Rigid vs. Flexible Pavements. 1.2 Truck Characteristics Affecting Pavements; [a] Axle Weights

Navigation: Page 1, Cover Page | Page 2.. Phase 1.1, Background. | 1.2 Truck Characteristics Affecting Pavements. (a) Axle Weights | Page 4, Section 1.2 [b] Tire Characteristics | Page 5, Section 1.2 (c) Suspension Systems | Page 6, Section 1.2; (d) Axle Spacing | Page 7, Section 1.2; (e) Liftable Axles | Page 8, Section 1.2; (f) Tridem Axles | Page 9, Section 2.1 Axle Weight Limits | Page 10, Section 2.2 Bridge Formula | Page 11, Section 2.3 - 80,000 Pound GVW Cap | Page 11, Section 2.4 Policies to Encourage Tridems | Page 11, section 2.5 Weight Limits Per Unit of Tire Width | Page 12, section 2.6 Turner Trucks | Page 13, section 2.7-New Approach; TRB Truck Weight Study | Page 14, section Section 3.0; Knowledge Gaps and Research Needs | Page 15, section 4.0 References for Pavements Working Paper

Comprehensive Truck Size and Weight (TS&W) Study

Phase 1—Synthesis

Working Paper 3—Pavements and TS&W Regulations

1.2 Truck Characteristics Affecting Pavements

(a) Axle Weights

Load equivalence factors measure the relative effects of different types of loadings on pavements. Pavement engineers generally use the concept of an equivalent single-axle load (ESAL) to measure the effects of axle loads on pavement. By convention, an 18,000-pound single axle is 1.00 ESAL. The ESAL values for other axles express their effect on pavement wear relative to the 18,000-pound single axle. Stating, for example, that a given vehicle on a given type of pavement is 3.0 ESALs means that one pass by the vehicle has the same effect on the pavement as three passes by an 18,000-pound single axle.

The American Association of State Highway Officials (AASHO) Road Test conducted in the 1950s provided sets of ESAL values for single and tandem axles on various types of pavements. In 1986, the Road Test results were extended by the American Association of State Highway and Transportation Officials (AASHTO) to provide load-equivalence factors for tridem axles (AASHTO 1986). The load-equivalence factors vary sharply with weight, following roughly a fourth-power relationship. On both flexible and rigid pavements, the load-equivalence factor for a 20,000-pound single axle is about 1.5 because (20/18) 4 is approximately equal to 1.5. Thus, 100 passes across a pavement by a 20,000-pound axle would have the same effect on pavement life as 150 passes by an 18,000-pound axle.

AASHTO provides separate sets of ESAL values for flexible and rigid pavements. The principal difference between the flexible and rigid pavement ESAL values is that tandem axles were found to have a greater effect on rigid pavements (Exhibit 1). For example, a 34,000-pound tandem axle is about 1.1 ESALs on flexible pavement and about 2.0 ESALs on rigid pavements.

[ed. note / relevant statement: the significance of the above means that:

(i) concrete pavement surfaces decay + deconstruct more rapidly than do asphalt pavement surfacing,

(ii) when they are exposed to identical t-indexes, traffic patterns, sizes and frequencies.

In other words:

"When all other factors are the same,

asphalt will last approximately 82% longer than will concrete pavement surfacing."

this study does not address: WHY the above is true, only that it is true ...

relatively more flexible pavement surfaces last longer than do rigid pavements.

definition of terms: in this UsDOT/FHWA (TS&W) study, the term 'rigid pavements' refers to concrete vehicular traffic driving surfaces, the term 'flexible pavements' refers to asphalt driving surfaces.

please note also: that when this study refers to 'the pavement' it is referring to the smoothe driving surface applied as the finish surfacing - on top of the pavement; i.e. the thin layer of asphalt or concrete.

more correctly: in pavement engineering and/or geotechnical fields, typically the term 'the pavement' more accurately is defined as: the entire engineered structure; which chiefly consists of the roadbed foundation, +various layers of drain rock, compacted composite aggregates, leveling or 'choker' layers, and so forth beneath the finish surfacing.

The asphalt or concrete surfacing, although serving the purpose of diverting rainfall into sheetwater runoff by preventing precipitation infiltrating into the roadbed, should be more correctly understood as 'the surface finishing' than as the pavement itself.

analogy - 1 | for the purpose of illustration: This is similar to the term 'the building' refering to much more than simply the coat of paint applied to the exterior of a newly constructed built environment.

relevance to actual/real pavement costs: Please note that it does not cost upwards of $10million/mile and more, simply to lay a coat of concrete or asphalt on an interstate highway; rather the asphalt or concrete "pavers" would be similar to the house painters, who are called in to finish the house exterior - after the house "has been built." ~m.a.r.]

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The effect of a given vehicle on pavements can be estimated by calculating the number of ESALs for each axle and summing to get total ESALs for the vehicle (Exhibit 2). However, a comparison of vehicles in terms of ESALs would not account for the fact that vehicles with higher weights, assuming more axles, require fewer trips to transport the same amount of freight, thereby offsetting part of the additional pavement wear caused by increased weight. To circumvent this problem, vehicles can be compared in terms of ESALs per unit of freight carried (Exhibits 3 to 6).

Click below to enlarge Exhibits 3, 4, 5 and 6.

Because of the fourth-power relationship from the AASHO Road Test, ESALs increase sharply with vehicle weight. The number of axles is also important: other things being equal, a vehicle with more axles has less effect on pavements. Thus, a nine-axle combination vehicle carrying 110,000 pounds has much less effect on pavements than a five-axle combination vehicle carrying 80,000 pounds.

Average ESALs per ton of payload were examined by Fekpe and Clayton under different assumptions about enforcement. They found ESALs per ton of payload to be lower for a six-axle combination with a rear tridem than for a conventional five-axle combination. They also found lower ESALs for seven- and eight-axle doubles than for five- and six-axle tractor-semitrailers.

Two recent studies have raised questions about the fourth power relationship between axle weight and pavement wear. In Road Work: A New Highway Policy, Small, Winston, and Evans present the results of their reanalysis of data from the AASHTO Road Test. Their analysis show a somewhat less steep relationship between pavement life and axle load—closer to a thirdpower law than the fourth-power law conventionally used to approximate the original AASHTO findings. Similar results are reported by Irick and ARE Inc. in their 1989 study for the Trucking Research Institute (TRI). The TRI Executive Summary notes that

"the study refutes the existence of a universal fourth power law of pavement damage. Rather than a fourth power relationship, ARE found significant scatter in the data depending upon pavement type, pavement thickness, and the type of distress being analyzed. Damage functions were generally found to be less than the fourth power, lying somewhere in the range of the second or third power in most cases."

The increase in pavement costs per added ESAL mile can vary by several orders of magnitude depending upon pavement thickness, quality of construction, and season of the year. Thinner pavements are much more vulnerable to traffic loadings than thicker pavements. Pavements are much more vulnerable to traffic loadings during spring thaw in areas that are subject to freeze-thaw cycles. The literature provides widely varying estimates of the marginal pavement cost per ESAL mile. The 1982 Final Report on the Federal Highway Cost Allocation Study estimated efficient pavement damage charges by functional system ranging from 8.7 cents per ESAL mile on rural Interstates to 69.1 cents per ESAL mile on local urban highways. In contrast, Hutchinson and Haas estimated the marginal pavement damage costs for a pavement with 500,000 annual ESALs as 2 cents per ESAL kilometer (3.3 cents per ESAL mile).

Deacon (1988) developed a model using the AASHTO pavement design and performance equations to estimate the changes in pavement rehabilitation costs resulting from increases or decreases in pavement loadings. In this model, each pavement section to be analyzed is described in terms of its thickness, base traffic loadings, and other design and environmental variables such as resilient modulus and drainage coefficient. The model then calculates the remaining life of the existing pavement and the annualized cost of all future resurfacings under base traffic and a ten percent increase in base traffic. The model indicated that there is surprisingly little variation in the additional cost associated with a ten percent increase in loadings under a very broad range of traffic and environmental conditions. Thus, when viewed in terms of cents per ESAL mile, pavement costs are much higher on low traffic roads than on high traffic roads. Very similar results are presented in Hutchinson and Haas. They show average and marginal costs per ESAL on highways with 500,000 to 2,000,000 ESALs per year. The cost per ESAL on highways designed for 500,000 ESALs per year is almost four times as great as the cost per ESAL on highways designed for 2,000,000 ESALs per year. One practical implication of this finding is that a policy which causes heavy trucks to shift from highways with thicker pavements to highways with thinner pavements can have adverse pavement cost impacts. An example of such a policy would be having more permissive axle-weight limits off the National Highway System (NHS), since this policy would encourage trucks with high axle weights to shift from the NHS to non-NHS highways.

[continued on next page, Page 4, Section 1.2 [b] Tire Characteristics ]

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