Axle Configurations Research Articles

Evaluation of New Mechanistic–Empirical Pavement Design Guide Rutting Models for Multiple-Axle Loads

The new FHWA Mechanistic–Empirical Pavement Design Guide (M-E PDG) does away with the AASHO-derived concept of the equivalent single-axle load and calculates damage caused by various axle configurations directly. Because multiple axles represent about half the axle configurations that the pavement will experience, there is a need to evaluate the M-E PDG procedure for predicting rutting due to multiple-axle configurations. Axle factors (AFs) based on rutting were calculated for different axle configurations by using three procedures: the M-E PDG, accounting for the effect of each individual axle within a group, and integrating the entire strain pulse. The AFs from these procedures were compared with laboratory-derived values for asphalt concrete. Also, layer rutting contributions were predicted for six in-service SPS-1 experiment sections from the Long-Term Pavement Performance Program with the M-E PDG software and were compared with the analysis of their transverse surface profiles. The results show that the M-E PDG procedure underestimates rutting prediction due to multiple axles. Calibration of the M-E PDG rut models with field data seems to improve their prediction, although it is still lower than expected for multiple axles. The best method for calculating rut depths under multiple axles appears to be integration of the entire strain pulse. This method shows that rutting damage is proportional to the number of axles within an axle group. This theory was confirmed in the laboratory for asphalt concrete. Also, although the M-E PDG rut models are superior to previous rut models, in that they are able to dissect the total surface rutting between all pavement layers, their prediction of the individual layer rutting contributions does not always agree with results from the analysis of measured transverse profiles.

Modeling the Response of Paved Low-Volume Roads under Various Traffic and Seasonal Conditions

Since low-volume road (LVR) systems were first designed and built, events such as deregulation and free-trade agreements and changes in economic growth patterns and in manufacturing and farming practices have resulted in increased truck movements and have accelerated the deterioration of paved LVRs, such as the ones in North Dakota, which were not designed for such heavy-truck traffic. The traditional empirical methods—use of the soil factor and R-value designs and the use of visual inspection and ride quality—to assess pavement performance are not adequate. A mechanistic model that uses material data and is capable of predicting pavement response under different traffic loading and seasonal conditions is deemed necessary. An analytical approach to determine and compare pavement response for different truck axle loadings under normal, thaw, and hot summer seasonal conditions on paved LVRs is discussed. A three-dimensional finite element program is used to model the pavement response in the form of total permanent deformation (TPD) and to calculate axle and truck damage factors. Various axle loadings representing U.S. axle weight limits on steering axles with single tires, single axles with dual tires, tandem axles, and tridem axles are used in this study. Seasonal variations are modeled through material strength parameters. The analysis of the TPD responses provides greater insight and understanding of how paved LVRs react to different axle configurations, levels of loading, and seasonal variations. This analysis ultimately may help to improve paved LVR design and planning practices and enhance seasonal load limit criteria.

Effect of Heavy Multiple Axle Trucks on Flexible Pavement Damage Using In-Service Pavement Performance Data

Truck axle configurations and weights have changed significantly since the AASHO road study was conducted in the late 1950s and early 1960s. Emerging concerns about the effects of new axle configurations on pavement damage, which is unaccounted for in the AASHTO procedure, have prompted several researchers to investigate the impacts of different axle and truck configurations on pavement performance. However, there is still a need to strengthen the mechanistic findings using field data. In this paper, actual in-service traffic and pavement performance data for flexible pavements in the state of Michigan are considered. Monitored truck traffic data for different truck configurations are used to identify their relative damaging effects on flexible pavements in terms of cracking, rutting, and roughness. The analysis included simple, multiple, and stepwise regression. The results indicated that trucks with multiple axles (tridem or more) appear to produce more rutting damage than those with only single and tandem axles. On the other hand, trucks with single and tandem axles tend to cause more cracking. Pavement roughness results did not show enough evidence to draw a firm conclusion.

The use of continuous high-frequency strain gauge measurements for the assessment of the role of ballast in stress reduction on steel railway bridge decks

Although orthotropic plated decks are most frequently used for road bridges, some examples of the use of this type of decking for railway bridges exist. In these cases, the deck will be subject to higher loads as compared to road bridges, yet a number of mechanisms will lead to a better structural performance and use of the structural capacity of the deck, especially concerning fatigue. This latter phenomenon limits the design possibilities quite largely for road bridges, certainly for the deckplate. For railway bridges, a more favourable situation is created which is mostly overlooked. This situation is created by the threefold action of the stone ballast layer. The effectiveness of this action can most efficiently be tested by continuous strain gauge measurements on an in-service bridge. This procedure does not disrupt the railway traffic, and allows the effects to be measured for various axle configurations at different speeds. However, it requires the use of continuous and high-speed measurements as the measured strains change very fast. In addition, the measurements have to be taken very close to high-voltage electrical fields which disturb the small electrical signals in the sensor wiring. This article describes in detail the test setup and the results from such tests carried out on two railway bridges in Belgium. In addition, the experimental results are linked to finite element calculations. This procedure permits not only a confirmation of the validity of the measured values and the calculation model, but also allows the calculations to be fine tuned.

Probabilistic Neural Networks Application for Vehicle Classification

Federal, state, and local agencies use vehicle classification data for planning, design, and conducting safety and operational evaluation of highway facilities. In conformity with federal reporting requirements, most states use the scheme which classifies vehicles based on their axle configurations; primarily the number of axles and the length of axle spacings. However, the scheme is prone to errors resulting from imprecise demarcation of class thresholds. To improve classification, the problem is hereby viewed as a pattern recognition problem in which statistical techniques such as probabilistic neural networks (PNN) can be used to assign vehicles to their correct classes. In this research, the network was trained and applied to field data composed of individual vehicle's axle spacing and number of axles per vehicle. The PNN reduced the error rate by 3.3% compared to an existing classification algorithm. The error rate was further reduced by 6.5% when the individual vehicle's gross weight was added as a classification variable. These results confirm the promise of neural networks in axle classification but the technique still requires additional field validation as well as exploration of additional variables to improve categorization of vehicles into the F or other schemes.

Recommendation of a new truck model for bridge evaluation

The Federal Bridge Formula calculates the maximum allowable load that can legally be imposed on a bridge by any group of two or more consecutive axles on a vehicle or combination of vehicles. In the United States, trucks are typically allowed unrestricted operation and are generally considered ‘legal’ provided they meet weight guidelines of Federal Bridge Formula B. Specialized Hauling Vehicles (SHVs) with short wheelbases have had difficulty complying with the Federal Bridge Formula B. The trucking industry has in recent years introduced SHVs with closely spaced multiple axles that make it possible for these short wheelbase trucks to carry the maximum load of up to 355.8 kN (80 000 lb) and still meet the Bridge Formula. The current AASHTO (Association of State Highway and Transportation Officials) legal loads do not represent these newer axle configurations and it is therefore considered likely that these specialized vehicles may be severely overstressing some non-posted bridges. A three-year NCHRP proje...

Quantification of Pavement Damage Caused by Dual and Wide-Base Tires

A study conducted in 2001 on the heavily instrumented Virginia Smart Road measured pavement responses to a new generation of single wide-base tire (445/50R22.5) and to dual tires (275/80R22.5). The new single wide-base tire has a wider tread and a greater load-carrying capacity than conventional wide-base tires. The potential fatigue damage resulting from different tire configurations was evaluated. After successful field testing, a finite element (FE) parametric study was conducted to investigate different failure mechanisms that were not evaluated in the field. In this study, dual tires and two new generations of wide-base tires (445/50R22.5 and 455/55R22.5) were evaluated. The main difference between the two generations of wide-base tires is that the 455/55R22.5 is wider than the 445/50R22.5; hence, it further reduces the contact stress at the pavement surface under the same nominal tire pressure. In the developed FE models, geometry and dimensions were selected to simulate accurately the axle configurations typically used in North America; actual tire tread sizes and applicable contact pressure for each tread were considered; laboratory-measured pavement material properties were incorporated; and models were calibrated and properly validated against stress and strain measurements obtained from the experimental program. Four failure mechanisms were considered: fatigue cracking, primary rutting, secondary rutting, and top-down cracking. Results indicated that the new generations of wide-base tire would cause the same or relatively greater pavement damage than conventional dual tires. Because overall truck weight is reduced by approximately 450 kg when wide-base tires are used, it is reasonable to implement the load limits currently applied to the dual-tire assembly on the 455/55R22.5 wide-base tire.

Quantification of Pavement Damage Caused by Dual and Wide-Base Tires

A study conducted in 2001 on the heavily instrumented Virginia Smart Road measured pavement responses to a new generation of single wide-base tire (445/50R22.5) and to dual tires (275/80R22.5). The new single wide-base tire has a wider tread and a greater load-carrying capacity than conventional wide-base tires. The potential fatigue damage resulting from different tire configurations was evaluated. After successful field testing, a finite element (FE) parametric study was conducted to investigate different failure mechanisms that were not evaluated in the field. In this study, dual tires and two new generations of wide-base tires (445/50R22.5 and 455/55R22.5) were evaluated. The main difference between the two generations of wide-base tires is that the 455/55R22.5 is wider than the 445/50R22.5; hence, it further reduces the contact stress at the pavement surface under the same nominal tire pressure. In the developed FE models, geometry and dimensions were selected to simulate accurately the axle configurations typically used in North America; actual tire tread sizes and applicable contact pressure for each tread were considered; laboratory-measured pavement material properties were incorporated; and models were calibrated and properly validated against stress and strain measurements obtained from the experimental program. Four failure mechanisms were considered: fatigue cracking, primary rutting, secondary rutting, and top-down cracking. Results indicated that the new generations of wide-base tire would cause the same or relatively greater pavement damage than conventional dual tires. Because overall truck weight is reduced by approximately 450 kg when wide-base tires are used, it is reasonable to implement the load limits currently applied to the dual-tire assembly on the 455/55R22.5 wide-base tire.

Effect of Different Axle Configurations on Fatigue Life of Asphalt Concrete Mixture

The fatigue life of an asphalt mixture under different truck axle configurations was determined directly from the indirect tensile cyclic load test by using load pulses that were equivalent to the passage of an entire axle group or truck. The dissipated energy approach was adopted in analyzing the results and determining the number of repetitions to failure for each case; a unique fatigue curve that can be used for multiaxle configurations was developed. Trucks consisting of up to 11 axles and axle groups of up to 8 axles were studied. The results indicated that the normalized damage per load carried decreased with an increasing number of axles within an axle group. Additionally, the fatigue lives predicted by using single load pulses were compared with the measured ones from the different axle groups and trucks.

Incremental Nonlinear Model for Predicting Pavement Serviceability

A recursive nonlinear model was developed for the prediction of pavement performance as a function of traffic characteristics, pavement structural properties, and environmental conditions. The model highlights some of the advantages of relaxing the linear restriction that is usually placed on the specification form of pavement performance models. First, a functional form that better represents the physical deterioration process can be used. Second, the estimated parameters are unbiased, owing to a proper specification and the use of sound statistical techniques. Finally, the standard error of the prediction is reduced by half that of the equivalent existing linear model. This improved accuracy has important economic implications in the context of pavement management. The model developed as part of this research enables the determination of an unbiased exponent of the so-called power law and of the equivalent loads for different axle configurations. The estimated exponent confirms the value of 4.2 traditionally used. However, it should be noted that this exponent is only to be used for determining damage in terms of serviceability. On the other hand, equivalent loads estimated for different axle configurations tend to differ from traditionally used values.

Model Validation for Bridge-Road-Vehicle Dynamic Interaction System

The dynamic response of highway bridges subjected to moving truckloads has been observed to be dependent on (1) dynamic characteristics of the bridge; (2) truck configuration, speed, and lane position on the bridge; and (3) road surface roughness profile of the bridge and its approach. Historically, truckloads were measured to determine the load spectra for girder bridges. However, truckload measurements are either made for a short period of time [for example, weigh-in-motion (WIM) data] or are statistically biased (for example, weigh stations) and cost prohibitive. The objective of this paper is to present results of a three-dimensional computer-based model for the simulation of multiple trucks on girder bridges. The model is based on the grillage approach and is applied to four steel girder bridges tested under normal truck traffic. Actual truckload data collected using a discrete bridge WIM system are used in the model. The data include axle loads, truck gross weight, axle configuration, and statistical data on multiple presence (side by side or following). The results are presented as a function of the static and dynamic stresses in each girder and compared with code provisions for dynamic load factor. The study provides an alternate method for the development of live-load models for bridge design and evaluation.

Analysis Procedure for Load-Zoning Pavements

Most load-zoned roads in Texas are still posted with a gross vehicle weight limit of about 260 kN, corresponding to the legal load limit at the time these roads were designed and built. Since the load from a vehicle is transmitted to the pavement through its axles, establishing load limits based on axle load and axle configuration is a more rational approach than the one currently used. In recognition of the need for a better methodology of load-zoning pavements, the Texas Department of Transportation funded a project to develop a procedure for evaluating load restrictions on the basis of axle load and axle configuration. Research efforts conducted at the Texas Transportation Institute led to the development of the Program for Load-Zoning Analysis (PLZA), which pavement engineers may use to evaluate the need for load restrictions and to determine, as appropriate, the single- and tandem-axle load limits based on a user-prescribed reliability level. To predict the induced pavement response under surface wheel loads, PLZA uses a layered elastic pavement model that permits users to characterize pavement materials as linear or nonlinear. The predicted horizontal strain at the bottom of the asphalt layer and the vertical strain at the top of the subgrade are used to evaluate pavement performance. To combine the effects of different axle loads and axle configurations, PLZA uses Miner’s hypothesis of cumulative damage to predict service life and the probability of pavement failure within a prescribed analysis period. The methodology for load-zoning is described, and its application using data collected on in-service pavements is demonstrated.

Influence of the axle load, tyre size and configuration on the compaction of a freshly tilled clayey soil

Enhancement of the potential root growth volume is the main objective of farmers when they establish a conventional tillage system. Therefore, the main function of primary tillage is to increase soil’s structural macroporosity. In spite of this, during secondary tillage operations on these freshly tilled soils, the traffic on seedbeds causes significant increases in soil compaction. The aim of this paper was to quantify soil compaction induced by tractor traffic on a recently tilled non consolidated soil, to match ballast and tyre size on the tractors used during secondary tillage. The work was performed in the South of the Rolling Pampa region, Argentina. Secondary tillage traffic was simulated by one pass of a conventional 2WD tractor, using four configurations of bias-ply rear tyres: 18.4×34, 23.1×30, 18.4×38 and 18.4×38 duals, two ballast conditions were used in each configuration. Soil bulk density and cone index in a 0 to 600 mm profile were measured before and after traffic. Topsoil compaction increased as did ground pressure. Subsoil compaction increased as total axle load increased and was independent from ground pressure. At heavy conditions, topsoil levels always showed higher cone index values. From 150 to 450 mm depth, the same tendency was found, but with smaller increases in the cone index parameter, 22 to 48%, averaging 35%. Finally, at the deepest layer considered, 600 mm, differential increases due to the axle load are great enough as to be considered similar to those found in the upper horizon, 36 to 64%, averaging 55%. On the other hand, bulk density tended to be less responsive than cone index to the traffic treatments. Topsoil compaction can be reduced by matching conventional bias-ply tyres with an optimized axle weight.

Estimation of Rutting Models by Combining Data from Different Sources

The accurate prediction of rutting development is an essential element for the efficient management of pavement systems. The objective of this paper is to demonstrate the effectiveness of the estimation of rutting models by combining the information from two data sources, the AASHO and the WesTrack road tests. Combined estimation with both data sources is used to identify parameters that are not identifiable from one data source alone. In addition, this estimation approach also yields more efficient parameter estimates. The results presented in this paper demonstrate that joint estimation produces more realistic parameter estimates than those obtained by using either data set alone. Furthermore, joint estimation allows us to account for the effects of pavement structure, axle load configuration, asphalt concrete mix properties, freeze-thaw cycles, and hot temperatures in a single model. Finally, it allows us to predict the relative contributions of rutting originating both in the asphalt concrete and in t...

Minimizing Distress on Flexible Pavements Using Variable Tire Pressure

The potential of variable tire pressure technology to minimize distress on flexible pavements with thin asphalt surfacing layers and peat soil subgrade was evaluated using in-situ stress-strain data. Pavement interfacial strains and corresponding subgrade stresses imposed by a three-axle truck were measured and used to estimate the fatigue life of the surfacing layer. Three levels of truck wheel loads in combination with four tire inflation pressures (350, 490, 630, and 770 kPa) were used to evaluate the potential distress by the single steering wheel and rear dual wheels in tandem. Results suggest that lateral strain due to the single steering wheel is the most critical to fatigue failure, which is influenced by the viscoelastic nature of asphalt, and therefore truck speed and axle configuration are important. Lower tire pressures increased the fatigue life of the surfacing layer with respect to the rear dual wheels and the steering wheel by up to 200 and 300%, respectively. Haulage trucks with systems for managing variable tire pressure such as the central tire inflation systems may therefore enhance the serviceability of pavements overlying peat or other soft soil foundations.

Simplified Load Distribution for Vehicles with Nonstandard Axle Gauges

Several simplified methods have been developed to determine the live-load distribution factors for overweight vehicles on slab-on-girder bridges; however, these methods were developed for vehicles with standard axles. Many vehicles exist with nonstandard axle configurations, such as two-wheel axles that are wider than 6 ft (1.83 m) and four-wheel axles with wheels that are evenly or unevenly spaced. For these vehicles, a rigorous analysis is generally desired but is often deemed uneconomical. Therefore, a simplified method should be an asset to the bridge community and the trucking industry. A simplified method for determining live-load distribution factors for vehicles with nonstandard axle configurations is presented. Distribution factor formulas for moment and shear in interior and exterior girders are given. These formulas account for the transverse axle configurations that compose a vehicle. Several two- and four-wheel axle configurations are considered. The distribution factor formulas for slab-on-girder bridges presented in the AASHTO LRFD Bridge Design Specifications are incorporated into the proposed simplified method. The simplified method formulas were developed to approximate the results from a rigorous finite strip method. Comparisons are presented as verification of the accuracy of the simplified method. The simplified method results are usually conservative and correlate reasonably well with the rigorous results. In general, simplified methods worked better for interior girders than for exterior girders, and moment was better predicted than shear.

Performance of Superpave Mixtures Under Accelerated Load Testing

The performances of two Superpave® test sections, constructed with 12.5 mm (0.5 in.) nominal maximum size Superpave mixture (SM-2A) with varying percentages of river sand at the Kansas Accelerated Testing Laboratory (K-ATL), are described. A 150-kN (34-kip) tandem axle with dual wheels and 620-kPa (90-psi) tire pressure was used for loading. After 10,000 repetitions, the sections were loaded by 160-kN (36-kip), 150-kN (34-kip), and 144-kN (32.5-kip) tandem axles and 98-kN (22-kip), 90-kN (20-kip), and 80-kN (18-kip) single axles, for estimating relative pavement damage due to different axle loads and configurations. Critical pavement responses were measured on each test section with the transverse strain gauges and soil pressure cells under both K-ATL tandem axle and falling weight deflectomter (FWD) loads. The pavement responses were also estimated from a multilayer elastic analysis program, ELSYM5. Laboratory fatigue tests were conducted on the SM-2A beams. Fatigue and rutting damage analyses were then conducted. The results indicated very little fatigue damage on these sections. The Superpave mixtures appeared to be less susceptible to fatigue damage, presumably due to better aggregate structure and higher binder content than the traditional asphalt mixtures. Rutting on the section with 15 percent river sand was mainly due to consolidation of the SM-2A and/or other layers since no significant flow of the Superpave mixture was evident. The AASHTO load equivalency factors were found to be much higher than those calculated in this study.

Issues in Superload Crossing of Three Steel Stringer Bridges in Toledo, Ohio

A 371-ton (816.7-kip) cooling unit crossed over three steel stringer bridges in Toledo, Ohio. Any bridge damage was compensated if scientifically documented. Researchers predicted the superload responses by using finite element analysis and experimental test results, and then measured the superload responses to establish whether the event caused any damage. Although the conservative results of the finite element model predicted an overbound of critical stresses, experimental results gave closer predictions of the measured response under superload. During the superload crossing, the maximum stress levels at the girders remained at about 34.5 MPa (5 ksi). The cross braces were stressed up to 68.9 MPa (10 ksi), exceeding the girder stresses. The study included questions about the real behavior versus design assumptions regarding cross braces in steel stringer bridges. (Cross braces are the weak links that fail first, causing lateral instability and failure.) Research revealed the possibility of predicting superload responses and behavior quite reliably by using a combination of diagnostic tests and finite element analysis. These tools permit the best design for superload axle configuration, crossing paths and positions on bridges, and possible options for effective bridge strengthening when needed.

Impact of High-Pressure Tires and Single-Tired Axles in Oregon

The growing concern in Oregon in the early 1990s that the use of single tires on the nonsteering axles of trucks may be growing and leading to increased damage to pavements is addressed. An associated concern was that tire pressures may be increasing and contributing to increased damage. Single tires are used in two ways on nonsteering axles. Wider-than-normal tires (wide-base) may be used instead of traditional dual tire assemblies, or singling out of dual tires may be done, where the inside tire of the duals is removed. An extensive survey of trucks was conducted at Oregon's five ports of entry in 1992 to establish the pattern of use of single tire and tire pressures. The data were compared with those available from studies conducted in Oregon in 1986 and 1989. There did not appear to be a growth in use of single tires or a trend of increasing tire pressures. However, the data collected previously for single tire use was limited. The surveys conducted in 1992 showed that single tire use varies throughout the state, depending on the commodities hauled in an area. Singling out tends to be used where relatively light articles are being hauled, whereas wide-base tires appeared to be concentrated with trucks hauling forest products. The surveys indicated that less than 10 percent of trucks used single tires on nonsteering axles. The use of single tires on nonsteering axles does not appear to be a significant problem. However, on the basis of the analysis conducted in this study, some control may be appropriate because such tires could cause more damage than similarly loaded dual tires. Partial singling out of tandem axles has the potential to be particularly damaging. Such axles have six tires instead of the usual eight. However, there are many options for using single tires and some are used with axle configurations that cause less pavement damage than alternatives using dual tires. An example of this would be a tridem axle with single tires versus a tandem axle of the same weight using dual tires. Any control of single tires should not limit their use with less damaging axles.

Field Verification of Moving Load Model for Pavement Response

A validation study undertaken to verify the predictive capability of a recently developed moving load model to predict pavement response is described. The full-scale field-measured responses of longitudinal strain at the bottom of the asphalt concrete (AC) layer were used in the verification. The field testing program, in which the strain responses induced by a semitrailer truck were measured as a function of vehicle speed, was carried out at the Pennsylvania State University test track. The material behavior of the AC layer, which was assumed to be viscoelastic, was deduced from the laboratory behavior of the AC and from the backcal culated AC modulus from falling-weight deflectometer data. The unbound material layer properties were assumed to be elastic. The moving load model reproduced many important general observations made from the field tests, such as the existence of a complex interaction in the case of a tandem axle configuration and the strong influence of vehicle speed on the strain response. G...