Dated 30.10.2015.Sub: Need for adopting a unified, integrated and comprehensive

pavement design procedure/software in order to obtain an economical, safe, durable and sustainable pavement structure in the country – reg.

A lot of developments in the field of pavement design and performance have taken place outside India during last 20 years which has given an insight into continued improvement in traffic characterization, material characterization and quality, mix designs, pavement design approaches and construction methodologies, performance prediction models and laboratory testing procedures, and maintenance approaches etc. Consequently, in India also, flexible or rigid pavement structures are designing using the Mechanistic-Empirical pavement design approach either as per IRC:37-2001/2012 or IRC:58-2002/2011/2015. Mechanistic-Empirical pavement design (MEPD) approach consist the following essential components:

a. Pavement structure and its Structural (Mathematical) Modelling,b. Traffic and Environmental loading,c. Material properties characterisation,d. Pavement (mechanical) response at critical locations,e. Pavement Distress model, critical pavement response and failure

criterion,f.Pavement Distress estimation and accumulation (Transfer functions),g. Performance prediction,h. Maintenance approaches, i.Life cycle cost analysis, andj.Pavement design strategy selection.

2. Now, time has come to adopt an integrated and unified approach to design a pavement structure for all types of new or reconstructed pavements as well as rehabilitated pavements irrespective of materials used in constituent layers besides precisely describing all aforementioned components of Mechanistic-Empirical pavement design approach. Till date, we are far behind to achieve this goal as no systematic and inclusive approach is adopted so far to frame proper pavement design guideline based on adequate lab research supported by field performance data. 3. A pavement structure need to be designed so that:

a. it must be structurally and functionally adequate during entire design life,

b. it must survive the pre-defined performance level at the end of its design life as per designer’s chosen certainty (reliability) level,

c. it should integrate design process with maintenance strategies, d. it should be constructible and use available local material to extent

possible, ande. it should result an optimal and sustainable pavement structure –

optimize the pavement thickness and material consumption, minimize the initial cost as well as life-cycle cost, and improve the pavement sustainability.

4. It is suggested that the following methodology need to be adopted:a. Pavement Structure and its Mathematical Modeling: Layered

elastic model is used in IRC guidelines. However, to consider the effect of temperature and traffic load rate variation on constituent layers, these layers should be divided into sub-layers. Pavement distress in

critical sub-layer should be considered to determine the defined critical pavement responses for distress calculation and accumulation.

b. Characterization of Traffic: Axle load spectra method should be used as it can be easily determined from the axle weight data obtained from weigh-in-motion (WIM) station or else which is now-a-days readily available. These spectra represent the percentage of the total axle applications within each load interval for single, tandem, tridem, and quad axles. Vehicle class distributions, daily traffic volume, and axle load distributions define the number of repetitions of each axle load group at each load level. For a given load group, the damage caused by each load, on each axle type, and under each climate condition during the year is simulated over the design life of the pavement.

c. Effective characterization of pavement materials: Dynamic modulus of bituminous materials and the resilient modulus of bound/unbound materials as a function of time and environmental influences over the entire design period and duly account for the variation in applied stress state, pavement depth etc. need to be considered to compute the pavement responses.

d. Environmental loading: Two main environmental variables namely moisture level and temperature changes need to be considered adequately to capture their impact on the strength, durability, load carrying capacity, service life and serviceability of the pavement structures.

e. Distress model (Surface-down and Bottom-up fatigue cracking; Permanent deformation (rutting) in bituminous layers, in base/sub-base layers or in the subgrade; fatigue fracture in chemically stabilized layers; thermal cracking; joint faulting, and slab cracking; smoothness; skid resistance etc.), Critical Pavement Responses (tensile horizontal strain at the bottom of lowest bituminous layer for bottom-up fatigue cracking; tensile horizontal stresses/strains at the bottom of dry lean concrete or stabilized base layer for bottom-up fatigue cracking; compressive vertical stresses/strains within bituminous layer for bituminous rutting failure; compressive vertical stresses/strains within base/ sub-base layer for base/sub-base rutting failure; compressive vertical stresses/strains at the top of subgrade layer (for subgrade rutting failure) etc.) and Failure criterion for each distress model need to be clearly spelt out in the guidelines after accounting for different service levels and importance of pavements on highway.

f. Pavement Performance and its Prediction: Pavement performance includes consideration of functional performance, structural performance, and safety. - Structural distress indicators includes fatigue (load-induced and thermal) cracking and rutting (in all layers) for flexible pavements, and joint faulting, and slab cracking for jointed plain concrete pavements. The functional performance of a pavement concerns how well the pavement serves the highway user. Riding comfort or ride quality and skid resistance (or surface friction) are the two dominant characteristic of functional performance. Riding comfort is quantified in terms of smoothness as express by International Roughness Index (IRI) which combines the effects of initial pavement/subgrade condition with the distresses developed over time.

g. Damage Accumulation Procedure: Accumulation of damage over the entire design period is a function of time, traffic and climate. Incremental Damage Accumulation Procedure should be used. - Attempts will be made to simulate how pavement damage occurs in nature, incrementally, load by load, over continuous time periods. To achieve this goal, design life is

divided into shorter design analysis periods or increments beginning with the traffic opening month. Within each increment (or analysis period), all factors (traffic and material characterization) that affect pavement responses and damage are held constant for simplification and computational speed. Critical pavement responses (stress and/or strain values) for each distress type are determined for each analysis increment and thereafter, are converted into incremental distresses either in absolute terms (e.g., incremental rut depth) or in terms of a damage index (e.g., fatigue cracking) by the distress prediction model. Incremental distresses and/or damage are summed over all increments and output at the end of each analysis period is used by the designer to compute the accumulated distress and later on, to compare them with respective design criteria for each distress.

h. Life cycle cost analysis (LCCA): LCCA is a tool to determine the most cost-effective and feasible pavement design alternatives to build and maintain them by analyzing initial costs and discounted future cost, such as pavement construction, maintenance, rehabilitation, and reconstruction cost as well as administrative cost to perform all these activities over the useful service life of pavement structure, and salvage value (all are grouped under Agency Costs); and User Costs (includes vehicle operating costs (VOC), crash costs, and user delay costs). - In view of huge investment made in India in highway infrastructures during forthcoming period and to ensure the best value of invested public money, LCCA is becoming the most inevitable component of pavement design process and should therefore, be made part of design process in India also. LCCA can also be used to evaluate the overall long-term economic efficiency between competing alternative investment options. Procedure for LCCA should be well defined in IRC guidelines.Development of integrated and comprehensive design procedure as well

as software is the essence of pavement design without which it is almost impossible to consider the variability of input parameters and thus, to optimize the pavement design based on LCCA and other considerations. After adopting above procedure, it will be possible to design an optimal and sustainable pavement structure based on indigenously developed design method which must not only be structurally and functionally adequate during entire design life but also survive the pre-defined performance level at the end of its design life as per designer’s chosen reliability level with minimum life-cycle cost. Improved design methodologies and treatment strategies for the preservation, repair and rehabilitation of existing pavements may also be evolved. A National long-term pavement performance and maintenance database should be established which will act as a knowledge base for future refinements of pavement design process.