strl-evaluation-overlay.pdf

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Flexible Pavement

Structural Evaluation

Overlay Design

M.N.Nagabhushana

Principal Scientist

CRRI, New Delhi-110 025

5thSeptember, 2012


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DESIGN OF OVERLAY

D

Benkelman Beam Deflection Method

IRC:81-1997


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x

Direction of traffic

Pavement Shoulder

x

Direction of traffic

Pavement Shoulder

Rebound Deflection Study by

Benkelman Beam

0.9 m for a two lane

1.5 m for a divided0.6 m for a single lane


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Criteria for Classification of Pavement

Sections based on Pavement Condition

Survey

Classification Pavement Condition

Good No Cracking, rutting 20 mm

Sections with cracking exceeding 20per cent shall be treated as failed


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DEFLECTION BY BENKELMAN

BEAM


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BBD Study


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BBD Study


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Determination of Pavement Crust

NH-11


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Benkelman Beam

Length of the probe arm from pivot to 244cm

probe pointLength of measurement arm from pivot 122cm to dial

Distance from pivot to front legs 25cm

Distance from pivot to rear legs 166cm

Lateral spacing of front support legs 33cm

Equipment


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Equipment

A 5 tonne truck is recommended on the reaction. The vehicle shall have

8170 kg rear axle load equally distributed over the two wheels,

equipped with dual tyres.

Tyre pressure measuring gaugeThermometer (0-100oC)

A mandrel for making 4.5cm deep hole


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Procedure

(1) The point on the pavement to be tested is selected

and marked. For highways, the points should be located 60cm from the

pavement edge if the lane

width is less than 3.5m and 90cm from the pavement edge for wider lanes.

(2) The dual wheels of the truck are centred above the

selected point


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(3) The probe of the Benkelman beam is inserted between

the duals and placed on the selected point

(4) The locking pin is removed from the beam and the spindle is in

contact with stem of the dial gauge. The beam pivot arms are checkedfor free movement.

(5) The dial gauge is set at approximately 100mm. The initial reading isrecorded when the rate of deformation of the pavement is equal or less

than 0.025mm per minute

Procedure


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(6) The truck is slowly driven a distance of 2.70m

and stopped

(7) An intermediate reading is recorded when the rate of

recovery of the pavement is equal to or less then0.025mm per minute

(8) The truck is driven forward a further 9m

Procedure


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Procedure

(9) The final reading is recorded when the rate of recovery of

pavement is equal to or less than 0.025mm per minute

(10) Pavement temperature is recorded at least once every hourinserting thermometer in the standard hole and filling up the holewith glycerol

(11) The tyre pressure is checked at two to three hour interval during

the day and adjusted to the standard, if necessary


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(1) Subtract final dial reading from initial dial reading,

Subtract the intermediate reading from the initial reading

(2) If the differential reading obtained compared within0.025mm the actual pavement deflection is twice the final differentialreading

(3) If the differential reading obtained do not compare to

0.025mm, twice the final differential dial reading represents apparent

pavement deflection

CALCULATIONS


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(4) Apparent deflection are corrected by means of the following formula:

XT= XA+ 2.91 Y , where

XT = True pavement deflection

XA = Apparent pavement deflection

Y = Vertical movement of the front legs i.e. twice the difference

between the final and immediate dial

reading

CALCULATIONS


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CHARACTERISTIC DEFLECTION

(1) Mean deflection

(2) Standard deviation

(3) Characteristic deflection=Dc

D = Individual deflection

Da= Mean deflection

N = Number of deflection measurements

= Standard deviation


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Traffic : A = P (1 + r)n+10

A = Number of commercial vehicles per day for design

P = Number of commercial vehicles per day at lastcount

r = Annual rate of increase of commercial vehicles

(7.5%)

n = Number of years between the last count and the

year of completion of overlay construction

CHARACTERISTIC DEFLECTION


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Collection of Soil Sample for Field Moisture


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Sandy/Gravelly Soil for Low Rainfall Area


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Overlay Thickness Design Chart


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Design Example

The following deflection readings were observed

over a 1 km section of a National Highway

Chainage Deflection Chainage Deflection(mm) (mm)

100.000 1.50 100.550 1.60

100.050 1.24 100.600 1.75

100.100 1.20 100.650 1.80100.150 1.10 100.700 1.98

100.200 1.76 100.750 1.45100.250 1.80 100.800 1.72

100.300 1.90 100.850 1.79

100.350 1.78 100.900 2.01

100.400 1.92 100.950 1.98100.450 1.75 101.000 1.80100.500 1.50


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Data

Sl.

No. Di Df Di~Df

Legcorrectio

n

Deflecti

on dc Temp

Corrected for

Temp.

Correct

ed forMoistur

e Mean

Std.

Devi.

Mean+2*Std.

Devi

Final

Deflection

1

5.5

6.5

1.0

0.020

0.13

0.13

22

0.26

0.27

0.52

0.186

0.89

0.89

2

17.0

19.0

2.0

0.040

0.38

0.38

0.51

0.53

3 10.5 12.0 1.5 0.030 0.24 0.24 0.37 0.38

4

13.5

15.5

2.0

0.040

0.31

0.31

0.44

0.45

5 12.0 18.0 6.0 0.120 0.36 0.71 0.84 0.87

6 10.5 13.5 3.0 0.060 0.27 0.44 0.57 0.59

7

18.0

21.5

3.5

0.070

0.43

0.63

0.76

0.79

8 9.0 11.5 2.5 0.050 0.23 0.23 0.36 0.37

9 10.5 13.0 2.5 0.050 0.26 0.26 0.39 0.40

10

10.0

13.5

3.5

0.070

0.27

0.47

0.60

0.62

11 11.0 13.5 2.5 0.050 0.27 0.27 0.40 0.41


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The Subgrade Soil is clay of low plasticity of

PI < 15 with a moisture content of 11 percent.

The pavement temperature was observedto be 40 C. The area has annual rainfall less

than 1300 mm. The existing traffic is 5600 cv

per day. Design life is required for 10 years

Traffic growth rate is 8 percent and VDF is 4.5

Example


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N = 365 x [(1+0.08)101] x 5600 x 0.75 x4.5

-----------------------------0.08

= 99.2 msa say 100 msa

Calculation of CSA in millions


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DA= 1.68 mm = 0 .25

Dc = DA+ 2 = 2.18 mm

Correction factor for temp = 0.01mm/ C rise-ve for higher than 35 C

+ve for lower than 35 C

Deflection corrected for temperature:

= 2.18

5 0.01 = 2.13 mm

Calculations


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Referring the chart for clayey soil with PI


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