Benkelman Beam Deflection Method.

General approach.

Benkelman Beam is less expensive deflection measuring equipment and the method of study is simple and easy to carry out. The Indian Roads Congress 9IRC) has issued guidelines for conducting ‘Benkelman Beam Deflection’ (BBD) studies for structural evaluation of flexible pavements and for the design of overlay thickness for strengthening the same. Therefore, this method of pavement evaluation and overlay design is generally followed in this country.

The magnitude of ‘rebound deflection’ of a flexible pavement when a standard wheel load of a truck is moved forward is measured using Benkelman Beam. On the selected road stretch, the rebound deflection values due to the standard wheel load are noted down at each point. These deflection values generally vary from point to point and the overall characteristics deflection value of the pavement stretch is estimated by statistical analysis of the deflection data collected.

Equipment.

In order to conduct BBD studies on a flexible pavement the following equipment and accessories are required:

[a] Standard Benkelman Beam with dial gauge.

[b] Loaded truck as specified.

[c] Accessories and tools for collection of subgrade soil sample, measurement of pavement temperature, etc.

The basic unit of standard Benkelman Beam consists of (1) datum frame supported on a rear leg with provision for quick adjustment and two front legs with adjustment screws (2) a slender beam of total length 3.66 m, hinged to the datum frame at a distance of 2.44 m from the probe end and a clamping arrangement and (3) a dial gauge fitted with extension rods to be in contact with the deflection beam fixed at a distance of 1.22 m from the hinge, the dial gauge measures deflection readings indicated by the beam, to an accuracy of 0.01 mm generally the least count of the large needle of the dial gauge is 0.01 mm and that of small needle is 2.0 mm.

The truck load is adjusted such that the load on the rear is 8170 kg, equally distributed over the two sets of dual wheels on either side with 4085 kg load on each pair of dual wheels. The clear space gap between the two tyres of the dual wheels should be 30 to 40 mm to insert the probe of the deflection beam, the tyres shall be 10 x 20 ply inflated to a pressure of 5.60 $kg/cm^2$, the use of tyres with tubes and rib treads is recommended.

The other accessories required are 30 m tape, chalk for marking on the pavement surface, tools to dig the bituminous pavement layers, glycerol, thermometer to measure temperature up to 100°C with 1°C accuracy, augurs to collect soil samples from the subgrade, plastic bags with provision to be sealed to carry the soil samples without loss of moisture, pressure gauge to check Tyre pressure and spanners. Figure 10.11 shows the Benkelman Beam with the essential components marked.