Perpetual Pavement: Way Forward from Design to Quality Construction

Atasi Das, Assistant Vice President, G R Infraprojects, explains the concept of the Perpetual Pavement in India’s expressway constructions and its practical implementation with due consideration of layers, materials, and thicknesses that are unique and relevant to Indian conditions, and are according to the codes of the Indian Road Congress.

Expressway construction in India is broadly based on Smart Pavement technology with the design and construction of Perpetual Pavement (PP) on a large scale. Perpetual Pavement is being instituted in India for the first time in the construction of the 8-lane Delhi Vadodara Expressway and many more greenfield corridors which are in the awarding or bidding stage by NHAI. These include the Bangalore-Chennai 4-lane Expressway, and the Delhi-Amritsar-Katra 4-lane Expressway.

Although the concept of a PP is based on the published data and literature from other countries (mostly UK and USA), the practical implementation of the concept requires consideration of layers, materials, and thicknesses that are unique and relevant to Indian conditions, and according to the mandate of the Indian Road Congress (IRC) codes. There is a need to walk through the design, specifications, construction methodology, quality control and quality assurance.

Perpetual Pavement
Perpetual Pavements are based on the concept that if the horizontal tensile strain at the bottom of the asphalt mix layer and the vertical compressive strain on top of the subgrade are kept below specific thresholds, no bottom-up fatigue cracking and subgrade related rutting can be expected in the pavement for a long period of time (perpetuity), and that relatively minor surface cracks and rutting can be periodically dispensed with thorough periodic maintenance. Such a pavement would not require major rehabilitation for a long period of time (for example, 50 years), and thus would result in significant savings in terms of agency and user-time, money, materials, and energy.

The criteria for the strains have evolved over time, although the most frequently used values are 80 με, and 200 με, for cracking and rutting respectively. A properly designed and constructed PP consists of a strong subgrade, subbase, a base with “rich” asphalt content that enhances its fatigue cracking tolerance, a rut-resistance upper layer, and finally a durable wearing course. The use of these layers requires the use of some premium materials/layers – such as Stone Matrix Asphalt (SMA) binder/wearing course. In the Indian context, this poses a challenge since some of the required materials/layers are either rarely used or are being just introduced (for example, SMA). Furthermore, there are specified layers that do not exist in specifications of the countries which have so far successfully built PPs. Hence, in this article, PPs with layers that are relevant to the Indian conditions only are considered and analyzed.

The success of the new technology of Perpetual Pavement and the SMA is attributed to strict quality control and quality assurance in the execution stage.


1.1. Design of Perpetual Pavement
A pavement composition that satisfies the design guidelines and which has been implemented successfully by GR Infraprojects in the Delhi-Vadodara expressway is presented. The project map is shown in Figure 1. The pavement composition of the Perpetual Pavement is based on endurance limit concept using the IITPAVE software (Figure 2, 3 & 4). As per the IRC 37-2018 (Design of Flexible Pavement), the endurance limit for bituminous layer is 80µ and granular layer is 200µ.


1.2. Pavement Composition
Based on the availability of borrow soil, pavement composition is designed with CBR of 15%, for the Delhi-Vadodara Expressway (Figure 5: TCS of one carriageway).

Layer	Thickness (mm)	Resilient Modulus (MPa)	Poisson’s Ratio
Stone Matrix Asphalt (SMA)	50	2650	0.35
Dense Graded Bituminous Macadam (DBM)	250
Wet Mix Macadam (WMM)	150	276	0.35
Granular Sub-base (GSB)	200
Effective Subgrade (15% CBR)	1000	100	0.35

The pavement composition may be modified as per the project requirements, budget constraints, material availability, and expertise of the contractor. Based on the design concept, DBM is divided into three layers:

• Top Layer- 80mm DBM Grade 1 (37.5 NMAS)
• Intermediate Layer- 100mm DBM Grade 1 (37.5 NMAS)
• Bottom Layer- 70mm DBM Grade 2 (26.5 NMAS)

We have deep seated DBM of 250 mm total thickness. Deliberations started with using 3 layers of DBM with Grade 1. We know that DBM Grade 1 can have high air voids, and the NMAS size is not commensurate with the essence of the Perpetual concept worldwide. The closest we could suggest after all the research and discussion was use of Grade 2 DBM with higher bitumen content and lower NMAS for the bottom-most DBM layer, which has least chance of secondary compaction. After another round of deliberations, we got confident to build a sustainable pavement with this system.

1.3. Renewable Surface Layer - Stone Matrix Asphalt (SMA)
To achieve the goal of a 20-year surface life, the use of stone matrix asphalt (SMA) was chosen to resist both rutting and durability problems for the renewable surface layer. SMA was developed in Germany in the mid-1960s and it has been used successfully by many countries in the world as a highly rut-resistant bituminous course, both for binder (intermediate) course and wearing courses, especially for heavy traffic loads. It is a gap graded bituminous mix with stone-on-stone contact for effective load transfer. (Fig 6).


Materials for SMA

• Bitumen (VG 40)
• Coarse Aggregate (Crushed Rock)
• Fine Aggregate
• Mineral Filler (Stone dust and / or hydrated lime)
• Cellulose Fiber in Pelletized Form & Anti Stripping Agent

The design of SMA is done as per the MoRTH 2013 and IRC SP 79 specifications. As SMA mix contains a higher bitumen content & lower fines which tends to drain down the bitumen from mix, Cellulose fibres are required to hold the bitumen from draining out of the mix (Figure 7 & 8).


2. Plant Production
Some of the key features practised for smooth running of the 3-layer DBM system and the SMA are noted here:

• Two plants (240 TPH) on each project in order to keep the haulage distance under control & to avoid excessive drop in the mix temperature.
• Each of the plants is equipped with minimum five numbers of bitumen storage / service tank in order to simultaneously maintain the bitumen temperature.
• Each of the HMPs is equipped with automatic cellulose fibre feeder system and it is integrated with Hot Mix Plant operating program.
• Utilisation of Baghouse filler produced during DBM production to save Environment.

3. Construction Methodology
3.1. Dense Bituminous Macadam (DBM): Once the bituminous mix is prepared, laying and compaction should be done promptly. The surface texture of the top-most layer was made rough to improve the bond between the DBM and the SMA layer. The entire DBM layer is laid in full width using 2 pavers that operated in tandem with each other. Advanced sensors are used to ensure that this operation is completed in a precise manner. No longitudinal cold joint is provided in the pavement structure.

The compaction pattern followed in the DBM is as follows:

• 1 plain pass of the steel roller
• 2 low vibratory passes of the steel roller
• 2 high vibratory passes of the steel roller
• 1 low vibration pass of the steel roller
• 2 passes of the pneumatic tire roller
• 1 pass of the pneumatic tire roller for generating a rough surface texture
• Finally, a rolling finish was applied at 95°C

3.2. Stone Matrix Asphalt: Once the SMA mix design is finalized, the mix must be produced sequentially to ensure that a good quality mix is prepared. Initially, the aggregates must be dosed followed by the dosing of the mineral filler. This is followed by dosing pelletized cellulose fibers in the mix for about 2-3 seconds to ensure that the fibers are properly dispersed. Then, VG 40 bitumen is added to the mix in the required amount. The entire mix is then mixed in a batch-type HMA plant for about 30 seconds per batch. Finally, the produced mix is transported to the site for laying and compaction.


The compaction of the SMA mix should be done immediately after laying the mix on the site. One low vibration pass of the steel-wheeled roller must be done as soon as the mix is laid to prevent the escape of heat from the mix. This is followed by 4 normal passes of the roller to achieve the necessary in-situ density of the mix. Pneumatic tired rollers must not be used to compact the SMA mix as the rubber tires tend to pick up the mortar, eventually leading to surface defects.

• Physical compaction doesn’t happen in SMA; only Re-orientation / Re-arrangement of aggregate takes place which results into compaction.
• Rolling pattern shall be followed very cautiously.
• Rate of sprinkling of water during rolling shall be minimum to avoid sticking of aggregate from drum and rapid loss of temperature in mix.

Two rollers for immediate breakdown rolling to prevent heat escape in loose mix and rest 4 rollers for secondary rolling have been used. Compaction pattern followed is noted below:

• 1 Low Vibration Pass (Breakdown Rolling)
• 4 Plain Pass
• No PTR

SMA mix with PMB requires to be produced at 190°C and finishing of rolling shall be done at 130°C which is quite tough to achieve. During paving with PMB mixes for SMA, due to high viscosity of bitumen & higher filler content mix becomes stiff & starts dragging which results into blank spots & honeycombing on the surface. While with VG 40 production, paving, rolling, compaction & finishing are possible at temperature ranging from 160-100°C.


4. Quality Control and Quality Assurance for Construction of Perpetual Pavements
A systematic quality assurance test plan is formulated for each layer of the 3-layer DBM system and the SMA layer.

Particular		Performance Parameter	Instrument Name	Frequency		Corrective Measure
				Frequency & Location	Remarks
Dense Bituminous Macadam (DBM)	Layer 1	Density / % Compaction	Nuclear / Non-Nuclear Density Gauge	5 test point considering one sided full width and per 100 running meter along wheel path.	1. Resilient Modulus (MR) test shall be conducted on DBM pre-moulded lab specimens on monthly basis.  2. Full depth thickness shall be checked using GPR on DBM top layer.  3. Full width bituminous layer paving shall be completed using two pavers simultaneously (i.e., no longitudinal cold joint shall be allowed). 4. Roughness shall be checked on top of each bituminous layer.	If there is any anomaly (such as density variations, localized voiding, vertical segregation, debonding, and moisture presence) found in GPR data then FWD test shall be conducted and MR value shall be estimated and all data shall be interpreted in conjunction. If the desired design parameters are not met then expert advice shall be taken.
	Layer 2
	Layer 3	Density / % Compaction	Nuclear / Non-Nuclear Density Gauge	5 test point considering one sided full width and per 100 running meter along wheel path.
		Thickness	Ground Penetration Radar	-

Density and thickness are considered as the performance parameters for the DBM layers and the SMA layer. It is estimated using a Nuclear/ Non-nuclear density gauge (Figure 9). The full depth thickness is checked using a Ground penetration radar (GPR) on the DBM top layer and the SMA layer (Figure 10). A Network Survey Vehicle (NSV) which deploys a laser profilometer (Figure 11), measures the surface unevenness and road roughness (IRI) of the DBM and SMA layers. Only non-destructive testing measures are adopted to ensure proper quality control of the mixes.


Conclusion
There is no defined guideline for mix design of asphalt mix, layer thickness, grading and performance tests for perpetual pavement. Considering past works carried out abroad, it is found that maximum size of aggregate used shall be 25 mm and 26.5 mm for Indian grading consideration. In India, 37.5 mm size of aggregate is used for DBM 1 but larger size of aggregate use has some deficiency that it entraps water in the mix which creates vapour during hot summer and breaks/segregates aggregates from bitumen causes pavement distress. Therefore, it is recommended that DBM Grade 1 should be avoided in perpetual pavement.

Super Pave Mix Design Procedure is suggested in place of the Marshall Method to ensure that 2-3 % air void or less/close to 2% is available at 205 gyrations as mentioned in Austroads 2017.

Top surface shall be rut resistant and fatigue resistant. There is chance of top-down crack propagation due to tyre’s heavy concentration of load transfer and there is a chance of rut in the bituminous layer.

Permeability of the mix shall be checked using laboratory sample and core sample or prepare mould at laboratory with field compaction by trial error method. Permeability at field shall be 125×10-5 cm/s or less.

Author
Atasi Das, Assistant Vice President, G R Infraprojects