---
title: "Flexible pavement failures; deflection overlay design"
subject: "Transportation Engineering II"
chapter: 4
key: "semester_7/transportation_engineering_ii:flexible-failures-deflection-overlay"
source: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/flexible-failures-deflection-overlay
site: BCT Engineering
---

# Flexible pavement failures; deflection overlay design

Transportation Engineering II · Chapter 4: Highway Maintenance, Repair, and Rehabilitation

- Asked in: 2079 Bhadra, 8 marks
- Video: [Distress in flexible pavement](https://www.youtube.com/watch?v=09bg3-2JSDI) — Dr. G. VENKATESAN
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/flexible-failures-deflection-overlay
- Whole chapter: https://www.bctengineer.com/md/semester_7/transportation_engineering_ii/chapter-4

## Question

What are the most common failures of flexible pavement? Describe the deflection approach for the design of overlay for flexible pavement.

## Short answer

Common flexible pavement failures include alligator cracking, rutting, shoving, potholes, ravelling, bleeding, depression, edge failure, longitudinal/reflection cracks, and stripping. The deflection approach measures elastic rebound deflection with a Benkelman beam under a standard 8170 kg axle load, corrects for temperature (to 35°C) and season, then calculates the overlay thickness needed to carry future traffic.

## Answer

#### Common failures of flexible pavement
A flexible pavement fails when it can no longer give a safe, smooth ride or carry the traffic. Common failures:

| Failure | Description | Main causes | Remedy |
|---|---|---|---|
| **Alligator (fatigue) cracking** | Interconnected cracks like crocodile skin in the wheel paths | Repeated loading, thin layers, weak subgrade or base, poor drainage, aged binder | Patching or removal and replacement, overlay, drainage improvement |
| **Rutting** | Longitudinal depressions in the wheel paths | Weak subgrade/base, insufficient compaction, overloading, soft mix in hot weather | Fill with a levelling course, overlay, reconstruct if deep |
| **Shoving and corrugation** | Bulges and waves across the road | Unstable mix, poor bond, braking and starting loads | Remove and replace, scarify and relay |
| **Potholes** | Bowl-shaped holes | Water entering through cracks, weak mix, traffic wear | Patching with premix |
| **Ravelling** | Loss of stone from the surface | Poor bitumen, stripping, ageing, insufficient binder | Seal coat, surface dressing |
| **Bleeding** | Excess bitumen forms a shiny film | Too much binder, low air voids, heat | Spread hot sand/chips, remove excess binder |
| **Depression and settlement** | Local sinking | Subgrade consolidation, poor compaction, utility trench | Fill and level, reconstruct base |
| **Edge failure** | Breaking at the pavement edge | Poor support, narrow shoulder, water | Strengthen shoulder, improve drainage |
| **Longitudinal / reflection cracks** | Cracks that follow joints or cracks in the layer below | Differential settlement, poor construction joints, old cracks | Seal, overlay with a crack relief layer |
| **Stripping** | Loss of bond between bitumen and aggregate | Water action, hydrophilic aggregate | Anti-stripping agent, replacement |

#### Deflection approach for overlay design
The deflection approach estimates the overlay needed to raise the structural capacity of an existing pavement to that required for future traffic. It is based on the elastic rebound deflection measured under a standard wheel load (Benkelman beam as per IRC:81-1997, or FWD). Steps of the Benkelman beam method:

1. **Select the section and test points**: divide the road into homogeneous sections and choose at least 10 test points in each, at about 50 m spacing, staggered between the lanes. The points lie in the outer wheel path, 0.9 m from the pavement edge (0.6 m for lanes narrower than 3.5 m).
2. **Standard test vehicle**: a truck with a rear axle load of 8170 kg (dual wheels, 4085 kg on each dual-wheel set) and a tyre pressure of 5.6 kg/cm².
3. **Measurement**: place the beam probe between the dual wheels at the test point and note the initial dial reading $d_0$. Move the truck slowly forward 2.7 m (intermediate reading $d_i$) and then to 9 m (final reading $d_f$). Record the pavement temperature.
4. **Rebound deflection**: $D=2\,(d_0-d_f)\times$ least count, where the factor 2 is the beam lever ratio. If $d_i-d_f$ exceeds 0.025 mm, the front legs of the beam were inside the deflection bowl, and a leg correction is applied: $D=2(d_0-d_f)+2K(d_i-d_f)$ with $K=2.91$.
5. **Corrections**:
   - **Temperature**: the standard temperature is 35 °C. Correct by 0.01 mm for each °C difference from 35 °C (add when the pavement is colder, subtract when it is hotter). This applies when the bituminous layers are 40 mm or thicker.
   - **Season**: multiply by a seasonal correction factor (from the IRC:81 charts, based on subgrade soil type, plasticity index, field moisture content and annual rainfall) to convert the deflection to the weakest, post-monsoon condition.
6. **Characteristic deflection**: $D_c=\bar D+2\sigma$ for national and state highways ($D_c=\bar D+\sigma$ for other roads), where $\bar D$ is the mean and $\sigma$ the standard deviation of the corrected deflections.
7. **Design traffic** in msa over the overlay design life (usually 10 years), from the present commercial vehicles per day, growth rate and VDF.
8. **Overlay thickness**: read from the IRC:81 chart of characteristic deflection against design traffic (msa), which gives the thickness in mm of bituminous macadam (BM). It can also be estimated by the approximate formula $h=550\log_{10}(D_c/D_a)$ (h in mm), where $D_a$ is the allowable deflection for the design traffic.
9. **Convert** to the actual overlay material using the IRC:81 equivalence (1 cm of BM = 1.5 cm of WBM/WMM = 0.7 cm of DBM/BC). Repair badly cracked areas before overlaying, and provide the profile-correction (levelling) course and wearing course in addition.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. The deflection approach to overlay design:
   A. Estimates overlay needed to bring structural capacity up to that required for future traffic, from rebound deflection under a standard load
   B. Measures only surface roughness
   C. Designs rigid slabs
   D. Selects binder grade
   Answer: A. Elastic rebound deflection indicates remaining capacity, and the overlay supplies what future traffic needs. (Key point: Purpose of deflection approach)

2. Benkelman beam tests are done:
   A. At 50-100 m intervals in the outer wheel path, at least 10 points per section
   B. One point per km in the centre
   C. At 1 m intervals on shoulders
   D. At the edge only
   Answer: A. Homogeneous sections are tested at 50-100 m intervals in the outer wheel path, minimum 10 points. (Key point: Test spacing)

3. The closed-form overlay thickness is:
   A. h = 550 log10(Dc/Da)
   B. h = Dc x Da
   C. h = 550(Dc - Da)
   D. h = log10(Dc) + Da
   Answer: A. Da is the allowable deflection for design traffic, so h = 550 log10(Dc/Da) in mm of BM. (Key point: Overlay thickness formula)

4. 1 mm of bituminous concrete is equivalent to how much BM?
   A. 0.5 mm
   B. 1.0 mm
   C. 1.5 mm
   D. 3 mm
   Answer: C. IRC:81 uses 1 mm BC = 1.5 mm BM when converting the overlay. (Key point: BC to BM equivalence)

## Related in Transportation Engineering II

- [Typical failures of flexible pavement](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aflexible-pavement-failures-causes) — Transportation Engineering II, chapter 4, asked 2×
- [Flexible pavement failures: causes and remedies](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aflexible-pavement-failure-cause-remedy) — Transportation Engineering II, chapter 4
- [Flexible pavement design with 8 CBR values](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aflexible-design-eight-cbr-340-cvd) — Transportation Engineering II, chapter 2
- [Structural evaluation methods; overlay from deflection](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Astructural-evaluation-deflection-overlay-1-5) — Transportation Engineering II, chapter 4
- [Flexible pavement design: four lane, ADT 8000 trucks](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aflexible-design-adt-8000-four-lane) — Transportation Engineering II, chapter 2
- [Overlay design by Benkelman beam method](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Abenkelman-overlay-design-process) — Transportation Engineering II, chapter 4

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