Rigid Pavement Design

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Rigid Pavement Design for roads in India

Text of Rigid Pavement Design

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The guidelines on design of flexible pavement were first brought out in 1970

The guidelines were based on

CBR of subgrade soilTraffic in terms of no. of commercial vehicles (> 3 tonne laden wt.)FLEXIBLE PAVEMENT DESIGN (IRC:37-2012)BACKGROUND IN BRIEF:

Then IRC:37-1970 was revised in 1984 in which design traffic was considered in terms of cumulative number of equivalent standard axle load of 80 kN in msa

In addition, design charts were provided for traffic up to 30 msa using an empirical approach

Once again, IRC:37-1984 was revised in 2001 when pavements were required to be designed for traffic as high as 150 msa


Once again, IRC:37-1984 was revised in 2001 when pavements were required to be designed for traffic as high as 150 msa

This particular guidelines used a semi-mechanistic approach based on the results of the MORT&Hs research scheme R-56 implemented at IIT Kharagpur

The software, FPAVE was developed for the analysis and design of flexible pavements.

Multilayer elastic theory was adopted for stress analysis of the layered elastic system.


The traffic pattern has changed since then and so has the technology

The volume of tandem, tridem and multi-axle vehicles has increased manifold and heavier axle loads are common

Experience has been gained on the use of new form of construction and materials such as stone matrix asphalt, modified bitumen, foamed bitumen, bitumen emulsion, warm mix asphalt, cementitious bases and sub-bases since the publication of the last revision of the guidelines

Conventional construction material like aggregates is becoming progressively scarce on account of environmental concerns as well as legal restrictions on quarrying while the construction activity has expanded phenomenally. This has shifted focus from large scale use of conventional aggregates to use of local, recycled and engineered marginal aggregates in constructionFLEXIBLE PAVEMENT DESIGN (IRC:37-2012)SCOPE OF THE GUIDELINES

The guidelines shall apply to the design of new flexible pavements for ExpresswayNational Highways

State Highways

MDRFor the purpose of guidelines, flexible pavements include pavements with bituminous surfacing over:

Granular base and sub-base

Cementitious bases and sub-bases with a crack relief layer of aggregate interlayer below the bituminous surfacing

Cementitious bases and sub-bases with SAMI in between bituminous surfacing and the cementitious base layer for retarding the reflection cracks into the bituminous layer

Reclaimed Asphalt Pavement (RAP) with or without addition of fresh aggregates treated with foamed bitumen/bitumen emulsionA brief introduction to foamed asphalt In 1956 Prof. Ladis Csanyi came up with the idea of introducing moisture into a stream of hot bitumen, which effects a spontaneous foaming of the bitumen (similar to spilling water into hot oil).

In the foam state the bitumen has a very large surface area and extremely low viscosity making it ideal for mixing with aggregates Why is foamed asphalt only now gaining popularity? Part of the answer lies in the fact that the original bitumen foaming process was a proprietary product, patented by Mobil Oil, with the associated restrictions on the general use of the technology.

Furthermore, the lack of standardized mix design procedures meant that foamed asphalt was overlooked in preference for more well documented and familiar products. Foamed bitumen is produced by injection of a small amount of tap water into hot bitumen.

The fine droplets of water come into contact with the hot bitumen (typically 160 C to 170 C).

After the nozzle (pressure reduction) the rapid evaporation of water produces a very large volume of foam:

Theoretically 1 liter of water forms about 1200 liters of steam.How Foam Bitumen is manufactured?The steam expands until a film of bitumen holds the steam and air in bubbles.An important factor in foaming is the

nozzle design and

the injection pressure in order to obtain a good water droplet spray in contact with the hot bitumen

The foaming characteristics of a specific bitumen are further influenced by numerous factors:Temperature of the bitumen. For most bitumens the foaming characteristics are improved with higher temperature.

The expansion ratio increases with an increase in the amount of water added, whilst the half-life decreases.

The water helps in creating the foam, but the foam can collapse rather fast due to rapidly escaping steam.

It is known that e.g. silicone compounds can be effective anti-foaming agents.

On the other hand, compounds have also been identified that can increase the expansion ratio and the half-life of the foam from seconds to minutes.

Illustration Showing How Foamed Asphalt WorksThe Wirtgen 2500 Is Used to Both Pulverize the Road Bed and Apply the Foamed AsphaltIllustration Showing How Foamed Asphalt is Applied Inside the Mixing ChamberWhat's so special about foamed asphalt? Foamed asphalt epitomizes the asphalt industry drive towards energy efficient, environmentally friendly and cost effective solutions for road-building. Some of the most striking advantages of foamed asphalt are as follows:

1. The foamed bitumen increases the shear strength and reduces the moisture susceptibility of granular materials. The strength characteristics of foamed asphalt approaches that of cemented materials, but foamed asphalt is flexible and fatigue resistant. 2. Foam treatment can be used with a wider range of aggregate types than other of cold mix processes 3. Lower binder and transportation costs because foamed asphalt requires less binder and water than other types of cold mixing. 4. Saving in time because foamed asphalt can be compacted immediately and can carry traffic almost immediately after compaction is completed. 5. Energy conservation because only the bitumen needs to be heated while the aggregates are mixed in cold and damp (no need for drying). 6. Environmental side-effects of the evaporation of volatiles from the mix is avoided since curing does not result in the release of volatiles. 7. Foamed asphalt can be stockpiled with no binder runoff or leeching. Since foamed asphalt remains workable for very extended periods the usual time constraints for achieving compaction, shaping and finishing the layer are avoided. 8. Foamed asphalt layers can be constructed even in adverse weather conditions, such as cold or light rain, without affecting the workability or the quality of the finished layer Sub-base layer (cemented/unbound)Subgrade/Stabilised SubgradeBase layer (cemented/unbound)Aggregate interlayer for cemented base/SAMI layerBituminous layerStress Absorbing Membrane Interlayer (SAMI)Basically, it is a reinforced layer. Researchers have used FiberMat as SAMI

FiberMat is a process that sandwiches strands of chopped fiberglass between two layers of polymer modified asphalt emulsion, and is applied using specialized equipment.The first layer of emulsion provides a bond to the existing hard surface, with random interweaving of the fiberglass strands providing tensile strength to the mix, the second application of asphalt emulsion encapsulates the fiberglass, ensures the existing pavement is sealed, and is quickly covered with a thin veil of aggregateThe aggregate is seated into this second layer of emulsion using traditional rolling techniques and the SAMI is capable of accepting traffic in approximately 20 minutesThis reinforced layer can be used as a temporary wearing surface, on high volume roads, and is usually covered with a thin layer of hot mix asphalt within 14 days.Once capped with hot mix, it becomes a true SAMI.Its function is to seal the existing pavement with a resilient waterproof membrane, reduce reflective cracking through the new wearing surface, and ultimately prolong the useful service life of the road.

Hot mix asphalt overlaySAMIPre-existing pavementSAMI within the pavement structure

1st layer of asphalt emulsion2nd layer of asphalt emulsionChopped fiber glass

Even distribution of materialsPRINCIPLES OF PAVEMENT DESIGNA flexible pavement is modelled as an elastic structure.

Stresses and strain at critical locations are computed using a linear layered elastic model.

IITPAVE has been used for the computation of stresses and strains in flexible pavementsTop Down Cracking in Bituminous Layer: Fatigue cracking is conventionally considered as a bottom-up cracking phenomenon. Top down cracking has also been observed on high volume roads in the country, because of excessive tensile stresses developing at the top surface due to heavy axle loads.PRINCIPLES OF PAVEMENT DESIGN

Tensile strain, t, at the bottom of the bituminous layer and the vertical subgrade strain,v, on the top of subgrade are conventionally considered as critical parameters for pavement design to limit cracking and rutting in the bituminous layers and non-bituminous layers respectively. DESIGN STIPULATIONSTRAFFICTRAFFIC GROWTH RATEDESIGN LIFEVEHICLE DAMAGE FACTOR (VDF)LANE DISTRIBUTION FACTORCOMPUTATION OF DESIGN TRAFFICVEHICLE DAMAGE FACTOR(VDF)It is a multiplier to convert the number of commercial vehicles of different axle loads and axle configuration to the number of standard axle load repetition

It is defined as equivalent number of standard axles per commercial vehicle

The VDF varies with the vehicle axle configuration, axle loading, terrain, type of road and from region to region