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2
D
IFFERENT WAYS AND DEVICES TO ANALYZE A ROAD
The foundation of good road analysis is good data. This is the reason why data collection procedures and devices are continuously studied and improved to increase the accuracy of the collected data and the speed during collection. The data collected are divided in two major groups: structural data and functional data. Functional data are used to understand the shallow behave, while structural ones explain the behave of the layers below. These features are not completely divided, because the functional behave is affected by structural one, and vice versa.
We need to be fast, to analyze and collect data on a higher number of road, without losing the accuracy of the data. These requirements become very important mostly when we are approaching a network level analysis of the road.
It´s needful to distinguish between project and network level of road analysis. In a project level analysis the target is just one road or section. This means that the accuracy of the measurements should be very high, and the time is often not a problem. A network level analysis is completely different. In this case the target is to collect data from all the roads of the network, like highways, major suburban roads, all the roads of a city and others. Hence the need of high efficiency devices, which can collect a high number of data in a short time, without losing accuracy.
These network analyses are used mostly during PMS (Pavement Management System), where the main aim is to have a screening of all the roads with the intend of optimizing resources, money and time into more efficient maintenance works and repairs. A more accurate description of PMS follows in a later chapter.
This is valid for both structural and functional devices, because a proper road analysis requires both kind of data.
2.1
Functional equipment
Functional performance is based on visual distress, surface friction, rutting and roughness. These main features of the functional behave of a road are very important because they are perceived by the user driving on a road and affects both safety and vehicle operational costs. Functional features are important too as they may contribute to the structural ones. For example, a high roughness road will have a structural collapse earlier, and vice versa because roughness is due to structural failings.
The differences between structural and functional failure are:
- Structural failure, includes a collapse of the pavement structure or a breakdown of one or more pavement layers to make the pavement incapable of supporting the loads. The main mechanism of that failure is the fatigue cracking of the bounded layers or permanent deformation in the unbound layers due to traffic loads. Its evaluation is performed through deflection studies.
- Functional failure, depending primarily on the severity of surface roughness and rutting. It usually consists in loss of regularity of the road surface or geometric alteration. It may or may not be accompanied by structural failure, but the pavement will not carry out its intended function without causing discomfort to passengers or without causing high stresses to the vehicles or airplanes running over it. A functional failure will also decrease the safety of a pavement, making it dangerous for the vehicles which are moving over it. This geometric alteration can be caused by an accumulation of permanent deformations in the pavement structure layers and in the subgrade, or by problems in the asphalt mix resulting displacement of the aggregate.
As we were saying, these two types of failure can appear as a consequence of each other. The main causes of a road failure, both structural or functional, are:
- Traffic loads, depending on weight, repetition and tire pressure can cause either structural or functional failure.
- Environmental factors, affecting the elastic moduli of the various layers. They may cause surface irregularities and structural weakness (frost lifting, expansion of soil due to wetting and drying, breakup and voids resulting after freezing, improper drainage).
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- Construction mistakes, compaction failure, improper moisture conditions during construction, materials quality, accurate layer thickness.
All the devices for friction data collection are evolving to continuous ones, instead of using the old static methods like BPN (British Pendular Number). These devices work with a wheel, rolling over a thin layer of water to simulate the worst road surface conditions, with different slipping percentages and different inclination of the wheel. The main benefits of these new devices are:
- Increased speed in data collecting, since they don’t need to stop in the test point but just driving on the stretches.
- Increased accuracy, with the BPM the data acquired are valid only on each tested spot while with continuous devices you can collect a value every centimeter. - Closer to a real situation, the rolling wheel is closer to a vehicle and due to the
slipping percentage the data collected are dynamic friction values, like the one between the vehicle’s tire and the pavement. Instead, the BPN provides a static friction value.
While friction devices are evolving to continuous ones, static devices are still used to collect roughness data. This is due to the roughness perception depending on the speed and on the distance between the wheels.
On roads the most used devices are inertial profilers. But on airport pavements things are completely different in terms of speed and wheelbase. This is why static methods and devices for roughness data collecting are still used. With these devices we can acquire bigger wavelength roughness data, which might be dangerous for a landing plane, but not perceived by a user on a car.
2.1.1
Static devices
In this chapter there is a brief overview about static devices to acquire functional data. They have been almost completely replaced by continuous ones, due to increase the speed and the accuracy of the data collected.
British Pendulum Number (BPN):
The British pendulum number is a pavement friction tester. The pendulum uses a standardized piece of rubber fixed on the pendulum foot, which is set up to travel across the testing floor for around 125 mm. The pendulum is placed on the surface and the arm is lifted until the horizontal position, as it’s shown in Figure 2.1. As soon as it’s released, the pendulum foot will move close to the pavement, a spring applies a pressure to the rubber so that it will rubs on the pavement. A pointer records the maximum height of the pendulum on the other side compared to the starting position. When the arm of the pendulum is set up to miss the testing surface completely, the arm swings up to parallel from where it started, and the pointer reads zero. Slippery surfaces produce readings close to zero, while rough ones give results further from zero.
Figure 2.1 – A British Pendulum on a road.
Nowadays the BPN is used mostly inside workshops for sample testing. The reasons why the BPN has been replaced are:
- Too slow, it takes a lot of time to have a reading and for a proper analysis the final number should be the average of at least three values.
- Small validity range, the final value is correct only where the rubber plate has been rubbed. For an accurate analysis the distance between two testing points on a road should be short, and this will increase the time it takes.
- Static friction, the final number is far from a real situation because the rubber is scratching on the pavement instead of rolling.
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The BPN has been replaced by continuous equipment which work using a rolling wheel. These devices can analyse continuously a pavement, with closer conditions to a real situation of a vehicle moving on a road. The final value is a dynamic friction, closer to the one between a vehicle’s tire and the pavement. The accuracy of these new devices is higher, since is possible to test the pavement every centimetre, and the data collection is faster. A more accurate description of this new devices in a following chapter.
Road and level:
It’s a static method to measure the roughness of a pavement. It is still used on airfield do detect roughness with longer wavelength that the inertial laser profilers cannot find. This longer wavelength roughness can be dangerous for airplanes moving on it, because the speed is higher comparing to a car and the distance between the wheels is higher too.
As the BPN, the testing speed is very low and it can detect short wavelength only if the distance between two pleasured points is very short. These are the reasons why it’s used only on airfield.
Dipstick:
The device utilizes a precision inclinometer to measure the difference in height between the two supports and so detect the roughness. The testing speed is very low and it cannot detect long wavelength.
Profilograph:
It is a long beam with wheels at each end, and a test wheel in the center. The road roughness is measured by the vertical movement of the test wheel. It’s a big equipment, hard to move, slow in testing and it cannot work on too narrow or wide pavement.
2.1.2
Multi-Functional Vehicle
The Multi-Functional Vehicle usually a van with two devices working on it. The MFV combines the functionality of the RSP (Road Surface Profilometer) with the Laser Crack Measurement System (LCMS). It´s the ideal tool for both airport and highway testing. Can be used to measure the IRI, longitudinal and transverse profile, macrotexture, cracks and other surface distresses and geometrics.
The MFV brings safety to the forefront, allowing surveys of roads and airports to be performed from a vehicle at normal traffic speeds up to 100 km/h, day or night, precluding the need for traffic management. Both the LCMS and the RSP can be mounted separately on different kind of vehicles, the MFV simply merges the benefits coming from the two tools.
Figure 2.2 – A Multi-Functional Vehicle testing on a road.
Main benefits:
- Rapid evaluation and screening of all sections of the network
- The crack detection and crack classification procedures allow for rapid, objective and accurate identification of cracks
- The 3D capability of the LCMS allows for the rating of raveling
- Surveys a 4m pavement width, producing a continuous image of the pavement surface in a single run
- Detects cracks as narrow as 1mm
- Allows also manual distress analysis of the pavement surface to be performed quickly and efficiently
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2.1.2.1 Road Surface Profilometer
The RSP is an inertial profiler, composed of a laser which measures the relative distance between the road surface and a computed inertial reference, an odometer for the longitudinal distance and an accelerometer to take into account vehicle vertical accelerations. The RSP measures characteristics including the longitudinal profile, the International Roughness Index (IRI), the transverse profile and so the rutting depth. It also has special lasers just in front of the wheels which can work with high frequency to measure the macrotexture of the road surface on the wheels´ path, where the friction is very important.
Figure 2.3 – The RSP bar supporting the lasers fixed in front of a van
Figure 2.3 shows the RSP laser sensors in front of a vehicle. The following Figure 2.4 contains all the different profiles which a RSP can provide.
Figure 2.4 – An illustration of all the profiles that a RSP can provide.
Rut depth is the distance between the bottom of the rut and the original road surface, before the damage. They are produced by traffic loads, and so placed in the wheel path. Depending on the wavelength is possible to identify which is the faulty layer, since a larger rut means a deep faulty layer.
The IRI represents the absolute sum of the relative vertical displacement experienced by the user when driving a fictitious model car over a section of the road at a constant speed of 80 km/h. The commonly recommended units are meters per kilometer [m/km] or millimeters per meter [mm/m]. A perfectly smooth road results in an IRI value of 0 (equivalent to driving on a plate of glass). Roads with severe roughness give IRI values of around 6 meters per kilometer (m/km), and in extreme cases a very bumpy unpaved road can result in IRI values up to 20 m/km. Each country has different threshold for starting maintenance repairs. The following Figure 2.5 shows the most common IRI ranges represented by different classes of road.
Figure 2.5 – IRI reference values, depending on speed and kind of pavement.
Once the RSP has acquired the longitudinal profile of the road surface, a quarter car simulator runs over the profile with a constant speed of 80 km/h. The quarter car simulator consists of: a wheel, the suspension system and a quarter of the vehicle mass. A simple scheme in Figure 2.6.
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Figure 2.6 – Schematic drawing of a quarter car simulator.
Where:
m1 is the mass of a quarter of the vehicle
m2 is the mass of wheel and suspension
k1 and c1 are the spring and dumper constants for the suspension
k2 and c2 are the spring and dumper constants for wheel and tire
While the simulator is running over the road profile the information about the vertical displacement experienced by the car´s frame, and so by the driver, are recorded. From this vertical displacement the IRI index is derived.
2.1.2.2 Laser Crack Measurement System (LCMS)
The LCMS is able to automatically measure, detect and quantify all the functional parameters of a pavement in a single pass, including cracking, rutting, potholes, shoving, raveling and roughness, macro-texture.
The LCMS acquires high resolution 3D profiles and 2D images of the pavement surface using two laser profiles and high-speed cameras, laser illumination projectors, and advanced optics receivers. The two lasers placed in the back of a van scan the pavement with high-resolution cameras which record pictures of the road. It is capable of performing a complete pavement condition inspection across 4 m width pavement lane at speeds up to 100 km/h
during day or night, with a transversal accuracy of 1 mm, a transversal resolution of 4160 points/profile and a depth accuracy/resolution from 0,25 to 1 mm. The LCMS allows for automatic detection and measurement of cracks, rutting, potholes, raveling, sealed cracks, joints in concrete and macro-texture measurements over the whole lane width.
Figure 2.7 – The LCMS cameras system sixed in the rear of a van.
Figure 2.8 – Illustration of the LCMS working principle.
The LCMS automatically rates the distresses, and its work can be checked and verified by software and an operator from an office.
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2.1.3
Friction tester
Pavement friction is the force that resists the relative motion between a tire and a pavement surface. This resistive force is generated as the tire rolls or slides over the pavement
surface.
Graph 2.1 – Coefficient of Friction depending on the tire slip percentage.
Friction is the result of a complex interplay between two principal forces, adhesion and hysteresis.
Adhesion is a force on the interface between the tire and the pavement, it depends mostly on micro-texture of the particles in the aggregate of the surface layer.
Adhesion is the bigger part of friction on smooth and dry pavements and low speed.
Hysteresis force depends mostly to the macro-texture of the surface layer. It´s the resulting of the contact forces coming from the rubber of the wheel deforming on over the aggregates. Hysteresis is the dominant component on wet and rough pavements, and high speed.
Usually we consider micro-texture a wavelength < 0,5 mm, while macro-texture 0,5 < wavelength < 50 mm.
Micro-texture depends on the kind of rocks used in the aggregate, on the percentage of crushed rocks and on the method for crushing the rocks.
Macrotexture can be measured with static methods, like the “sand height”, or with continuous ones, like the MPD (Mean Profile Depth). The MPD is a high frequency laser scanner which can measure the average of the profile depth of the macro-texture. Can be fixed on a van or on a MFV, like previously written, and it measures the macro-texture while the van is moving at normal driving speed.
To measure the friction there are a lot of different methods and instruments. Since in this thesis we´re talking about continuous methods to test roads to evaluate structural and functional features, in this chapter we´re going to talk only about continuous methods of friction testing.
Actually there are three big groups of devices: - Locked wheel devices,
They provide a skid number (SN). - Oblique wheel devices
They provide a slide force coefficient (SFC), like a transversal friction coefficient, in addition to the longitudinal one.
- Fixed tire slip devices,
They provide a brake slip number (BSN)
They can be trailers towed by a vehicle or trucks (or cars) with an additional special wheel in the frame used for the measurements.
Each of these devices have a water spray system to simulate the worst condition of the road, talking about friction, with a 0.5 mm thick water layer. The way they work is the same for all the three groups, the load of the trailer (or of the vehicle) push down the testing wheel on the pavement, over the water layer. The testing wheel can be flat or grooved, straight or oblique and with different percentages of slip. A force transducer measure instantaneously the force applied by the pavement on the wheel, so the program calculates the friction.
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Figure 2.10 – A friction tester on a van.
Many devices and methods have been developed around the world to measure the friction and texture of paved road surfaces. The International Experiment by PIARC was conducted for the purpose of comparing and harmonizing the test results obtained from various testing devices. As a result, the International Friction Index (IFI) was developed. The IFI consists of a Friction Number (F60) and a Speed Constant (Sp) and is reported as IFI (F60, Sp). Once the IFI is obtained, it is possible to calculate Coefficient of Friction F(s) at any speed. The Friction Number F60 indicates friction at a slip speed of 60km/h. There are several methods to measure friction, one is to directly measure the coefficient of friction between tires and road surfaces. On the other hand, the Speed Number (Sp) indicates the speed dependency which is calculated from the macrotexture of the pavement.
2.2
Structural equipment
For a proper road analysis we need also structural data, in addition to functional ones. This means that we need to know exactly all the layers under the surface, their thickness and moduli. With structural data we can obtain information about residual life of the pavement, the weakest layer, so the one that is going to fail for first, and determines the optimum rehabilitation alternatives for that specific pavement.
Only in this way a complete analysis of the road can be made.
Figure 2.11 – Schematic representation of different pavement failures.
Figure 2.11 shows three different layers failure. As we can see, if we only look at the surface these three damages show the same problem. Only with data analysis we can understand where the problem is coming from and consequently solve the problem. Otherwise we may waste a lot of money with useless maintenance works without knowing the real problem. Nowadays, with lesser road to build, the importance of road maintenance is growing more and more. All the bigger road administrations are implementing PMS (Pavement Management Systems) to optimize pavement maintenance and rehabilitation, to help funding allocation and to minimize pavement life-cycle costs.
We´ve learned from PMS that is better to spend some money for them and later save money with correct maintenance works, instead of saving money on PMS but spend a huge amount of money in expensive maintenance.
Anyway we are going to talk about PMS also later, since this is one of the main purposes of the RWD. PMS needs both functional and structural data, and so the necessity to collect data faster and faster.
From this needing new devices have been developed, and they are called “high efficiency” devices. The main aim of these new tools is a network level analysis, using nondestructive methods and without having to close a road causing a lot of problem to the drivers. In addition, data collection will be cheaper and the accuracy increased.
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There are three different kind of devices for measuring the bearing capacity of roads: - Almost static load devices, like Benkelmann´s deflectometer
- Sinusoidal load devices, like Road Rater - Impulsive load devices, like FWD
The main benefit of this last kind of devices is their ability in reproducing the loads coming from a heavy vehicle in terms of load value and time of application. This is the reason why they are the most used devices for collecting structural data, so we will talk about those in this thesis.
2.2.1
The Falling Weight Deflectometer (FWD)
The Falling Weight Deflectometer (FWD) is a nondestructive structural testing device. It applies a dynamic load that simulates the loading of a moving wheel. It has a wide loading range, from 4 up to 120 kN, suitable for all kinds of paved and unpaved roads, and also for airport surfaces.
The FWD is available as a trailer or a truck mounted version (USA only). Using a FWD a single operator can record data about many test points, up to 60 per hour.
Figure 2.12 – A Falling Weight Deflectometer trailer.
The load is produced by dropping a large weight onto a set of rubber buffers on a bracket connected to a circular load plate. The load plate is divided into four segments to
accommodate uneven or rutted pavement surfaces. A hydraulic system lifts the weight to different heights, according to the load in the settings of the drops.
Figure 2.13 – The hydraulic system used to lift the weights on a FWD.
A load cell mounted on top of the plate measures the imparted load. Deflection sensors (geophones) mounted radially from the center of the load plate measure the deformation of the pavement in response to the load, also called deflection basin. The accuracy of the geophones is ±2 µm.
In the Dynatest’s FWDs the minimal configuration uses 7 geophones, but the FWD can accommodate up to 15 sensors.
The load cell has a resolution of 0.1 kPa, and the geophones have a resolution of 0.1μm.
Figure 2.14 – Scheme of the typical Dynatest FWD geophones configuration.
Figure 2.14 is an example of a typical Dynatest configuration with 9 geophones, but it can be changed based on the kind of pavement and the layer thicknesses.
Usually the geophones are fixed on a single beam, but it also allows different geophones configurations for various kinds of pavement.
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Figure 2.15 and 2.16 – Geophones configuration used to test a longitudinal joint, scheme and picture.
For example, this different configuration shown in figure 2.15 and 2.16 can be used to test the longitudinal joint between two slabs in a concrete pavement. We can use the deflections before and after the joint to calculate the Load Transfer Efficiency index, to have information about how the joint is working.
Other configuration, like the one in the following Figure 2.17, can be used to calculate the Load Transfer Efficiency index of a joint also in the corner.
The FWD data can also be used to detect voids under slabs in rigid pavements.
The standard equipment of a FWD trailer includes also a DMI (Distance Measuring Instrument) and air and surface temperature sensors.
The DMI is used to record the distance traveled and show it to the operator, in this way he can stop exactly in the right spot to have a measure.
Middepth asphalt temperature is very important for calculating the E moduli of layers at reference temperature, especially for asphalt concrete layers which are very temperature sensitive. During a testing day the temperature of these layers can change up to ±20˚C, causing a variation in the E moduli up to ±3500 MPa. The middepth asphalt temperature has to be recorded manually.
There are also other upgrade tools available. A GPS for tracking the data, additional sensors (up to 15), Camera system for plate location or photo-logging, on board generator for standalone operation, trailer mounted lights or strobes, GPR system for layer thickness.
Data analysis:
The pavement response is analyzed with Dynatest's ELMOD (Evaluation of Layer Moduli and Overlay Design) software to determine the deflection basin, and later to calculate elastic moduli, stresses and strains of each modeled layer.
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Starting from the deflection basin there are four important parameters to evaluate:
- The deflection under the load plate, representing the total deflection of the entire set of layers.
- The slope of the basin or the difference between deflections close to the load plate, which gives you information about the top layers.
- The slope of the basin or the difference between deflections in the middle part of the basin, for the stiffness of the subbase of a road or however the layer under the surface. - The deflections at the end of the basin, related to the subgrade stiffness.
Back-analysis:
During the back-analysis process the collected data about the deflection basin are elaborated to find the E moduli of the different layers of the pavement.
The process is made by four steps: 1) Inserting thickness of the layers.
GPR tests or cores extraction are used to find the layer thickness. Seed moduli can be added as input or estimated automatically using a simple model.
2) Calculation of the deflection basin, using the input values. As standard a Boussinesq-Odemark model is used supplied with a non-linear subgrade, but linear elastic models are also available.
3) Comparison between calculated and measured deflections.
4) Adjustment of moduli values, until the difference between the two different deflection basins is under a predetermined threshold.
From point 2) to 4) is an iterative process.
ELMOD reports the weakest layer of failure, residual life and determines the required overlay to comply with the design life.
Some of the challenges of this back-analysis are the following: - Input values for E moduli, and variation range of these values.
- Layers thickness, the relative accuracy of thin layers is very low and adjacent layers with same level of moduli can be difficult to backcalulate independently.
An error of 15% in the measure of the layers thickness may cause a 50% error in the E moduli calculation.
- Temperature sensitive behavior of asphalt concrete layers. ±20°C may cause a ±3500 MPa error.
- Deflection matching checks. Results are different between minimizing the absolute or the percentage error.
- Non-linearity of some layers, especially the subgrade with a stress-softening behavior. This problem can be handled with non-linear subgrade model in combination with the Boussinesq-Odemark procedure.
In addition, the final result isn´t unique but there may be different combination of E moduli for the same deflection basin.
Despite these problems, nowadays the FWD is the most widely accepted and consolidated structural testing method on the market. It´s the reference device used for all the comparisons and the validations of the other testing devices (AASHTO R32-11 calibration protocol compliant and Passes TRL “Transport Research Laboratory” correlation trials).
Here is a summary of all his main features and benefits: - Nondestructive testing device
- Wide loading range (4-120 kN) to test on every kind of pavement - Economical and needs only one operator
- Compact and easy to move on all sites, also small ones - Excellent repeatability and stability
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2.2.1.1 The Fast Falling Weight Deflectometer (FFWD)
The Fast Falling Weight Deflectometer was developed to solve the main issue of the previous FWD, the slowness. Stopping the towing vehicle and the FWD trailer every time to collect the measure cost a lot of time. So, instead of creating a completely new continuous device to solve the problem, like the rolling weight deflectometer, they tried to make a standard FWD as fast as possible.
This means that the trailer and the way it works is exactly the same, imparting a load pulse to the pavement surface using a load plate and weights dropped from a certain height. The range of loads is the same (4-120 kN). Geophones will record the deflection basin.
Figure 2.19 – Picture of a Fast Falling Weight Deflectometer.
But the FastFWD is 5 times faster per drop than the FWD. Therefore the number of testing points per hour is increased up to 160. It isn´t five times the test points number of the FWD because the moving time is the same, but it´s definitely faster.
This is possible thanks to a completely new way to lift the weights. It consists in a single 3-phase torque motor and ball screw assembly drives the new system, replacing the hydraulic system in the old FWD.
Another advantage of this new system, the latest FWD trailers can be upgraded to the Fast version simply changing the weight lifter. In addition, replacing the old hydraulic system means also less maintenance costs and stopping time due to repairs. In the following Figure 2.20 there is the Fast FWD, as we can see from the weight lifting system, used for the comparison tests with the Dynatest RAPTOR.
Figure 2.20 – A FWD connected to a van while testing.
Due to this easy-upload feature from a normal FWD to a fast one simply changing the weight lifting system, nowadays the bigger part of the Dynatest´s FWD have been upgraded, to increase the benefits coming from this device.
2.2.2
Heavy Weight Deflectometer (HWD)
The original commercial developer of the Falling Weight Deflectometer (FWD) technology and the world’s largest supplier of FWD deflection based equipment is Dynatest. They had designed the Heavy Weight Deflectometer (HWD) with much higher loading capability to meet the needs of airports and pavement agencies with extra thick, stiff pavement structures.
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Similar to the FWD, the HWD is a nondestructive structural testing device, designed to impart a load pulse to the pavement surface simulating the load produced by a rolling vehicle wheel.
The load is produced by dropping a large weight on a set of rubber buffers on a bracket connected to a circular load plate. The load plate is divided into four segments, to accommodate paved and unpaved roads.
A load cell mounted on top of the plate measures the imparted load. Geophones mounted radially in and from the center of the load plate measure the deformation of the pavement in response to the load.
As the FWD, all the instruments are mounted on a trailer that make it easy to be carried and used to test also in tiny sites. Using a HWD a single operator can record data about many test points, up to 60 per hour.
Figure 2.22 – A HWD while testing in an airport.
Dynatest HWD can easily simulate and measure the load levels and response of large aircraft such as the Boeing 747 or 777 and the Airbus A380. The HWD can produce higher and wider range of load levels suitable to both the highway and airfield applications, from 30 to 320 kN.
Usually the geophones are fixed on a single beam in the direction of the towing vehicle but, as the FWD, it also allows different geophones configurations for different kind of pavement. Since the HWD is mostly used to test airport pavements, one of the most useful upgrade is the rear or the rear and transverse extension bars, to allow different configurations for the geophones. Configurations like the one described allow you to test the longitudinal joint between two slabs in a concrete pavement, and also close to the corner. Suddenly the
deflections before and after the joint can be used to calculate the Load Transfer Efficiency index, to have information about how the joint is working, in the area close to the corner or in the middle of the slab. It can be used also to detect voids under a concrete pavement.
Figure 2.23 – A HWD while testing in an airport.
The data collected have an excellent accuracy, repeatability and reproducibility. The HWD is AASHTO R32-11 calibration protocol compliant and it has passed TRL correlation trials.
The standard equipment also includes:
- Air and pavement temperature sensors, as in the FWD these data are very important for the back analysis.
- Distance measuring instrument (DMI), to record the distance traveled and show it to the operator, in this way he can stop exactly in the right spot to have a measure. Other upgrades are available, like a GPS, additional geophones camera system for plate location, on board generator, lights or strobes, additional beam to support more geophones (also transversal ones) and a GPR system for layer thickness.
Data analysis:
Just as in the FWD the pavement response is analyzed with Dynatest's ELMOD software to determine the deflection basin, and later to calculate elastic moduli, stresses and strains of each modeled layer. For a more accurate description of the deflection basin parameters and back analysis refer to chapter 2.2.1. The results coming from this process can effectively be used for the evaluation of pavement structural condition and residual life of the pavement.
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For maintenance and rehabilitation the software allows the user to select the optimum solution for a pavement section according to cost/benefit ratios and required overlay thickness for a given service life.
Since the FWD is mostly used to test airfield pavements, the optional PCN module in Elmod calculates PCN in accordance with the ACN/PCN methods, as described in the ICAO and FAA design manuals.
2.2.3
Light Weight Deflectometer (LWD)
The Light Weight Deflectometer (LWD) is a portable dynamic plate loading device designed for compaction quality assurance and determination of the modulus of unbound or partially bound material. The LWD is a portable version of the Falling Weight Deflectometer (FWD). The LWD uses a load cell and geophones with the same accuracy as the FWD.
A weight of 10 kg is lifted following a metal stick as a guide, and suddenly released. The falling weight hit a buffer over the loading plate and it cause an impulsive load to the layer. A load cell over the load plate will measure the load, while a geophone inside the plate will measure the deflection.
Figure 2.24 and 2.25 – A Light Weight Deflectometer and a detail of the plate with the buffers and the weight.
There are different weights, 10, 15 and 20 kg which can be lifted for different heights along the guide-stick. The available diameters for the load plate are: 100, 150, 200 and 300 mm. whith all the combinations of different height, weight and diameter this device can produce a load up to 15 kN
Because of this small load the FWD can be used to test thin asphalt layers, recycled materials bound with foamed bitumen, unbound granular aggregate base, partially stabilized base layers, unbound subbase and subgrade soil. The LWD is suitable for testing in difficult to access areas, e.g. trenches, narrow channels, etc.
It’s designed to determine the modulus of up to two layers. Two additional geophones can be mounted.
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2.2.4
Ground Penetrating Radar (GPR)
The ground penetrating radar is a nondestructive, high efficiency device to collect information about the layer thickness of the pavement.
It´s a geophysical method that uses radar pulses and changes in material properties to image the subsurface and detects the reflected signals from subsurface structures and layers.
The GPR is a supplementary device which can easily be installed on almost every van or other measuring devices. It´s made only by three components, a radar pulse emitter, a receiver and a support.
Figure 2.27 – A GPR antenna mounted on the rear of a van.
A radar signal goes from the emitter to the road, while a receiver collects the reflected signal. The signal is reflected on the surface and every time it meets an interface between two different layers, due to the different properties of the two materials. Analyzing the amplitude and the arriving times is possible to calculate the different layer thicknesses.
Figure 2.28 – Scheme of the operating principle of the GPR.
The GPR is a tool that complete the data collection coming from other analysis devices. It´s used together with the FWD data, to insert the layer thickness into ELMOD software, to complete the back analysis of the pavement and to obtain the E moduli of the layers.
The GPR is a continuous and high efficiency device, but it needs some calibration. Unfortunately the calibration can be made only with core extraction or other destructive methods. This makes the GPR data collection little bit slow, but is not the only issue with this device.
The data analysis requires a lot of times to check if the program is identifying correctly the interfaces between layers. In addition, sometime is hard to find an interface between layers with similar properties, and is quite impossible to distinguish very thin layers. This is the reason why it cannot be used to evaluate only asphalt concrete layers.
Despite to these problems, the GPR is the higher efficiency device on the market, compared to other tools like boroscope, Cone Penetration Test or core extraction, and so the most used.
Except for the GPR, we have continuous devices to collect functional data but not any continuous devices for structural data. This is the reason why we need a new device like the Rolling Weight Deflectometer and why Dynatest is investing money and time in research for developing this new device which can really improve the future of the road testing.
