PAVEMENT DESIGN

Highway Engineering

Pavement Types

1. Flexible pavement2. Rigid pavement3. Variation of Pavement Types

1. Soil cement and stabilized bases that have cemented aggregate

2. Composite pavements(made of flexible and rigid layers)

3. Continuously reinforced pavements4. Post-tensioned pavements

Pavement function

To distribute the traffic load stresses to the soil (subgrade) at a magnitude that will not shear or distort the soil.

Typical soil bearing capacity

Can be less than 50 psi (345 kPa) Can be as low as 2 to 3 psi (14 to 21

kPa) If soil is saturated with water it can be

very low

Typical weight of vehicle

Automobile = 3500 lb (15.5 kN), tire pressure =35 psi (241 kPa)

Tractor-semi-trailer truck = 80 kips (355.8 kN), tire pressure =100 psi (690 kPa)

Flexible Pavement

Is constructed with asphaltic cement and aggregates and usually consists of several layers( subgrade, subbase, base, and wearing)

Subgrade-the soil itself, he upper 6-8” (152-203 mm) is usually scarified and blended to provide a uniform material before it is compacted.

Flexible pavement

Subbase – crushed aggregates (rock) Base – crush aggregates of higher

strength than the subbase, w/c either stabilized/unstabilized with a cementing material.

Wearing – asphaltic cement( asphalt cement + aggregates), its purpose is to protect the base layer from wheel abrasion and to waterproof the entire road structure.

Flexible Pavement

Rigid pavement

Is constructed with PCC and aggregates. Use of base layer is optional depending on the

properties of the subgrade. Transverse contraction joints are built into the

pavement to control cracking due to shrinkage of concrete during the curing process.

Load transfer devices (dowels) are placed in the joints to minimize deflections and reduce stresses near the edges of the slabs.

Slab thickness = 8-12” (200 to 300 mm)

Typical Rigid Pavement

Pavement system design: Principles for flexible pavement

Primary function of the pavement structure is to reduce and distribute the surface stresses (contact tire pressure) to an acceptable level at the subgrade ( to a level that prevents permanent deformation).

Flexible pavement reduces the stresses by distributing the traffic wheel loads over greater and greater areas, through the individual layers, until the stress at subgrade is at an acceptable low level. The traffic loads are transmitted to the subgrade by aggregate to aggregate particle contact.Confining pressures(lateral forces due to material weight) in the subbase and base layers increase the bearing strength of these materials. A cone of distributed loads reduces and spreads the stresses to the subgrade.

Load Distribution

Calculation of Flexible pavement stresses and deflection

Boussinesq theory assumes that the pavement is one layer thick and the material is elastic, homogeneous, and isotropic.

Where

σz = stress at a point in kPa

P = wheel load in N z = depth of the point in question in mm K = variable defined as

Boussinesq Theory

Calculation of Flexible pavement stresses and deflection

Considering the tire foot print,

Where a = equivalent load radius of the tire

footprint in mm P = tire load in N, and p = tire pressure in kPa

Ahlvin and Ulery’s Equation

For radial-horizontal stress(w/c is caused of pavement cracking,

The equation for deflection

Ahlvin and Ulery’s Equation

Where, σz = vertical stress in psi/kPa

σr = radial-horizontal stress in psi/kPa

Δz = deflection, at depth z in inches/mm

p = pressure due to tire load in psi/kPa μ = Poisson ratio E = modulus of elasticity in psi/kPaA,B,C,F, and H + function values, as presented in Table 4.1, that depend on z/a and, the

depth in radii and offset distance in radii, respectively,Where z = depth of a point in question in mm r = radial distance in mm from the center of the circular load area to point in question

and, a = equivalent load radius of the tire footprint in mm

Function A

Function B

Function C

Function F

Function H

Example 1

A tire with 100 psi air pressure distributes a load over an area with a circular contact radius, a, of 5”. The pavement was constructed with a material that has a modulus of elasticity of 50 ksi and a Poisson ratio of 0.45. Calculate the radial-horizontal stress and deflection at a point a depth of 20” and a radial distance of 10” from the center of the tire load. (Use the Ahlvin and Ulery equations)

AASHTO Flexible-Pavement Design Procedure

Flexible Pavement Design Equation,

Where, W18 = 18 kip equivalent single axle load ZR = reliability( z-statistics from the standard normal curve) SO = overall standard deviation of traffic SN = structural number ΔPSI = loss in serviceability from the time the pavement is new until it reaches its

TSI MR = soil resilient modulus of the subgrade in psi

AASHTO Flexible-Pavement Design Procedure

MR = 1500 x CBR Coefficient of 1500 is used for CBR values less than 10 CBR = ratio of load bearing capacity of the soil to the

load bearing capacity of a high quality aggregate, multiplied by 100

SN = a1D1 + a2D2M2 + a3D3M3

a1,a2,a3 = structural layer coefficients of the wearing surface, base, and subbase layers

D1,D2,D3 = thickness of the wearing surface, base, and subbase layers in inches

M2,M3 = drainage coefficients for the base and subbase

Example 2

A pavement is to be designed to last 10 years. The initial PSI is 4.2 and the TSI (the final PSI) is determined to be 2.5. The subgrade has a soil resilient modulus of 15 ksi. Reliability is 95% with an overall standard deviation of 0.4. For design, the daily car, pickup truck, and light van traffic is 30,000, and the daily truck traffic consists of 1000 passes of single-unit trucks with two single axles and 350 passes of tractor semi-trailer trucks with single, tandem, and triple axles. The axle weights are

Cars, pickups, light vans = two 2-kip single axles Single-unit truck = 8-kip steering, single axle = 22-kip drive, single axle tractor semi-trailer truck = 10-kip steering, single axle = 16-kip drive, tandem axle = 44-kip trailer, triple axle M2 and M3 are equal to 1.0 for the materials in the pavement structure. Four

inches of hot-mix asphalt is to be used as the wearing surface and 10” of crushed stone as the subbase. Determine the thickness required for the base if soil cement is the material to be used.

Example 3

A flexible pavement is constructed with 4” of hot-mix asphalt wearing surface, 8” of emulsion/aggregate-bituminous base, and 8” of crushed stone subbase. The subgrade has a soil resilient modulus of 10 ksi, and M2 and M3 are equal to 1.0 for the materials in the pavement structure. The overall standard deviation is 0.5, the initial PSI is 4.5, and the TSI is 2.5. The daily traffic has 1080 20-kip single axles, 400 24-kip single axles, and 680 40-kip tandem axles. How many years would you estimate this pavement would last(how long before its PSI drops below a TSI of 2.5) if you wanted to be 90% confident that your estimate was not too high, and if you wanted to be 99% confident that your estimate was not too high?

Pavement System Design: Principles for Rigid Pavements

Calculation of Rigid-Pavement Stresses and Deflections

Westergaard equationsΔ = p/k

Δ = slab deflection in inches p = tire pressure in psi k = modulus of subgrade reaction in psi

Ioannides, Thompson, and Barenberg

Interior loading,

Where σi = interior bending stress in psi Δi = interior deflection in inches P = total load in lb μ = Poisson ratio

Ioannides, Thompson, and Barenberg

h = slab thickness in inches k = modulus of subgrade reaction in psi a = radius of circular load in inches (the tire footprint

radius) γ = Euler’s constant, equal to 0.577215, and l = radius of relative stiifness ( a measure of the slab thickness in

inches), defined as

Where E = modulus of elasticity in psi

Ioannides, Thompson, and Barenberg

For edge loading,

Ioannides, Thompson, and Barenberg

For the corner loading,

Where al = distance to the point of action of the resulting

load on a common angle bisection at the slab corner equal to √2 x a

Example 4

A 15-kip wheel load is placed on a portland cement concrete (PCC) slab that is 10” thick. The concrete has a modulus of elasticity of 4,500,000 psi with a Poisson ratio of 0.18. The modulus of subgrade reaction is 200 pci. Tire pressure is 100 psi. Using the revised Westergaard equations, calculate the stress and deflection if the load is placed on the corner of the slab.

The AASHTO Rigid Pavement Procedure

The regression equation used to determine the thickness of a Rigid Pavement PCC slab

D = PCC slab thickness in inches S’c = concrete modulus of rupture Cd = drainage coefficient J = load transfer coefficient

Example 5

A rigid pavement is to be designed to provide a service life of 20 years and has an initial PSI of 4.4 and a TSI of 2.5. The modulus of subgrade reaction is determined to be 300pci. For design, the daily car, pickup truck, and light van traffic is 20,000; and the daily truck traffic consists of 200 passes of single-unit trucks with single and tandem axles, and 410 passes of tractor semi-trailer trucks with single, tandem, and triple axles. The axle weights are

Cars, pickup, light vans = two 2-kip single axle Single-unit trucks = 10-kip steering, single axle = 22-kip drive, tandem axle Tractor semi-trailer trucks = 12-kip steering, single axle = 18-kip drive, tandem axle = 50-kip trailer, triple axle Reliability is 95% the overall standard deviation is .45, the concrete’s

modulus of elascticity is 4,500 ksi, the concrete’s modulus is 900 psi, the load transfer coefficient is 3.2, and the drainage coefficient is 1.0. Determine the required slab thickness.

Example 6

In 1986, a rigid pavement on a northbound section of the interstate highway was designed with a 12” PCC slab, an Ec = 6 x 106 psi, a concrete modulus of rupture = 800 psi a load transfer coefficient of 3.0, an initial PSI of 4.5, and a TSI of 2.5. The So = 0.45, k = 190 pci, and a reliability of 95% was used along with a drainage coefficient of 1.0. The pavement was designed for 20-year life, and traffic was assumed to be composed entirely of tractor emi-trailer trucks with one 16-kip single axle, one 20-kip single axle, and one 35-kip tandem axle (the effect of all other vehicles was ignored). The interstate has four northbound lanes and was conservatively designed. How many tractor semi-trailer trucks, per day, were assumed to be traveling in the northbound direction?

Measuring Pavement Quality and Performance

International Roughness Index Friction Measurements Rut Depth