The present invention relates to a method for pavement testing, in particular accelerated pavement testing.

There has in the past three decades or so been an increased interest in Accelerated Pavement Testing (APT) that is to say an accelerated, generally destructive testing of pavements, such as roads or airfield runways. In such tests the pavement is repeatedly subjected to loads, thus simulating in a shortened time the normal use over a longer time span.

One known device for this is the widely used mobile device Heavy Vehicle Simulator (HVS), which was developed by the Council for Scientific and Industrial Research in Pretoria, South Africa starting in the late 1970's for which the most recent model, the Mark VI owned and operated by the Univer- sity of California, Davis.

Full-scale APT involves the continuous application of a moving wheel load to either a specially prepared pavement structure or to an in-service pavement, with the objective of achieving a predefined condition designated as "failure". Failure is typically defined as an unacceptable level of rutting or cracking on the pavement surface.

There are several advantages of using a light weight mobile device vs. the alternatives such as the permanent, circular track at CEDEX in Spain . A light weight mobile device can be operated 24/7, whereby fixed tracks may require the use of trucks to traffic the pavement surface, which in turn implies the costly employment of truck drivers. Furthermore, a light weight mobile device can more easily be moved around and deployed when testing in- service pavements, compared to other test devices which require the use of heavy duty machinery to transport and a more extensive amount of work to deploy once at the test site. The facility and staff requirements are much less for a light weight mobile device as compared to the fixed-track option.

Still, however, these prior art devices are quite big and bulky, and not as easily transported as could be desired. Accordingly it is the object of the present invention to provide a method for accelerated pavement testing, which uses smaller and more easily transported testing devices and less personnel.

According to the present invention this object is achieved by a method for accelerated pavement testing comprising the steps of placing a light weight mobile test device at a predetermined section of pavement to be tested, repeatedly subjecting said predetermined section of pavement to an impact creating an impact force using the test device, and inspecting after a predetermined number of impacts the pavement for signs of degradation. Using impacts allows the testing to be carried out very localized compared to prior art test methods, where space was required for the reciprocating movement of a load along the pavement to be tested. Moreover, using impacts e.g. from lifting and dropping a weight may be performed at a much higher repeti- tion rate than the reciprocating movement of a load along the pavement. This accelerated testing also requires very little personnel, much of it may even be done unattended. The step of inspecting the pavement can include a simple visual inspection to check the test surface for visual signs of failure or imminent failure. Other non-visual parameters when inspecting for degradation can be layer moduli, stresses, and strains. These can be tested by various methods such as permanent deflection of the pavement after a series of impacts, e.g. by using a laser to measure deflection distance, or measuring the condition of the upper and lower pavement, e.g. by using geophones to measure impact propagation in the pavement.

It should be noted, that "light weight mobile" in this context is considered a test device of less than five metric tons, excluding the means for transportation, which requires little or no fastening to the test surface. Such a test device will have the benefit of being easily transportable, e.g. by trailer or airplane, while also being quickly operational once at the test site.

According to a preferred embodiment, the test device is a falling weight deflectometer adapted to repeatedly lifting and dropping a weight. Such devices are much smaller than the prior art test rigs for reciprocating movements along the pavement, and may be provided as a self-contained unit on a single-axle trailer towable by cars the size of an ordinary passenger car, e.g. of a total weight of less than 3,500 kg, and hence by holders of a normal driver's license.

Alternatively to dropping the weight the falling weight deflectometer may be adapted to repeatedly lifting a weight and accelerating said weight towards the ground. By employing an accelerated rather than a free fall drop of the weight, the falling weight deflectometer can perform a full impact cycle even faster and produce a greater range of impact forces.

According to a further preferred embodiment, the weight is lifted using an electric motor in direct engagement with said weight. This lifting facility available in recent developments of falling weight deflectometers, cf. WO2015/051798 incorporated herein by reference, allows a short cycle time of lifts and drops.

According to a preferred embodiment, the typicall cycle time is less than 5 seconds, and preferably less than 2 seconds. Such short cycle times, in turn, allows the accelerated pavement testing to be performed in a very short time as compared to the prior art. As an example 200,000 cycles with a cycle time of 5 seconds could be performed in less than a fortnight, and if as experiments indicate, the cycle time can be reduced to 2 seconds, the test could be performed in less than one week.

According to a another preferred embodiment the inspection comprises a deformation depth measurement of the predetermined section with respect to the surrounding pavement after said predetermined number of im- pacts. Thus, at suitable intervals the progress of a destructive testing may be inspected, and the test possibly terminated if damage indicates that further testing with further impacts is unnecessary, thus shortening the test time. Additionally or alternatively, the inspection may comprise visual inspection for cracks and/or fractures. Again the test could be terminated, but also other actions could be taken, e.g. the testing could be paused for a period, e.g. in order to determine whether the pavement is capable of recovering.

According to yet a further preferred embodiment, pavement deflec- tion in response to the impact force is measured at the predetermined section of pavement to be tested and/or one or more distances from the predetermined section of pavement to be tested. This is advantageous as it allows the continuous monitoring of changes of the pavement during the accelerated pavement testing, thus in addition to the visual inspection at predetermined intervals, the response of the pavement to the impacts may be followed. This, in turn, gives knowledge of how the pavement changes, e.g. up to the failure, when the accelerated pavement testing is performed as a destructive pavement testing.

According to a further embodiment, the pavement deflection is measured using geophones placed on the pavement at said one or more distances from the pavement section to be tested. Using such geophones is facilitated by the fact these are normal as a part of a falling weight deflectome- ter setup, and often form an integral part of such falling weight deflectome- ters, e.g. mounted on a beam that may be lowered onto the pavement in a radial direction from the predetermined sector of pavement to be tested, i.e. from the centre of the load plate of the of the falling weight deflectometer.

According to yet a further preferred embodiment, the impact force may exceed 30 kN, and can exceed 100 kN. This suffices for the impacts necessary for the testing, but the skilled person will know that evidently other loads may be used. Furthermore, loads may be varied during the testing.

According to another embodiment, said deformation depth measurements and/or said visual are performed after one or more predetermined numbers of impacts. Inspecting the depth provides knowledge about how the failure of the pavement in the destructive testing progresses, and allows for early termination of the test as explained above.

Preferably, said one or more predetermined numbers of impacts exceeds a number selected from the group consisting of 100, 1 ,000, 2,000, 5,000, 10,000, 20,000, 50,000, 100,000 and/or 200,000. In particular the higher numbers among the above, allow a realistic simulation of the influence of road traffic on a paved road.

According to an embodiment of the invention the steps of repeatedly subjecting said predetermined section of pavement to an impact creating an impact force, and inspecting after a predetermined number of impacts the pavement for signs of degradation is repeated until failure of said predetermined section of pavement. This embodiment is particularly favourable as it was considered impossible to perform using traditional testing methods as these required unreasonable time-spans to complete. By running the APT to failure, it is possible to simulate the performance of the pavement for its entire lifespan, as well as projecting said life span from the expected usage of the pavement.

According to another embodiment, the method further comprises the step of measuring the ambient temperature and/or temperature of said predetermined section of pavement. As the properties of the pavement may vary depending on its temperature, it might be desirable to record temperature data for the data processing following the accelerated pavement testing. Tem- perature measurements may be taken either continuously or at discrete times during the accelerated pavement testing.

The present invention will now be described in greater detail based on preferred embodiments and tests performed and with reference to the drawings on which:

Fig. 1 a and b shows a falling weight deflectometer suitable for use in the method according to the present invention,

Figs. 2a - 2f schematically show the setting up of the falling weight deflectometer of fig. 1 for the test method according to the present invention, and

Fig. 3 shows the drop time cycle of the drop weight 13 of the falling weight deflectometer of fig. 1 for the test method according to the present invention.

Turning first to Fig. 1 a and b a falling weight deflectometer 1 corresponding to the disclosure of WO2015/051798 mounted on a trailer 2 for form a self contained testing unit 3. The trailer 2 comprises standard features of a trailer, such as a frame 4 with attachment means 5 at the leading end for attachment to a towing hook on a towing vehicle and a bumper 6 with lights, indicators, reflectors, license plate etc, to make the trailer 2 street legal. The trailer 2 is a single-axle trailer with a pair of road wheels 7 and a nose wheel 8 of the swivel type which may be lowered for support of the trailer 2 when the trailer 2 is not attached to a towing vehicle. On the frame of the trailer 2 the lifting and dropping mechanism of the falling weight deflectometer 1 is mounted. The lifting and dropping mechanism comprises support columns 9, a spindle 10 driven by a direct drive electric motor 12 to lift and drop a weight 13. The electric motor 12 and other electrical equipment, such as measuring devices and computers, can be powered by an electric generator (not shown) mounted on the frame 4 of the trailer 2 in order to make the test unit 1 a self- contained autonomous unit. As shown in fig. 1 b, the falling weight deflectometer 1 furthermore comprises a beam 15 on which a number of sensors 16 such as geophones or accelerometers are located. The beam 15 may be lowered from the frame 4 of the trailer 2 to bring the sensors 16 closer or into contact with the pavement 17 in radial alignment at predetermined distances from the centre of the sector of pavement to be tested, i.e. from the centre of the load plate 18 when lowered onto the pavement 17 as illustrated in Figs. 2c-2f.

The above device is already commercially available for pavement de- flection testing as described in WO2015/051798, and the invention does not reside in the apparatus, but in the realisation by the inventors that with the short cycle times of less than 5 seconds for a lift and drop cycle, an entirely different testing may be performed, namely accelerated pavement testing. Where the above pavement deflection testing is a non-destructive test of the current condition of a pavement, the accelerated pavement testing is a long- term test of a pavement in principle until failure or until conditions defined as failure occur. Such accelerated pavement testing has in the prior art been performed using large and bulky equipment allowing only slow cycles of horizontally reciprocating loads on the pavement to be tested. Accordingly, part of the invention also resides in the switch from the use of reciprocating horizontal loads on a stretch of pavement to be tested to an entirely new method using repeated pounding of a section to be tested using vertical drops of a weight in rapid succession.

The skilled person will understand that such testing may be made in situ on an existing pavement or on a specific test pavement. The following description will be given based on an in situ test.

For the test the test unit is towed to a designated test site behind a towing vehicle. Because of the relatively small size of the trailer 2 comprised in the test unit 1 , the towing vehicle need only be a passenger car, a van or a small lorry, in particular with a total weight of less than 3,500 kg, where the driver needs only an ordinary driver's licence.

Arriving at the test site, the test unit may be detached from the towing vehicle, the nose wheel 8 lowered, and the test unit and made to rest on the road wheels 7 and the nose wheel 8, as illustrated in Fig. 2a.

At the test site the falling weight deflectometer 1 with the load plate

18 is lowered onto the surface of the pavement to be tested, the load plate 18 thereby defining a test sector of the pavement to be tested, as illustrated in

Figs 2b to 2c.

Having set up the test device 1 this way which can be done in a matter of minutes if one neglects the transportation time to the test site, the electric motor 12 may be energized to turn the spindle 10. The spindle 10 is threaded and in permanent engagement with the weight 13 allowing the weight 13 to act as a wandering nut on the spindle 10 so as to be lifted when the electric motor 12 is energized, and dropped when the electric motor 12 is not energized. If a real free fall condition is to be achieved, the electric motor 12 should evidently be somewhat energized in the moving direction so as not to be rotated by the energy from the falling weight. Alternatively, the electric motor 12 may accelerate the spindle to produce an accelerated drop.

However, for the accelerated pavement testing the drop conditions are not necessarily important, as the emphasis is on the repeated number of impacts, rather than on well defined impacts as would be the case in deflec- tion measurements.

After the drop, the accumulated kinetic energy of the weight 13 is transferred via a loading transfer plate to the load plate 18 from where it can propagate into the pavement 17. Preferably, the impact between the weight 13 and the loading transfer plate is cushioned by a number of elastic buffers 19 typically made of rubber in order to shape the impact force. In the embodiment shown in fig. 2a-f, the elastic buffers 19 are mounted on the bottom of the drop weight 8, but the skilled person will realize that the elastic buffers could also be mounted on top of the impact plate.

The full cycle of the weight 13 is shown in figure 3. A weight 13 lifted from its resting position h ₀ to the predetermined height hi . From there the weight falls in a virtually free fall, and impacts the loading transfer plate at the impact time t,. After the impact the weight 13 bounces of the elastic buffers 19. The motor is then re-engaged at time ti to catch the weight and lift it back to hi . When the weight reaches hi at time t _n the drop can be repeated. It is the short time between impact, which is less than 5 seconds and possibly even less than 2 or 1 second, which allows for thousands of impacts to be done in a viable amount of time and thereby enables APT.

The effect of the impact on the pavement can then be inspected during and after each impact. This inspection may include several different tests such as deflection, deformation, and visible indications of failure of the surface and substructure of the pavement. These tests may be performed using one or more different sensors, such as lasers, geophones, and cameras in order to provide additional information on non-visible properties and responses of the pavement.

The non-visible properties and responses, e.g. layer moduli, stresses, and strains, are of interest as they provide insight into the behaviour of the change in the pavement with accumulated load applications both during its lifespan and prior to failure. This can in turn be applied to measure other pavements of similar composition using less destructive methods. These measurements may also be used to determine whether a failure has occurred or is about to occur. This, in turn, allows the computer control and measure- ment unit of the testing device to summon a person to perform visual inspection, if it is determined that failure has occurred or is about to occur much earlier than expected. Inspection need not be confined to the testing site defined by the loading plate 18. Depending on the impact force, the pavement within several meters of the impact location will be affected. Therefore, the inspection is ideally performed not only at the localized test site but also at one or more in- spection sites a distance from the predetermined section of pavement.

It should be noted that the pavement does not have to be inspected after every impact cycle of the APT, though this is possible. In fact, as the number of impacts required to perform a simulation of the total lifespan of a pavement is often in the number of hundreds of thousands, the predeter- mined number of impacts between each inspection may be in the order of hundreds or even thousands and still produce sufficient data. The step of inspecting the pavement may be more frequent during stages of the test, where the rate of permanent damage is expected to be higher.

It should be further noted that the impact force can vary during a full scale APT, i.e. the impact force can be changed after each or a number of repeated impacts. Such a variation can be accomplished by changing the weight, the lift height or the drop acceleration. Evidently, the data from the non-visual measurements may be used to determine suitable changes in the impact force and the suitable time for such changes.

Both the weight and the number of impacts between inspections may evidently also be changed during the course of the APT in response to the results of a visual inspection. This may be required if the tested pavement is more durable or fragile than an initial estimate, as indicated above.

Needless to say the method for accelerated pavement test according to the invention may also be carried out without the falling weight deflectome- ter being integrated in a unit comprising a trailer or a self-propelled vehicle, but it will then obviously be less easy to set up the test.

Although the method for accelerated pavement testing has herein been described through use of a known falling weight deflectometer, it should be understood that alternative test devices which could carry out the method of the invention could also be conceived. In particular it is envisaged that falling weight deflectometers will improve in the future towards even faster and/or heavier lifting, and such faster and/or heavier falling weight deflecto- meters will evidently also be suitable for the new and inventive accelerated pavement testing method of the present invention

While being intended for pavement, the described method may also be used to test the strength and/or durability of un-paved surfaces and surfaces with weaker surfaces than concrete or asphalt, such as gravel roads or floors subjected to high loads, e.g. factory floors.