Key Findings From LTPP Analysis 2000-2003

MATERIAL CHARACTERIZATION

The knowledge of pavement layer material properties is important for modeling pavement behaviors. The following are summaries from several several LTPP material studies.

1.  
2. Backcalculation of Pavement Layer Material Properties
   • Backcalculation of Rigid Pavement Layer Parameters
   • Backcalculation of Flexible and Rigid Layer Parameters
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3. FWD Data For Quality Control/ Quality Assurance Measure

Backcalculation of Pavement Layer Material Properties

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.

1. 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.
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   • For the SPS and GPS test sections studied
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     • 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.
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   • 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 parameters.
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   • Poor correlation was found between backcalculated and laboratory elastic moduli of concrete.
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2. 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.
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   • The backcalculated elastic layer moduli obtained in this study were found to be consistent with earlier studies.
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     • Seasonal effects: Moduli of AC surface layers, base layers, and subgrade increase for the winter months and decrease in the summer months.
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     • Temperature effects: Moduli of AC surface layers increase as mid-depth pavement temperature decreases.
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     • Time effects: Moduli of the AC and PCC surface layers increase with respect to pavement age. This is due to hardening and curing.
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   • No significant difference in the computed layer moduli (Young's) was found between the wheel path and non-wheel path deflection measurements.

FWD Data for Quality Control/ Quality Assurance Measure

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 pavement quality.
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• 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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