Strengthening of Flexible Pavements using Benkelman Beam Deflection TechniqueAs per IRC 81:1997 By Prof. S. L. Bodas Pavement Overlay-1 • A distressed pavement requires maintenance. • Its expenditure can be optimized by proper planning,construction & quality control. • Eg: By providing proper drainage / Preventing overloading beyond legal limits. • Necessary Maintenance measures at an early distress stage can obviate major Maintenance expenditures in future. Pavement Overlay-2 • Pavement Maintenance measures can be classified in two categories. • 1.Maintenance measures other than overlay & 2.Maintenance measures with overlay • • Maintenance measures other than overlay are minor maintenance or repair works.They do not enhance the structural strength of the pavement. • They either improve functional standards or check the rate of deterioration. Pavement Overlay-3 • The Overlay is the extra thickness provided on the pavement surface which strengthens the pavement structurally& thereby enhances its longevity. • The overlay design comprises the determination of the thickness and the type of material to be laid over the existing surface to extend its longevity. • Prior to 1960,(in India 1980) overlay design used to be based on judgement.& experience. The BB method is based on deflection approach & that by FWD is based on the mechanistic approach. C. .History of Benkelman Beam Deflection Technique-1 • Devised by A. Benkelman during WASHO road test in 1953 study R-6. • Benkelman Beam deflection technique is • a popular method due to its simplicity & low cost. guidelines were revised by IRC in 1997. History of Benkelman Beam Deflection Technique-2 • Development of methods such as Benkelman Beam Deflection Method for evaluation of structural capacity of existing Flexible Pavements and also for estimation and design of overlays for strengthening of any weak pavement. . moisture and their deflection on pavement deflection & service behaviour. • Also based on Most R-6 research study.History of Benkelman Beam Deflection Technique-3 • It was undertaken to collect Indian data on pavement deflection values before & after overlaying & other parameters like temperature. sub-grade soil type. . • Imagine a standard loaded truck standing at point A on the pavement surface.44 m long probe arm & 1.6 kg/cm2 is used in a BBD Study. • A standard loaded truck with rear axle load of 8100 kg and fitted with dual tyres. Probe point of probe arm of BBD is placed between the dual tyres. pressure 5. Due to load.22 m long probe arm rotating around the pivot.66 m long beam consisting of 2. A acquires a position A'.Principles of BBD Study-1 • BBD is a slender 3. . .Principles of BBD Study-II • By suitably placing the probe between the dual wheels of a loaded truck. • Rebound deflection can be related to performance of pavement. it is possible to measure the rebound and residual deflections of the pavement structure. • Residual deflection may be due to non-recoverable pavement deflection. Rear axle load of 8170 kg. Pressure of 5.(+/-1% tolerance of total load) Spacing between tyre walls: 30-40 mm.Description of Benkelman Beam-III • • • • • • Details of Benkelman Beam: Length of probe arm from pivot to probe point: 244 cm Length of measurement arm from pivot to dial: 122cm Distance from pivot to front legs: 25 cm Distance from pivot to rear legs: 166 cm Lateral spacing of front support legs: 33 cm • Equipment needed: • A 5 tonne truck with dual tyres as the reaction.5% tolerance) .12 ply. Tyres will be 10x20.6 kg/cm2(+/. Benkelman Beam . Diameter of hole 1.5 cm deep hole in the pavement for temperature measurement. Tyre pressure measuring gauge A mandrel for making 4. .25 cm at surface. 1 cm at bottom.Description of Benkelman Beam-IV • • • • Equipments (contd): Use of tyre with tubes & ribs recommended. Steps in Overlay study • Step 1: Pavement condition Survey • Step 2: Deflection Measurements & calculation of true deflections • Step 3: Collection of subsidiary data • Step 4: Application of corrections for temp. & seasonal variation to true deflections • Step 5: Measurement of traffic data & computation of Design traffic • Step 6: Determination of characteristic deflection • Step 7: Design of Overlay . Transverse Cracking . Fatigue Cracking . Block Cracking . Rutting . .Pavement condition Survey • Conducted primarily by visual observations & rutdepth measurements. • Criteria for classification of pavement sections classification: • Good: No cracking.rutting less than 10 mm • Fair: No cracking or cracking confined crack in the wheel track with rutting between 10-20 mm. Sections with cracking exceeding 20% shall be taken as failed. • Poor: Extensive cracking &/or rutting >20 mm. • Or at a distance of 1. • Each section of uniform performance. the measurement point should be at a distance of 60 cm (for road width < 3. • In transverse direction.5 from pavement edges (For 4-lane divided highway) .Deflection Measurements-I • Minimum section: 1 km. minimum 10 points.5 m).5 m lane) or at a distance of 90 cm (for road widths >3.should be marked at equal distance (not exceeding 50 m) for making deflection observations in the outer wheel path. It should read exactly one half thickness of the metallic block. A number of metallic blocks of different thicknesses (measured with a micrometer. Place the beam on a hard levelled ground. Otherwise check dial gauge. . with perfect face) are placed under probe & dial gauge readings noted.Calibration of Benkelman Beam • Calibrate Benkelman Beam before use. Benkelman Beam dimensions Probe Arm Dial Gauge arm 2.22 m .44 m 1. C B A 9m 2.7 m . Initial Deflection Reading A . 30-40mm Tyres Beam Tip of beam . Intermediate Deflection Reading B A . Final Deflection Reading C B A . . . (Recovery rate less than or equal to 0. • After moving the truck to C (9 m away from B) next deflection reading called final deflection should be noted.Deflection Measurments-I • Intial deflection was noted when rear axle of the truck was at A.7 m away from A) called intermediate deflection. .025 mm/min.) • Note next deflection reading after slowly moving the truck to B (2. 025 mm. every hour at a small hole made with mandrel & filled with glyceral. • If the second difference is less than 0.91 times ' Y' . True Pavement deflection • = Apparent Pavement deflection + 2. • Determine twice the difference between final & initial (Apparent Pavement deflection) as well as between intermediate & final readings (Y). then pavement deflection is twice of final reading.Deflection Measurments-II • Record pavement temp. • Otherwise. 49 mm True Pavement deflection =Apperent Pavement deflection + 2.Deflection Measurments-III • • • • • • • • Sample calculation Initial deflection 0.0) + 2.91x2x(0.155 .49 .46) =0.74 + 2.49 .91 times Y =2x(0.0.91x0.46 mm Final deflection 0.0 mm Intermediate deflection 0.06 = 1. Correction not applicable for roads with thin toppings.Correction for Temperature Variations-I • The stiffness of bituminous layers changes with temperature of the binder & consequently the surface deflection of a given pavement will vary as per binder temperature. • For areas in the country having tropical climate the standard temperature recommended is 35C. . It is necessary that the measured deflections be corrected to a common standard temperature. higher than 35C. • The Correction: • Positive for temp.Correction for Temperature Variations-II • Correction should be 0. No correction.01 mm per degree centigrade change. • For altitudes > 1000 m. . • Negative for temp. deflection measurments shold be done only when ambient temperature is 20C only.lower than 35C. soil moisture content goes on changing. (6 graphs) . different types of plasticity & different rainfall intensities. different moisture corrections are needed. • During different times in a year. • Actual soil moisture content should be measured.Correction for Seasonal Variations-I • For different types of soil. Appropriate moisture corrections corresponding to different data should be calculated using proper graph. • For N.Determination of Characteristic deflection • From series of True deflection values.H. its mean & Standard deviation values should be determined. • Characteristic deflection=Mean + 2*SD • For other roads • Characteristic deflection=Mean + SD .H & S. . . . . . . . . If the pavement is to sustain further 20 MSA of traffic repetitions. field moisture content.Problem 1 • The following are the BBD. The average annual rainfall of the area is 1200 mm & the soil is of clayey nature with average plasticity index of 12. design an overlay thickness for the stretch. and temperture readings at equidistant points obtained along a stretch of a major road. . . • No specific methodology has been suggested in IRC81 except asking to do this visually & minimum length of a section should be less than 1 km. divide it into number of parts where deflection is somewhat same in magnitude. Best fit line should be drawn. • AASHTO has suggested that the cumulative data points to be plotted on a km scale.Seletion of homogenous section • When BBD survey is conducted on a long stretch of road. .