DETERIORATION MODELS FOR INSERVICE FLEXIBLE PAVEMENTS IN INDIA

by

B. B. REDDYResearch Fellow

A. VEERARAGAVANAssociate Professor

Dept. of Civil Engineering (UVCE)Bangalore University

Bangalore - 56India

Paper prepared for presentationat the PAVEMENTS Session

of the 1997 XIIIth IRF World MeetingToronto, Ontario, Canada

ACKNOWLEDGMENTS

The work reported in the paper is part of research projects on `Benkelman beam DeflectionStudies', sponsored by the Ministry of Surface Transport, and `Development of DeteriorationModels for Flexible Pavements', sponsored by Ministry of Human Resource Development,Government of India. The data collected under these projects have been utilised in the presentinvestigation. The authors thank the National Highway departments of the states of AndhraPradesh, Karnataka, Kerala and Tamilnadu for their cooperation and help during datacollection. The help rendered by Prof. C.E.G. Justo, Bangalore University, during the analysisis greatfully acknowledged.

ABSTRACTWith the growing interest in the development of maintenance management models for optimumutilisation of available resources in developing countries, to decide the most economicalstrategies based on flow of funds, one of the most essential inputs is development ofdeterioration models for structural and functional conditions of flexible pavements. This paperdescribes the development of deterioration models for the prediction of deflection growth, rutdepth progression, crack area progression, unevenness(roughness) growth and the variation inriding comfort with increase in unevenness of the pavement surface. The deflection histories ofoverlaid flexible pavements have been analysed to find the appropriate stabilised deflectionvalue which has been used as a strength parameter in the deterioration modelling. The presentinvestigation has been presented in few key elements viz., need for flexible pavementdeterioration models, performance studies on flexible pavements, strength parameter foroverlaid flexible pavements, structural condition deterioration models and functional conditiondeterioration models.

INTRODUCTION

The road and traffic conditions in developing countries are distinctly different from developedcountries. The development of roads in developing countries is undertaken in a phasedmanner. The funds available for maintaining the roads, in traffic worthy conditions are notadequate and no tools are available for making the inputs into maintenance managementplanning in a scientific manner. A road pavement suffers progressive structural deteriorationfrom the day it is opened to traffic. Structural deterioration manifests itself in terms ofdeformation and cracking. In case of in-service pavements, the undulations and ruts are foundto develop excessively along the wheel paths and they progressively increase in magnitude dueto the combined effects of traffic loads, climatic and environmental factors, inherent defects inthe quality control during construction and improper maintenance measures during service life.The riding quality of vast majority of road length in India is found to be very poor. The ridingquality is unsatisfactory on a large portions of road sections even on the primary road systemconsisting of National and State highways.

The functional performance of a pavement is found to be directly related to its structuralcondition. A strong pavement may not show signs of distress in the form of wide cracks, rutsetc. whereas a weak pavement will exhibit signs of failure in the form of ruts, wide cracks,poor riding quality etc. There is a need to periodically evaluate the structural and functionaldeterioration of the flexible pavements and to study the causative factors. Any loss in pavementstrength is found to have adverse impact on the road user cost. In view of the importance ofsurface undulations mentioned above, it is essential to measure quantitatively the surfaceundulations including rut depths and cracking of bituminous pavement surface at suitableintervals. The flexible pavement deterioration can be predicted mainly through the followingtypes of response parameters viz., deflection due to the applied load (as related to structuraladequacy), rutting and cracking of pavement surface, unevenness (as related to serviceability orriding comfort).NEED FOR FLEXIBLE PAVEMENT DETERIORATION MODELS

Pavement Maintenance Management System(PMMS) based on the concept of totaltransportation cost model is one of the important topics on which studies are in progress indeveloping and as well as developed countries. A successful pavement management systemrequires structural and functional evaluation at regular intervals and suitable steps for the designand construction of overlays of required thickness at the appropriate time before allowing anyof the pavement layers to be excessively distressed. The application of formal managementsystems to the maintenance of road networks, and the application of economic criteria for theevaluation of appropriate standards, alternative maintenance policies and pricing of road usehave created demands for reliable, well quantified and validated means for predicting roaddeterioration.

Vehicle operation cost rapidly outpace the cost of road repair as the condition of roadpasses from good to fair to poor. As vehicle operation costs reduce with improvement inpavement unevenness and serviceability, total returns in terms of savings in road user costs forunit investment in road improvement would be higher for the highly trafficked roads. Thisstrategy requires a systems approach comprising regular monitoring of performance ofpavement stretches on road networks, development of pavement deterioration models, analysisof life cycle cost of different pavement maintenance strategies and development of maintenancemanagement models. The economic evaluation of any road rehabilitation or maintenancestrategy has to take pavement undulations into account and hence, the model for the predictionof unevenness need to be developed. The main applications of deterioration models inpavement management system are to:

* decide useful service life of pavement

* evaluate and decide various alternative design and rehabilitation strategies

* estimate remaining effective life of pavement

* determine life cycle cost of road pavements

Pavement evaluation studies leading to development of pavement condition deteriorationmodels will yield the decision as to when and where the improvements are needed to extend thelife of pavement. Hence there is a need for the development of pavement condition deteriorationmodels .

PAVEMENT PERFORMANCE STUDIES

Test sections of flexible pavements carrying varying traffic were selected with severalsub-sections having various types and thickness values of bituminous layers on Nationalhighway network in India. Overlay construction of pavement sections was done as a part ofR-6 research scheme sponsored by the Ministry of Surface Transport(MOST), Govt. ofIndia(2). Two types of overlay materials such as Bituminous Macadam plus BituminousConcrete (BM+BC) of 75 to 125 mm thick and Bituminous Macadam plus PremixCarpet(BM+PC) of 70 to 120 mm thick were used in the overlay construction. Extensive fieldstudies were carried out on these test sections under varying conditions of loading rate bysystematically monitoring for a period of over ten years. The structural condition evaluationwas in terms of rebound deflection using Benkelman beam(5), rut depth was measured by 3 mstraight edge and crack area measurements were made on deflection observation points;functional condition evaluation was in terms of unevenness(roughness) measurements usingBump Integrator, riding rating over 0-5 point scale by a panel of raters selected across the roaduser population(11). The crack area was evaluated by measuring length and width of cracksfound around deflection observation points within one sq.m area. Additional studies like trafficvolume data collection, wheel load studies using a portable wheel weigh bridge designed anddeveloped at the Department of Civil Engineering, Bangalore University, Bangalore andtransverse placement of wheel loads have been conducted(9,10). Periodic condition surveys oneach experimental overlaid pavement stretch generated detailed records of above performanceindicators.

MODEL FORM AND PARAMETERS

In life cycle prediction phase of Pavement Management System(PMS) or in pricing of road use,the models need to predict the expected condition in future over a given period of time or under

the transit of extra axle loads, when the current condition is known. The model formconsidered in the present study is a deterministic as shown below:

Future condition = { present condition, pavement strength, incremental traffic and agecharacteristics, and climate}

The data collected from the test stretches for different performance and influencingparameters were analysed. The performance condition of pavement has been represented interms of characteristic rebound deflection ( average plus standard deviation ) corrected forpavement temperature and subgrade soil moisture (9), Unevenness Index(UI) in mm/km andRiding Comfort Index(RCI) which is an average value of ride ratings from the panel of raters,crack area (%) and rut depth (mm). Vehicle Damage Factors (VDF) were determined using thedata on wheel loads of commercial traffic (buses and trucks) and AASHO load equivalencyfactors. Transverse Distribution Factors (TDF) were calculated as a percentage of therepetitions of standard axle load along the outer wheel path, based on the data on transversedistribution of commercial vehicle traffic. The data on traffic volume (commercial vehicles perday), traffic growth factor of 7.5%, values of VDF and TDF, have been employed to determinethe cumulative standard axles (CSA) of 8.2 tonnes along the outer wheel path at any time.Pavement age since last renewal/strengthening to different performance observation cycles wasobtained.

STRENGTH PARAMETER FOR OVERLAID FLEXIBLE PAVEMENTS

In order to study the structural behaviour of overlaid flexible pavements in terms of Benkelmanbeam rebound deflection, it is necessary to establish a relationship between the reduction indeflection due to the construction of overlays of various thicknesses and types and also thedeflection-performance of the overlaid pavements. A flexible pavement which has been inservice for a reasonable period may be assumed to function as an elastic layer system. Thus itfollows that there will be an elastic deflection of the pavement surface under a wheel load,which will be followed by an elastic recovery or rebound when the load is removed or movedforward. The magnitude of the deflection or the elastic rebound of a flexible pavement due to awheel load depends on the structural stability of the pavement system and also on themagnitude of the load.

It is a conventional practice in all the pavement deterioration models developed in othercountries(3,7,12,13) and in India(1) to consider the structural number(13) derived fromAASHO road test results as the parameter representing the pavement strength. Highwaymaterials and construction practices in India are very different from AASHO test conditions.Hence in the absence of pavement layer equivalencies for Indian conditions, the surfacerebound deflection under a standard wheel load has been considered as a parameterrepresenting the pavement structural strength. The field data suggest that the rebound deflectionof a pavement after the construction of overlay starts decreasing under appropriate rates andstabilises after certain period. This is called the "Stabilisation period". Subsequently thedeflection increases as the traffic load repetitions increase. This stage has been defined asinitiation of structural condition deterioration. Decreasing trend of deflection at initial stagedepends on the deflection state before overlay construction, type and thickness of overlay, rateof traffic loading etc. The deflection data collected were analysed and trend curves were drawnfor different pavement sections. The conceptual trend in deflection growth of a overlaidpavement is as shown in Fig. 1 . The deflection state at the stabilization period in turn indicatesthe stabilised deflection value (initial deflection, `iDEF') of pavement after construction of theoverlay. In the present study the value of `iDEF' has been considered as the strength parameter.The data collected were analysed and equations were developed to find the initial deflection`iDEF' and the initiation of structural deterioration (stabilisation period, Ni) in terms ofcumulative standard axles, as given below.

To Find Initial(Stabilised) deflectioni) BM+PC Surface

(bDEF - iDEF) = 0.2169 log[ (H*CSAYR)bDEF ] - 0.824 .... (1)R2 =0.64, SE =0.27

ii) BM+BC Surface(bDEF - iDEF) = 0.2995 + 0.1169 log[ (H*CSAYR)bDEF ] .... (2)R2 =0.53, SE =0.21

To Find Initiation of Structural Deterioration

Ni = 1.6715 [ ( bDEF - iDEF ) 0.565 csayr] .... (3)R2 = 0.89, SE = 0.42

Where, bDEF = deflection(mm) before overlay construction, iDEF = initial deflection (mm), H = overlay thickness (mm), CSAYR = cumulative standard axles per year (millions)STRUCTURAL CONDITION DETERIORATION MODELS

Deflection Growth ModelsDeflection, a measure of pavement behaviour, represents an immediate response to load.Although deflection is not a measure of pavement deterioration, extensive engineeringexperience has shown that it influences the rate of pavement deterioration. Therefore, it isuseful to predict deflections after an overlay has been constructed. An attempt has been made inthe present study to obtain the relation between rebound deflection and cumulative standard axleload repetitions. Rebound deflection data collected at different ages were grouped into fourcategories based on the initial deflection ranges. Cumulative standard axles calculated at thecorresponding ages were used in the statistical analysis. The resulted significant model formsare presented in Table 1. The deflection growth of flexible pavements with different initialdeflection values (iDEF = 0.5, 0.7, 0.9 and 1.15 mm ) and a traffic loading of 3000 standardaxles per day(S/d) has been shown in Fig. 2.Rut Depth Progression ModelsThe rut depth is not found to affects the riding quality significantly. This may be because of thereason that in developing countries, due to lateral wander of commercial vehicles in the absenceof lane discipline, the rut depth values seldom exceeds 15 mm. However the rut depth isgenerally accepted as a measure of pavement distress due to permanent deformation of thesubgrade and pavement layers caused by the repeated application of wheel loads along theconcentrated wheel paths. Therefore the magnitude of rut depth is considered to represent theextent of structural deterioration of the flexible pavements, even to the extent of deciding theterminal pavement condition before resorting to strengthening of pavements. In the presentstudy the average values of rut depths measured at deflection observation points were correlatedwith cumulative standard axles. The resulted model forms which are found to be significantare presented in Table 2. The rut depth progression from different initial rut depths of 3, 4, 5and 6 mm and an initial deflection of 0.75 mm, have been shown in Fig. 3 and Fig. 4respectively for (BM+PC) and (BM+BC) surfaced pavements.

TABLE 1 DEFLECTION GROWTH MODELS

iDEF Range(mm) Model Form R2 SE 0.44 < iDEF < 0.61 Dt = iDEF + 0.07884[(Nt * Age)iDEF ] 0.92 0.11 0.66 < iDEF < 0.8 Dt = iDEF + 0.0027 exp [(iDEF * Nt)iDEF ] +

0.0859(Age) 0.69 0.29

0.84 < iDEF < 1.05 Dt = iDEF + 0.04513 (exp Nt)0.45 + 0.0924 (exp Age)log iDEF

0.82 0.82

1.10 < iDEF < 1.25 Dt = iDEF + 0.03658[exp (iDEF * Nt)]0.5 +0.19864 (Age)0.26

0.82 0.2

Here, Dt = corrected characteristic rebound deflection(mm) at any time `t', iDEF = initialdeflection(mm), Nt = cumulative standard axles(millions) at `t', Age = age of pavement at `t',years.

Crack Area Progression ModelsCrack in the form of map is a defect in the bituminous pavement surface which weakens thepavement structure. Cracking of flexible pavement surface is considered to significantly affectfurther deterioration of the pavement due to ingress of water from the surface during rains and aconsequent deterioration of the pavement component due to the combined action of traffic andwater under this weakened pavement condition. Therefore pavement cracking may beconsidered mainly as a possible measure of pavement deterioration for the overall evaluation ofpavement. Development of alligator crack at the top of the bituminous surface course is anindication of the structural failure of the pavement layers.

In the present study the crack area determined in one sq.m area on deflectionobservation points considering different widths of cracks, were correlated with cumulativestandard axle repetitions. The model forms found to be significant are as given in Table 2.Crack area progression in (BM+PC) and (BM+BC) surfaced pavements with different initialcrack areas of 1, 2, 3 and 4% and an initial deflection of 0.75 mm are shown in Fig. 5 and Fig.6 respectively.

TABLE 2 RUT DEPTH AND CRACK AREA PROGRESSION MODELS

MODEL TYPE MODEL FORM R2 SERUT DEPTHPROGRESSION

(BM+PC) surfaced pavementsRdt = iRD [1+0.461(iDEF * Nt)0.62 + 0.1817 log(Nt)]

0.90.5

(BM+BC) surfaced pavements Rdt = iRD [1+0.1794(Nt * iDEF)1.05 + 0.0289(Nt)]

0.80.3

CRACK AREAPROGRESSION

(BM+PC) surfaced pavementsCAt = iCA [ 1+ 0.7436 (iDEF * Nt)0.32 + 0.0054exp(Nt)]

0.8 0.9

(BM+BC) surfaced pavements CAt = iCA [ 1+ 1.49(iDEF * Nt)0.15 + 0.0547 exp(Nt)]

0.72.9

Here, Rdt = rut depth(mm) at time 't', iRD = initial rut depth(mm), iDEF = initialdeflection(mm), CAt = progressed crack area in percent at time 't, iCA = initial crack area(%),Nt = cumulative standard axles in millions at time 't'.

FUNCTIONAL CONDITION DETERIORATION MODELS

Riding Comfort Index(RCI) ModelThe condition or performance of a pavement is generally determined by some formal ratingsystem, the complexity of which varies with the needs and experience of the agency using theinformation. For quick evaluation of the riding quality of long stretches of flexible highwaypavements, it may not be practicable to measure the classified cracked area, rut depth and thearea of pot holes etc., along with unevenness. Therefore for making a reasonably dependableevaluation of pavements in a relatively short time it will be desirable to determine the ridingquality in terms of RCI based on UI value alone which takes into account overall riding qualityof the pavement surface.

In view of the above discussion, it was decided to develop RCI model considering onlyone major factor affecting the riding quality of pavement, measured in terms of UI. The testsections of length 1074 km selected for the development of RCI model has UI values rangingfrom 1930 to 15880 mm/km( 2.7 to 17.5 m/km of IRI) and the corresponding mean RCIvalues ranging from 4.5 to 0.1. The best correlation between the two parameters was obtainedand is given in Table 3. The variation of riding comfort index with unevenness index is asshown in Fig. 7.

Unevenness Progression ModelOne of the primary functional characteristics of a road pavement is the level of service itprovides to the users. Road pavement unevenness is highly correlated with serviceability andis the principal measurement of pavement condition. The importance of unevenness is not justrestricted to its effects on the riding comfort of the road user, but also it is the principal measureof pavement condition, directly related to the vehicle operation cost. Predicting the trend ofunevenness progression over the life cycle of a road pavement is undoubtedly the most criticalof the various pavement performance predictions. Keeping in view the importance ofunevenness deterioration model to study the functional behaviour of flexible pavements, amathematical model has been developed. The rebound deflection values measured under astandard wheel load using a Benkelman beam has been considered as a parameter to representthe structural condition of the flexible pavement. An equation was developed for predicting theunevenness index based on initial unevenness index, cumulative standard axles, age andstructural condition of pavement in terms of rebound deflection. The resulted model formfound to be significant is as presented in Table 3. Unevenness progression in flexiblepavements having initial unevenness of 3000 mm/km, different structural condition values(deflection of 0.5, 0.7, 0.9 and 1.1 mm), with a traffic loading of 3000 S/d is shown in Fig. 8.

TABLE 3 FUNCTIONAL CONDITION DETERIORATION MODELS

MODEL TYPE MODEL FORM R2 SERCI MODEL RCIt = 18.8135 - 1.93113 log (UIt) 0.94 0.21UI GROWTH MODEL UIt = iUI [ 1 + 0.3012 ( Nt * DEFo)0.08 Age

]0.70 0.21

Here, RCIt = Riding comfort index at any time `t', UIt = Unevenness index(mm/km) at anytime `t', iUI = Initial Unevenness Index (mm/km), DEFo = Deflection(mm) at the time ofiUI, Nt = cumulative standard axles in millions at time 't', Age = age of the pavement(years).

The criteria that have been considered in the selection of structural condition and functionalcondition model forms is their ability to satisfy the initial and possibly the end-of-life boundaryconditions of data considered in the present study, in addition to yielding a reasonably lowstandard error of estimate.

SUMMARY AND CONCLUSIONS

Pavements deteriorate with time due to the combined influence of traffic loads, climate andenvironment and the knowledge of these factors is essential in order to make reliablepredictions of the future performance. Modelling pavement deterioration is an essential activityof the PMS. The models play a crucial role in several aspects of the PMS, including financialplanning and budgeting as well as pavement design and life-cycle economic analysis. Keepingin view the importance of deterioration models for Indian condition to develop maintenancemanagement system, simplified prediction models were developed for structural and functionalcondition of flexible pavements. These models are based on the initial condition of respectiveparameter, initial strength of pavement and the increment in traffic loading and age.

The models developed based on the data collected from inservice flexible pavements representsthe behaviour of pavements during the study period of its life time. Therefore, the variablequantifying the initial structural condition of the pavement viz. initial deflection has beenconsidered in the development of present models. Width of the cracking which indicatesseverity of distress level has been considered in the development of crack area models. Theinitial deflection as determined in the present study can be used as a strength parameter in themodelling of pavement performance.

The models designed to forecast pavement changes, are based essentially on statistical laws.These are derived from observations and measurements on different types of pavements inservice; therefore, they reflect actual traffic and climatic condition. The models developed forRCI and UI growth can be used to study the functional behaviour of inservice flexiblepavements. The values of UI at any critical RCI values can be determined, from the RCImodel. The UI growth model is useful to estimate the unevenness after an anticipated trafficloading/ age, from an initial unevenness and deflection so that the increment in vehicleoperation cost of various pavements can be evaluated.

The developed models are simple and can be used to estimate the structural as well as functionalbehaviour of flexible pavements after anticipated traffic loading and to find the allowable trafficloading at different limiting values of deflection, rut depth, crack area, RCI and UI. Thus thephasing of maintenance/rehabilitation activities can be planned scientifically.

REFERENCES

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