interpretations of road profile roughness data: review

62~.7 ~esearch Report (94) ~RR 295 AUS

ARR295

Interpretations of road profileroughness data: Review and research needs

John McLean Euan Ramsay

Interpretations of road profile-roughness data: Review and research needs

John McLean and Euan Ramsay

t:1f' I r-~~3£

ARR 295 October 1996

ARAB Transport Research Ltd ACN 004 620 GS 1

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lriformation Retrieval

MCLEAN, J.:and RAMSAY, R. (1996): INTERPRETATIONS OF ROAD PROFILE-ROUGHNESS OAT A: REVIEW AND RESEARCH NEED.S. ARRB Transport Research Ltd. Research Report No. 295. 24 pages including 3 figures and 3 tables, and appendiCes.

ABSTRACT: The report first reviews the development of road surface roughfless as a measure of overall road· condition and of roughness measurement technologies. The International Roughness Index (IRI) was adopted in Australia as a calibration reference for NAASRA Roughness Meters. However, highway speed. profilers are now used for most network level roughness surveys, with ]RI being calculated directly from the profile data, and the calibration equation used to convert the IRI value to an equivalent NAASRA Roughn·ess Count. While IRI or NAASRA roughness should provide a good objective measure of the ride quality experienced by a passenger car occupant,. the profile data should be able to provide additional information relevant to managing a road network. The report explores three other types of information that could be extracted from the profile data: truck ride quality; dynamic lo<Jding of pavements and bridges; and the distress modes contributing to the roughness. Research directions are outlined for each of these. Finally, the report recommends the adoption of the World Bank definition of lane-IRI for profile-based roughness measurement in Australia. ·

ARR 295 October 1996

ISBN 0 869 "10 722 4 ISSN 0518-0728

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ARR 295 Interpretations of road profile-roughness data: Review and research needs .

Contents

1. INTRODUCTION 1

2. A SHORT HISTORY OF ROAD ROUGHNESS 1 2.1 Roughness Mea~urement 1 2.2 The Need for a Profile Statistic- Development of IRI 2

3. ROAD PROFILES AND VEHICLE RESPONSE 3 3.1 Note on Units 3 3.2 The roughness Frequency Domain 3 3.3 Profile Frequency Characteristics and Vehicle Response 4

4. RIDE QUALITY 4 4.1 Vibration and Human comfort 4 4.2 Car Ride 4 4.3 Truck Ride 6 4.4 Research Needs 7·

5. DYNAMIC LOADING OF PAVEMENTS AND BRIDGES 8 5.1 Current Position 8 5.2 Research Needs 9

6. ROUGHNESS AND STRUCTURAL CONDITION 10 6.1 The TRRL High speed Profilometer 10 6.2 Relation to Structural Condition 10 6.3 Research Needs 11

7. ROUGHNESS REPORTING UNITS 11

8. CONCLUSIONS AND RECOMMENDATIONS 12

REFERENCES 13

APPENDIXES 15

ARR 295 Interpretations of road profile-roughness data: Review and research needs

Executive Summary

Road pavement roughness has trarlitionr1lly been regarded as a measure of the ride quality perceived by occupants of passenger cars. It has also been found to provide a . good measure of overall pavement condition and to pr~vide a link variabie for relating vehicle operating costs to pavement condition. For these reasons, and because of the relatively low cost of data collection, road roughness has become the most commonly used measure of pavement condition at the network level. ·

The International Roughness Index (IRI) was originally developed as a time-stable, transportable calibration reference for traditional roughness meters (eg, the NAASRA Roughness Meter). Calibration equations between IRI and NAASRA Roughness Counts have been developed and these are used as the calibration reference for NAASRA roughness measurement.

Calculation of IRI requires measurement of road longitudinal profile. Developments in road profile measurement technology have improved the reliability and affordability of profile measuring systems to the extent that they are being employed for most network-level roughness surveys in Australia: In these cases, IRI is calculated from the measured· profile, and the calibration equation used to convert ·IRI to NAASRA roughness. However, the road profile data may contain additional information which could be useful for managing the road system. ·

As summarised below, the paper discusses possible interpretations of road· profile data other than car ride and addresses the form of IRI to be adopted for reporting purposes.

Truck Ride

While IRI and NAASRA roughness provide good indicators of the ride quality perceived by occupants of passenger cars, they are not necessarily good indicators of the ride perceived by truck drivers, or of possible damage to· goods carried. There is evidence that truck drivers are more sensitive to roughness than car drivers, and anecdotal cases of truck drivers complaining of poor ride on roads having reasonable levels of NAASRA roughness. The question or Lr uck ride is complicate~ by the wide range ot truck suspension and response characteristics and a general move in the industry towards use of air suspensions. ·

There are indications that truck pitch and roll response modes m<;~y be 'important contributors to perceived ride discomfort. These are not included in quarter-vehicle models of truck· ride.

It is recommended that any future research on truck ride should commence with relatively low-cost investigations aimed at establishing truck response modes. contributing to discomfort and the profile spatial frequencies which affect them. Such investigations would include interviews of truck drivers, profile measurement of road sections identified as providing poor ride, and vehicle simulation.

Executive Summary continued

Dynamic Loading

The dynamic component of the loads truck axles apply to road infrastructure increase . ' with increasing road roughness. The IRI for a length of road provides a good indicator ·

of the average dynamic load (as given by a Dynamic Load Coefficient, DLC) applied . over that length of road. However, as both IRI and DLC are average measures for the road length, they cannot be used to identify particular profile features which stimulate suspension response and, hence, dynamic loading ..

Dynamic loads are of particular concern for short-span bridges which are common in Australia.

The OECD Road Transport Program Project IR6 is coordinating an international research effort aimed at quantifying dynamic loads and their impact on infrastructure wear. An NSRP project (N9514 - Road Roughness and Level of pavement Defect) is contributing to this effort, with particular attention to identifying profile teatures on bridge approaches which accentuate dynamic loads applied to the bridge.

It is recommended that consideration of research needs in this area await the results of the OEco· Project.

Road Profile and Structural Condition

Intuitively we cim expect a relation between the spatial frequency components of roughness and causal factors. For example, surfacing failures are likely to contribute to short wave length components whereas reactive subgrades are likely to contribute to long wave length components.

The UK TRRL have established a relationship between increases in short wave length roughness and visually assessed str_uctural condition for asphalt and asphalt over lean mix concrete pavements. However, the criterion recommended for identifying structurally deficient pavements from profile data correspond to very advanced states of deterioration. While .:~ road profile statistic similar to the UK measure of high fre~ quency rqughness could be incorporated into the processing software for the ARRB TR laser profiler, it should be regarded as an indicator of a pavement in near terminal condition rather than a predictor of future structural deficiencies.

Considerable empirical research would be required to relate roughness frequency characteristics to distress modes. It is recommended that any such research commence with a feasibility study to determine whether there are routinely discernible differences in frequency characteristics for pavements exhibiting· different distress modes.

Reporting of IRI

IRI is essentially a single wheel path concept, and some form of averaging is required to apply it to both wheel paths in a lane. The form of lane IRI adopted as a calibration reference for NAASRA roughness was that which correlated best, and is different from that assumed for the World Bank HDM models and related literature. HoVI(ever, the correlation between NAASRA roughness and either form of lane IRI is very high. To avoid future confusion, it is recommended that the HDM form of lane IRI be adopted as the calibration reference for NAASRA roughness and for reporting lane IRI in Australia.

1 Research Report ARR 295

1. INTRODUCTION This report was prepared in response to a number of questions raised by the Austroads Asset Management Reference Group (AMRG) related to the reporting imd interpretation of road roughness (profile) data. The report commences with a brief review of developments in the measurement and interpretation of road roughness, followed by a description of the frequency characteristics of roughness and their relationship to vehicle response. Interpretations of road roughness in terms of ride quality, dynamic loading, and relation to structural distress are then considered.

Three issues were identified for AMRG consideration:

(a) · Possible future research on truck ride and its relation to profile characteristics.

(b) Possible future research on profile characteristics related to modes of pavement failure and distress.

(c) The form of International Roughness Index (IRI) ·to be used for calibration and re.porting · purposes.

2. A SHORT HISTORY OF ROAD ROUGHNESS

2.1 Roughness Measurement

Early efforts at quantifying and measuring road surface roughness, or unevenness as it is referred to in Europe, were aimed at having an objective measure which reflected the ride quality perceiveci-by users of the road. As early as 1930, the Annual Report of the Victorian Country Roads B~ard . described a road roughness measuring instrument fitted to a modified model T Ford.

The relationship between an objective measure of road roughness and subjective ratings of ride quality (or seryiceability) was first formalised in the AASHO Road Test (Carey and Irick 1960). Here, the subjective assessment of "serviceability" as provided by a panel of raters was related to objective measures of surface condition employed for the tests. Slope variance (the objective measure of road roughness employed in the test) accounted for over 80 per cent of the variance in subjective "serviceability"ratings between road sections. The 0 to·S serviceability scale developed by Carey and Irick still provides the basis for quantifying pavement condition in the USA.

Following the AASHO Road Tests, there was considerable interest in dev~loping systems which could measure roughness at highway speeds. These included the PCA and Mays road meters in the USA and the Bump Integrator in the UK. The NAASRA Roughness Meter followed from . developments in the USA. In its basic form, the NAASRA meter is a simple, mechanical device whkh sums the relative motion between the rear axle and body of the vehicle in which it is mounted (Scala and Potter 1977). The development of these affordable, reliable and efficient meters caused roughness to become the predominant measure of road pavement condition at the network level.

All of the above roughness meters are based on a mechanical system which produce relative motion in response to the road surface profile, and a measuring system which aggregates the relative motion. This type of roughness measuring system has become known ~s response-type road roughness meters (RTRRMs). In the case of the NAASRA Meter,.the_ mechanical system is the host vehicle itself ..

At about the same time as these meters were being developed, it became apparent-that measures of- · road roughness provided a link variable to quantify the effects of road surface condition on a number

ARRB Transport Research Ltd


of vehicle operating cost components such as fuel consumption, tyre wear and vehicle repair and maintenance. The pioneering work of Claffey ( 1971) was carried. out without objective roughne55· measurements, but included qualitative;: dt:sl:riptions of the road surfaces to which "rea5onable" roughness values were subsequently assigned. Subsequent vehicle operating cost studies in deveJoping countries, sponsored by The World Bank, employed objective measuresofroad roughness (eg, Hide et al1975).

2.2 The Need For A Profile Statistic - Development of IRI

The RTRRMs were developed in the absence of a physical definition of roughness. In effect, roughness was that which was measured by the roughness meter. Two problems emerged from this absence of a physical definition. The first was calibration. RTRRMs wer~ typically caiibrated against a '!standard" RJRRM of the same type, but there was no way of checking whether there were changes in the response characteristics ofthe "standard". This was of particular concern for NAASRA roughness, where the host vehjcle provided the response characteristics, The second was transferability ofresearch results.on pavem~nt pt:rformance and vehicle operating costs. As each type of RTRRM had its own response characteristics, there was no analytical way of converting roughness measures produced by one type to those produced by another.

. . . #- ' ~ • •• • •

..

The International Roughness lndex (IRI) was developed from the World Bank sponsored International Road Roughness Experiment which was designed to resolve.the above problems (Sayers, Gillespie and Queiroz 1986). The IRI is a road profile statistic which represents the vertical response to the measured road profile ofa hypothetical quarter-car (as represented by a mathematical model) travelling ·at 80 kin/h: In effect, the quarter car model filters out the profile frequency components which have little effect on the ride quality experienced by a ·car, and the response parameters employed in the model define IRI. Prem (1989) gives regression equations which relate NAASRA Roughness to single wheel track and two wheel track formulations ofiRI. The development and basis ofiRI is described further in Appendix B. As ·explained in this Appendix the reference quarter car for com.puting IRI is not intended to be representative of real cars. ·

It was originally envisaged that IRI would be a fully-transportable, machine-independent reference standard for calibrating RTRRMs. The actual profile of roughness calibration sections could be measured either by rod ·and level survey, or by automated road profile measuring systems such as the GM Profilometer. 'rhe mathematically derived IRI values for the measured profiles would then provide the reference against which the outputs of a particular RTRRM could be calibrated. Subseque·nt technological developments have reduced the cost, and increased the reliability, of profilometers to the extent that they are now used directly for network level roughness surveys. In . this case, IRI is computed from the profile data collected in the survey, and a calibration equation employed to convert the IRI values to the equivalent output of the RTRRM which the pi-ofilometer has replaced.

Now we have a situation where detailed data on road longitudinal profile is being collected in routine surveys, but the data is being processed to provide a single output representing the technically constrained outputs of the RTRRMs developed more than 75 years ago. While this has provided improved time stability and accuracy for roughness measures intended to reflect the ride quality experienced by a paSsenger car, the potential for additional outputs frorri the profile data has largely been ignored. Possible additional outputs include:

(a) information on the fr_equency characteristics of the profile;



(b) information on the causal components of the roughness ( eg, surface defects vs reactive subgrade);

(c) measures of the ride quality experienced by trucks; and

(d) similar to (c), measures of the dynamic loading applied by trucks as a result of the pavement profile.

3. ROAD PROFILES AND VEHICLE RESPONSE

3.1 Note On Units

The vehicle dynamics literature refers to suspension frequency characteristics in units of cycles/s, or Hz. The literature relating vehicle response to road surface profile typically uses the unit of wave number (cycles/m) to describe the spatial frequency characteristics of the profile. On the other hand, road managers often conceptualise the frequency characteristics of the profile in terms of the wave length. The relationships between these measures are:

and

where:

f = frequency (Hz);

w = wave number (cycles/m);

v = vehicle speed (m/s);

V = vehicle speed (km/h); and

Lw = wave length (m)

w = f/v = 3.6f/V

For a vehicle travelling at 100 km/h these become:

w = f/27.8

and

Lw = 27.8 ff

3.2 The Roughness Frequency Domain

The PlARC Technical Committee on Surface Characteristics has specified frequency domains for. various surface characteristics as given in Table 1 (Descornet 1990). The frequency nirige for roughness is that which induces relative motion in road vehicle suspension systems over a reasonable range of operating speeds. · ·



TADLE 1.

PIARC SPF.C.TFICATION OF FREQUENCY RANGES FOR ROAD SURFACE CHARACTERISTICS

Surface· Frequency Range Characteristic Wave Length WaveNumber

(cycle/m)

Microtexture <0.5 mm > 2,000

Macrotexture 0.5-50 mm 20- 2,000

Megatexture 50-500 mm 2-20

_Roughness 0.5 ~50 Ill 0.02- 2 - -··

3.3 Profile Frequency Characteristics And Vehicle Response

Appendix A provides a brief summary of frequency characterisation of road profiles and frequency domain analysis of vehicle response to the profile. In essence, the road profile can be characterised as a power spectral density (PSD). This shows the variance in road profile elevation (or slope) as related to spatial frequency.

A vehicle traversing an uneven road surface responds in various ways, including vertical motion, pitch, and roll. However, as a first-order approximation of ride quality, the vehicle can be simplified as a quarter-car (or truck), thus limiting response to vertical motion (Fig. A4). ·The quarter vehicle

- -representation has·two respon~e modes:cbody bounce, atfrequcncies typically around -.I to 4cHz; and 0 c

axle hop, at frequencies around 10 to 18 Hz. The vehicle response to road profile can be modelled in the frequency domain as a response function.

The vehicle response characteristics serve to amplify profile frequencies around the natural frequencies of the response modes, and attenuate profile frequencies well rt:moved from those of the response modes. In mathematical terms, the vehicle frequencyresponse function acts as a multiplier to the input road profile PSD to give the PSD of the vehicle response.

The vehicle response can be represented in different ways, and interpretation of response PSDs requires that they are based on the appropriate response function. For example, the relative displacement response between axle and body is used to define IRl, whereas ride quality is related to the absolute motion response of the body.

4. RIDE QUALITY

4.1 Vibration And Human Comfort

There is an extensive literature on the effects of vibraliuu ou human comfort and fatigue, the findings .from which are summarised in ISO Standard 2631/1 (see SAA 1990). The magnit~des ofthe vibration are typically expre·ssed as accelerations. The ISO Standard implies that, for-vertical vibrations, humans are most sensitive to those in the frequency range 4 to 8Hz, this being the resonant (requency range for the human body. However, there is contention as to the validity of the ISO Standard and the lack of agreement between laboratory studies, on which the standard was based, and reported field studies ( eg, Oborne 1976).



4.2 Car Ride

Relevant Frequency Range

There is evidence that both body bounce and the .vibrations produced by the wheel hop mode are important. factors affecting subjective ride quality. Gillespie et al (1980) cite an unpublished study in which subjective ride quality was related to roughness frequency components. The roughness contained in the spatial frequency range 0.066 to 1.64 cycles/m (1.5 to 37Hz at 80 km/h) correlated most strongly with subjective rating, implying contributions from body bounce mode, wheel hop mode, and vibratory responses at frequencies greater than the wheel hop mode. (The latter could arise from body vibration modes not represented in quarter-car models). From a similar study, Janoff and Hayhoe (1990) report the highest correlations for the range 0.41 to 2.1 cycles/m (9 to 46Hz at 80 km/h), implying contributions from wheel hop mode and higher frequency vibratory response.

Figure 1 shows the body acceleration response PSD of the quarter-car representation of a typical passenger car (Chevrolet Impala), and the Prem quarter truck, for a typical profile PSD. The form of the vertical vibration tolerance relation given in the ISO standard is shown for comparison. As the response PSDs and the vibration tolerance relation have different physical interpretations, they cannot be compared absolutely. However, the relative similarity at both the body bounce and axle hop frequencies imply that both will be important in determining ride comfort. ·

Taken together, the above findings indicate that both body bounce and wheel hop contribute to subjective perceptions of car ride comfort. For highway speeds, this corresponds to profile spatial frequencies in the range 0.05 to 0.7 cycles/m. The existence of higher frequencies in the subjective rating correlation studies suggests that their are other higher frequency noise and vibration modes affecting perceptions of ride quality that are not represented by the quarter-car suspension model. However, given the form of the ISO vibration tolerance relations, these are likely to be more of an irritant than a cause of discomfort.

I

-----~----------------------1-~---c~ \---~-~-----r---

' 0 (/) a. Q) Ill ' c: 0 a. Ill Q)

a:: c:

. ' --------------- -,-- -------- - .. --- -.---- "'------ ~

/ ' ' -------------------~---------------------~--1 '

0 .r· :;:::. ~ Q)

Q) 0

---------- /------------ -.------- -----------/

------- -1.., Truck ---------- ·-------- -·-- --------' /

0 -<t: -- +--------------------------------- -·- :---->- /

"C 0 Ill

0.1 1 10 100 Frequency (Hz)

Fig.l Body acceleration response PSD for passenger car and truck compared with form of ISO vibration tolerance relation.



IRI as a Measure of Car Rido Comfort

lRl is given by the cumulative summation of axlt:-body relative displacement of the reference · quarter~car. Figure 2 compares the PSD of this response for a typical road profile PSD with the body acceleration respons.e for quarter car representations of a typical car and truck. There is reasonable similarity be~een the form of car body acceleration response and that of the IRI response, implying that, overall, IRI should provide a reasonable measure of the ride comfort percei~ed by occupants of passenger cars. However, IRI would underestimate the contributions of the higher freqminc.y wheel· hop vibrations to discomfort:

There is an extensive literature relating subjective rating of ride quality with various measures of road roughness, includi11g IRI, with generally high levels of correlation ( eg, Potter et a! 1992 found · correlations between mean panel rating arid NAASRA roughness with r2 values between 0.78 and 0.94).

4.3 Truck Ride

The Issues

There are two aspects totruck ride:

(a) damage to goo~s; and

(b) driver/occupant ride comfqrt ..

The ~uthor is unaware of any quantitative research relating either of these to road profile roughness, although such work could be reported outside of the tradition11l road research literature. Hence, the following discussion is largely. based; on interpretation of vehicle response functions, anecdotal evidence, and opinion.

Cl (/') a.. Q) 1/) c: 0 .C. ·I/) Q)

0::: C:. 0

~ . Q) Q)

~ >.

"'C 0

CD

--------------~-~---------------------' - ' Truck · I -- -..,.

~=-------- ----------------~----------------- / . -- ... ,;- .... , ...............

- ;_-;.......:.- --- ~ ... ::::-----~---·car~·-.-----:-----------' . . ------------------·------------- --------------- '-~ . . ...

------------------------~----------- -------' ... ...

---------------------------~----------------', IRI ... . .

-------------------------------~------------. - . ... ... ... -----------------------------------~--------

'

0.01 0.1 1 10 Spatial Fre.quency (cycles/m)

Fig. 2. ,Body acceleration response PSD for pas.senger car and truck compared with IRI. quarter car axle-body displacemen_t response.

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The following can be surmised:

(a) For a given road, truck drivers are more sensitive to ride quality than are car drivers. For example, an NRMA survey of users of the Newell Highway found that over 60 per cent of, truck drivers rated the road surface as poor compared with 26 per cent of car drivers. (This is different to the sensitivity to the vibrations actually experienced by the driver. Research conducted at the_ TRRL during the 1970s found that truck drivers tolerated a higher level of vibration than car drivers (Cooper and Young 1980). However, because of different · .SUSpension characteristics, for a given ro~d surface truck drivers experie'}C.e a much higher level ofvertical vibration.) ·

(b) There are instances of truck drivers complaining of particularly poor ride on sections of road - showing reasonable roughness count values. These instances are typically associated with low frequency roughness. ·

(c) · Some industry groups; parlicularly growers of fruit and vegetables, claim thafrough roads result in product dam.age during transport.

Vertical Truck Response

Consistent with the above, the Fig. 1 and 2 comparisons indicate that the vertical ~esponse of a . typical ~ruck to a ro(ld profile is greater than that of a car over the frequency range affecting vibratory comfort, and particularly so at wheel hop frequencies. Moreover, the IRI underprediction ofthe wheel hop vibration contribution to discomfort ~ill be greater for trucks than for cars. However, this does not explain the association of truck ride problems with low frequency roughness.

Other Response Modes

From discussions with people associated with the truck industry, it appears that truck tide problems associated with low frequency roughness may result from pitch and roll response· rriodes .which are excluded in the quarter truck formulation. These are considered to be a particular problem for short wheel base, high centre-of-gravity prime movers. The response frequencies for pitch and roll are 'in· the same range as body bounce ( 1 to 4 Hz).

The ISO standard on vibration exposure is also consistent with pitch· arid roll modes contributing to _truck ride discomfort. Fora driver positioned above the suspension, pitch-and-rolhesponse produces

longitudinal and transverse oscillations. The ISO standard indicates that, relative to vertical vibration, humans are more sensitive to these motions and the frequency range for highest sensitivity

. is in the range· I to 2 Hz:. . · · . . · · IRI as a Measure of Truck Ride

Referring to Fig. 2, as the relative difference between IRI response and vehicle vertical acceleration response i~ greater for trucks than cars, IRI as a measure of vertical-response contributions to ride disc~mfort will be poorer for trucks than fof cars~-

Further investiga_tions oftruckresponse in pitch and roll would be required to assess IRI as a mea,sure of the contributions to ride discomfort from these. response modes ..

4.4 Research Needs . . . .,.

Car Ride_

If we accept thatiRI (or NAASRA roughness) provides a good overall measure of the ride qualitY perceived by car occupants, then the only research question is whether we need to improve our



understanding ofthe relationship between subjective measures ofriut:: yuality and IRI. The current Austroad's view is tha~ this is not reyuired.

Truck Ride

The situation is much more complex for truck ride. We are not sure as to which response modes contripute. to ride discomfort, let alone how well IRI reflects those responses. We also need to consider the ride provided for the goods carried as well as the driver. The issue is further complicated by the wide range of"truck and suspension characteristics, and the general move towards air suspen~ions which is likely to be accelerated by proposals to allow higher mass limits for axle groups fitted with air suspensions (NRTC 1996).

While a panel rating study similar to that adopted for the car Pavement User Rating pilot studies is an obvious approach, such an exercise would be very expensive and may be inconclusive if either the tru<?k or the range of rated profiles were inappropriate for the response modes which most affect ride discomfort. ·It is recommended that any research in this area should start with lower cost investigations which would enable us to home in on the particular profile characteristics which affect truck ride. The following investigations are suggested.

(a) Inter-Views with truck drivers to obtain a better understanding of the responses they perceive as affecting ride ·comfort and, possibly, to identify road sections they regard as providing particularly poor ride. (Interviews are proposed as part ofNSRP Project N9514 -Road Roughness and Level of pavement Defect- with the objective of identifying sites with high dynamic loading. These same interviews may provide some information on perceived ride.)

(b) Profile measurements and detailed analysis for road sections identified by truck drivers as providing particularly poor ride. (Again, some of this may be undertaken as part ofNSRP Project N9514).

(c) Vehicle simulation studies for a range of input profiles.

Similarly, any work on ride for the goods carried should commence with discussion with people in · the packaging industry to learn from their experience regarding critical frequencies and acceleration

levels.

5. DYNAMIC LOADING OF PAVEMENTS AND BRIDGES

5.1 Current Position

The truck response to changes in road profile elevation introduces a dynamic component to the loads truck wheels apply to the riding surface. The magnitude of this dynamic loading. typically expressed as a dynamic loading coefficient (DLC) increases with increasing road roughness and vehicle speed (eg, de Pont 1994, Sweatman 1983). Figure 3 shows an example of results from Sweatman (1983). The DLC is also dependent on suspension type, with air suspensions consistently producing lower DLCs than steel sprung suspensions. Hence, air suspensions are being referred to as road friendly · suspensions.

De Pont (1994) considered various profile statistics aspredictors ofDLC, including statistics like IRI but based on a quarter truck model. However, IRI provided the best overall correlation with measured DLC.



The relationships between suspension types, dynamic loading and road infrastructure wear are being investigated by the OECD Road Transport Research Program. OECD (1992) provides a state-of-art review prepared by an expert group which concludes that dynamic loads have a significant impact on road infrastructure wear, and that there is potential for shifts to road friendly suspensions to reduce this wear. A current OECD project is coordinating research in a number of countries aimed at providing improved quantification of the effects of suspension type on infrastructure wear.

Cebon ( 1993) provides a comprehensive review of the research history and state-of-art of tuck dynamic loads and their effect on road infrastructure wear.

There are current concerns in Australia regarding suspension type and dynamic loading of bridges. While air suspensions typically have lower DLCs and are considered to be less damaging to pavements, they have response frequencies similar to those for short span bridges common in Au!>trnlin. This can result in dynamic coupling betwt:t:u lht: lruck and the bridge which serves to amplify the dynamic load applied to the bridge (Heywood 1995). NSRP Project N9514 (Road Roughness and level of pavement Defect) is aimed at identifying surface defects, particularly on bridges and bridge approaches, which excite suspensio·n response and increase dynamic loads.

5.2 Research Needs

The OECD initiative has generated considerable international research in this area which is bein~ monitored through NSRP Project N9514. The most appropriate course of action m·ay 'be to await the results from the OECD Project and N9514, and then decide if further research is required to make the results relevant to AMRG interests. It is likely that such ~ork would extend the N9514 ·approach of developing maintenance practices to minimise dynamic loads and their effects rather than refining:·· predictions of dynamic loads.'·

-c Q)

"(3

!E 0.2 Q) 0 (.)

"C ro 0

...J 0 ·E o.1 ro c >

CI

---42 km/h ......... 63 km/h

78 km/h

0 +---~---+--~~--+---~---+----~~ 0 50 100 150

NAASRA Roughness (c/km) 200

Fig. 3 Example of results from Sweatman (1983) relating DLC to speed and roughness ·(walking beam suspension RT380).



6. ROUGHNESS AND STRUCTURAL CONDITION Rese~rch at the UK Transport Resear~h Laboratory has demonstrated a relation between change in high frequency roughness and visual assessment of structural condition (Jordan and Cooper 1989). This section of the report examines this rehition in terms of roughness units used in Australia.

s: 1 The TRRL High Speed Profilometer

The TRRL High Speed Profilometer is inco~orated in the High-speed Road Monitor (HRM) (Jordan 1990). The profilometer differs from the more common inertial systems in that it builds the profile from displacements measured at points along a towed longitudinal profile beam. The calculated profile covers the spatial frequency range 0.01 to 5 cycles/m (wavelength of0.2 to 100m).

Roughness is rep~rted as pro~le variances about a moving average base length, with values for the latter typically ~eing 3, 5 10 and 20m. The moving average base length serves to filter out longer wavelength profile. components. There are reaso.nable, non-linear correlations between IRI and variances about the short~r 3 and 5 m base lengths. Appendix C gives approximate conversion relations between profile variance about the 3 m base and IRI and NAASRA roughness.

6.2 Relation To Structural Condition

Jordan and Cooper (1989) found a relation between visual assessment of pavement condition and . '

rel~tive increase in_ profile variance about~ ,3 m base length for 400 test sections comprising asphalt and asp_halt over le~ ~oncr~te pavement types. The. results, presented in terms of changes over a 2 year period to reflect the frequency of condition surveys, are summarised in Table'2. A relative change in profile variance of 120 per cent etfectively separated pavements which were satisfactory from those which were exhibiting surface manifestations of structural distress. It was recommended that this value be used as a screen which could be applied to routinely collected HRM condition data to identify sections of the network warranting more detailed investigation.

TABLE2.

RELATION BETWEEN TWO YEAR RELATIVE CHANGE IN PROFILE VARIANCE AND VISUAL ASSESSMENT

Rei. change in Visual Assessment

3 m base profile Category Description . variance (%)

<60 1 sound

60 to 120 2 fine cracks/some fretting

120 to 240 3 dey«?loped cracks/crazing/fretting

>240 4 wide cracks/spalling/area crazing

To better appreciate the UK relation, the 120 per cent incre~se in profile variance was converted to equivalent increases in NAASRA Roughness Counts for near new (NRC = 50), worn (NRC= 75) and tired (NRC= 100) pavements, with the results given in Table 3. Given that the rate of deterioration

ARRB Transport Research .Ltd


for Australian arterial roads is typically 1.5 NAASRA counts per year, the roughness increases corresponding to the UK criterion represent substantial levels of deterioration which would normally be associated with a pavement approaching its terminal state. While there would be some benefit in identifying such sections from routine condition data, one would expect that, at the operational level, they would have already been identified by other means.

TABLE3.

NAASRA ROUGHNESS EQUIVALENTS OF A 120 PER CENT INCREASE IN 3 M BASE PROFILE VARIANCE

Initial roughness Roughness after Equiv. annual 2 years increase

50 68 9

75 105 15

100 141 20

6.3 Research Needs

Given the forms in which structural distress is manifested for the pavement types considered in the UK study, the strong correlation with high frequency roughness can be expected .. While this correlation will probably not apply to other pavement types or failure modes, intuitively we can expect that different failure modes will result in roughness increases in different frequency ranges. For Australian granular, chip sealed pavements, we can expect surfacing failures to produce high frequency roughness, subgrade reactions to produce low frequency roughness, and base failures to produce roughness of a frequency in between. However, considerable empirical research would be required to establish the frequency ranges and reasonable levels for roughness increases. Hence it is recommended that any research in this area commence with a feasibility study to determine whether there are discernible differences in roughness frequency characteristics for pavements exhibiting different distress modes.

At a much lower level of effort, a high frequency roughness statistic, similar to the TRRL 3 m moving average base, could be incorporated into the post-processing software for the ARRB TR profilometer.

7. ROUGHNESS REPORTING UNITS As explained in Appendix B, the lam: IRI as calculated by the standard processing software for the ARRB TR laser profilometer as a calibration standard for NAASRA roughness is different from the lam: IRI assumed for the World Bank HDM mo'dels and'related literature. This was not a concern when the profilometer was being used soleiy to c~librate NAASRA roughness vehicles. However, now that profilometers are being used for most netWork ro,ughness surveys, and some. agencies are considering reporting roughness in IRI units, there is potential for future confusion.

The form of Jane IRI currently employed in for the ARRB profilometer was originally chosen on the basis of providing the best calibration reference for NAASRA roughness. However, as shown in


12 Research Report ARR- 295

Table B 1, the difference in the correlation between NAASRA roughness and the two forms of lane IRI is marginal, with both having r2 values sufficiently high as to imply a one-to~o~e correspondence between NAASRA Roughness Coun~s and either forin of lane IRI;

In order to avoid future confusion regarding the fonrt of lane IRI, it would be appropriate to modify the ARRB profilometer processing software to use the World Bank form of lane IRI.

8. 'CONCLUSIONS AND RECOMMENDATIONS Over the past 15 years, there has been a merging of road engineering and vehicle engineering interests in measuring ~nd characterising road profiles. From this, IRI has emerged as a fully transportable and time-:stable reference for-calibrating roughness meters. With the development of affordable and reliable highway-speed profiling systems, many network level roughness surveys are now being carried out by pro"filometers with roughness recorded in IRI units.

·. .

There is a very strong correlation between NAASRA Roughness Counts and two forms of Jane IRI. The form of Jane IRI currently employed in the ARRB TR profilometer software as a calibration reference for NAASRA roughness is different from that assumed for the World Bank HDM models and associated literature. Possible confusion arising from these two forms of hme IRI could be averted by adopting the World Bank form of lane IRI as the calibration reference with only a marginal difference in the accuracy of translating between Jane IRI and NAASRA roughness.

IRI,-and hence NAASRA roughness,. provid~s a good measqre oftheride quality perceived by car . occupants. However, it is likely that it provides a.poorer measure of truck ride quality. Jn particular, truck ride is likely to be affected by pitch and roll response as well as vertical motion ..

IRI, and hence NAASRA rough~es~, provides-a good' measure of the ove~all increase in the .. dynamic loading applied to the riding. surface by tru~k axles with increasing roughness: However, as both are average measures for a length of road, they cannot-be us~d to identify ipe_cific. elem~enjs. which excite truck suspension response and subseq'l;lent peak dynamic loads ..

While research in. the UK has demonstrated a relation betwe_en increase in high frequency roughness and stnictu~al dlstre~s f~r asph~lt.pavements, the leve-ls of distress identified are such that. the rel~tion must be regarded as in indicator of structural failure rather than a predictor of imminent failure. It is likely that there are further relations between profile frequency characteristics and other failure and distress m~des. . - . . ·

It is recommended that AMRG:

(a)

(b)

(c) .

agree to adopting the World Bank form of lane IRI as the calibration reference for NAASRA roughness and request that ARRB tR mo-dify the-laser profilometer proc~ssi'tl.g·s~ftware

. accordingly; .. '

consider the need for research into the relation-betWeen truck ride and IRI and, if necessary, · the development of a profile statistic which better reflects truck ride; and· . . . . . '

co~sider the ne~d for r~~earch. into relation~ betw·e~n profiie frequency ch~racteri~tics ~~d different pavement failure and distress modes.


•..

Research Report ARR 295

REFERENCES CAREY, ·W.N. and IRICK P.E. (1960). The pavement serviceability-performance concept. Highway Research Board Bulletin, 250, pp.40-58.

CEBON, D. (1993). Interaction between heavy vehicles and roads. The39th L. Ray Buckendale Lecture. (SAE: Pennsylvania).

CLAFFEY, P.J. (1971). Running costs of motor vehicles as affected by road de!)ign ancl traffic. National Cooperative I~igJzway Research Prugrum Report 111.

COOPER, D. and YOUNG, J. (1980). Road surface irregularity and vehicle ride Part 3 ~ Riding comfort in coaches and heavy goods vehicles. TRRL Report SR 560.

DE PONT, J. (1994). Road profile characterisation. Transit New Zealand Research Report No. 29.

DESCORNET, G. (1990). Reference rand surfaces for vehicle testing. Roads PIARC, No.272.

GILLESPIE, T., SAYERS, M. and SEGEL, L. (1980). Calibration of response-type road roughness measuring systems. National Cooperative Highway Research Program Report 228.

GILLESPIE, T. et al (1993). Effects of heavy-vehicle characteristics on pavement response and performance. National Cooperative Highway Research Program Report 353.

GRIFFIN, M. (1990). Handbook of Human Vibration. (Academic Press: London)

HEYWOOD, R. (1995). 'Road-friendly' suspensions and short span bridges. In: K. Sharpe (ed.) Dynamic Interaction of Vehicles and Infrastructure, Proceedings of DIVINE Special Session: 17th ARRB Conf., pp.4I-65.

HIDE, H., ABA YNA Y AKA, S. W., SAYER, I and WYATT, R.J. (1975). The Kenya road transport cost study: Research on vehicle operating costs. Transport and Road Research Laboratory,

Laboratory Report 672.

JANOFF, M. and HA YHOE, G. (1990). The development of a simple instrument for measuring pavement roughness and predicting pavement rideability. In Meyer, Wand Reichert J. (Eds). Surface Characteristics of Roadways: International Research and Technologies. ASTM, STP 1031, pp 171-183 .

.JORDAN,-P. (1990).- Equipmentand-methods forassessing surface-characteristics of a road-network.· In Meyer, Wand Reichert J. (Eds). Surface Characteristics of Roadways: International Research and Technologies. ASTM, STP I 031, pp.l84-197.

JORDAN, P. and COOPER, D. (1989). Road profile deterioration as an indicator of structural

condition. TRRL Research Report 183.

NATIONAL ROAD TRANSPORT COMMISSION (1996). Mass Limit"s Review Report & Recommendations of the Steering Committee. (NRTC: Melbourne)

OECD (1992). Dynamic Loading of pavements. (OECD: Paris)

ORB ORNE, D.J. ( 1976). A critical assessment of studies relating whole-body vibration to passenger

comfort. Ergonomics, 19(6), pp.751-774.

POTTER, D. et al (1992). An investigation of car users' perception of ride quality of roads. Road & Transport Research, 1 (1 ), pp.6-26.

PREM, H. (1987). Road roughness influence on suspension performance. Proc. Symposium on Heavy Vehicle Suspension Characteristics, 23-25 March, 1987, Canberra, pp.365-389. (ARRB:

Melbourne)



SAYERS, M.W., GILLESPIE, T.D. and QUEIROZ,C.A.V. (1986). The international ro::td roughness experiment: establishing correlation and a calibration standard for measurements. World Bank Technical Paper 45.

SCALA, A.J. and POTTER, D.W. (1977). Ivlt:asurt::tllt::IIL uf JUaJ toughness. Australian Road Research Board, Technical Manual ATM I.

STANDARDS ASSOCIATION OF AUSTRALIA (1990). Evaluation of human exposure to whole~ body vibration, Part 1: General requirements. Australian Standard AS 2670.1.

SWEATMAN, P. (1983). A study of dynamic wheel forces in axle group suspensions of heavy vehicles. ARRB Special Report SR 27. - ·


I I I .

I

i


APPENDIX A

PROFILE FREQUENCY CHARACTERISTICS AND VEHICLE RESPONSE

Spatial Frequency Components Of Roughness

Spectral analysis can be employed to determine the frequency components of a road surface profile, with the results expressed as a Power Spectral Density (PSD) function• . The area under the PSD of, say, profile elevation within a frequency band w+~w is the variance in profile elevation attributable to frequency components within that band.

To complicate the picture further, two different forms of road profile PSD are commonly reported in the literature:

(a) The PSD of elevation, or displacement; and

(b) the PSD of slope.

As an example, Fig AI shows the profiles measured on a section of road prior to, and after, the application of an overlay, while Figs A2 and A3 show the elevation and slope PSDs for these profiles

Slope PSD is preferred for inspecting details ofthe road surface PSD (eg, growth of roughness within a particular frequency band. There is a simple transformation between the two forms ofPSD. The slope PSD is the differential of the displacement PSD, and the mathematics are such that:

Slope PSD(w) = Disp. PSD(w) x (2nw)2 (AI)

where w is the spatial frequency (cycles/m).

While spectral analysis of road surface profiles is relatively new to road engineering, it has been employed by automotive engineers for several decades. Here, characteristic road surface PSDs are employed for suspension design and testing. One simple characterisation of elevation PSD translates to a straight line on the typical log-log presentation, and this forms the basis for part of a draft ISO Standard for reporting road profiles. With this characterisation, the slope of the PSD line remains constant, but its position shifts upward with increasing roughness. With this characterisation, the slope PSD is a constant.

Other research suggests that the straight line representation of road profile PSD is overly simplistic. Figure A4 shows characteristic PSDs developed for recent research in the USA (Gillespie et at 1993). These indicate that, while the PSDs increase across the whote frequency range with increasing roughness, the increase is not uniform for all frequencies. For Australian granular pavements with chip seal, Prem (1987) reported increases in PSD at around 0.4 cycles/m for rough pavements .

• The term Power Spectral Density reflects the early communications engineering genesis of spectral analysis. Spectral analysis was applied to an electrical sig~al: the area under the PSD function within a frequency band had the units ofpower and represented the signal power in that frequency band.



w .... t .. uo Hwy (!i) near Lllllmur, 416.2 to 417 km

0.25

0.20

0.15

E 0.10 c 0

iii ~ 0.05

0.00

-o.os

100 200 300 600 /UO BUU

Distance (m)

1-Before Rel1atBlitHI.iu1 -Ane.-Reh~

Fig. Al Profiles for a road section before and after overlay.

Western Hwy (5) near Lillimur, 416.2 to 417 km

1E+OO

1E.01 ~

" 1E-02 .!!

F 1E.03 g 0

"' ... 1E-04 ' ::---- .......

...... , .......

' c

i 1E-05

j\

""' ... .!! 0 1E-o6

I ~ i'--1\- :

1E.07

1E.08

'~ ~ 'r.l~ ~ l 'i I I I -~ N I

'I

I ! I I

0.001 0.01 0.1

SJN!Ual P1tftllm1~J ILr~hra/ui)

1-Before Rehallililalion -After Rehabilitation 1

Fig. A2 Elevation PSDs for the Fi~. Al profiles.

I .0

·-~ 8. 0

1E-04

1E.OS

iO 1E-o6

1E.07 0.001

" "I/

....... ... Y


Western Hwy (5) near Llllimur, 416.2 to 417 km

~~I I

Jl I I

-.......... .......... ,/ fll ! I

~ I , v I r

i ~ ""'-,- \ ·~ n :

~ I ~

J .,

v ~ ....... v

0.01 0.1

Spatial Frequency (cycleslm)

1--Before Rehabilitation After RehabiUlation I

Fig. A3 Slope PSDs for the Fig. Al profiles.

10

10


0.001

0.0001

0 (fJ

~ 1E-05 c. 0

U5 1E-06

1E-07

[---------------------------------------

r ~~------------ -...... _____________ _

- - - -i- - 1- -I- 1- .. 1-1-1 t - - - + - -I- 1 -.1-1-1-1 + 1- - - - 1- - .. - .... -11 +

0.01 0.1 1 Spatial Frequency (cycle/m)

10

1- Smooth AC . Rough AC . - -. Smooth PCC - Rough PCC

Fig. A4 Gillespie et al (1993) characteristic slope PSDs

Vehicle Respons~ Characteristics

A vehicle traversing an uneven road surface responds in various ways, including vertical motion, pitch, and roll. However, as a first-order approximation of ride quality, the vehicle can be simplified as a quarter-car (or truck), thus limiting response to vertical motion (Fig. AS). The quarter-vehicle has two response modes:

(a) low frequency movement of the vehicle body bouncing on the suspension (referred to as body bounce or heave mode); and

(b) higher frequency wheel oscillation with the.unsprung mass bouncing between the suspension and the springing provided by the tyrc (referred to as wheel hop mode). ------- ~-

With the road profile as input, three displacement response functions can be defined for the quartervehicle: ·

(a) displacement of the body;

(b) displacement of the axle; and

(c) relative displacement between axle and body.

The latter best captures the effects of both body bounce and wheel hop modes.



displacement of I the sprung +

massz 5

sprung mass m

5

.. linear~ spring k

5

-------1 unsprung mass ·

displacement of ~ mu the unsprung . ·

ma:3:3 z u

linear damper c

.s

profile input ·1-b-- flx~d contact length b

y (x)

Fig. AS Quarter-vehicle representation of respoo,se to. uneven road surfa.ce.

For trucks, body bounce frequencies ar'e typicaliy in the range 1 to 4 Hz, which for a truck speed of 100 krri/h translate's to a spatial frequency of 0.035 to 0.14 cycles/in, or wavelengths of 7 to 29 m. With current suspension designs, the lower part ofthis frequency range is associated with air.bag · suspensions 1 .•. Wheel hop frequencies are typically in the range 10 to 18Hz, which for 100 km/h

~~·~=tran·slates~tb~0:35~to-"0~65~cycles/m~or-1"Sto~3~m. -~~~~~=~~~~--=~-~~ -----

·Figure A6 compares the· body and axle-body displacement frequency response· functions for quartervehicle models representing a car (Chevrolet Impala model parameters given in Gillespie et al 1980) and a steel spring suspended truck (model parameters given in Prem 1987).

It should be noted that the car response characteristics are not an accident, but have evolved over time to minimise occupant discomfort. Humans are particularly sens.itive t9.vertical oscillatory motion in the frequency ranges 4 to 8Hz (vibratory discomfort) and 0.1 to 0.5 Hz (motion sickness) (Griffin 1990). As seats typically attenuate vibrations at frequencies above about 6 Hz, we can expectcar suspensions to be designed to minimise· body motion in the frequency ranges;O. ~ to 0.5 Hz and 4 to 8Hz. Until recently, howev~r, truck suspensions were largely designe~ for robustness and minimal cost.

A response frequency of less than 1.5 Hz is one of the criteria used in the EEC definition of road friendly suspension. The definition was designed to differentiate between damped air bag suspensions and conventional steel spring suspensions.

ARRB Transport Research Ltd··


7 ~------------------------------------------~

6 . !:

~ 5 c: o·· :u 4 c: ::1

LL. 3 Q) 1/J c: &. 2 1/J Q)

0::: 1

____________________ f\_ __ -I Body Response ~ _________ _

.II · .. ___________________ J4--------------------------

' 1 'Truck --------------~----L-

I

-------------- ----~-~--------------~----------Car 1 I

- - -.- - - - - --;- - - -;'- - - - ~ -. ~ - - -- -- - - - - - - - - - - ------

------~-----------------------

' ...... 0 +----+--+-+-~++~--~~~~~-9·A-~'~--+--+-+~~~


4 ~------------------------------------------~

c:: ·n; 3 <.9 . c:: 0

u § 2 u.. Q) !/)

c:: 0

~1 Q)

0::

·1 Axle-Body Response j

• Truck I \

--,-l-------1

- .... I

0 +--=~~+-~rT++T...-~--~~~~~HH+----+~~~~~~


Fig. A6 Comparison of quarter-yehicle model displacement response functions for typical car and truck.

ARRB .Transport Re.search Ltd


Vehicle Response To Profile

The PSD of the vehicle response to a profile PSD is given by:

Response PSD(f) = [Response Function Gain(f)]2 >< Elevation PSD(f)

This is shown graphically in Fig. A 7 for the body displacement response of the quarter car representation of a Chevrolet Impala travelling at 100 km/h.

0 Ul a..

~ ~

Elevation PSD

~,

0.1

= X= =· = = = = = = = = = = -----~------------------- ------------------- ------------------- -

1 10 Frequency (Hz)

100

X

0.1

[Resp. Fn Gain]2

10 100 F,.,.,..,cy{HZ)

0 Ul a.. :ll

~ ., a:

0.1

Response PSD

1 10 Frequency (Hz)

}lig. A 7 Development of body displacement response PSD.


(A2)

100


APPENDIX 8

BASIS OF IRI

Reference Vehicl.e

While the International Roughness Index (IRI) was developed from the World Bank sponsored International Road Roughness Experiment (Sayers et a! 1986), the theoretical underpinning came from an earlier NCHRP Project (Gillespie eta! 1980). This earlier study produced a very similar roughness statistic, referred to as the i-ISRI Reference, using the same quarter-car model as is employed for IRI.

The objective behind the HSRI Reference and IRI was to develop a roughness statistic which would provide a time-stable calibratiun rt:ft:renct: for conventional response-type road roughness meters (RTRRMs) .. The Gillespie eta! study sought a statistic which correlated well with the outputs of RTRRMs, and this was not necessarily the best representation of real vehicles or perceived ride quality.

As most of the existing RTRRMs were based on measuring the relative displacement between a sprungmass (body) and unsprung mass (axle), Gillespie eta! decided that the calibration statistic should be based on a quarter-vehicle representation of the same. Three formulations of the quartervehicle were employed:

(a) quarter-vehicle representation of a typical car (Chevrolet Impala);

(b) a hypothetical quarter-car with very stiff suspension damping; and

(c) a band pass filter which provided a flat response over the frequency range of interest.

Figure B 1 compares the axle-body response functions for these fqrmulations.

3.5 '

3 c:

·ro 2.5 C)

c: 0

+= 2 C) c: ::I u.

1.5 Q) (/) c:

I I I

-----------~11---------------~--------------

l I I I ------------,-,-------------t-,-------------

1 \ I \ ---------------------------~----------------1 \ I

I \ I I ---------------- ---------t I \

0 1 a. (/) Q)

0::: 0.5

I -------------- r--------------- ------------/ . i ~ ...... \

~

I ,' -------- -,~-, -..-

0 ,~ ...

----;::.....


1- IRI quarter car _ .Impala - _ . Band pass filter I



Fig. Dl Axle-body· response functions for the qunrtcr~vchiclc formulations considered~ by GiH~spi~ P.t al (19RO).

The hypothetical quarter-car was chos~n as the reference vehicle (or, more correctly, the reference formulation) because it provided the best overall correlations with the RTRRMs considered in the study (r2 between 0.90 and 0.96). While these correlations were only marginally better than those for the band pass filter (r2 between 0.87 and 0.96), they were substantially better than those for the quarter-car representation of the Impala (r2 between 0.80 and 0.90). .

The poor correlations for .the Impala formulation were attributed to the "peaky" frequency response, and the tendency for the vehicle to tune into roughness spectral components corresponding to the body bounce and wheel hop modes. While this is a characteristic of real vehicles, the mode frequencies are vehicle dependent, so that slightly ditlerent vehicles will respond quite differently to a given ruall profile.

Quarter-Car IRI

IRI is defined as the accumulated axle-body displacement response of the reference vehicle to a single wheel track road profile. This can be readily computed for a known profile. JRI is usually repo~ed in units of mlkm, but inches/mile is still used in parts of North America. Figure B2 shows the axle-body displacement (or IRI) response PSD of the reference quarter car for a typical road profile PSD.

Lane IRI

Quarter-car IRI is not directly applicable to RT~s which measure the combined effects of roughness in both wheel paths, or to profilometers which measure profile in each wheel path. The so called lane IRI (also referred to as half-car IRI) was developed so·that the IRI concept could be applied to two wheel path roughness measurement. Two different methods have evolved which has resulted in some confusion.

ARRB Transport· Research Ltd


0 C/) c.. Ql en

.c:: 0 c. en Ql a:: a:: . ·~.

-------------~--~--------~----- ----------------------------------------------- --------. .

0.01 0.1 1 10 Spatial Frequency (cycles/m)

Fig. B2 Axle-body displacement response PSD of the IRI reference qu~rter car for a typical road profile PSD.

Method 1: Take a point-by-point average of the road profile elevation in each wheel path as input to the quarter-car model.

Method 2: Compute a quarter-car IRI for each wheel path and average to give lane IRI.

Method I provides the better representation of RTRRMs that respond to roughness in both wheel paths (eg, roughness measuring car or towed two wheel trailer) It was first proposed by Gillespie et al (1980) as a calibration reference for those RTRRMs. It also provides the current reference standard for NAASRA roughness (see below).

The World Bank HDM-3 model assumes that lane IRI is derived from Method 2. This method was considered to provide a better measure of overall pavement condition and to relate more closely to the relationship between direct vehicle operating costs and surface condition.

Relationship Between Naasra Roughness And IRI.

In developing an IRI-based calibration standard for NAASRA roughness, Prem (1989) developed regression relationships between the NAASRA roughness counts from the, then, standard roughness vehicle and quarter-car IRI and both forms of lane IRI. These are summarised in Table B 1. Prem recommended that the Method 1 lane IRI be used as the calibration reference for NAASRA roughness on the basis of both the better physical representation of the response of a roughness

· measuring car and better r2 value.



TABLEBl.

REGRESSION RELATIONS BETWEEN NAASRA ROUGHNESS COUNTS (NRC) AND V ARlO US FORMS OF IRI

Form ofiRI Relations r2

OWP quarter car IRI ~ -0.009 + 0.042 NRC 0.955 NRC = -3.47 + 27.50 IRI- 0.557 IRJ2

Lane Method 1 IRI = 0.072 + 0.030 NRC 0.994 NRC = -1.95 + 33.67 IRI

Lane Method 2 IRI = 0.037 + 0.037 NRC· 0.990 NRC = -1.27 + 26.49 NRC

ARRB ·-r:ransport Res_earch· Ltd ·


APPENDIX C

RELATIONS BETWEEN UK PROFILE VARIANCE, IRI, AND NAASRA ROUGHNESS

· The UK High-speed Road Monitor (HRM) produces absolute measures of single wheel path road profile in the spatial frequency range 0.01 to 5 cycles/m. Roughness is reported as profile variance about a moving average base length, typically 3, 5, I 0 or 20m. Variances about the shorter base lengths have been found to correlate reasonably with IRI.

Jordan (1990) gives a scatter.plot of IRI against the square root of the profile variance about a 3 m base length. An eyeball fit to this data gave the form,

(Cl)

where Var3 is the profile variance about a 3m moving average base (mm2).

The IRI given by eqn (Cl) is for a single (outer) wheel path, and the quarter-car outer wheel path regression relations in Table B I are appropriate for translating between this and NAASRA Roughness Counts.


Latest Publications Research Report No. 279

Review and enhancement of vehicle operating cost models Thorolf Thoresen and Ron Roper

Research Report No. 282 A review of existing pavement performance relationships Tim C. Martin

Research Report No. 283 Rural pavement improvement prediction due to rehabilitation Tim C. Martin and Euan D. Ramsay

Research Report No. 284 The electronic accident report Dr. David Andreassen and Simon Cusack

Research Report No. 288 Oinder removal from pavement samples: evaluation of the iynitiun uvtm and a modified bitumen recovery procedure John Oliver, Stephen Co/grave and Peter Tredrea .

Research Report No. 289 The change in properties of PMBs with simulated field exposure John Oliver and Peter Tredrea

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APRG Report No. 14/Research Report No. 275 Installation and commissioning of a pavement heating system for the Accelerated Loading Facility .Jim clohnson-Giarke and Oa!tid Fossey

APRG Report No. 15/Research Report No. 281 Performance of cement-stabilised flyash under accelerated loading: the Eraring ALF Trial 1995 Geoffrey Jameson, Richard Yeo, Kieran Sharp and Nicholas Vertessy

APRG Report No. 16/Research Report No. 286 Performance of unbound and stabilised pavement materials under loading: summary report of Beerburrum II ALF trial B. T. Vuong, K.G. Sharp, E.J. Baran, N.J. Vertessy, J.R. Johnson-Clarke and I.N. Reeves

~ APRG Report No. 17/Research Report No. 287 ~ Rut-resistant properties of asphalt mixes under accelerated loading: final summary ~ report ~ K. G. Sharp, G. W Jameson, J. W H. Oliver, N.J. Vertessy, J. R. Johnson-Clarke and !;i A.J. Alderson

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