6-19) what properties of aggregates are important to ensure a high degree of ski
ID: 1713389 • Letter: 6
Question
6-19) what properties of aggregates are important to ensure a high degree of skid resistance in a pavement surface? Why?6-29) what are the three design levels used in the SHRP methods? What criteria are used to decide on the method to be used? Briefly describe what is involved in each design method. 6-19) what properties of aggregates are important to ensure a high degree of skid resistance in a pavement surface? Why?
6-29) what are the three design levels used in the SHRP methods? What criteria are used to decide on the method to be used? Briefly describe what is involved in each design method.
6-29) what are the three design levels used in the SHRP methods? What criteria are used to decide on the method to be used? Briefly describe what is involved in each design method.
Explanation / Answer
Ans.6-19) Aggregate Properties which are important to ensure a high degree of skit resistance in pavement surface are --frictional resistance and wheel-polishing resistance which can be measured through :--
6-29} The Superpave Mix Design System Manual of Specifications, Test Methods and Practices. The Superpave software program (The Superpave Specification, Mix Design and Support Program) and its users manual are also necessary to take full advantage of the mix design system. This software is designed to run on an 80386-based or, preferably, an 80486-based personal computer. 1.2 The Superpave Mix Design System The Superpave mix design system is a comprehensive method of designing paving mixes tailored to the unique performance requirements dictated by the traffic, environment (climate), and structural section at a particular pavement site. It facilitates selecting and combining asphalt binder, aggregate, and any necessary modifier to achieve the required level of pavement performance. The Superpave system is applicable to virgin and recycled, dense-graded, hot mix asphalt (HMA), with or without modification. In addition, the Superpave performance tests are applicable to the characterization of a variety of specialized paving mixes such as stone matrix asphalt (SMA). It can be used when constructing new surface, binder, and base layers, as well as overlays on existing pavements. Through materials selection and mix design, it directly addresses the reduction and control of permanent deformation, fatigue cracking, and low-temperature cracking. It also explicitly considers the effects of aging and moisture sensitivity in promoting or arresting the development of these three distresses. The objective of the Superpave mix design system is to define an economical blend of asphalt binder and aggregate that yields a paving mix having * sufficient asphalt binder, * sufficient voids in the mineral aggregate (VMA) and air voids, • sufficient workability, and ,, satisfactory performance characteristics over the service life of the pavement. The Superpave mix design system has several distinctive features. First, only performance-based and performance-related properties are used as the criteria for selection of a mix design. Performance-based properties directly govern the response of the pavement to load; performance can be predicted from this response. Performance-related properties are those which are indirectly related to performance. They affect performance, but do not, in themselves, control it. As the traffic and environmental demands on the pavement increase, the Superpave mix design system relies more on performance-based properties to select the optimum mix design. The maximum possible contribution of the materials in the paving mix to pavement performance can be achieved regardless of its structure. Second, the Superpave performance-based specification for asphalt-aggregate mixtures is based on the predicted performance of the pavement built with the paving mix under design. The Superpave system provides a uniquely tailored specification, expressed in terms of the rut depth, area of fatigue cracking, and spacing of low-temperature cracking expected over a selected service life, for any paving project with significant traffic and environmental demands. These specifications may be compared to criteria presented in this manual or to criteria established by agency policy for discrete classes of traffic. Third, in determining pavement performance, Superpave explicitly considers the interaction of pavement structure, traffic, and environment with the paving mix. Thus, the mix design and structural design can be integrated into an single system. Finally, the design method provides a truly objective measure of the benefits or penalties associated with the use of materials of varying levels of quality. 1.2.1 Concept of the Superpave Mix Design The concept behind the Superpave mix design system is straightforward: use available materials to prepare a mix design that achieves a level of performance commensurate with the demands of traffic, environment, structure, and reliability (or, conversely, risk) on the pavement. This concept is illustrated by the general flow of a Superpave mix design engineering evaluation of trial mix designs to field control, decisions must be made on the basis of satisfying the specific pavement performance requirements to the extent possible. _ 1.2.2 Low-, Intermediate-, and High-Traffic Level Designs The Superpave mix design system contains three distinct levels of design, termed level 1, level 2, and level 3. This feature permits the agency to select a design process that is appropriate for the traffic loads and volume (expressed as total 80 kN equivalent single axle loads (ESALs) over the service life of the pavement) expected for the paving project. General recommendations for applying the three design levels are presented These traffic levels are presented as suggested guidelines only. Higher or lower transition points may be set on the basis of individual agency policy. In addition, all three design levels explicitly consider the effects of the climate (environment) on pavement performance. Selection of the performance grade of asphalt binder is guided by the high and low pavement design temperatures at the paving project, the traffic speeds, and the design traffic levels. Candidate paving mixes are evaluated for unacceptable moisture sensitivity. Both asphalt binders and candidate paving mixes are aged in the laboratory to simulate the effects of both short- and long-term aging in pavement service. The complexity of the design process increases significantly from level 1 to level 3. Level 3 requires a greater number of tests, more test specimens, and more time to complete a design. In return, the reliability of the design -- that is, the probability that the paving _By contrast, the widely used Marshall and Hveem methods of mix design are neither performance-based nor performance-related. They are concerned primarily with achieving a mix design with a stable, economical balance of aggregate and asphalt binder that features sufficient workability to permit efficient placement of the mix. Both methods attempt to gauge anticipated performance with empirical properties, such as Marshall stability and flow, but neither method can ensure that a trial mix design will meet specific pavement performance criteria. 4 mix will provide satisfactory pavement service under the anticipated conditions of traffic and climate-increases proportionally. The Superpave mix design system provides flexibility to deal with one, two, or all three of the major distress types which it addresses. As examples, at the discretion of the agency, designs for warm weather climates can concentrate on permanent deformation only or on permanent deformation and fatigue cracking. In extreme cold weather climates, designs can be aimed exclusively toward preventing low-temperature cracking without regard for the development of other distresses. In a climate that experiences extremes of both heat and cold, the agency can choose to use a level 3 design for permanent deformation, while addressing fatigue and low-temperature cracking with the level 2 design process.
1.2.2.1. Level 1 (Low Traffic) Mix Design Level 1 mix design employs a performance-based asphalt binder specification with empirical, performance-related aggregate specifications, and principles of volumetric mix design to obtain a paving mix with satisfactory performance for low-traffic paving projects without the need for performance-based testing. Final selection of the design asphalt content is based upon attaining specified levels of air voids, voids in mineral aggregate, and voids filled with asphalt at initial, design, and maximum levels of compaction. It is not possible to estimate the pavement performance of level 1 mix designs with regard to permanent deformation, fatigue cracking, or low-temperature cracking without the level 2 or 3 performance-based tests. However, the level 1 mix design provides a reasonable guarantee of adequate performance if all of the specified criteria are met. Gyratory compaction is the key to successful level 1 mix designs. In addition, this design level explicitly considers the effects of moisture sensitivity and aging in selecting the final mix design.
1.2.2.2 Level 2 (Intermediate Traffic) Mix Design The level 2 mix design incorporates the selection of a design asphalt content with the volumetric (level 1) design procedure. Candidate mixes prepared at the design asphalt content and at a high and a low asphalt content bracketing the design value are subjected to a series of performance-based tests selected for use in routine mix designs. Figure 1.2. Level 1 Superpave Mix Design (Note: All specimens are compacted from paving mix that has been short-term aged (SHRPM-007)). Select aggregate based upon Select asphalt binder grades specification criteria based upon high and low - gradation limits pavement design temperatures. (1-2) - coarse aggregate angul ari ty I - fine aggregate angularity Conduct optional net adsorption clay content test to evaluate asphalt-aggregate thin and elongated compatibility. (2 specimens) (1-2A) particles (1-1) ....................................... IPrepare I three or four trial gradations determined from control points established on 0.45 power curve. (I-3) Calculate an initial trial asphalt content for each trial gradation (Two specimens compacted to N_esign per gradation = 6 or 8 specimens) (1-4) Evaluate compacted trial gradations; select a design aggregate structure; calculate an estimated design asphalt content. (1-5) Determine moisture Compact specimens at 4 Aes around susceptibility of design the estimated design asphalt content• asphalt content and design (2 compacted specimens per AC = 8 gradation using AASHTOT 283 specimens) (I-6) (Compact 6 specimens at 7% AV. Test three dry and three wet.) (1-8) The Superpave software uses these test results to estimate pavement performance predictions for permanent deformation, fatigue cracking, and low-temperature cracking. The reliability of these predictions is consistent with the designation of level 2 as the method of choice for routine mix designs. The optimum asphalt content is determined from these performance predictions. The use of gyratory compaction and the Strategic Highway Research Program (SHRP) shear test device is essential to successful level 2 mix designs. This design level also introduces a test for tertiary creep to screen out early in the design process trial mixes that may be susceptible to catastrophic rutting failures or unacceptable long-term permanent deformation 2. In addition, this design level explicitly considers the effects of moisture sensitivity and aging in selecting the final mix design. Together, level 2 and level 1 mix designs should be suitable for 95 percent or more of the mix design work conducted by a state agency in a typical year; in many states, these design levels will be satisfactory for all mix designs.
1.2.2.3 Level 3 (High Traffic) Mix Design The level 3 mix design incorporates the selection of a design asphalt content with the volumetric (level 1) design procedure. Candidate mixes prepared at the design asphalt content and at a high and a low asphalt content bracketing the design value are subjected to a series of performance-based tests selected to develop mix designs suitable for very heavy traffic, severe climates, or any situation where only a minimal design risk is tolerable. Level 3 mix designs require considerably more time and a greater number of specimens than level 2 designs. The Superpave software uses these test results to estimate pavement performance for permanent deformation, fatigue cracking, and low-temperature cracking. The degree of reliability of these estimates is consistent with the designation of level 3 as the method of choice for mix designs where high performance is mandatory. The optimum asphalt content is determined from these performance estimates. The use of gyratory compaction and the SHRP shear test device is essential to successful level 3 mix designs. This design level also uses a test for tertiary creep to screen out early in the design process those trial mixes that may be susceptible to catastrophic 2 It is anticipated that the test for tertiary creep will not be needed if the compactibility of the mix conforms to the Superpave guidelines. rutting failures or unacceptable long-term permanent deformation. In addition, this design level explicitly considers the effects of moisture sensitivity and aging in selecting the final mix design. The level 3 mix design also provides a battery of optional proof tests recommended to confirm the results of the performance-based test
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