TANUJ CHOPRAAssistant Professor

Thapar University, Patiala

DESIGN OF OVERLAY FOR FLEXIBLE ROAD & RUNWAY PAVEMENTS

PAVEMENT EVALUATION FOR OVERLAY DESIGN

“Pavement evaluation is a technique of assessing the condition of a pavement, both structurally and from the point of view of surface & functional characteristics”

Pavement Evaluation is a Measure of Pavement Performance

Parameters to be Evaluated:-

RoughnessSurface Distress

Rutting Fatigue cracking Potholes Transverse & Longitudinal Cracking

Skid ResistanceStructural Evaluation:- Deflection Value.

Objective of various parameter measurements: (roughness, deflection, skid resistance, visual distress survey like potholes....etc.)

Establish maintenance priorities. Determine maintenance and rehabilitation

strategies. Predict pavement performance at any time.

CONSEQUENCES OF DELAYED MAINTENANCE

.

Excellent

Failed Age

Patching, thinner overlays

Thicker overlays

Reconstruction

Joint and crack sealing, surface seals

OPTIMIZED MAINTENANCE ALTERNATIVE

Trigger Point for Treatment 1

Age or Traffic Loads

S

ervi

cea

bili

ty

Trigger Point for Treatment 2

Treatment 2 in Year Y at 10X Cost

Existing Performance Treatment 1 in Year A and Z at X Cost

STRUCTURAL EVALUATION WITH BENKELMAN BEAM FOR ROAD PAVEMENT

PRINCIPLE OF DEFLECTION METHOD FOR OVERLAY DESIGN

The rebound deflection method of pavement evaluation & Overlay design is based on the concept that stretches of flexible pavements that are in service and have been conditioned by traffic, would deform elastically under a wheel load and when the load is removed or is moved forward, the deflected pavement surface would rebound.

Performance of Flexible Pavement is closely related to elastic deflection of pavement under the wheel loads.

Pavement deflection is measured by using Benkelman beam which consists of beam of 3.66 m length pivoted at a distance of 2.44 m from the tip and a dial gauge is used for measuring deflection.

By suitably placing the beam between the dual wheels

of the standard axle, it is possible to measure the rebound deflection of pavement structure.

DEFLECTION SURVEY

Deflection Survey consists of two operations:- Pavement Condition survey for collecting

the basic information of the pavement. Actual Deflection Measurements by using

Benkelman beam, Falling Weight Deflectometer (FWD) or Loadman readings for Runway & Road Pavements.

PAVEMENT CONDITION SURVEY

Good: No cracking, rutting less than 10 mm Fair: No cracking or single crack in the wheel

track with rutting between 10 mm to 20 mm Poor: Extensive cracking and / or rutting

greater than 20 mm (sections with cracking exceeding 20 per cent shall be treated as a failed)

FACTORS AFFECTING REBOUND DEFLECTION

Subgrade soil type Moisture content of the subgrade soil Type and thickness of the pavement

component layer Temperature of the bituminous layers Previous loading history of traffic load on the

Pavement Magnitude, contact pressure and

configuration of the applied load and Other environmental factors

 

21 points in each Km Interval between the points should not be more

than 50 m Marking of points should be staggered for the

roads having more than one lane Transverse distance from the edge of pavement 60 cm if lane width is less than 3.5 m 90 cm if lane width is more than 3.5 m 150 cm for divided four lane highway Rear axle load 8170 kg Tyre pressure 5.6 kg / cm2

DEFLECTION MEASUREMENTS (IRC : 81-1997)

MEASUREMENTS ON FOUR LANE DIVIDED CARRIAGEWAY

CALCULATIONS

D0 Initial reading (obtained when the rate of deformation of

pavement is less than 0.025mm per minute)

Di Intermediate reading (Truck is slowly moved to a distance of

270 cm and stopped and reading obtained when the rate of deformation of pavement is less than 0.025mm per minute)

Df Final reading (Truck is further driven to 9m & final reading

is obtained when the rate of deformation of pavement is less than 0 .025mm per minute)

Pavement temperature and tire pressure should be checked regularly & necessary correction should be applied

Overlay Deflection when Di – Df < 0.025 mm D= 2 (Do – Df) Overlay Deflection when Di– Df > 0.025 mm D= 2 (Do – Df) + 2K (Di – Df) Where K = 2.91

 

Deflection measurements are generally taken at 350C.

If the deflection measurements are taken at any other temperature, the deflection values are corrected by using correction factor of 0.01 mm/0C .

The correction is positive (+ve) for pavement temperature is lower than the 350C and negative (-ve) for pavement temperature higher than the 350C.

TEMPERATURE CORRECTION

Pavement deflection = f (change in climatic season of year)

Desirable to take deflection measurement during the season when the pavement is in its weakest condition

In India this period occurs soon after monsoon

When deflection are measured during the dry months, a correction factor is required

CF = maximum defection immediately after monsoon / minimum deflection in the dry months

SEASONAL VARIATION CORRECTION

Characteristics Deflection (DC) for major roads NH and SH 

DC = x + 2* SD Characteristics Deflection for all other roads  

DC = x + SD

Where x = Mean deflection, mm

CHARACTERISTIC DEFLECTION

COMPUTATION OF DESIGN TRAFFIC AS PER IRC:37-2001

365 * A [ (1+r)X – 1 ]NS = --------------------------------------- * F* D

r

NS = The cumulative number of standard axles for the design in

msa

A = Initial Traffic, in the year of completion of construction,

in terms of number of CVPD.

r = Annual growth rate of commercial traffic

x = Design life in years

F = Vehicle damage factor

D = Lane Distribution Factor.

DESIGN OF OVERLAY

The overlay thickness in terms of bituminous macadam (BM). In case other compositions appropriate equivalency factors are :

  1 cm of BM = 1.5 cm of WBM / WMM / BUSG1 cm of BM = 0.7 cm of DBM / AC / SDC

Where structural deficiency is not indicated thin surfacing may be provided to improve the riding quality as required.

The type of material to be used in overlay construction will depend on several factors such as the importance of the road, the design traffic, the thickness and condition of existing bituminous surfacing, construction convenience and relative economics. For heavily trafficked roads, it will be desirable to provide bituminous overlays.

Before implementing the overlay, the existing surface shall be corrected and brought to proper profile by filling the cracks, pot holes, ruts and undulations. No part of the overlay design thickness shall be used for correcting the surface irregularities.

GENERAL GUIDELINES (IRC:81-1997)

OVERLAY FOR RUNWAY PAVEMENT USING FWD DEFLECTIONS

Factory calibrated geophones are used to register peak deflections

The operating sequence is completely automated Pulse loads between 1500 to 2700 lb (7 to 120

kN) are produced with the model 8000 FWD Optimum features available with various

Dynatest FWD model include automated pavement temperature sensing and automated air temperature sensing