Chris A Bell

EFFECT OF PERCENT COMPACTION ON ASPHALT MIXTURE LIFE

One of the major difficulties in asphalt concrete pavement construction is the control of materials and the subsequent level of payment to contractors for materials not conforming to specifications. The effect of this noncompliance on pavement serviceability is not fully established; however, it frequently results in reduced payments to contractors, which, in turn, causes much controversy between the two parties. Specific mixture properties outside specification tolerance, which are accepted by most agencies through reduced payment include percent compaction, asphalt content, and mix gradation. The reduced payment used normally relies heavily on the experience and judgment of the agencies; therefore, there is often wide disparity in the pay adjustment factors used. The paper summarizes currently available techniques for estimating the modulus and fatigue life of asphalt mixtures and shows how these, together with existing failure criteria for fatigue and permanent deformation may be used to estimate the effects of changes in compaction on pavement life. Secondly, the results of extensive laboratory tests on asphalt mixtures from three projects in Oregon are reported. Tests were performed to evaluate the effect of mix variations (including density) on mix properties (modulus, fatigue life, and resistance to permanent deformation). Tests were run using both as-compacted and conditioned (vacuum saturated followed by freezing then heat soaking) specimens. The test used throughout was the ASTM Repeated Load Diametral Test (D 4123-82) modified slightly such that fatigue life could be evaluated. The results of the test program indicated that percent compaction (or void content) was the most significant factor affecting mix performance. An increase in void content is associated with a decrease in modulus, fatigue life, and resistance to permanent deformation. The result of the study findings was the development of new asphalt concrete compaction specifications for the state of Oregon. Not only does the Oregon Department of Transportation (DOT) now require a higher standard of compaction (91% of American Association of State Highway and Transportation Officials (AASHTO) T209 maximum specific gravity), but they have also spelled out clearly the penalties for not achieving density. (Author)

The Effect of Moisture on the Performance of Asphalt Mixtures

This paper presents the results of a laboratory study to investigate the effects of mixing moisture on mechanical properties of asphalt mixtures. The potential benefits of lime and Pavebond Special were also evaluated. The repeated load diametral test device was used to measure the mixture performance in terms of the resilient modulus, fatigue, and permanent deformation characteristics of laboratory specimens prepared with and without moisture (0, 1, and 3%) and with and without lime (1%) and Pavebond (0.5%). Mixtures were prepared that were representative of two projects for which considerable field data were available. One project utilized low quality and high absorptive aggregate and the other good quality aggregate. To evaluate the long-term durability of mixtures, they were tested before and after conditioning using the Lottman approach. The test results showed that inferior performance occurred for mixtures with 3% moisture but was most pronounced in mixtures with higher air-void contents. However, the mixtures with low quality and high absorptive aggregate showed improved performance at 1% moisture content, associated with their lower air-void contents, which may be due to absorbed moisture preventing asphalt absorption and the higher asphalt content of these mixtures. The addition of lime resulted in distinct improvement of performance for moist specimens from the project, which had good quality aggregate, but high air-void contents. However, neither additive showed substantial benefit for moist samples from the project with low quality aggregate and low air-void content.

A study of Commercial Vehicle Safety Alliance's out-of-service criteria

This paper summarizes a two-phase project that reviewed the Commercial Vehicle Safety Alliances out-of-service criteria for vehicles. The first phase examined relevant background information and conducted a questionnaire survey of CVSA inspectors and industry representatives. The second phase of the project involved extensive collection and evaluation of accident data. The results of both phases show a high level of support, in terms of contribution to vehicle accidents, for four of the vehicle criteria (regarding brakes, load securement, tires, and wheels and rims). There was some support for coupling devices, fuel systems, lighting devices, steering and suspension. The support for the remaining vehicle criteria (exhaust systems, frames, van and open top trailer bodies, and windshield wipers) was little to none. Further research continuing the study of accident data is recommended to confirm these findings. However, the data must be collected in a consistent and detailed manner if accurate information on the relationship of accidents and vehicle criteria is to be established.

Study of Aircraft Accident Related to Asphalt Runway Skid Resistance

On February 20, 1991, a brand-new British Aerospace BAE 146-200 aircraft, operated by a commercial airline, landed at Guardiamarina Airport in Port Williams, Chile. The aircraft landed under good weather conditions on an asphalt surface treatment runway and skidded more than 1,000 m into the Beagle Channel. It was a charter flight with 60 tourists on board; 40 survived. A long investigation provided evidence on why the aircraft skidded off the end of the runway.

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 Oregons 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 throughou...