USE OF HMA IN RAIL TRACK: INEFFICIENCIES AND REDUNDANCY OF THE PRESENT SYSTEM

Luv Sehgal 77 Water Street, ARUP, 4th Floor, NY, NY,10005 Tel: 217-904-5962 Cell: 646-341-2927; Email: Luv.Sehgal@arup.com Amit Garg Chief Engineer, Ministry of Railways, India Construction Office, South Western Railway Bengaluru, India Tel: +919717647825; Email: amitgargbangalore@gmail.com William Buttlar Glen Barton Chair of Flexible Pavement Technology Civil and Environmental Engineering, University of Missouri 31 Heinkel Bldg, Columbia, MO 65211 Email: buttlarw@missouri.edu ABSTRACT

Considering that adoption of asphalt underlayment has the potential of becoming a standard component of all railway trackbed (be it conventional or high-speed or heavy haul), a study was conducted on the current standards and practices for use of asphalt all over the world. Several interesting and prominent conclusions were arrived at.

This study attempts to create specifications for the asphalt mix for the underlayment and combination using superpave-mix design in reference to different imposed load. Sensitivity analysis has been carried out between different PG binder grades, imposed load, aggregate gradation, air voids and seasons with different thicknesses of asphalt layer and different models, to predict the most economical option.

KENTRAK 4.0 FEM model has been used to evaluate stresses, strains and the Dynamic complex modulus. The E* values peak at around 2% of the void ratio at which the 4” asphalt layer shows excellent performance. Ideally a high modulus asphalt, a low range PG binder with void ratio of 2% and aggregate gradation as available, can be used to create a mix which can fulfill all the key performance criteria. Findings also reveal that small thickness of granular subballast combination works better than 8-inch asphalt layer without subballast. Also, there is a strength redundancy in the system which can be exploited by adding in sustainable recyclable material (RAP/RAS).

This paper is an attempt to propose the findings to regulatory bodies such as AREMA and FRA for their opinion and future incorporation.

Keywords: Superpave mix design, Dynamic complex modulus, Sensitivity analysis, Stiffness, Performance characteristics, subballast

INTRODUCTION

Fifty years since the inauguration of the world’s first high speed rail line between Tokyo and Osaka and more than 30 years after the building of the first European high speed (Paris to Lyon), high speed rail has proven to be one of the most competitive means of medium distance intercity transport (1).

With the assumption that the conventional railway track structure with a concrete tie and rail resting on 12 inches of ballast has reached a level which has matured beyond the possibility of any classical enrichment from research, use of basic track structure has been a no-go area (2). However, with unprecedented growth in speed and wheel loads, the rail industry has to find new technologies to provide a stronger, safer and low maintenance structural solution.

Background

HMA layer is used mostly for uniform load distribution to the subgrade by providing a layer of consistent stiffness, reduced subgrade pressures, waterproofing and confining the subgrade & ballast, thereby providing consistent load-carrying capability – even on subgrades of marginal quality. The waterproofing effects are particularly important since the impermeable asphalt layer essentially eliminates subgrade moisture fluctuations, which effectively improves and maintains the underlying support. Additionally, the resilient asphalt layer provides a positive separation of ballast from the subgrade, thereby eliminating subgrade pumping without substantially increasing the stiffness of the trackbed (3).

The resulting stable trackbed has the potential to provide increased operating efficiency and decreased maintenance costs that results in long-term economic benefits for the railroad and rail transit industries. HMA (hot mix asphalt) test tracks and specific special trackwork problem-solving installations are performing extremely well (4). The increased cost of using HMA is most often minimal, and the indications are that at many sites, the long-term savings may be substantial when compared to conventional construction, maintenance and rehabilitation techniques.

High Speed Track structure: Global Scenario Globally most of the high-speed nations are using asphalt bases as underlayment or as combination in tracks. Japan’s actual thickness for HMA base is just 2-4 inches and is adjoined with a concrete slab (5). Germany is directly placing its track on HMA layer. In USA, combination of asphalt underlayment and combination are used at special places only (6). Different case studies conclude that the most preferred choice of HMA layer thickness is 6-8 in with subballast layer in European countries and Asia. Specifically, for USA, the preferred design thickness is 8-in or more without subballast layer. Moreover, there are no standard design guidelines for design of HMA layer and is used primarily for special trackwork. Research on HMA placement on transitions is still under trial at railway testing facilities.

Country Slab/Ballasted Layer 1 Layer 2 Layer 3 Layer 4 Japan

Slab Slab thickness (7.5-in)

Asphalt layer (6-in),

Crushed stone well graded (6-in)

Subgrade

Ballasted Ballast (11.5-in) Asphalt layer (2-in)

crushed stone layer (6-24 in)

Subgrade

Italy Ballasted Ballast (14-in) Asphalt layer 5-in

Supercompatto layer (12-in)

Subgrade

Spain Ballasted Ballast (14-in) Asphalt layer (5-6 in)

Frost protection layer (12-16 in)

Subgrade

Germany Ballasted slab - Asphalt layer (8- in)

Asphalt base multilayer

Subbase (20-in)

France Ballasted Ballast (12-in) Asphalt layer (6-in)

Adjustment layer (8-in)

Subgrade

USA

Ballasted Ballast (8-12 in) Asphalt layer (5-6 in)

No subballast Subgrade

Ballasted (rare) Ballast (8-12 in) Asphalt layer (8-in)

Sub ballast Subgrade

India Ballasted Ballast (14-in) Blanket layer (upto 24-in)

Subgrade -

TABLE 1 Global scenario for use of HMA in Tracks (7) (13) (14) (15)

STRUCTURAL MODELS AND STANDARD INPUTS

Before a design mix is done for HSR (High Speed Rail) it is essential to understand the loading environment, deterioration conditions and failure mechanisms of asphalt layer. Following are the performance requirements of an asphalt layer subjected to loading of HSR.

a. Durability: Asphalt layer should resist water seeping through the ballast and mud pumping. b. Temperature Resistance: For HSR, the asphalt layer is situated between the ballast and subgrade.

Due to such confinement, HMA experiences lesser temperature variation as compared to other layers which are directly exposed to atmosphere.

c. High Modulus: HMA should not distort (rut) or deform (shove) under traffic loading. HMA deformation is related to various reasons. It can be attained through use of angular aggregates, optimum binder content and specifying binder with minimum high temperature viscosity, thereby ensuring higher modulus.

d. Fatigue Resistance: HSR loading may not be very high in magnitude, yet, it can cause fatigue failures. The use of an asphalt binder with a lower stiffness will increase the HMA’s fatigue life by providing greater flexibility.

e. Fine Graded: Open graded is more permeable and is used in surface courses for pavements. In tracks, the asphalt layer is required to perform sufficient drainage functions, hence fine graded is an optimal choice.

Track models used in US using HMA The uses of HMA as a sub ballast layer within railroad track structures for new trackbed construction and trackbed maintenance applications have grown steadily in the United States during the past 25 years. The “Underlayment” is typically 6 to 8-in thick and 12 ft. (3.6 m) wide. A base mix having a 1.0-in maximum aggregate size, that is slightly “over asphalted”, with about 0.5 percent additional asphalt binder than normally specified for a highway pavement mix is considered the ideal mix for railway trackbed applications. The specified thickness of the overlying ballast is normally 8 to 12-in (3).

However, it is important to note that the load on the base layer of a highway pavement is of the order of 435 psi. whereas for a dedicated high-speed railway, this load is only 116 psi, nearly a third in comparison. HMA for railway trackbed is designed to be a medium modulus, flexible, low void, fatigue resistant layer that will accommodate high tensile strains without cracking. The second is the “Asphalt Combination” trackbed which includes both the asphalt layer and the granular subballast layer. The asphalt layer thickness can be reduced since a relatively thick subballast layer exists below. Subballast, performing as an improved subgrade, is considered as an additional protection for the subgrade and a support to the asphalt layer for improved performance characteristics.

FIGURE 1 - Trackbed models for special trackwork (3) & (8).

The standard inputs for loading conditions for the study are taken as applicable to High Speed Rail systems. The input axle load is taken as 22 tons (t) which is higher than that in the Japanese Shinkansen High Speed Rail system by around 6 tons. For determining the stresses on the ballast and subgrade, Talbot analysis is

used. As input for the trainset dimensions, a typical high-speed rail trainset is considered. The tie spacing of 27” is considered for concrete ties in accordance with AREMA guidelines. Stress at the top of asphalt layer comes out to be 20-22 psi which is approximately equal to the stresses by heavy haul trains, at the same level. Though there is a difference in axle load (wheel load of heavy haul train being 32,000 lbs. and high-speed train being, 18000 lbs.), the dynamic effects of the high speed make the stresses comparable to those in heavy haul. The effect of speed is not factored into KENTRACK software. Due to this, higher value of axle-load and load frequency has been considered in the analysis. The results shown in this paper may vary from actual conditions and will need validation from field tests and experimentation. However, the trends and observations derived from this analysis have led to reliable conclusions. HMA TRACKBED STUDY USING KENTRACK SOFTWARE The KENTRACK is a program which uses a layer elastic finite element model (FEM) to measure the structural performance of railway trackbeds. It is developed to analyze trackbeds having various combinations of all-granular and asphalt-bound layered support. It can be used for calculating compressive stresses at the top of subgrade which is indicative of potential long-term trackbed settlement failure, for trackbeds containing asphalt layer, for calculating tensile strains at the bottom of the asphalt layer, indicative of potential fatigue cracking and predicting trackbed service life considering that variances in subgrade modulus and axle loads and the incorporation of a layer of asphalt within the track structure have significant effects on subgrade vertical compressive stresses (7). The latest version of KENTRACK uses properties of performance graded (PG) asphalt binders and the Witczak E* predictive model for dynamic modulus calculation (7). Standard Inputs for KENTRACK 4.0 Software The model divides the track system into the following components from top to bottom: rails, springs (tie plates/pads), ties, and layered support system. Rails and ties, considered as beam elements, are orthogonal to each other. Spring connections between rails and ties are used to account for looseness between rail and tie. A linear spring constant is specified to indicate the rigidity of the connections. A wheel load is converted to a circular load applied on the top layer. Burmister multi-layer system theory is applied to calculate stresses and strains in the trackbed. Stresses and strains at subgrade or asphalt layer under multiple wheel loads are obtained by load superposition theory. Subballast and subgrade are considered as linear elastic materials. The bedrock is assumed incompressible with a Poisson’s ratio of 0.5. Important input parameters for the KENTRACK are as follows:

1. Rail type: RE136 2. Rail Young’s modulus: 30000000 psi 3. Rail Section Modulus: 23.9 in 4. Rail moment of Inertia: 94.9 x 10^4 in^4 5. Rail Tie Spring constant (simulating the rail tie pad): 7000000 lbs./in 6. Tie: Concrete 7. Tie spacing: 27 inches 8. Number of seasons for output: 4 9. Tolerance for Vertical deflections: 0.00001 10. Tolerance for Tensile stress: 0.01 psi 11. Subgrade Modulus: 12000 psi/sq-in/in 12. Ballast modulus: 18000 psi/sq-in/in

13. Temperature for Asphalt for various seasons: Season 1: 50, Season 2: 67, Season 3: 33, Season 4: 20 (in Fahrenheit)

SENSITIVITY ANALYSIS The two most important performance criteria when the layer of asphalt (HMA) is introduced in the track structure are the compressive stresses on the top of subgrade and the tensile strains at the bottom of the asphalt layer. These are studied in detail in the sensitivity analysis done using the FEM program KENTRACK and the results are presented herein. Their effect on the residual design life is also reflected in the analysis. All the results are studied for understanding the changes in stresses and strains in varying thicknesses of the asphalt layer. Variation with PG Binder Grade Superpave performance grading (PG) is based on the idea that an HMA asphalt binder’s properties should be related to the conditions under which it is used. For asphalt binders, this involves expected climatic conditions as well as aging considerations. Therefore, the PG system uses a common battery of tests (as the older penetration and viscosity grading systems do) but specifies that a particular asphalt binder must pass these tests at specific temperatures that are dependent upon the specific climatic conditions in the area of use. KENTRACK has calculated and used following values for strain and stress calculations for different binder grades and different seasons. The KENTRACK software permits use of only 3 PG binder grades as default values i.e. PG 64-22, PG 70-28 and PG 76-34. These 3 binder grades have been chosen for determination of tensile strains at the bottom of asphalt layer in varying thicknesses of asphalt underlayment and combination systems (see figure 2). Also, the Dynamic modulus (E*) is one of the primary material parameters for mechanistic–empirical pavement design and performance prediction. KENTRACK evaluates dynamic modulus using Witzack model and gives value for every season (8). To evaluate changes of binder grades on E*, the upper and lower grades of the binders are changed. The upper grade represents the highest temperature in which the binder can operate, and it is mainly considered for rutting. On the other hand, the lower grade represents the lowest temperature a binder can operate in and is mainly considered for thermal cracking. Therefore, it is expected that the stiffness of a upper grade binder should be higher than a lower upper grade binder. Research and recent development have shown that PG binders have become less sensitive to high temperature than old Asphalt Concrete. The upper grade of asphalt binders controls the highest temperature that asphalt can operate, therefore, even if the upper grade increases, the asphalt performance, such as viscosity, dynamic modulus, etc., would not vary significantly in a trackbed environment. Research has also shown that varying the lower asphalt binder grades has a more significant effect on the service life than varying upper grade (8). The analysis on KENTRACK gives stresses and strains and design life Fof subgrade and asphalt layer. The graphic shown below (Figure 2) shows variation of stresses and strains for different HMA layer thicknesses and different PG binders. The design life is the reciprocal of the repetition ratio. If a year is divided into four seasons, the design life for each distress mode can be written as:

𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 𝐿𝐿𝑆𝑆𝐿𝐿𝑆𝑆 𝑆𝑆𝑖𝑖 𝑎𝑎 𝑠𝑠𝑠𝑠𝑆𝑆𝑆𝑆𝑆𝑆𝐿𝐿𝑆𝑆𝑆𝑆 𝑙𝑙𝑎𝑎𝑙𝑙𝑆𝑆𝑆𝑆 = 1

𝑃𝑃𝑆𝑆𝑆𝑆𝑃𝑃𝑆𝑆𝑆𝑆𝑃𝑃𝑆𝑆𝑃𝑃 𝑖𝑖𝑛𝑛𝑛𝑛𝑛𝑛𝑆𝑆𝑆𝑆 𝑜𝑜𝐿𝐿 𝑙𝑙𝑜𝑜𝑎𝑎𝑃𝑃 𝑆𝑆𝑆𝑆𝑠𝑠𝑆𝑆𝑃𝑃𝑆𝑆𝑃𝑃𝑆𝑆𝑜𝑜𝑖𝑖𝑠𝑠 𝑆𝑆𝑎𝑎𝑆𝑆ℎ 𝑠𝑠𝑆𝑆𝑎𝑎𝑠𝑠𝑜𝑜𝑖𝑖𝐴𝐴𝑙𝑙𝑙𝑙𝑜𝑜𝐴𝐴𝑎𝑎𝑛𝑛𝑙𝑙𝑆𝑆 𝑖𝑖𝑛𝑛𝑛𝑛𝑛𝑛𝑆𝑆𝑆𝑆 𝑜𝑜𝐿𝐿 𝑙𝑙𝑜𝑜𝑎𝑎𝑃𝑃 𝑆𝑆𝑆𝑆𝑠𝑠𝑆𝑆𝑃𝑃𝑆𝑆𝑃𝑃𝑆𝑆𝑜𝑜𝑖𝑖𝑠𝑠

FIGURE 2 (a) Variation of compressive stress with different PG binder grades

FIGURE 2 (b) Variation of Tensile strains in asphalt with different PG binder grades

-8

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Thickness of Asphalt Layer as Asphalt Underlayment and CombinationCompressive Stresses (in psi) for different PG Binder grades

PG 64-22 Asphalt PG 64-22 Combination PG 70-28 AsphaltPG 70-28 Combination PG 76-34 Asphalt PG 76-34 Combination

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Thickness of Asphalt layer as Asphalt Underlayment and Combination

Tensile Strain (in 10^-5) for different PG Binder Grades

PG 64-22 Asphalt PG 64-22 Combination PG 70-28 Asphalt

PG 70-28 Combination PG 76-34 Asphalt PG 76-34 Combination

Figure 3: Variation of compressive stress and tensile strain with binders and thickness

Observations from the sensitivity analysis (Figures 2 & 3):

• Increasing the upper grade of PG Binder decreases the compressive stresses in subgrade and tensile strains in HMA in both trackbeds that utilize asphalt. As expected, the service life of subgrade and asphalt layers are enhanced.

• It should be noted that varying the lower asphalt binder grade has a more significant effect on the service life than varying the upper grade. However, these changing trends are opposite to varying the lower grade. This means lowering the lower value of PG binder grade decreases the design life. This can also be substantiated from reference no 8.

• Performance of the combination model is better than the underlayment model, with reference to both compressive stresses and tensile strains for all thicknesses of asphalt mix. This is expected because of the additional support provided by the sub ballast layer to the HMA layer above it.

• The PG 64-22, the most economic binder grade has a relatively better performance when compared to other grades for an asphalt layer thickness. This complies with previous research results.

• At lower thicknesses of HMA, difference in stresses for a particular thickness of HMA and different binder grades is greater than at 8-in HMA Layer. At 8-in stresses for all three binder grades are closer in value.

• Slope is severe between 2-in and 4-in HMA Layer as compared to other thickness beyond 4-in. • Impact of PG Binder is more prominent for design life of asphalt layer as compared to that of the

subgrade. • For a binder grade, design life/stresses of 8-in combination models are higher by just 15% to 25%

compared to 4-in or higher combination model.

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2 in 4 in 5 in 6 in 7 in 8 in 2 in 4 in 5 in 6 in 7 in 8 in

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Design life of Subgrade and Asphalt layer

PG 64-22 Asphalt Underlayment PG 64-22 CombinationPG 70-28 Asphalt Underlayment PG 70-28 CombinationPG 76-34 Asphalt Underlayment PG 76-34 Combination

• The most economic model is a 4-inch asphalt underlayment with PG 64-22 binder grade. This model passes the tests of compressive stresses and tensile strains without any reservation. Since the design life for such a model is around 50 years, introduction of cheaper and sustainable option i.e. use of recycled material can be considered.

Variation with Air Voids Requirement of impermeability is very important for the HMA layer for efficient drainage and performance as protective layer to the subgrade. A decrease in the void ratio of the aggregate structure contributes to significant decrease in porosity and permeability. Consequently, the indirect tensile strength and durability generally increases as the mixture porosity decreases. For these, three values of air voids were chosen i.e. 2%,4% and 9%, covering the widest spectrum of air voids in practice. It was varied with different thickness of asphalt layer for asphalt underlayment and combination models. The results are represented below:

Figure 4(a): Variation of compressive stress with different air voids percentage of asphalt layer

Figure 4(b): Variation of tensile strain with different air voids percentage of asphalt layer

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Compressive Stresses variation with Percent Air Voids

4% Asphalt 4% Combination 2% Asphalt2% Combination 9% Asphalt 9% Combination

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Tensile Strain in Asphalt variation with Percent Air Voids

4% Asphalt 4% Combination 2% Asphalt2% Combination 9% Asphalt 9% Combination

Observations from the sensitivity analysis (Figure 4a & 4b) –

• For any air void, combination model is performing better than asphalt underlayment method. • More compact asphalt mix is performing better. Higher thickness with lower air voids leads to lesser

stress on subgrade and lesser strains on asphalt. • Difference in stresses/strains between 2% and 4% air void mix is lesser as compared to the

difference between 4% and 9% air voids mix. • For any air void ratio, design life/stresses of 8-in combination models are higher by just 15% to

25% compared to 4-in or higher combination model.

Variation with Aggregate Gradation

Rut resistance is highly dependent on aggregate grading and that HMA made with the best possible materials would fail without a proper gradation in case of highway pavements. However as discussed earlier, in the case of rail track underlayment, the HMA layer is highly insulated and protected from weather and temperature variations and is also not in direct contact with load. Therefore, failure due to rutting is not an expected phenomenon in the subject study. Sensitivity analysis was done for 2 different types of gradations. By varying gradations for different thickness of asphalt layer, asphalt underlayment- gradation I (i.e. 2,56,40,16) is performing similar to gradation II (i.e. 6,65,50,10) for asphalt underlayment. Previous studies have also shown that gradation has minor impact on subgrade and asphalt layer performance when used in trackbeds (9). Well graded aggregate mix. in 4-inch combination is performing much better than 8-inch asphalt underlayment. Again, the compressive stresses in subgrade are within the permissible limits. Therefore, use of most economical gradation can be used. Also, 8-inch asphalt layer is performing better than 4-in asphalt layer model. In general, there is not much difference in design life of HMA layer with gradation, it varies only with different trackbed models. Design life of HMA in combination is 2-3 times higher as compared to underlayment for respective cases. The study indicates that asphalt combination model performs superior to the asphalt underlayment model, buttressing our earlier observations. Use of inferior recycled material can be considered as an economic and viable option. USE OF RECLAIMED MATERIALS Apparently from all the variations studied above, it is explicit that the asphalt layer performs successfully in all parameters of performance. The order of magnitude of the stresses and strains that are developed in the asphalt layer and the subgrade for all cases of variations are on the lower side and there appears to be a built-in redundancy in the ballast-asphalt-sub ballast-subgrade system. Thus, it provides a significant window of opportunity to use recyclable materials as much as possible to reduce both, cost and carbon footprint. Though recycled materials were not considered as part of sensitivity analysis, the results of sensitivity analysis show the it will be economically viable to use these. As stated, the main parameter measured in the Dynamic modulus test is HMA stiffness. Previous studies indicated increased stiffness of mixture of HMA containing Recycled Asphalt Reclaimed Asphalt Pavement (RAP), Reclaimed Asphalt Shingles (RAS), or a combination of both. Stiffness modulus, toughness, moisture sensitivity, resistance to rutting, and fatigue resistance of the mixtures were studied (10). Results of Dynamic modulus tests showed higher modulus for higher RAP content. Recycled asphalt mixtures with high RAP percentages lead to increased stiffness. Experience in evaluating fatigue life of HMA mixtures including RAP and RAS is mixed. Results showed that inclusion of RAP may shorten fatigue life of HMA mixtures (11). However, some researchers have reported similar or better fatigue performance of recycled mixtures with RAP if proper mix design was considered.

From past research, it can be inferred that RAP/ RAS can suitably be adapted to ballasted railway trackbed. Considering railway performance characteristics are vastly inferior to those of highways, it can safely be deduced that these recyclable materials have huge potential for being adopted as replacement to virgin binder and aggregates bringing down cost and bringing in sustainability (12). CONCLUSIONS The literature review and results of the analysis of FEM model KENTRACK and insights achieved on the subject in the past independent study on the background of this subject (which has been presented and published in (Institution of Permanent Way Engineers in India (I.P.W.E.) 2017,Mumbai) have led us to the following conclusions and suggestions:

1. Use of asphalt as a component of railway track in a modern railway system is an undisputed opportunity with limitless possibilities and has the potential to change the way the railway track is looked at.

2. The US has adopted the asphalt underlayment model at selected locations and problem spots, primarily for mitigating variation in stiffness. For some reason, its use has been restricted and thus potential valuable benefits are being shunned.

3. KENTRACK needs to be modified for incorporation of speed of the train as a vital input in the analysis. At present, only the static axle load and the number of repetitions is fed into the software but speed, which contributes heavily to the dynamic loading (aka Talbot) is conspicuous by its absence. However, the inputs in the form of load and load frequency were on conservative side.

4. The PG binder grades which are a default entry in the software but are for only three of the 25 grades that are given in the PG nomenclature matrix. For the other grades, the default properties have to be researched and then used which is sometimes difficult. The software should permit use of all the 25 standard PG grades to be used as a default setting.

5. The comparative study between the two models have revealed some remarkable facts. It is observed that the 4-inch asphalt combination, which the cheapest model in adoption of HMA in railway track performs as good as all other options which are more capital intensive. Apparently, the combination model would bring in a level of redundancy which may never be exploited. The combination model thus can be used with a thickness of asphalt layer of around 4-in with a 10-in granular layer supporting it- the model which is adopted only by Japan.

6. The two failure modes in this model are: a. failure by settlement due to excessive compressive stresses in the subgrade and b. failure by fatigue cracking due to excessive tensile strains in the asphalt layer. AREMA has given the values of limited stress on subgrade which can be compared with the in-service stress. However, there are no values for limiting strain at the bottom of the asphalt layer with which the actual strain levels can be compared.

7. This is also indicative of the general indifference to the use of asphalt as a useful alternative to the granular subballast by the AREMA and by FRA. It is recommended that standard specifications be prepared for use of asphalt in rail trackbed along with various tolerance parameters.

8. The use of RAP/RAS can be maximized in railway use considering that the stress levels are low in comparison to highway and also that the asphalt layer is sufficiently insulated from the elements as well direct load. Also, of note is the fact that there is redundancy in the system which can be exploited by adding in sustainable recyclable material.

9. Based on the sensitivity analysis from this paper, inferences from international case studies and the E* values indicating stiffness of the binder grades peaks at around 2% of the void ratio at which the 4-in asphalt layer in combination with 4 to 6-in subballast shows excellent performance. Thus, ideally a high modulus HMA, a low range PG binder, with a void ratio of 2% and well graded aggregates as available can be used to create a hot mix asphalt which can have a sufficiently long service life fulfilling all the key performance criteria.

ACKNOWLEDGEMNTS This research was a result of independent studies carried out by first two authors with help of Prof William J. Buttlar. We sincerely want to thank Prof Jerry Rose, who has been a source of inspiration and constant help throughout our research. REFERENCES 1. European Asphalt Pavement Association (2003) Asphalt in Railway Tracks, www.eapa.org, Accessed

July 2016. 2. Hay, W. Railroad Engineering. John Wiley & Sons, New York, 1982. 3. Rose, Reginald R, Asphalt Institute (2007) The Asphalt Handbook, MS-4, 7th Edition, Chapter 15.3

Railway Roadbeds, 832 pp. 4. Read, D., and D. Li. TCRP Research Results Digest 79: Design of Track Transitions. Transportation

Research Board of the National Academies, Washington, D.C., Oct. 2006. 5. Momoya, Y., Horiike, T., and Ando, K. (2002) Development of Solid Bed Track on Asphalt Pavement,

Quarterly Report, Railway Technical Research Institute, Vol. 43, No. 3, September, pp. 113-118. 6. Rose, Teixeira, Ridgway (2010) Utilization of Asphalt/Bituminous Layers and Coatings in Railway

Trackbeds – A Compendium of International Applications. JRC 2010-36146. Proceedings JRC 2010. 7. Su, Bei, (2003) KENTRACK: A Finite Element Computer Program for the Structural Design and

Analysis of Railroad Track Structures, MSCE Thesis, Department of Civil Engineering, University of Kentucky.

8. Rose, Liu and Souleyrette (2014) KENTRACK 4.0: A Railway Trackbed Structural Design Program. JRC 2014-3752, Proceedings JRC 2014.

9. Anderson, R. M., and H. U. Bahia. Evaluation and Selection of Aggregate Gradations for Asphalt Mixtures Using Superpave. In Transportation Research Record 1583, TRB, National Research Council, Washington, D.C., 1997, pp. 91-97.

10. Anderson and Daniel. Long Term Performance of Pavement with High Recycled Asphalt Content-Case Studies. In Transportation Research Record: Journal of the Transportation Research Board, No 2371, Transportation Research Board of the National Academies, Washington, D.C., 2013, pp. 1-12.

11. Shu, Huang and Vukosavljevic. Laboratory evaluation of fatigue characteristics of recycled asphalt mixture. Science Direct: Construction and Building Materials, Vol 22, 2008, pp. 1323–1330.

12. Pradyumna, Mittal and Jain. Characterization of Reclaimed Asphalt Pavement (RAP) for Use in Bituminous Road Construction. Procedia - Social and Behavioral Sciences, Vol 104, 2013, pp. 1149 – 1157

13. Sehgal, Garg and Buttlar (2017), Hot Mix Asphalt in Ballasted Railway Track: International Experience and Inferences, IPWE-2017-1, pp 521-539

14. Rose, Teixeira and Veit, (2011) International Design Practices, Applications, and Performances of Asphalt/Bituminous Railway Trackbeds.

15. Rose, Teixeira, Ridgway (2010) Utilization of Asphalt/Bituminous Layers and Coatings in Railway Trackbeds – A Compendium of International Applications. JRC 2010-36146. Proceedings JRC 2010

LUV SEHGALRail Engineer, ARUP

AMIT GARGChief Engineer| Indian Railways

USE OF HOT MIX ASPHALT IN BALLASTED RAIL TRACK: INEFFICIENCIES AND REDUNDANCY OF THE PRESENT SYSTEM

September 22-25, 2019 - Minneapolis

USE OF HOT MIX ASPHALT IN BALLASTED RAILWAY TRACKA TECHNO-ECONOMICAL & SUSTAINABLE SOLUTION

IPWE MUMBAI- 2017

IPWE 2017 AREMA 2016

AGENDA• Past Studies• Why Ballasted track?• Advantages of HMA in Ballasted track• KENTRACK program• Sensitivity Analysis • Lessons Learned/ Conclusions

CAUSE & EFFECT

MORE SPEED, FREQUENCY AND LOAD

HIGH SPEED RAIL

TYPICAL TRACK STRUCTURES

• Economical Option• Frequent maintenance

• 6-7 times costly• Less maintenance

HSR NATIONS OF THE WORLDRailway line % Tunnels % Bridges or

Viaducts Total Main Track Type (Ballasted/ Slab)

Japanese HSRTokaido: Tokyo–Osaka (515 km) 13 33 46 BallastedSanyo: Osaka–Hakata (554 km 51 38 89 SlabTohoku: Tokyo–Morioka(497 km) 24 71 95 SlabJoetsu: Tokyo–Niigata (270 km) 40 60 100 Slab

European HSRParis–Lyon (480 km) .8 .7 1.5 BallastedTGV Atlantique (280 km) 4.7 1.1 5.8 BallastedValence–Marseille (295 km) 5.1 6.8 11.9 BallastedHannover–Würzburg (326 km) 19.3 10.4 29.7 BallastedKöln–Frankfurt (177 km) 22.6 3.4 26 SlabRoma–Napoli (220 km) 11.4 15.9 27.3 BallastedMadrid–Sevilla (471 km) 1.9 3.4 5.3 Ballasted

MORE BRIDGES MORE TUNNELS MORE SLAB TRACK

GLOBAL SCENARIO- TRACK STRUCTURECountry Slab/Ballasted Layer 1 Layer 2 Layer 3 Layer 4

JapanSlab

Slab thickness (7.5-in)

Asphalt layer (6-in),Crushed stone well graded (6-

in)Subgrade

Ballasted Ballast (11.5-in) Asphalt layer (2-in)crushed stone layer (6-24 in)

Subgrade

Italy Ballasted Ballast (14-in) Asphalt layer (5-in)Supercompatto

layer (12-in)Subgrade

Spain Ballasted Ballast (14-in) Asphalt layer (5-6 in)Frost protection layer (12-16 in)

Subgrade

Germany Ballasted slab - Asphalt layer (8- in)Asphalt base

multilayerSubbase (20-

in)

France Ballasted Ballast (12-in) Asphalt layer (6-in)Adjustment layer (8-in)

Subgrade

USABallasted Ballast (8-12 in) Asphalt layer (6-8 in) No subballast Subgrade

Ballasted (rare) Ballast (8-12 in) Asphalt layer (8-in) Sub ballast Subgrade

India Ballasted Ballast (14-in)Blanket layer (upto 24-

in)Subgrade -

WHY ASPHALT (HMA) ?• Reduced track settlement and differential

settlements• Smoothening out variations in vertical stiffness.

The track deterioration in transition zones is 5-7 times

• Improved ballast behavior• Consistent support, thus reducing vertical

accelerations and displacements.• Completely water resistant

TRACK STRUCTURES USING HMA

Asphalt Underlayment Asphalt Combination

DESIGN PHILOSOPHY FOR HMA LAYER–BASED ON SUPERPAVE MIX DESIGN

• No design guidelines for design of HMA mix layer in US (AREMA/FRA).• Similar to a bottom layer of a perpetual highway pavement• HMA layer is insulated from sharp temperature variations, direct wheel

loads.Recommended parameters:• Medium modulus• Fatigue resistant (to accommodate high strains without cracking)• Low void• Fine grade mix (for moisture resistivity)• Fine grained binder• Durability

KENTRACK 4.0 ANALYSIS AND INPUTS• Compressive stresses at

the top of subgrade, (potential long-term trackbed settlement failure)

• Tensile strains, indicative of potential fatigue cracking.

• Predicting trackbedservice lives

Parameter Name Parameter ValuesWheel Load (pound force) 22000Distance between Loads (inch) 70Ballast Modulus (psi) 18000Subballast Thickness (inch) 4Subballast Modulus (psi) 12000Poisson’s Ratio for Subballast 0.35Poisson’s Ratio for HMA 0.45Volume of Voids for HMA(%) 3Temperature for HMA (°F) 50 (spring) 37 (autumn),

20 (winter) 77 (summer)HMA Modulus (lb per foot sq) 698000 (spring)Subgrade Thickness (inch) 200Poisson’s Ratio for Subgrade 0.4Poisson’s Ratio for Bedrock 0.5Ballast Thickness (inch) 8 inHMA Thickness (inch) VARIESSubgrade Modulus (psi) 10000

SENSITIVITY ANALYSIS• Variation with PG Binder Grade VS stresses &

Strains• Variation with Air Voids VS stresses & Strains• Variation with Aggregate Gradation VS stresses &

Strains• 𝑆𝑆𝑒𝑒𝑟𝑟𝑣𝑣𝑖𝑖𝑐𝑐𝑒𝑒 𝐿𝐿𝑖𝑖𝑓𝑓𝑒𝑒 𝑖𝑖𝑛𝑛 𝑎𝑎 𝑠𝑠𝑝𝑝𝑒𝑒𝑐𝑐𝑖𝑖𝑓𝑓𝑖𝑖𝑐𝑐 𝑙𝑙𝑎𝑎𝑦𝑦𝑒𝑒𝑟𝑟 = 1/((𝑃𝑃𝑟𝑟𝑒𝑒𝑑𝑑𝑖𝑖𝑐𝑐𝑡𝑡𝑒𝑒𝑑𝑑 𝑛𝑛𝑢𝑢𝑚𝑚𝑏𝑏𝑒𝑒𝑟𝑟 𝑜𝑜𝑓𝑓 𝑙𝑙𝑜𝑜𝑎𝑎𝑑𝑑 𝑟𝑟𝑒𝑒𝑝𝑝𝑖𝑖𝑡𝑡𝑖𝑖𝑡𝑡𝑖𝑖𝑜𝑜𝑛𝑛𝑠𝑠 𝑒𝑒𝑎𝑎𝑐𝑐ℎ𝑠𝑠𝑒𝑒𝑎𝑎𝑠𝑠𝑜𝑜𝑛𝑛)/(𝐴𝐴𝑙𝑙𝑙𝑙𝑜𝑜𝑤𝑤𝑎𝑎𝑏𝑏𝑙𝑙𝑒𝑒 𝑛𝑛𝑢𝑢𝑚𝑚𝑏𝑏𝑒𝑒𝑟𝑟 𝑜𝑜𝑓𝑓 𝑙𝑙𝑜𝑜𝑎𝑎𝑑𝑑 𝑟𝑟𝑒𝑒𝑝𝑝𝑖𝑖𝑡𝑡𝑖𝑖𝑡𝑡𝑖𝑖𝑜𝑜𝑛𝑛𝑠𝑠))

VARIATION WITH PG BINDER

-8.5

-8

-7.5

-7

-6.5

-6

-5.5

-5

-4.52 in 4 in 5 in 6 in 7 in 8 in

Thickness of Asphalt Layer as Asphalt Underlayment and Combination

Compressive Stresses (in psi) for different PG Binder grades

PG 64-22 Asphalt PG 70-28 Asphalt PG 76-34 Asphalt

PG 64-22 Combination PG 70-28 Combination PG 76-34 Combination

ALL GRANULAR

Compressive stress in Subgrade in psi

VARIATION WITH PG BINDER

5

5.5

6

6.5

7

7.5

8

8.5

2 in 4 in 5 in 6 in 7 in 8 in

Tens

ile st

rain

at B

otto

m o

f Asp

halt

Laye

r

(in

10-5

)

Thickness of Asphalt layer as Asphalt Underlayment and Combination

Tensile Strain (in 10-5) for different PG Binder GradesPG 64-22 Asphalt PG 64-22 Combination PG 70-28 AsphaltPG 70-28 Combination PG 76-34 Asphalt PG 76-34 Combination

VARIATION WITH PG BINDER

0.010.020.030.040.050.060.070.080.090.0

100.0

2 in 4 in 5 in 6 in 7 in 8 in 2 in 4 in 5 in 6 in 7 in 8 in

Desi

gn L

ife in

yea

rsDesign life of Subgrade and Asphalt layer

PG 64-22 Asphalt Underlayment PG 64-22 Combination PG 70-28 Asphalt Underlayment

PG 70-28 Combination PG 76-34 Asphalt Underlayment PG 76-34 Combination

Subgrade Asphalt Layer

ALL GRANULAR

OBSERVATIONS• Performance of the combination model is better than the underlayment model.• The PG 64-22, the most economic binder grade has a relatively better performance• For a binder grade, design life/stresses of 8-in combination models are higher by

just 15% to 20% compared to 4-in or higher combination model.• Since the design life for such a model is around 50 years, introduction recycled

material can be considered.• Increasing the upper grade of PG Binder decreases the stresses and strains.• Varying the lower asphalt binder grade has a more significant effect on the service

lives than varying the upper grade. However, these changing trends are opposite tovarying the lower grade. This means lowering the lower value of PG binder gradedecreases the design life.

VARIATION WITH AIR VOIDS

-8.5

-8

-7.5

-7

-6.5

-6

-5.5

-5

-4.52 in 4 in 5 in 6 in 7 in 8 in

Compressive Stresses variation with Percent Air Voids

2% Asphalt 4% Asphalt 9% Asphalt2% Combination 4% Combination 9% Combination

Thickness of HMA Layer as Asphalt Underlayment and Combination

ALL GRANULARCom

pres

sive

stre

ss in

Sub

grad

e in

psi

VARIATION WITH AIR VOIDS

5

5.5

6

6.5

7

7.5

8

2 in 4 in 5 in 6 in 7 in 8 in

Tens

ile S

trai

n at

Bot

tom

of A

spha

lt La

yer (

10-5

)Tensile Strain in Asphalt variation with Percent Air Voids

2% Asphalt 4% Asphalt 9% Asphalt2% Combination 4% Combination 9% Combination

OBSERVATIONS• For any air void, combination model is performing better than asphalt

underlayment method.

• More compact asphalt mix is performing better. Higher thickness with lower air voids leads to lesser stress on subgrade and lesser strains on asphalt.

• Difference in stresses/strains between 2% and 4% air void mix is lesser as compared to difference between 4% and 9% air voids mix.

• For any air void ratio, design life/stresses of 8-in combination models are higher by just 15% to 20% compared to 4-in or higher combination model.

VARIATION WITH AGGREGATE GRADATION• By varying gradations for different thickness of asphalt layer, asphalt

underlayment- gradation I (i.e. 2,56,40,16) is performing similar to gradationII (i.e. 6,65,50,10) for asphalt underlayment.

• Well graded aggregate mix. in 4-inch combination is performing better than8-inch asphalt underlayment.

• Design life of HMA in combination is 1.5-2 times higher as compared tounderlayment for respective cases.

• Use of inferior recycled material could be an economic and viable option• Consistent with past research results

COMMITMENT TO SUSTAINABILITY• Austria has used it in large stretches and have concluded a 17-25% long

term savings in maintenance

• The use of RAP/RAS can be maximized in railway use considering that thestress levels are low and asphalt layer is sufficiently insulated

• 33% reduction in energy loss with 50% RAP, as compared to HMA

• Depth of Ballast and subballast can be reduced, materials saving

RESULTS & CONCLUSIONS1. Use of asphalt as a component of railway track in a modern railwaysystem is an undisputed opportunity with limitless possibilities and hasthe potential to change the way the railway track is looked at.

2. The US has adopted the asphalt underlayment model at selectedlocations and problem spots, primarily for mitigating variation in stiffness.For some reason, its use has been restricted and thus potential valuablebenefits are being shunned.

CONCLUSIONS- KENTRACK3. KENTRACK needs to be modified for incorporation of speed of the trainas a vital input in the analysis. At present, only the static axle load and thenumber of repetitions is fed

4. The PG binder grades which are a default entry in the software but arefor only three of the 25 grades that are given in the PG nomenclaturematrix.

CONCLUSIONS5. It is observed that the 4-inch asphalt combination, which is the cheapest

model in adoption of HMA in railway track performs as good as all otheroptions.

6. AREMA has given the values of limited stress on subgrade which can becompared with the in-service stress. However, there are no values for limitingstrain at the bottom of the asphalt layer with which the actual strain levelscan be compared.

7. It is recommended that standard design specifications are prepared usinglatest Superpave method

8. The use of RAP/RAS can be maximized in railway use considering that thestress levels are low in comparison to highway and also that the asphalt layeris sufficiently insulated from the elements as well direct load.

CONCLUSIONS9. Based on the sensitivity analysis from this paper, inferences from

international case studies and the E* values indicating stiffness of the binder grades peaks at around 2% of the void ratio at which the 4-in asphalt layer in combination with 4 to 6-in subballast shows excellent performance.

10. Thus, ideally a high modulus HMA, a low range PG binder, with a void ratio of 2% and well graded aggregates as available can be used to create a hot mix asphalt which can have a sufficiently long service life fulfilling all the key performance criteria.

ACKNOWLEDGEMENT We sincerely want to thank Prof Jerry Rose, who has been a source of inspiration and constant help throughout our research.

Prof. Bill ButtlarGlen Barton Chair of Flexible Pavement Technology

College of EngineeringUniversity of Missouri

Conrad RuppertSenior Research Engineer

College of EngineeringUniversity of Illinois at Urbana-Champaign

LUV SEHGALRail Engineer, ArupLuv.Sehgal@arup.com646-341-2927

AMIT GARGChief Engineer| Indian Railwaysamitgargbangalore@gmail.com

THANK YOU