4.STATE-OF-THE-ART FLEXIBLE PAVEMENT DESIGN IN INDIA – IRC:37–2001[5]: To overcome limitations and empiricism in pavement design as discussed in paragraphs 1.1, 2 and 3, attempts were made under the patronage of Ministry of Road Transport and Highways (MORT&H), Government of India via Research Schemes R-6, R-19 and R-56 which gave birth to IRC:37–2001 and thus, laid down the foundation for Mechanistic-Empirical Pavement Design Method (MEPDM) for flexible pavement designs in India and open a new chapter in the history of pavement designs with ample scopes for further improvements and refinements in future. Salient features of the IRC:37–2001 are briefly described below: Only conventional standard flexible pavement structure as shown in figure 1 has been considered for pavement design, which has been modeled as a three layer structure consisting of binder layer (BM or DBM) plus surface layer (PC, MSS, SDBC, or BC) as layer 1, granular sub-base layer (GSB) plus base layer (WBM or WMM) as layer 2, and compacted subgrade as layer 3. After taking (i) a typical fixed value of elastic modulus (E1) at average annual pavement temperature of 35 0C and Poisson’s ratio (μ1) of 0.50 for bituminous layers having DBM/BC constructed with 60/70 grade bitumen, (ii) μ2 = 0.40 for granular layers and a restricted composite elastic modulus of sub-base and base course (E2) determined by the empirical equation 3(a) and, (iii) μ3 = 0.40 for subgrade layer and elastic modulus of subgrade (E3 ) determined empirically from the index property, CBR value through equation 3(b) and 3(c), the flexible pavement structures were analyzed by ‘FPAVE’ software. where, E2 (MPa) h E3 (MPa) = E3 * 0.20 * h0.45, = thickness of granular layers, mm = 10 * CBR for CBR ≤ 5, and = 17.6 * (CBR)0.64 for CBR > 5 …(3a) …(3b) …(3c) 4.1. 4.2. The pavement responses, in terms of the critical strains [(a) vertical compressing strain (εc) at the top of the subgrade – to avoid excessive strain and hence, permanent deformation (or rutting) in subgrade layer during design life, and (b) horizontal tensile strain (εt) at the bottom of the bituminous layers – to avoid the bottom-up fatigue cracking] at pre-defined locations, have been computed using the linear elastic model “FPAVE” developed under MORT&H’s Research Scheme R56 “Analytical Design of Flexible Pavements”. Rutting within the bituminous layer(s) was avoided or controlled by meeting the mix design requirements as per the MORT&H’s Specifications. These strains were then, used to predict the performance level as defined in terms of two classical modes of structural distresses namely bottom-up fatigue (alligator) cracking and rutting in subgrade layer resulting from repeated (cyclic) application of traffic loads as per the following two failure criterions which ensure a specified level of pavement performance at the end of design life. 4.3.1 Fatigue Criteria: The distress prediction model was calibrated to develop the following fatigue cracking failure criterion which relates allowable number of load repetitions (the fatigue life of the pavement) to horizontal tensile strain at the bottom of the bituminous layer (εt) for a pre-defined performance level (as considered in the form of fatigue cracking in 20% of the design lane area). 4.3. * Superintending Engineer & Regional officer, M/o RT&H, Regional Office (C), Bengaluru (India) –560001. Email –
[email protected],
[email protected] However.) in which it was developed.5 3 3 7 …(5) 4. design (failure) criterion. Further. And.2 1 x 1 0 −4 1 εt 3 . it is not known to the designer what will happen or in what way will he analysis and design the pavement structure.5. 4.3. Neither FPAVE nor the analysis and design approach is available in public domain either for free or some cost.1. an average annual pavement temperature of 35 0C. to optimize the design.2 N f = 2 .5% becomes 150. material quality. and climatic conditions etc. these design catalogue or tables were applicable only for a fixed set of conditions namely a standard flexible pavement structure as shown in figure 1. It becomes difficult for a designer to design the pavement structure for an expressways which will carry traffic volume certainly more than 3000 CVD as IRC:37–2001 is applicable only for cumulative design traffic up to 150 msa. it gives one feasible solution only leaving no scope for the designer to optimize the pavement structure economically or in terms of material consumption and/or quality as available at site. CBR value ranging from 2% to 10% and c.8 5 4 …(4) in which. if any of these variables will vary? Absence of any pavement design software is the biggest difficulty as a designer is unable to perform the analysis and design the pavement structure with user-defined (or project-specific) input variables and thus. Design life for flexible pavements needs to be enhanced to 30 to 40 years in line with practices in USA and Europe.4. Pavement design catalogue as outlined in IRC:37–2001 provide one of the easiest method in the world to design the flexible pavement on the basis of the Mechanistic-Empirical Pavement Design philosophy. traffic volume and loading.3 msa for initial traffic volume after project construction of 2825 CVD only. material properties of bituminous mixture.8 9 1 E 0 . for a given set of traffic volume and subgrade strength. the approach suggested in IRC:37–2001 for dealing traffic more than 150 msa needs to be reviewed as IRC:81–1997 was based on empirical method which has very limited applicability due to changes in the conditions (such as pavement structure. sub-grade material characterized as before in terms of index property. construction methods. for limiting the permanent deformation in subgrade layer up to 20 mm. 4. and annual average pavement temperature (35 0C) as pointed out also in para 4. for most of the projects constructed under BOT model or PPP model or any similar financing model with usual range of concessioner period of 25 to 30 years. the rutting failure criterion relates allowable number of cumulative standard axles (Nr) to vertical compressive strain (εc) at the top of the subgrade layer as: N r = 4 . Nf is the allowable number of load repetitions to control fatigue cracking and E is the effective elastic modulus of all bituminous layers.6. Therefore. b. Consequently. . the current IRC guidelines are unable to provide an optimal pavement design which commensurate with service life. Similarly. 4. the pavement design tables or catalogues for the conventional standard flexible pavement structure in terms of total pavement thickness and constituent layer thickness were developed to cater for: a.5 and traffic growth rate of 7. Cumulative traffic for 20 years design period and two lane highways with vehicle damage factor of 4. design traffic (evaluated as before by equation 2 except with slight modification in vehicle damage factor) ranging from 1 msa to 150 msa.1 6 5 6 x 1 0 −8 1 εc 4 .2 Rutting Criteria: Similarly. N x = { } r where. then Nx = 42. then • A = 225 CVD.49 msa and T = 697 mm.5% and traffic after construction.42 msa and T = 500 mm. . then • A = 730 CVD. then Nx = 4.25 msa and T = 635 mm. then • A = 730 CVD. Nx = 38. A = 225 CVD. then • A = 225 CVD. A = 730 CVD. from which it is clearly evident that pavement thickness for a pavement structure designed as per IRC:37–2001 increased by 13% to 23. . how much level of pavement performance or service life can be ensured? Design Life 10 years • • • 15 years • • • Table 2 Pavement design details for Example 3 Pavement Design as per Pavement Design as per IRC:37–1984 (F-2. However. then • A = 2183 CVD. then • A = 2183 CVD.24 msa and T = 738 mm. 730 CVD and 2183 CVD. traffic growth rate.75. mm 365* A * (1 + r ) − 1 * D * F x Design traffic (in msa). then Nx = 7. for a two lane NH/SH.04 msa and T = 718 mm.35 msa and T = 585 mm.40 msa and T = 460 mm. then Nx = 23. A = 730 CVD.75) IRC:37–2001 (F-4.7% over the pavement design as per IRC:37–1984 primarily to account for the increased share of heavy axle loads and ensuring some certainty in pavement performance against two classical modes of pavement failure i. Design details are given in table 2.78 msa and T = 540 mm. then Nx = 2. subgrade CBR = 5%. Nx = 3. Total pavement thickness = T.72 msa and T = 668 mm. then Nx = 14. r = 7.e. D = 0. A = 2183 CVD. Nx = 7.92 msa and T = 553 mm. bottom-up fatigue cracking and subgrade rutting. in view of current developments it is questionable whether such enhancement in pavement thickness is justifiable? Will it lead to overdesign? For a given pavement design. A = 225 CVD. Nx = 70.92 msa and T = 665 mm. lane distribution factor.3 Example 3: Given that.24 msa and T = 616 mm. Nx = 23.50) A = 225 CVD. Nx = 12. Solution: Let. A = 2183 CVD. Design flexible pavement for 10 and 15 years for two lane NH in plain terrain as per IRC:37-1984 and IRC:37-2001.