SAPEM Chapter 2 - Sanral

South African Pavement Engineering Manual
Chapter 2: Pavement Composition and Behaviour
12. VARIABILITY
Designing a pavement is not an exact science because of all of the variability. This variability occurs in four
predominant areas:
Materials:
Random variability exists in all materials, which is apparent when viewing quality and strength results.
Behaviour of materials is complex, resulting in variability in the form of random error.
The tests that are used do not entirely simulate field conditions and follow simplified protocols, thus
introducing more variability in the form of systematic error.
Material models are simplified for expedience and thus unable to fully capture complex material behaviour,
resulting in more variability and uncertainty.
Climate:
Temperature varies diurnally and seasonally, and prediction models are not exact.
Moisture varies seasonally and regionally, and prediction models are also not exact.
Weathering is a function of climatic conditions and material properties, which are both variable.
Durability phenomena that influence performance, e.g., oxidation of binders and carbonation of stabilisers,
change with climate and time and are co-dependent on many other factors, e.g., voids in an HMA mix.
Traffic:
Axle loads and wheel loads are not constant.
Tyre pressure, contact stresses at the surface, and contact area are all variable so the stresses experienced
by the road materials are not constant.
Traffic speeds, which influence the response of some materials, are variable.
Models to take account of different traffic loads, e.g., damage factor calculations and equivalent standard axle
loads, are over-simplified.
Construction:
Layer thicknesses in the pavement structure must meet specification requirements, but are nevertheless
variable.
Compaction of layers must meet specification requirements, but the achieved density is still variable.
Laboratory simulations of material behaviour cannot exactly simulate construction behaviour, thus introducing
systematic error.
It is important to take account of variability in a pavement during the design. A design that uses average results for
all of the input variables will have 50% reliability when the design life is reached. In addition, it is important to
reduce variability wherever possible, which can be achieved through improved quality management during the
material investigation phase, construction and testing. This limits the occurrence of failures under in-service
conditions.
At this point, it is opportune to introduce the concept of reliability. Reliability is a measure of precision, e.g., high
reliability of results implies high precision, and low random error, which is part of variability. Reliability does not
imply accuracy, however. Accuracy is the validity of the results, i.e., how closely do they resemble reality? For
example, high accuracy implies high validity and low systematic error, which is part of variability. In pavement
design, reliability is used to assess the confidence of a design. In important roads, high design reliability is required,
typically 90 to 95%. This ensures the likelihood of premature failure is
reduced. See also Chapter 10, Section 3.2.
Precision vs Accuracy
Accuracy is the degree of
closeness of measurements to
the actual value. Precision is the
reproducibility or repeatability of
the measurements.
Section 12: Variability
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