Resarch areas

In conjunction with development of rolling resistance models it has been brought up to date that the knowledge of the size of variation of road surface characteristics data across a road section is not well known.

In a rather limited VTI study it was shown that the MPD (Mean Profile Index) level varies with lateral position, especially on smaller roads which are often used as test sections in model development. Also other road surface data such as IRI (International Roughness Index) and megatexture vary. Another conclusion is that data used for model development should be collected

Experiments show that the deep drainage system is an effective method to control the groundwater table and, concerning road construction, it is recommended as a method when the road is located in earth cut below the ground surface in combination with high groundwater table. However, it is important to maintain the drainage system.

Along route 126 near Torpsbruk, the Swedish Transport Administration for many years had problems with bearing capacity, potholes and other deterioration. The problem obviously exists due to a combination of factors such as poor construction material, and high water content in the construction.

VTI experiments show that the deep drainage system is a very effective method to control the groundwater table. This was supported by the plugging trials. The water levels increased and decreased rapidly in connection with plugging and opening of the deep drainage system. Deep drainage is also a very effective method to control the water content in the road when the groundwater table is high in the road construction. Sealing of the road verge and embankment slope affected the dynamics of water content in the road. To investigate the infiltration of rain and road water some part of the road shoulder was sealed with a plastic tarpaulin.

The results can be used to evaluate if the use of sealed road shoulders might be an appropriate maintenance action. To evaluate the long-term effect of this action on the road construction and durability the measurements have to continue for a longer period of time.

The road network in large parts of the Nordic countries is exposed to severe environmental variations throughout the year. During winter, pavement structures are exposed to subzero temperatures for a sufficiently long period to result in frost penetration in pavement materials and subgrade soil. During the winter water accumulates in the pavement structure and forms ice lenses, mainly through lateral moisture transfer, precipitation infiltration and capillary rise from the water table. The accumulated water then transforms back to the liquid phase during the spring thaw. Thus, the pavement structure is subjected to relatively high moisture content. The excess water then gradually drains out from the system during the so-called recovery period.

The increase in moisture content in unbound pavement layers results in reduction of shear strength and loss of internal friction forces between the aggregates. Therefore a considerable loss in unbound pavement material stiffness and road bearing capacity in general is observed during the thawing period. Excessive fatigue damage in the bitumen bound surface layer can also be experienced due to the loss of support in the underlying layers. Many authors doing research in cold region pavement engineering have mentioned spring thaw as the most significant deterioration factor for pavements subjected to frost penetration. Thus, investigation of the pavement structural capacity during the spring thaw period and its connection to the climatic factors seemed necessary.

As a case study, the structural performance of the county road 126 pavement system during spring thaw has been investigated. This road is a typical two lane two direction rural road with flexible pavement and it was opened to traffic in July 1985. The field study measurements were performed near Torpsbruk, which lies about 5 km north of the town of Moheda. Torpsbruk is a locality situated in the Alvesta Municipality in the province of Småland in southern Sweden.

In order to provide the basis for understanding the pavement climatic condition during the freezing and thawing period the test site was equipped with moisture content and temperature measurement probes. These parameters were continuously measured in the pavement structure profile in depth. The pavement moisture content measurements were done using high frequency capacitance probes. The frost zone and the thaw progression in the pavement were measured based on the profile temperature registrations. A Tjäl2004 probe, developed at VTI, was used for this study.

The mechanical response of the pavement structure during this period was investigated using a KUAB 50 Falling Weight Deflectometer (FWD). In order to evaluate the relationship between the environmental factors and structural characteristics of the pavement, a series of FWD tests, mainly during the spring thaw period, were performed. Eleven sets of FWD tests were performed in total, seven of them being between mid-March and May 2010. Layer moduli backcalculation as well as deflection basin analyses were performed using the FWD measurement data. Both the deflection basin indices and backcalculated layer moduli indicated that the pavement was weakest during the subgrade thawing phase.

Furthermore, the falling weight deflectometer load history data were used to classify the general pavement response and investigate the dissipated work during the spring thaw. Finally, a compatibility study of the backcalculated layer moduli and the pavement measured moisture content versus the predictive moduli-moisture content model for unbound materials was performed. The measured field data from the test road pavement in Torpsbruk showed promising agreement with the resilient modulus predictive model, both for granular layer and subgrade material. Similar models using FWD and moisture content measurements could be developed or calibrated for other soils and granular materials if sufficient data are to become available in the future. This database could assist pavement engineers to choose more realistic design values for pavement materials that take into account the seasonal variations.

The influence of road surface properties, such as macrotexture and unevenness, on rolling resistance and fuel consumption is an important factor to consider when deter¬mining the coating of a road surface. Results of a VTI study show that the effect of unevenness is in general significantly smaller than that of macrotexture.

The relative smallness of this influence makes measurements of it a challenging task. In literature a wide range of results can be found and there is still much confusion and uncertainty about how large the influence actually is. In this study, an attempt is made to obtain more reliable estimates of how macrotexture and unevenness affect rolling resistance. The primary method used here is the coastdown method. It has been applied to a private car and to a heavy goods vehicle (HGV). For a private car, macrotexture effect on rolling resistance (characterized by the coefficient CrMPD) can also be estimated by two alternative measurement methods: a specially equipped trailer for rolling resistance measurements on road, and a laboratory drum with different drum surfaces. Data from both have been made available to us from TUG in Gdansk.

Due to different premises for the three methods, results are not fully comparable. Still, results from the three methods clearly point in the same direction. The coefficient CrMPD, estimated from coastdown, trailer and drum measurement data, agreed very well: CrMPD=0.0017. From the drum measurement data, covering 90 different tyres, an idea of the variation among tyres can be obtained. The standard deviation for CrMPD was 0.0002.

Concerning the effect of unevenness on rolling resistance, only the coastdown method provides any information. Results show that the effect of unevenness is in general significantly smaller than that of macrotexture.

The coastdown measurements for the private car included both normal tyres and studded winter tyres.

The coastdown method provides, besides information about rolling resistance, other useful data for the vehicle, such as air resistance coefficients, temperature coefficients and transmission resistance.

For the HGV, only coastdown data has been available, and no possibility to compare with other methods existed. The extent of the measurements was much smaller than for the private car. Results are unstable and it is difficult to draw any definitive conclusions.

Focus has been on the coastdown method. A serious disadvantage with the method, at least when applied to road surface effects, is that it can be implemented in many different ways and that results may differ for different implementations. The difficulty to trace any instabilities in results (regression coefficients) to their sources (measurement errors) is a further weakness of the method. Extreme care must be taken in order to obtain reliable and stable results.

The aim of the project is to explore a concept based on performance optimization of road pavement structures with various fractions of subgrade layer and asphalt pavement thickness.

The project has a total of 14 test sections located in K1 and K2 in the northbound direction on road E4 between Skånes Fagerhult and Markaryd. These were observed with measurements 2004 to 2010. The follow-up was effected by cross profile measurements and falling weight measurements.

The follow-up was carried out on 8 test sections á 100 m and 6 test sections á 40 m. The asphalt pavement in the right (slow) lane consists of 40 mm wearing course, 45 mm bitumen base course and 95 mm bitumen roadbase with 80 mm gravel roadbase beneath. The asphalt pavement in the left (fast) lane has the same wearing and bitumen base course but not any bitumen roadbase. The subbase vary between the sections from 60 cm crushed rock 0–300 mm to 42 cm crushed rock 0–150 mm or crushed rock
0–90 mm. All the test sections are situated on embankment of varied height.

The road was opened for traffic in 2004, but was then trafficked on the bitumen base course. The wearing course was constructed late in summer 2005 after about one year of traffic. The road served 4 years until the summer of 2009 when the wearing course of the right lane was maintained with a remixing. In the left lane no maintenance was done.

The first measurement at the test sections with the new wearing course was conducted in the spring of 2006 after it had been in service approximately 1 year. In spring 2009 a profile measurement was done before the maintenance operation. The latest profile measurement was carried out in autumn 2010. The rut depth has been calculated according the thread principle.

An analysis of the profile measurements until the maintenance operation 2009 shows that the measured profiles in the right lane have a significant rutting with a rut depth on average per test section of 9.2–11.8 mm in left wheel path and 8.8–13.0 mm in the right wheel path. The ruts are relatively wide and the distance between the wheel paths is about 2 m which means that the rutting is caused by heavy traffic. The rut deformation in the left lane has always been very small. The ruts calculated from the cross profiles are just a few millimetres deep.

Although the cross profiles in the right lane had no rut deformation directly after the maintenance operation there was some unevenness at the profile edges, the sideline included. This has been taken into account when calculating the rut deformation. The rut propagation from 2009 until the latest measurement 2010 has been relatively high; around 2–4 mm during the first year after the maintenance operation with a significant rut deformation. The rut propagation of the left lane is still very small with only a few millimetres of total rut depth.

Falling weight test was carried out in the right wheel path in both lanes. The test sections were measured at 4 times annually from 2004 to 2007. Measurements in 2004 and 2005 were carried out on bitumen base course while subsequent measurements were carried out on wearing course. In the left lane, measurements were conducted only in 2006 and 2007.

Analysis of the results of falling weight measurements has been made by backcalculation of E-modulus of the different layers in the pavement structure.

E-modulus for asphalt pavement in the right lane at the last measurement occasion is approximately 4,000–5,000 MPa, corrected to15°C pavement temperature. The E-modulus in the left lane is much higher than in the right lane with an average of approximately 17,000 MPa. It's a very high calculated E-modulus, which can be explained by the fact that the bitumen pavement is very stiff, but probably also by the actual layer thickness being slightly larger than the nominal layer thickness used in the backcalculation.

The stiffness of the unbounded pavement layers is slightly higher on the southern test sections than on the northern sections and generally between 172 and 297 MPa at the latest measurement. The result of the measurements shows that the unbound layer of 0–150 material has a slightly higher stiffness than layer of 0–90 material. The 0–300 material has the same stiffness as 0–150 material.

The response of the pavement structure when loading by truck and falling weight deflectometer was measured with sensors installed in the structure. Response measurement was carried out on two occasions, in 2005 and 2006. The results of the measurements are of varying quality. There is little variation in the measured values between the passes with the truck which means a good repeatability. But there is major difference in the measured values between the different sensors at the same test section and primarily in the measured stresses. This makes it difficult to draw any clear conclusions on the differences in stresses between the test structures.

Most obvious conclusion from the follow-up of the test sections on the E4 Skånes Fagerhult is that there are good reasons to optimize road structure depending on the traffic in respective lanes. The relatively thin asphalt pavement which is located in the left lane has performed perfectly well so far. The rut propagation in the left lane after about 5 years of service is only marginal, while it is much more prominent in the right lane.

The clearest indication of the differences between the test structures is shown in the results from the falling weight measurement where the subbase layer 0–150 mm has higher stiffness than subbase layer with 0–90 mm granular size. To increase the granular size further to 0–300 mm did not contribute to increased stiffness.