CONFIGURATIONS OF TIRE PRESSURE ON THE PAVEMENT FOR COMMERCIAL VEHICLES: CALCULATION OF THE ‘N’ NUMBER AND THE CONSEQUENCES ON PAVEMENT PERFORMANCE

Road traffic has a high variety of types of vehicles and cargo transported. One of the main difficulties in pavement design is forecasting the changes in traffic over time and evaluating the comparative deterioration power of the various loads with axle and tire settings and different levels of repetition. The road environment in Brazil, as we know, is the main means of the country's cargo transportation. Throughout the 1990s and 2000s the road modal accounted for over 60% of the total cargo transported in the country leaving the unbalanced Brazilian transport matrix with almost exclusive investments in such means of transportation.


INTRODUCTION
Roads are the country's primary means of cargo transportation. Over the 1990s and 2000s this modal accounted for over 60% of the total cargo transported in the country (IPEA, 2010). Excluding the transportation of iron ore that is made by railway, roads account for more than 70% of general cargo.
In 1950, roads were responsible for 38% of the national cargo transportation (BNDES, 2008). When Juscelino Kubitscheck's government defined the Target Plan, roads were prioritized in order to, among other goals, stimulate the manufacturing industry through the auto industry. Since then the Brazilian transport matrix has shown this unbalance, with investments almost exclusively in this type of modal.
The significant dependence of this modal for the Brazilian cargo transportation is emphasized when compared to other countries with similar dimensions. According to Bartholomeu (2006), in the United States, roads account for 26% of cargo transportation, while in Australia this figure is 24% and in China only 8%.
According to the National Confederation of Transport (CNT, 2012), the Brazilian road network consists of 1,366,578 km of unpaved roads and 214,414 km http://www.ijmp.jor.br v. 7, n. 5, Special Edition IFLOG 2015ISSN: 2236 of paved roads, which represents one of the world's biggest road networks. Despite its large extension, only about 13% of the network is paved.

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According to a survey conducted by CNT (2012) in approximately 45% of the paved network, evaluating functional pavement conditions, traffic signs and geometry, 46% of the country's road network show some kinds of pavement deficiency.
IPEA (2010) conducted a study to identify bottlenecks and deficiencies of national roads and they concluded it would take 183.5 billion reais in investments to solve the problems and boost the road sector, increasing its efficiency and its impact on the country's economy.
Within this context, it is clear that the country's economy depends on this modal and that Brazilian roads are in poor conditions. Road pavement stands out mainly because it represents the highest costs, either of deployment, restoration and maintenance, as well as providing users a proper traffic-ability with comfort and safety, resisting the actions of traffic and distributing the efforts to its underlying layers so that the entire set leads to savings in fuel costs, risk of accidents, cargo damages, among others.
One of the main factors to determine the pavement's service life is the type and volume of the traffic to which it will be exposed over its lifetime and its project.
Therefore, studies are needed to contribute to a better understanding of the complex influence of the traffic on the pavements sizing and service life (MEDINA; MOTTA, 2005).
In order to standardize the measurement of the factors that influence the pavement sizing, combining the current traffic, the projected traffic and the consequences of its use, mainly through commercial loads, on the fatigue of the pavement structure, computer programs were created that verify the project's viability and durability. Among the standards established and accepted by the projects of the National Department of Transport Infrastructure (DNIT) and its state representatives according to the Paving Manual -IPR -719 (2006) or the IP-DE-P00/001 -Paving Projects of DER/SP, the following parameters were defined: the standard single axle dual wheel of 80 kN and the tire-pavement contact pressure of 0.56 MPa. http://www.ijmp.jor.br v. 7, n. 5, Special Edition IFLOG 2015ISSN: 2236 Considering the variation of ambient temperature, the differences of transported loads that generate more or less friction between the tires and the pavement. The absence of automatic pressure calibration equipment in the vehicle, the driver's negligence regarding the equipment calibration and the load allowed by law, as well as the quality of tires, leaks, etc., we can relate the variation of tire pressure on the pavement with the resulting different structure fatigue through a study where loads are altered in the mechanistic analysis program and the structure behavior is verified. Therefore, the general goal of this work is to analyze the structural condition of a standard pavement used in projects of Brazilian roads with predefined and constant dimensions, changing the tire pressure applied to the pavement used in standard methodology and verifying the effect on the pavement layers through fatigue equations standardized by DNIT.

LITERATURE REVIEW
According to National Federation of Automotive Vehicles Distribution (FENABRAVE), the commercial vehicle industry starts growing from the year 2000, associated to the growth of the Brazilian economy and the increase in the transport of commodities. Given the high average age of the Brazilian fleet (Trucks: 17.2 years and Buses: 14.4 years), loan portfolios were created to finance and stimulate the commercial vehicle market. Table 1 shows the production by size from 2008 to 2014.

Pavement and traffic
According to Huang (1993), traffic is the main factor when designing a pavement structure. The traffic variable should include load magnitude, the configuration and the number of load repetitions. According to Yoder and Witczak (1975), the main factors to evaluate the structure of a pavement design are the effects of the type of vehicle, traffic volume and the type of vehicle. The type of vehicle should include specific factors such as axle configuration, wheelbase, wheel load and tire inflation pressure.
According to DNIT (2006), trucks are the largest consumers of the pavement structure because they apply the highest loads on its surface. Among the heavy trucks, not all cause the same damage due to the variations in wheel load (static and dynamic), the number and location of axles, the types of suspension, the number of wheels, the type and pressure of tires and other factors. Balbo (2007) adds that road commercial vehicles (trucks and buses) are the ones that actually matter for designing the pavement since light vehicles cause minor damages to the pavement structure when compared to the load of commercial vehicles.
The pavement is a structure of multiple layers of finite thicknesses, implemented over a final surface of earthwork or sub base, technically and economically intended to resist the efforts resulting from vehicle traffic and weather. It is designed and built to resist the volume of motor vehicles under different climate conditions and to improve the rolling conditions in terms of comfort and safety (SENÇO, 1997;BERNUCCI, et al., 2006). According to Pinto and Preussler (2002), the variations of axle overload and the corresponding destructive effects on the pavements take place exponentially. http://www.ijmp.jor.br v. 7, n. 5, Special Edition IFLOG 2015ISSN: 2236 They point out the importance of avoiding using overloaded vehicles. Regarding the use of the tire, the pavement design guide considers the tire inflation pressure of 750 kPa, however it adds that data from Tasmania indicate that the tire inflation pressure can vary from 500 to 1200 kPa (AL-QADI; ELSEIFI, 2007). Kawa et al. (1997)  In the road test of The American Association of State Highway Officials-AASHO, the tire inflation pressure varied from 520 to 550 kPa (75-80 psi). Today trucks usually use tires with pressure ranging from 600 to 800 kPa (85 to 115 psi) and, in some cases the tire pressure can reach 900 to 1000 kPa (130 to 145 psi).

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Literature review shows that high tire pressures can potentially cause more damages to the pavements.
According to Feng (2005) the tire pressure of commercial vehicles has increased in recent decades. A more visible effect of increasing tire pressure would be the reduction of the tire/pavement contact area, which in turn can result in an increase of the tire/pavement contact and another damaging effect on the structure.

Types of Brazilian road traffic and their characteristics
The Brazilian legislation establishes some parameters for the circulation of  http://www.ijmp.jor.br v. 7, n. 5, Special Edition IFLOG 2015ISSN: 2236 Within the scope of the present research, the Brazilian Traffic Code establishes the following provisions:

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• In order to travel on roads the vehicle must comply with the limits defined by CONTRAN regarding weight and dimensions, and the overweight should be measured by weighing equipment or by checking tax documents. Additionally, a percentage over the limits of the total gross weight and the gross weight transmitted by vehicle axle to the road surface is tolerated.
• Passenger or cargo vehicles will not travel with capacity or load above the limit defined by the manufacturer, and neither exceed the maximum traction capacity of the traction unit.
• Driving the vehicle with overweight, considering the tolerance percentage when measured by a weighing equipment, will result in a moderately serious offense with a fine increased every two hundred kilograms or a fraction of verified overweight, according to the law.
The Resolution No 210 of November 13, 2006, "defines the weight and dimension limits for vehicles travelling on roads and offers other provisions".
According to DNIT (2015), the resolution establishes the maximum limits of Total Gross Weight and Total Gross Weight Combined (Table 2). in Art. 97, 99 and 314 of the Brazilian Traffic Code, establishing, within the context of this research that the combinations with more than two units, including the traction unit, with total gross weight over 57 tons or total length over 19.80 meters, will only be able to circulate with a Special Traffic Authorization (STA).
The Article II of the Resolution defines the minimum requirements for a special traffic authorization, which will not be the object of this research because it represents an insignificant share in terms of road traffic. 258/07 accepted a tolerance of 7.5% (seven and a half percent) by axle. After that, the tolerance by axle will be of 5% (five percent).
The Resolution nº 258/07 also establishes, regardless of the type of load, limits by axle that allow the vehicle's travel to continue without the need of relocation or overflow after an overweight fine is applied. The Resolution establishes that the tolerance is valid for the equipment ("scale") and cannot be incorporated during the loading. Whoever uses the tolerance during loading would be illegally increasing its http://www.ijmp.jor.br v. 7, n. 5, Special Edition IFLOG 2015ISSN: 2236 limits, thus reducing the tolerance intended for scale faults and considerably increasing the risk of getting a fine. Additionally, the 5% (five percent) tolerance was abolished during invoice checking; this tolerance was previously defined by the now revoked Resolution nº 104/99, which only allowed verification through the invoice on roads where there were no scales. Now, when there is no scale, the measurement can be made at any time or place through the weight declared in the invoice, waybill or cargo manifest, and no tolerance is allowed regarding the declared weight.

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Besides the changes mentioned above, the Resolution nº 258/07 established a fine to vehicles that travel with a load above the Maximum Traction Capacity Therefore, the maximum tolerance of 7.5 % (seven and a half percent) over the limits of gross weight transferred by the axles to road surfaces will still be allowed. After that date, the maximum tolerance will again be 5% (five percent).

Weighing of commercial vehicles
According to DNIT (2006) the weighing operation consists of weight checking by axle or set of axles and of the total gross weight, performed at the weighing sites using mobile or fixed scales located in an area of the road that doesn't disturb the flow of vehicles. Also, according to DNIT, the weighing shall follow these guidelines:  http://www.ijmp.jor.br v. 7, n. 5, Special Edition IFLOG 2015ISSN: 2236

Tires
The inflation pressure range recommended by Pirelli, between 750 and 800 kPa, is the most significant within the universe of commercialized tires. In order for an actual verification it would be necessary to know the sales percentage of each type of tire and to check, using other tools, the actual inflation pressure. However, this gives us a comprehensive parameter of the actual inflation pressure acting on the pavement.
To better visualize the inflation pressures recommended by Pirelli a chart was prepared showing the pressure range vs. the percentage of commercialized tires,

Methods to calculate the "N" number and its variables
The "N" number is calculated using the following Equation 1: (1) Where, N = number of solicitations equivalent to the standard axle; P= project period; VDM= daily average traffic volume; FV= vehicle factor; FR = regional climate http://www.ijmp.jor.br v. 7, n. 5, Special Edition IFLOG 2015ISSN: 2236 , 1972 and 1983), the former is proposed by DER/SP and the latter for the purpose of mechanistic evaluation.

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According to the sizing procedure of DER/SP, in order to consider the variations of humidity of the materials that constitute the pavement during the seasons of the year (resulting in variations of the capacity of material support), one must multiply the equivalent number of solicitations of the standard axle (or traffic parameter) "N" by a "FR" coefficient, called Regional factor, which, on the experimental AASHTO lane, ranged from 0.2 (low level of humidity) to 5.0 (materials are virtually saturated).

Verification of the pavement structures through the elasticity theory
The vehicles' loads generate tensions and deformations inside the pavement structure. These tensions and deformations are a function of the load magnitude, the resilient modules, the thickness of the pavement layers and the sub base's support capacity.
Therefore, the acting shifts and deformations that originate inside the loaded pavement were determined in order to be compared with the allowable shifts and deformations value and that are a function of the type of material used in the pavement structure.
In order to determine the internal efforts, the deformations and shifts of the flexible pavement structure the computer program ELSYM 5 (Elastic Layered System) was used, which takes into account the constant elastic characteristics for each layer of the pavement structure. The parameters were used to calculation: In order to determine the allowable efforts the following fatigue equations 3, 4 and 5 were used.

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The values obtained by the software are used in fatigue equations shown above for each layer of the pavement and, as indicated, the "N" number is found for the period. The mechanistic analysis of the pavement structure uses material fatigue models that calculate their limitations through the above-mentioned parameters.

Influence of the calculation variables on the result of the N number
It is necessary to understand how and under which conditions the mathematical expressions of material fatigue were obtained in order to consider them in the evaluation of the quality and performance of a particular pavement. http://www.ijmp.jor.br v. 7, n. 5, Special Edition IFLOG 2015ISSN: 2236

Performance of asphaltic pavements
The pavement performance evaluation was developed based on the criterion of deform-ability of flexible pavements, expressed by measuring the recoverable deflections since its experience has shown there is a correlation between the magnitude of the deflections (and of the corresponding radius of curvature) and the occurrence of failures in flexible pavements.
The behavior of a well-built pavement depends on the various solicitations it suffers throughout its lifespan, including the traffic that results in different efforts of compression, shearing and flection, as well as climate factors such as rains and temperature changes that cause fatigue on the entire structure.
Regarding the deflection, the following phases of a pavement life cycle can be observed: • usually the result is a degradation process. Figure 2 illustrates the phases of a pavement life cycle described above.  The flexible pavement will be technically and economically better than longer elastic phase.

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The allowable deflection value depends on the materials that constitute the coating and the base of the pavement, as well as on the N number of solicitations of axels that equivalent to the standard axle of 80 kN. Figure 3 shows the criteria for a structural evaluation, associating the allowable deflections with the pavement state.  http://www.ijmp.jor.br v. 7, n. 5, Special Edition IFLOG 2015ISSN: 2236 This study used the AASHTO and USACE methodologies. The pavement structure chosen for the study consists of a 5.0 cm thick layer of CBUQ Band III, followed by a 5.0 cm thick layer of CBUQ Band II, a 20.0 cm thick layer of Simple Graded Gravel as base and a 30.0 cm thick layer of Dry Bound Macadam as sub base. Figure 4 illustrates the pavement of this study.  Table 3 shows the simulation of road cost per linear meter using the DER/SP single price table of 03/31/2015, considering the structure analyzed in this study.  http://www.ijmp.jor.br v. 7, n. 5, Special Edition IFLOG 2015ISSN: 2236 As can be seen in Table 3 and Figures 5, 6, 7, 8, 9 and 10 shown below, there is a significant decrease of the service life of the pavement that is the object of this study. As the axle load applied on the contact pressure between the tires and the road surface increases, the durability calculation projected on the N number decreases, resulting in premature fatigue of the pavement structure and a possible need to repair or even rebuild the road before the 10-year period required by the rule in effect.