--- title: "Structural evaluation methods; overlay from deflection" subject: "Transportation Engineering II" chapter: 4 key: "semester_7/transportation_engineering_ii:structural-evaluation-deflection-overlay-1-5" source: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/structural-evaluation-deflection-overlay-1-5 site: BCT Engineering --- # Structural evaluation methods; overlay from deflection Transportation Engineering II · Chapter 4: Highway Maintenance, Repair, and Rehabilitation - Asked in: 2066 Bhadra, 2+6 marks - Video: [Module 3 Design of overlay part 2](https://www.youtube.com/watch?v=NgM9KmAkP3E) — Gowtham B - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/structural-evaluation-deflection-overlay-1-5 - Whole chapter: https://www.bctengineer.com/md/semester_7/transportation_engineering_ii/chapter-4 ## Question Write down the methods of structural evaluation of pavement. A number of deflection readings were taken on a pavement. The mean and standard deviation were 1.5 and 0.2 respectively. The allowable deflection is 1.0mm. Determine overlay thickness. ## Short answer Structural evaluation uses deflection measurements (Benkelman beam, FWD), destructive tests, and non‑destructive techniques. Characteristic deflection = 1.5 + 2×0.2 = 1.90 mm. Overlay thickness = 550 log10(1.9) = 153 mm of bituminous macadam. ## Answer #### 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which is a structural evaluation method? A. Pendulum skid test B. Falling Weight Deflectometer C. Bump integrator D. Roadroid Answer: B. FWD, Benkelman beam and deflectograph measure deflection under standard load. (Key point: Structural methods) 2. With mean 1.5 mm and SD 0.2 mm, the characteristic deflection Dc = mean + 2 SD is: A. 1.7 mm B. 1.9 mm C. 2.3 mm D. 1.1 mm Answer: B. Dc = 1.5 + 2 x 0.2 = 1.90 mm. (Key point: Characteristic deflection) 3. Dc = 1.9 mm vs allowable 1.0 mm means: A. No strengthening needed B. Pavement needs strengthening because Dc > Da C. Reconstruction mandatory D. Deflection is too low Answer: B. Since Dc exceeds the allowable deflection, an overlay is required. (Key point: Need for overlay) 4. The overlay thickness of BM is h = 550 log10(Dc/Da) = A. about 153 mm B. about 550 mm C. about 19 mm D. about 275 mm Answer: A. 550 x log10(1.9) = 550 x 0.2788 = 153 mm, about 155 mm BM. 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