Since the 1960's, the Asphalt Institute has recommended voids in the mineral aggregate (VMA) to control minimum asphalt content in asphalt aggregate mixtures. VMA criteria are based on aggregate nominal maximum sieve size. Use of VMA criteria requires a clear understanding of the relationship of VMA to aggregate gradation. Nominal maximum size and maximum density lines must also be understood. In the asphalt industry today, significant confusion exists concerning different methods used to draw “maximum” density lines. A need exists to determine which method represents the maximum packing of aggregate materials. Maximum density lines are required to modify gradations and change VMA. Closely related to maximum density lines is the definition of aggregate nominal maximum size which also requires clear definition. This paper reports the relationship of VMA to aggregate gradation and particle characteristics for a controlled experiment. Using results from this experiment plus three other databases of mix designs, different methods of drawing maximum density lines are evaluated. A definition of nominal maximum sieve size which is more specific than the current ASTM definition is proposed.
Mechanical properties of asphalts containing styrenic block copolymers and properties of dense-graded asphalt concrete produced from these binders are presented. Materials studied include unmodified AC-5 and AC-20 asphalts and AC-5 containing 3 and 6% styrenic block copolymers. Dynamic rheology of the binders was studied as a function of temperature and deformation rate. Complex viscosity of the polymer-modified asphalts exhibits less temperature susceptibility than that of control asphalts from 0°C (32°F) to 93°C (200°F) and slightly higher temperature susceptibility above 93°C (200°F). The modified asphalts are viscoelastic throughout the pavement operating temperature range with a significant elastic component. However, the unmodified asphalts are essentially nonelastic above 38°C (100°F). Increases in polymer content increase viscosity, ductility, toughness, tenacity, and elasticity of the materials tested. However, shear-thinning characteristics of the polymer-modified asphalts allow handling by familiar techniques at conventional temperatures. Asphalt concrete was evaluated by resilient modulus and indirect tension over a range of temperatures. Results indicate tensile modulus is lowered at low temperatures and raised at high temperatures by addition of the polymer.
A full-scale experimental pavement was constructed to compare low temperature cracking performance of asphalt concrete containing two types of styrene block copolymers and an ethylene-based polymer each at two levels of concentration in a relatively soft asphalt cement. The experiment was designed to compare field performance with observed behavior in the laboratory. Samples of all binders and mixtures were collected during construction and laboratory tests were conducted using actual materials placed in the field. Mixture tests were limited to Marshall, resilient modulus, and ASTM D4867 moisture sensitivity. Results indicate a high resistance to water damage and relatively high Marshall stabilities for all mixtures evaluated. Marshall test results were highly variable and no apparent differences between materials could be resolved using Marshall parameters. Resilient modulus was conducted at three temperatures, but small differences in this property were not conclusive regarding effects of polymer modification on temperature susceptibility of the mixtures. Variability in resilient modulus measurements was high for certain mixtures. Experience gained in the laboratory and field during design and construction of this experimental pavement has provided information regarding recommended practices when using certain polymer modifiers during construction of asphalt concrete. For example, conventional viscosity-temperature relationships developed with capillary viscometers were misleading regarding establishment of mixing and compaction temperatures. The predicted temperatures were higher than required for mixing and compaction of the polymer modified mixtures. This experience suggests that new techniques should be developed for establishing appropriate mixing, laydown and compaction temperatures for polymer modified asphalt mixtures.
The physical behavior of asphalt concrete is related to the volumetric characteristics of the mixture. A significant challenge facing asphalt pavement engineers is predicting the future density and air voids of asphalt paving mixtures after construction and during service. How rapidly changes in density and voids occur during service can be related to the ability of the pavement to withstand forces imposed from vehicles and the environment. Densification of asphalt concrete after construction and during trafficking varies between mixtures and is related to the method of laboratory compaction, design criteria, and construction technique. This study describes a procedure for quantifying compactibility of mixtures and suggests a procedure for considering densification rate as a criteria for describing optimum asphalt mixtures. The so-called refusal density of six types of asphalt concrete mixtures was evaluated in this study as function of vibratory hammer compaction. Three aggregate gradings ranging from 1–1/2 inch to 3/8 inch (37.5 to 9.5 mm) maximum aggregate size were compacted in the laboratory using three models of portable vibratory compactors administered by two operators. The resulting factorial experiment was analyzed to determine effect on volumetric properties due to hammer, aggregate grading, asphalt content and operator. Vibratory compaction was compared with Marshall, kneading, and gyratory compaction procedures to determine differences between each method and for each grading with respect to density and voids characteristics. A new procedure is outlined for using the vibratory compaction technique for development of a new asphalt concrete design method and for adapting the technique for determining the sensitivity of asphalt concrete mixtures to further densification under traffic. Results of the study indicate that all three vibratory hammers evaluated compacted asphalt concrete to higher levels of density than achievable by conventional Marshall, Hveem or gyratory methods and therefore, should be capable of producing mixtures with higher density than could be obtained after trafficking. The implication of this efficiency during laboratory compaction is a design tool that could provide information regarding the potential for mixtures to reach a plastic condition in service as related to void content.
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