Structural evaluation methods; overlay from deflection

Methods of structural evaluation of pavement

1. Deflection measurement - Benkelman beam (IRC:81), Falling Weight Deflectometer (FWD), deflectograph; the deflection under a standard load indicates the strength.
2. Destructive tests - test pits and cores to find layer thickness, material properties and subgrade CBR.
3. Non-destructive and in-situ tests - Dynamic Cone Penetrometer (DCP), plate load test, Ground Penetrating Radar, and back-calculation of layer moduli from FWD data.
4. Pavement condition survey - cracking, rutting and patching (a condition index) to support the structural findings.

Overlay thickness from deflection (IRC:81)

Mean deflection $\bar D=1.5$ mm, standard deviation $\sigma=0.2$ mm, allowable deflection $D_a=1.0$ mm (all taken as corrected readings).

Characteristic deflection (national/state highway):

$$D_c=\bar D+2\sigma=1.5+2\times0.2=1.90\ \text{mm}$$

Since $D_c>D_a$, the pavement needs strengthening. Overlay thickness of bituminous macadam:

$$h=550\log_{10}\frac{D_c}{D_a}=550\log_{10}\frac{1.90}{1.0}=153\ \text{mm}$$

Answer: about 155 mm of bituminous macadam overlay (about 103 mm of bituminous concrete equivalent, taking 1 mm of BC = 1.5 mm of BM). For a lower-category road, where $D_c=\bar D+\sigma=1.7$ mm, $h=550\log_{10}1.7\approx127$ mm. If the IRC:81 chart is available, the thickness for $D_c=1.9$ mm is read from it for the design msa.