from 1976 and therefore had a maximum aggregate size of 40 mm (AC F 40 old). In the original investigation, all surface and binder or upper base courses apart from the binder courses of sections 18 and 19 were totally new. As for the investigated first and second base courses, six were totally new, while another six consisted of existing layers containing mixtures according to the Swiss standard from 1976 [15]. For nearly all road sections, traffic data, e.g. the average daily traffic in vehicles (ADT) and the percentage of heavy vehicles (vehicles >3.5 t) are given in Table 1 [11]. The coring for the investigation of the long-term performance was conducted a few meters away from the original coring site. For every road section 5 cores were taken inside and another five outside the wheel track.
3. Testing The layer parallel direct shear (LPDS) test device (Fig. 2), [10,13] is a modified version of equipment developed in Germany by Leutner [12] being more versatile in the geometry and more defined in the clamping mechanism. The modified LPDS test device fits into an ordinary servo-hydraulic Marshall testing machine and allows the testing of cores with a diameter of about 150 mm and rectangular specimens; no normal force can be applied. One part of the core (up to the shear plane to be tested) is placed on a circular u-bearing and held with a well defined pressure by a semicircular pneumatic clamp. The other part, the core head in Fig. 2, remains unsuspended. Shear load is induced to the core head by a semicircular shear yoke with a deformation rate of 50.8 mm/min, thus producing fracture within the pre-defined shear plane of 2 mm width [10,13]. The specimens were conditioned in a climate chamber for 8 h and all tests were conducted at a temperature of 20 °C.
Table 1 Sites with pavement characteristics and traffic data. Site
Mixture types Surface
2 3 5 6 7 8 10 11 12 13 14 15 18 19
SMA 11 SMA 11 SMA 11 SMA 11 SMA 11 SMA 11 AC 11 HRA 11 MA 11 PA 11 SMA 11 AC 11 AC 16 AC 16
Traffic Binder
AC 16
MA 16 AC 16
AC 10 old AC 10 old
1. Base
2. Base
AC AC AC AC AC AC AC AC AC AC AC AC AC AC
unknown
32 32 22 22 22 32 32 22 22 32 old 22 32 16 old 16 old
3. Base
AC 32 CSHM 22 AC 32 AC 32 AC 30 old AC 22 AC 30 old AC 32
Subbase
AC F 22 AC 32
AC F 22 ACF 40 old
ADT (veh/d)
>3.5 t (%)
– 18,350 32,700 94,990 19,800 31,450 77,890 6800 32,700 31,500 28,050 64,230 – –
Buses 9.8 7.6 4.4 4.4 5.5 11.1 2.6 7.6 4.6 5.8 8.2
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Fig. 2. LPDS test device, schematic drawing.
From the LPDS test the shear force F as a function of shear deformation w is obtained. Additional to the shear force the maximum slope from the diagram of shear force F versus shear deformation w can be used to define the LPDS maximum shear ‘‘stiffness” value SLPDS as follows:
SLPDS ¼
DF Dw
ð1Þ
Note, that a comparison of shear loads is only valid in case of similar diameters. If diameters change, the nominate shear stress F/A can be used. 4. Test results 4.1. Shear force Fig. 3 shows the LPDS test results at 20 °C for the different test sections and top layer materials. The figure represents the comparison of the maximum shear force values immediately after construction (before) and the maximum shear force values after about 10 years later (after). In case of the values immediately after construction (before) all points are mean values of at least seven specimens. In case of the values after several years of trafficking (after) in the upper part of the figure the mean value of all five cores taken in the wheel track is given, while the lower part depicts the mean value of all five cores taken outside the wheel track. The standard deviation is also presented. In comparison with Figs. 1 and 3 shows that the long term behaviour of the bond between binder on upper base and lower base on subbase courses is different to the one between surface and binder or upper base courses. When in the latter case an improvement can be generally stated (see Fig. 1), for the lower layers the trend towards an improvement is less obvious. Comparing the upper and lower part of Fig. 3 also reveals that there are considerable differences between the shear force values inside and outside the wheel track. As visible in the figure, from 14 tested cases overall only six show a clear improvement of shear forces over time and therefore their values can always be found above the 45° line of equality. While in two cases, AC 22 upper base (sections 12) and AC 32 lower base (section 13), the values before and after (in and outside the wheel track) more or less remain the same (values on or near the 45° line), in one case a distinct deterioration of the shear force value of more than 50% can be found (AC 10 binder, sections 18, wheel track). For sections 2, 7, 10, 11, 14 and 19 the behaviour changes with respect to the values inside or outside the wheel track. Here, a dramatic decrease in the wheel track, such as for AC 32 upper base (sections 10) and AC 22 upper base (section 7), or outside the wheel track, AC 22 upper base (section 14), is observed.
Fig. 3. Comparison of maximum shear forces at 20 °C between lower pavement layers directly after construction (before) and about 10 years later in (after, wheel track) and outside the wheel track (after, outside).
The differences between the values in and outside the wheel track appear to be higher for the binder on upper base courses, which are more affected by the traffic loads than the lower base on subbase courses. Nevertheless, it is necessary to mention that the number of tested binder and upper base courses is considerably higher than the one for lower base on subbase courses. When looking at the different mixture types and aggregate sizes, regarding an increase or decrease of the shear force with time, no difference can be found between them. This fact is not surprising since all investigated layers consist of dense graded AC mixtures. In order to see if a difference in long-term performance regarding the age of the structure can be found, Fig. 4 was composed. In this figure the same results are depicted as in Fig. 3. But this time the focus is on whether the construction was totally new (new/
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Shear Force [kN]
before
new/new
50
after, wheel track
old/old
new/old
Base/Subbase new/new old/old
after, outside
40 30 20 10 0
5
7
8
12
14
15
2
10
11 14 Site
13
18
19
5
12
13
Fig. 4. Comparison of maximum shear forces at 20 °C between lower pavement layers directly after construction (before) and about 10 years later in outside the wheel track, grouped according to the age of the pavement structure.
new), or a rehabilitation on an existing construction (new/old) or whether the bond between existing layers had been tested (old/ old). Again, the results inside and outside the wheel track are shown. Fig. 4 illustrates that, similar to the behaviour of surface courses (which were always new), new constructions of binder and upper base courses seem to have the greatest potential for an improvement of the shear force values. According to Fig. 4, this finding also appears true for the bond between lower base and subbase courses. Since in this investigation only lower layers were evaluated, it was not possible to detect surface distress phenomena. Sections 3 and 15 which showed distresses such as cracking and rutting, had either no further layers (section 3) or their lower layers appeared to be in order (section 15). Regarding the traffic data of section 2, which had a high number of bus traffic, but relatively high shear force values for the bond between surface and upper base layer, obviously had more problems with the bond between upper and lower base layer. The same proves to be the case for section 10, where the daily traffic volume (ADT) of 77,000 veh/d and the percentage of heavy vehicles (11%) were extremely high and a considerable decrease in shear force be-
Fig. 5. Comparison of maximum shear forces at 20 °C between lower pavement layers in and outside the wheel track.
Fig. 6. Comparison of maximum shear stiffness at 20 °C between lower pavement layers directly after construction (before) and 10 years later in (after, wheel track) and outside the wheel track (after, outside).
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Shear Stiffness [kN/mm]
50
new/new
Base/Subbase old/old
old/old
new/old
new/new
40
before after
30 20 10 0
5
7
8
12
14
15
2
10
11 14 Site
13
18
19
5
12
13
Fig. 7. Comparison of maximum shear stiffness at 20 °C between lower pavement layers directly after construction (before) and about 10 years later (after), grouped according to the age of the pavement structure.
tween first and second base course in the wheel track could be found. The observation, that section 5 (AC 22) had three times higher shear force values after 10 years of trafficking can be attributed to the fact that this section had severe bonding problems directly after construction. When taking cores, already many of them broke before testing. This finding demonstrated that also in case of newly constructed lower layers the bond may improve over time due to traffic induced compaction and settlement. Fig. 5 shows the differences in shear forces inside and outside the wheel track for binder on upper base courses and upper on lower base courses. The figure distinguishes between combinations which had been new at the time of construction (new/new) and others where binder or upper base course had been put over an existing lower base course layer (new/old). It is not surprising that for the lower layers no trend regarding the range of the values inside or outside the wheel track can be established. As seen earlier, the values vary for the single sites and seem only to depend on the condition and situation of the investigated layers. Here, it even does not matter if the structure had been totally new at the time of surface layer construction or if a new layer had been put on an existing one. 4.2. Shear stiffness Figs. 6 and 7 depict the shear stiffness at 20 °C directly after construction (before) and about 10 years later (after). In Fig. 6, again the evaluated data are depicted according to the top layer material. Fig. 7 assembles the different data according to the age of the construction (new/new, new/old or old/old). In Fig. 6 the upper part displays the values in the wheel track, while the lower part shows those outside. Since for some sites no stiffness data had been determined, these diagrams have less data than the shear force diagrams. Contrary to the measured shear forces, the shear stiffness seems to improve for nearly all investigated sites and layers over the regarded 10 years period independently if the shear forces were determined inside or outside the wheel track. As visible in Fig. 6, from eleven tested sections, nine values can be found above the 45° line of equality and therefore show an improvement of shear stiffness over time. As depicted in Fig. 7, the two sections (sections 13 and 18) with decreasing stiffness are both constructions older than the time of the first investigation (before). This finding leads to the conclusion that the potential of improvement of the shear stiffness is greater in case of new constructions. Regarding the shear force and shear stiffness results it is interesting to see, that section 18, with decreasing stiffness over the years, also showed a decrease in shear force over the years by nearly 50%. Whereas, section 5, with 30% increase of shear stiffness, also revealed a strong increase in shear forces over time.
Fig. 8. Comparison maximum shear stiffness at 20 °C between lower pavement layers in and outside the wheel track.
In Fig. 8 the differences in shear stiffness inside and outside the wheel track for binder on upper base courses and upper on lower base courses are depicted. This time, the values again show sites, where binder and base courses had been constructed either at the same time (new/new) or where AC courses had been put on existing base layers (new/old). Again, for the lower layers no trend regarding the range of the stiffness values inside or outside can be observed. The stiffness values vary for the single sites and seem only to depend on the condition and situation of the investigated layers. 5. Conclusions From the results of this extensive, but still limited Swiss longterm performance study of bonding properties of binder, base and subbase asphalt concrete layers the following main conclusions can be drawn: 1. Traffic induced compaction and settlement after construction has a positive effect on bonding properties of carefully designed and manufactured roads. For new constructed binder, base and subbase courses an increase in shear resistance in terms of shear force between these layers after 10 years can be assumed, but is not always the case.
C. Raab, M.N. Partl / Construction and Building Materials 23 (2009) 2926–2931
2. For rehabilitations or old constructions an improvement cannot be expected and especially in case of old constructions seems to depend on their service life. For the bond between lower layers decreases of shear force values down to 50% are possible, a situation which for surfaces courses would most probably lead to complete damage. 3. Shear force and shear stiffness values inside and outside the wheel track in the lower pavement layers vary for the single sites and seem only to depend on the condition and situation of the investigated layers, but no trend can be derived. 4. In terms of shear stiffness, a general increase can be observed for nearly all tested combinations (binder/upper base, upper base/lower base and lower base/subbase). This finding proves to be valid, particularly for new constructions due to aging. 5. A high percentage of heavy vehicles over a long period of time can cause bonding problems. It is interesting to note, that even in case of increasing shear force values between surface and binder courses, for the same site, a decrease in shear forces may occur in the lower layers. 6. This investigation provides valuable quantitative practical input, e.g. for standardization purposes. It shows that interlayer shear properties are clearly subjected to change over the lifetime of a pavement. Hence, further investigations will also have to focus on the continuous monitoring of the whole process of change, i.e. the development of interlayer properties over time as a function of traffic and climatic history as well as of the evolution of volumetric and chemo-physical properties. This future task will certainly not only be a challenge in modelling but also in sensoring and monitoring techniques.
References [1] Choi Y, Collop A, Airey G, Elliot RA. Comparison between interface properties measured using the Leutner test and the torque test. J Ass Asphalt Paving Technol 2005;74E [ISSN: 1553–5576, CD-ROM].
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[2] Diakhate M, Phelipot A, Millien A, Petit C. Shear fatigue behaviour of tack coats in pavements. Road Mater Pavement Des 2006;7:201–22. [3] Kruntcheva MR, Collop AC, Thom NH. Properties of asphalt concrete layer interfaces. J Mater Civil Eng 2006;18(3):467–71. [4] Tschegg EK, Macht J, Jamek M, Steigenberger J. Mechanical and fracturemechanical properties of asphalt–concrete interfaces. ACI Mater J 2007;104:474–80. [5] Canestrari F, Ferrotti G, Partl MN, Santagata F. Advanced testing and characterization of interlayer shear resistance. Transport Res Record 1929; 2005: p. 68–78. [6] Rabiot D, Morizur M. Polymer-modified bitumen emulsions an advantage for various road applications. In: Proceedings of the Euroasphalt and Eurobitumen congress, Strasbourg, France; 7–10 May 1996. [7] Stöckert U. Schichtenverbund – Prüfung und Bewertungshintergrund [Interlayer bonding – testing and assessment background], Straße + Autobahn [Road and Motorway], 2001;11:324–631. [8] Raab C, Partl MN. Langzeiterfassung des Schichtenverbunds – Relation zwischen Prüfwert nach Einbau und Langzeitverhalten [Long-term performance of the adhesion properties – relation between test value after construction and long-term performance]. ASTRA-Project FA No. 1195; 2007. 56p. [9] Raab C, Partl MN. Investigation on long-term interlayer bonding of asphalt pavements. Baltic J Road Bridge Eng 2008;3(2):65–70. [10] Raab C, Partl MN. Methoden zur Beurteilung des Schichtenverbunds von Asphaltbelägen [Methods to determine the bond of asphalt pavements]. ASTRA-Project FA 12/94, Report No. 442; 1999. 118p. [11] Bundesamt für Straßen ASTRA; Bundesamt für Statistik BFS [Switzerland Federal Road Office (FEDRO), Switzerland Federal Office for Statistics (FSO)] 2006. Schweizerische Straßenverkehrszählung 2005/Comtage Suisse de la circulation routière 2005 [Swiss Traffic Survey 2005], Bern, 56p. [12] Leutner R. Untersuchungen des Schichtenverbunds beim bituminösen Oberbau [Investigation of the adhesion of bituminous pavements]. Bitumen 1979:84–91. [13] Partl MN, Raab C. Shear adhesion between top layers of fresh asphalt pavements in Switzerland. In: Proceedings of the 7th conference on asphalt pavements for Southern Africa, Victoria Falls, Zimbabwe; 29th August–2nd September 1999. p. 5.130–7 [CAPSA’99]. [14] Swiss Standard SN 640431a, Asphaltbetonbeläge. Konzeption, Anforderung, Ausführung/Revêtement en béton bituminiex. Conception, exigences, exécution; 1988. 32p. [in German and French, invalidated 2001]. [15] Swiss Standard SN 640431, Asphaltbetonbeläge. Konzeption, Anforderung, Ausführung/Revêtement en béton bituminiex. Conception, exigences, exécution; 1976. 32p. [in German and French, invalidated 1988].