CEDR Transnational Road Research ProgrammeCall 2013: Aging Infrastructure Management

funded by Denmark, Germany, Ireland, Netherlands, UK and Slovenia

HiSPEQ: GUIDANCE FOR ROAD ADMINISTRATIONS FOR SPECIFYING

NETWORK SURVEYS

Version ControlDate Version

NumberDescription Authors

28/06/16 0.1 Draft for review by PEB, after completion of HiSPEQ7, excluding all other text

HiSPEQ Project Team

HiSPEQ0 INTRODUCTION

HiSPEQ1SPECIFICATION FOR PAVEMENT CONDITION MEASUREMENT

HiSPEQ2 SPECIFICATION FOR REFERENCING DATA TO THE NETWORK

HiSPEQ3 SPECIFICATION FOR PAVEMENT TRANSVERSE EVENNESS MEASUREMENT

HiSPEQ4 SPECIFICATION FOR PAVEMENT LONGITUDINAL EVENNESS MEASUREMENT

HiSPEQ5 SPECIFICATION FOR PAVEMENT SURFACE DETERIORATION MEASUREMENT

HiSPEQ6 SPECIFICATION FOR PAVEMENT STRUCTURE MEASUREMENT

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HiSPEQ7 SPECIFICATION FOR TRAFFIC SPEED PAVEMENT DEFLECTION MEASUREMENT

HiSPEQ7: 1 Why collect Traffic Speed Pavement Deflection Data?

The amount that a pavement flexes under load is linked to how resilient the pavement is to this loading/unloading i.e. the pavement’s bearing capacity. Road authorities need information on structural pavement condition and structure to be able to deliver safe, effective and sustainable pavements to their customers, and to support structural pavement performance prediction. In general bearing capacity information is more important at the project level than at the network level. However, road authorities have a desire for high-speed testing devices to deliver information on the bearing capacity of their networks at the network level.

A direct measure of a pavement’s bearing capacity would include in situ measurement of stresses and strains within the pavement. This would require strain gauges, stress cells, or similar, to be installed within all layers in the pavement’s structure. Given the length of the road network in Europe and the process needed to instrument a pavement (which in itself would be likely to influence the structure), such measurement would be impractical. Therefore, the response of the pavement to the load applied is measured instead. This is then combined with knowledge of the materials used in each pavement layer, the thickness of each layer, and the behaviour of that material under load, in order to estimate pavement stiffness and/or strains within the structure and hence the length of time that the pavement is likely to endure traffic travelling on it before excessive deformation or an unacceptable level of service.

The most common devices for bearing capacity testing are the Curviameter, the Deflectograph and the falling weight deflectometer (FWD). None of these operate at high-speed. However, development in high-speed measuring techniques has delivered a promising high-speed device called the Traffic Speed Deflectometer – TSD.

Many of the needs of European road authorities in relation to structural condition measurement can be met by the TSD. Road authorities can obtain pavement bearing capacity information collected at high or traffic speed. This data can support the authorities in obtaining an overview of the current condition of the road networks, and provide valuable input to prediction of future pavement condition and hence future pavement maintenance needs and costs. Currently, most applications of the TSD focus on network level pavement evaluation (as opposed to project level). This could be linked to the continued use of the slower speed devices, e.g. the FWD, for project level bearing capacity testing or the need for other data (surface characteristic parameters like evenness or visual condition) that is collected during project level assessment. Recent studies in South Africa indicate that the TSD can be used for both network and project level investigations. Using the TSD for both network and project level testing will be very efficient, especially if this means that the same data can be used for both purposes. Applying the same data for two purposes could be accomplished by performing two different data analysis schemes:

a) A crude method for network level investigations spanning over a relatively long section with low testing density, i.e. characteristic values for each 100m length.

b) A detailed and accurate analysis of a shorter road length with high testing density, i.e. average values every metre.

Project level testing with the TSD would allow easy access to detailed bearing capacity information at locations where it is not likely that FWD testing would be performed due to

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high traffic intensities. In this way, the decision basis for pavement investigations will be better informed.

Bearing capacity testing in urban areas poses a special challenge. With the FWD, it is difficult to conduct the tests due to the disturbance of traffic, while with the TSD the challenges include low testing speeds, frequent stops, and the size of the testing vehicle. For the present, TSD testing in urban areas will be limited to major (arterial and collector) roads.

HiSPEQ7: 1.1 General introduction to Traffic Speed Deflectometer Surveys

The Traffic Speed Deflectometer (Figure 1) is developed, manufactured and sold by Greenwood Engineering A/S in Denmark. The TSD measures the vertical velocity of the deflected pavement surface based on an advanced Doppler laser technique using 7 to 10 laser sensors. Loading is provided by the rear axle of a semi-trailer. This way the TSD is different from the FWD: while the FWD simulates a moving wheel load, the TSD actually applies a moving wheel load to the pavement. The measuring equipment is placed in a tractor-semi trailer combination, with the measurement instrumentation in the trailer and the operator in the driver’s cabin. Measurements are conducted continuously at driving speeds generally between 40 km/h and 80 km/h. Two people are required for the testing: one driver and one operator.

Figure 1: The ANAS (Italy) TSD. Picture downloaded from http://www.ndtoolbox.org/content/pavement/d-description on 17

December 2014.Table 1 shows an overview of the eight TSDs currently existing. A further three machines are expected to be delivered during 2016. The Danish and UK machines constitute the first generation of the TSD. In the last ten years, Greenwood Engineering has delivered six TSD in a second generation format.

Intensive use of the TSD for practical testing purposes has been carried out in Denmark, the UK, Poland, Italy and Australia. In Denmark and the UK the TSD is used for network maintenance management and especially evaluation of the need for structural maintenance. The purpose of the Australian TSD activities seems similar. In Italy, the TSD is used for acceptance testing in connection with construction and performance contracts.

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Table 1 Overview of TSDs currently in operationDelivery number

Generation Country Owner Year of delivery

Number of Doppler sensors

Additional systems

Network surveys

1 1 Denmark DRD 2000 4 Prototype State road network each year since 2005

2 1 United Kingdom

Highways Agency (now Highways England)

2005 4 GPR from 2015 Annually since 2010

3 2 Italy ANAS 2010 7 ROW imaging, longitudinal profiler (one sensor), GPS, new hardware and software

Network surveys on ANAS road network

4 2 Poland IBDiM 2011 8 As TSD#3 Silesian region and possibly more

5 2 South Africa

SANRAL 2012/2013 10 As TSD#3, pavement distress/crack detection, LIDAR

Planned

6 2 China RIOH 2013 7 As TSD#3 Not known

7 2 USA/Denmark

Greenwood Engineering

2013 7 TSD#3, built for experimental and demonstration purposes

Demonstrations in 9 US states.-

8 2 Australia ARRB 2013 7 As TSD#3, pavement distress/crack detection, imaging

Since 2014; network testing in Australia and New Zealand conducted. Annual testing planned.

Source: www.greenwood.dk/tsd.php and publications from the owner organisations.

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HiSPEQ7: 2 Measurement of Pavement Deflection

Unlike other pavement deflection measuring equipment such as the Falling Weight Deflectometer the TSD does not measure the vertical deflection response of the road surface but the velocity of the road surface along the axis of the sensor. The sensor is set to be at roughly 2 degrees ahead of the vertical in a plane aligned with the direction of travel. Thus the sensor responds not solely to the vertical movement of the pavement under the wheel loads but also to the horizontal survey speed and the extraneous movement of the measuring frame on which the sensors are mounted. The unwanted velocity components are removed from the measured signal by subtracting the measurements made by the reference sensor mounted away from the loading wheels. Despite this, the resulting vertical measurements of pavement deflection velocity, which are regarded as the main raw measurement from this equipment, will still be significantly dependent on the survey velocity.

In order to nullify the effect of driving speed of the TSD vehicle on the value of the vertical deflection velocity, the base TSD data vertical deflection velocities (Vv) of the deflected pavement is divided by horizontal velocity (travel speed of the vehicle, Vh) of the TSD vehicle, to produce a quantity termed 'the slope' (Vh/Vv). The resultant value can be regarded as an estimate of the instantaneous slope of the pavement surface where the laser contacts the road i.e. the actual physical slope of the pavement surface within the deflection bowl centred under the moving TSD load wheel. In its rawest form this slope value is provided for each of the measurement sensors at various offsets from the loading wheels at up to 1000 times per second and these slope values, averaged over 10m, can be regarded as the measured data for the TSD (Figure 2).

Figure 2: The TSD slope concept (www.greenwood.dk/tsdres.php accessed 11/12/15).

It is possible to utilise these slope measurements directly in the evaluation of structural condition by correlating such values to traditional deflection measures such as the maximum deflection measured by the Deflectograph. However, if a number of slope values are measured at a range of offsets from the loading wheels it is possible to estimate the shape of the deflection bowl or basin. The simplest approach to this derivation is to integrate the slope versus offset relationship to generate the deflection/offset relationship i.e. the deflection ‘bowl’, as illustrated in Figure 3 and discussed later in section HiSPEQ7: 3.1. These estimated deflection values at various offsets from the load or deflection ‘bowl’ can be regarded as primary parameters.

Some of the parameters used by engineers to relate to structural condition are based on the difference between the measured deflections at specific offset locations, e.g. the central deflection minus that at 300mm from the load, termed SCI300 (surface curvature index over 300mm). This index, also regarded as a primary parameter, is discussed further in section

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HiSPEQ7: 3.2.2. The above described integration technique is particularly useful for the estimation of surface curvature indices since the only inputs required are the slope values between the specified offset locations and not the whole bowl shape.

Figure 3: Relation between pavement deflection (y) at any wheel load offset (x) and the cumulative area of the plot of slope (dy/dx =Vv/Vh), versus

offset (x)1.A more comprehensive approach to deriving the deflection bowl is to model a generic pavement structure and thus derive the general form for a deflection bowl and hence derive the general form for a ‘slope’ bowl. This latter equation can then be fitted to the measured slopes and the best estimate of the associated deflection bowl can be derived. This approach has been utilised by the TSD manufacturer, Greenwood Engineering A/S, using various ever more complicated and accurate pavement models as explained later in sections HiSPEQ7: 3.1.2 and HiSPEQ7: 3.1.3.

HiSPEQ7: 2.1 Measurement requirementsTo ensure that reliable, consistent data is obtained that will allow comparisons between a number of roads in a network, it is necessary to specify a number of requirements for the TSD data collection. These will also help ensure that the quality of the results from several runs with one TSD at the same road section, or one or more runs with several TSDs at the same road section, is acceptable.

A description of the TSD, and the basic principles of how it works, is given in section HiSPEQ7E of the Equipment Guidance document.

HiSPEQ7: 2.1.1Measurement positionCurrently all TSDs measure in the nearside wheel path only, in the direction of travel (noting therefore that the UK TSD measurement is thus on the opposite side to other TSDs in Europe). For research purposes and versatility of testing devices it has been suggested that an ability to mount the TSD Doppler sensor beam such that it could be moved across the width of the measurement trailer would be desirable. This would allow an assessment of the structural capacity of the pavement in any line across the width of a road lane. The moveable beam would also increase the versatility of the TSD as there would no longer be a need for a

1 Austroads, 'Traffic Speed Deflectometer Data Review and Lessons learnt', Technical Report AP-T279-14, 2014.

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'right side' mounted as well as a 'left side' (e.g. UK) mounted TSD. However, at the time of producing this guidance a TSD with this capability was not yet available.

It is common practice to base pavement designs on testing results obtained in the nearside wheel path, but testing should really link to the design standard. Thus you should state a measurement line for collection of TSD data that links to your design standard.

Whether you need more than one line of measurement will depend on how variable the construction of the roads on your network are likely to be: If you know that the construction on the left of the lane is, in general, the same as the construction in the right of the lane, then one measurement line should be sufficient (Case Study 1).

If you do require measurement of deflection in a wheelpath, the location of the wheel path to be measured should be defined, in order to ensure consistency of measurement.

Case Study 1: Which lines to measure deflection in?An investigation into the difference between deflection measurements from the nearside and offside wheel paths was carried out using Deflectograph measurements from the motorway and primary road network in England. The Deflectograph measures deflection response in both the nearside and offside wheelpaths. Almost 3,000km of deflection and construction data was used to calculate residual life and to also categorise each 100m as “in good to fair condition” or “in need of further investigation”.The data analysis suggested that 1.2 times as much of the network could be considered likely to have poor structural strength if data from both wheel paths is considered, compared to measurements from only the nearside wheel path. This factor is likely to increase for roads where the construction varies more across the width of the pavement than the variation seen on the English motorway and primary road network.

HiSPEQ7: 2.1.2 Number of Doppler laser sensors The review of equipment development has highlighted significant progress in the development of the TSD since the first generation devices became available. Current devices delivered are equipped with at least 7 Doppler sensors (including 1 reference sensor) and some as many as 11 sensors.

The number of necessary Doppler sensors depends on the use of the TSD. For network testing at a relatively basic level of accuracy (screening of pavement bearing capacity), as little as one Doppler sensor may be sufficient, as currently used in the UK, while many more sensors are needed if the purpose of the TSD testing is estimation of the full deflection bowl shape.

With the constantly increasing computing power at decreasing costs, there seems no reason to equip a TSD to meet a limited scope of work, and hence we recommend using the TSD with enough Doppler laser sensors to allow deflection bowl analysis. The current standard regarding number of Doppler laser sensors is 7 (including a reference sensor), and this number should be suited for deflection bowl analysis. Hence, it is recommended that 7 Doppler laser sensors are used. However, the potential ability to measure the shape of the bowl both before and after the load position and hence any asymmetry of the bowl may require a higher number of sensors.

HiSPEQ7: 2.1.3 Sample rate for Doppler laser sensorsExperience with the current TSDs in operation has shown that the quality of results depends on the sampling rate. This is defined as the number of valid measurement samples reported per second. Good quality TSD results are usually obtained at sampling rates of at least 600 samples/s. Both 1st and 2nd generation devices normally deliver this rate when the sensors are in good condition and correctly set up. New, shiny black asphalt surfaces provide a special case where it has been found difficult to obtain these data rates of 600 samples/s. In these specific cases the problem may be mitigated by reducing the driving speed or repeating the survey some months later when the reflectivity of the surface has reduced significantly.

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HiSPEQ7: 2.1.4Measured data intervalA sufficient amount of data is a pre-requisite for obtaining reliable results from any measuring equipment. The Doppler laser system collects road surface velocity at around 1000 samples per second i.e. around every 22mm at 80 km/h. the interval at which the measured data is reported can be selected by the TSD user: data intervals can be specified from 10 metres upwards.

The measured data (i.e. deflection slopes) are normally reported at 10m intervals, which is generally considered suitable for most network and project level applications.

HiSPEQ7: 2.1.5Data formatWith the focus being on network level assessment the format for the delivered data should be specified, so as to allow simple integration into the road authority’s Pavement Management Systems or other data handling tools (see section HiSPEQ1: 5.3).

HiSPEQ7: 2.1.6 Applied load on TSDThe rear axle loading of the TSD device should ideally correspond to the local design load. This will normally lie in the range from 8 to 13 tonnes (80 to 130kN). The most common load is currently 10 tonnes.

Typically, the TSD is equipped with a lead ballast load mounted in two odd-sized frames providing the total axle load range. The two odd-sized frames give the user an opportunity to vary the axle load. The static load can easily be checked using standard load cells, ideally measuring all four wheel assemblies simultaneously.

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HiSPEQ7: 2.1.7 Longitudinal profileAlthough the measurement of longitudinal profile is not a necessary input to the estimation of structural condition it can provide a useful explanation of unexpected results such as those caused by dynamic loading. Since the standard TSD includes the capability to measure profile as part of the system controlling the height of the Doppler lasers mounting beam it is recommended that the road's longitudinal profile is measured and recorded. This should be measured in the same line as the Doppler sensors. If the measurement of longitudinal profile is also intended for evenness evaluation, the specifications for the longitudinal profile should follow those given in HiSPEQ4. However, if the longitudinal profile will be used for other purposes, e.g., control of TSD measuring instruments, other specifications than those in HiSPEQ4 can be specified.

HiSPEQ7: 2.1.8Additional reference information to be recorded during TSD testingVery accurate testing speed is a vital measurement for the TSD, as it a vital component of the deflection slope estimation process as well as an important element in assessing the validity of the measured data.

Unevenness in the road surface may cause the loading to vary significantly from the static load, due to dynamic effects whilst surveying at speed. Thus the manufacturer has provided an option to fit strain gauges to the rear axle close to the loaded wheels, which enable estimates of the dynamic loading to be made. This facility is now fitted to many of the existing TSD machines enabling equivalent deflections under the known static load to be estimated on the basis of the actual deflection recorded under a measured dynamic load. This is a recommended option.

The equipment can also include instrumentation to monitor all of the key aspects, which affect data collection and analysis, such as vehicle speed, measuring beam temperature, air and road temperatures, tyre pressures and tyre temperatures. This additional information can be used to control the proper measuring conditions, and hence discard data outside acceptable thresholds. For example, results registered at driving speeds below 40 km/h could be discarded, and so can results obtained with a too low or varying tyre pressure

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(Case Study 2). Such additional data can also be helpful in the interpretation of test results and may provide input to formulae allowing for correction of measured data to standard test conditions. For instance, deflections could be corrected for load variations, or deflections/stiffness moduli could be adjusted for temperature. Therefore, the collection of this data is recommended.

Static information, such as the offset location of the Doppler sensors, the tyre and wheel configuration and tyre condition should also all be recorded for each survey session.

Case Study 2: Invalidation of TSD dataWhen calculating parameters from TSD surveys on the English primary road network, the measured data is checked to ensure that survey conditions fall within predetermined limits. This includes checks on :

• The data rates of each of the Doppler lasers: This check evaluates the rate at which data points have been recorded by each laser over predetermined lengths, and compares against a threshold above which the data rate must fall. Any individual Doppler laser data collected in lengths where the Data rates for that laser fall below a predetermined threshold (500points/s) are considered Invalid.

• Survey speeds: Data collected at survey speeds falling outside a predetermined range (30-80km/h) are considered Invalid.

• Temperatures of the measuring beam equipment: Data collected outside a predetermined range (20ºC ± 5ºC) are considered Invalid.

• Data collected where there was a difference in temperature of greater than 5ºC reported between any of the sensors installed on the measuring Equipment beam is considered Invalid.

• Data collected where there was a difference in tyre pressure of greater than 10psi between the rear wheels of the Equipment is considered Invalid.

• Data collected over lengths of pavement where the temperature of the pavement at a depth of 40mm was estimated to be less than 5ºC, or greater than 35ºC, is considered Invalid.

HiSPEQ7: 2.1.9Location referencingHigh quality measurement data has little value if it cannot be referenced back to the exact position on the road network on which it was measured. Therefore there is a need for all TSD measurements to be location referenced to the network. See HiSPEQ2 for the requirements for location referencing.

HiSPEQ7: 2.2 Survey conditions affecting measurement of pavement deflection (TSD)

HiSPEQ7: 2.2.1 Damp or wet road surfaceThe TSD uses Doppler lasers to measure the velocity of the pavement’s vertical movement when loaded. Laser measurement systems depend on the form of the reflected signal from the road surface and so the results are affected by the amount of water present on the road surface and measurements made with these systems, when the road is damp or wet, can generally be considered unreliable. This reduced performance is generally indicated by a reduced sample rate so the guidance provided in HiSPEQ7: 2.1 regarding sample data rate should ensure recording under such conditions are not possible. However, it is recommended for optimum results, that TSD surveys should only be performed when the road surface is dry.

HiSPEQ7: 2.2.2Cross winds, road geometry and pavement moisture contentThe loading on the pavement will be affected by a number of issues including the static weight and external effects such as dynamic loading and load transfer. For example, strong cross winds will act on the large exposed vehicle body to transfer load from one wheel assembly to the other as discussed in Zofka’s paper2 and hence affect the measured 2 Zofka A., Graczyk M., Rafa J., Qualitative evaluation of stochastic factors affecting the Traffic Speed Deflectometer results. Presented at TRB 94th Annual Meeting, January 1-15, 2015, Washington D.C.,

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deflection response. Extreme road geometry can also cause imbalance in the load applied to the wheels and Zofka’s paper also discusses the effect of various road geometry parameters on the measured pavement response. At present, acceptable limits to these effects have not been determined. It is therefore recommended that surveys do not take place when extreme cross winds are present, and results obtained in conditions of extreme geometry should be treated with caution.

Deflection measurements can be affected by changes in the moisture content of the pavement that can occur through seasonal rainfall, although this effect is less noticeable on a pavement with a thick bound layer (e.g. motorway construction). The subgrade moisture content varies in relation to seasonal changes of water table, drainage malfunction and ingress of water through the pavement. Therefore it is recommended for optimum results that measurements are collected at the same time of year for each road section, or that surveys are only carried out in certain seasons, e.g. spring or autumn when the water table is more likely to be relatively high.

HiSPEQ7: 2.2.3 TemperatureThe temperature of the pavement structure will often affect the response of the pavement to the applied load (Case Study 3). Flexible bituminous layers of the pavement are likely to be softer when warm, compared to when they are cold. For hydraulically bound or concrete layers the reverse can be the case. Therefore, ideally tests should be made under comparable climatic conditions, and avoiding high temperatures where excessive (often plastic) deformations of the flexible pavement layers could occur. The latter is especially relevant for testing of thick asphalt pavements in Europe and North America, where temperature has a major influence on pavement performance. A lower temperature limit should also be defined to avoid testing of very cold, brittle and stiff pavements not representative of most pavement situations. Furthermore, pavement response is expected to be very low at low temperatures.

It is recommended to conduct TSD tests at pavement temperatures (at a depth of 100 mm below the surface) between 5 °C and 35 °C. This in-depth pavement temperature is impossible to measure directly efficiently when carrying out a traffic speed survey. However, there are techniques to estimate such in-depth temperature from measurements of surface temperature and air temperature both of which can be obtained efficiently whilst surveying. For example, the surface temperature of the pavement can be measured during the survey using suitably calibrated infrared thermometers and the air temperature can be easily obtained by conventional equipment. The in-depth pavement temperature can then be estimated from the measured surface temperature and air temperature using appropriate algorithms such as those given in the Example 1.

USA.

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Case Study 3: Effect of temperature on TSD dataTRL have carried out a number of repeat surveys over a five month period during 2014 with the Highways England TSD on two test sites, TT1 and TT2. These sections are each of around 50m in length and with 200mm of bitumen bound material but with very different stiffness characteristics. The following plot shows the mean section results of each of these surveys with the slope of the P300 sensors (mm/m) given on the y-axis and the pavement temperature measured at a depth of 100mm on the x-axis. As would be expected the weaker structure, TT1, shows higher slope values than TT2 and a higher sensitivity to temperature3.

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f(x) = 0.0207154978150616 x + 0.0631052446026537R² = 0.643460023637058

f(x) = 0.00764912619158438 x + 0.0644825245787266R² = 0.536621275361977

Average of P300 slopes for sections TT1 and TT2 @70kmph

TT2Linear (TT2)TT1Linear (TT1)

3 Meitei, B., P. Langdale, N Elsworth and F Coyle (2016): “Traffic Speed Deflectometer: A comparative study of the 1st and the 2nd generation TSDs”. TRL published report RPN3386

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Example 1: BELLS3 equation for estimating temperature at depth4

Td = 0.95 + 0.892 * IR + {log(d) - 1.25}{-0.448 * IR + 0.621 * (1-day) + 1.83 * sin(hr18 - 15.5)} + 0.042 * IR * sin(hr18 - 13.5)

Where:

Td = Pavement temperature at depth d, °C

IR = Pavement surface temperature, °C

log = Base 10 logarithm

d = Depth at which mat temperature is to be predicted, mm

1-day = Average air temperature the day before testing, °C

sin = Sine function on an 18-hr clock system, with 2π radians equal to one 18-hr cycle

hr18 = Time of day, in a 24-hr clock

The BELLS3 equation is developed for routine testing with up to 30 seconds of shade due to pavement (FWD) testing. The US Federal Highway Administration recommends the use of the BELLS3 model for routine bearing capacity testing. It is worth noting that the BELLS3 expression is based on data from North America and it would need local calibration before used in other geographical areas.

Even if TSD tests are conducted within the recommended temperature range, TSD results are still likely to be influenced by the pavement temperature if the stiffness properties of the pavement materials are temperature sensitive and interpretation of the results will be challenging. A temperature correction procedure that converts relevant TSD results from any temperature to a given reference temperature has yet to be developed. Once a generic model is developed, it would need adjustment to local/regional climate conditions.

ANAS in Italy have developed a simple method for temperature correction of the TSD parameter SCI300 (See HiSPEQ7: 3.2.2) for the structures on their network (Case Study 4).

Case Study 4: Temperature correction of TSD data5

The Surface Curvature Index, SCI300 can be adjusted for pavement temperature influence using the expression:

SCIref = SCI*exp(k*(14-tair))

where

SCIref is the Surface Curvature Index at the reference temperature 14 °CSCI is the Surface Curvature Index at the tair during TSD testingk is a coefficient depending on type of asphalt material type and material ageing (k = 0,25 can be used as an average reference value)tair is the air temperature during the TSD survey.

4 “LTPP Guide to Asphalt Temperature Prediction and Correction”, Publication Number: FHWA-RD-98-085, Federal Highway Administration, US Department of Transportation, http://www.fhwa.dot.gov/publications/research/infrastructure/pavements/ltpp/fwdcd/tempred.cfm, accessed on the Internet on 10 August 2015.5 Personal communication with Stefano Drusin, ANAS S.p.A., Italy, 10th February 2016.

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Temperature correction formulae exist for deflections measured by the FWD and similar equipment, but the influence of temperature on TSD results is not well researched. Until temperature correction procedures are established for the TSD we recommend that a narrow range of allowable pavement temperature are used within specific geographic/climatic areas unless it is known that the deflection response of the structures to be surveyed are relatively insensitive to the effect of pavement temperature. This could be like the ranges shown in Table 2.

Table 2 Recommended temperature range for different climatic areasCold climate Hot climate

Recommended pavement temperature at 100 mm depth 5 – 20 °C 20 – 35 °C

Using the narrow pavement temperature ranges of Table 2 limits the potential error from not performing any temperature correction of TSD results related to asphalt materials. However, these would need to be adapted to the types of pavement and local climate. For example, for network purposes a wider temperature range without correction to a standard temperature could be acceptable whereas for project level usage either a much smaller acceptable temperature range would be needed or a robust temperature correction procedure.

HiSPEQ7: 2.2.4SpeedSpeed can affect the TSD measurements, due to visco-elastic phenomena of any asphaltic materials in the pavement and vibrations of the TSD itself (Case Study 5). Therefore, it is necessary to define an acceptable range of speeds for the TSD survey. The recommended testing speed of the TSD is in the range 40-80 km/h. TSD data measured at low driving speeds will include significant contributions from eventual speed dependent visco-elastic pavement response, while data measured at speeds above 30-40km/h will contain almost purely elastic pavement response, which is not speed dependent. To keep the data analysis relatively simple for flexible pavements the lower limit for network surveys is recommended to be approximately 40 km/h. The reason for the upper speed limit is that the TSD was developed in Denmark where the general legal speed limit for trucks is 80 km/h. This upper limit coincides well with the optimal performance of the Doppler laser. Decreasing Doppler data rates on new asphalt binder-rich (dark) pavement surfaces have been observed from 60 km/h upwards. Furthermore, 40-80km/h driving speed is comparable to the speeds of Heavy Goods Vehicles (HGVs); a fact that improves the representativeness of the TSD results.

Two studies looking at the effects of speed on TSD data are shown in Case Study 5 and Case Study 6. As for the effect of pavement temperature, discussed in the previous section, the effect of survey speed on deflection response values will depend to some extent on the pavement construction including the age of the materials. Thus the acceptable speed range may need to be adjusted depending on the expected construction of the network to be surveyed and the usage to which the data is to be put.

In order to reduce the effects of driving speed on measured results, it is recommended that the survey be specified so that a narrow a range of driving speeds of the TSD is achieved. This may be relaxed once the subject has been studied more thoroughly and a possible correction formula has been developed. One way of handling the possible effects of driving speed could be to link speed to road class.

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Case Study 5: Effect of speed on TSD data - 1Studies have been carried out on a first generation TSD in the UK into the effect of speed on the deflection slope data. As can be seen from the graph below, the results for speeds of 40km/h and above are fairly consistent. However, at the lower speeds, the results are quite different.

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e (m

m/m

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Events

Figure 4: Deflection slope measured at different speedsWith the introduction of second generation devices, it is anticipated that speed affects will be reduced but an NRA may wish to confirm this when testing TSDs on their network.

Case Study 6: Effect of speed on TSD data - 2The US Federal Highway Administration studied the response lag measured with the TSD as a function of vehicle speed and pavement stiffness. At similar temperature, the study found significant difference in TSD response in tests conducted at 48km/h (30 mph) and 96km/h (60 mph), respectively. Results from the FHWA study are in the two figures below. The left-hand figure shows the influence of driving speed as a function of deflection while the right-hand figure shows the influence of testing speed on deflection slope. Note that the speeds are given in miles per hour and that the x-axis shows the offset from the centre of the load.

Reference: Rada, G.R., Nazarian, S., Visintine, B. A., Siddharthan, R. and Thyagarajan, S., "Pavement Structural Evaluation at the Network Level” (In publication), Federal Highway Administration, Washington, DC, USA, 2015.

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HiSPEQ7: 2.3 The need for accurate construction dataAnalysis of the data provided by the TSD, as discussed further in HiSPEQ7: 3, can be used to estimate bearing capacity (structural condition). However, the pavement layer thickness and material type has a strong influence on this derivation. There is therefore a clear need for any road owner, wishing to undertake a network level structural condition survey, to have a robust understanding of the pavement structure to improve the estimation of structural condition. Unfortunately most road agencies lack detailed information on pavement layer thickness and material type, and the slow speed method to provide this data (coring and test pits) is expensive, impractical and negates much of the benefit of traffic-speed deflection measurement.

Therefore, it is recommended that if TSD surveys are to be commissioned, GPR surveys should also be commissioned, in order to obtain this construction data. GPR surveys are discussed in HiSPEQ6. While pavement deflection is dynamic in nature, pavement structure is on the other hand static, and hence TSD testing should normally need to take place more often than GPR surveys. If economics justify it then the ideal solution might be to mount the GPR on the TSD enabling both deflection response and construction information to be collected simultaneously.

HiSPEQ7: 3 ParametersAs discussed earlier at the beginning of HiSPEQ7: 2, there are several parameters that can be derived from the measured TSD slopes to aid the interpretation of the structural condition of a pavement. The most basic derivation is to estimate the vertical deflection response of the pavement at various offsets from the centre of loading i.e. the shape of the deflection bowl. Some of the current methods of determining these primary parameters are presented in HiSPEQ7: 3.1. There are several indices that can be estimated from the deflection slopes or the derived deflection bowl that can be utilized by the highways engineer to assess the structural condition of the pavement. Some of these secondary parameters are presented in HiSPEQ7: 3.2.

HiSPEQ7: 3.1 Deflection bowlsHiSPEQ7: 3.1.1The AUTC MethodsBased on TSD tests in Australia, Australia Road Research Board (ARRB) developed an analysis method called 'Area Under the Curve' (AUTC) method, which enables an estimate of deflections at various offsets from the load to be made i.e. a TSD deflection bowl6.

By plotting slope value against offsets from the load point as a slope profile curve, it is possible to show that the cumulative area under the slope profile starting at the tail is equal to the vertical deflection at that point, as shown earlier in Figure 3. The difference between any two deflection points is equal to the area under the slope profile curve between these two points.

Various versions of this AUTC method have been developed depending on the comprehensiveness of the available data. The AUTC Adjustments (AUTC+) method was developed to satisfactorily model the deflection bowl and its maximum deflection value only using the data from the TSD. The method considers the slope profile in terms of three zones, the 'head', 'body, and 'tail:

6 Roberts, J. and Byrne, M. 2008, 'An initial review of the Greenwood traffic speed deflectometer (TSD) and its potential applicability for the RTA', ARRB contract report RC73952, ARRB Group, Vermont South, Vic;Muller, W. B. and Roberts, J. 2013, 'Revised approach to assessing traffic speed deflectometer data and field validation of deflection bowl predictions', International Journal of Pavement Engineering, vol. 14, no. 4, pp. 388-402.

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The area of the 'head' is known and defined by the TSD lasers in the vicinity of the maximum value;

The area of the 'body' is bounded by the slope values at locations between 300 and 900mm from the load and by the shape of the curve between the these points (S300 and S900);

The 'tail' area is equal to the deflection 900mm from the load estimated independently.

A TSD slope profile curve-fitting model was developed, with its controlling parameters designed to be automatically responsive to the difference in the S300 and S900 slope values. The model fits a slope profile curve between the S300 and S900 points and consequently gives modelled estimates of the values at S450, S600 and S750 locations, as shown in Figure 5. Other versions of the AUTC method can used when more or less sensor data is available.

Figure 5: Conceptual representation of the TSD slope profile gap filling model7

HiSPEQ7: 3.1.2Initial Greenwood pavement modelThis pavement model was proposed as the foundation for a modelling method for TSD data, as illustrated in Figure 6.

Figure 6: The pavement modelThe model is governed by the following equation:

(EI ∂4∂ X4 +k 0)u (X )=−Fδ ( X ) , X∈R

where, EI is the bending stiffness or flexural rigidity of the beam, representing the pavement, as given by the Young's modulus E [Pa] and the second moment of inertia I [m4]; F [N/m] is

7 Austroads, 'Traffic Speed Deflectometer Data Review and Lessons learnt', Technical Report AP-T279-14, 2014

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the load distribution and k0 [N/m2] is the spring coefficient per length of the Winkler foundation8. The beam model could be further extended to include more parameters and consider the effect of different assumptions regarding the loading. For instance, Graczyk et al9 developed an analytical solution for the pavement deflections in one-layer pavement system. The pavement is modelled as the Euler-Bernoulli beam supported by the viscoelastic foundation while the loading is assumed as a set of concentrated forces moving with a constant speed. Compared to other available solutions, this solution is formulated in the full analytical form that allows a quick implementation and calculation of desirable parameters, such as deflection velocities, bending moments and deflections.

HiSPEQ7: 3.1.3Refined Greenwood pavement modelAs a result of a PhD project of Louis Pedersen10 for Greenwood Engineering, the Greenwood Engineering TSD processing software has been released with both the original beam model, and a more refined deflection bowl model. The latter can benefit from Doppler lasers positioned behind the wheel load and reveal both asymmetry (visco-elastic) and deflection delay between centre of load and maximum deflection (inertia) (see Figure 7). The user can select which model to apply11.

Figure 7: Schematic of TSD deflection bowl (grey curve), TSD slopes (green curve), pavement structure and pavement parameters, E: modulus,

and h: layer thickness (https:// www.greenwood.dk/tsdres.php accessed 11/12/15).

HiSPEQ7: 3.1.4 Selecting an analysis method to estimate deflectionsThe above text, in sections HiSPEQ7: 3.1.1 to HiSPEQ7: 3.1.3, has discussed various alternatives to analysis of TSD data as well as application of TSD data for pavement condition evaluation.

Three analysis methods have been presented to derive pavement deflection:

The Australian AUTC method

8 Pedersen, L., Hjorth, P. G. and Knudsen, K., 'Viscoelastic modelling of road deflections for use with the traffic speed deflectometer', Technical University of Denmark, 20139 Graczyk, M. et al., 'Analytical Solution of Pavement Deflections and its Application to the TSD Measurements', Poland, 201410 Pedersen, L., Hjorth, P. G., & Knudsen, K. (2013). Viscoelastic Modelling of Road Deflections for use with the Traffic Speed Deflectometer. Kgs. Lyngby: Technical University of Denmark. (IMM-PHD-2013; No. 310). 11 www.greenwood.dk/tsdres.php (accessed on 11th December 2015);Personal communication with Jørgen Krarup, Greenwood Engineering, 12th March 2015.

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Greenwood Engineering's beam model Greenwood Engineering's refined model.

The AUTC model is simple, easy to understand and based on practical experience from TSD testing and data analysis in Australia. The model is empirical and hence not based on a theoretical model, which would allow determination of further pavement parameters than deflection. It also exists in various versions depending on the comprehensiveness of the sensor data available.

The Greenwood beam model is a simple model, which is easy to apply on data and it has seen significant use. The beam model is a theoretical model describing a one-layer pavement, and as such it is limited in its potential regarding a description of a 'real' pavement.

The Greenwood refined model is based on a thorough theoretical study involving finite element modelling of a layer structure. The refined model does not apply the finite element method per se, though, since this would be too heavy regarding computational power. The model is a synthesized model based on theoretical and practical work and hence combines solid knowledge and computational performance.

Overall, the AUTC model can be recommended for network testing with relatively limited needs for descriptive pavement parameters. For more detailed investigations with higher requirements to pavement parameters, we recommend the Greenwood refined model. Since the two Greenwood analysis models are delivered with a TSD, it is recommended that the Greenwood refined model would normally be used.

HiSPEQ7: 3.2 Structural indices from TSD dataWith estimates of the absolute deflection bowl available, many new ways of analysing TSD data may be possible. The deflection bowl should also make it possible to apply a wealth of already existing methods to analyse pavements. One of the interesting possibilities will be to apply TSD data using FWD back-calculation procedures and hence determine elastic layer moduli, etc. With GPR data from the same pavement section, it should be possible to determine elastic layer moduli for every, say 10 m.

The following sections describe various methods for utilizing the deflection slopes or derived deflection bowls to provide indices, which the highway engineer can use to assess the structural condition of a pavement when calibrated to the network under consideration.

HiSPEQ7: 3.2.1 Network structural condition categoriesIn the UK, TRL have developed a methodology to convert a single TSD slope value into an equivalent Deflectograph value and thence into one of four Network Structural Condition (NSC) categories. Since 2010 routine network TSD surveys have thus provided agents responsible for the English Primary road network with valuable structural information to guide their maintenance planning and investigations. As illustrated in Case Study 7.

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Case Study 7: Deflection slope and UK DeflectographThe procedure for interpretation of TSD measurements on the English Primary Road Network is based on a relation between deflection slope, as measured by the TSD, and deflection, as measured by the UK Deflectograph. By using the existing UK relation between Deflectograph deflection and residual life, the structural condition of the road pavement could then be estimated12.

The analysis procedure uses an algorithm to convert each of the 1m TSD slopes to an estimated peak Deflectograph value. By using the estimated peak Deflectograph values together with the construction and traffic information, a measure of structural condition could be obtained for each 100 m length. These measures are then used to assign one of four levels of Network Structural Condition (NSC) category to each 100 m reporting length using criteria that depend on the characteristic base type. The definition of each of the NSC categories is provided in Table 3.

Table 3: UK Network Structural Condition categories

Category Description

1 Flexible pavements without any need for structural maintenance

2 Flexible pavements unlikely to need structural maintenance

3 Flexible pavements likely to need structural maintenance

4 Flexible pavements very likely to need structural maintenance

HiSPEQ7: 3.2.2Deflection difference indicesA number of studies have shown that differences between selected deflections within the deflection bowl are strongly related to the structural condition of certain layers within a pavement. For example, the difference between the central deflection and that 300mm from the centre of the load relates strongly to the condition of the upper layers of a pavement.

This structural curvature index 300 (SCI300) can then be used to evaluate the characteristics of the upper layers. The SCI300 is defined as

SCI300 = d(0) – d(300)

where,

d(0) = the maximum deflection at the centre of the loading, and

d(300) = the absolute deflection 300 mm in front of the centre of loading.

The SCI300 can be determined by a number of means from the raw slope measurements as discussed earlier including direct derivation from the estimated deflection bowl. However, one simpler approach is by using the deflection slope from the TSD measurements, based on the assumption that the slope at the location of the applied load is zero. This assumption might not be exactly true due to the viscoelastic nature of pavement, but it is expected to be a good assumption. The following equation was used to calculate the SCI from the TSD13:

SCI∈(mm )=0.5×( S1001,000 )×100+0.5×( S 100+S3001,000 )×200where,

S100 = TSD measured slope deflection (mm/m) at 100 mm, and

12 Ferne, B., Langdale, P., and Wright, M. A., 'Developing and implementing traffic-speed network level structural condition pavement surveys'. 9th International Conference on the Bearing Capacity of Roads, Railways and Airfields, Trondheim, Norway - 25 to 27 June 2013, 2013.13 SHRP2, 'Assessment of Continuous Pavement Deflection Measuring Technologies', Report S2-R06F-RW-1, 2013.

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S300 = TSD measured slope deflection (mm/m) at 300 mm.

Current versions of the Greenwood software provided with the TSD provide estimates of SCI300 based on their various models of the pavement deflection basin or bowl as discussed earlier in HiSPEQ7: 3.1.2 and HiSPEQ7: 3.1.3.

The SCI300 estimates can then be compared to limits for good and poor bearing capacity, respectively. These limits depend on the traffic loads on the specific section. Locations with an SCI300 above the poor bearing capacity limit have an indicated need for strengthening. The needed overlay thickness can then be determined based on the FWD tests.

Figure 8 presents an example from the Danish Road Directorate's pavement management system, showing TSD tests (purple line) on a 2 km long road section14. The red curve shows the limit for poor bearing capacity with strengthening need, while the green line represents the limit between good and fair bearing capacity. Apparently, the first 1.1 km mostly lay above the red line, which indicates the need for strength. The correspondence between TSD indications of poor bearing capacity and strengthening needs to be confirmed by FWD testing.

The limits characterizing poor and good bearing capacity have been developed based on similar limits for FWD measurements, and the TSD limits have been adjusted according to comparative tests within FWD and TSD. Application to other networks would require appropriate recalibration. Other deflection difference indices further away from the load can be calculated from the derived deflection bowls and studies have suggested these can relate to the condition of lower layers in the pavement.

Figure 8: Extract from The Danish Road Directorate’s pavement management system showing TSD tests (purple line) on a 2 km long section. The red curve

shows the limit for poor bearing capacity with strengthening need, while the green line represents the limit between good and fair bearing capacity14.

HiSPEQ7: 3.2.3 Layer stiffnessesIn some cases sufficient raw slope measurements can be analysed with an adequate pavement model to enable a deflection bowl to be derived that is of sufficient accuracy to be used with a back analysis technique similar to that used with FWD deflection bowls as discussed earlier in section HiSPEQ7: 3.1.3. This should enable the estimation of the stiffness of the component pavement layers but as yet little work has been carried out to validate the robust of the technique. If reliable layer stiffness estimates could be derived

14 Personal communication with Susanne Baltzer, Danish Road Directorate, 11th December 2015.

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then these could be compared to acceptable criteria to make judgements on the structural condition of the various layers of the pavement.

HiSPEQ7: 3.2.4 Effective structural number SNeffThe effective structural number could be estimated by the method developed by Rohde15, which is based on the pavement thickness and the difference in deflections between two points. It is advantageous for applying this method to the TSD because it is possible to integrate the deflection slope to find the difference in deflection between the two points. The existing pavement SNeff could be determined as

SN eff=k1SIPk2H p

k3

where, for asphalt pavements, k1 = 0.4728, k2 = -0.4810, and k3 = 0.7581. Hp is the depth of the pavement (mm) and SIP = Do – D1.5Hp

To calculate the SIP (Structural Index of the Pavement) from the TSD deflection slope, the deflection slope at 1.5 times the pavement depth should be estimated. After that, the area under the curve defined by the deflection slope could be calculated, which defines the difference in deflections between two points between which the area is calculated. The calculation of this area is using the assumption that the deflection slope varies linearly throughout the deflection bowl, and the deflection slope at the location of the applied load is 0, as discussed earlier in section HiSPEQ7: 3.2.2.

There are also methods to derive SNeff from estimated layer stiffnesses, which could utilize those derived from the TSD bowl described in section HiSPEQ7: 3.2.3.

HiSPEQ7: 3.2.5 Critical pavement strainsMany pavement design and assessment systems utilize strains at critical positions within the pavement to assess the remaining life and any required strengthening treatments. Such strains could be derived by several means. The most obvious is to use the estimated layer stiffnesses, derived from the deflection bowls, in a suitable model to estimate the strains. Studies by Senthil et al16 have also shown that the critical strains can be strongly correlated to certain deflection bowl parameters such as SCI300, which can be easily obtained from the raw deflections slopes or the deflection bowl as discussed earlier in HiSPEQ7: 3.2.2.

15 Rohde, G., 'Determining Pavement Structural Number from FWD Testing’. In Transportation Research Record: Journal of the Transportation Research Board, No. 1448, TRB, National Research Council, Washington, D.C., PP. 61-68.16 Senthilmurugan Thyagarajan, Nadarajah Sivaneswaran, Katherine Petros and Balasingam Muhunthan “Development of a simplified method for interpreting surface deflections for in-service flexible pavement evaluation” TRB Jan 2012 and Senthilmurugan Thyagarajan, Nadarajah Sivaneswaran, Katherine Petros “Methodologies for incorporating traffic speed deflection devices based flexible pavement structural evaluation within a pavement management application TRB Jan 2015)

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HiSPEQ7: 4 AccreditationThis section should include a description of the surveys that will need to be carried out, in order to obtain data for the accreditation of TSD data and any parameters derived from the TSD data.

The data assessed during the accreditation tests should be provided by surveys that are performed in a manner that is representative of how surveys on the whole network are performed. Therefore, the usual crew configuration (i.e. a driver with their usual operator) should be used for the accreditation tests where possible.

Choosing survey sites to provide measurement data for the assessment: It is likely that reference deflection measurements will be provided by a slow speed method. Thus a survey of a test track or private road system would be beneficial, to eliminate the need for road closure or risk to operator’s health. Also, if tests are performed, to assess the effect of speed on the measurements or parameters, then it would be safer for this data to be collected by surveys of a test track or private road network, where public access can be restricted. It is recommended that the test track site is 1km or more in length.

Since the test track is unlikely to contain road construction or conditions that are fully representative of the network to be surveyed, there is also a need to assess data collected on lengths of the road network, particularly to assess system repeatability and fleet consistency. The road network sites used to test measured TSD data or the parameters should include a range of condition that is representative of that found on your network i.e. there should be a range of constructions included (e.g. fully flexible, semi-flexible, rigid), lengths where the road is in good structural condition, through to lengths where there are structural issues. It is recommended that a minimum of 10km of road network sites be surveyed, with 100km preferred.

Requirement for calibration: Before accreditation tests occur, calibration of the angles of the Doppler sensors should be performed on a structurally uniform length of road of appropriate strength, according to the manufacturer’s recommendations.

Repeat survey requirement: You should state how many times the equipment is required to survey each test site: It would be recommended that this is a minimum of two repeat surveys, per site, with three repeat surveys used if the repeatability of the data is to be tested.

Accreditation survey conditions: Ideally, the surveys should be carried out under normal survey conditions, however, you may wish to specify extra requirements for this and these should be specified here.

Testing for the effect of speed on the measurements: TSD measurements can be affected by the speed of the vehicle. Therefore, it is recommended that you include repeat surveys of the test track at a range of speeds, so that the range of acceptable speeds can be determined during the first accreditation test e.g. repeat surveys of the test track at 40, 50, 60, 70 and 80km/h, with two runs performed for each speed.

Extent of tests: You should also briefly describe the extent of the tests i.e. whether accuracy, repeatability and/or fleet consistency will be tested. The benefits of testing each aspect are discussed in HiSPEQ1: 6.2.4. If multiple survey crews will be used to operate the equipment, then it would be recommended that each of the survey crews are required to perform the surveys of the network routes.

Re-accreditation: This section should also specify if subsequent accreditation (i.e. re-accreditation) tests are different to the accreditation tests i.e. will there be less testing carried out on the data?

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HiSPEQ7: 4.1 Contractor calibration of the TSDThe TSD will need regular calibration in order to secure the collection of valid measurements. The original TSD calibration procedure was designed by Greenwood Engineering and aimed at establishing the difference between the angles of the Doppler lasers. The principle behind the method was described by Ferne et al. (2009)17. In an ideal situation, the Doppler sensors should all be positioned at the exact same angle, since this would allow easy determination of the deflection velocity response at each Doppler sensor location (by subtracting the response of the reference sensor from the response of each of the respective Doppler sensors). However, as the Doppler sensors are not mounted at the exact same angle, a calibration is carried out to establish the factors which allow a correction from the real situation to the ideal situation. The calibration procedure for a 1st generation TSD is presented in Case Study 8. For the 2nd generation TSD, the calibration procedure is much different and much improved compared to the 1st generation procedure (Ferne et al., 201518) and this is presented in Case Study 9.

Case Study 8: Calibration of 1st generation TSDFor the 1st generation TSD, the calibration procedure involves removal of the ballast load and testing on a stiff rigid pavement. The reduced TSD load, in combination with the stiff pavement, is intended to provide a negligible deflection velocity and hence a zero deflection slope. The calibration is used to determine angle correction factors for the Doppler sensors, which provide zero deflection slope. The procedure is carried out with repeat runs at different driving velocities.

Case Study 9: Calibration of 2nd generation TSDThe new calibration method for the 2nd generation TSDs utilises the fact that the Doppler sensors are mounted on a moveable beam. According to the new procedure, the beam is progressively displaced in its longitudinal direction during multiple test runs, which allows measurement of pavement response at different offsets from the axle load. The limited study conducted by Ferne et al. (2015)18 showed that the calibration angles from tests conducted in one day on two different rigid pavement structures resulted in a maximum variation of 0.003 degrees during the tests using a 2nd generation TSD, while the two 1st generation TSDs gave maximum variations of up to 0.007 degrees.

TRL has investigated the calibration procedure further to take different shortcomings of the original procedure into account. Ferne et al. (2009)17 concluded that a method using an accelerometer installed in the pavement can reliably reproduce deflection velocity and hence be used to determine the sensor angles.

HiSPEQ7: 4.2 Accreditation of TSD deflection dataData for assessment: There is a need to define what data will be used to perform the assessment of TSD deflection data e.g. surveys on a test track, surveys of the road network.

We would recommend that the surveys are performed in normal survey conditions (e.g. dry, clean road) but you may wish to specify additional or alternative conditions and these should be stated here.

17 Ferne, B. W., P. Langdale, N. Round and R. Fairclough (2009): "Development of a calibration procedure for the UK Highways Agency Traffic Speed Deflectometer", Transportation Research Record, Journal of the Transportation Research Board, December 2009, Vol. 2093, pp. 111-117, Washington, DC, USA.18 Ferne, B. W., S. Drusin, S. Baltzer, P. Langdale, B. Meitei (2015): "UK Trial to compare 1st and 2nd Generation Traffic Speed Deflectometers", International Symposium Non-Destructive Testing in Civil Engineering (NDT-CE), September 15-17, 2015, Berlin, Germany.

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HiSPEQ7: 4.2.1Accreditation tests and requirements for accuracy of TSD deflection data

Devices and methods used to provide reference data: In order to test the fundamental accuracy of data, there is a need to obtain reference data. Unlike some pavement condition measuring systems, no “golden” device exists to provide reference measurements against which to compare the TSD data. Slow speed/stationary devices do exist for measuring pavement deflection, e.g. FWD, Deflectograph, Curviameter, and these can be used to provide some level of reference data. However, none of the slow speed or stationary devices measure the deflection under a load moving at traffic speed and therefore the measurements cannot be considered to be the same and cannot be directly compared. Therefore, the initial accuracy of the device can be checked by confirming whether lengths that have higher than average deflection reported by the TSD also have higher than average deflection reported by the slow speed device and vice versa for lengths with smaller than average deflection (Example 2, Example 3).

As well as the slow speed data not being directly comparable to TSD data, surveying the road network with slow speed devices requires traffic management and potential road closures. This has a high cost associated with it, both in terms of inconvenience to road users and safety risks for the operators, and thus it is suggested that TSD data from a previous survey is used as the reference (Example 3).

Example 2The performance of the equipment will be assessed against previous measurements collected on the same site using accredited equipment. Where such data does not exist, the performance of the equipment will be compared with reference data, collected by a Deflectograph, FWD, or other recognised slow speed device. Each survey dataset collected during the accreditation tests will be tested for accuracy.

Example 3Where previous TSD data is available, the difference between the current and previous deflection slope data will be calculated for each 10m length of the test site. The differences are required to lie within ±0.050 for 95% of the lengths on the test track and 90% of the lengths on the network routes.

If previous TSD data is not available, the TSD data will need to be compared with data from a slow speed device. When comparing TSD deflection slope data with data from a slow speed device, the shape of the data will be compared, to ensure that high and low levels of deflection are reported in similar locations. Note the values of the data will not be directly comparable, since the two data sets are likely to have a different scale.

How the measured data will be compared to the reference: It is not essential for you to inform the survey contractor as to how the measured data will be compared to the reference, in order to determine its accuracy. However, this information may help potential survey contractors to know whether their equipment is likely to meet the accuracy requirements.

Requirements for accuracy: State what the requirements for accuracy are and these should be based on your use for the data e.g. if the data is used to obtain a network-level indication of condition, the requirements for accuracy should be less stringent than if the use is for project-level investigation.

Valid survey speeds: You may also wish to inform the survey contractor as to how you will determine the range of speeds for which measurements are valid (Example 4).

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Example 4For each repeat survey, the deflection slope data will be calculated for each 10m length of the test site. The differences between each run and the survey performed at 50km/h will be calculated, along with the standard deviation of deflection slopes for each 10m length.

Any speed for which the differences lie within ±0.050 for 95% of the lengths and the standard deviation is ≤0.025 will be considered to be a valid survey speed.

HiSPEQ7: 4.2.2Accreditation tests and requirements for system repeatability of TSD deflection data

If you wish to test system repeatability, you will need to state what data will be used for the assessment (e.g. repeat surveys of the test track, repeat surveys of a road site) and how the data will be compared. You will also need to state the requirements for system repeatability of the measured data (Example 5).

Example 5Five repeat runs will be performed by the same crew on the test track at survey speeds exceeding 30km/h.

The difference between the repeat and original measurements of deflection slope data will be calculated for each 10m length of the test site, along with the standard deviation of values for each 10m length.

The differences are required to lie within ±0.045 for 95% of the lengths on the test track and the network routes and the standard deviations should be ≤0.025

HiSPEQ7: 4.2.3Accreditation tests and requirements for fleet consistency of TSD deflection data

If you wish to test fleet consistency, you will need to state how this will be tested e.g. what surveys will be needed, how the data will be assessed, what the requirements for consistency are (Example 6).

Example 6Each vehicle in the fleet will be required to perform two surveys on each of the network routes used for Accreditation tests.

For each vehicle and each survey, the deflection slope value will be calculated for each 10m length. An average will then be calculated for all values for each 10m length.

The difference between the measurements of deflection slope for individual surveys and the average will be calculated for each 10m length of the test site, along with the standard deviation of values for each 10m length.

The differences are required to lie within ±0.05 for 95% of the lengths and the standard deviations should be ≤0.025

HiSPEQ7: 4.3 Accreditation of TSD longitudinal profileIf the measurement of longitudinal profile is intended for evenness evaluation, the accreditation tests and requirements for the longitudinal profile should follow those given in HiSPEQ4. If, however, the longitudinal profile is to be used for other purposes, e.g., control of TSD measuring instruments, it may be more appropriate to determine less onerous testing for this measure.

In this case you will need to state what data will be used for the assessment, how the assessment will be achieved and what requirements you have.

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Data for assessment: If the measured longitudinal profile data is to be tested within the accreditation, then there is a need to define what data will be used to perform the assessment e.g. slow speed tests, surveys on a test track, surveys of the road network.

Slow speed tests of calibration surfaces, with a known shape e.g. a square or triangular wave can be useful to demonstrate the equipment’s vertical resolution and accuracy. For example, calibration surfaces similar to those described in ISO 13473-3:2002 (but with features large enough for the system to measure).

The slow speed tests do not demonstrate the equipment’s ability to measure longitudinal shape under normal survey conditions and therefore we recommend high speed surveys of a test track or private road network in addition.

We would recommend that the surveys are performed in normal survey conditions (e.g. dry, clean road) but you may wish to specify additional or alternative conditions and these should be stated here.

HiSPEQ7: 4.3.1Accreditation tests and requirements for accuracy of longitudinal profile

Devices and methods used to provide reference data: To test the accuracy of the longitudinal profile measurements, you need to know what the true measurements are and, to obtain these, you will need to use a reference device or method. It is not necessary to inform the survey contractor of the device or method used to obtain the reference data but they will find it informative. Therefore, it has been suggested that the reference data is described in this section i.e. what method(s)/equipment will be used, along with how often the reference dataset will be updated (Example 7). A number of reference devices for longitudinal profile are discussed in section HiSPEQ4: 3.1.1, which can be found in Part 1 of this guidance.

Example 7Calibration surfaces:Two calibration surfaces will be used:

Rectangular profile (each level 30mm wide, with peak-bottom 10mm, 20mm, 30mm, 40mm and 50mm)

Staircase (30mm wide steps, each 1mm high).

Reference Measure for longitudinal profile:The longitudinal profile of the test track being surveyed will be measured with the ARRB Walking Profiler each time an accreditation test is carried out. The Walking Profiler records a single line of longitudinal profile at a spacing of approximately 0.25m.

How the measured data will be compared to the reference: It is suggested that an explanation is given of how the measured data will be compared to the reference data and assessed for accuracy, including whether individual points will be compared, or whether you will e.g. calculate an average value for every 10m on the survey route (Example 8).

Example 8The longitudinal profile will be measured in each wheel path of the test track. A 3m moving average filter will be applied to both the measured and the reference profiles. The filtered measured longitudinal profile and the corresponding reference profile will be visually aligned and then the error between the measured profile points and the reference profile points calculated. 95% of the measurements should lie within 2.5mm of the reference, for both the nearside and the offside profiles.

The correlation coefficient of the (aligned and filtered) reference and measured longitudinal profiles for the whole site will be calculated. The requirement is that r2≥0.7 for both the nearside and the offside profiles.

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Valid survey speeds: You may also wish to inform the survey contractor as to how you will determine the range of speeds for which measurements are valid (Example 9).

Example 9A 3m moving average filter will be applied to the profiles collected during the repeat surveys, carried out at different fixed speeds on the test track. These filtered profiles will be compared with the filtered reference profile. The minimum and maximum speed for which less than 95% of the measurements lie within 2.5mm of the reference will be calculated. Data will only be considered valid if the survey has been performed within this range of speeds.

HiSPEQ7: 4.3.2Accreditation tests and requirements for system repeatability for longitudinal profile

If you wish to test system repeatability, you will need to state what data will be used for the assessment (e.g. repeat surveys of the test track, repeat surveys of a road site) and how the data will be compared. You will also need to state the requirements for system repeatability of the measured data (Example 10).

Example 10Five repeat runs will be performed on the test track at survey speeds exceeding 30km/h.

A 3m moving average filter will be applied to the longitudinal profiles from each of the repeat runs. An average profile will be calculated from all of these profiles. The cross correlation between each profile and the average profile will be calculated. The cross correlation should exceed 0.75 for all profiles.

HiSPEQ7: 4.4 Accreditation of TSD parametersIf the quality of the parameters is to be tested within the accreditation, then there is a need to define what data will be used to perform the assessment e.g. surveys on a test track, surveys of the road network. We recommend that the surveys are performed in normal survey conditions (e.g. dry, clean road) but you may wish to specify additional or alternative conditions and these should be stated here. State how many repeat surveys are required and whether the data will be assessed for accuracy and/or system repeatability. If there are multiple survey vehicles, you should also state if a test of fleet consistency will be performed.

HiSPEQ7: 4.4.1Accreditation tests and requirements for accuracy of TSD parametersIf the accuracy of the parameters is to be tested, there is a need to define how the tests will be performed. Beneficial, but not essential, information includes what method(s) or equipment will be used to provide reference data; how often the reference data will be updated; how the data will be compared to/assessed against the reference, including the assessment length (we would recommend that the assessment length used is the same as the reporting length for the parameter).

If you have included a test for measured data (section HiSPEQ7: 4.2.1), you will need to consider this when setting a requirement for the accuracy of your parameter(s): The accuracy of the parameter cannot exceed the implied accuracy from the measured data.

As discussed above (HiSPEQ7: 4.2.1), no “golden” device exists to provide reference measurements against which to compare the TSD data. Also slow speed/stationary devices do not measure the deflection under a load moving at traffic speed and therefore the measurements cannot be directly compared with those from the TSD. Therefore, the initial accuracy of the device can be checked by confirming whether lengths that have higher than average deflection reported by the TSD also have higher than average deflection reported by the slow speed device and vice versa for lengths with smaller than average deflection. Alternatively, TSD data from previous a survey can be used as the reference.

Essential information includes the requirements for the accuracy of each parameter (whether delivered by the contractor, or calculated from measured data) (Example 11).

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Example 11The test sites will be surveyed with a Deflectograph at the same time as the equipment. Accurate construction data will also be obtained for the survey site. The average of three repeat surveys will be used to provide the Network Structural Condition (NSC) values for the reference data, reported over 100m lengths.

The NSC value, calculated from TSD data should match that for the reference for 75% of lengths, for all repeat surveys and be within 1 category for over 90% for the lengths i.e. if the NSC category from the Deflectograph data is 2, then the NSC from the TSD can have a value of 1, 2, or 3.

To determine if the data is affected by speed, repeat surveys at different speeds should be performed. You will then need to describe how this data will be analysed and how the range of speeds, for which sufficiently accurate data is obtained, will be determined (Example 12).

Example 12For each survey performed, the difference between the measured and reference SCI300 values will be calculated. The data will be considered to be unaffected by speed if at least 65% of the differences fall within ±60μm, whilst at least 95% of the differences fall within ±100μm.

If these requirements are not met for some speeds, then this may suggest that the data is speed dependent and further tests will be completed to determine the range of speeds for which accurate data should be delivered.

HiSPEQ7: 4.4.2 Accreditation tests and requirements for system repeatability for TSD parameters

If you wish to test system repeatability for the TSD parameters, you will need to state what data will be used for the assessment (e.g. repeat surveys of the test track, repeat surveys of a road site) and how the data will be compared. You will also need to state the requirements for system repeatability of the TSD parameters (Example 13).

Example 13The data from the repeat runs, performed on the test track, and on the road routes, will be compared. The mode of all NSC values will be calculated for each 10m reporting lengths. When individual NSC values are compared with this mode, it will be expected that

The NSC value is the same as the mode for at least 65% of the lengths. The NSC value is equal to the mode±1 for at least 90% of the lengths.

HiSPEQ7: 4.4.3 Accreditation tests and requirements for fleet consistency for TSD parameters

If you wish to fleet consistency for the TSD parameters, you will need to state what data will be used for the assessment (e.g. surveys by each of the vehicles of the test track, or of a road site) and how the data will be compared. You will also need to state the requirements for fleet consistency of the TSD parameters (Example 14).

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Example 14Each survey vehicle will be required to survey the two road routes twice and SCI300 values reported for each 10m. Average SCI300 values, shall be calculated for each 10m length using all runs from every vehicle. The difference between this average SCI300 value and the individually measured SCI300 values shall be calculated for each survey, each vehicle and each 10m length on the survey route. The fleet will be considered to be consistent if

At least 65% of the differences between the repeat measurements and the average fall within 40μm.

At least 95% of the differences between the repeat measurements and the average fall within 60μm.

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HiSPEQ7: 5 Quality AssuranceSome parameters that can be calculated from TSD data require knowledge of pavement structure. Since the pavement structure data may not be available on site, it would thus be difficult to assess these parameters during QA. Thus, for TSD data, HiSPEQ recommends that a QA process is applied to test the quality of just the deflection slopes.

HiSPEQ7: 5.1 QA for TSD deflection dataHiSPEQ7: 5.1.1QA tests and requirements for accuracy of deflection slopesQA should focus on the ability to ensure consistency, as accuracy is checked under the accreditation regime. Therefore, checking the accuracy during QA could be considered optional, or at least, could be performed less frequently than consistency tests.

If the accuracy of the deflection slopes is to be tested, there is a need to define how the tests will be performed. Beneficial, but not essential, information includes what method(s) or equipment will be used to provide reference data; how often the reference data will be updated; how the data will be compared to/assessed against the reference, including the assessment length (we would recommend that the assessment length used is the same as the reporting length for the parameter).

As discussed above (HiSPEQ7: 4.2.1), no “golden” device exists to provide reference measurements against which to compare the TSD data. Also slow speed/stationary devices do not measure the deflection under a load moving at traffic speed and therefore the measurements cannot be directly compared with those from the TSD. Therefore, it would be recommended that TSD data from previous a survey should be used as the reference. Essential information includes the requirements for the accuracy of the deflection slopes (Example 15).

Example 15The TSD will be expected to survey several sites, spread around the network to be surveyed, within 1 month of Accreditation. The data collected during these surveys will then be provided to the Auditor and used as reference data.

When the TSD then surveys these sites as part of its routine survey of the network, this data will then be compared with the reference: The difference between the current and previous deflection slope data will be calculated for each 10m length of the test site. The differences are required to lie within ±0.050 for 90% of the lengths.

HiSPEQ7: 5.1.2QA tests and requirements for system repeatability for deflection slopes

The recommended approach to monitor the repeatability and consistency of the survey equipment throughout the duration of the survey is to require the contractor to survey a number of QA reference sites soon after accreditation and then at regular intervals throughout the duration of the survey contract.

The reference sites should be representative of the pavement construction and condition found on the network and be evenly spread around the network to be surveyed. This will mean that the contractor will not have large travel times to get from the area in which they are working, to one of the QA reference sites. The contractor should be required to survey one of the reference sites on at least a monthly basis (preferably more frequently, if large distances are being surveyed).

The data from these surveys can then be compared to the original data (the data measured soon after accreditation) in a similar way to the accuracy tests used for accreditation (using the original data as the reference) (Case Study 10). Note that, since TSD data is affected by

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both the survey speed and the temperature of the pavement, it may be necessary to compare the relative values of the deflection slope, rather than the absolute values.

Case Study 10: QA procedure for TSD surveys of English Primary Road networkThe approach to QA of TSD data is based on four levels of checking, using three types of site:

• Primary Check Site (One site 10-20,000km long, surveyed no more than 7 days following Accreditation/Re-Accreditation);

• Secondary Check Site (sites chosen by the Auditor, on the network);• Daily Checks (Several sites at least 400m long, surveyed at the start and end of every

survey day).

Primary Checks: The aim of the Primary Check is to provide an ongoing check of data consistency using a well-established test site. To perform a Primary Check the Consultant shall carry out, every week, at least three Surveys on the Primary Check site.

The Consultant shall calculate the differences between the Primary Check Reference Data and the parameters calculated from the Survey Data obtained during the Primary Check of the Primary Check site under test.

The differences between the deflection slopes (assessed over 10m lengths) shall fall within ±0.050 for 90% of the lengths, with all differences lying within ±0.200. If the tolerance level is exceeded in all three repeat runs the auditor/survey commissioner shall be consulted, who may require the equipment to undergo re-Accreditation.

Secondary Checks: The Secondary Check utilises a set of known test lengths located across the network, referred to as Secondary Sites and is carried out as part of the routine survey regime. The Secondary Check compares the Survey Data from the current survey against the data collected previously on the Secondary Sites to provide an on-going check of data consistency using known test lengths.

For a Secondary Check the survey contractor carries out a survey on the network as part of the routine survey. They then:

Process the Survey Data. Identify whether a Secondary Check Reference site has been surveyed within that survey

route. If a site is contained within the route, the data collected over the Secondary Check

Reference site should be compared with previous data collected for that Secondary Check Reference site.

The requirement for the deflection data is that 75% of the differences between deflection slopes, calculated for each 10m length on the site, are within ±0.050, with all differences lying within ±0.200.

Secondary checks are assessed on a week-by-week basis by summarising the performance over all Secondary Check sites surveyed in each survey week. Each parameter must successfully meet the requirements of the Secondary Check in at least one Secondary Check carried out in each survey week. If this requirement is not satisfied then the Auditor shall be informed and no further Surveys carried out until the problem is resolved.

Daily Checks: The objective of a Daily Check is to check the day to day consistency of the Equipment. The following procedure is used for Daily Checks:

Equipment Daily Check: Each day the consultant will undertake checks on the general safety and functionality of the equipment including the vehicle, trailer and the measurement systems.

Data Daily Check: The consultant selects a suitable test site, close to the facility used for the overnight storage of the equipment for carrying out Daily Checks on the equipment both when travelling from the depot and after carrying out the day’s surveys. The same site shall be surveyed in the same direction both at the start and the end of the day.

A survey of the test site is carried out at the start of the survey day. The Survey Data is processed and termed the Daily Check Reference. It is noted that a new set of Daily Check Reference Data is generated each survey day, each set only being used for the

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survey day on which it was collected. After completing the day’s work a measurement is carried out over the same test length as

that carried out at the start of the survey day. The processed data collected from the second test site Survey is compared with the Daily

Check Reference. The requirement for the deflection data is that 75% of the differences between deflection slopes, calculated for each 10m length on the site, are within ±0.050, with all differences lying within ±0.200.

If the tolerance level is exceeded for any parameter then the test run should be repeated. If the problem remains then the auditor/survey commissioner shall be informed and no further Surveys carried out until the problem is resolved. It may be necessary to reject all data collected since the last successful Primary Check.

HiSPEQ7: 5.1.3QA tests and requirements for fleet consistency for TSD deflection dataFleet consistency can be checked during the accreditation tests and, if equipment can be shown to be consistent with itself for the duration of the survey contract, it can be assumed that it will also remain consistent with other equipment in the fleet. Also, to test fleet consistency would require each survey equipment vehicle to survey the same length of road, which may not be convenient if they are spread around the country, surveying different areas. Therefore, checking the fleet consistency during QA could be considered optional, or at least, could be performed less frequently than repeatability tests.

If fleet consistency is to be tested, a test site or sites should be chosen and all vehicles required to survey this in a short space of time (e.g. within 2 weeks) on a regular basis. A similar test to that used for the accreditation can then be used to assess the quality of the data (Example 16).

Example 16Every two months, the contractor will chose a site (>10km long) on the road network on which to perform the fleet consistency tests. Each survey vehicle will be required to survey this site within a two week period.

Average deflection slope values, will be calculated using all runs from every vehicle. The difference between these average deflection slope values and the individually measured values shall be calculated for each survey, each vehicle and each 10m length on the survey route. The fleet will be considered to be consistent if

• At least 65% of the differences between the repeat measurements and the average fall within 0.050.

• All differences between the repeat measurements and the average fall within 0.200.

HiSPEQ7: 5.2 QA for TSD longitudinal profile dataFor practicality it is recommended that the performance of parameters, calculated from the longitudinal profile, are assessed within QA. As with Accreditation for TSD longitudinal profile, if the measurement of longitudinal profile is intended for evenness evaluation, the accreditation tests and requirements for the longitudinal profile should follow those given in HiSPEQ4. If, however, the longitudinal profile is to be used for other purposes, e.g., control of TSD measuring instruments, it may be more appropriate to determine less onerous testing for this measure.

In this case you will need to state what data will be used for the assessment, how the assessment will be achieved and what requirements you have.

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HiSPEQ7: 5.2.1QA tests and requirements for accuracy of ride quality parametersQA should focus on the ability to ensure consistency, as accuracy is checked under the accreditation regime. Therefore, checking the accuracy during QA could be considered optional, or at least, could be performed less frequently than consistency tests.

If the accuracy of the ride quality parameters is to be tested, there is a need to define how the tests will be performed (Example 17).

Example 17Network sites, already surveyed by the contractor, will be selected on which to carry out accuracy testing. The sites will comprise up to 5% of the network surveyed and will be surveyed with the reference device within 2 weeks of being surveyed by the equipment. The average of two repeat surveys will be used to provide the reference data and the longitudinal profiles from these surveys will be processed with ProVal to obtain reference IRI values for each 10m reporting length on the sites.

The difference between the measured IRI values and reference IRI values for each 10m length will be calculated. The accuracy of the equipment will be acceptable if:

65% of the differences lie within ±1.5mm/m 95% of the differences lie within ±3mm/m.

HiSPEQ7: 5.2.2QA tests and requirements for system repeatability for ride quality parameters

The recommended approach to monitor the repeatability and consistency of the survey equipment throughout the duration of the survey is to require the contractor to survey a number of QA reference sites soon after accreditation and then at regular intervals throughout the duration of the survey contract.

The reference sites should be representative of the condition found on the network and be evenly spread around the network to be surveyed. This will mean that the contractor will not have large travel times to get from the area in which they are working, to one of the QA reference sites. The contractor should be required to survey one of the reference sites on at least a monthly basis (preferably more frequently, if large distances are being surveyed).

The data from these surveys can then be compared to the original data (the data measured soon after accreditation) in a similar way to the accuracy tests used for accreditation (using the original data as the reference) (Example 18, Case Study 11).

Example 18The equipment will survey four sites on the road network within two weeks of accreditation. The average IRI values, from the nearside and offside wheel paths will be calculated for each 10m reporting length on the sites. This data will be known as the “original data”.

The equipment will be required to survey at least one of these sites on a monthly basis and the contractor will perform the following comparison:

Calculate the difference between the measured and original nearside IRI values; Calculate the difference between the measured and original offside IRI values;

The results of this comparison should then be provided to the Survey Commissioner. The data will be considered to be acceptable if:

At least 65% of the differences fall within ±1.0mm/m and At least 95% of the differences fall within ±2.0mm/m, for both the nearside and the offside

data.

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Case Study 11: QA of parameters, calculated from TSD longitudinal profile dataThe approach to QA of TSD data is based on four levels of checking, using three types of site: Primary, Secondary and Daily, as discussed in Case Study 10. The requirements for the LPV parameters are:

Primary:• 90% of 3m eLPV values lie within 0.60 of the reference• 90% of 10m eLPV values lie within 0.70 of the reference

Secondary and Daily:• 75% of 3m eLPV values lie within 0.60 of the reference• 75% of 10m eLPV values lie within 0.70 of the reference.

HiSPEQ7: 5.2.3QA tests and requirements for fleet consistency for ride quality parameters

Checking the fleet consistency during QA can be considered optional, or at least, could be performed less frequently than repeatability tests.

If fleet consistency is to be tested, a test site or sites should be chosen and all vehicles required to survey this in a short space of time (e.g. within 2 weeks) on a regular basis. A similar test to that used for the accreditation can then be used to assess the quality of the data (Example 19).

Example 19Every two months, the contractor will chose a site (>10km long) on the road network on which to perform the fleet consistency tests. Each survey vehicle will be required to survey this site within a two week period.

Average IRI values, for each wheel path, shall be calculated using all runs from every vehicle. The difference between this average IRI value and the individually measured IRI values shall be calculated for each survey, each vehicle and each 10m length on the survey route. The standard deviation of all values shall be calculated for each 10m length. The fleet will be considered to be consistent if

At least 65% of the differences between the repeat measurements and the average fall within 1.0mm/m.

At least 95% of the differences between the repeat measurements and the average fall within 2.0mm/m.

95% of the standard deviations shall be less than 2.5mm/m.For IRI measurements from both the nearside and the offside wheel paths.

This survey specification guidance was developed by the HiSPEQ project, the research for which was carried out as part of the CEDR Transnational Road Research Programme Call 2013. The funding for the research was provided by the national road administrations of Denmark, Germany, Ireland, Netherlands, UK and Slovenia.

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