EFFECT OF HEAVY TRUCKS WITH LARGE AXLE GROUPS ON ASPHALT PAVEMENT DAMAGE

Fatigue damage caused by multiple axles, when normalized by the load they carry, decreases with increasing number of axles per axle group. Therefore, multiple axles are more economically efficient from the point of view of damage caused by the amount of goods transported.

Rutting damage caused by multiple axles increases with increasing number of axles per axle group. When normalized to the load each axle carry, the results were inconclusive.

Several factors such as traffic, environment, material and design considerations affect the pavement damage over time. Traffic loads play a key role in pavement deterioration. Trucks are the major consumers of the pavement network as they apply the heaviest loads to the pavement surface. Truck loads are transferred to the pavements through various combinations of axle configurations depending on the truck type. The current AASHTO pavement design guide converts different axle load configurations to a standard axle load (18 kips) using Load Equivalency Factors (LEFs). These LEFs are based on loss of Pavement Serviceability Index (PSI), and were developed for a limited number of pavement and axle types, load magnitudes, load applications, age and environment. The PSI is widely based on the functional performance of the road Surface (rideability), and accounts to a low degree for other key performance measures such as fatigue and rutting for flexible, and faulting for rigid pavements. Also, increased demands due to economic growth have led to changes in the designs of heavy vehicles and in their weights. Therefore, there is a need to examine damage caused by newer axle and truck configurations using laboratory as well as field data from in-service pavements.

EFFECTS OF TRUCK SIZE AND WEIGHTS ON HIGHWAY INFRASTRUCTURE AND OPERATIONS A SYNTHESIS REPORT

Finding 3. The pavement damage from vehicle traffic depends mainly on the number of axle passes over the pavement and axle weights. The consensus in engineering literature is that pavement damage is a function of the number of axle passes over the pavement and axle weights. As Crockford (1993) put it:

The fundamental cause of pavement failure is the application of a tire contact pressure that exceeds the load carrying capacity of the pavement. The tire contact pressure (or the next best indicator, axle load) is important to the minimization of damage. To the trucking industry, this means that the gross vehicle weight is almost unlimited by the pavement structure (within reason of course)… The reason gross vehicle weight is almost unlimited by pavement structure is that tire contact pressure can be reduced by increasing the number of axles, the number of tires, or by using low inflation pressure tires.

An increase in the weight of a given vehicle will, of course, exacerbate the stresses on pavement by adding to axle weights. If a switch to a vehicle with additional axles accompanies an increase in gross weight, however, the pavement can be neutral or even benign.

Finding 4. An increase in axle weight generally causes a more than proportional increase in pavement damage. The relationship appears to approximate an exponential function, and various studies have assumed the power of the exponent to be about 4 as a rule. Estimates of the exponent’s power vary substantially, however.

Finding 5. The effects of axle spacing on pavement damage are complex and generalizations elusive.

Finding 6. An increase in truck speed tends to have mixed effects on pavements. For a truck moving over a smooth pavement, the load transmitted to the pavement would be static. An

increase in the vehicle’s speed would not affect the intensity of the stress on the pavement, but would reduce its duration and, hence, the amount of pavement damage.

The pavements of actual roads are somewhat uneven, however, which causes vehicles traversing them to move up and down. These movements cause the load transmitted to the pavement to vary, increasing as the vehicle moves down and makes greater contact with the pavement, and decreasing as the vehicle is lifted up. Because pavement damage tends to increase more than proportionally with vehicle load (finding 4), these dynamic fluctuations add to pavement damage.

Finding 7. The pavement cost per mile traveled by a heavy vehicle varies greatly between pavements, being greater on pavements designed for light duty than on sturdier pavements.

Finding 8. Increases in TS&W limits that lead to higher axle weights can have quite large pavement costs.

Finding 9. Increases to TS&W limits that encourage the use of trucks with more axles do not necessarily lead to higher pavement costs; they can even produce savings in pavement costs.

EVALUATION OF TRUCK IMPACTS ON PAVEMENT MAINTENANCE COSTS

“This study thus establishes that one heavy truck is approximately equivalent to 90 light trucks or passenger cars in terms of its impact on pavement maintenance cost”

“The most important finding is that the coefficient of heavy truck annual average daily traffic is positive and highly significant (heavy trucks are defined in this study as those with 5 or more axles). In fact this variable has the largest t-statistic, implying that it is the single most important variable that influences pavement maintenance costs. The estimated coefficients also indicate light truck and passenger car traffic does not significantly contribute to pavement maintenance costs.”

“The most important finding is that heavy truck traffic has much larger impact on pavement maintenance cost than does light truck or passenger car traffic” (…) “In addition, the model indicates that the effect of weather on pavement maintenance costs is relatively small, with maintenance cost decreasing with the average annual temperature. The model also indicates, other things being equal, fewer funds are spent per mile on pavement maintenance in mountain areas.” (…) “As expected, maintenance cost increases with the age of the pavement. The study found, however, that this increase is small, presumably because routine pavement maintenance is performed at a certain rate regardless of the gage of pavements.”

MAJOR RECOMMENDATIONS

“Extending the conclusions one can arrive at three major recommendations. First, the State of California should review its highway taxation policies. Second, effort should be directed to refine and improve this analysis and results contained herein. Finally, a national study should be taken to evaluate the applicability of this approach for other states.”

GUIDELINES ON MAXIMUN WEIGHTS AND DIMENSIONS OF MECHANICALLY PROPELLED VEHICLES AND TRAILERS, INCLUDING MANOEUVRABILITY CRITERIA

HEAVY VEHICLES VS URBAN PAVEMENTS

“The damage to a pavement structure is directly related to the magnitude and frequency of the load applied. Pavement performance (and design) is governed by environmental conditions as well as truck, buses and other heavy vehicles to the exclusion of light, passenger vehicles. The heavier a vehicle utilizing a pavement, the more extensive the damage induced. In pavement design, all axle loads (ESAL’s) representing the standard 18,000 pound single axle design load to simplify analysis. Result of the AASHO Road Test concluded that the ratio of damage induced by an axle load is proportional of that axle load to a standard 10,000 pound single axle load raised to the fourth power.” (…) “For many of the streets evaluated which had already been in place for 30 or more years, the reduction in pavement life induced by the overweight buses is negligible because they are close to or past their design life”.

“Bus system routing should recognize that some of the thinner asphalt concrete streets in the urban area are less capable of sustaining the heavy loading induced by overweight vehicles. Routes should be developed to avoid these streets if at all possible. If the street cannot be avoided, the routes utilizing the street should be using lighter vehicles in the bus fleet, or consideration should be made for rebuilding the street to sustain the increased loadings.”

“Future street designs should accommodate the overweight vehicles such as buses, fire trucks and waste vehicles which utilize them. During the course or the study it was found that the loads induced by the City of Seattle fire trucks are the greatest of any vehicle considered (although because these loads are generally infrequent, their contribution to pavement deterioration is usually not significant). Pavements can be designed to accommodate for heavier loads of the bus fleet and other overweight vehicles. The increased initial cost of the slightly thicker pavement sections which might be required would be a more effective means of dealing with overweight vehicles rather than frequent, disruptive rehabilitation.”

“As new vehicles are added to the bus and other fleets, every effort should be made to ensure they will meet legal axle loads. While future pavements can be designed to meet heavier axle loads, many local streets are composed of relatively thin asphalt concrete pavements to heavier loads, their service life is shortened, thereby requiring expenditure of rehabilitation funds which are in short supply.”

HOW VEHICLE LOADS AFFECT PAVEMENT PERFORMANCE (Me parece que todo el artículo es importante, conviene leerlo)

“Three elements work to cause road deterioration: traffic loads, the environment, and aging. While we have little or no control over the environment and aging, we can control traffic loads.”

“Pavement damage increases rapidly with higher axle loads, and actually increases faster than the loads increase. One nine-ton axle load, for example, causes about ten times more damage than a five-ton axle load.”

“Tandem axles can carry much greater pay loads with little increase in pavement damage.” (…) “Tridem (three) axles are even better for reducing road damage.” (…) “Since different truck configurations can carry greater loads without necessarily causing more road damage, it

makes sense to post several load limits on roads which regularly carry different truck configurations.”

“Changing from dual tires to single tires, on a single axle load truck, will increase the pavement stress by 10-% because the load is now concentrated in one spot rather than in two. The effect of dual wheels depends in part on pavement thickness. As depth increases, the stress caused by dual wheel loads becomes equivalent to single wheel loads.” (…) “Tire pressure is also important. Tire pressure increases with truck weight and the higher the pressure the greater the stress on the pavement. Furthermore, the effects of tire pressure are more pronounced in upper pavement layers.”

“… the strength of the aggregate base becomes very important in the road’s ability to support loads. A good base increases the overall strength of the road by distributing load effects to the soil beneath it and provides drainage to help protect against frost heave (see figure 4). Any weakening of the underlying soil by moisture of freeze-thaw action will greatly diminish the road’s strength. Because underlying soils, field conditions, and pavement materials vary, a deflection (strength) test can be very helpful in evaluating a pavement for future maintenance and improvement.”

“The major component of fatigue is deflection or bending. Thicker pavements suffer less stress and deflection, and therefore, will last longer under heavy loads. Weakened roads are commonly strengthened by adding thickness through overlays or complete rebuilding.”

THE INFLUENCE OF OVERLOADING TRUCK TO THE ROAD CONDITION

The result of analysis of sensitivity show that 150% overloading of single, dual, and triple axle truck, will bring about 500, 135, and 122% level of damage respectively.

Overloading is very often assumed as the factor that affects the level of pavement structure damage. Even though that assumption is not wrong but the other factors need to be thoroughly and proportionally studied before take the conclusion. Generally factors that influence the road damage can be described as follows:1. Traffic load (overloading of the heavy vehicle or truck and frequency of the traffic).2. Stress on the surface layer of the pavement (cause by higher tyre pressure which

account for the higher stress on surface layer)3. Characteristic of pavement materials (their quality that expressed as number of

relative strength)4. Thickness layers factor

5. Subgrade or roadbed soil (the bearing capacity of soil)6. Regional or environment factor7. Failure criterion

Traffic load is dominant function because the function of the pavement is to directly resist the traffic load. The mention of traffic load covers traffic volume or frequency of the traffic, and weight of vehicle as well as intensity of the vehicle. Traffic volume accumulatively shows the number of repetition of the load and function the time (service life). Intensity of the weight of vehicle is depending on weight of axle load, axle configuration, and wheel configuration.

Tyre pressure is remarkably influence the stress to the surface layer of pavement under tyre contact area. The higher of tyre pressure, the higher the stress on surface layer of the pavement.

Characteristic of pavement layers materials (strength, stiffness, elasticity) extremely influential to the performance of pavement layers to response the traffic load. The higher quality of the materials, the higher the ability to response the load will be.

Improperly of pavement structure construction often cause the layer thickness are not fulfill the thickness as required in the design specification. The thickness of one layer unquestionably will influence performance of entirely pavement structure. The thicker of the pavement layer over the design requirement, the higher the performance of the pavement to response traffic loading will be.

Change of the properties of road-bed soil is extremely affecting the performance of pavement structure. The smaller of ability or the smaller of bearing capacity of existing road-bed soil, the weaker ability of pavement to response traffic loading, and on the contrary.

Condition of road-bed soil is extremely influence by environment condition that specifically is influenced by change in water content. Such as the graph in relation between water content and density, the highest or the lowest of water content from the optimum condition, the smaller of the bearing capacity. This condition is not only occurring in road-bed soil but also in base and sub-base layer.

Parameter of failure criterion use in this paper is permanent deformation or rutting. Rutting is the signal of pavement failure as a result of excessive of the fatigue strain on asphalt surface pavement or because of excessive of vertical compressive stress on the top of base, sub-base, and of roadbed soil layer.

Besides using analysis of sensitivity, the influence of overloading truck to the road condition also can be shown by using vehicle damage factor (VDF). VDF is the ratio between capacities to damage by axle load of vehicle to the standard axle load. This ratio is not linear but exponential ratio (Source: DGH) and expressed as follows:

The detrimental effect of single axle trucks is high compare to dual or triple axle trucks. Relative strength of pavement structure cause by overloading of single axle truck

decrease much more than cause by overloading of dual or triple axle truck. The higher the overloading, the higher decreasing of relative strength of pavement

structure.To prevent early damage of pavement structure, some efforts are needed to be conducted:

1. Anticipation occurring of early damage on pavement structure caused by overloading truck, especially single axle truck, is needed.2. Strict on quality control during construction period to insure that all specifications are met the requirement is required and is very important.3. Strict control on overloading truck by controlling limited truck load is required.4. Regulation to call for using multi-axle truck instead of single-axle truck is needed to be considered.

USING REGIONAL FREIGHT TRAFFIC ASSIGNMENT MODELING TO QUANTIFY THE VARIABILITY OF PAVEMENT DAMAGE FOR HIGHWAY COST ALLOCATION AND

REVENUE ANALYSIS

It was confirmed that pavement segments that are subjected to higher traffic loading levels or located in a region which has higher climatic severity (higher precipitation and/or freeze index) deteriorate faster and thus incur higher M&R expenditures albeit in magnitudes that vary significantly across the pavement segments.

The parameter estimate for total traffic (total ESALs over treatment service life) was found to be normally distributed with a statistically significant mean and standard deviation (random parameter), indicating that the influence of the traffic on the response variable is significantly different for the different pavement segments. This is an

important result because it indicates that it is appropriate to charge highway users different fees not only across the different highway functional classes but also across the different road segments in a given functional class.

The results also suggest that the life-cycle damage cost (life cycle cost of pavement upkeep associated with the traffic loads), on average, is approximately $85,000/lane-mile, $70,000/lane-mile and $55,000 per lane-mile, for the vehicles that use the pavements on Interstates, non-Interstate NHS, and non-NHS, respectively.

It is necessary to address the lingering question on how best to achieve reliable future traffic volumes. This can be realized using regional freight assignment models, particularly those of a dynamic nature. Appropriate and accurate assignment of future freight traffic on the highway network system on the basis of projected socio-economic developments, could yield more reliable estimates of truck traffic volumes at each individual link on the highway system. That way, it will be possible to report the total damage costs not for families of pavements but for individual pavement segments within a family highway agencies can establish appropriate segment-specific costs of pavement damage and thus establish a foundation upon which existing fees for overweight vehicles could be reviewed.PAVEMENT LESSONS LEARNED FROM THE AASHO ROAD TEST AND

PERFORMANCE OF THE INTERSTATE HIGHWAY SYSTEM

MAJOR TECHNICAL FINDINGS OF THE AASHO ROAD TESTSurface ThicknessThe AASHO Road Test gave quantitative value to the importance of pavement surface thickness in increasing the number of load repetitions that can be carried to pavement failure. It tied pavement surface thickness to pavement performance, where “performance” is defined as the service provided by the pavement or the number of load repetitions that can be carried to an unserviceable level.

Load EquivalencyPavement engineers had long had trouble dealing with various axle loads in pavement design. Some methods used only the heaviest load (CBR), and others including the Texas Design Method used the average of the 10 heaviest loads that were expected to be carried on the pavement. The AASHO Road Test provided quantitative information about the relative damaging effect of heavy loads, and immediately after the Road Test, Paul Irick and Frank Scrivner used the Road Test equations to generate load equivalencies called ESALs. Francis Hveem of the California DOT had earlier hypothesized a load equivalency concept tied to 10-kip axles. The Road Test equivalencies validated and extended the Hveem hypothesis statistically. The load equivalency concept (ESAL) is by far the most widely used pavement concept in the world. We as authors have collectively visited more than 50 countries and all 50 states in the United States. All of these agencies use the ESAL concept in pavement design.

PSI: Performance ConceptBefore the AASHO Road Test there was no good definition of pavement failure. This seems hard to believe but please check the literature; you will find it to be true. After the WASHO Road

Test, Paul Irick and Bill Carey developed the Present Serviceability Index (PSI) concept and defined “performance” as “accumulated traffic to a fixed level of PSI.” The selected level of PSI was “failure.” While many agencies adopted this concept, some have continued to refer to “roughness.” Therefore, a defined level of roughness is sometimes accepted as failure in the form of an International Roughness Index (IRI) level. The technical literature shows that IRI and PSI are inversely related to each other.The present serviceability concept (PSI) relates pavement failure directly to riding quality and the acceptance or satisfaction of the riding public. It is indeed more definitive of true performance than roughness alone and strong consideration should be given to resurrecting it in pavement studies and designs.

Layer Equivalencies: Material PropertiesThe AASHO Road Test included four types of base under asphalt pavements: (a) river gravel, (b) cement stabilized, (c) asphalt stabilized, and (d) crushed stone. These were compared to define the levels of performance that resulted from improving the quality of the base layer. Francis Hveem had also hypothesized such relative benefit of stronger layers as part of a “gravel equivalency concept” and he was instrumental in getting the wedge-shaped base sections added to the Road Test to validate that concept. The structural number concept, developed based on layer equivalencies, is widely used around the world and is the basis for layer selection in all AASHTO Pavement Design Guides up to 2002.The Road Test of course was not perfect because it was impossible to make it large enough to solve all possible factors. We don’t know if these layer equivalencies would be the same with different subgrades and in a different environment. These questions have been the subject of considerable research in the past 50 years.

Value of Subbase to Reduce Pumping in Rigid PavementsAt the Road Test those PCC pavement sections that had a gravel subbase under the slab performed much better than those that were placed directly on the clay subgrade. This occurred regardless of the thickness of the gravel subbase layer. However, there were no stabilized subbases used on the rigid pavements and we can only hypothesize what improvement would have resulted.

Pumping of Subbase and Subgrade MaterialsBefore the Road Test the PCC paving industry had strongly hypothesized that the problem of pumping of subgrade material from beneath pavements could be solved by placing a granular subbase beneath the slab. This was proved to be incorrect at the Road Test, where under heavy loads and high rainfall, even the gravel subbase layer pumped and caused early slab failure.

Effectiveness of Dowels for Load TransferBefore 1960 most people were of the opinion that it was necessary to put some form of positive load transfer across joints and cracks in PCC pavements. Yet the concrete industry continued to claim that thicker pavements would solve the problem. The Road Test used load transfer dowels in all pavement sections. There was no faulting at the AASHO Road Test at cracks or joints, thus validating the effectiveness of dowels for load transfer under extremely heavy loads up to 30,000 pounds on a single axle.

Joint SpacingTwo joint spacings were used at the AASHO Road Test: 15-ft joint spacing with no reinforcement steel and 40-ft joint spacing with mild reinforcement. Both of these joint spacings performed well under heavy loads up to 30-kip single axle and both contained dowels across the joints. The 40-ft slabs cracked at approximately 12- to 15-ft spacing, and no faulting occurred at those cracks during the test. However, 15 years later, field studies of some of these same sections left in service on IH 80 did show faulting as the mild reinforcement steel rusted and lost its effectiveness.