2017.06.19 FwdMan2.8.17.docx 10-73
Decrease
It is commonly accepted in pavement engineering that the measured pavement deflections
should decrease as the distance from the centre of the load and point of measurement
increases. In other words, the farther a deflector is positioned from the centre of a load, the
smaller deflection it should measure. This is true in theory, but sometimes not in practice.
Pavement cracks, joints, and other irregularities (such as defective deflectors) result in “non-
decreasing deflections” whereby some outermost sensor records a higher deflection than its
neighbour who might be closer to the load impact point. This results in data that is extremely
difficult or impossible to analyze.
Roll Off
At the end of the sampling interval (60 msec), the program expects the deflection time history
to return to less than 90% of the peak value. Roll-off errors may occur when a deflector is
lowered onto a piece of gravel or some other unstable surface, then falls off when the weights
drop. A roll-off error can also occur if the pavement is experiencing excessive vibrations due
to heavy traffic in an adjoining lane. Finally, a roll-off error may be caused by a defective
deflector.
Overflow
Most deflectors delivered with Dynatest FWD/HWDs are capable of measuring deflections of
up to 2000 microns (80 mils). Deflectors delivered with some systems are rated at 2450
microns (100 mils). If the range is exceeded, the deflectors may exceed their stated accuracy
and the quality of the data may be in question. Overflows can occur on soft pavements, near
joints and corners on Portland Cement Concrete, or may be caused by defective deflectors.
Repeatability
Some agencies (most notably the FHWA LTPP group in the USA) utilize a repeatability
check as an additional quality assurance measure. Repeatability specifications require that a
series of consecutive similar drops give similar results. These specifications are intended to
alert the operator in the event that the testing is in some way affecting the physical properties
of the pavement structure, or that the surface of the pavement is unstable. They also alert the
operator to variations in loads or deflections caused by defective equipment.
The program allows the operator to specify the allowable variation in load and/or deflection,
both in terms of the actual measurement units as well as in percent. Seating drops can also be
included in the repeatability check. Note that the test only applies to a series of drops that are
conducted from the same drop height (or the same target value).
To Next Page
Loading...