Benkelman Beam Deflection Studies

Transportation Engineering Section

Introduction

Benkelman Deflection Studies(IRC-81:1997)

Pavement

Benkelman Deflection Studies(IRC-81:1997)

Static Load Test Procedure

wheel

Benkelman Beam

Evaluation Of Structural Capacity Of

Existing Pavement

Estimation And Design Of Over Lay

For Strengthening Of Weak Pavement

Need of Evaluation

• Pavement deteriorate functionally and structurally with time due to traffic loading and the different climatic condition.

• It is necessary to evaluate the condition of existing pavement in terms of functionally and structurally.

OVERLAY• Pavement that do not have adequate structural

strength to carry out the projected future traffic will have to be reinforced by providing additional pavement layer

Scope

Performance Of Flexible Pavement

Elastic Deflection Of Pavement Under Wheel Load

Sub Grade Soil Type And Its Moisture Content

Compaction

Thickness

Quality Of Pavement Course

Drainage Condition Etc.

• This test procedure covers the determination of the rebound deflection of a pavement under a standard wheel load and tyre pressure.

Rebound deflection

Deflected Pavement Structure Bounce Back To Original Shape

Deflected Surface Original Position

LOAD

LOAD

Load removed

Equipment

1. Benkelman beam –

• Consist of slender beam of length 3.66 m.

• Pivoted at 2.44 m from probe.

• distance from pivot to dial gauge 1.22 m.

• Distance from pivot to front leg 25 cm.

• Distance from pivot to rear leg 1.66 m.

2. Loaded truck • Weight of truck 12 t• Rear axle load 8170 kg(dual tyre)• Spacing between tyres 30-40 mm.• Inflation pressure 5.6 kg/sq.cm.

3. Accessories –• Tyre pressure measuring gauge.• Thermometer(0-100 °c) with 1 ° division.• Measuring tape.etc

Deflection survey

Deflection Survey

1 .Pavement Condition Survey

2.Measurement Of Deflection

Done As Per IRC-81:1997

• Deflection Measurement –• Point selection –

• 1 km road stretch .• Minimum 10 points at 50 m interval .

• Location Of Point –

• Lane Width(m)• Distance From Lane Edge(cm)• Less than 3.5 • 60• More than 3.5 • 90

Procedure

A B C2.7 m 9 m

1. Select the points and marked.2. The dual wheel of the truck is centered above the mark.3. The probe of the benkelman beam is placed between the

dual tyres at the marked position.

4. Dial gauge is set at 1 cm.5. Initial reading (s) is recorded when rate of deformation is less

than or equal to .025 mm/min.

6. Truck is slowly driven (at speed 8-10 m/s appr.) at a distance of 2.7 m. and stopped.

7. Intermediate reading (I) is recorded.8. Truck is driven forward a further 9 m.9. Final reading (F) is recorded.10.Pavement temperature is recorded atleast once in

each hour.11. Tyre pressure is checked at 2-3 hrs interval

during a day.

Calculation

•Find (S-I) & (S-F)

• If deferential Reading ≤ 0.025 mm (2.5 Divisions), then True Rebound Deflection At Temp. T Is

XT = 2(S-F)

•If deferential reading ≥ 0.025mm, then XT = 2(S-F) + 5.82 (I-F)

The pavement rebound deflection at a standard temperature of 20°C shall be calculated

Effect of temperature

• Standard temp 35 °c.• Correction is applied when min thickness of the pavement is

40 mm.• No correction for

– thin bituminous surfacing– severe cracking in pavement– bituminous layer is striped.– Cold and high altitude regions where daily temp. < 20 °c

• Correction will be positive when temp blow stand. Temperature• Correction will be negative when temperature above standard

temperature.• Correction factor is 0.01mm/ °c variation from standard temp.

Analysis of data

Over Lay Design For A Given Section Is Based Not On Individual Deflection Value But On Statistical

Analysis Of All Measurements In The Section

1.Mean Deflection

2.Standard Deviation

3.Charecteristic Deflection

DC = M + 2S, For NH & SHDC = M +S, for other roads

Concept of Over Lay

Undulated Surface Cracked Surface

Surface

Over Lay Surface

Thank you