The Benkelman beam is the simplest and the oldest deflection test device, developed in the United States in the mid-1950s. Its used to measure the structural capacity of a flexible pavement.

1. Benkelman Beam Deflection Study
Method to access pavement condition and overlay design
Priyansh Singh
(Research Scholar)
catchpriyansh@gmail.com
Departemt of Civil Engineering
Indian Institute of Technology Delhi 110016
March 29, 2016

2. Introduction NDT Benkelman Beam Deflection Measurement Calculation Correction Overlay
Overview
Introduction
NDT
Benkelman Beam
Deflection Measurement
Calculation
Correction
Overlay
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3. Introduction NDT Benkelman Beam Deflection Measurement Calculation Correction Overlay
Pavement Maintinance
Pavement maintenance rehabilitation and strengthening 63
Levelofservice
Warning level
Minimum allowable (action) level
Action period
No maintenance
(curve A)
Maintenance
(curve B)
Planned
rehabilitation
(curve C)
Inevitable
rehabilitation
End of extension
period
Life duration of pavement (years)
Figure 15.1 Effect of pavement maintenance and rehabilitation to pavement level of service level and pave
ment life duration.
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4. Introduction NDT Benkelman Beam Deflection Measurement Calculation Correction Overlay
Structural Evaluation of Pavement
Pavement performance is a function of its relative ability to
serve traffic over a period of time.
To check this ability Structural Evaluation of Pavement is
necessary.
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Structural Evaluation of Pavement
The methods of structural evaluation can be classified in two
categories:
Destructive Testing
Non Destructive Testing (NDT)
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6. Introduction NDT Benkelman Beam Deflection Measurement Calculation Correction Overlay
Overview
Introduction
NDT
Benkelman Beam
Deflection Measurement
Calculation
Correction
Overlay
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7. Introduction NDT Benkelman Beam Deflection Measurement Calculation Correction Overlay
Non Destructive Testing (NDT)
NDT devises are Instruments imposing a load to the pavement
surface and measuring the oncoming surface deflection;
so-called deflection measuring devices.
The magnitude of the deflection is an indicator of the
pavement capacity to withstand further traffic loading.
The greater the oncoming deflection, the lesser the
structural capacity
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8. Introduction NDT Benkelman Beam Deflection Measurement Calculation Correction Overlay
Non-Destructive Structural Evaluation
1. Static devices
have to stop to take measurements
2. Semi-static devices
moving slowly during measurements
3. Moving devices
taking deflection measurements as they move
(a) Benkelmen Beam (b) Curviameter (c) Rolling dynamic de-
flectometer
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9. Introduction NDT Benkelman Beam Deflection Measurement Calculation Correction Overlay
Overview
Introduction
NDT
Benkelman Beam
Deflection Measurement
Calculation
Correction
Overlay
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Benkelmen Beam
The Benkelman beam is the simplest and the oldest deflection
test device, developed in the United States in the mid-1950s
Simplicity in taking measurements.
lowest purchasing cost.Pavement evaluation and measurement of functional and structural characteristics
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11. Introduction NDT Benkelman Beam Deflection Measurement Calculation Correction Overlay
Benkelmen Beam
loading wheel axle (back axle). Details of the Benkelman beam and deflection taking read-
ings are shown in Figures 16.39 and 16.40.
The truck is loaded so that the rear single axle imparts a standard axle load, typically
80 kN (18,000 lb) with the dual tyres inflated to 480 to 550 kPa (70 to 80 psi); loading con-
ditions to the previous one may vary in some countries.
2.44 m or 2.50 m
Approx. 1.30 m
Pivot
Pivot
Varies
VariesUsually 1.22 m or 1.25 m
Road surface
Elevation
Arm tip
Arm of
the beam
Initial position
of wheels
Twin supporting
feet
Dial gauge
Supporting
frame
Single foot
Plan view
Figure 16.39 Diagrammatic representation of the principle of the Benkelman beam. (Adapted from Norman,
P.J. et al., Pavement Deflection Measurements and Their Application to Structural Maintenance and
Overlay Design, TRRL Report LR 571. Transport and Road Research Laboratory. Crowthorne,
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12. Introduction NDT Benkelman Beam Deflection Measurement Calculation Correction Overlay
Selection of Test Point
In each road section of uniform performance minimum of ten
points should be marked at equal distance in each lane of traffic
for making the deflection observations in the outer wheel path.
The interval between the points should not be more than 50m.
For more than two lanes point marked should be
staggered.
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Overview
Introduction
NDT
Benkelman Beam
Deflection Measurement
Calculation
Correction
Overlay
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Static Deflection Measurement
The point on the pavement to be tested is selected and
marked. For highways, the point should be located 60cm
from the pavement edge if the lanewidth is less than 3.5m
and 90cm from the pavement edge for wider lanes. For
divided four lane highway, the measurement points
should be 1.5m from the pavement edge.
The dual wheels of the truck are centered above the
selected point.
The probe of the Benkelman beam is inserted between the
duals and placed on the selected point.
The locking pin is removed from the beam and the legs are
adjusted so that the plunger of the beam is in contact with
the stem of the dial gauge. The beam pivot arms are
checked for free movement.
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Static Deflection Measurement
The dial gauge is set at approximately 1cm. The initial
reading is recorded when the rate of deformation of the
pavement is equal or less than 0.025mm per minute.
The truck is slowly driven a distance of 270cm and slopped.
An intermediate reading is recorded when the rate of
recovery of the pavement is equal to or less than 0.025mm
per minute.
The truck is driven forward a further 9m.
The final reading is recorded when the rate of recovery of
pavement is equal to or less than 0.025mm per minute.
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Overview
Introduction
NDT
Benkelman Beam
Deflection Measurement
Calculation
Correction
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Calculation of Deflection
Subtract the final dial reading from the initial dial reading.
Also subtract the intermediate reading from the initial
reading.
If the differential readings obtained compare within
0.025mm the actual pavement deflection is twice the final
differential reading.
If the differential readings obtained do not compare to
0.025 mm, twice the final differential dial reading
represents apparent pavement deflection.
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Calculation of Deflection
Apparent deflections are corrected by means of the following
formula:
XT = XA + 2.91Y (1)
Where, XT is True pavement deflection
XA is Apparent pavement deflection
and, Y is twice the difference between the final and
intermediate dial readings.
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Overview
Introduction
NDT
Benkelman Beam
Deflection Measurement
Calculation
Correction
Overlay
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Temperature Correction
The standard temperature is recommended to be 35 .
It is recommended to conduct study when temperature is
in range of 30-35
Above 30 temperature and deflection has linear
relationship.
if the deflection is measured at a pavement temperature of
37 , the correction factor will be 0.02mm(=2x.01) which
should be subtracted from the measured deflection.
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Seasonal Correction
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Overview
Introduction
NDT
Benkelman Beam
Deflection Measurement
Calculation
Correction
Overlay
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Calculation of Characteristic deflection
Mean Deflection : ¯X =
ΣX
n
(2)
Standard Diviation : σ =
Σ(X − ¯X)2
n − 1
(3)
Characteristic deflection in mm:
For Major Roads : DC = ¯X + 2σ (4)
For other Roads : DC = ¯X + σ (5)
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Overlay Design
400
Chorocteristic Deflection, mm
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Thanks
Priyansh Singh
catchpriyansh@gmail.com
End of the Slide
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