Slide 1

FLEXIBLE PAVEMENT DESIGN (IRC:37-2012)BACKGROUND IN BRIEF:

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

FLEXIBLE PAVEMENT DESIGN (IRC:37-2012)BACKGROUND IN BRIEF:

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.

FLEXIBLE PAVEMENT DESIGN (IRC:37-2012)BACKGROUND IN BRIEF:

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

The VDF is arrived from axle load surveys

LANE DISTRIBUTION FACTOREXAMPLE 1: Design the pavement for construction of a new flexible pavement with the following data:

4 lane divided carriagewayInitial traffic in the year of completion of construction = 5000 CV/day (both directions)% of single, tandem & tridem axles are 45%, 45% and 10% respectively

Traffic growth rate per annum (r) = 6.0 %Design life = 20 yearsVehicle damage factor (based on axle load survey) = 5.2CBR soil below the 500 mm of the subgrade = 3%CBR of the 500 mm of the subgrade from borrow pits = 10%DESIGN CALCULATIONSLDF for 4 lane divided carriageway = 0.75

Initial traffic = 2500 CVPD assuming 50% in each direction

VDF = 5.2 (given)

Cumulative no. of standard axles to be catered for in the design

N = 2500 x 365 x { ( 1+0.06)20 1}0.06x 0.75 x 5.2 = 131 msa4. Since there is a large difference between CBR of the embankment material (3%) and CBR of 500 mm subgrade (10%), effective CBR of the subgrade should be obtained

Effective CBR of the subgrade from Fig. above = 7 % Now, find the relevant resilient modulus for a known effective CBR :

MR (MPa) = 10 x CBR for CBR 5

MR (MPa) = 17.6 x (CBR)0.64 for CBR > 5

(1)(2)Since effective CBR > 7%, using eqn (2)

Resilient modulus (MR) is calculated as below:

MR = 17.6 x (7) 0.64 = 61.14 say 62 MPa Thickness of proposed Bituminous layer with VG 40 bitumen (40/60 as per IS:73-2006) with bottom DBM layer having air void of 3% (0.5% to 0.6% additional bitumen over OBC) over WMM and GSB = 185 mm at reliability of 90 %

Two fatigue equations were fitted , one in which the computed strains in 80% of the actual data in the scatter plot were higher than the limiting strains predicted by the model (and termed as 80% reliability level) and the other corresponding to 90% reliability level

Two equations for conventional bituminous mixes designed by Marshall method are as given below:

Nf = 2.21 x 10 04 x [1/t] 3.89 x [1/MR]0.854 (80% reliability) --- (1)

Nf = 0.711 x 10 04 x [1/t] 3.89 x [1/MR]0.854 (90% reliability) --- (2)Nf = fatigue life in number of standard axles

t = maximum tensile strain at the bottom of the bituminous layerMR = resilient modulus of the bituminous layerAs per the prevailing practice, the mixes used in the pavements under study section were generally designed for 4.5% air voids and bitumen content of 4.5% by wt. of the mix (which in terms of volume should come to 11.5%)Most literature recommend a factor C to be introduced in fatigue models to take into account the effect of air voids (Va) and volume of bitumen (Vb), which is given by the relationships

C = 10M, and M = 4.84 (

VbVa +Vb- 0.69)Corresponding to the values of Va & Vb as stated above, introduction of C in eqn (2) leads to Eqn (3)

Nf = 0.5161 x C x 10-04 x [ 1/t]3.89 x [1/MR]0.854----------- (3) recommended for 90% reliability Rutting model also established and calibrated with the R-56 studies using the pavement performance data collected during the R-6 and R-19 studies at 80% and 90% reliability levels.

Two equations are given below:

N = 4.1656 x 10 08 x [1/v] 4.5337 (80% reliability) --- (4)

RUTTING MODELN = 1.41 x 10 08 x [1/v] 4.5337 (90% reliability) --- (5)N = Cumulative no. of standard axles to produce rutting of 20 mm

v = Vertical strain in the subgradeSubgrade/Stabilised SubgradeGranular Sub-base (GSB) = 230 mmWet Mix Macadam (WMM) = 250 mmDense Bituminous Macadam (DBM) = 140 mmBituminous concrete = 50 mm90 mm50 mm150 mm100 mmFor BC or SDBC = in no case a single layer thickness should be less than 25 mm & not more than 100 mm For DBM or DGBM = in no case a single layer thickness should be less than 50 mm & not more than 100 mm For WMM = in no case a single layer thickness should be less than 75 mm & not more than 200 mm For GSB = in no case a single layer thickness should be less than 100 mm & not more than 225 mm 130 mm100 mm

SELECTION OF SUBGRADE CBR FOR PAVEMENT DESIGNThe CBR values of the subgrade soil varies along a highway alignment even on a homogenous section. 90th percentile CBR is recommended in the guidelines.

Method of determination of the 90th percentile is shown below

Say Sixteen CBR values have been obtained from different chainages of the road section. 3.55.28.06.88.84.26.44.69.05.78.48.27.38.68.97.6SELECTION OF SUBGRADE CBR FOR PAVEMENT DESIGN3.55.28.06.88.84.26.44.69.05.78.48.27.38.68.97.6Arrange the above 16 values in ascending order3.54.24.65.25.76.46.87.37.68.08.28.48.68.88.99.0 Now, Calculate the percentage greater than equal to each of the values:For CBR value of 3.5, % of values greater than equal to 3.5 = 16/16 * 100 = 100

For CBR value of 4.2, % of values greater than equal to 4.2 = 15/16 * 100 = 93.75SELECTION OF SUBGRADE CBR FOR PAVEMENT DESIGN3.54.24.65.25.76.46.87.37.68.08.28.48.68.88.99.0For CBR value of 4.6, % of values greater than equal to 4.6 = 14/16 * 100 = 87.5

For CBR value of 5.2, % of values greater than equal to 5.2 = 13/16 * 100 = 81.25Similarly for :For CBR value of 5.7, % of values greater than equal to 5.7 = 12/16 * 100 = 75.00For CBR value of 6.4, % of values greater than equal to 6.4 = 11/16 * 100 = 68.75For CBR value of 6.4, % of values greater than equal to 6.4 = 10/16 * 100 = 62.50

So onNow a plot is made between percentages of values greater than equal and the CBR values versus the CBR as follows

The 90th percentile CBR value = 4.7, and 80th percentile CBR = 5.7.According to the Asphalt Institute, USA, 87.5 percentile subgrade modulus is recommended for design traffic greater than one msa PAVEMENT DESIGN CATALOGUES FORBITUMINOUS SURFACING WITH GRANULAR BASE & GRANULAR SUB-BASEFive different combinations of traffic and material properties have been considered for which pavement composition has been suggested in the form of design charts presented in platesThe five combinations are as follows:Granular Base and Granular Subbase (Plate 1 to 8)

Cementitious Base and Cementitious Subbase with aggregate interlayer for crack relief. Upper 100 mm of the cementitious subbase is the drainage layer (Plate 9 to 12)3. Cementitious base and subbase with SAMI at the interface of base and the bituminous layer (Plate 13 to 16)4. Foamed bitumen/bitumen emulsion treated RAP or fresh aggregates over 250 mm cementitious subbase (Plate 17 to 20)5. Cementitious base and granular subbase with crack relief layer of aggregate layer above the cementitious base (Plate 21 to 24)

Treated as single granular layerSemi-infinite subgradeCROSS SECTION OF BITUMINOUS PAVEMENT WITH GRANULAR BASE AND GRANULAR SUB-BASE1. GRAULAR BASE & GRANULAR SUB-BASE

Note:

These charts are to be used for traffic of 2 msa and above. For traffic below 2 msa, refer IRC SP 72-2007. City roads should be designed for minimum 2 msa traffic.

Thickness design for traffic between 2 and 30 msa is exactly as per IRC 37-2001

In all cases of cementitious sub-bases, the top 100 mm thickness of sub-base is to be porous and act as drainage layer It is considered as a three layer elastic structure consisting of bituminous surfacing, granular base and subbase and the subgrade

The granular layers are treated as a single layer

Strain and stresses are required only for three layer elastic system

The critical locations are shown in the Fig. above

For traffic > 30 msa, VG 40 bitumen is recommended for BC as well as DBM for plains in India

Thickness combinations up to 30 msa are the same as those adopted in IRC:37-2001.BITUMINOUS PAVEMENTS WITH CEMENTED BASE AND CEMENTED SUBBASE WITH CRACK RELIEF INTERLAYER OF AGGREGATE

Fig. shows a five layer elastic structure consisting of bituminous surfacing, aggregate interlayer layer, cemented base, cemented subbase and the subgradeImportant points:

Material properties such as modulus and poissions ratio are the input parameters apart from loads and geometry of the pavement for the IITPAVE software.

For traffic > 30 msa, VG 40 bitumen is used for preventing rutting

DBM has air void of 3% after rolling (bitumen content is 0.5 % to 0.6% higher than the optimum)

Cracking of cemented base is taken as the design life of a pavement

For traffic > 30 msa, minimum thickness of bituminous layer consisting of DBM and BC layers is taken as 100 mm (AASTHO-1993) even though the thickness requirement may be less from structural consideration

Residual life of the bituminous layer against fatigue cracking is not considered since it cracks faster after the fracture of the cemented base.

Allowable horizontal tensile strain in bituminous layer is 153 x 10-6 for VG 40 mixes whereas as per IRC: 37 2001 this value is 178 x 10-6 (eqn 1) for a mix with VG 30

Allowable vertical compressive strain on subrade is 291 x 10-6 whereas as per IRC: 37-2001 this value is 370 x 10-6 (eqn.4).

Allowable tensile strain in cementitious layer is 64.77 x 10-6

Illustration

For traffic 150 msa

Subgrade CBR = 10 %

Since CBR value is > 5% therefore, use eqn MR= 17.6 x (CBR)0.64MR subgrade = 17.6 x (10) 0.64 = 75 Mpa

Pavement composition for 90% reliability is

DBM + BC = 100 mm

Aggregate inter layer = 100 mm (MR = 450 MPa)

Cemented base = 110 mm (E= 5000 MPa)Cemented Subbase = 250 mm (E= 600 MPa)Design criteria adoptedCorresponding to the values of Va & Vb as stated above, introduction of C

1. Nf = 0.5161 x C x 10-04 x [ 1/t]3.89 x [1/MR]0.8542. N = 1.41 x 10 08 x [1/v] 4.5337 (90% reliability) 3. N = RF

(11300/E0.0804 + 191)t12Design criteria adoptedN = RF

(11300/E0.0804 + 191)t12RF = Reliability factor for cementitious materials for failure against fatigue

= 1 for expressways, NHs & other heavy vol roads

= 2 for others carrying less than 1500 trucks per day

N= Fatigue life of the cementitious material

E= Elastic modulus of cementitious material

t = tensile strain in the cementitious layer, microstrainCEMENTED BASE AND CEMENTED SUBBASE WITH SAMI AT THE INTERFACE OF CEMENTED BASE AND THE BITUMINOUS LAYER

Fig shows a four layer pavement consisting of bituminous surfacing, cemented base, cemented subbase and the subgrade

Upper 100 mm of the cemented subbase having the gradation 4 shown in Table below is provided over the cemented lower subbase

For the given composition of pavement thicknesses, 90% reliability is adopted

SAMI is provided on top of cemented base

The reduction in thickness of the cemented base increases the bending stresses considerably because it is inversely proportional to the square of the thickness. Hence, design should be checked against fatigue damage.

Design criteria adoptedCorresponding to the values of Va & Vb as stated above, introduction of C

1. Nf = 0.5161 x C x 10-04 x [ 1/t]3.89 x [1/MR]0.8542. N = 1.41 x 10 08 x [1/v] 4.5337 (90% reliability) 3. N = RF

(11300/E0.0804 + 191)t12

FOAMED BITUMEN/BITUMEN EMULSION TREATED RAP/ AGGREGATE OVER CEMENTED SUBBASE

Fig shows four layer pavement consisting of bituminous surfacing recycled layer Reclaimed asphalt pavement, cemented subbase and the subgrade

Illustration:Traffic 150 msaSubgrade CBR = 10%, E subgrade = 17.6 (CBR).64 = 75 MpaMR = 3000 Mpa, MR of RAP = 600 MPa, E of cemented subbase = 600 MPaFrom the plate shown above,BC+DBM = 100, RAP = 160, Cemented subbase = 250 mm

Design traffic = 150 msa therefore, 90% reliability is adoptedDesign criteria adoptedCorresponding to the values of Va & Vb as stated above, introduction of C

1. Nf = 0.5161 x C x 10-04 x [ 1/t]3.89 x [1/MR]0.8542. N = 1.41 x 10 08 x [1/v] 4.5337 (90% reliability) CEMENTED BASE AND GRANULAR SUBBASE WITH CRACK RELIEF LAYER OF AGGREGATE INTERLAYER ABOVE THE CEMENTED BASE

Critical location for vertical subgrade strainIt is modelled as a five layer elastic structure in IITPAVE softwareFor reconstruction of a highway , designers may have a choice of bituminous surface, aggregate interlayer, cemented base while retaining the existing granular subbase.

The drainage layer in GSB is required to be restored in area where rainfall may damage the pavements

Using IITPAVE, it has been modelled as five layer elastic structure

The aggregate interlayer acting as a crack relief should meet the specifications of Wet Mix Macadam and if required, it may contain about 1 to 2 % bitumen emulsion if the surface of the granular layer is likely to be disturbed by construction traffic

Emulsion can be mixed with water to make the fluid equal to optimum water content and added to the WMM during the processing.

Design criteria adoptedCorresponding to the values of Va & Vb as stated above, introduction of C

1. Nf = 0.5161 x C x 10-04 x [ 1/t]3.89 x [1/MR]0.8542. N = 1.41 x 10 08 x [1/v] 4.5337 (90% reliability) 3. N = RF

(11300/E0.0804 + 191)t12

PERPETUAL PAVEMENTThe pavement having a life of 50 years or longer is termed as a perpetual pavement.

If the tensile strain caused by the traffic in the bituminous layer is less than 70 micro strains, the endurance limit of the material, the bituminous layer never cracks (Asphalt Institute, MS-4, 7th Edition 2007).

Similarly, if vertical subgrade strain is less than 200 micro strains, there will be little rutting in subgrade.

In such pavement design concept, different layers are so designed and constructed that only the surface layer is the sacrificial layer which is to be scrapped and replaced with a new layer from time to time.