overlay design by benkelman beam

Strengthening of Flexible Pavements using

Benkelman Beam Deflection Technique

As per IRC 81:1997

ByProf. S. L. Bodas

Pavement Overlay-1• A distressed pavement requires

maintenance.

• Its expenditure can be optimized by proper planning,construction & quality control.

• Eg: By providing proper drainage / Preventing overloading beyond legal limits.

• Necessary Maintenance measures at an early distress stage can obviate major Maintenance expenditures in future.

Pavement Overlay-2• Pavement Maintenance measures can be

classified in two categories.• 1.Maintenance measures other than overlay

& 2.Maintenance measures with overlay• • Maintenance measures other than overlay

are minor maintenance or repair works.They do not enhance the structural strength of the pavement.

• They either improve functional standards or check the rate of deterioration.

Pavement Overlay-3• The Overlay is the extra thickness provided on

the pavement surface which strengthens the pavement structurally& thereby enhances its longevity.

• The overlay design comprises the determination of the thickness and the type of material to be laid over the existing surface to extend its longevity.

• Prior to 1960,(in India 1980) overlay design used to be based on judgement.& experience. The BB method is based on deflection approach & that by FWD is based on the mechanistic approach.

History of Benkelman Beam Deflection Technique-1

• Devised by A.C. Benkelman during WASHO road test in 1953 study R-6; guidelines were revised by IRC in 1997.

• Benkelman Beam deflection technique is • a popular method due to its simplicity &

low cost.

History of Benkelman Beam Deflection

Technique-2• Development of methods such as Benkelman Beam Deflection Method for evaluation of structural capacity of existing Flexible Pavements and also for estimation and design of overlays for strengthening of any weak pavement.

History of Benkelman Beam Deflection

Technique-3• It was undertaken to collect Indian data on

pavement deflection values before & after overlaying & other parameters like temperature, sub-grade soil type, moisture and their deflection on pavement deflection & service behaviour.

• Also based on Most R-6 research study.

Principles of BBD Study-1

• BBD is a slender 3.66 m long beam consisting of 2.44 m long probe arm & 1.22 m long probe arm rotating around the pivot.

• A standard loaded truck with rear axle load of 8100 kg and fitted with dual tyres, pressure 5.6 kg/cm2 is used in a BBD Study.

• Imagine a standard loaded truck standing at point A on the pavement surface. Due to load, A acquires a position A'. Probe point of probe arm of BBD is placed between the dual tyres.

Principles of BBD Study-II

• By suitably placing the probe between the dual wheels of a loaded truck, it is possible to measure the rebound and residual deflections of the pavement structure.

• Rebound deflection can be related to performance of pavement.

• Residual deflection may be due to non-recoverable pavement deflection.

Description of Benkelman Beam-III

• Details of Benkelman Beam:• Length of probe arm from pivot to probe point: 244 cm• Length of measurement arm from pivot to dial: 122cm• Distance from pivot to front legs: 25 cm• Distance from pivot to rear legs: 166 cm• Lateral spacing of front support legs: 33 cm

• Equipment needed:• A 5 tonne truck with dual tyres as the reaction. Rear

axle load of 8170 kg.(+/-1% tolerance of total load) Spacing between tyre walls: 30-40 mm, Tyres will be 10x20,12 ply. Pressure of 5.6 kg/cm2(+/- 5% tolerance)

Benkelman Beam

Description of Benkelman Beam-IV

• Equipments (contd):• Use of tyre with tubes & ribs recommended.• Tyre pressure measuring gauge• A mandrel for making 4.5 cm deep hole in the

pavement for temperature measurement. Diameter of hole 1.25 cm at surface, 1 cm at bottom.

Steps in Overlay study

• Step 1: Pavement condition Survey• Step 2: Deflection Measurements &

calculation of true deflections• Step 3: Collection of subsidiary data• Step 4: Application of corrections for temp.

& seasonal variation to true deflections

• Step 5: Measurement of traffic data & computation of Design traffic

• Step 6: Determination of characteristic deflection

• Step 7: Design of Overlay

Transverse Cracking

Fatigue Cracking

Block Cracking

Rutting

Pavement condition Survey

• Conducted primarily by visual observations & rut-depth measurements.

• Criteria for classification of pavement sections classification:

• Good: No cracking,rutting less than 10 mm

• Fair: No cracking or cracking confined crack in the wheel track with rutting between 10-20 mm.

• Poor: Extensive cracking &/or rutting >20 mm. Sections with cracking exceeding 20% shall be taken as failed.

Deflection Measurements-I

• Minimum section: 1 km.• Each section of uniform performance,

minimum 10 points,should be marked at equal distance (not exceeding 50 m) for making deflection observations in the outer wheel path.

• In transverse direction, the measurement point should be at a distance of 60 cm (for road width < 3.5 m lane) or at a distance of 90 cm (for road widths >3.5 m).

• Or at a distance of 1.5 from pavement edges (For 4-lane divided highway)

Calibration of Benkelman Beam

• Calibrate Benkelman Beam before use.Place the beam on a hard levelled ground. A number of metallic blocks of different thicknesses (measured with a micrometer, with perfect face) are placed under probe & dial gauge readings noted. It should read exactly one half thickness of the metallic block. Otherwise check dial gauge.

Benkelman Beam dimensions

2.44 m

1.22 m

Probe Arm Dial Gauge arm

2.7 m9 m

ABC

A

Initial Deflection Reading

30-40mm

BeamTyres

Tip of beam

AB

Intermediate Deflection Reading

ABC

Final Deflection Reading

Deflection Measurments-I

• Intial deflection was noted when rear axle of the truck was at A. (Recovery rate less than or equal to 0.025 mm/min.)

• Note next deflection reading after slowly moving the truck to B (2.7 m away from A) called intermediate deflection.

• After moving the truck to C (9 m away from B) next deflection reading called final deflection should be noted.

Deflection Measurments-II

• Record pavement temp. every hour at a small hole made with mandrel & filled with glyceral.

• Determine twice the difference between final & initial (Apparent Pavement deflection) as well as between intermediate & final readings (Y).

• If the second difference is less than 0.025 mm, then pavement deflection is twice of final reading.

• Otherwise, True Pavement deflection • = Apparent Pavement deflection + 2.91

times ' Y'

Deflection Measurments-III

• Sample calculation• Initial deflection 0.0 mm• Intermediate deflection 0.46 mm• Final deflection 0.49 mm• True Pavement deflection • =Apperent Pavement deflection + 2.91

times Y• =2x(0.49 - 0) + 2.91x2x(0.49 - 0.46)• =0.74 + 2.91x0.06 = 1.155

Correction for Temperature Variations-I

• The stiffness of bituminous layers changes with temperature of the binder & consequently the surface deflection of a given pavement will vary as per binder temperature. It is necessary that the measured deflections be corrected to a common standard temperature.

• For areas in the country having tropical climate the standard temperature recommended is 35C. Correction not applicable for roads with thin toppings.

Correction for Temperature Variations-II

• Correction should be 0.01 mm per degree centigrade change.

• The Correction:• Positive for temp.lower than 35C. • Negative for temp.higher than 35C.

• For altitudes > 1000 m, deflection measurments shold be done only when ambient temperature is 20C only. No correction.

Correction for Seasonal Variations-I

• For different types of soil, different types of plasticity & different rainfall intensities, different moisture corrections are needed.

• During different times in a year, soil moisture content goes on changing.

• Actual soil moisture content should be measured. Appropriate moisture corrections corresponding to different data should be calculated using proper graph. (6 graphs)

Determination of Characteristic deflection

• From series of True deflection values, its mean & Standard deviation values should be determined.

• For N.H & S.H.• Characteristic deflection=Mean +

2*SD

• For other roads• Characteristic deflection=Mean + SD

Problem 1

• The following are the BBD, field moisture content, and temperture readings at equidistant points obtained along a stretch of a major road. If the pavement is to sustain further 20 MSA of traffic repetitions, design an overlay thickness for the stretch. The average annual rainfall of the area is 1200 mm & the soil is of clayey nature with average plasticity index of 12.

Seletion of homogenous section

• When BBD survey is conducted on a long stretch of road, divide it into number of parts where deflection is somewhat same in magnitude.

• No specific methodology has been suggested in IRC81 except asking to do this visually & minimum length of a section should be less than 1 km.

• AASHTO has suggested that the cumulative data points to be plotted on a km scale. Best fit line should be drawn.