In-situ Pavement Deflection TestThe Benkelman Beam MethodLec. Israa SaeedPavement Maintenance- 4th stage- 2019-2020Highway and Transportation Engineering

Purpose

This test procedure covers the determination of the rebound deflection of apavement under a standard wheel load and tire pressure. This test isnecessary for pavement structural evaluation and in overlay design.

Apparatus A Benkelman Beam: The beam must be fitted with a satisfactory locking

device designed to secure the beam when moving to a new site and asuitable vibrator mounted at the pivot point. In sunny weather the beammay pass from shade into sunshine as the vehicle moves away. Therefore ashield should be used.

A truck or trailer: The weight of truck should be 12 tons, with rear axle load of 8.20 ±0.15 tones equally distributed on two dual tired wheels.

A thermometer with a range of 0-100°C in 1°C divisions.

Measuring Tape.

Procedure

Selection of Test Points: A minimum of 10 points should be marked at equal distance in each lane in

the outer wheel path. The interval between the points should not be more than 50m.

The test point shall be preselected and marked. For highway pavements, test points shall be located 6o cm from the pavement edge if the lane width is less than 3.5m and 90 cm or wider lanes.

Outer lane

Outer lane

Median

Rear outer wheel

Deflection Measurements The truck shall initially be positioned with the test wheel between 100 and

150mm to the rear of the test spot, i.e. position A. The probe of the beam shall be inserted between the dual tires of the test

wheel with the toe located on the test spot. The locking device shall be released and the rear of the beam adjusted so that

the plunger is in contact with the dial gauge. The dial gauge shall be set to read 1cm. The initial reading (S) is recorded when the rate of deformation is less than or

equal to 0.025mm/min. The truck is slowly driven at a speed of 8m/sec for a distance of 2.7m. An intermediate reading (I) is recorded when the rate of recovery is less than or

equal to 0.025mm/min. The truck is slowly driven at a speed of 8m/sec for a distance of 9m. A fin reading (F) is recorded when the rate of recovery is less than or equal to

0.025mm/min.

Calculations

The rebound deflection of the pavement shall be calculated in thefollowing manner:

Find (S-I) and (S-F) by subtracting the intermediate and the final readingfrom the initial reading.

If differential reading ≤ 0.025mm the true rebound deflection attemperature T shall be calculated as:

𝑿𝑿𝑻𝑻

= 𝟐𝟐(𝑺𝑺 − 𝑭𝑭)

If the indicators (S - I) and (S - F) differ by more than 0.025mm the truerebound deflection at temperature T shall be calculated as:

𝑿𝑿𝑻𝑻

= 𝟐𝟐(𝑺𝑺 − 𝑭𝑭) + 𝟓𝟓.𝟖𝟖𝟐𝟐(𝑰𝑰 − 𝑭𝑭)

Temperature Measurements

Temperature measurements must be made when the top layer of thepavement consists of 40mm or more of bitumen bound material.

The pavement rebound deflection at 20°C shall be calculated as:

𝑿𝑿𝟐𝟐𝟐𝟐 = 𝑿𝑿𝑻𝑻

+𝟐𝟐𝟐𝟐 − 𝒕𝒕𝟏𝟏𝟏𝟏𝟐𝟐

where:

X20 = temperature corrected rebound deflection in millimeters, and

t = temperature in degrees Celsius 40mm below the surface of thepavement.

ReportData Sheet for Benkelman Beam Test

Analyst: Date:

Location: Lane:

Wheel Path Distance:

Temperature

Pavement Temp.: Depth: Time:

Correction factor:

Test

Location S I F S-I S-F XT X20

1

2

3

4

H.W

Determine the average rebound deflection for this pavement section, thedeflection measurements were measured at 27°C pavement temperature.

Location S I F

1 1.00 0.977 0.927

2 1.00 0.939 0.858

3 1.00 0.982 0.968

4 1.00 0.889 0.862