backcalculation _ pavement interactive

3
 Design » Structural Design » Backcalculation Publish date: August 13, 2007 | Author: Pavement Interactive Backcalculation “Backca lculation” is a mechanistic evaluation of paveme nt su rface deflection basins generated b y various pave ment deflecti device s. Backcalculation takes a measured surface deflection and attempts to match it (to with in some tolerable error) with calculated surface deflection generated from an identical paveme nt structure using assumed layer stiffnesses (moduli). The assumed layer moduli in the calculated model are adjusted until they produce a surface deflection that closely matches the measured one. The combination of assumed layer stiffnesses that results in this match is th en assumed to be near the actu in situ modu li for th e vari ous paveme nt layers. The backcalculatio n p rocess is usually iterative and normally done with computer software. Typical Flowchart  A basic flowc hart (patterned after L ytton, 1989 [1]) that represents the fundamental elements in all known backcalculation programs is shown as Figure 1. Briefly , these elements include: Measured deflections. Includ es the measured pavement surface deflections and associate d distances from th e load. Layer thicknesses and loads . Includ es all l ayer thicknesses and load levels for a specific test location. Seed moduli . The seed moduli are the initial moduli u sed in the computer p rogram to calculate surface deflections. The moduli are u sually estimated from user experi ence or variou s equations. Deflection calculation. Layered elastic computer programs are generally used to calculate a deflection basin. Error check . This element simply compares the measure d and calculated basins. There are various error measures whi can be used to make such comparisons (more on this in a subsequent p aragraph in this section). Search for new moduli . Various methods h ave been employed within t he variou s b ackcalculatio n p rograms to converge a set of layer moduli which produces an acceptable error between the measured and calculated defl ection basins. Controls on the range of moduli . In some backcalculation programs, a range (minimum and maximum) of moduli are selected or calculated to p rev ent program convergence to unreasonable moduli l eve ls (either high or low). C Backcalculation | Pavement Inter active http://www.pavementinteractive.org/article/backcalculation/ 1 of 3 6/ 23/2015 4: 31 PM

Upload: varunsingh214761

Post on 05-Nov-2015

214 views

Category:

Documents


0 download

DESCRIPTION

Backcalculation for FWD

TRANSCRIPT

  • Design Structural Design Backcalculation

    Publish date: August 13, 2007 | Author: Pavement Interactive

    Backcalculation

    Backcalculation is a mechanistic evaluation of pavement surface deflection basins generated by various pavement deflection

    devices. Backcalculation takes a measured surface deflection and attempts to match it (to within some tolerable error) with a

    calculated surface deflection generated from an identical pavement structure using assumed layer stiffnesses (moduli). The

    assumed layer moduli in the calculated model are adjusted until they produce a surface deflection that closely matches the

    measured one. The combination of assumed layer stiffnesses that results in this match is then assumed to be near the actual

    in situ moduli for the various pavement layers. The backcalculation process is usually iterative and normally done with

    computer software.

    Typical Flowchart

    A basic flowchart (patterned after Lytton, 1989[1]) that represents the fundamental elements in all known backcalculation

    programs is shown as Figure 1. Briefly, these elements include:

    Measured deflections. Includes the measured pavement surface deflections and associated distances from the load.

    Layer thicknesses and loads. Includes all layer thicknesses and load levels for a specific test location.

    Seed moduli. The seed moduli are the initial moduli used in the computer program to calculate surface deflections. These

    moduli are usually estimated from user experience or various equations.

    Deflection calculation. Layered elastic computer programs are generally used to calculate a deflection basin.

    Error check. This element simply compares the measured and calculated basins. There are various error measures which

    can be used to make such comparisons (more on this in a subsequent paragraph in this section).

    Search for new moduli. Various methods have been employed within the various backcalculation programs to converge on

    a set of layer moduli which produces an acceptable error between the measured and calculated deflection basins.

    Controls on the range of moduli. In some backcalculation programs, a range (minimum and maximum) of moduli are

    selected or calculated to prevent program convergence to unreasonable moduli levels (either high or low).

    Cite

    Backcalculation | Pavement Interactive http://www.pavementinteractive.org/article/backcalculation/

    1 of 3 6/23/2015 4:31 PM

  • Figure 1. Backcalculation Flowchart

    Measure of Convergence

    In backcalculating layer moduli, the measure of how well the calculated deflection basin matches (or converges to) the

    measured deflection basin was previously described as the error check. This is also referred to as the goodness of fit or

    convergence error. The primary measure of convergence is typically Root Mean Square (RMS) or Root Mean Square Error

    (RMSE). Both terms are taken to be the same. EVERCALC uses RMSE.

    where: nd = total number of deflection sensors used

    dci = calculated pavement surface deflection at sensor i

    dmi = measured pavement surface deflection at sensor i

    Example Measure of Convergence Calculation

    Given the following table of measured and calculated deflections, calculate the RMSE.

    Table 1. Measured and Calculated Deflections

    Deflection Sensor Distance fromLoad Center

    Deflection (mils)

    Measured Calculated

    1 0 mm (0 inches) 5.07 4.90

    2 200 mm (8 inches) 4.32 3.94

    3 300 mm (12 inches) 3.67 3.50

    4 450 mm (18 inches) 2.99 3.06

    5 600 mm (24 inches) 2.40 2.62

    6 900 mm (36 inches) 1.69 1.86

    7 1500 mm (60 inches) 1.01 0.95

    Solution:

    Backcalculation | Pavement Interactive http://www.pavementinteractive.org/article/backcalculation/

    2 of 3 6/23/2015 4:31 PM

  • Copyright 2012 Pavia Systems, Inc.

    In practice, this RMSE is quite high for pavement backcalculation. Generally an adequate range of RMSE is 1 to 2 percent.

    Footnotes ( returns to text)

    Backcalculation of Pavement Layer Properties. Nondestructive Testing of Pavements and Backcalculation of Moduli. ASTM STP 1026. American Society for Testing and

    Materials. Philadelphia, PA. pp. 7-38.

    1.

    Backcalculation | Pavement Interactive http://www.pavementinteractive.org/article/backcalculation/

    3 of 3 6/23/2015 4:31 PM