--- title: "Overlay design by Benkelman beam method" subject: "Transportation Engineering II" chapter: 4 key: "semester_7/transportation_engineering_ii:benkelman-overlay-design-process" source: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/benkelman-overlay-design-process site: BCT Engineering --- # Overlay design by Benkelman beam method Transportation Engineering II · Chapter 4: Highway Maintenance, Repair, and Rehabilitation - Asked in: 2081 Bhadra, 8 marks - Video: [Benkelman Beam Deflection | Highway maintenance | Transportation Engineering II | PoU](https://www.youtube.com/watch?v=p9klOFBXPSw) — BB Teaches - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/benkelman-overlay-design-process - Whole chapter: https://www.bctengineer.com/md/semester_7/transportation_engineering_ii/chapter-4 ## Question Write down the process of overlay design of pavement by Benkelman beam Deflection Method. ## Short answer Test points in outer wheel path at 50 m spacing; standard dual-wheel load 4085 kg at 5.6 kg/cm². Rebound deflection D=2(d0−df)×0.01 mm (leg correction if di−df>0.025 mm). Correct for temperature (0.01 mm/°C to 35°C) and seasonal factor. Characteristic deflection Dc = mean + 2σ (highways) or mean + σ (others). Overlay thickness from IRC:81 chart or h=550 log10(Dc/Da) in mm BM. ## Answer The Benkelman beam deflection method (IRC:81-1997) designs a bituminous overlay from the rebound deflection of the existing pavement under a standard load. 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 standard test vehicle for Benkelman beam (IRC:81) has: A. Rear axle load 8170 kg (dual wheels) and tyre pressure 5.6 kg/cm2 B. Axle load 2000 kg and pressure 1 kg/cm2 C. Single wheel 10 t at 8 kg/cm2 D. Front axle 4085 kg only Answer: A. The standard truck has an 8170 kg rear axle (4085 kg per dual wheel set) with 5.6 kg/cm2 tyre pressure. (Key point: Test vehicle) 2. Rebound deflection is calculated as: A. D = (d0 - df) x least count B. D = 2(d0 - df) x least count C. D = (d0 + df) x least count D. D = 2(di - df) Answer: B. The factor 2 is the beam lever ratio, applied to initial minus final readings. (Key point: Rebound deflection) 3. For national and state highways characteristic deflection is: A. Mean + 1.65 sigma B. Mean + 2 sigma C. Mean only D. Mean - 2 sigma Answer: B. Dc = mean + 2 sigma for NH/SH; mean + 1.65 sigma applies to other roads. (Key point: Characteristic deflection) 4. Which two corrections are applied to the measured deflection? A. Pavement temperature and seasonal subgrade moisture B. Wind speed and traffic colour C. Slab thickness and joint spacing D. Axle spacing and aggregate size Answer: A. Deflections are corrected to standard temperature (35 C) and for seasonal variation in subgrade moisture. (Key point: Corrections) ## Related in Transportation Engineering II - [Benkelman beam overlay thickness numerical](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Abenkelman-overlay-numerical-4085kg) — Transportation Engineering II, chapter 4 - [Flexible pavement failures; deflection overlay design](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aflexible-failures-deflection-overlay) — Transportation Engineering II, chapter 4 - [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 - [Pavement design factors; IRC rigid design steps](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Apavement-factors-irc-rigid-steps) — Transportation Engineering II, chapter 2, asked 2× - [Temperature stresses in rigid pavement; dowel bar design](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Atemperature-stress-rigid-dowel-bar-procedure) — Transportation Engineering II, chapter 2 ## Related in other subjects - [Beam design with deflection, 5.5 m span](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Abeam-design-with-deflection-5-5) — Design of RCC Structure, chapter 4 - [Intermediate beam design for BM and deflection](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aintermediate-beam-design-for-bm-and) — Design of RCC Structure, chapter 4 - [Slope deflection: three-span beam with settlement](https://www.bctengineer.com/question.md?key=semester_5%2Ftheory_of_structures_ii%3Aslope-deflection-three-span-fixed-ends) — Theory of Structures II, chapter 3 - [Slope deflection: beam with rotations and settlements](https://www.bctengineer.com/question.md?key=semester_5%2Ftheory_of_structures_ii%3Aslope-direction-beam-fbd-bmd-sfd) — Theory of Structures II, chapter 3