Scholarly article on topic 'Perpetual Pavement – A Boon for the Indian Roads'

Perpetual Pavement – A Boon for the Indian Roads Academic research paper on "Civil engineering"

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Abstract of research paper on Civil engineering, author of scientific article — Chandan Basu, Atasi Das, Pavanaram Thirumalasetty, Tanmay Das

Abstract Dwindling resources and increasing cost of new construction and maintenance are prompting the highway agencies and concessionaires to step beyond the conventional and look for value engineered options for pavement type selection involving alternative design and pavement materials. Balancing short- and long-term performance with direct and life-cycle costs using optimum aggregate quantity for high-volume traffic corridor of multi-lane highway tends to be a challenging task. Thus, an engineered full-depth asphalt pavement is proposed as a sustainable perpetual solution to heavy traffic and the best socio- economic interest. A total of three scenarios depending on different levels of traffic or laning and different categories of subgrade strength are studied in this paper. Value Engineering is incorporated via life-cycle cost analysis. Life-cycle cost is considered to include the initial or direct cost of construction, cost of rehabilitation/reconstruction, and cost of potential maintenance regime. This paper studies a total of seven types of pavement including conventional, semi-rigid, recycled white- top and rigid pavement sections along with their perpetual counterparts followed by respective life cycle cost analyses. Both initial construction and life cycle costs do reflect their dependence on the type of the pavement, the traffic category and cost of materials in the particular project area. At the same time, with the progressive ban on mining, consideration of future availability of aggregates is emphasized in the paper. The paper aims in spreading awareness that a holistic approach considering all pavement types in conjunction with life cycle cost analyses and availability of aggregate materials shall be duly considered as a socio-economical option for our future highway projects.

Academic research paper on topic "Perpetual Pavement – A Boon for the Indian Roads"

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Social and Behavioral Sciences

Procedia - Social and Behavioral Sciences 104 (2013) 139 - 148

2nd Conference of Transportation Research Group of India (2nd CTRG)

Perpetual Pavement - A Boon for the Indian Roads

Chandan Basu, Atasi Das1, Pavanaram Thirumalasetty, Tanmay Das

Intercontinental Consultants & Technocrats Pvt. Ltd., A-8 Green Park, New Delhi 110016, India

Abstract

Dwindling resources and increasing cost of new construction and maintenance are prompting the highway agencies and concessionaires to step beyond the conventional and look for value engineered options for pavement type selection involving alternative design and pavement materials. Balancing short- and long-term performance with direct and life-cycle costs using optimum aggregate quantity for high-volume traffic corridor of multi-lane highway tends to be a challenging task. Thus, an engineered full-depth asphalt pavement is proposed as a sustainable perpetual solution to heavy traffic and the best socioeconomic interest. A total of three scenarios depending on different levels of traffic or laning and different categories of subgrade strength are studied in this paper. Value Engineering is incorporated via life-cycle cost analysis. Life-cycle cost is considered to include the initial or direct cost of construction, cost of rehabilitation / reconstruction, and cost of potential maintenance regime. This paper studies a total of seven types of pavement including conventional, semi-rigid, recycled white-top and rigid pavement sections along with their perpetual counterparts followed by respective life cycle cost analyses. Both initial construction and life cycle costs do reflect their dependence on the type of the pavement, the traffic category and cost of materials in the particular project area. At the same time, with the progressive ban on mining, consideration of future availability of aggregates is emphasized in the paper. The paper aims in spreading awareness that a holistic approach considering all pavement types in conjunction with life cycle cost analyses and availability of aggregate materials shall be duly considered as a socio-economical option for our future highway projects.

© 2013 The Authors. Published by Elsevier Ltd.

Selectionand peer-reviewunder responsibilityoflnternationalScientificCommittee. Keywords: Semi-rigid; rigid; perpetual pavement; RAP; aggregate savings; life cycle cost analyses

1. Introduction

Expressways and highways have made road transportation much faster and efficient in India. Massive projects are underway to expand the highway network with the addition of 18,637 km of expressways by the year 2022. To build hundreds of kilometers of roadway undoubtedly involves colossal use of natural resources. The ban on

* Corresponding author. Tel.: +91-011-40863000; fax: +91-011-26855252. E-mail address: atasid@gmail.com

1877-0428 © 2013 The Authors. Published by Elsevier Ltd.

Selection and peer-review under responsibility of International Scientific Committee. doi: 10.1016/j.sbspro.2013.11.106

mining in several states and the increasing cost of bitumen used in flexible pavements is having an exorbitant upward implication in the total project cost and future maintenance. Finding quality construction material within reasonable vicinity of the project road is increasingly becoming a hurdle in meeting the ambitious target of highway construction in India. Thus, it is time to revisit the concept of "perpetual pavement" and find its applicability and potentiality in the Indian context as a means of sustainable solution.

Traditionally, conventional flexible pavements for the National highways and expressways are designed for a 20-year life, whereas perpetual pavements (PP) are expected to perform for 50 years or more. A perpetual pavement in this study considers a full-depth asphalt pavement designed and built to last 50 years or more without requiring major structural rehabilitation or reconstruction. With perpetual pavements, the potential for traditional fatigue cracking is reduced, and pavement distress is typically confined to the upper layer of the structure. Thus, when surface distress reaches a critical level, an economical rehabilitation required is the removal of the distressed surface layer and resurfacing with an asphalt overlay.

Perpetual pavements have been around for a long time in the western countries. But, due to lack of appropriate codal provisions, there was a gap in our understanding of the design of this pavement. However, with the publish of the latest guidelines by the Indian Roads Congress (IRC 37-2012), the design to satisfy 50 year design periods can be adopted with mechanistic pavement design methodology, appropriate materials selection to improve rutting and fatigue resistance, and prediction of performance. Yet, with economic melt-down, the option of constructing a sustainable or perpetual pavement with full-depth asphalt involving higher initial investment expenditure than conventional would appear luxurious. But, the perpetual pavement outweighs the conventional pavement section in terms of savings in aggregates.

This paper aims at studying the potential perpetual and non-conventional pavement types applicable for highways and compares with the conventional types in terms of the economics of construction. To meet this objective, three scenarios of 2-lane, 4-lane and 6-lane highways are envisaged and hypothetical but, realistic design parameters adopted. With the current demanding rates of construction, the life cycle cost of conventional and perpetual pavements are worked out. The common belief of perpetual pavements being a clear-cut winner in terms of life cycle cost is analyzed to be misfit in this current study. The paper figures out the system that is socio-economically preferred for the Indian roads.

1.1. Objectives and Scope

The specific objectives of this study are to:

• Determine optimal perpetual pavement structure using the design philosophy of IRC 37-2012 to develop and evaluate design alternatives based on pavement layer, stiffness, and thickness for various levels of traffic and various strengths of subgrade; also, to perform a life-cycle cost analyses (LCCA) on the alternative designs;

• Examine the effect of using treated base and subbase and unbound subbase and recycled (emulsified) base with that of conventional perpetual; and

• Examine the best suited option for the best suited traffic category and the strength of subgrade.

1.2. Highlights of IRC 37-2012

The design guidelines apply to new flexible pavements for Expressways, National Highways, State Highways, Major District Roads and other categories of roads carrying predominantly motorized vehicles. Flexible pavements include bituminous surfacing over granular bases and sub-bases, cementitious bases and sub-bases, reclaimed asphalt pavement with foamed bitumen or bitumen emulsion, and perpetual pavement. The guidelines stress on better performing pavements with the use of rut and fatigue resistant surface and bottom bituminous layers respectively.

2. Project Conception

To meet the above objectives, a conceptual project road with various levels of traffic is envisaged and hypothetical yet, realistic design parameters adopted. There are a total of seven pavement types that are studied for the project road under various levels of traffic and lane configuration. These options are (i) conventional flexible Type 1, (ii) flexible pavement incorporating a semi-rigid layer of Cement Treated Base (CTB), (iii) flexible pavement incorporating recycled asphalt base layer (henceforth referred as RAP option), (iv) Perpetual (PP) conventional, (v) perpetual semirigid with CTB, (vi) perpetual RAP and (vii) conventional rigid or concrete pavement. For high volume roads, IRC: 37-2012 restricts the design CBR to 8 % and above. Keeping this in view, the relevant design parameters adopted for the study are indicated in Table 1.

3. Project Recourse

Initially, the concepts of the currently-introduced and less-used options of the semi-rigid and recycled and perpetual types of pavement are briefly described, design methodology presented, material properties suggested, and pavement composition presented. Comparison of these non-conventional types of pavement is conducted with the conventional flexible and rigid types in terms of value engineering and life cycle costing in the subsequent sections.

3.1. Design inputs and methodology

In general, the Mechanistic-Empirical (M-E) procedure as suggested in IRC: 37 - 2012 have been adopted. Some of the highlights of the procedure are noted below:

• For high volume highways, 90 % reliability is used i.e. only 10 % of the area will have 20 % cracks to avoid frequent maintenance; the fatigue equation (1) indicated in Table 2 represents air void around 3 % and volume of bitumen of about 13 %

• Equation (2) in Table 2 limits rutting in subgrade and granular layers to 20 mm at 90 % reliability with the use of VG-40 grade or equivalent modified bitumen in both the bituminous wearing course of Bituminous Concrete (BC) and base course of Dense Bituminous Macadam (DBM)

• The treatment of fatigue cracking of cementitious layer is considered at two levels - cumulative standard axles (Equation (3)) and cumulative fatigue damage due to individual axles (Equation (4))

• For an assumed pavement section, the imimum tensile strain (et), the vertical sttain in subgrade (£v), tensile strain in the cementitious layer (ect) are obtained using the IITPAVE program and compared with the allowable strains for the particular level of the adopted traffic

• For perpetual pavement, et and ev are limited as indicated in Table 2. The performance model is based on the criteria shown in Table 2.

Table 1. Project Road Properties

Pavement Type Lane (L) Configuration Design Traffic Design Remarks

in both directions (MSA1) CBR (%)

Now Future 20 years 50 years

2L 2L 50 320 8 Design traffic of 20 years

All seven types 4L 6L and 8L 150 785 12 projected to 50 years at 5 %

growth rate

6L 8L 200 1050 12

'Million Standard Axles

Table 2. Design Equations and Criteria

Failure Criteria Design Equation Remarks

Fatigue criteria of bituminous layer Nf = 2.021 x 10-4 x [1/eJ389 x [1/E]0 854 (1)

Rutting criteria Nr = 1.41 x 10-8 xfl/Ev]45337 (2)

Fatigue criteria of cementitious layer N = [(11300/Ea804+191)/ Ect]12 (3) Standard Axles

Cumulative Damage Analysis of cementitious layer Log Nfi={0.972-(st / Mrup)}/0.0825 (4) Axle Load Spectrum

Perpetual Using (1), Et (max) = 70 m Using (2), £Z (max) = 200 m

As per the guidelines of IRC: 37-2012, the elastic moduli and poisons ratios of different pavement layers considered in the pavement design are presented in Table 3.

Table 3. Design Inputs for Structural Pavement Design

Pavement Layer E Value (MPa) m Material Properties

Bituminous Surfacing (BC, DBM) 3000 0.5 VG-40 or equivalent Modified Bitumen

Cement Treated Base (CTB) 5000 0.25 28 day UCS1 of 5 MPa

Cement Treated Sub-base (CTSB) 600 0.25 7 day UCS1 of 0.75 to 1.5 MPa

Aggregate Interlayer 450 0.35

Recycled Asphalt Base (RAP) 600 0.35 Bitumen Emulsion / Foamed bitumen treated RAP / aggregates

Granular Base and Sub-base E2 = Ei * 0.2 * h045 0.35

Sub-grade Ei = 17.6 * (CBR)064 0.35

1Unconfined Compressive Strength

3.2. Design period

The design period for the perpetual pavement is 50 years. Hence, to provide a parallel platform for comparison, all the types of pavements have been designed for a total life of 50 years. Accordingly, maintenance regime has been set for the non-perpetual types of pavement for a longer life and is included in Table 4. It includes both rehabilitation and reconstruction (partial and full) works.

3.3. Pavement composition

The various pavement compositions for each of the three scenarios have been designed using IITPave software as per IRC 37-2012. Comparative plots of the various pavement compositions for initial construction are presented in Fig 1, Fig 2 and Fig 3 respectively.

It would be interesting to see from all the three figures that the RAP Type 3 of pavement involves the least pavement section with the lowest usage of aggregates whilst the Semi-rigid Type 2 involves the thickest pavement section with the highest usage of aggregates when designed conventionally for 20 years of design life.

However, the case is exactly opposite in case of perpetual pavement. Perpetual pavement Type 5 with CTB involves the least pavement section and the Type 6 with RAP shows the highest pavement section.

Fig. 1. Comparison of pavement composition for all types at initial construction for 2-lane section (50 MSA)

Fig. 2. Comparison of pavement composition for all types at initial construction for 4-lane section (150 MSA)

Fig. 3. Comparison of pavement composition for all types at initial construction for 6-lane section (200 MSA)

4. Pavement Type Selection

Selection of the appropriate pavement type is driven by several factors including life cycle cost of the pavement type, availability of quality raw materials, constructability, stimulation of competition between industries, and nonetheless political regime. On a Design Build Finance Operate and Transfer (DBFOT) model, Concessionaires are looking for value engineering options in pavement selection type to get maximum economy by minimizing the use of natural resources. Ministry and government are also supporting such positive steps which not only save the natural resources but also reduce the carbon footprints of infrastructure project. However, the engineering aspect, sustainability and durability of non-conventional pavement types are very important parameters and must be dealt comprehensively with proper field and laboratory testing. Also, the results must be verified for localized climatic conditions to ensure the long term durability of the selected pavement types. The subsequent sections illustrate value engineering options for the selection of the optimum pavement holistically.

5. Value Engineering

Value engineering is treated as future investment for future gaining technology leadership in industry. It triggers a complete overhaul of the system, alternate design, and alternative material, design verification for strength, durability and safety. Thus, all the pavement types are subjected to Life Cycle Cost Analysis (LCCA) to select the type that will provide a satisfactory level of service at the lowest cost with the optimum usage of natural resources over time. Life-cycle cost is considered to include the initial or direct cost of construction, cost of stage construction, and cost of potential maintenance regime. Ideally, scheduled preventive maintenance and periodic renewal of the sacrificial friction course would be the only work required on a perpetual pavement after initial construction.

The analysis is most sensitive to the factors of inflation, discount rate, and analysis period. Here, in this paper the long term economic viability of pavement types using Present-Worth method of analysis has been studied.

The discount rate is the interest rate by which future costs will be converted to present value, and has been adopted as 12 % in this study. The interest rate is given by 4 %.

5.1. Rehabilitation and maintenance regime for Life Cycle Cost Analysis (LCCA)

The rehabilitation and maintenance regime adopted for the concept project based on the minimal requirement for highway standards is given in Table 4. To treat all pavement types on the same platform, widening concept is made applicable to all the road types in the same year over the period of study. For the perpetual pavement, widening has been considered for 15 years to 20 years design life as the case may be and hence remaining life of the widened portion is not applicable.

Table 4. Rehabilitation and Maintenance Regime for 50 years

Pavement Type Initial Lane Configuration Widening Full or Partial Reconstruction Strengthening Functional Overlay (times) Diamond Grinding Sealant Renewal

Conventional - Type 1 2L - Once Once Six - -

4L 6L & 8L Once Twice Five - -

6L 8L Once Twice Four - -

Semi-rigid — Type 2 2L - - Twice Four - -

4L 6L & 8L Once Twice Four - -

6L 8L Once Twice Four - -

RAP - Type 3 2L - Once Once Four - -

4L 6L & 8L Once Twice Five - -

6L 8L Once Twice Five

PP Conventional Type 4, 2L - - - Four - -

PP Semi-rigid Type 5 and PP RAP Type 6 4L 6L 6L & 8L 8L - - Three Three - -

Rigid Type (7) 2L - - Once (concrete) - Once in Once in

4L 6L & 8L - Once (concrete) - 10 years 10 years

6L 8L - Once (concrete) -

5.2. Cost comparison

The total cost summary per km for the concept project in terms of initial construction cost and life cycle cost for the various traffic categories or laning system have been worked out. The prevalent standard unit costs of materials have been considered for the estimation.

The comparison of savings in initial cost and life cycle cost for the various pavement sections for each of the three scenarios is graphically presented in Fig 4 and Fig 5 respectively with Type 1 pavement as base.

5.3. Material comparison

Today's alarming fact is that the naturally occurring materials are fast depleting because of their overexploitation to meet the huge demand for construction of infrastructure projects. To cope with the huge demand

of these materials at present and in the future, sufficient reserves have to be ensured and these reserves are non-replenishable. Unless we fulfill this task now, the existing reserves of natural resources of materials will ultimately disappear for which the next generation will not pardon us. Besides, the amount of energy consumed for blasting the hills for quarrying operations, crushing the rocks, transportation of this material to plants, mixing, laying etc. is doing unspeakable damage to the environment. Thus, keeping the economics of construction aside, let us evaluate the amount of materials involved for the concept project road for each of the scenarios and if at all in future, we will be left with sufficient material for the stage construction, rehabilitation, reconstruction, etc.

The volume / quantity of aggregates involved in construction of any of the pavement layers is directly proportional to the designed thickness for a given project corridor. Thus, the savings in aggregates have been computed based on the quantity of aggregates involved for the particular pavement composition for each pavement type. The quantity of material is evaluated for all stages of construction, widening and rehabilitation for the entire design period of 50 years.

Fig 6 presents the materials savings involved in using each of the pavement types for each of the scenarios with Type 1 pavement as the base.

Fig. 4. Comparison of savings (%) in initial cost for the various pavement types with Type 1 as base for all three scenarios

Savings in Li fe Cycle Cost with Type 1 agbase(%)

Type - 2 Type - 3 Type - 4 Type - 5 Type - 6 Type - 7

Conventional wittl CTB Conventional RAP Peipetual Pavement conventional Perpetual Pavement CTB Peipetual Pavement RAP Rigid Pavement

H 6- lane / 200 MSA 24 9 -20 4 14 27

Q 4-lane / 150 MSA 23 12 -22 0 -19 17

B2-Lane/ 50 MSA 19 21 -58 -20 44 -66

Fig. 5. Comparison of savings (%) in life cycle cost for the various pavement types with Type 1 as base for all three scenarios

g 60 ê 50 ¡J 40 0 s 30 ri S. 20 £ .3 M 1 ° rJ~A III 1 1! ! 111 1 11 m

Conventional with CTB Type-2 Conventional RAP Type - 3 Peipetual Pavement conventional Type-4 Perpetual Pavement CTB Type-5 Peipetual Pavanent RAP Type - 6 Rigid Pavement Type-7

Si Lane 50MSA 33 67 38 40 35 33

□ 4-lane / 150 MSA 10 60 50 37 63 37

H 6-lane / 200 MSA 14 55 51 51 66 43

Fig. 6. Comparison of savings (%) in aggregate quantities for the various pavement types with Type 1 as base for all three scenarios

6. Results and Discussions

Based on the output of analysis noted above, a summary is presented in Table 5, which indicates the optimum pavement type for the Indian roads depending on the funding situation of the project.

Types 3 and 6, which represents use of RAP in conventional and perpetual type have not been considered in the evaluation as these options are constrained with the availability of adequate quantity of RAP on the project. If sufficient quantity is available to meet the project requirements, then undoubtedly Type 3 or Type 6 shall be preferred depending on the traffic category. If two types meet the positive requirements, then the optimum section is selected on the basis of maximizing on the aggregate savings. There is an average of 30% to 40 % savings in aggregates with the use of concrete or any perpetual pavement for traffic of 150 MSA and above. Higher the design traffic, higher is the savings in aggregates with the use of perpetual pavement.

Table 5. Observation on best suited pavement type based on initial cost, LCC and material savings

Lane configuration Criteria Type 1 Type 2 Type 3 Type 4 Type 5 Type 6 Type 7 Remarks / Observation

or traffic category Adequate fund Fund constraint

50 MSA (2-lane) Initial cost LCC V X V V X X X X X X Semi-rigid CTB type best

Aggregate savings X V V V V

150 MSA (4-lane) Initial cost LCC Aggregate savings V X X V V V Not applicable X X V X V Not applicable V V V Rigid (Concrete) best Semi-rigid CTB type best

200 MSA (6-lane) Initial cost LCC Aggregate savings V X X V V V X X V X V V V V V Any Perpetual pavement best Perpetual pavement with semirigid CTB type best

Note: 'X' denotes benefits are negative; W' denotes positive benefits; '-' denotes borderline case

Conclusion

It may be concluded that the costing has been carried out with the present rate of aggregates. However, the pace of unit rate for aggregates is anticipated to be accelerated in the days to come keeping the progressive ban on mining in view. Thus, who knows, if in future, perpetual pavement will not come out as the unanimous winner both in initial cost and life cycle cost.

Acknowledgements

The authors are thankful to Mr. K. K. Kapila, Chairman and Managing Director, Intercontinental Consultants and Technocrats Pvt. Ltd., New Delhi for guidance and providing all facilities in carrying out the reported work.

References

Tentative Guidelines for the Design of Flexible Pavements, IRC publication, No. 37, The Indian Roads Congress, New Delhi, 2012.