Pavement Design

 INTRODUCTION : Based on structural behavior pavements are generally classified into two categories.

Difference Between  Flexible And Rigid Pavements:

The major differences between the type of pavements can be tabulated below:

A) Flexible pavements

• It consists of a series of layers with the highest quality materials at or near the surface. 
• It reflects the deformations of sub grade and subsequent layers on the surface.
• Its stability depends upon aggregate interlock particle friction and cohesion.
• Pavement design is greatly influenced by the subgrade strength.
• It functions by way of load distribution through the component layers.
• Design is based on IRC: 37-2001

B) Rigid pavement

• It consists of one course Portland concrete slab of relatively high bending resistance.
• It is able to bridge over localized failures and areas of inadequate support.
• Its structural capacity is supplied by the pavement slab itself by beam action.
• Flexural strength of concrete is major factor for design.
• It distributes load over a wide area of sub grade because of its rigidity and high modulus of elasticity.
• Design is based on IRC: 58-2002

COMPONENTS OF PAVEMENTS:

Sub grade: The lowest layer. A layer of natural soil properly prepared. The strength evaluation of a subgrade is highly decisive in pavement design. The common tests conducted are CBR test, California resistance Value test, Plate bearing test etc.

Sub base: Stabilized soil, gravel subbase, broken stone etc. For load dispersion from base course to subgrade.

Base Course: Improve the load carrying capacity. A layer between wearing course and subbase. Can be of graded stone, WBM and Bituminous layer. Under rigid pavements,                        1) it prevents mud pumping

 ii) Products the subgrade against frost action.

Wearing Course: To give a smooth riding surface. Dense materials. It resists pressure exerted by tyres and takes up wear and tear due to traffic. Water light layer against surface water infiltration. Generally of Bituminous or asphaltic.

DESIGN FACTORS:

(1) Design life: The number of standard axles that can be carried before strengthening of the pavement is necessary

(a) Flexible Pavements: Expressways 20 years; NH and SH: 15 years; Other categories 10 to 15 years

(b) Rigid Pavements: High volume roads 30 years; Low volume roads 20 years

(2) Anticipated traffic: The following formulae are used.

A = P [l+r]ⁿ

Where
A = Design traffic intensity in terms of number of commercial vehicles (laden weight > 3 tonnes) per day.
P = Number of commercial vehicles per day at last count.
r = Rate of growth of traffic (generally 7.5%) 
n = Number of years between the last count and till the end of life of pavement.

(3) Design Traffic: Design traffic is based on 7 day 24 hour traffic count as per IRC - 9

(4) Design Wheel Load:

Type of load	Flexible pavements	Rigid pavements
Maximum legal axle load	8200 kg	10.2 t
Maximum equivalent single wheel load	4100 kg	5.1 t
Maximum tendem axle load	14500 kg	19 t
Maximum tridem axle load	-	24 t

(a) Maximum Wheel Load:

>> Total load influences the thickness requirements of pavements.

>>  Tyre pressure influences the quality of surface course.

>>  Design of pavement is passed on 98^th percentile of axle load

(b) Contact Pressure:

Contact pressure = Load on wheel

Contact area or area of Imprint

>> Contact area is assumed as a circle though it is mostly ellipse.

>> At greater depth the effect of tyre pressure diminishes and the total load exhibits a considerable influence on the vertical stress magnitudes.

Three terms:

Tyre pressure	Exactly the same
Inflation pressure	Exactly the same

Contact Pressure:

For road vehicles 5 to 7 kg / cm2                            

Contact Pressure is found to be more than tyre pressure is less than 7 kg / cm2 and is vice versa when tyre pressure exceeds this value.

RIGIDITY FACTOR:

Rigidity factor =  Contact pressure/Tyre pressure

= 1, for tyre pressure of 7 kg / cm²                                                                                                                          

> 1, for low tyre pressure < 7 kg / cm²                                                                                                                              

 < 1, for high tyre pressure> 7 kg /cm²

>>  tyre pressure for the design is 0.8 MPa (8 kg I cm2) in the design of rigid pavements as per 1RC58

(c) EQUIVALENT SINGLE WHEEL LOAD (ESWL):

>> To maintain the maximum wheel load with in the specified limit and to carry greater load, it is necessary to provide dual wheel assembly to the rear axles of road vehicles.

>> The effect of dual wheel assembly is not equal to two times the load on any wheel. The pressure at any depth lies between single load and two times load carried by any one wheel.

>> ESWL may be calculated either by equivalent deflection or equivalent stress criterion.

>>  Equivalent deflection criteria is more reliable.

Repetition of Loads:

P₁ N₁= P₂ N₂

Where

N₁, N₂ = Number of repetitions

P₁, P2 = Corresponding loads

Mç = load method is based on this

-10₆ load repetitions = 1 million repetitions

FLEXIBLE PAVEMENT DESIGN METHODS

(A)  Empirical: Based on physical properties or strength pavements of soil sub grades.

> GI method, CBR, Stabilito meter and MC load methods.

(B) Semiempirical or Semi theoretical: Triaxial test method modified by Kansas state highway department.

(C) Theoretical Method: But-mister method

CBR Method

CBR design curves   

It is a simple Method:

Basis: A material with a given CBR requires a certain thickness of pavement layer as a cover.

Higher the load — higher the thickness of pavement

Thickness of sub base = total thickness — thickness over base.

Thickness of base = thickness over sub base — thickness over base.                                                                                                                   

IRC RECOMMENDATIONS:

• CBR test performed in laboratory test. In situ test are not recommended.
• For new roads compacted to OMC.
• Existing roads to FMC.
• Four days soaked CBR.
• Minimum 3 samples.
• Top 50 cm of subgrade be compacted up to 95 to 100 % of proctor density.
• Pavement of major roads be designed for 10 years life.
• Thin layer of wearing course shall not be treated as crust.

DRAW BACKS OF CBR METHOD: The current Indian practice suffers from the following drawbacks.

>> It does not take fully into account the damaging effects of heavier wheel loads and their frequency in the wheel load spectrum.

>> This does not consider whether the road is for multi — lane single carriage way or dual carriage way.

>> The design curves only give the value for the total thickness of pavement for different traffic intensity and CBR values of sub grade. These curves do not specify the thickness of sub-base, base and surfacing separately which are needed to evolve the most economic design pavement.

>>  It permits an equivalency factor of up to 2 for bituminous constructions to equate the thickness of bound base to that of conventional water bound macadam. But it is questionable to use an equivalency factor of up to 2 for the better load spreading properties of bitumen bound bases irrespective of the type of mix used. The equivalency factors for different types of such bases need to be evaluated under different conditions of pavement composition and environment.

Modified CBR Method (as per IRC 37-2001)

Design is based on cumulative no of standard axles, in the lane carrying maximum traffic

N= 365 {(1l+r)ⁿ -1} /n x AXDXF

Where: N = msa;

A = initial traffic n the year of completion of construction in terms of no of cv /day

n= design life in year

F= vehicle damage factor (VDF)

D= lane distribution factor (LDF)

LDF for various types of roads:

Type of traffic	LDF
Single lane traffic (cv in both the directions should be considered)	1.0
Two lane single carriage way roads (cv in both the directions should be considered)	0.75
Four- lane single carriage way roads (cv in both the directions should be considered)	0.4
Duel two lane carriage way roads (cv in one directions should be considered)	0.75
Duel three lane / four lane carriage way roads (cv in one directions should be considered)	0.60/0.45

                                                     

                                                                                                                             

VDF values: the designer should take realistic values of VDF after conducting axle load surveys

Note: Traffic in one direction is equal to half of the total traffic in both the directions. If significant difference between two streams occur then maximum traffic should be considered for the design.

Subgrade: For expressways , NH, SH and MDR should be compacted to 97% dry density with heavy compaction (modified proctor compaction) other cases 97% of dry density of standard proctor compaction

>> For Ex was. NH and SH sub-grade material should have the dry density < 1.75 g/cc

>> CBR value should be based on remoulded soils tested in the lab only, preferably by static compaction

>> CBR test samples should be soaked for 4 days

>> Minimum 3 samples should be tested with the maximum variation as shown below

CBR %	Max variation
5	± 1
5 - 10	± 2
11 - 30	± 3
> 31	± 5

>> Where the variations are more than above specified values 6 test samples are required

Equivalency factor: Damaging factor for different axle loads with respect to standard load.

Single axle load = {axle load in kg / 8200}⁴

Tandom axle load = {axle load in kg / 14500}⁴

Fatigue Criteria: Bituminous surfacing of pavements display flexural fatigue cracking of the tensile strain at the bottom of the bituminous layer is beyond certain limit:

Where

N_f  = number of csa to produce 20% cracked surface area

e = tensile strain at the bottom on BC layer (micro strain)

E = modulus of elasticity (MPa)

>The above equation is calibrated at 35 °C  with 80/100 bitumen it can be safely used for temperature ranges 20⁰ C to 40⁰ C.

>The poisson’s ratio of bituminous layer is 0.5 at temp. 35 °C to 40⁰ C and 0.35 for temp 20⁰  C to 30°C

Rutting criteria:

Nr=4.1656 * 10^-8 [1/e]^4.5337

Nr = number of cumulative standard axles to produce rutting of 20 mm

e = vertical subgrade strain in micro seconds

Note: The total cumulative standard axles to be used for the design of the pavement should include fatigue and rutting criteria also

Modulus of elasticity of subgrade Sub grade:

E  (in MPa) = 10 * CBR for CBR s< 5 and = 176 * (CBR)^0.64 for CBR> 5

Granular sub base and base:

E₂ (in MPa) = Ei*O.2*h^0.45

E₂=composite elastic modulus of granular sub base and base (MPa)

E₁= elastic modulus of subgrade (MPa)

h = thickness of granular layer (mm)

Note: Poisson’s ratio for both granular layer as well as sub grade layer is taken as 0.4