3
H
ISTORY OF
R
OLLING
W
EIGHT
D
EFLECTOMETERS
All the devices shown in the previous chapter have been used until now to collect data during road analysis. Between these there´s also the FWD, the most widely accepted and consolidated device to collect structural data and calculate bearing capacity of roads. But all these devices, including the FWD, have a common issue. They all have to stop on a test point for a while to acquire data and information. This make them unsuitable when there’s a needing of a data collection over a road network, with a lot of km of road to be tested.
A fast way to analyze a road is what a PMS needs, and since the demand of new road decreases day by day while the amount of road maintenance is increasing, slow tools for data collection become a bigger problem.
We’re not building so much new road, but every country has a huge heritage of “old” road which need to last in good conditions for years.
Road maintenance is becoming increasingly important and so the need to have high efficiency instruments capable to collect accurate data, without stopping in every point and without the need to close road, causing traffic disorder. All the bigger road administrations are developing PMS to handle all their network, which need a big amount of data, both structural and functional.
Since we already have some continuous, high efficiency devices for functional data, nowadays the market needs a new continuous device to collect structural data.
All these needs have led the development of a new generation of devices, called Rolling Weight Deflectometer (RWD).
The RWD is a nondestructive pavement test device that continuously measures the maximum pavement deflection near a loaded test wheel while travelling along the pavement. The main advantages of a rolling load are:
- It can measure continuously, no traffic disorder and the safety during tests is increased.
- It creates a wave-like response in the pavement, similar to the one generated by heavy vehicles in traffic.
3.1
The first attempt
The first idea of a RWD dates back to the middle of 1970, when dr. Milton Harr at Purdue University pioneered the measurement method used in the RWD. The technique promised deflection measurements at highway speeds. A suite of four noncontact optical pavement sensors mounted on a beam form the basic instrument. One sensor is close to the load tire, while the other three are mounted ahead of it and out of the influence of the deflection basin. These laser sensors operate on the principal of optical triangulation and measure the distance down to the pavement as the RWD moves forward. A simple algorithm determines pavement deflection as the beam is transported over the pavement.
This idea also defined the common shape of all the following attempts to build a RWD prototype. Since we need a long and longitudinal support for the sensors all the RWD are modified trailers long approximately 10 m and 2,5 m width. A ladder like structure with parallel support rails and transversal ones at a constant distance forms a perfect support for the sensors. On one end the structure converges to a V-shaped structure supported on an axle or directly connected to the towing vehicle with a trailer hitch. At the opposite end of the trailer is mounted a load platform directly connected to the load axle with single wheels.
Figure 3.1 – Drawing of the first trailer prototype.
The deflection measurement first identified by Harr is a two-step process. In its simplest form the distance from a reference datum down to the pavement surface is measured using
noncontact optical sensors in three equally spaced sensors. When the RWD has moved forward a distance equal to the sensor separation distance, the same three points on the pavement are measured, but now using the second, third, and fourth sensor. In this second step the load wheel is now located at the rearmost of the three points.
Figure 3.2 – Illustration of how the Harr algorithm works, with the spatial coincidence method.
The four sensors mounted on the beam are termed A, B, C and D. A is the lead sensor in the direction of travel. Equal distances separate the sensors. The data from each sensor is a positive number that is equal to the distance from the reference datum provided by the physical beam on which the sensors are mounted, to the pavement surface. An important requirement for accurate deflection measurement is that sensors A, B and C be situated out of the depression basin created by the load wheel. Sensor C is the critical one because it is nearest the load wheel. The sensors have to be positioned far enough away such that
vertical measurement errors caused by sensor C being in the influence of the depression basin are less than or equal to the measurement errors of the individual sensors.
The first measurement step uses the front three sensors, A, B, and C, and is made on the pavement surface at points P1, P2 and P3, respectively. These points are presumed to be
beyond the influence of the depression basin. The RWD described here has a sensor separation distance of 274 cm, enough to be out of the deflection basin in most of the cases. This distance can be increased or decreased in the field. The second measurement is made over the same three points on the pavement, but now made with pavement sensors B, C, and D. In other words, during the second measurement step sensor B is located where sensor A was previously, sensor C is located where sensor B was, etc. A high-resolution encoder mounted on the axle of one of the RWD tires commands the system to take data after moving the specified distance between the sensors.
It can be noticed that the quantity A-B is the difference between the slope of the beam and the pavement measured at points P1 and P2. Likewise, the quantity B-C is the slope between the beam and points P2 and P3, respectively. The difference between these two slopes, (A-B) - (B-C) = A-2B+C, is the change in pavement slope across three points and is independent of beam height or angle.
The measurement is determined from a line drawn from P1, through P2 and intersecting sensor C’s axis. The distance from this intersection point to the pavement is a virtual height because is not a physically measurable distance.
Two virtual heights are needed for a deflection measurement: one taken on undisturbed pavement and the other taken with the load wheel at point P3. Sensors B, C, and D are used to make this second virtual height measurement. Subtracting the two values will provide pavement deflection next to the load wheel:
At time T1: A - (A - B) = C - h + (A - B) and h = A - 2B + C At time T2: B' - (B' - C') = D' - h' + (B' - C') and h' = B' - 2C' + D'
Deflection is determined as: h - h’
The primes indicate the measurements are taken in different times.
For the deflection measurement method to work properly, the following constraints apply at all times:
- The pavement sensors must sequentially pass over identical areas of pavement. - The pavement sensors must never move with respect to each other.
For the algorithm to work, requirement number one is certainly a requirement. In both measurement steps the sensors must pass exactly over the same pavement region. This is why the RWD does not work well on corners.
The second one is a harder issue to solve. Previous investigators were aware that any thermal or vibrational bending of the beam corrupted the deflection signal. No practical means were discovered to solve the problem. Proposed and tested solutions tended to fall into three broad categories: the use of different beam mounting methods; making the beam stiffer; maintaining the beam at a constant temperature with some cooling methods. These methods didn´t work due to the fact that the exact nature of the amount and shape of beam bending cannot be controlled to the degree needed for accuracy compared to the deflectometer. An infinitely stiff beam made of material that also has a zero coefficient of expansion does not exist.
These problems didn’t find a solution until the first years of 1990, so the idea of Harr didn’t end with a prototype of a RWD.
3.2
First functional prototype by Dynatest and Quest
In 1992 the development of a RWD had a big improvement, since that Dynatest together with Quest found the solution to the beam-bending problem. Quest is a supplier of laser alignment system, so they decided to use a laser ray as a reference datum that never bends. Instead of trying to develop a beam that is very stiff, the beam is allowed to bend and the amount of bending is measured using these additional optical alignment sensors mounted over each pavement sensor.
The method developed to compensate for thermal gradients or vibration of the beam is to monitor the amount of sensor movement in real time. An alignment laser beam aimed down the central cavity of the steel support beam is used as the reference datum. The amount of vertical movement of each pavement sensor is measured using optical position sensors mounted intimately to each pavement sensor.
They work with a beamsplitter fixed to the frame of the vehicle, which reflects about 15% of the laser beam onto the position detector. This detector can measure the distance between him and the laser reference datum. The position of the laser beam, as determined by the position sensor, is added to the height data measured by the pavement sensor. The addition of these two variables produces pavement sensor data that is compensated for thermal or vibrational bending.
Using a laser beam as the reference datum allows accurate deflection measurements in the presence of thermal or vibration bending of the physical beam.
Figure 3.3 – Laser alignment system over each sensor.
The sampling control system of the apparatus is controlled by an odometer, directly linked to the load wheel which sends electrical pulses to a computer to trigger the taking of measurements by the sensors. The odometer is set to send a pulse to the laser sensors so that they can collect measurements at a spacing equal to the sensor-spacing distance. After the trailer has moved forward by one sensor-spacing distance, the sampling control system is again activated by the rotation of the odometer. In this first prototype the sampling distance is 274 cm, because this was the distance between sensors.
The odometer used for measuring the distance and for the identification of the exact same point between the moved loaded wheel and the preceding distance sensors needs to be extremely precise to collect data exactly in the same position of the previous sensor. Distance measurement are taken at about 1 millisecond intervals while driving at 6 km/h, a higher speed requires shorter sampling intervals.
Making sure that the lasers “hit” the same exact surface point is the main technological challenge to let the algorithm to yield a precise and useful result.
In addition, also the track of the RWD needs to be precise and follow a straight line, in order not to measure little on the left or right in the second position.
Many attempts have been made to try to solve this problem but without a solution. At the beginning they tried to mark the pavement with reflecting paint spot spaced by the sampling distance, so that a laser can identify them and send the pulse to collect the data over every spot. The idea was to create a points path, so that all the sensors will hit the same point on the pavement to measure the deflection.
The same solution was made using thermal marks on the pavement and infra-red sensor to detect them. Both the attempts weren’t used because the accuracy of these points on the pavement was not so high, and the detecting system was introducing more errors.
This problem has been solved using a large, elliptical laser footprint. The accuracy of a conventional optical triangulation sensor is related to the spot’s size on the target surface, a smaller spot size means a more accurate reading of the sensor. However, the RWD pavement sensor uses dual optical triangulation optics to greatly improve accuracy and allow for the use of large diameter laser footprints. The dual optics eliminate measurement errors caused when the pavement surface is very textured or has high contrast, like a typical asphalt concrete pavement. A large footprint also removes the error of measuring different heights in close spots due to the surface’s macrotexture.
The laser footprint used was 50 mm wide perpendicular to direction of carriage travel and 25 mm in the direction of carriage travel. This was enough for small deviations from a straight line while testing, but still not enough to test on ninety degrees turns, like in an urban area.
The RWD sensor makes two measurements at the same time: one to measure the distance to the pavement, the other to measure the vertical displacement of the sensor with respect to the laser beam reference datum. The accuracy of the RWD pavement sensor is 20 microns, while the accuracy of the optical alignment sensor is 5 microns.
The project ended with a functional prototype in 1994, developed by Dynatest together with Quest, for the U.S. Air Force.
Figure 3.4 – The first RWD trailer ever built.
The trailer was around 12 m long and 2,5 m wide, with a weight close to 4 tons.
The maximum speed for data collection was 8 km/h, and the maximum load applied by the single tire was 220 kN. The load was very high because this first prototype was created to test on hard and stiff airfield pavements. This first prototype has no suspension system and no brakes.
The RWD also contains additional sensors. The instantaneous static load applied load of the rolling weight deflectometer is measured with a load cell. A vertical accelerometer attached to the load platform is used to aid in correlating deflections with the actual applied load. Since the load can bounce, it is important to know the vertical acceleration for each data point to correct the static load with the inertial one.
Pavement temperature is continuously taken at sensor with a Raytek IR pyrometer with an accuracy of 0.5°C.
The entire suite of sensors transfers data onto a computer for data collection. All sensors make the same number of discrete measurements for the same time period. The number of measurements can be set from 1 to 1024. They are averaged after the measurement interval is over. The average of an individual sensor’s measurements is the value sent to the computer. This whole process is repeated for every measurement interval.
The PC also has an operator interface that allows viewing of real-time deflection data. The data are stored in the RWD’s computer during a scan of the pavement and suddenly can be transferred to process the data. The data are stored as comma separated variables to automated entry to spreadsheets or data analysis programs.
It was tested and compared to the FWD several times. The following Graph 3.1 is a comparison made over an airport taxiway composed by 10 cm of asphalt concrete and 30 cm of base course. The stretch was 700 m long. The RWD collected deflection data in 1-foot intervals and the FWD collected data in 50-1-foot intervals. Note that the FWD and RWD plots do not overlap, as the FWD was set for 10000 pounds (approximately 4,5 tons) and the RWD for 5000 pounds (approx. 2,2 tons), but anyway the matching of the two results was good and promising.
Graph 3.1 – Deflection comparison between the first RWD trailer and the FWD.
While the comparison results were promising, this first RWD prototype had some defects which make it difficult to be used on road testing.
First of all, it was too long. Due to the 274 cm between each sensor the trailer was more than 12 m long. This huge space between the sensor was due to place only one sensor inside the deflection basin, while the others were outside. So, the results can provide only information
about the whole pavement, considering all the layers, but without any information about particular layers under the surface.
The needing of an additional system to compensate the bending of the beam from a reference datum increased the amount of data and the post processing work.
The testing speed was very low, up to maximum 8 km/h, allowing the RWD to test only on airfield or big closed road, due to avoid traffic disturb.
Due to his big length and his low testing speed, this first RWD was intended to test only on airfield where the size is not a problem, used as a screening device for locating pavement sections with similar deflections. Since several comparisons showed good matching results, it can be used to subsection pavement networks into uniform sections, to simplify the work of the more accurate but slower FWD or HWD, which will test only on the bad sections founded by the RWD instead of the whole network. This will reduce significantly the time spent in testing activities.
In 1997 the same team of developers made by Dynatest and Quest produced a new functional prototype, with some improvements.
The concept was actually the same, a trailer with four laser sensors equally spaced. The dimensions were the same, with a trailer around 11 m long and 2,5 m wide. The separation distance between pavement sensors was still 274 cm and they were using the same Harr algorithm to evaluate deflections.
To monitor the bending of the support beam for the sensors a laser alignment system used as a reference datum was still the best solution. It works exactly as in the previous model, adding the height of the sensors from the reference datum to the distance of the surface measured by the sensors.
They changed only the laser footprint of the sensors, due to an improvement in the laser sensors technology. The new footprint was 38 mm wide and 5 mm long in the direction of travel. This allowed the RWD to work with higher speed, up to 32 km/h, with a maximum sampling frequency of 1000 Hz. In addition, as explained in the previous chapter, a larger footprint allows the RWD not to drive on a prefect straight line. The accuracy of the new laser sensor was still 20 microns, as the previous prototype.
To collect data while the trailer was moving at 32 km/h the developers had to improve the stability too. This was made with a double, still single tire, axle in the back part of the trailer, close to the load platform.
Driving at the top speed of 32 km/h the maximum load allowed is 40 kN on the same single load tire. This second prototype was also able to operate with higher load, up to 220 kN as the previous one, but with a maximum speed of 10 km/h.
A load cell monitors the static load, and the same set of additional sensors of the previous prototype complete the data collection tools. So, also on this trailer we can find accelerometer, to measure inertial load due to bouncing, to correct instantaneously the applied load, infra-red sensor for pavement temperature and a high accuracy odometer to measure the exact distance travelled by the RWD.
This second RWD utilizes a standard laptop PC for operation and data collection. All these tools to collect measurements on the trailer are triggered by a very accurate odometer directly connected to the wheel, sending an electrical pulse every 274 cm (since this is the spacing distance of the sensors and Harr algorithm requires this distance between two readings). The number of measurements can be set from 1 to 1024 for each interval. At the end of each interval they are averaged and sent to the PC. As the previous model, also this prototype has no suspension system and no brakes.
Figure 3.5 – Second RWD trailer with new upgraded features.
Looking at Figure 3.5 we can notice the beam under the frame supporting the laser sensors (three, black, box-shaped, the fourth is hidden close to the load wheel), and the double axle with single tires in the back part.
This prototype was submitted to several tests over airfield pavement with the Dynatest FWD to compare the deflections collected. The following graph shows the result of one of these tests over a runway made by 165 mm of asphalt concrete and 400 mm of crushed stones base, with a load of 40 kN. As we can notice from Graph 3.2, the results are promising.
Graph 3.2 – Test results of a comparison against the FWD.
Another very important parameter to compare is the time, not shown in Graph 3.2. The test section was 300 m long, and it took less than one minute to run the prototype RWD over it, while it took almost two hours to run the FWD with test points spaced by 3 meters.
In this new prototype some improvements have been made. The laser sensors technology was improving year by year in that period. The speed and the stability of the trailer was raised. On the other side also this new trailer still had some defects of the previous one. The trailer was still too long to be used on normal roads. Due to his length the only other field where it can be used are highways, but only during closing periods of these. The speed was increased, but it wasn’t enough for testing in the traffic.
At the end a prototype Rolling Weight Deflectometer for airfield pavements has been developed and tested. The results of these tests indicate that the device compares favorably to a Falling Weight Deflectometer, which is commonly used for project level deflection testing.
As currently constructed, the device is not suitable for collecting data on highways open to traffic. Future research and development will be conducted to adapt the system for use on highways and other roads. The electronics will be improved to increase data collection rate.
Because the accuracy of the deflection measurement for this device will be on the order of 25 microns, the RWD is intended as a screening device for locating pavement sections with similar deflections.
At the network level, it can be used to subsection pavement networks into uniform sections, and to detect the worst sections in terms of deflections. These worst sections can be later tested with the slower, more accurate FWD for more accurate results.
The RWD used as a screening device, as explained in a previous comparison, can reduce significantly the time spent in testing activities.
The RWD can also be used to monitor seasonal variations in deflections on the highway network.
This second prototype was tested and used on airfield pavement, comparing the results with other devices, like the FWD or other devices for structural data.
The shape and the tools on the RWD didn’t change, and no more attempts or improvements have been made until the first years on 2000.
But starting from this new millennium, two new devices have been developed, and both of them introduced new technologies and new ideas, trying to improve these first two prototype.
These new generation of devices was made by the ARA Rolling Weight Deflectometer and by the Greenwood Traffic Speed Deflectometer.
3.3
The ARA Rolling Weight Deflectometer
During the first years of 2000 Applied Research Associates (ARA), Inc. developed a RWD which was designed to test flexible pavements and the majority of roads made by asphalt concrete (AC)-surfaced pavements. Although the RWD was not designed specifically to test concrete (PCC) pavements, but a few PCC pavement sections have been tested.
The concept behind this new device was basically the same, a special trailer with a single axle imparting the load, while four laser sensors are measuring the deflection.
Figure 3.6 and 3.7 – The ARA RWD trailer testing on a road.
The current prototype was developed jointly by the Federal Highway Administration (FHWA) Office of Asset Management and Applied Research Associates (ARA), Inc. It uses four triangulation lasers mounted underneath the bed of a semi-trailer to measure a continuous deflection profile of the pavement when loaded by the trailer's 18-kip (80 kN or 8 tons) single axle load.
The system has undergone extensive field testing, including pilot studies for numerous state highway agencies, including Texas, Indiana, Virginia, New Jersey, and Minnesota DOTs. The first tests result was shown in 2006. Field testing has verified the RWD's capability to measure pavement deflections at highway speeds. Currently, ARA is enhancing and improving the device to make it available for commercial data collection services.
Because of these tests made in the US the following reported data are using US customary units.
The ARA RWD can also collect continuous digital images of each road tested in addition to deflections. With additional effort, the RWD could be enhanced to collect inertial longitudinal profiles for use in calculating the International Roughness Index (IRI). The combination of pavement deflection, condition rating, and IRI would make the RWD a powerful single device for the collection of multiple PMS data types.
3.3.1
ARA RWD description
Equipment:
The RWD is comprised of a set of four triangulation lasers attached to an aluminum beam mounted on a custom designed 53-ft (approximately 16 m) trailer. The trailer is sufficiently long to isolate the deflection basin produced by the RWD trailer's 18-kip (80 kN or 8 tons), dual tire, single axle from deflections produced by the towing tractor. In addition, the natural frequency of the trailer's suspension of 1.45 to 1.8 Hz is low enough that it does not couple with the high-frequency vibration of the 25.5-ft (approx. 7,77 m) aluminum beam used to support the lasers. The beam uses a curved extension to pass under and between the dual tires, placing the rearmost laser approximately 6 inches rear of the axle centerline and 7 inches above the roadway surface. The wheels have been spaced a safe distance from the laser and the beam using custom lugs and a spacer shown in figure 3.10.
Figure 3.8 and 3.9 – Particulars of the beam supporting the sensors and of the spacer between the twin-wheel.
Measurement Methodology:
The RWD utilizes the same "spatially coincident" methodology for measuring pavement deflection as the first two prototypes.
This method was originally developed by the Transportation and Road Research Laboratory (TRRL) and furthered by Dr. Milton Harr at Purdue University.
Three lasers placed forward of the loaded axle are used to define the unloaded pavement surface profile and a fourth laser placed between the dual tires measures the deflected pavement surface. Deflection is calculated by comparing the undeflected pavement surface with the deflected pavement profile at the same location. At 55 mph, the RWD's 2-kHz lasers take readings approximately every 0.5 in (1,5 cm), resulting in extremely large data sets. To make the data set manageable and to reduce the random error of individual readings, data are averaged over an interval suitable for pavement management purposes, typically at 0.1-mi (528ft or 160 m) intervals. At normal highway speeds, a 0.1-0.1-mi average contains approximately 60,000 individual laser readings.
The RWD was operated by two people: a driver and an operator. During data collection the operator entered event markers that corresponded to bridges, changes in pavement surface type, and zones of significant acceleration/deceleration. Due to the absence of a clearly-posted mile marker system that could be used for referencing during high-speed data collection, the RWD data was referenced by measuring distance in miles from the start point.
Event markers are used during data processing for filtering of outlier data due to localized events. In addition to deflection data, the RWD also records continuous digital images and GPS coordinates for each road.
Raw data from the RWD were processed in the to calculate and display the following parameters per sample unit (0.1-mi):
- RWD deflection and deflection deviation - Truck speed and speed deviation.
- Pavement surface temperature.
- Linear referencing based on the Distance Measuring Instrument (DMI), GPS, and physical mile markers.
- Event markers, such as bridges, intersections, or other references.
3.3.2
ARA RWD results
Figures in this chapter show average RWD deflections calculated at 0.1-mi intervals. In several figures, the representative RWD deflection is shown as well. This corresponds to the 98th percentile (meaning 98 percent of the data is lower than this value) and it is considered more representative of expected pavement structural capacity than just the mean RWD deflection itself, as it considers pavement variability.
These following tests have been made over three different kinds of pavement: fair, sufficient and very poor.
The roadway chosen for these tests carries significant truck traffic and this is reflected in its current condition, which ranges from fair to very poor. The pavement is made by asphalt concrete. The RWD deflections clearly indicate three distinct pavement structures. From mile 0 to 4.6, the deflections are relatively low and uniform, with a representative deflection of 12.5 mils. Then, from mile 4.6 to 19 the condition of the road deteriorates and the representative deflection increases to 23.4 mils. Finally, the section from mile 19 to 32 exhibits high and variable deflections, and this section is also in poor condition, with significant fatigue cracking. The visual conditions for all sections show the roadway needs improvement. However, the RWD deflections show that the section from mile 0 to 4.6 already has good structural capacity and only needs surface improvement, while the remainder of the road may require structural improvement.
Graph 3.3 – Results over different kinds of pavement.
As shown by Graph 3.3, low and uniform deflections are measured on asphalt concrete roads in good conditions. On the other side, asphalt concrete roads in poor conditions show high and very variable deflections. These roads, with the weaker pavement (higher deflection) are expected to show structural deterioration first.
Defining the section's representative deflection as its 98th percentile, we can use RWD output data to rank roads and order them from the weakest to the strongest.
Other tests with various speeds on the same section have been made. The results show a dependence of them from the speed of the test, but not in all the sections. Thus, a good repeatability is not expected if the tests are made with different speed.
3.3.3
ARA RWD Pros & Cons
Pros:
- The lasers are fixed on a beam situated just in the middle of the dual tire, 8 tons single axle. In this way the load is very close to the first laser sensors. This produces a higher deflection and a deeper basin under the wheel. A higher deflection significantly decreases the error rate. This was basically the bigger improvement from the old models.
- A climate-controlled chamber maintains the measurement system at a constant temperature. This is to avoid a wrong data collection, due to the dependency of the measured distance with the temperature.
- distance-measuring instrument (DMI) to longitudinally reference collected data and a global positioning system (GPS).
- infrared thermometer to measure pavement surface temperature.
Cons:
- Also if the beam supporting the lasers is shorter (almost 7,7 m) the trailer is still too long to be easily guided into city roads.
- It cannot be used on wet surfaces because of the refraction of lasers, which can generate wrong results in distance data.
- No bending beam control system.
- No accelerometer to measure the real load on the wheel instantaneously, due to the bending of the trailer.
- The repeatability was affected by a high speed-dependency of the results.
As explained before, the bigger improvement was the use of a dual tire on the single axle imparting the load. This allows the measure of the deflection with a laser sensor closer to the load, comparing to the previous prototypes.
Another important improvement was about the testing speed of this trailer. That was increased up to almost 90 km/h, allowing the ARA RWD to test on roads surrounded by traffic.
On the other side, this RWD was still using the Harr algorithm to calculate the deflection. This algorithm requires that three sensors out of four have to be out of the deflection basin caused by the load. The problem in this new trailer is that these three sensors should also be out of the deflection basin caused by the towing vehicle. Consequently the length of the trailer is 16 meters, too long to operate in small areas and city zones. The fields of application of this device still were airports or major roads. Due to the increased testing speed it was allowed to test on roads opened to traffic.
3.4
The Greenwood Traffic Speed Deflectometer (TSD)
Like the other prototypes of a rolling weight deflectometer, also the Greenwood TSD has looked into the past.
One of the first attempt to measure roads deflection with a moving device was the Benkelmann Beam. It was developed and refined so that deflection could be measured during a slow rolling wheel loading. The wheel velocity was “walking speed” and the dynamics of the pavement still quite different from what would happen under a real truck in traffic.
From 1985 to 1990 the Danish Highway Network developed a Deflectograph designed by the Danish Road Institute, with a survey speed of 5km/h. The measure of the deflection was made according to the Benkelmann principle, using the load of a truck axle. When a truck in 1990 crashed into the Danish Deflectograph it was damaged and never reconstructed.
Figure 3.10 – One of the first attempt to build a Rolling Weight Deflectometer. It was based on an automatization of the Benkelmann’s beam.
The next demand was to develop a device that would follow traffic flow, increasing the safety for the drivers and for the operators. For this reason new studies have been made to increase the testing speed of a defectograph, trying to develop a high speed version.
The purpose of the Traffic Speed Deflectograph was to provide a quick, safe and reliable tool for the evaluation of the structural condition of roads at network level. As opposed to stationary measuring systems the device provides continuous measurements making it feasible to survey a whole network at regular intervals. The device can be used as a screening tool revealing discontinuities in the bearing capacity. Using suitable models for the interpretation of the data, information about the current structural condition can be obtained making it possible to detect problems at an early stage. The data from the Traffic Speed Deflectograph can be used as a supplement to data from other measuring systems, providing input to pavement management systems, improving planning and cost effective-ness of maintenance work.
From 2006 the name “Traffic Speed Deflectometer” (TSD) replaces the former “High-Speed Deflectograph” (HSD). The TSD has a loaded truck axle travelling at traffic speed. The main difference of the TSD, compared with other devices already described, is that it uses Laser Doppler sensors for non-contact deflection measurement from the rolling truck.
3.4.1
TSD trailer description
Vehicle:
The vehicle is a standard truck with a modified trailer. The modifications are the load compartment under the container and the double tire with a little extra spacing between the two rims so the laser light is not blocked by the tires. The trailer has standard fittings for a normal freight container. Inside the isolated container the deflection sensor system and the data acquisition system are installed.
Figure 3.11 and 3.12 – TSD prototype and newer versions built on a standard truck.
Deflection Sensor System:
The Traffic Speed Deflectometer measures the pavement deflection velocity using Doppler laser sensors. Laser Doppler sensors measure the velocity of the pavement surface when the pavement moves during the load passage. The Doppler principle delivers an instant measurement where the change in laser light frequency is caused by the reflecting surface velocity. The change in frequency gives the surface velocity.
∙ Where:
Fdoppler is the frequency shift at the receiver, Fsource is the emitted frequency,
c is the wave propagation speed,
v is the relative velocity between source and receiver.
The laser beam emitted from the Doppler sensor strikes the road surface and the sensor measures the velocity in the direction of the laser beam. The loading of the road surface by the wheel yields a deformation of the road surface and the Doppler sensor registers the velocity of this deformation. The concept is well-suited for testing at high driving speeds as the deflection velocity increases with increasing driving speed.
Since this method is non-contact and does not compare two measurements from the same surface point, then the equipment has no problem measuring in curves.
Figure 3.14 and 3.15 – Inside view of the TSD, with a focus on the servo system supporting the sensors.
The Doppler sensors are mounted on a rigid beam inside a refrigerated trailer such that the sensors move together. A Doppler laser measures velocity in the direction of its light beam.
Because the Doppler sensor is not positioned exactly perpendicular to the road surface the laser beam does not strike the road surface at an angle α of incident of exactly 0 degrees. Hence, the Doppler sensor registers, in addition to the desired deflection velocity, also a component of the driving speed Vds.
+ sin
To correct for this, the angle of incident of the laser beam on the road surface needs to be measured accurately together with the driving speed. The angle is measured by means of a second laser Doppler sensor placed outside the deflection bowl. The sensors are mounted on a stiff beam to ensure that the relative angle between the sensors remains fixed.
If α1 and α2 are the angles of incident of the reference sensor and the deflection sensor respectively and Vm1 and Vm2 are the values measured by the sensors, then:
sin
+ sin
In the case where α1=α2=α the deflection velocity can be found directly by subtracting the velocity measured by the reference sensor from the velocity measured by the deflection sensor.
Unfortunately obtaining the same angle of incident for both sensors is not practical. If αc= α2- α1, then:
sin + sin
The angle αc is determined during a calibration procedure and the driving speed Vds is measured using an odometer.
As the Doppler sensors register the relative velocity between the sensors and the road surface it is necessary to adjust the data for the movement of the sensors. Thus, the movement of each sensor has to be measured and subtracted from the deflection velocity output. The movements are determined using an inertial unit composed of three accelerometers and three gyros. Movement of the laser sensors is limited and controlled by a servo system on the mounting beam to ensure that the laser sensors are focused at all times. The inertial unit and one distance-measuring laser in each end of the mounting beam provide input data for the servo system.
A TSD can have a Doppler sensor configuration as a FWD if the same number of points describing the deflection basin is required. The minimum number of sensors required is two. One sensor is positioned 250 mm in front of the load centre where deflection data are to be obtained. The other sensor is a reference sensor and is placed further away in a position where no deflection occurs. Sudden changes in deflection velocity reveal discontinuities in the structure. Thus, when equipped with two laser Doppler sensors the TSD can be used for screening purposes.
To obtain more detailed information the device could be fitted with additional sensors. When a larger number of sensors are employed the deflection velocities are measured in several points in the deflection bowl. By integration the absolute deflections can be obtained giving access to existing interpretation methods used with the FWD. Up to ten Doppler laser sensors can be mounted on the beam inside the trailer.
Special designed trailer and wheel hubs allow measuring close to the load, in the middle of the double tire wheel. This is very important if the aim is to have a comparison with the FWD, which has geophones for measuring the deflection close to the load plate.
Figure 3.16 – Detail of the sensors acquiring data close to the loading area, in the middle of a twin-wheel.
Deflection Slope is determined as Deflection Velocity divided by Driving Velocity. As the slope of the deflection is the derivative of the displacement it is possible to calculate the displacement integrating the slope deflection. This means, it is possible to obtain bearing capacity characteristics such as Structural Curvature Index 300 (d0 – d300) and Center Deflection (d0) based on the measured deflection slopes.
Figure 3.17 – Illustration of how to calculate deflection from the data acquired.
Load:
The Load on the rolling axle is composed by weight of the trailer, the measuring system and weights in a detachable load compartment. A special ballast load mounted below the trailer, can be removed or rearranged. This feature allows the TSD to measure pavement responses to various load levels, up to 5 tons on each wheel. Inside the load compartment bags filled with lead beads are stored. The total vehicle weight and the axle loads are adjusted by use of portable weigh pads under the wheels. The operator must unload/reload lead bags until the axle load is as required.
The maximum load allowed is 10 tons on a single load axle, driving up to 80 km/h. Using this configuration the wheel impacts the road with a force magnitude of 50 kN. On smooth roads it provides reliable results at speeds up to 90 km/h. The minimum operational speed is 20 km/h. The reason of a lower limit for the speed is related to what the Doppler sensors are measuring. Since they need to measure a deflection speed, if the TSD is moving slowly the value registered by the sensors will be too small, within the range of the accuracy of the sensors. Therefore to collect useful data which are not affected by the accuracy of the sensors the trailer need to be towed at 20 km/h or more.
Modelling of the data:
In the model used for data analysis, it is assumed that the pavement construction behaves like an elastic beam on a Winkler-foundation of linear springs. This is expressed in the Euler–Bernoulli beam equation, where F is the point force, E the elasticity, I the moment of inertia, h the pavement thickness and k is the spring constant.
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The solution to this differential equation is a two-parameter model in A and B, where x ≥ 0, A> 0 and B > 0.
Using the equation for the deflection slope, is possible to estimate the constants A and B. These can be used in the deflection equation to evaluate the deflection basin.
Model limitations:
The model is limited in accuracy because it has only two parameters. It will produce trustworthy results only at positions in the deflection basin where data are available. With sensors placed at 100, 200 and 300 mm from the center of load the model is suitable for describing the deflection basin in the range 0–300 mm from the load.
The model is defined for x ≥ 0, this implies that the measured slopes of the deflection basin cannot be negative. When measuring on roads with high bearing capacity some measured slopes are negative due to signal noise. A solution for this could be to accept a lower limit for the slopes that can be measured. Subsequently, road sections that produce very shallow slopes could be considered as one group with good bearing capacity.
Employing additional Doppler sensors will increase the accuracy of the system in general and of the results from the model as well. Extra sensors will offer the possibility of using models with more parameters.
Furthermore, if the additional sensors are placed in positions further away from the load the
reach of the model is extended and it becomes possible to distinguish between effects from the different layers.
3.4.2
TSD tests and results
In this chapter are shown some of the results coming from different test sites.
The very first test have been made in Denmark, where the TSD has been developed. In August 2001 a TSD prototype was tested on a section of motorway M30 between Maribo and Rødbyhavn. Motorway M30 is a typical older Danish motorway with two lanes in each direction and a narrow median. The pavement consists of 230 mm to 280 mm asphalt concrete on top, and more than one metre of base and subbase materials composed of cement concrete, gravel and sand. The natural subgrade is moraine clay.
The purpose of these tests was to document the measuring concept. All measurements were conducted at driving speeds in the range 70-80 km/h. Several road sections were measured two to three times, approximately one hour apart, to check also the repeatability of the data. Falling Weight Deflectometer data from June 2000 were available for certain road sections, allowing a comparison between the two devices.
One of the main aspect to test, already known by the technicians, was the driving speed influence in the overall level of the deflection velocities. Thus, with these tests it is possible to observe changes in the deflection velocity in the measurements.
Repeatability tests:
These kinds of tests have been made with repeated measurements over the same road section. The Doppler sensors collect approximately 1000 samples/second corresponding to one sample every 20 mm when the driving speed is 20 m/s or 72 km/h. The graphs show a moving average of 500 samples corresponding to approximately 10 metres. The x axis represents the location in kilometres and the y axis the deflection velocity in m/s. Both measurements are from motorway M30 near Rødby in Denmark. Graph 3.5 indicates good repeatability as the same fluctuations can easily be identified in the repeated measurement.
The differences in the overall level of the measurements appear because the driving speed was not exactly the same in each measurement. The influence of the driving speed on the deflection velocities is partly due to the fact that a higher driving speed causes a higher deflection velocity.
Reliability tests:
The reliability can be investigated by comparing the Traffic Speed Deflectograph data with results from measurements conducted with the Falling Weight Deflectometer. The following Graph 3.6 shows the Traffic Speed Deflectograph data and maximum centre deflections obtained with the FWD. The left y axis shows the deflection velocity in m/s and corresponds to the two lower curves from the Traffic Speed Deflectograph. The y axis to the right shows absolute deflection in 1/1000 mm and corresponds to the upper curve based on FWD data. Some correlation between the deflection velocities and the maximum centre deflections can be observed.
Graph 3.6 – Comparison between the two previous runs and the FWD.
To avoid comparing distances with velocities, the slope of the FWD deflection bowl at an offset from the centre of loading corresponding to the position of the rear Doppler sensor of the Traffic Speed Deflectograph is calculated from the FWD data. The measured deflection velocities should be directly proportional with the slope. The Traffic Speed Deflectograph measures at a position 250 mm in front of the centre of the wheel load. The slope is estimated
from the maximum centre deflections measured with FWD in the positions 200 mm and 300 mm from the centre of the load. Knowing that the slope is the ratio between deflection speed and driving speed, the deflection speed of the FWD can be estimated using the following equation, multiplying the slope with the driving speed the deflection velocities:
()* +,+-. 1// 0// // 0// Where:
Vdefl-FWD is the estimated deflection velocity from the FWD data, Vdriving is the driving speed,
d200 is the maximum deflection in the position 200 mm from the centre of the load, d300 is the maximum deflection in the position 300 mm from the centre of the load, l200-300 is the distance between the two sensors at 200 and 300 mm.
Graph 3.7 - Comparison between two runs of the TSD and the FWD.
These discrete points have been connected with a curve, so the values between the actual test points do not represent actual test results. For a better comparison could be desirable to have the same distance between FWD and Traffic Speed Deflectograph points.
The second test trial have been made in Australia in 2010, with a final amount close to 18000 km. Since in Australia they use to drive on the left, they had to change the towing truck with a different one, approved for operating in Australia. There were also considerations on making changes to the TSD, by for instance moving the lasers to the left side of the trailer, instead of having them in the right side. But it was considered wiser using the system as it was, being well tested, instead of making changes that may introduce unknown characteristics.
In December 2009, prior to arrival of the TSD, five test sections were tested with a Deflectograph and a FWD which are the equipment traditionally used in Australia to provide information about pavement bearing capacity. Suddenly the same test sections have been tested with the TSD. The aims of this comparison were:
- Check the validity of TSD measurements, to see if it provides similar information to traditional equipment like deflectograph and FWD, and how should results be interpreted as compared to traditional methods.
- Speed dependency of the collected data.
- Parameters for processing of results and how should TSD measurements be processed to obtain the best fit with data delivered from traditional equipment. - Check the repeatability of TSD measurements.
To check the repeatability and the speed dependency of the data, three runs at three driving speeds have been made. These nine measurements have been made in a test site on Illawarra Highway. At approximately 760 m one of the measurements at 80 km/h and one on 60 km/h identifies a relatively soft spot, perhaps due to a crack not registered during other measurements.
Besides showing good repeatability, these measurements also illustrate that the production measurements would not be limited to a certain driving speed interval, as measurements do not show speed dependency. In Denmark, there has only been very limited testing of speed dependency, and these showed a little dependency of practical meaning. Investigations in the UK by TRL however, have demonstrated speed dependency, so it seems that there is some viscous effect of speed on thick asphalt pavements, which hasn’t been tested neither in Denmark or Australia. Any possible speed dependency was considered to be negligible for practical purposes in this project.
Graph 3.8 – Speed dependency tests.
The comparison of the TSD with the deflectograph and the FWD has been made with this procedure. The test sections were tested first with a deflectograph and FWD. The deflectograph delivers a value of maximum deflection for every 4 m. FWD measurements were conducted at the first half of each test section to limit the time used on each site. They were taken for every 10 m, and the output is given as maximum deflection to compare with other equipment. TSD data is continuous and up to 1000 data points are registered every second. To remove the excessive noise and to create practical file sizes and comparable output, data was processed to give an output value for every 4 m as is the case for the deflectograph. The output value given is the running average of the preceding 4 m.
For these tests the FWD measured the inner wheel path, to match the measuring line of the TSD. The deflectograph measured in both wheel paths simultaneously.
The TSD data plotted are average values of three TSD runs at 60 km/h. While it may have been possible to produce closer agreement, generally by choosing shorter reporting lengths it was decided to adopt a 5 m interval as the reporting convention. Due to differences between devices in parameters including loading, test speed and tyre pressures it should not be expected that absolute agreement would be achieved.
Graph 3.9 – Comparison between deflectograph, TSD and FWD.
The three measuring devices show virtually identical profiles of bearing capacity. The deflectograph and TSD show profiles very similar to each other in both shape and magnitude. These two devices also provide the same type of loading to the pavement, although the deflections are measured by different principles and at different speeds. Even though the FWD simulates the load of a rolling wheel, the application of load is slightly different. Generally for these measurements, the FWD results were of slightly higher magnitude than results from deflectograph and TSD. It does seem clear however that the pavement responses recorded by different devices, using different testing principles, are quite consistent. At this stage no equations are proposed that would directly translate between devices.
In Graph 3.10 is shown a comparison of the deflectograph and the TSD. The measurements are performed on a 20 km test section on Highway 4. Only the first 5 km are shown in the figure. In this case both measurements are performed on the same day. The TSD profile plotted is just one measurement, instead of an average of several runs. The two devices very well identify the same strong sections and weak spots. However the deflectograph seems slightly more sensitive to changes, as the deflectograph shows a few more peaks than the TSD results.
Graph 3.10 – Comparison between TSD and Deflectograph
Graph 3.11 of this Australian tests shows results from measurements at three different dates on Illawarra Highway, done with the purpose to check the repeatability. All measurements identify similar variation in pavement strength. The measurement on March 1st shows a slightly stiffer pavement. Pavement surface temperatures onJanuary 17th, January 29th and March 1st were 28°C, 31°C and 15°C respectively. There is no temperature correction to the measurements, so this difference in magnitude could be due to temperature differences.
Cost-benefit analysis:
The development of the Traffic Speed Deflectograph prototype had a project budget of approximately 1.3 million euro. The Agency for Trade and Industry provided financial support corresponding to 80 percent of the project budget, while the remaining costs were financed by the Road Directorate itself. Of the total budget of 1.3 million euro, the work carried out by the main contractor, Greenwood Engineering A/S, accounted for a approximately 80 percent, while the remaining part of the budget was allocated to the Danish Road Institute's own activities or other suppliers. Originally, the project was intended to run from the middle of 1996 until the end of 1999, but due mainly to unforeseen problems with a technology, the project was not terminated until the end of 2001.
The costs related to the Traffic Speed Deflectograph is primarily the development costs. During routine use of the device for scanning of road networks the Traffic Speed Deflectograph results should be supplemented by detailed Falling Weight Deflectometer tests at roughly ten percent of the measured road length.
Tests with the Falling Weight Deflectometer at one third of the Danish state network (1,629 km) can be done in 27 days and costs approximately 130,000 euro including the use of a hazard warning vehicle as well as analysis and reporting of data.
Scanning of the same road length with the Traffic Speed Deflectograph and following supplementing Falling Weight Deflectometer tests at 10 percent of the measured distance would take approximately two days and cost approximately 25,000 euro including hazard warning vehicle for the Falling Weight Deflectometer as well as analysis and reporting of data.
The primary quantifiable benefit of the Traffic Speed Deflectograph is its production capacity. With a driving speed during measurement around 70 km/h and five effective work hours per day, 1,750 km can be tested within a week. This means that the all lane-km at the Danish state road network (approximately 6,000 km) can be tested in four weeks. Similarly, it will require approximately 12 weeks to test all lane-km at the county roads in Denmark (approximately 20,000 km). Thus, the total period needed to monitor the structural condition of the major road networks in Denmark is approximately 16 weeks.
During the considerable part of the year, where the Traffic Speed Deflectograph is not used for testing in Denmark, it will be possible and desirable to perform testing in other countries. With a daily lease price of 10,000 euro, the Traffic Speed Deflectograph will potentially be able to earn 1.75 million euro per year when the device is operating for 35 weeks per year.
3.4.3
Conclusions:
The TSD developed by Greenwood Engineering A/S is nowadays a used and validated device for road analysis, mostly in screening, network level ones. It delivers continuous measurements of maximum deflection a lot faster than traditional equipment, and with no disturbance to traffic. The system operates successfully with non-contact deflection measurements. The high data resolution gives opportunity to obtain detailed output that can be regarded as continuous. With the traditional 100m to 200m distance between stationary measuring points, a limitation of 0.02m distance between output values is equal to a continuous output.
The TSD output is reliable and has reproduced output variations that compares with FWD. The TSD has reproduced output patterns on a test section when measuring at different driving speed level, showing a little speed dependency of the results. This dependency will be subject of future studies.
As in the ARA RWD, the laser sensors are close to the load due to a special wheel hub. This allow an easier comparison with the FWD, in which the deflections are measured close to the load too.
Since this method is non-contact and does not compare two measurements from the same surface point, then the equipment has no problem measuring in curves.
With the 4-laser TSD it is possible to get 3 points from which can be calculated the deflection basin, the fourth sensor is a reference sensor. The 4-laser TSDs provides deflection velocity readings in positions 100mm, 200mm, and 300mm from the center of the load. This configuration gives measuring points concentrated at the center of the load so that the results are reliable only in that distance from the load.
The model is reliable in the proximity of the data points and provides traditional bearing characteristics like SCI300 and D0. With more sensors (up to ten) more advanced response models can be applied and more accurate results obtained.
On the other side, because it has to measure a deflection velocity of the pavement, the minimum driving speed required is around 40 km/h. With lower speeds the Doppler sensors are not able to measure the speed of the pavement deflection. This reduce the use of the TSD in urban areas.
The air condition system in the trailer can maintain the indoor temperature at a constant level that is good for lasers and computers. In addition, no surfaces have created problems, except for very shiny new asphalt surfaces which are known to give problems, as the laser light is not reflected back but is scattered from the surface. Otherwise it is necessary to state that the TSD do not measure on gravel or dirt roads.
The advantages coming from a benefits costs comparison with the FWD are undisputable, in terms of time and costs for the analysis.
Because of the length of the Traffic Speed Deflectograph and the limited widths and curve radii of most rural, municipal roads it will not be easy to perform high-speed tests at small country roads. The bearing capacity is, however, not the main factor in the deterioration of these roads.
Nowadays there are five TSD trailers, delivered all over the world by Greenwood, which are working and testing with a lot of network analysis on the road networks of these countries. The countries which have an own TSD are: South Africa, Poland, of course Denmark and also Italy, where A.N.A.S. is using it, mostly on the highways network.
All these countries provide a lot of information, data and experience, to keep improving this device.
