truck loads effect.pdf

Emirates Journal for Engineering Research, 12 (2), 65-73 (2007) (Regular Paper)

65

IMPACT OF HIGH-PRESSURE TRUCK TIRES ON PAVEMENT DESIGN IN EGYPT

M.E. Abdel-Motaleb

Zagazig University, Egypt, [email protected]

(Received September 2006 and accepted January 2007)

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In the past, damage resulted from load application to highway pavements focused primarily on the magnitude and frequency of axle loads. In recent years, the effect of increased truck tire pressure on flexible pavements responses has become a subject of great concern. This paper aims to evaluate the effects of axle load and high tire pressure on the determination of equivalent axle load factors (EALF) for flexible pavement design in Egypt. The study showed that the operational levels of tire inflation pressure of trucks in Egypt are as high as 140 psi. The results of pavement analysis using KENLAYER program showed that the effect of increased tire pressure on asphalt pavement depends on axle load. At low to intermediate axle loads, the increase in tire pressure causes marked increase in EALF, while fatigue failure is the predominant failure mode. At high axle loads, the failure mode turns to rutting and, in such case the variation of EALF with tire pressure becomes insignificant. The EALFs at the operational level of tire contact pressure of 130 psi along with the legal values of axle loads were found approximately twice and triple the AASHTO factors for single and tandem, respectively. So, the calculated truck factor with adjustment for operational level of tire pressure was found of about two times the traditional truck factor using the AASHTO load equivalency factors. The study also concluded that using the traditional AASHTO load equivalency factors may produce under-designed pavement structures and the premature failure may occur. Finally the study recommended that the detrimental effects of high tire contact pressure should be considered in the determination of reliable truck factor for pavement design purposes using the adjusted values of EALFs developed in this study. Keywords: Tire pressure, Load equivalency factors, Pavement design, and KENLAYER

M.E. Abdel-Motaleb

66 Emirates Journal for Engineering Research, Vol. 12, No.2, 2007

1. INTRODUCTION

Premature failure of flexible pavements has more circulation in many roads in Egypt as a result of the drastic changes in truck axle loads as well as tire pressures. Important findings of a recent study[1] have indicated that, rutting is the major distress modes surveyed in Egypt due to its high severity and extent levels. Another study[2] concluded that tire pressure has more significant effect on rutting tendency of surface asphalt layer than wheel loads. In the past, damage resulted from load application to highway pavements focused primarily on the magnitude and frequency of axle loads. In recent years, the effect of increased truck tire pressure on flexible pavements responses has become a subject of great concern. Analytical study[3] to investigate the effects of truck-tire pressure on pavement responses found that tire pressure was significantly related to tensile strain; t at the bottom of the asphalt layer and stresses near the pavement surface for both the thick and thin pavements. However, tire pressure effects on vertical compressive strain; c at the top of the subgrade were minor, especially in the thick pavement. The increased rutting, decreased fatigue life and accelerated serviciability loss of the pavement have been attributed to the effect of increased truck tire presssure as well as increased axle loads[4-6].

Existing practice assumes the tire pressure to be uniform over the contact area. The size of contact area is then calculated depending on the contact pressure. The contact pressure is greater than tire inflation pressure for low-pressure tires, because the wall of tires is in compression and the sum of vertical forces due to wall and tire pressure must be equal to the force due to contact pressure. On the other hand, the contact pressure is smaller than tire inflation pressure for high-pressure tire, since the wall of tires is in tension[7]. Whatever, a computer program called TireView was developed[8] that provides estimates of tire contact area as a function of tire type, tire load, and tire inflation pressure and predicts the stress distribution at the tire-pavement interface based on polynomial interpolations of measured tire contact stresses in the data base.

In Egypt, AASHTO pavement design guide has been widely accepted for flexible pavements design. Traffic is one of the most important factors in pavement design. In pavement design, the thickness of pavement structure is much affected by the number of repetitions of a standard axle load, usually the 18-kip single axle load. All axle loads are converted to the standard 18-kip single axle load by an equivalent axle load factor (EALF). A summation of the equivalent factors of all axle loads during the design period, usually expressed as Equivalent Single Axle Load (ESAL). The equivalent axle load factor (EALF) for different traffic loadings presented in AASHTO guide are based mainly on data derived from AASHTO Road Test, in which tire contact pressure of 70 psi (equivalent to inflation pressure of 80 psi) was used.

The equivalent axle load factor EALF is defined as the damage per pass to a pavement by the axle in question relative to the damage per pass of a standard axle load, usually the 18-kip single axle load. The values of EALF are computed as the ratio of the strain caused by a particular load to the strain caused by the reference load (usually the 18-kip single axle load) raised to a power[9]. However, inflation pressure used in truck tires has increased significantly over the years because increasing tire pressure lead to decrease in Vehicle Operating Cost[4,5]. That is, therefore using the AASHTO load equivalency factors in mechanistic analysis is only valid when the truck tire contact pressure is of about 70 psi, and also when the behaviour of the used material is similar to the behaviour of the materials at the AASHTO Road Test. If another material is used for a pavement structure under higher tire pressure, then new values of EALF should be developed[10-12]. It is the AASHTOs recommendation that pavement engineers and designers need to keep apprised of possible changes which can influence pavement performance[9]. The effects of high tire inflation pressure on flexible pavement have been widely accepted as an important factor for flexible pavement design. Asphalt Institute[13] has adopted an Equivalent Single Axle Load (ESAL) adjustment factor for tire pressure in its asphalt pavement thickness design manual published in 1991. If actual truck tire measurements indicate that contact pressures are significantly above the standard loading condition (70 psi), then the adjustment factors may be used to modify the design traffic for this additional stress. The adjustment is made by multiplying the initial design ESAL by the ESAL adjustment factor for each individual vehicle type or for the average truck distribution.

2. STUDY OBJECTIVE The main objective of this study is to investigate the effects of high tire pressure as well as axle loads on pavement response and to develop new reliable equivalent axle load factors (EALFs) can be used for structural design of flexible pavements under the actual traffic characteristics in Egypt. To achieve this objective: a) field measurements of truck-tire inflation pressure was carried out to recognize the operational level of truck-tire inflation pressure in Egypt. b) the detrimental effects of high tire pressure on the typical sections of flexible pavement used in Egypt were investigated by computing the tensile strain (t) at the bottom of the asphalt layer and the compressive strain (c) at the top of the subgrade by using computer program KENLAYER. Then, pavement performance models were introduced to develop new equivalent axle load factors (EALFs) based on the two critical strains. c) The effect of the high operational level of truck-tire inflation pressure on truck factor was evaluated and introduced for practical applications in Egypt.

Impact of High-Pressure Truck Tires on Pavement Design in Egypt

Emirates Journal for Engineering Research, Vol. 12, No.2, 2007 67

Table 1. Classification of Trucks and legal axle loads on Egyptian roads[14]

Truck Code Truck Type

Legal Axle Load (ton) Gross Weight (ton)

T F

1st 2nd 3rd 4th

2D

6 10 - - 16 2.445

3A

6 16 - - 22 1.505

2-2

6 10 10 10 36 6.805

2-3 6 10 10 16 42 5.865

3-2

6 16 10 10 42 5.865

3-3

6 16 10 16 48 4.925

2-S1

6 10 10 - 26 4.625

2-S2

6 10 10 10 36 6.805

2-S3

6 16 10 10 42 5.865

3-S1

6 16 10 - 32 3.685

3-S2

6 16 16 - 38 2.745

3. TRAFFIC CHARACTERISTICS The Egyptian Ministry of Transportation determinde the legal axle load limit as 6-ton for single axle single tire, 10-ton for single axle double tires, and 16-ton for tandem axles (< 2.0 m distance between axles). The legal axle load limits and guide Truck Factor (TF) for the different types of trucks in Egypt are shown in Table (1). The values of TF are calculated using the legal values of axle loads by multiplying the number of axles in each truck type by the corresponding EALF given in AASHTO design guide. These data are used in pavement analysis and design if no detailed axle load data are available[14].

4. TIRE PRESSURE SURVEY In order to guess the actual tire pressure in use, interviews were made on owners of several tire stores in Egypt. It was found that tire inflation pressures used for the majority of trucks are in the range of 120 to 140 psi. So, a sample field survey to measure the actual inflation truck-tire pressure was carried out at two rest-stations located on Cairo-Suez road and Cairo-Alexandria desert road. A total of 1618 tires in 117 trucks from different categories were measured for tire inflation pressure. The collected data from the two stations are assembled and presented in Table (2) which illustrates that the tire inflation pressures of trucks in Egypt varies from 93 to 141 psi with mean

M.E. Abdel-Motaleb


value of 121 psi and standard deviation of 13.35. An important observation was noticed during measuring the inflation pressure of the assembly dual-tire. It is that, the dual-tire assembly showed difference in inflation pressure between the two tires in the dual-tire assembly ranged from 10 to 50 psi. Also, as can be seen in the Table a difference from 20 to 60 psi between inflation pressures of the same truck tires was noticed. This variation in tire inflation pressure may be attributed to the difference between the two tires quality, as it was noticed that the stronger and more durable the tire, the higher the inflation pressure and vice versa.

Table 2. Measured inflation tire pressures for different truck types on Egyptian roads

Truck Code

No. of Trucks

No. of Tires

Measurements of tire inflation pressure (psi)

Min. value

Max. value

Mean value

Stand. Devi

2D 19 114 74 132 112 15.92 3A 25 250 87 146 121 14.63 2-2 7 98 72 132 113 13.96 2-3 19 342 92 140 122 13.12 3-2 27 486 85 145 120 12.42 3-3 6 132 96 144 125 14.11

2-S1 7 70 80 135 115 13.34 2-S3 5 90 125 146 129 11.14 3-S2 2 36 128 148 130 11.52 Tot.

Aver. 117 1618 93.2 140.9 120.8 13.35

Table 3. The structural properties of the investigated pavement cross sections

Section Layer Thick.

(in) Modulus of

elasticity (psi)Poissonsratio ()

Strong

Asphalt concrete layer

Granular base layer

Subgrade

4

14

-

400,000

25,000

8,000

0.35

0.40

0.45

Weak

Asphalt concrete layer

Granular base layer

Subgrade

2

10

-

400,000

25,000

8,000

0.35

0.40

0.45 The collected data from the two stations were assembled and presented in Figure (1) which shows the distribution of the measured tire inflation pressure of the investigated sample. It can be seen from the Figure that 97% of tires operate with tire inflation pressure greater than 80 psi, 59% with tire inflation pressure greater than 120 psi and 2% with tire inflation pressure greater than 140 psi. The fact that cannot be ignored that the majorty of truck tires (74%)

Figure 1. Distribution of measured truck-tire inflation pressures

operates with inflation pressure range from 120 to 140 psi. It is quite obvious that a tire inflation pressure of 80 psi for pavement analysis cannot reflect the field situation. Pavement design based on the standard tire inflation pressure of 80 psi certainly would not suffice. Typically, truck tire contact pressure is approximately 90 % of the tire inflation pressure[7]. So, a conservative value of the operational truck-tire contact pressure of 130 psi (equivalent to the tire inflation pressure of 140 psi.) is recommende in Egypt for pavement analysis and design.

5. PAVEMENT CROSS SECTIONS Two pavement cross sections are considered for analysis. The first represents, in general, the strong section used in major roads, while the second section represents the weak section used in minor and local roads; according to the Egyption Specificatios of Roads and Bridges Authority (RBA). The structural properties of the investigated pavement sections are shown in Table (3).

6. PAVEMENT RESPONSE ANALYSIS METHODOLOGY

Flexible pavement is typically taken as a multi-layered elastic system in the analysis of pavement response. Materials in each layer are characterized by a modulus of elasticity (E) and a Poissons ratio (). Traffic is expressed in terms of repetitions of single axle load (from 8 to 36-kip) applied to the pavement on two sets of dual tires and (from 18 to 62kip) on two sets of tandem axles. The investigated contact pressures are varied from 70 psi to 160 psi. The dual tire is approximated by two circular plates (with variable radius according to axle load and tire pressure) and spaced at 13.60-in. center to center. The tandem axles are represented by two axles spaced 48-in. center to center. The geometry of axle's configuration along with the locations for the determination of maximum strains are illustrated in Figure (2).

Tire pressure (psi)

0

4

8

12

16

20

24

28

32

36

Perc

ent (

%)

70-80 80-90 90-100 100-110 110-120 120-130 130-140 140-150

26

2

31

17

11

8

23



Figure 2. Geometry of axles configuration

The strong pavement section was used in pavement analysis with all investigated tire pressures (from 70 to 160 psi) to evaluate the detrimental effect of increased tire pressure on pavement responses. While the weak pavement section was used in pavement analysis at the operational level of tire contact pressure; 130 psi only for practical application in Egypt. A circular tire imprint is assumed and the radius of contact is calculated as follow:

a = ( P/ pt) (1) where: a= Radius of contact, P= Wheel load, pt : Contact pressure.

7. DAMAGE ANALYSIS In pavement analysis, loads on the surface of the pavement produce two strains which are believed to be critical for design purposes. These are the horizontal tensile strain, t, at the bottom of the asphalt layer and the vertical compressive strain, c, at the top of the subgrade layer. The two critical strains are determined by using computer program KENLAYER. If the horizontal tensile strain, t, is excessive, cracking of the surface layer will occur, and the pavement distresses due to fatigue. If the vertical compressive strain, c, is excessive, permanent deformation occurs at the surface of the pavement structure from overloading the subgrade, and the pavement distresses due to rutting. Damage analysis was performed for both fatigue cracking and permanent deformation as follows:

Fatigue Criteria

The relationship between fatigue failure of asphalt concrete and tensile strain t, at the bottom of asphalt layer is represented by the number of repetitions as suggested by Asphalt Institute[15] in the following form:

Nf = 0.0796 (1/t) 3.291 (1/E1) 0.854 (2) where:

Nf = number of load repetitions to prevent fatigue cracking.

t = tensile strain at the bottom of asphalt layer.

E1 = elastic modulus of asphalt layer.

The equivalent axle load factor (EALF) on the basis of fatigue failure with the same material is the ratio of Nfss to Nfij :

EALF = Nfss /Nfij

EALF = (tij /tss ) 3.291 (3) where: Nfss = the number of standard load and pressure

applications. Nfij = the number of arbitrary load and pressure

applications. tss = the maximum tensile strain at the underside of

asphalt layer under the standard single axle load of 18-kip and tire inflation pressure of 80 psi.

tij = the maximum tensile strain at the underside of asphalt layer for the i axle load and j tire inflation pressure.

Rutting Criteria

The relationship between rutting failure and compressive strain c , at the top of subgrade is represented by the number of load applications as suggested by Asphalt Institute[15] in the following form:

Nd = 1.365 * 10-9 (1/ c) 4.477 (4) where:

Nd = number of load applications to limit permanent deformation.

c = vertical compressive strain, at the top of subgrade The equivalent axle load factor (EALF) on the

basis of rutting failure with the same material is the ratio of NcS to NcL

EALF = NcS /NcL

EALF = (cij /css )4.477 (5) where: NcS = the number of standard load and pressure

applications NcL = the number of arbitrary load and pressure

applications css = the maximum vertical compressive strain at the

top of subgrade under the standard single axle load of 18- kip and tire inflation pressure of 80 psi.

cij = the maximum vertical compressive strain at the top of subgrade for the i axle load and j tire pressure.

13.6 in

13.6 in

48 in

Location of responses

* Single axle

* Tandem axle

M.E. Abdel-Motaleb


8. DETERMINATION OF EQUIVALENT AXLE LOAD FACTORS

An equivalent axle load factor (EALF) defines the damage per pass to a pavement by the axle in question relative to the damage per pass of a standard axle load, usually the 18-kip (80 kN) single axle load. KENLAYER, a popular pavement analysis program developed by Huang[16], was used to determine the levels of strain in the flexible pavement under increased tire pressures and axle loads. With the maximum tensile strain at the bottom of asphalt layer and the maximum vertical compressive strain at the top of subgrade determined by KENLAYER, the equivalent factors with reference to any tire and axle load can be computed using Equations 3 and 5. The maximum tensile strain at the bottom of asphalt layer was used to calculate the equivalency factor due to fatigue, EALFF (Eq. 3). Similarly, the maximum vertical compressive strain on the top of subgrade was used to calculate the equivalency factor due to rutting, EALFR (Eq. 5). The strong section was used for this analysis. Table (4) presents the equivalency factors due to the two failure modes, EALFF and EALFR, for single-axle loads ranging from 8 to 36 kip. Table (5) presents the equivalency factors due to the two failure modes, EALFF and EALFR, for tandem-axle loads ranging from 18 to 62 kip. In Tables (4 and 5), the greater equivalency factors between EALFF and EALFR were marked in bold, such that the control failure mode can be manifested.

9. ANALYSIS OF RESULTS It can be observed from Tables (4 and 5) that the effect of increased tire pressure on asphalt pavement depends on axle load. As the axle load increases, high tire pressure causes marked increase in the EALFF, while fatigue failure is the predominant pavement failure mode. As the axle load continues to increase, the failure mode turns to a rutting one and, in such case the effects of increases in tire pressure can be ignored. If EALFF is the greater equivalency, increases in tire pressure are accompanied by significant increases in equivalent factor EALFF. This is because equivalency factors were derived based on fatigue criteria, and would certainly be influenced by tire pressure. In cases where EALFR is the greater equivalency, the change in EALFR with tire pressure is almost negligible. This is reasonable because equivalency factors were derived using rutting criteria. The compressive strain at the top of subgrade, being over 18-in. from pavement surface, is relatively insensitive to tire pressure. Conversely, the compressive strain is very sensitive to axle loads. That is, when the single-axle load is greater than 26 kip or tandem-axle load greater than 58 kip, rutting criteria control the determination of EALFR.

Table 4. Variation of equivalency factors with tire pressures and axle loads based on fatigue and rutting criteria (single axle)

Axle Load(kip)

DamageCriteria

Contact Pressure (psi)

70 80 90 100 110 120 130 140 150 160

8 Fatigue 0.19 0.22 0.24 0.26 0.29 0.30 0.32 0.34 0.36 0.38

Rutting 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03

10 Fatigue 0.31 0.36 0.40 0.45 0.49 0.54 0.56 0.60 0.63 0.67

Rutting 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08

12 Fatigue 0.45 0.54 0.61 0.68 0.76 0.82 0.88 0.94 1.00 1.05

Rutting 0.17 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18

14 Fatigue 0.62 0.73 0.85 0.96 1.06 1.17 1.27 1.36 1.46 1.54

Rutting 0.34 0.34 0.35 0.35 0.35 0.36 0.36 0.36 0.36 0.36

16 Fatigue 0.81 0.96 1.12 1.27 1.42 1.58 1.71 1.86 1.97 2.11

Rutting 0.61 0.61 0.62 0.63 0.63 0.64 0.64 0.65 0.65 0.66

18 Fatigue 1.00 1.21 1.42 1.63 1.83 2.04 2.22 2.41 2.59 2.76

Rutting 1.00 1.02 1.04 1.06 1.07 1.08 1.08 1.09 1.09 1.09

20 Fatigue 1.22 1.48 1.75 2.00 2.25 2.52 2.78 3.02 3.29 3.49

Rutting 1.59 1.62 1.66 1.67 1.67 1.69 1.71 1.71 1.72 1.73

22 Fatigue 1.44 1.77 2.09 2.43 2.74 3.07 3.39 3.72 4.04 4.34

Rutting 2.39 2.46 2.49 2.56 2.56 2.58 2.60 2.64 2.66 2.66

24 Fatigue 1.79 2.07 2.46 2.85 3.26 3.65 4.03 4.42 4.84 5.25

Rutting 3.46 3.59 3.65 3.68 3.76 3.95 3.78 3.80 3.87 3.93

26 Fatigue 2.23 2.42 2.85 3.31 3.80 4.26 4.75 5.25 5.67 6.19

Rutting 4.92 5.07 5.18 5.23 5.30 5.42 5.45 5.51 5.53 5.57

28 Fatigue 2.68 2.95 3.25 3.79 4.35 4.93 5.49 6.06 6.66 7.17

Rutting 6.70 6.94 7.11 7.18 7.28 7.39 7.45 7.53 7.60 7.67

30 Fatigue 3.20 3.52 3.79 4.31 4.96 5.61 6.30 6.94 7.61 8.24

Rutting 9.00 9.24 9.49 9.74 9.85 9.90 10.1610.1710.2510.25

32 Fatigue 3.91 4.12 4.42 4.85 5.58 6.41 7.15 7.92 8.55 9.32

Rutting 11.50 11.7512.0012.3012.3512.4012.5012.5112.5512.56

34 Fatigue 4.52 4.73 5.20 5.41 6.31 7.28 8.05 8.98 9.45 10.45

Rutting 14.20 14.5014.7515.0515.1015.1515.2015.2215.2615.27

36 Fatigue 5.12 5.45 5.95 6.01 6.98 8.27 9.07 9.12 10.4011.56

Rutting 17.30 17.7 18.0018.3018.3018.3118.3518.3718.3818.40



Table 5. Variation of equivalency factors with tire pressures and axle loads based on fatigue and rutting criteria (tandem axle)

Axle Load (kip)

Damage Contact Pressure (psi)

Criteria 70 80 90 100 110 120 130 140 150 160

18 Fatigue 0.42 0.49 0.55 0.61 0.66 0.71 0.77 0.81 0.86 0.90

Rutting 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06

22 Fatigue 0.64 0.76 0.86 0.97 1.06 1.16 1.26 1.33 1.43 1.49

Rutting 0.14 0.14 0.14 0.14 0.14 0.14 0.15 0.14 0.15 0.15

26 Fatigue 0.90 1.07 1.23 1.40 1.56 1.71 1.85 1.99 2.13 2.23

Rutting 0.29 0.29 0.29 0.30 0.30 0.30 0.30 0.30 0.31 0.31

30 Fatigue 1.18 1.41 1.65 1.90 2.10 2.32 2.55 2.76 2.96 3.15

Rutting 0.53 0.54 0.54 0.55 0.56 0.56 0.56 0.57 0.57 0.57

34 Fatigue 1.48 1.79 2.12 2.43 2.75 3.06 3.37 3.65 3.91 4.22

Rutting 0.92 0.93 0.95 0.96 0.97 0.98 0.99 0.99 0.99 1.00

38 Fatigue 1.80 2.21 2.61 3.03 3.44 3.84 4.23 4.65 5.04 5.35

Rutting 1.48 1.51 1.53 1.56 1.58 1.58 1.59 1.62 1.64 1.65

42 Fatigue 2.14 2.63 3.15 3.66 4.18 4.68 5.23 5.74 6.22 6.67

Rutting 2.28 2.32 2.37 2.41 2.44 2.44 2.50 2.52 2.52 2.53

46 Fatigue 2.45 3.08 3.70 4.51 4.97 5.63 6.22 6.89 7.46 8.08

Rutting 3.33 3.41 3.51 3.56 3.61 3.68 3.70 3.75 3.77 3.78

50 Fatigue 3.07 3.56 4.28 5.03 5.78 6.56 7.32 8.10 8.84 9.66

Rutting 4.77 4.94 5.00 5.10 5.15 5.22 5.27 5.34 5.35 5.47

54 Fatigue 3.74 4.09 4.90 5.79 6.67 7.57 8.53 9.44 10.3811.30

Rutting 6.63 6.80 7.04 7.18 7.33 7.30 7.47 7.51 7.62 7.68

58 Fatigue 4.47 4.95 5.38 6.55 7.57 8.67 9.68 10.7911.8512.97

Rutting 8.94 9.25 9.59 9.79 9.83 10.0710.3010.1910.2210.41

62 Fatigue 5.28 5.86 6.42 7.32 8.53 9.77 11.0012.2413.5414.79

Rutting 11.9312.2312.7112.9613.0213.4613.5713.6813.8813.96

Comparing the values of EALFs adopted by the

RBA (the AASHTO load equivalency factors) with those in Tables (4 and 5) at standard tire contact pressure of 70 psi, it can be seen that the AASHTO factors for axle load up to 18 kip, 34 kip for single and tandem, respectevly are smaller than EALFF (the control failure mode) whereas EALFR agree with AASHTO factors. This means that the AASHTO factors consider the rutting failure mode only. It can be said that using the AASHTO factors in this range of axle load even at tire contact pressure of 70 psi may produce under-designed pavement sections. For pavement design purposes, a single value of equivalent factors should be used. So, the greater

equivalency factors between EALFF and EALFR were listed in Tables (6 and 7). It can be noticed also that the values of the EALFs in Tables (6 and 7) at the operational level of tire contact pressure of 130 psi along with the legel values of axle loads in Egypt are approximately twice and triple the AASHTO factors for single and tandem axle, respectively.

Table 6. Equivelant axle load factors for single axles

Axle Load AASHTO Contact Pressure (psi)

(kip)(ton) EALF 70 80 90 100 110 120 130 140 150 160

8 3.6 0.034 0.19 0.22 0.24 0.26 0.29 0.30 0.32 0.34 0.36 0.38

10 4.5 0.088 0.31 0.36 0.40 0.45 0.49 0.54 0.56 0.60 0.63 0.67

12 5.4 0.189 0.45 0.54 0.61 0.68 0.76 0.82 0.88 0.94 1.00 1.05

14 6.4 0.360 0.62 0.73 0.85 0.96 1.06 1.17 1.27 1.36 1.46 1.54

16 7.3 0.623 0.81 0.96 1.12 1.27 1.42 1.58 1.71 1.86 1.97 2.11

18 8.2 1.000 1.00 1.21 1.42 1.63 1.83 2.04 2.22 2.41 2.59 2.76

20 9.1 1.510 1.59 1.62 1.75 2.00 2.25 2.52 2.78 3.02 3.29 3.49

22 10 2.180 2.39 2.46 2.49 2.56 2.74 3.07 3.39 3.72 4.04 4.34

24 10.9 3.030 3.46 3.59 3.65 3.68 3.76 3.95 4.03 4.42 4.84 5.25

26 11.8 4.090 4.92 5.07 5.18 5.23 5.30 5.42 5.45 5.51 5.67 6.19

28 12.7 5.390 6.70 6.94 7.11 7.18 7.28 7.39 7.45 7.53 7.60 7.67

30 13.6 6.970 9.00 9.24 9.49 9.74 9.80 9.90 10.1610.1710.2510.25

32 14.5 8.880 11.5011.7512.0012.3012.3512.4012.5012.5112.5512.56

34 15.4 11.180 14.2014.5014.7515.0515.1015.1515.2015.2215.2615.27

36 16.3 13.930 17.30 17.7 18.0018.3018.3018.3118.3518.3718.3818.40

Table 7. Equivelant axle load factors for tandem axles

Axle Load AASHTO Contact Pressure (psi)

(kip)(ton) EALF 70 80 90 100 110 120 130 140 150 160

18 8.2 0.077 0.42 0.49 0.55 0.61 0.66 0.71 0.77 0.81 0.86 0.90

22 10 0.180 0.64 0.76 0.86 0.97 1.06 1.16 1.26 1.33 1.43 1.49

26 11.8 0.363 0.90 1.07 1.23 1.40 1.56 1.71 1.85 1.99 2.13 2.23

30 13.6 0.658 1.18 1.41 1.65 1.90 2.10 2.32 2.55 2.76 2.96 3.15

34 15.4 1.095 1.48 1.79 2.12 2.43 2.75 3.06 3.37 3.65 3.91 4.22

38 17.2 1.700 1.80 2.21 2.61 3.03 3.44 3.84 4.23 4.65 5.04 5.35

42 19.1 2.510 2.28 2.63 3.15 3.66 4.18 4.68 5.23 5.74 6.22 6.67

46 20.9 3.550 3.33 3.41 3.70 4.51 4.97 5.63 6.22 6.89 7.46 8.08

50 22.7 4.860 4.77 4.94 5.00 5.10 5.78 6.56 7.32 8.10 8.84 9.66

54 24.5 6.470 6.63 6.80 7.04 7.18 7.33 7.57 8.53 9.44 10.3811.30

58 26.3 8.450 8.94 9.25 9.59 9.79 9.83 10.0710.3010.7911.8512.97

62 28.1 10.84 11.9312.2312.7112.9613.0213.4613.5713.6813.8814.79

M.E. Abdel-Motaleb


Table 8. Adjusted EALFs for operational level of tire pressure; 130 psi

Axle Load

Strong Section

Weak Section

(kip) (ton) Single Axle Tandem

Axle Single Axle

Tandem Axle

8 3.6 0.32 - 0.59 - 10 4.5 0.56 - 0.85 - 12 5.4 0.88 - 1.11 - 14 6.4 1.27 - 1.36 - 16 7.3 1.71 0.62 1.61 1.14 18 8.2 2.22 0.77 1.84 1.46 20 9.1 2.78 1.10 2.06 1.75 22 10 3.39 1.26 2.64 1.97 24 10.9 4.03 1.51 3.82 2.18 26 11.8 5.45 1.85 5.45 2.47 28 12.7 7.45 2.13 7.50 2.69 30 13.6 10.16 2.55 10.28 2.97 32 14.5 12.50 2.86 13.87 3.16 34 15.4 15.20 3.37 17.62 3.50 36 16.3 18.35 3.71 22.33 3.62 38 17.2 - 4.23 - 3.92 40 18.1 - 4.68 - 4.03 42 19.1 - 5.23 - 4.35 44 20 - 5.77 - 4.78 46 20.9 - 6.22 - 5.11 48 21.8 - 7.00 - 6.38 50 22.7 - 7.32 - 7.34 52 23.6 - 8.35 - 9.36 54 24.5 - 8.53 - 10.38 56 25.4 - 9.84 - 12.41 58 26.3 - 10.30 - 13.89 60 27.2 - 11.46 - 16.21 62 28.1 - 13.57 - 18.76

Table 9. Adjusted truck factor (TF) at tire contact pressure of 130

psi versus traditional TF for different truck types in Egypt.

Truck Code

Adjusted TF * Traditional ** TF

Adjusted TF / Traditional TF

Strong Section

Weak Section

Strong Section

Weak Section

2D 4.47 3.92 2.445 1.828 1.603 3A 4.58 4.84 1.505 3.043 3.216 2-2 11.25 9.20 6.805 1.653 1.352 2-3 11.36 10.11 5.865 1.937 1.724 3-2 11.36 10.11 5.865 1.937 1.724 3-3 11.47 11.02 4.925 2.33 2.24

2-S1 7.86 6.56 4.625 1.70 1.42 2-S2 11.25 9.20 6.805 1.653 1.35 2-S3 11.36 10.11 5.865 1.937 1.724 3-S1 7.97 7.47 3.685 2.17 2.03 3-S2 8.08 8.38 2.745 2.944 3.053

Average 9.183 8.265 4.649 2.10 1.95

* Calculated based on the legal axle loads using the adjusted EALFs. ** Calculated based on the legal axle loads using AASHTO equivalency

factors.

10. PRACTICAL APPLICATION From the previous analysis of data, it is become aboviously that truck tire inflation pressure has experienced significant increases over the years. Doubtlessly, these high tire pressures are much more damaging to pavements, especially when combined with heavy axle load. Pavement designers should be aware of the fact that the traffic analysis based on empirical equations developed under a tire inflation pressure of 80 psi would not satisfy the traffic demand under a tire inflation pressure over 140 psi. A reasonable scheme to consider the effects of high tire inflation pressure on pavement performance is highly desirable. That is therefore, adjusted EALFs are needed for proper design of Egyptian pavements during calculating the design number of ESAL. So, adjusted values of EALF were determined for the weak section using tire contact pressure of 130 psi. The adjusted EALFs at the operational level of truck-tire contact pressure of 130 psi were summarized in Table (8) for both strong and weak pavement sections, to be used if detailed data for truck distribution and axle loads survey are available. Comparing the values of adjusted EALFs for strong and weak sections in Table (8), one can notice that the EALFs of weak section are mildly greater than those of strong section at light and heavy axle loads, while at intermediate axle loads the reverse is true for both single and tandem axles.

Furthermore, adjusted truck factors (TF) for all truck categories at the operational level of truck-tire pressure of 130 psi were calculated and presented in Table (9) for both strong and weak sections, to be used for approximate estimates of ESALs, if no detailed traffic distribution data are available. Comparing the values of traditional TF with those adjusted, one can found that the adjusted values are 1.35 to 3.22 times the traditional values with average of 2 for both strong and weak pavement sections. To explain this finding, consider the most used truck in Egypt (code 3-2) with four axles (12 kip single, 34 kip tandem, 22 kip single and 22 kip single) with the operational level of tire contact pressure of 130 psi. The traditional TF using the AASHTO factors is 0.189 + 1.095 + 2.18 + 2.18 = 5.644 compared with the adjusted TF of 11.03 = 0.88 + 3.37 + 3.39 + 3.39 at tire contact pressure of 130 psi. This means that the calculated TF with adjustment for operational level of tire pressure is about 2 times the traditional TF which did not take the effect of high tire pressure into consideration. This finding can explain the premature deterioration of Egyptian roads.

11. CONCLUSIONS AND RECOMMENDATIONS

Based on the methodology and analysis of results of this study, the following conclusions were drawn:



1. Field measurements of tire inflation pressure showed that the tire inflation pressures for trucks in Egypt are as high as 140 psi. with a difference from 20 to 60 psi between the same truck tires, and from 10 to 50 psi between the two tires in the dual-tire assembly.

2. The effect of increased tire pressure on the pavement depends on axle load. At low to intermediate axle loads, high tire pressure can cause marked increase in EALFs. At high axle loads, variation of EALFs with tire pressure becomes insignificant.

3. The calculated TF with adjustment for operational level of tire pressure is two times the traditional TF which did not take the effect of high tire pressure into consideration.

4. It is recommended that the detrimental effects of high tire pressure should be considered in the determination of ESAL for pavement design in Egypt using the adjusted value of EALF or TF at tire pressure of 130 psi. Also, a proposed study to investigate the effects of difference in inflation pressure between the tires of the same truck and the two tires in the dual-tire assembly on pavement responses is recommended.

REFERENCES 1. Gab-Allah, A.A. (1995). Rutting of Asphalt Pavements in

Egypt Roads and Methods of its Prediction and Evaluation, Ph. D. Faculty of Eng., Zagazig Univ.

2. Eisenmann, J., and Hilmer, A. (1987). Influence of Wheel Load and Inflation Pressure on the Rutting effect at Asphalt Pavements (Experimental and Theoretical Investigations), Proc., 6th International Conf. on the Structural Design of Asphalt Pavements, Vol. I, Ann Arbor, 392-403.

3. Machemehl, R.B., Wang, F. and Prozzi, J.A. (2005). Analytical Study of Effects of Truck Tire Pressure on Pavements with Measured Tire-Pavement Contact Stress Data, 111-120.

4. El-Hamrawy, S. (2000). Effect of Wheel Load, Tire Pressure and Subgrade Stiffness on Flexible Pavements Responses, Al-Azhar Engineering 6th International Conference, 489-502.

5. Southgate, H.F. and Deen, R.C. (1987). Effects of Load Distributions and Axle Tire Configurations on Pavement Fatigue, Proc., 6th Int. Conf. on Struct. Design of Asphalt Pavements, Michigan, 82-93.

6. Peiwen, H. and Yoshitaka, H. (2004). Stress Analysis of Asphalt Pavement under Non-Uniform Load, Proceedings of 8th International ASCE Conf. on Applications of Advanced Technologies in Transportation Eng.

7. Yoder, E.J. and Witczak, M.W. (1975). Pricipals of Pavement Design, 2nd ed., Wiley, New York.

8. Fernando, E.G., Musani, D., Park, D. and Liu, W. (2006). Evaluation of Effects of Tire Size and Inflation Pressure on Tire Contact Stresses and Pavement Response, Texas Transportation Institute; Texas Department of Transportation; Federal Highway Administration.

9. American Association of State Highway and Transportation Officials, Guide for Design of Pavement Structures, Volume 1, AASHTO, 1993.

10. Huang, W.H., Sung, Y.L., Lin, J.D. and Hung, C.T. (2001). Effect of Heavy Vehicle and Tire Pressure on Flixable Pavement Design in Taiwan, TRB, 80th Annual Meeting. 1-21.

11. Hass, R., Hudson R.W. and Zaniewski, J. (1994). Moderen Pavement Management, Malabar, Florida, 1st Edition, 1994.

12. Sharaf, E. A., and Darwish, G. S. (1997). Analysis of the Effect of Load Type on Truck Factors, Arab Roads, 2nd Edition, 15-29.

13. Asphalt Institute, (1991). Thickness Design- Asphalt Pavement for Highways and Streets, 9th ed., the Asphalt Institute, Manual Series No.1 (MS-1).

14. Egyptian Code for Urban and Rural Highways (2003). Part 6, Structural Design of Highways, 1st ed.

15. Asphalt Institute (1982). Researsh and Development of the Asphalt Institutes Thickness Design Manual, 9th Edition, Researsh Report 82-2, the Asphalt Institute, (MS-1).

16. Yang H. (1993). Pavement Analysis and Design, Englewood Cliffs, New Jersey.

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