Introduction
The aging structure as well as the considerable increase of heavy-traffic load on Germany’s motorways and trunk roads encourages the use of innovative, sound and reliable methods for the structural assessment on network level as well as on project level. Essential elements for this are data, which allow a reliable assessment.
The challenge of determining the input variables increases with the size of the project to be evaluated or the complexity of the network. Because of this complexity, the associated needed financial budget and also due to technical limitations, current pavement assessment procedures—especially at network level—rely on write-down models and surface characteristics.
Write-down models have the advantage that the required database is relatively simple to aggregate at a large scale. The implementation at network level, especially for strategically motivated decisions is given with this approach. The existing German system of periodical measurements of surface characteristics of the federal road network, so called ZEB, forms a suitable database to give the write-down models a closer relationship to structurally engineered issues. Because ZEB measurement system work non-destructive and with traffic speed the application on network level is given. Perspective techniques with advanced resolution and better routines will be used, which will allow a more in-depth interpretation, in particular for the classification of damage characteristics and causes of damage.
For a holistic approach to structural pavement assessment performance orientated measurements will be necessary. In combination with the above mentioned functional parameters as well as the write-down models, strategically motivated decision making processes will be useful combined with technically motivated decision processes.
For the application at network level, the available methods for performance orientated measurements are still challenging, as they are based either on testing drill-cores or on non-traffic speed methods. In recent years significant innovation steps have been made to bring traffic speed bearing capacity measurements and methods for evaluating pavement structures on the road.
Approach to structural pavement assessment
Structural pavement assessment can be described by the hereinafter mentioned four modules, see also Fig. 1.
(1) Structural assessment based on laboratory examination. Based on drill core examinations the current condition of the pavement and its residual lifetime can be derived. The in-draft guidelines for the evaluation of the structural condition of asphalt pavements form the basis for this at present in Germany.
(2) Structural assessment based on non-destructive measurements. The condition of the pavement (surface and inner structure) can be detected by non-destructive measurement methods and then will be evaluated. For the detection of the surface characteristics the ‘Road Monitoring and Assessment’ system ZEB has been established in Germany. For the assessment of the inner structure, different techniques (moving and stationary working) are available or are still being under development. Their possibilities and limitations are currently being evaluated.
(3) Structural assessment based on write-down models. Pavements are a capital item, which can be written off like any other investment good. Such approach is currently used primarily for the evaluation at strategic-level network-wide in the context of systematic planning of pavement maintenance.
(4) Structural assessment on network level. The structural pavement assessment on network level is a particular challenge, since in this case not only the technical implementation and evaluation are at the forefront but also the provision of appropriate strategies and routines. The ZEB system is an example of successful implementation.
The basis for each approach to structural pavement assessment is an appropriate and complete data base. Because among other things the effects from traffic, climate and building materials have a very complex impact on the aging and thus the condition of pavements, the intersection of multiple data sources is advantageous. A perfect data mix could thus consist of the elements shown in Fig. 2. Among other things, from an economic point of view, it should be considered how far a complete data mix for a variety of cases in situ is necessary. For example an extensive sampling using core samples and subsequent laboratory tests at a very old pavement with obvious surface distresses should be questioned to its usefulness.
STATUS OF RESEARCH AND TECHNOLOGY
Road Monitoring and Assessment
Since 1997 the ‘Road Monitoring and Assessment’ system ZEB (Zustandserfassung und-bewertung) is been established in Germany [
1‒
3]. Every fourth year the federal network will be monitored with high-speed non-destructive devices, seeFig. 3. These measurements include the longitudinal- and transversal-evenness, surface images and skid resistance. With the help of evenness and surface distress data a scaled structural number between one and five will be derived [
4].
The advantage of the ZEB is that it is well implemented into the German pavement management for years and that it covers the whole federal network. The disadvantage is that it is based on surface distress and therefore only gives limited information about structural damages.
Write-down model
A method for structural assessment of pavements that is also already applied is the so-called ‘thickness-equivalence-method’ [
5]. By means of equivalence factors the existing layer thicknesses of the given structure will be written down during their lifetime. This reduced and so-called ‘equivalent layer thickness’ is then compared with the required thickness which comes from an actual pavement design evaluation with current traffic loads. The normalized structural-number will be derived by comparing the equivalent layer thickness (is) with the needed layer thickness (should) from the pavement design, see Fig. 4.
This model is in use in the context of systematic pavement maintenance and pavement-management-systems. It is very advantageous that the number of input parameters for this method is relatively low. This allows an easy utilization for network-wide applications. However, it should be noted that in situ influences on the aging behavior of pavement constructions cannot be considered with this model alone.
Bearing capacity measurements
Pavement’s bearing capacity is an essential attribute of pavement condition. The bearing capacity can be measured only by a direct mechanical response of the pavement, or be derived therefrom. Worldwide a number of measurement principles and systems exist. The also in Germany [
6] most common are the Falling-Weight-Deflectometer, the Deflectograph Lacroix and the Curviameter, see Fig. 5.
The Falling-Weight-Deflectometer FWD works stationary [
7‒
10]. At a measuring point defined load impulses are applied to the pavements surface. By means thereof a geophone in the centre of loading and at defined intervals up to a distance of approximate two meters, the short-term deformation (deflection) of the surface will be detected. The needed dwell time per measurement point is approximately one to two minutes. The measuring point intervals must be chosen depending on the task. Common intervals are 25 or 50 meters.
The Deflectograph Lacroix moves continuously with a speed of ca. 5 km/h [
11,
12]. The measuring principle is based on Benkelman beam measurements. A measuring beam with a probe tip will be placed on the pavements surface. The beam will be rolled over between the dual wheels of the rear axle of a truck with defined load. The deformation of the pavement is recorded. The measuring point interval is three to six meters.
The Curviameter moves continuously with an average speed of 18 km/h [
13]. A chain with several geophones will be discharged to the pavements surface in the rolling track of the truck. The chain is placed between the dual wheel of the rear axle with defined load. By means of the geophones the deformation of the pavements surface will be recorded. The measuring point interval is five meters.
All three mentioned measurement systems need increased demands on traffic safety arrangements, thus they work significantly slower than the moving traffic. In addition the daily performance (kilometer per day) is not very high. Therefore the systems are more usable at project level than on network level, especially in the context of federal highway networks. Only within the last decade a fast-moving bearing capacity measuring device has been made ready for the market. The so-called Traffic-Speed-Deflectometer TSD allows non-destructive bearing capacity measurements at speeds up to 80 km/h. The TSD applies a defined axle load on the pavement via the rear axle of a truck trailer [
14,
15]. Using Doppler laser sensors the deformation of the pavement can be recorded right under the load as well as at several positions apart from the axle. The current generation of this system can be found eight times worldwide, additional systems will be delivered soon.
Ground-Penetration-Radar
The knowledge about the thickness of pavement layers and possibly present inhomogeneities inside the road construction is important for the structural assessment. For this purpose the Ground-Penetration-Radar GPR can be used, see Fig. 6. With the GPR it is possible to derive a traffic speed and non-destructive inventory of the road network. Measuring speeds of up to 100 km/h are possible. Systems with an array of antennas will be more and more used in future. With this it is possible to capture the entire width of the traffic lane with a single pass. Also three dimensional images of the inner structure of the pavement will be possible.
It should be emphasized that the evaluation of GPR measurements has to be done almost entirely manually. The requirements for qualifying staff for this task should be high in order to obtain reliable results. In addition it must be ensured that the right equipment depending on the task will be used. In Germany guidelines with requirements have been published recently [
16].
Structural assessment based on laboratory examination
For the structural assessment based on laboratory examination four main steps have to be done, see Fig. 7. First homogenous sections have to be defined. For this several non-destructive measurements (ZEB, bearing capacity, GPR, etc.) and an analysis of different data sources (age, traffic load, climate impact, etc.) has to be done. This data base will only be used for the finding of homogenous sections and not for the assessment itself. Within the homogenous sections then drill core are to be taken. Sixteen cores have to be taken for the first kilometer. For each additional kilometer of a homogenous section, five more drill cores are necessary. In the laboratory stiffness and fatigue functions will be determined with the indirect tensile strength test. With this data base reversed pavement design calculations will be made to derive the residual lifetime of the pavement.
Due to the direct mechanical testing of asphalt specimens under laboratory conditions, relatively accurate results are expected. The process is due to the extensive drill core extraction and the time-consuming laboratory tests in particular for the project level.
The guidelines [
17] for this approach for asphalt pavements have currently an in-draft status. Guidelines for the assessment of concrete pavements will be drafted next.
Multifunctional measurement vehicle
All of the above mentioned assessment methods and measuring principles have their limits and potentials. When thinking of an assessment on network level, there are two challenging tasks:(1)collecting high-precision data at traffic speed, which should be at least 80 km/h; (2) synchronizing these data to a common time slot and exactly the same geographic position.
The well established German ZEB system already uses the state-of-the-art to collect surface distress data at a very high precision level. Quality standards and quality control by the Federal Highway Research Institute BASt face the above mentioned challenges very well.
However the collection of structural data like bearing capacity data and inventory data was limited to project level or small networks, mainly because of traffic safety matters during the measurements. As mentioned before, there have been a massive development steps been done in the last decade which allowed to bring traffic-speed measuring systems to the market.
BASt accompanied these developments in recent years with several research projects. Based on this the procurement of a multifunctional measurement vehicle (MESAS) is initiated. MESAS will combine surface characteristics measurement systems with structural measurement equipment. Because of its sufficient dimensions, the TSD will be used as basis for MESAS. Figure 8 gives an idea of the project. Similar solutions for a multifunctional vehicle can be found in South-Africa and Australia.
TSD evaluation
Several research projects had been undertaken by or with BASt to evaluate the applicability, the comparability and the repeatability of the TSD. First tests have been made back in 2006 with the first generation TSD (prototype) which is still operated by the Danish Road Institute. Figure 9 gives an overview about the projects. Hereinafter the projects done with the recent 2nd TSD generation will be summarized.
In 2012 the state of Bavaria organized a TSD measurement campaign on 300 km of different road classifications. Motorways (approx. 34 cm asphalt), federal highways (approx. 30 cm asphalt) and state highways (approx. down to 22 cm asphalt) have been taking into account. On some tracks, the TSD measurements have been supported by FWD, Deflectograph Lacroix or Curviameter measurements. A detailed description of this project can be found in [
18].
The overall result of this project has been that the TSD gives valuable information and has the needed potentials. The advantage of the measurements seems to increase with decreasing road classification. The inspected 137 km of motorways don’t seem to have any bearing capacity problems. The comparison to the other devices has been satisfying, especially when comparing to rolling wheel devices. At the time of the measurements the used TSD was not equipped with a dynamic axle monitoring device. The combined evaluation of the TSD data, evenness data and front camera images shows, that the information about the actual applied axle load is crucial for the assessment of bearing capacity measurements with the TSD.
In 2014 a comparative testing of two in service TSD has been organized and performed on an old but stiff motorway pavement (30 cm asphalt on 15 cm cement stabilization) in northern Germany. Both 2
nd generation TSD devices have been manufactured by Greenwood Engineering and had the same specifications. Several runs on a total length of 50 km (25 km north- and 25 km southbound) have been made. One TSD was driving in front, the second followed directly in the same line as good as the driver of the second could do, see Fig. 10. The data has been analyzed afterwards by BASt. The full results can be found in [
19].
The data analysis showed that in general the measurements are comparable. At some data a shift could be observed (see Fig. 11) that could be lead back to the calibration procedure which needs some improvements, especially when measuring on stiff asphalt pavements. Additionally it has been observed, that the algorithm to derive deflection values out of slope values has its weak points, particularly when small deflections will be expected, see Fig. 12.
As a result of these measurements some improvements and recalibration have been made and the manufacture reconsidered the calibration method. Even knowledge groups like the FEHRL (Federal European Highway Research Laboratories) group BeCaTS (Bearing Capacity at Traffic Speed) started intensive discussions about calibration issues.
It is clear, that one of the best quality assurance tools is and will be direct comparisons of devices. In case of the TSD the problem is the minor number of available devices and their worldwide distribution. But for example for Europe future comparative tests can be possible, as the number of available devices will possible rise in the next few years.
When looking at the repeatability of in situ pavement measurements the challenging question is how to differ between the changes which have happened at the measurement device and the changes of the pavement itself. In particular bearing capacity measurements are very sensitive to environmental impacts like the changing temperature over a day. Every kind of temperature normalization or other ‘post-processed data fixing’ induces an inaccuracy to the raw data.
Taking this premise into account a R&D project, funded by BASt, has started in summer of 2015. This project aims to the short time repeatability of the TSD. Therefore repeatedly TSD measurements will be made at different types of asphalt pavements within a day and also at different seasons. The (interim) results will be published at future related conferences and journals.
Another big challenge is the derivation of assessment procedures of the measurement results and the implementation of these into pavement management procedures. R&D programs like the German ‘Innovation Program’ (hosted by BASt) are facing this challenge. The in progress In-Motion project, realized by the RWTH Aachen University, therefore evaluates the deflection status of an asphalt pavement under a rolling wheel and at different fatigue conditions. Therefore the BASt indoor test-track has been loaded with thousands of 50 kN loads by means of the Mobile Load Simulator MLS30 (aka MLS10). After defined loading cycles, geophones have been applied to the surface and the pavement has been loaded by a passing truck wheels at different speeds. With this setup the TSD measuring principle has been simulated, because the indoor test-track is too short to make real TSD runs. The deflections will be used to setup and calibrate a calculation model, which will be used to estimate the residual lifetime of a pavement, see Figure 13. The project will be finished in 2016.
Conclusions
Applicable methods and techniques for pavement assessment exist or are being on the way to be created. Within the last decade there is a clear boost to innovation in the further development of non-destructive measurement methods. The subject of ongoing research is among other things the unambiguous identification of the advantages and disadvantages and therefore the applicability of these innovative methods and techniques. The overall aim should be to provide technically and economically optimized processes for a lookahead structural pavement assessment.
The collection of more experience with the drafted structural assessment based on laboratory examination for asphalt pavements and the drafting of such process descriptions for concrete pavements are among the priority timely goals. Moreover, the knowledge of damage mechanisms within the pavement has to be expanded constantly, for example with further MLS30 applications.
As a long-term goal, the periodic monitoring of the condition in the whole federal network, by means of non-destructive technologies and the (partial) automated evaluation and assessment will be seen by doing defined intersections of various data sources. It can be assumed that nevertheless drill core extractions and subsequent laboratory tests at selected sampling points will be still needed for reliable pavement management actions.
Higher Education Press and Springer-Verlag Berlin Heidelberg
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