Damaging Effect of Armoured Vehicles with Rubber Tires on Flexible Pavement

Presented in this paper is a new study of the damaging effect of military armoured vehicles with rubber tires on flexible pavements. Two types of military armoured vehicles with rubber tires were studied, namely LAV-AT four-axle and LAV-300 triple-axle. A measure of the da maging effect of military arm oured vehicles with rubber tires loads was achieved by correlating their equi valent loads with the AASH TO equivalency factors. The equivalent load was developed on the basis of mechanistic - empirical approach. It was found that the damaging effect of the studied military armoured vehicles with rubber tires loads is 0. 0.200 to 4.736 times the damaging effect of the standard 18 kips (80 kN) axle load depending on the thickness of asphalt layer.


Introduction
Determining the pavement life under given structural, environmental, and traffic conditions is considered the main objective in the pavement design and analysis.The American Association of State Highway and Transportation Officials (AASHTO) Design Guide estimates pavement life in terms of the number of equivalent single axle loads (ESAL's).Its design equation was established through empirical analysis primarily based on the AASHO Road Test in Ottawa, Illinois conducted during the late 1950s.The deterioration of a roadway is accelerated over time by the repeated application of loads generated by heavy vehicles.Consequently projected maintenance and preservation costs increase.Pavement deterioration is further intensified by an incentive for overweight trucks due to economic benefits of an increased payload (Paxson and Glickert, 1982).
The effect of the traffic using these roads should be focused upon carefully from the standpoint of pavement structural design.Yoder and Witczak (1975) reported that this effect includes among other considerations, the expected vehicle type and the corresponding number of repetitions of each type during the design life of the pavement.The effect of various types of vehicles (axles) on the structural design of road pavement is considered by means of the approach of axle load equivalency factor.In this approach, a standard axle load is usually used as a reference and the damaging effect of other axle loads (corresponding to various types of axles) is expressed in terms of number of repetitions of the standard axle.The AASHTO standard axle is the 18 kips (80 kN) single axle with dual tires on each side (Yoder and Witczak, 1975).Thus, the AASHTO equivalency factor defines the number of repetitions of the 18 kips (80 kN) standard axle load which causes the same damage on pavement as caused by one pass of the axle in question moving on the same pavement under the same conditions.The AASHTO equivalency factor depends on the axle type (single, tandem, or triple), axle load magnitude, structural number (SN), and the terminal level of serviceability (pt).The effect of structural number (SN) and the terminal level of serviceability (pt) are rather small; however, the effect of axle type and load magnitude is pronounced (Razouki and Hussain, 1985).There are types of vehicle loads that not included in the AASHTO road test such as the military armoured vehicles that move on paved roads occasionally during peace times and frequently during war times.The effect of the military armoured vehicle loads on flexible pavements is not known, and not mentioned in the literature up to the capacity of the author's knowledge.Therefore, this research was carried out to find the AASHTO equivalency factors and the damaging effect of military armoured vehicles that move frequently on our roads network (even on small local paved streets) on daily bases for more than six years up to now.There are two main approaches used by researchers to determine the equivalency factors, the experimental and the mechanistic (theoretical) approach.A combination of two approaches was also used by Wang and Anderson (1979).
In the mechanistic approach, some researchers adopted the fatigue concept analysis for determining the destructive effect (Havens et al., 1979), while others adopted the equivalent single wheel load procedure for such purposes (Kamaludeen, 1987).The mechanistic empirical approach is used in this research depending on fatigue concept.Following Yoder and Witczak (1975), AASHTO design method recommended the use of 18 kips (80 kN) standard axle with dual tires on each side, thus, the AASHTO equivalency factor F j is:  1) above for different pavement structures.Huang (1993) reported that in fatigue analysis, the horizontal minor principal strain is used instead of the overall minor principal strain.This strain is called minor because tensile strain is considered negative.Horizontal principal tensile strain is used because it is the strain that causes the crack to initiate at the bottom of asphalt layer.The horizontal principal tensile strain is determined from: where, ε r = the horizontal principal tensile strain at the bottom of asphalt layer, ε x = the strain in the x direction, ε y = the strain in the y direction, γ xy = the shear strain on the plane x in the y direction.Therefore, ( ε r ) of equation ( 2) represents (ε j ) of equation ( 1) and will be used in fatigue analysis in this research.These two criteria were used in this research to determine the AASHTO equivalency factors of military armoured vehicles.The tensile strains at the bottom fiber of asphalt concrete and vertical compressive strains on sub-grade surface of similar pavement structures to that of AASHTO road test as reported by AASHTO (1986)

Characteristics of military armoured vehicles with rubber tires
The characteristics of military armoured vehicles which required in this research are their three dimensions (height, length, and width) in addition to the weight.These features were obtained from the brochure of their manufacturing company (General Dynamics Land Systems, 2010) and the website ( The Federation of American Scientists, 2010).Two types of military armoured vehicles with rubber tiers were taken for the purpose of this study as follows (see Figure (1) and Figure ( 2)): 1. LAV-AT four-axle eight-wheel military armoured vehicle was chosen to represent the family of four-axle military armoured vehicles with rubber tiers because it is widely used and can be converted to any other type and purpose.2. LAV-300 triple-axle six-wheel military armoured vehicle was chosen to represent the family of triple-axle military armoured vehicles with rubber tiers because it is widely used and can be converted to any other type and purpose.(Kamaludeen, (1987).AASHTO Poisson's ratios of 0.4 for asphalt layer, 0.35 for base layer, and 0.4 for sub-grade layer were taken for the purpose of this analysis.Two types of military armoured vehicles with rubber tires were studied, namely LAV-AT four-axle and LAV-300 triple-axle as shown in Table (1).

AASHTO equivalency factors of LAV-AT military armoured vehicle LAV-AT
four-axle eight-wheel multipurpose military armoured vehicle was used to represent the family of four-axle military armoured vehicles that is widely used world wide.Three-layer pavement structure was taken as mentioned in the introduction above to simulate AASHTO original road test as shown in Figure (1).The contact areas of the eight wheels were calculated using three values for tire pressure namely, 0.828, 0.69, and 0.552 MPa respectively to study the effect of tire pressure on the AASHTO equivalency factors of these military armoured vehicle loads.The total combat weight of 12.55 tons was distributed equally on the eight wheels because these vehicles have load distribution mechanism on equal bases. .5572 (Huang, 1993).Figure (6) was prepared to show the calculated vertical compressive strains on the surface of sub-grade layer of AASHTO pavement structure shown in Figure (1) under LAV-AT armoured vehicle with a tire pressure (contact pressure) of 0.828 MPa.These strains were obtained for 400 calculating points using KENLAYER computer program (Huang, 1993).It was found that the calculated tensile strains in the direction of x, y, and r at the bottom fiber of asphalt concrete layer are much more conservative than calculated vertical compressive strains on the surface of sub-grade layer under LAV-AT military armoured vehicle in comparison with their similar type of strains reported by AASHTO (1986), as shown in Figure (3) to Figure (6).
Therefore, the fatigue criterion governed and was used to calculate the AASHTO equivalency factors of LAV-AT military armoured vehicle.The maximum calculated horizontal principal tensile strains ( ε r ) at the bottom fiber of asphalt concrete layer under LAV-AT military armoured vehicle for the AASHTO (1986) pavement structures are summarized in Table (2).The AASHTO (1986) reported maximum tensile strains ( ε t ) at the bottom fiber of asphalt concrete layer for the AASHTO pavement structures under the standard 18 kips (80 kN) are shown also in Table (2).The values for the constant c of equation ( 4) for each of AASHTO (1986) pavement structure were obtained from the values of Asphalt Institute as mentioned by Huang (1993).The AASHTO equivalency factors of LAV-AT military armoured vehicle were calculated using equation ( 1) as shown in Table (2).The maximum tensile strains in the direction of x and y at the bottom fiber of asphalt concrete layer and the vertical compressive strains on the surface of sub-grade layer under LAV-AT military armoured vehicle for the AASHTO (1986) pavement structures were recalculated using different tire pressure values of LAV-AT military armoured vehicle to study the effect on strain values as shown Table (3).These strains were calculated using only one AASHTO pavement structure shown in Figure (1) above.It was found that the tire pressure has very small effect on the value of strain and later on the value of AASHTO equivalency factors of LAV-AT military armoured vehicle loads.This can be attributed to the high load magnitude and the interlocking of the effects of eight loaded tires in three dimensions.3

Discussion of results and Conclusions
It was found that the military armoured vehicles with rubber tires have a pronounced damaging effect on flexible pavements in terms of AASHTO equivalency factors as follows: 1-The AASHTO equivalency factors of LAV-AT military armoured vehicle load were found to be from 0.200 to 3.488 based on fatigue criterion.Increasing the thickness of the asphalt layer pavement decreases the AASHTO equivalency factors of LAV-AT military armoured vehicle load.This means that the structural damaging effect LAV-AT military armoured vehicle load on flexible pavements of secondary and local roads is higher than its damaging effect on the flexible pavement of major roads and highways.It was found that increasing the tire pressure has very small effect on the AASHTO equivalency factors of LAV-AT military armoured vehicle load from the theoretical point of view due to the high magnitude of LAV-AT military armoured vehicle load.2-The AASHTO equivalency factors of LAV-300 military armoured vehicle load were found to be from 0.875 to 4.736 based on fatigue criterion.Increasing the thickness of the asphalt layer pavement decreases the AASHTO equivalency factors of CM31 military armoured vehicle load.This means that the structural damaging effect CM31 military armoured vehicle load on flexible pavements of secondary and local roads is higher than its damaging effect on the flexible pavement of major roads and highways.

Recommendations
Based on the results of this study, an economic evaluation for the cost of damage that had been caused by the frequent movement of military armoured vehicles with rubber tires on the whole national road network during the last six years is required.Also, another study is necessary to determine the damaging effect of military armoured vehicles with rubber tires on the national road network during summer seasons.

Notations
F j AASHTO equivalency factor.c regression constant.E 1 the modulus of asphalt layer.E 2 the modulus of the base layer.E 3 the modulus of subgrade layer.t 1 thickness of asphalt layer.t 2 thickness of base layer.

Greek letters
ε j the maximum principal tensile strain for the jth axle.ε s the maximum principal tensile strain for the 18 kips standard single axle.ε r the horizontal principal tensile strain at the bottom of asphalt layer.ε x the strain in the x direction.ε y the strain in the y direction.γ xy the shear strain on the plane x in the y direction.ε v compressive strain on the top of subgrade soil.

3. 1 1 =
AASHTO equivalency factors of military armoured vehicles Three-layer pavement structure was taken as mentioned in the introduction above to simulate AASHTO original road test pavements as shown in Figure (1) and Figure (2).Only one set of values for the modulus of asphalt layer (E 1035.5 MPa), the base layer (E 2 = 103.5 MPa), and the sub-grade modulus (E 3 = 51.7 MPa) was taken from the original AASHTO road test because it is similar to the modulus values of local materials in practice ]

Figure ( 3 )
, Figure(4), and Figure(5) were prepared to show the calculated tensile strains in the direction of x, y, and r at the bottom fiber of asphalt concrete layer respectively under LAV-AT military armoured vehicle.These strains were obtained for 400 calculating points for each one of these figures with a tire pressure of 0.828 MPa and using KENLAYER computer program PDF created with pdfFactory Pro trial version www.pdffactory.comEng.& Tech.Journal, Vol.28, No.

& Tech. Journal, Vol.28, No.17, 2010 Damaging Effect of Armoured Vehicles with Rubber Tires on Flexible Pavement
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AASHTO (1986) maximum horizontal strain ( ε t) at the bottom fiber of asphalt layer under the standard 18 kips (80 kN) axle load for serviceability (Pt) of 2.0. (2) Calculated maximum horizontal principal tensile strain ( ε r) at the bottom of asphalt layer under LAV-AT for load layout shown in Figure (1) above.
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