section 1.2 Truck Characteristics Affecting Pavements; [b] Tire Characteristics

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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

(b) Tire Characteristics

Tires mounted on the AASHO Road Test trucks were representative of those in use in the late 1950s: they were of bias-ply construction with inflation pressures of 75 to 80 pounds per square inch (psi). Since then, bias-ply tires have been replaced with radial tires and inflation pressures have increased. A study by Bartholomew (1989) summarized surveys of tire pressure conducted in seven states from 1984 to 1986 and found that 70 to 80 percent of the truck tires used were radials and that average tire pressures were about 100 psi. As a result of these and similar studies, concern has been raised about the possibility of accelerated pavement wear, particularly rutting, as a result of increased tire pressure.

Higher tire pressure reduces the size of the tire "footprint" on the pavement, so that the weight of the wheel is distributed over a smaller area. The increased pressures hasten the wear of flexible pavements, increasing both the rate of rutting and the rate of cracking. During highway operations, the rolling of the tire results in a temperature rise that in turn causes the inflation pressure to increase. Inflation pressures of hot tires can be 10 to 20 psi greater than pressures of cold tires for bias-ply and 5 to 15 psi greater for radials (Sharp 1987). Results from other studies (Southgate and Deen 1987; Bonaquist et al. 1988a, 1988b) suggest that, for 20,000-pound single axles on thicker pavements characteristic of major highways, an increase in tire pressure from 75 to 100 psi increases pavement wear by about 15 percent. Taken together, these results suggest that, other things being equal, pavement wear effects of hot tires are 3 to 12 percent greater than pavement wear effects of cold tires.

The AASHTO load-equivalency factors strictly apply only to axles supported at each end by dual tires. Recent increase in steering-axle loadings and more extensive use of single tires on load-bearing axles have precipitated efforts to examine the effect on pavement wear of substituting single for dual tires. Both standard and wide-based tires have been considered. Past investigations of the pavement wear effects of single versus dual tires have found that single tires induce more pavement wear than dual tires, but that the differential wear effect diminishes with increases in pavement stiffness, in the width of the single tire, and in tire load.

Gillespie (1993) found that a steering axle carrying 12,000 pounds with conventional single tires is more damaging to flexible pavement than a 20,000-pound axle with conventional dual tires. He states further that

"road damage from vehicles currently operating at the 80,000-pound gross weight limit would be decreased approximately 10 percent by modifying road use laws to favor a load distribution of 10,000 pounds on the steering axle with allowance for 35,000 pounds on tandems."

Without disputing Gillespie's assessment of the relative pavement costs for steering axles and tandems at different weights, it should be noted that weight-limited five-axle tractor-semitrailers usually have steering axle weights below 11,000 pounds (even though truck weight limits would allow 12,000-pound steering axles). Hence, the practical effect of Gillespie's suggested change in limits for most weightlimited trucks would be to increase tandem axle weights without a compensating decrease in steering axle weights.

Bauer (1994) summarized several recent studies on the effects of single vs. dual tires:

"Smith (1989), in a synthesis of several studies dealing with the roadway-tire relationship, evaluated at 1.5 on average the relationship of the damage caused by wide base single assemblies and that caused by traditional dual tire assemblies with identical loading at the axle.

Sebaaly and Tabataee (1992) found rutting damage ratios between wide base and dual tire assemblies varying between 1.4 and 1.6. This was a study carried out at the University of Pennsylvania on two coatings, with 2 types of axle (single and tandem) and four sizes of tire (two dual mounted and two wide based).

Bonaquist (1992), reporting on results obtained from a study carried out on the road simulator of the Turner-Fairbank Highway Research Center at McLean (Virginia), on two types of roadway, using a dual tire assembly with 11 R 22.5 and a wide base with 425/65 R 22.5, indicates rutting damage ratios varying from 1.1 to 1.5, depending on the layers of the roadway."

In summary, Bauer states that the wide-base single tire would seem to cause around 1.5 times more rutting than the dual tire on roadways that do not possess good resistance qualities to rutting. However, Bauer also noted that one of the wheels in a dual tire assembly is frequently overloaded due to the road. He noted that the average overload for a dual wheel causes an increase in rutting similar to that which exists between a wide-base single and a dual tire assembly, so that the real advantage of dual tire assemblies is therefore undoubtably lower than the theoretical advantage with which they are attributed.

Conflicting results were reported by Akram et. al. They used multidepth deflectometers to estimate the damage effects of dual versus wide base tires. Deflections measured at several depths within the pavement under dual and wide-base single tires were used to calculate average vertical compressive strains. The Asphalt Institute subgrade limiting strain criteria were then used to estimate the reduction in pavement life that will occur by using the widebase single tires in place of duals. At a speed of 55 miles per hour and equivalent axle loading, they found that the wide-base single tires (trailer axle) reduced the anticipated pavement life by a factor of between 2.5 to 2.8 over that predicted for standard dual tires.

Molenaar, Huurman, and Naus examined the combined effects of tire pressure and super single versus dual wheel tires on rutting. They found a roughly tenfold increase in rutting for a super-single with a tire pressure of 1.00 MPa as compared with a dual tire with a tire pressure of 0.60 MPa.

Although it is undoubtedly true that, other things being equal, single tires have more adverse effects on pavements than dual tires, it appears likely that past investigations have overstated the adverse effects of single tires by neglecting two potentially important effects: unbalanced loads between the two tires of a dual set and the effect of randomness in the lateral placement of the truck on the highway.

Unbalanced loads between the tires of a dual set can occur as a result of unequal tire pressures, uneven tire wear, and pavement crown. As with unequal loads on axles within a multiaxle group, pavement wear increases as the loads on the two dual tires become more unbalanced.

The second neglected factor, sometimes termed "wander," is the effect of randomness in the lateral placement of trucks within and sometimes beyond lane boundaries. Less perfect tracking is beneficial to pavement wear: the fatiguing effect is diminished because the repetitive traffic loads are distributed over wider areas of the pavement surface. Because the greater overall width of dual tires naturally subjects a greater width of pavement to destructive stresses, wander is expected to have a smaller beneficial effect for dual than for single tires. Once rutting begins, however, tires—especially radial tires—tend to remain in the rut, thereby greatly reducing the beneficial effects of wander for both single and dual tires.

TRB's Truck Weight Study undertook a special analysis to examine the importance of loading imbalances and wander as part of its examination of vehicle characteristics affecting pavement wear (Deacon 1988b). Two types of pavement wear were considered: surface cracking due to fatigue and permanent deformation or rutting in the wheel tracks. Fatigue was found to be more sensitive to the difference between single and dual tires than rutting, and was selected as the basis for pavement wear comparisons.

Both balanced and unbalanced dual-tire loads were considered. In the unbalanced case, one of the tires carried a 5 percent greater-than-average load and the other carried a 5 percent less-than-average load. Wander was described by a normal probability distribution. In the absence of definitive field data, three standard deviations were considered: 4, 6, and 8 inches. For these values, approximately 99 percent of truck operations would track within a 2-, 3-, and 4-foot pavement width, respectively.

Analysis of these data showed that taking wander into account reduced the adverse effects of single tires on pavement wear, but that these effects were still significant (Exhibit 7). Without wander, the ESAL equivalent for an 18,000-pound axle with single tires was estimated to be 2.23. When wander with a standard deviation of 8 in. is assumed, the ESAL equivalent drops to 1.31. At least for the +5 percent case considered in this study, the effects of imbalance in dual-tire sets on ESALs were found to be very small relative to the effect of wander.

Research summarized by the Midwest Research Institute (MRI) also suggests that dynamic loadings are a consideration in assessing the relative merits of wide-base single vs. dual tires. MRI notes that

"the dynamic component of pavement loading arises from vertical movements of the truck caused by surface roughness. Thus, peak loads are applied to the pavement that are greater than the average static load. Gyenes and Mitchell report that the magnitude of the added dynamic components was earlier thought to increase road damage over that of the static loading alone by 13 percent to 38 percent, according to research reported by Eisenmann.

"Many recent studies have pointed out the fallacy in the earlier work, which assumed that the dynamic component of loading was distributed uniformly over the pavement in the direction of travel. What those researchers found, instead, is that the dynamic component is very localized. Because it arises from pavement surface irregularities, the dynamic loading is spatially correlated with these irregularities. Indeed, signs of pavement damage are typically localized, at least initially.

"Because of the localized nature of the dynamic loading, its severity is much greater than thought earlier. Gillespie et al. estimate that damage due to the combination of static and dynamic loading can be locally two to four times that due to static loading. Von Becker estimates that the combined loading produces a "shock factor" from 1.3 to 1.55, depending upon suspension characteristics. applying the fourth power law would translate these figures into relative damage estimates ranging from 2.8 to 4.8 time s the static loading damage. Gyenes and Mitchell suggest impact factors of 1.3 to 1.5, for relative damage estimates of 2.8 to 5.1."

Midwest Research Institute noted further that

"parallel research has shown that a wide base tire, having only two sidewalls, is much more flexible than a pair of dual tires with four sidewalls. This flexibility means that the tire absorbs more of the dynamic bouncing of the truck, so less of the dynamic load is transmitted to the pavement."

In summarizing their assessment of wide-base tires, MRI states that

"taking all of these findings into consideration suggests that the relative damage potential is much less than commonly believed, and conceivably the wide-base tires might be less damaging than duals."

[continued on next page, Page 5, Section 1.2 (c) Suspension Systems ]

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