Benkelman beam

Benkelman Beam Deflection StudyMethod to access pavement condition and overlay design

Priyansh Singh(Research Scholar)

[email protected]

Departemt of Civil EngineeringIndian Institute of Technology Delhi 110016

March 29, 2016

[email protected]

Introduction NDT Benkelman Beam Deflection Measurement Calculation Correction Overlay

Overview

Introduction

NDT

Benkelman Beam

Deflection Measurement

Calculation

Correction

Overlay

2/25


Pavement Maintinance

Pavement maintenance rehabilitation and strengthening 635

compatibility of estimated loading with actual traffic loading, structural behaviour of layer material inclusive of the subgrade used, efficiency/insufficiency of drainage system provided, reliability of pavement design methodology used and unforeseen traffic and environmental conditions) affect the pavement’s performance and hence the time when rehabilitation is needed. Rare is the case where rehabilitation is needed after the expiration of the design life.

One way to determine the rehabilitation time is by monitoring the provided level of ser-vice related to some surface distresses, mainly transversal/longitudinal evenness and crack-ing. Another way is by structural evaluation survey of the pavement.

The first approach, provided a minimum allowable level of service is set, can only give you the time when rehabilitation is inevitable. The second approach is more preferable since it is possible to determine the pavement’s remaining life and decide for planned rehabilita-tion before the stage of inevitable rehabilitation. Planned rehabilitation is particularly useful when extension of pavement design life is required.

Planned rehabilitation always requires less capital investment in comparison to ‘inevi-table’ rehabilitation, since it utilises the remaining life of the pavement found at the time of structural evaluation.

Figure 15.1 shows the effect of maintenance and rehabilitation actions to the pavement’s level of service and pavement’s life. It has been assumed that for a certain period, no main-tenance is required for, say, a 20-year design period pavement. Figure 15.1 also shows the warning level and the minimum allowable level, or action, of service set.

When maintenance (curve A) is applied, it will be impossible for the pavement to reach its design life. With periodic maintenance (curve B), the pavement will require an ‘inevitable’ rehabilitation a few years before the end of its design life. When planned rehabilitation is elected (curve C), the rehabilitation cost is less than the cost of rehabilitation applied later on (‘inevitable’ rehabilitation), even if pavement is maintained once to reach the end of the decided extension period.

15.4 TYPICAL SURFACE DISTRESSES IN FLEXIBLE PAVEMENTS

All types of surface distresses occurring in flexible pavements can be classified into four categories: cracking, distortion, disintegration and loss of skid resistance.

Leve

l of s

ervi

ce

Warning level

Minimum allowable (action) level Action period

No maintenance(curve A)

Maintenance(curve B)

Plannedrehabilitation

(curve C)

Inevitablerehabilitation

End of extensionperiod

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.

3/25


Structural Evaluation of Pavement

Pavement performance is a function of its relative ability toserve traffic over a period of time.

I To check this ability Structural Evaluation of Pavement isnecessary.

4/25


Structural Evaluation of Pavement

The methods of structural evaluation can be classified in twocategories:

I Destructive TestingI Non Destructive Testing (NDT)

5/25


Overview

Introduction

NDT

Benkelman Beam


Calculation

Correction

Overlay

6/25


Non Destructive Testing (NDT)

NDT devises are Instruments imposing a load to the pavementsurface and measuring the oncoming surface deflection;so-called deflection measuring devices.

I The magnitude of the deflection is an indicator of thepavement capacity to withstand further traffic loading.

I The greater the oncoming deflection, the lesser thestructural capacity

7/25


Non-Destructive Structural Evaluation1. Static devices

I have to stop to take measurements2. Semi-static devices

I moving slowly during measurements3. Moving devices

I taking deflection measurements as they move

(a) Benkelmen Beam (b) Curviameter (c) Rolling dynamic de-flectometer

8/25


Overview

Introduction

NDT

Benkelman Beam


Calculation

Correction

Overlay

9/25


Benkelmen BeamThe Benkelman beam is the simplest and the oldest deflectiontest device, developed in the United States in the mid-1950s

I Simplicity in taking measurements.I lowest purchasing cost.Pavement evaluation and measurement of functional and structural characteristics 759

After positioning the device, the truck is slowly driven forward (creep speed) and the maxi-mum deflection is recorded as the tyres pass the tip of the arm. The vehicle continues to move forward for some metres (approximately 7 more metres) and the minimum deflection is recorded.

The surface deflection is calculated using the manufacturer’s recommended formula, which is based on the configuration of the pivot on the beam. When a ratio of 2:1 is used (i.e. length of arm, 2.44 m; length of beam from pivot to the gauge, 1.22 m), the difference of the maximum and minimum reading is multiplied by 2 and this is the rebound deflection (RD) caused by the loaded wheel.

Deflection measurements should preferably be made when the road temperature is close to 20°C; measurements outside the range 10°C–30°C should be avoided because of the large temperature correction likely to be necessary. The tyre pressure and tyre dimensions should be in accordance with the specification followed.

The spacing between measurements is usually between 20 and 50 m.More information can be found in Norman et al. (1973), ASTM D 4695 (2008), AASHTO

T 256 (2011) or other related national guidelines/specifications.

16.4.1.1.1 Data processing and analysis

The aim of Benkelman beam testing, like any other deflection testing procedure, is to esti-mate the integrity, or remaining life, of the pavement and then to determine the thickness of the overlay, if required. This is obtained by correlating measured deflection with remaining life, expressed as number of standard loads.

One of the procedures to follow is the one described in the Asphalt Institute MS-17 (Asphalt Institute 3rd Edition). According to this procedure, the representative rebound deflection (RRD) value needs to be determined first. The RRD value is the mean of the deflection measurements (rebound reflections) adjusted for temperature and period of exe-cuting the measurements. The RRD can be computed from the following equation:

RRD = + ×( )x s c2 ,

where x is the arithmetic mean of the individual values that have been adjusted for tempera-

ture, xx f

n

i i=∑

; s is the standard deviation, sx x x

n=

−

−∑ ∑2

1; c is the critical period

Figure 16.40 Benkelman beam device. (Courtesy of Controls Srl.)

10/25


Benkelmen Beam

758 Highway engineering

rear single axle with the tip of the probe positioned approximately 1.38 m in front of the 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.

Table 16.14 Deflection measuring devices

Type of deflection measuring device Type of equipment Type of loading

Typical speed at testing (km/h)

Static Benkelman beam Rolling wheel 0Dynaflect Vibration load 0Road rater

FWDs Impulse loadingSemi-static (moving measurement vehicles with stationary measurement apparatus)

LaCroix deflectograph and other deflectograph versions (UK, PaSE Australian, etc.)

Rolling wheel load

2.5 or 3.5

Flash deflectograph (Vectra France) 3.5 or 7.0Curviameter (Euro Consulting Group) 18

Moving (moving measurement vehicles with non-stationary measurement apparatus)

Rolling dynamic deflectometer (RDD) (Texas)

Vibrating mass 5

Airfield rolling weight deflectometer (ARWD)

Rolling wheel 35

Rolling wheel deflectometer (RWD) (ARA for FHWA)

Up to 80

Traffic speed deflectometer (TSD) (Greenwood, DK)

60 to 80

2.44 m or 2.50 m

Approx. 1.30 mPivot

Pivot

Varies

VariesUsually 1.22 m or 1.25 m

Road surface

Elevation

Arm tipArm of

the beam

Initial positionof wheels

Twin supportingfeet

Dial gauge

Supportingframe

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, UK: Transport Research Laboratory, 1973.)

11/25


Selection of Test Point

In each road section of uniform performance minimum of tenpoints should be marked at equal distance in each lane of trafficfor making the deflection observations in the outer wheel path.The interval between the points should not be more than 50m.

I For more than two lanes point marked should bestaggered.

12/25


Overview

Introduction

NDT

Benkelman Beam


Calculation

Correction

Overlay

13/25


Static Deflection MeasurementI The point on the pavement to be tested is selected and

marked. For highways, the point should be located 60cmfrom the pavement edge if the lanewidth is less than 3.5mand 90cm from the pavement edge for wider lanes. Fordivided four lane highway, the measurement pointsshould be 1.5m from the pavement edge.

I The dual wheels of the truck are centered above theselected point.

I The probe of the Benkelman beam is inserted between theduals and placed on the selected point.

I The locking pin is removed from the beam and the legs areadjusted so that the plunger of the beam is in contact withthe stem of the dial gauge. The beam pivot arms arechecked for free movement.

14/25


Static Deflection Measurement

I The dial gauge is set at approximately 1cm. The initialreading is recorded when the rate of deformation of thepavement is equal or less than 0.025mm per minute.

I The truck is slowly driven a distance of 270cm and slopped.I An intermediate reading is recorded when the rate of

recovery of the pavement is equal to or less than 0.025mmper minute.

I The truck is driven forward a further 9m.I The final reading is recorded when the rate of recovery of

pavement is equal to or less than 0.025mm per minute.

15/25


Overview

Introduction

NDT

Benkelman Beam


Calculation

Correction

Overlay

16/25


Calculation of Deflection

I Subtract the final dial reading from the initial dial reading.Also subtract the intermediate reading from the initialreading.

I If the differential readings obtained compare within0.025mm the actual pavement deflection is twice the finaldifferential reading.

I If the differential readings obtained do not compare to0.025 mm, twice the final differential dial readingrepresents apparent pavement deflection.

17/25


Calculation of Deflection

Apparent deflections are corrected by means of the followingformula:

XT = XA + 2.91Y (1)

Where, XT is True pavement deflectionXA is Apparent pavement deflectionand, Y is twice the difference between the final andintermediate dial readings.

18/25


Overview

Introduction

NDT

Benkelman Beam


Calculation

Correction

Overlay

19/25


Temperature Correction

The standard temperature is recommended to be 35 ℃.I It is recommended to conduct study when temperature is

in range of 30-35℃I Above 30℃temperature and deflection has linear

relationship.I if the deflection is measured at a pavement temperature of

37℃, the correction factor will be 0.02mm(=2x.01) whichshould be subtracted from the measured deflection.

20/25


Seasonal Correction

Fig. 3. Moisture correction factor for Sandy/Gravelly subgrade for high rainfall areas

(Anrual rainfall > 1300 mm)

10

21/25


Overview

Introduction

NDT

Benkelman Beam


Calculation

Correction

Overlay

22/25


Calculation of Characteristic deflection

Mean Deflection : X̄ =ΣXn

(2)

Standard Diviation : σ =

√Σ(X − X̄)2

n − 1 (3)

Characteristic deflection in mm:

For Major Roads : DC = X̄ + 2σ (4)

For other Roads : DC = X̄ + σ (5)

23/25


Overlay Design400

Chorocteristic Deflection, mm

Fig. 9. Overlay Thickness Design Curves

19

Figure:24/25


Thanks

Priyansh [email protected]

End of the Slide

25/25