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3.1 Description of Dynaflect Test System
3.1.1 Description of Equipment
The Dynaflect, as previously mentioned, belongs to the dynamic
steady-state force-deflection group of NDT equipment. It is an electro
mechanical device for measuring the dynamic deflection of a pavement
caused by oscillatory loading. The testing system (84,104,108) consists
of a dynamic force generator mounted on a small two-wheel trailer, a
control unit, a sensor assembly and a sensor (geophone) calibration
unit. The Dynaflect can be towed by and operated from any conventional
passenger carrying vehicle having a rigid trailer hitch and a 12-volt
battery system.
The oscillatory load is produced by a pair of counter weights
rotating in opposing directions and phased in such a manner that each
contributes to the vertical force of the other, but opposes the horizon
tal force of the other, thereby canceling horizontal forces. The weight
of the unbalanced masses varies sinusoidally from 2500 lbs. to 1500
lbs., thereby producing a cyclic force of 1000 lbs. peak-to-peak at a
frequency of 8 Hz (see Figure 3.1). The cyclic force is alternately
added to and subtracted from the 2000-lb. static weight of the trailer.
The 1000-lb. cyclic force is transmitted to the pavement through a pair
of polyurethane-coated steel wheels that are 4-in. wide and 16-in. in
outside diameter. These rigid wheels are spaced 20-in. center to center
(see Figure 3.2).
The pavement response to the dynamically applied load is measured
by five geophones located as shown in Figure 3.2. The first geophone
measures the deflection at a point midway between the rigid wheels while
the remaining four sensors measure the deflection occurring directly