In this day and age, we’ve come to expect a lot from our roadways. We rely on roads to carry all kinds of things from place to place, from people and goods to emergency services. When it comes down to it, roads are the real heavy lifters of society. But how do they stand up to all this weight? We know roads deteriorate over time, but how do vehicle loads themselves impact pavement, and what factors affect the extent to which pavement is damaged? In this edition of the RoadReady Newsletter, we’re going to explore roadway loading, including load distribution in a pavement, which kinds of vehicles cause the most damage, and other issues associated with pavement design.
When a vehicle drives over a pavement, it induces bending stresses in the pavement layers below. Induced stresses vary with the relative location of the load to a given point in the pavement. Imagine using your hands to bend a pencil. The wood on one side of the pencil will be pushed together, while the wood on the other side will be pulled apart. Likewise, directly underneath a vehicle load, top layers of the pavement are compressed while those beneath it experience tensile stress. Away from the location of impact, this is reversed. with tension in the upper layers and compression in those below it. Damage occurs through repetitive cycles of loading and unloading. Cracking is generally a result of tensile stresses, as the pavement is pulled apart leaving voids. In flexible pavements, the asphalt concrete and base layers provides the resistance to both compressive and tensile loads. In rigid pavements, tensile load is carried by both the concrete and any reinforcing steel present in the pavement.
Trucks and buses are the largest contributors of roadway damage
Personal automobiles make up the vast majority of mileage traveled on roadways. Despite this, these vehicles contribute minimally to pavement deterioration. Heavier loads tend to damage a pavement much more severely than smaller ones. As a general rule, the relative damage that a load causes to a pavement structure is calculated by taking the ratio of load magnitude to the fourth power. In other words, your local highway would much rather do a lot of low-weight reps than max-out on the bench press. As a result, the damage caused by a modestly loaded freight truck is equivalent to that caused by thousands of passenger cars. Because of this, trucks and buses represent the real heavy lifting for our roadways. In fact, loads from motorcycles, passenger cars, pickup trucks, and vans are seen as negligible and are not typically factored into pavement designs.
Equivalent Single Axle Load (ESAL)
As discussed in the last section, different types of vehicles contribute differently to pavement damage. Because of this, pavement engineers use a metric that combines the contributions of these different sources into a single number. This number is then used as the basis for the structural design of the pavement. The equivalent single axle load, or ESAL, is such a metric. One ESAL is defined as a single axle with a downward force of 18,000 lbs. Other values of ESALs for different loads indicate the relative damage they would cause compared to this base load. For instance, an axle with an ESAL value of five is about five more damaging to a roadway structure. The ESAL value relative to one is also known as a load equivalency factor, or LEF. The calculation of load equivalency factor for a given load depends on the type of pavement (flexible or rigid) as well as the strength of the pavement structure. Designers will use traffic data and growth patterns to estimate the number of ESALs that a roadway will be subjected to in its lifetime. This number is then used to determine how strong the roadway structure needs to be.
The ESAL system was developed in the early 1960’s and is still widely used. However, the 2002 Guide for the Design of New and Rehabilitated Pavement Structures, developed by the National Cooperative Highway Research Program, recommends a new approach known as load spectra. Load spectra rely on the same data inputs as ESALs, but rather than aggregating the data into a single number, individual counts of weight and axle distribution are maintained. For instance, load estimation based on ESALs will produce a single number (typically in the millions) to be used for pavement design. In contrast, a load spectra approach separates vehicles by different types of loads, and totals frequencies for each category.
An example of data input for a load spectra approach
Axles and Tires
In addition to the total weight of a vehicle, the number of axles and tires has an important effect on how a load is distributed to a pavement, and therefore how the pavement is damaged. Generally, the more contact points that the load makes, the smaller each individual load, and the smaller the cumulative damage to the pavement. As a result, tables for ESALs are generally given for both single axles and tandem axles. Tandem axles are pairs of axles which are spaced close together. For the same axle load, these greatly reduce damage to a roadway. For instance, for an 18,000 pound load, a tandem axle contributes about one tenth the ESALs that a single axle does. In addition to single and tandem axles, axles can also be classified as dual tire or single tire, corresponding to the number of tires on each side of the vehicle. The distinction between dual and single tire is not as critical as single and tandem axles, but use of dual tire axles translates to a 10-20% reduction of pavement stress for a given single axle load.
Tire characteristics also affect the severity of roadway damage for a given load. For instance, wider tires will distribute loads over a larger section of a pavement. Tire pressure also impacts load distribution. Highly inflated tires will concentrate loads on the pavement surface, while tires under low pressure will tend to spread out the loads more.
Diagram of a typical axle and tire arrangement
In addition to determining the total amount of vehicle loads a road will experience over its lifetime, pavement engineers must also estimate how these loads will be distributed among the individual lanes of a roadway. If you work out your arms all day, they’re going to get tired before the rest of your body. The same goes for roads, and designers must take this into account. For multi-lane roads, a “design lane” is typically selected which carries the critical (maximum) traffic loads. These loads are used to design the entire roadway. Loads can vary between lanes in a number of different ways. For instance, opposite direction of travel will not necessarily be expected to carry the same amount of traffic. In addition, for multi-lane highways, large trucks are more likely to travel in the slower lanes. As a result, the outside lanes of a highway are generally subject to higher loads than those closer to the median.
Carrying the Load
Accurate characterization of roadway loads is critical to intelligent roadway design. On one hand, underestimating loads will lead to premature roadway failure, and increased maintenance and rehabilitation costs. Overestimating loads, on the other hand, will lead to over-designed roadways, which means increased and unnecessary upfront costs. To get the most out of roadway spending and meet the needs of the traveling public, design and construction activities must be guided by intelligent load estimation.