Pavimentos

Spectrum of Axles Approach

Typical Axle Load Spectrum
Axle Load (kN) 50 – 60 61 – 80 81 – 100 101 – 120 121 – 140 Number of Axles Single 5,000 3,000 200 50 6 Tandem 400 2,000 5,000 4,000 2,000 Tridem 100 500 800 1,000 1,500 Quad 5 10 30 80 100

Distress Model
• Distress models relate the structural responses to various types of distresses • These are equations relating the allowablenumber of repetitions of standard axle to the appropriate pavement response as per the failure criteria adopted

Fatigue Cracking and Rutting
Wheel Load

Asphalt Surface Course Asphalt Binder Course Unbound base Granular Subbase Compacted Subgrade

εt

Fatigue Cracking

εc

Natural Subgrade

Rutting

Fatigue Cracking Model
1 1 N f = k3      εt   E 
Nf
εt = = = = =
k1k2

No. of cumulative standard axles to produce 20% cracked surface area Tensile strain at the bottom of Bituminous Concrete layer Elastic Modulus of Bituminous Surface (MPa) Laboratory calibrated parameters Transfer parameter

E k1, k2 k3

Rutting Failure Model
1 N R = k5   εc 
k4

NR = No. of Repetitions to Rutting failure
εc = Vertical subgrade strain

k4, k5 = Calibratedparameters

Damage Ratio
• Allowable number of repetitions (Ni) are computed separately for each axle type i applying the distress model • Expected number of repetitions (ni) of each axle type i are obtained from traffic cum axle load survey • Damage Ratio (DR), which is the ratio between the expected repetitions and allowable repetitions, is worked out for each axle type • The cumulative DR ofall axles should be less than 1

Damage Analysis Considering Seasonal Variations
• Damage Ratio (DR)
Ratio between the predicted and allowable number of repetitions computed for each axle type in each season and summed over the year

DR = ∑∑
i j

nij N ij

nij=Predicted number of repetitions of axle type j for season i Nij=Allowable number of repetitions of axle type j for season iMaterial Characterisation
subgrade, subbase and unbound bases

Definition of CBR
California bearing ratio is defined as the ratio (expressed as percentage) between the load sustained by the soil sample at a specified penetration of a standard plunger (50 mm diameter) and the load sustained by the standard crushed stones at the same penetration.

Standard Load values on Crushed Stones forDifferent Penetration Values
Penetration, Standard Unit Standard mm Load, kg Load, kg/cm2 2.5 1370 70 5.0 7.5 10.0 12.5 2055 2630 3180 3600 105 134 162 183

Basic Test
• This consists of causing a plunger of 50 mm diameter to penetrate a soil sample at the rate of 1.25 mm/min. • The force (load) required to cause the penetration is plotted against measured penetration. • The loads at 2.5 mmand 5 mm penetration are recorded. • This load corresponding to 2.5 mm or 5 mm penetration is expressed as a percentage of standard load sustained by the crushed aggregates at the same penetration to obtain CBR value.

Load Vs Penetration Curve
160 140 120 100 Load 80 60 40 20 0 0 2.5 5 7.5 10 12.5 Penetration

Initial Concavity
• The load – penetration curve may show initial concavity dueto the following reasons:
– The top layer of the sample might have become too soft due to soaking in water – The surface of the plunger or the surface of the sample might not be horizontal

Correction
• Draw a tangent to the load-penetration curve where it changes concavity to convexity • The point of intersection of this tangent line with the x-axis is taken as the new origin • Shift theorigin to this point (new origin) and correct all the penetration values

Corrected Penetration Values
160 140 120 Load, kg 100 80 60 40 20 0 0 2.5 2.5 5

5

7.5

10

12.5

Penetration, mm

Variation in CBR Values
• At least three samples should be tested on each type of soil at the same density and moisture content to take care of the variation in the values • This will enable a...