Highway Engineering

deflection that occurs at the time and position of application of the load is

termed the maximum deflection. The deflection that remains after the load is

removed (permanent deflection) is termed the recovery deflection. It is the cumu-

lative effect of the latter type of deflection that leads to cracking, rutting and

ultimately failure of a pavement.

The rear axle of a dual-axle lorry is loaded symmetrically to 6250 kg. It has

two standard closely spaced twin wheel assemblies at its rear, with approximately

45 mm between their walls in each case. The test commences with the loaded

wheels moved into the starting position (see Fig. 9.1). An initial reading is then

taken. The measurement cycle consists of the vehicle moving forward past the

front end or tip of the beam to a point where the wheels are a minimum of 3 m

past the end of the tip. During this time the deflection of the beam is monitored

and the maximum value recorded. The deflection remaining at the end of the

test is the recovery deflection for that point. The readings are temperature cor-

rected. Half the sum of the maximum deflection plus the recovery deflection

yields the deflection value, expressed in hundredths of millimetres.

256 Highway Engineering

Pavement surface

Pivoting

beam tip Beam

Pivot

Gauge

Starting position of

twin wheels

Figure 9.1Measurement of pavement deflection using the Benkleman beam.

The deflection beam is used on short lengths of highway of 1 km or less.

The deflectograph provides a quicker method of assessment and is more suit-

able for the assessment of long sections of highway. It operates on the same

principle as the deflection beam, but the measurements are made using trans-

ducers instead of dial gauges and all measurements are made in pairs, one in

each of the wheel tracks. Loading is provided by two beams mounted on an

assembly attached to the vehicle. An operating cable connects the deflection

beam system to the lorry (Position A in Fig. 9.2). As the vehicle moves, the cable

is let out at such a rate that the beam assembly remains stationary on the

highway allowing the deflection under the rear wheels to be estimated (Position