Key Findings From LTPP Analysis 2000-2003
The knowledge of pavement layer material properties is important for modeling pavement behaviors. The following are summaries from several
several LTPP material studies.
- Backcalculation of Pavement Layer Material Properties
- FWD Data For Quality Control/ Quality Assurance Measure
Pavement material properties such as stiffness (modulus of elasticity)
can be backcalculated from FWD layer-deflection data by
three approaches: the slab on elastic solid (ES) foundation, the
slab on dense liquid (DL) foundation, and the elastic layer procedures.
While the ES and DL approaches are used for rigid pavements
only, the elastic layer approach can be used for both rigid
and flexible pavements.
The LTPP program conducted two studies to demonstrate how
the LTPP deflection data can be used for backcalculation of pavement
layer material properties. In the first study, the ES and DL
approaches were used; the elastic layer approach was used in the
second study for backcalculation of pavement material properties.
The following findings are obtained from the two studies.
- Backcalculation of Rigid Pavement Layer Parameters
Report No. FHWA-RD-00-086
The layer material properties for rigid pavements are backcalculated using FWD deflection data from the LTPP database bythe ES and DL foundation approaches.
- For the SPS and GPS test sections studied
- PCC moduli:
The majority of the backcalculated PCC moduli using either the ES or DL subgrade approach fall in
the range of 25,000 to 55,000 MPa (3,625 to 7,977 ksi). It seems that the two backcalculation
approaches can generate acceptable PCC moduli.
The PCC moduli obtained by the ES approach are consistently lower than the DL approach. This
is expected because an ES foundation provides significant shear load redistribution while the DL
approach provides no shear load redistribution.
- Base moduli:
For treated base materials, the backcalculated base moduli were found to be within reasonable
ranges using either the ES or DL approach. The backcalculated base moduli obtained by the ES
approach appear to be higher than the DL approach.
For untreated base materials, the backcalculated base moduli obtained by the ES approach are
usually lower than the DL approach. Base moduli backcalculated using either the ES or DL approach fall in reasonable ranges.
- For the SMP test sections studied, both backcalculated moduli of elasticity of the subgrade using the ES approach
and moduli of subgrade reaction (k-value) using the DL approach fall in reasonable ranges, as do backcalculated radii of relative stiffness.
- For the SPS, GPS, and SMP test sections studied, the backcalculation results using either the ES or DL model do not
depend on FWD load level. This supports a similar finding obtained from earlier studies.
- Slab curling due to temperature increase during the day increases the variability in the backcalculation results.
Conducting FWD basin testing early in the morning when temperature gradients are low will reduce variability in backcalculated
- Poor correlation was found between backcalculated and laboratory elastic moduli of concrete.
- Backcalculation of Flexible and Rigid Pavement Layer Parameters
Report No. FHWA-RD-01-113
Layer properties of flexible and rigid pavements are backcalculated
from FWD deflection measurements in the LTPP database
using an elastic layer approach.
- The backcalculated elastic layer moduli obtained in this
study were found to be consistent with earlier studies.
- Seasonal effects: Moduli of AC surface layers, base
layers, and subgrade increase for the winter months
and decrease in the summer months.
- Temperature effects: Moduli of AC surface layers increase as mid-depth pavement temperature
- Time effects: Moduli of the AC and PCC surface layers increase with respect to pavement age. This is
due to hardening and curing.
- No significant difference in the computed layer moduli (Young's) was found between the wheel path and non-wheel
path deflection measurements.
Report No. NCHRP 20-50(9)
The feasibility of using FWD data as a quality control and qualityassurance measure during the construction of pavement
structures has been an interesting topic. The key findings of
NCHRP Project 20-50(9) are presented below.
- In general, FWD test results provide data that can be used with confidence to estimate material properties
mainly stiffnesses or moduliand their variations at each layer interface during new or reconstructed pavement
construction. These values generally follow a similar deflection pattern from layer to layer with respect to stationing.
Moreover, these FWD data are moderately well correlated to other measures (e.g., material densities) of
- The correlations between the FWD-derived unbound material parameters (e.g., stiffness) and many of the traditional
unbound material parameters were found to be fair to good. Similarly, the correlation between the FWDderived
bound layer parameters and some other available bound layer parameters also were found to be good.
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