Pavement Design Methods 2.pdf

Structural Design Of Pavements Dr. Md. Mizanur Rahman

CE 6505

Staged Construction of Flexible  Pavement

Staged Construction of Flexible  Pavement

Staged Construction of Flexible  Pavement

Staged Construction of Flexible  Pavement

Staged Construction of Flexible  Pavement Example:

Solution: Determine ESAL for first 10 years and ESAL for the 20 years design period. Growth factor for 10 yrs = 12.58 (from Table 18.6) Growth factor for 20 yrs = 33.06 (from Table 18.6)

n1 = 60000*12.58 60000*12 58 = 754800 n2 = 60000*(33.0660000*(33.06-12.58) = 1228800 N1 = 1.67 * 754800 = 1260516 N2 = 2.5 2 5 * 1228800 = 3072000 From Figure 18.5 Required thickness for first stage h1=7.5 in R Required i d thickness thi k for f second d stage t h2=9.5 9 5 iin Depth of Overlay = (9.5(9.5-7.5) = 2 in

AASHTO Design Method of Flexible  Pavement # The AASHTO method for design g of highway g yp pavements is based primarily on the results of the AASHTO road test test..

# The factors considered in the AASHTO procedure for the design of flexible Pavement are are::

Pavement performance Traffic Subgrade materials Materials of Construction Environment a age Drainage Reliability

AASHTO Design Method of Flexible  Pavement (Example) (Example) A Flexible pavement for an urban interstate highway g y is to be designed g using AASHTO Design method to carry a design ESAL of 2 X 106. It is estimated that it takes about a Week for water to be drained from within the pavement and the pavement structure Will be exposed to moisture levels approaching saturation for 30% 30% of the time time.. The following additional information is available: available: Resilient modulus of asphalt concrete at 68 F = 450000 psi CBR value of base course materials = 100 CBR value of subbase course materials = 22 CBR value of subgrade materials = 6 Initial Serviceability index Pi = 4.5 T Terminal i l serviceability i bili index i d Pt =2.5 Determine a suitable pavement structure. structure.

The Structural Number (SN) represents the overall structural requirement needed to sustain the design’s traffic loadings loadings.. It is an abstract number that expresses the structural strength of a pavement required for given combinations of soil support (MR), total traffic expressed in ESALs, terminal serviceability and environment. The following equation can be used to relate individual material t types and d thicknesses thi k t the to th structural t t l number number: b :

SN = aa1D1 + aa2D2m2 + aa3D3m3 SN  a1, a2, a3 = structural-layer y coefficients of the wearing g surface, base, and subbase layers, respectively, D1, D2, D3 = thickness of the wearing surface, base, and subbase layers in inches, respectively, and m2, m3 = drainage coefficients for the base and subbase, respectively.

Solution: Reliability level ( R ) = 99 percent from Table 18.17 Standard deviations (So) = 0.49 (For Flexible Pavement range 0.4 – 0.5) Mr = 9000 psi for roadbed/subgrade S Serviceability i bilit Loss L = Pi – Pt = 4.5 4 5 - 2.5 25=2 Determine Design Structural Number SN = 4.4 from Figure 18.20 Determine the appropriate structural layer coefficient for each construction Materials : a1 = 0.44 (Mr of asphalt cement = 450000 psi, From Figure 18.17) a2 = 0.14 (CBR of base course material = 100, From Figure 18.16) a3 = 0.10 (CBR of subbase course material = 22, From Figure 18.15) Determine appropriate pp p drainage g coefficient mi from Table 18.15 and 18.16. Value of m2 = m3 =0.8 (Since only one set of conditions is given for both the Base and Subbase) Several values of D1 D2 D3 can be obtained to satisfy the SN value of 4.40. Layer thickness, however, are usually rounded up to the nearest 0.5 in.

For Mr = 31000 psi SN1 = 2.6 from figure 18.20 D1 = SN1/a1 = 2.6/.44 = 5.9 in Using 6 in Surface course SN1* = 0.44*6 =2.64 For Mr = 13500 psi SN2 = 3.8 from Figure 18.20 D*2 (SN2 SN1*)/ 2 2 =10.36 D*2=(SN2-SN1*)/a2m2 10 36 iin Use 12 in SN2*=0.14*.8*12 + 2.64 = 1.34 + 2.64 = 3.98 D3*= (SN3 – SN2*)/a3m3 = 5.25 in Use 6 in SN3*= 2.64+1.34+.8*.1 = 4.46 > 4.4

99

Standard Deviation, So Flexible Pavements Rigid Pavements

0.49

0.40 – 0.50 0.30 – 0.40

Reliability level, R = 99%, Overall Standard Deviation, So = 0.49, ESAL = 2 X 106 Resilient Modulus of Subgrade = 9000psi, Serviceability Loss PSI = 4.54.5-2.5 = 2

Design SN = 4.4

0.44

Figure 18.17

0.14

Figure 18.16

0.10

Figure:  18.15

Table 18.15

6” 12” 6”

THE DESIGN PROCESS There are three main steps to be followed in designing a new road pavement These are: are: (i) estimating the amount of traffic and the cumulative number of equivalent standard axles that will use the road over the selected design life; (ii) assessing the strength of the subgrade soil over which the road is to be built; (iii) selecting the most economical combination of pavement materials and layer thicknesses that will provide satisfactory service over the design life of the pavement (It is usually necessary to assume that an appropriate level of maintenance is also carried out).

This method considers each of these steps in turn and puts special emphasis on five aspects of design that are of major j significance i ifi i designing in d i i roads d in i mostt tropical t i l countries: • The influence of tropical p climates on moisture conditions in road subgrades. • The severe conditions imposed on exposed bituminous surfacing materials by tropical climates and the implications of this for the design of such surfacings. • The interrelationship between design and maintenance. If an appropriate i t level l l off maintenance i t cannott be b assumed. d it is not possible to produce designs that will carry the anticipated p traffic loading g without high g costs to vehicle operators through increased road deterioration. • The high axle loads and tyre pressures which are common in most countries. countries • The influence of tropical climates on the nature of the soils and rocks used in road building.

Definition of Pavement Layers

The e Pavem ment De esign P Process

Traffic Classes used in Design

Subgrade Classes used in Design

Index o of Structtural Cattalogue

Structural Catalogue (1) St t l C t l (1)

Structural Catalogue (2) St t l C t l (2)

Structural Catalogue (3) St t l C t l (3)

Structural Catalogue (4) St t l C t l (4)

Structural Catalogue (5) St t l C t l (5)

Structural Catalogue (6) St t l C t l (6)

Structural Catalogue (7) St t l C t l (7)

Structural Catalogue (8) St t l C t l (8)

CBR Requirements of various Pavements layers in the RHD Specifications

Design Life and Traffic Growth Rates

Determining Cumulative ESAs Over the Pavement Design Life

Design Chart of RHD (Flexible Pavements)

Determination of Improved  f d Sub‐‐grade Thickness Sub