A Manual for Design of Hot-Mix Asphalt with Commentary (2011) / Chapter Skim
Currently Skimming:
Pages 4-14
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 4... ...
Engineers and technicians with a broad range of experience in materials testing and the design of HMA mixtures and flexible pavement structures need not read this chapter in detail. Individuals who are relatively new to asphalt pavement technology will find the information on materials, asphalt pavements, asphalt concrete mixtures, and mix design methods helpful when reading the later chapters of this manual.
|
From page 5... ...
However, if the pavement is not compacted adequately during construction, compaction under traffic will fail to reduce the air void content to the design value and, as a result, the pavement will be permeable to air and water, potentially leading to moisture damage and excessive age hardening. Asphalt Concrete Pavements Asphalt concrete pavements are not simply a thin covering of asphalt concrete over soil -- they are engineered structures composed of several different layers.
|
From page 6... ...
Figure 2-2 is a cross section of a typical flexible pavement system. 6 A Manual for Design of Hot Mix Asphalt with Commentary Surface course Intermediate course Base course Granular subbase Subgrade 40-75 mm 50-100 mm 100-300 mm 100-300 mm Figure 2-2.
|
From page 7... ...
Excessive permanent deformation in one or more of the bound layers is the result of an asphalt concrete mixture that lacks strength and stiffness at high temperatures. Several problems with a mix design, such as selecting an asphalt binder that is too soft for the given climate and traffic level, can make it prone to rutting and other forms of permanent deformation.
|
From page 8... ...
High in-place air void content will also increase pavement permeability, which will then allow air and water into the pavement, both of which can damage the pavement and increase the rate of fatigue cracking. Relationships between fatigue cracking and HMA mix design are discussed in more detail in Chapter 6.
|
From page 9... ...
The use of moisture resistance testing in HMA mix design is discussed in greater detail in Chapter 8. Background 9 Figure 2-5.
|
From page 10... ...
Therefore, many "dense-graded" HMA mixtures do not use a true maximum density gradation, but use somewhat "open" gradations that deviate slightly from maximum density; such mixtures have more space between the aggregate particles and can be designed to contain more asphalt binder. Mixtures that are somewhat coarser than the maximum density gradation are called coarse10 A Manual for Design of Hot Mix Asphalt with Commentary
|
From page 11... ...
suggests that a range of gradations be evaluated during the mix design process and that the gradation most effective in meeting the given mixture specifications should be selected. The suggested volumetric requirements do include a slight increase in the allowable range for dust-to-binder ratio and an optional table for high-durability mixtures that includes an even higher dust-to-binder ratio and an increase in minimum VMA; both of these changes will probably reduce the number of dense/coarse-graded HMA mixtures being designed under this system.
|
From page 12... ...
during World War II and was modified by that agency and by the many state highway departments that eventually chose the Marshall method of HMA mix design. Briefly, the Marshall mix design procedure relies on compacting specimens using a standard drop hammer over a range of asphalt binder contents.
|
From page 13... ...
At one time, a cohesiometer test was also used to evaluate HMA properties in the Hveem method. As with the Marshall mix design method, evaluation of mixture volumetrics was an important part of the Hveem procedure.
|
From page 14... ...
A major research effort, continued since the implementation of SHRP, is under way to refine various aspects of this system, including volumetric requirements, compaction levels, and specifications for aggregate properties and gradation and finally implement practical mixture performance tests and methods of analysis. This mix design manual is largely based on the Superpave system of mixture design and analysis, but an attempt has been made to address the perceived performance problems associated with some mixtures designed using this system.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.