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Evaluation of Time Loading Influence on Asphalt Pavement Rutting

The rutting of asphalt pavement structures during their exploitation is considered to be one of the main problems in Lithuania, as well as in the entire world. This kind of pavement distress makes a negative impact to the exploitation characteristics of the asphalt pavement, to the residual life of pavement structure, also to the safety and quality of the traffic. The main purpose of this analysis is to define the resistance to rutting of various kinds of pavement structures and to assess the variation of rut depth in dependence on the traffic loading (number of equivalent standard axel loads – ESAL‘s). The analysis is performed and is still continued at the road of experimental pavement structures, where 27 different pavement structures were installed. The statistical analysis of the rut depth let to define the most precise criterion for the assessment of every kind of pavement – meridian. © 2013 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of the Vilnius Gediminas Technical University.


Introduction
Safwan and Tamer [1] analysed the rut formation prognosis model (The Ohio state model). This model is the linear dependence between the stress and repeated loading number logarithm, which parameters depend on the kind of material. The Ohio state model can be used to forecast the rut formation and to compare the behaviour of various asphalt mixes during the rut formation process. Using this model, the analysis and assessment of four different asphalt mixes rutting potential, using asphalt mixtures with narrow and wide aggregate particle distribution, was made. Rashid K. Abu Al-Rub [2] also analysed the prediction of rutting in asphalt pavements under repeated loading conditions comparing finite element and constitutive modelling techniques, i.e. he evaluated the difference between 2D and 3D simulations and the effect of different performance loading assumptions (e.g. pulse loading and equivalent loading) on the predicted rut depth. Results showed that the 2D plane strain FE simulations significantly overestimate rutting as compared with the rutting performance predictions from more realistic 3D FE simulations. 3D FE models have also been used for the evaluation of the effects of various bonding conditions on the overlay response as well as the interface behaviour. This study found that as pavement temperature increases, the interface bonding effects on the overlay response are amplified. This could be the main contributing factor to overlay rutting [3]. Motiejūnas [4] made an assessment of temperature distribution in the different layers of asphalt pavement dependence on environmental temperature as well as the influence of the temperature variation for the stiffness of asphalt layers. Temperature is one of the most dominant factors that impact the variation in modulus of asphalt layers. Nazarian and Alvarado [14] analysed the relationship between temperature and modulus of AC layers. Getautis and Sivilevičius [5] statistical analysis showed that the highest rut depth is formed at the first traffic lane in the pathway zone of the right wheel of the vehicle. As well as in this zone the rutting was the most uneven.
It is known that variations in material characteristics are a major factor to influence the rutting resistance of asphalt mixtures [17]. Many researchers have performed a lot of work on it [15], [18]. At the same time, different test methods were utilized to estimate the rutting performance of asphalt pavement. Visco-elasticity theory, especially linear visco-elasticity models, have been developed to predict the rutting in asphalt pavements [19].
Arabani [6] suggested two ways how to increase the durability of asphalt pavement structure: first is to increase the total thickness of asphalt layers, second is to use the polymer modified bitumen during the construction process of asphalt pavement. Jan Ritter [7], [20] defined, that at the thinner asphalt layers the considerable decrease the strength of pavement structure as well as increases the crack formation in the asphalt base layers. The modification of asphalt mix with polymer or other additives increase the durability of asphalt pavement and decrease the rutting [8][9]. Vaitkus [10] ascertained that the usage of particular supplements in warm mix asphalt increases the stiffness of the binder and at the same time improves the resistance to rutting. The dynamic creep compression tests for bituminous materials with different air void contents under different stress levels were carried out and the study indicated that air void contents have significant influence on the behaviour of rutting under wheel loadings [15]. The pavement structures of various kinds were constructed at these sectors. The experimental road was constructed on the way to gravel quarry where the empty heavy vehicle are going in and the loaded are going out. In this case, the one traffic lane of the experimental road is loaded much more than the other [11].
The thickness and materials for every pavement construction were chosen according to reference pavement structure, which was made from asphalt wearing layer 0/11 S-V (AC11VS); asphalt binder layer 0/16-A (AC16AS); asphalt base layer 0/32-C (AC32PS); base layer -crushed dolomite 0/56; frost blanket layer ԟ sand 0/11, see Fig. 1. The all combinations of the materials of different sectors are presented in Table 1 [13]. The traffic flow is registering consistently after the first car passed after opening of the road. For the classification of traffic flow the induction loops, installed into road pavement, were used as well as the Loop profiler classificator. All vehicle were normed under ESAL's 100 =100 kN. ESAL's 100 distribution at the time of experimental sector's use is presented in Fig. 2. The experimental road is used by 70000 ESAL's 100 in average annually, total amount of ESAL's100 since the beginning of road exploitation until the end of 2011 is 300000.  This article analysis the results of asphalt pavement structure rutting then the experimental sector was used by ESAL's 100 =180000, ESAL's 100 =230000, ESAL's 100 =250000, ESAL's 100 =290000.

The experimental research and analysis of the results
The rut depth of asphalt pavement structure of experimental road is measuring in both traffic lanes -loaded and unloaded. The measurements are made by mobile road research laboratory RST-28. The rut depth is measured using laser sensors that measure the transverse road profile at particular intervals and ensure the sufficient reliability for the definition of rut depth.
The measurements of rut depth are made every 1 m, 30 values in total. The average depth of the right, the left rut and the maximum clearance was counted at every sector. To increase the precision of the measurements and to get the reliable analysis results the additional statistical analysis was made. This analysis let to define the most relevant statistical index which shows the proper meaning of the rut depth in every measured sector. For the definition of rut depth proper meaning the statistical analysis of these grip description criterion was made: median, mode, the average of 20 section measurement data, the average of 30 section measurement data.
The one kind of statistical analysis covered the assessment of 20 meters average measurements of every section. In this case in every 30 m long section was assessed only these values which were received rejecting the first 5 and the last 5 meters of measurement data. So the possible errors, which could occur because of the connection of different pavement structures, were eliminated. The distribution of rut depth in different pavement structures dependent on statistical criterion when the experimental road was passed by ESAL's 100 =180000, ESAL's 100 =230000, ESAL's 100 =250000, ESAL's 100 =290000 are presented in Fig. 3-6. The assessment of the loaded traffic lane right rut depth measurement, i.e. comparing the differences between the minimal and maximal values, the average of rut depth in 30 m and 20 m sections, the median and mode the extreme points were defined. The extreme points most visible at ESAL's 100 =290000, see Fig. 6. These pavement structures were defined: 4th, 8th, 9th, 16th, 21th and 26th (Table 1.). The analysis of rut depth dependence on ESAL's let to define that rut depth increases proportionally ESAL's. The highest rut depth were measured at ESAL's 100 =290000; the lowest at ESAL's 100 =180000. For the further analysis the results of the 2011 year autumn measurements of rut depth were chosen at ESAL's 100 =290000. All the 27 pavement structures were analysed and the most proper rut depth measurement statistical criterion was defined. The Fig. 7 presents the comparison of rut depth statistical criterion assessment of pavement structure (sector no. 19). The statistical analysis of the rut depth let to define that median has a 0.30 mm (0.08 mm in average) distinction from mode, the difference from median and average value at 30 m is up to 0.58 mm (0.11 mm in average), the difference between 30 m and 20 m average values is up to 0.51 mm (0.15 mm in average), the difference between the minimal and maximal values -up to 11.30 mm (average 2.66 mm). Also it was defined: • There is a great difference between minimal and maximal values of rut depth; • There is almost no difference of the average value at 30 m and 20 m (then 5 m from the beginning and the end of the section were eliminated), it means that connection of different pavement structure has no influence to rut depth; • The median and the average value of rut depth at 30 m has a great difference (up to 0.58 mm), this is because there is a great difference between the minimal and the maximal rut depth values that misrepresent the total value of 30 m rut depth. • The median and the mode have a little difference (up to 0.30 mm), but evaluating all rut depth values the median is more precise than mode. For the further analysis the median criterion of every section was accepted.
So it can be affirmed that the rut is formed at the asphalt wearing course. Minimum rut depth was formed in these pavement structures that had asphalt pavement wearing course constructed from Confalt, stone mastic asphalt SMA11S and stone mastic asphalt SMA11S with PMB. The maximum rut depth is formed at these pavement structures which had an asphalt wearing course constructed from AC11VS and asphalt binder layer AC16AS with bitumen 50/70.

Conclusions
The asphalt pavement distresses impact the traffic loading, climate and environmental conditions. The most often and negative type of pavement distress is rutting. It significantly depends on type of pavement structure, kind and quality of materials used for different types of structure layers, also pavement construction techniques and quality.
The assessment of loaded traffic line right rut depth measurements i.e. comparing the differences between the minimal and maximal values, the average of rut depth in 30 m and 20 m sections, the average of rut depth at 30 m, median and mode at different number of ESAL's 100 , let to define that the most accurate way for the rut depth assessment is the median calculation.
So it could be affirmed that the rut is formed in asphalt wearing course. The minimum rut depth was formed in pavement structures were asphalt wearing course was built from Confalt, stone mastic asphalt SMA11S, stone mastic asphalt SMA11S with PMB. The maximum rut depth was formed in pavement structures were the asphalt wearing course was built from AC11VS and asphalt binder layer AC16AS with bitumen 50/70.