Cover Image

Not for Sale

View/Hide Left Panel
Click for next page ( 51

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement

Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 50
50 number of states and provinces have adopted freezing some confusion exists regarding its practical application. and thawing tests similar to AASHTO T103. Where One use of the 0.45 power chart is to estimate available VMA freezing-and-thawing is a concern, a test that actually of compacted mixtures. Increased VMA is obtained by mov- reproduces freezing and thawing may be preferable over ing further from the maximum density line, but several meth- a sulfate soundness test. ods exist for drawing maximum density lines. Aggregate polish resistance is of concern to agencies The packing characteristics of coated aggregate particles in with a predominance of carbonate aggregates. The an asphalt mixture are related to aggregate surface character- polish stone value test is the most widely used test to istics and gradation. Aggregate surface characteristics of the assess polish resistance of aggregates. When setting particles include angularity and surface texture. Gradation is specifications, agencies need to consider the interaction the size distribution of the particles. When selecting aggregate between tests for abrasion resistance and durability-- for a project, surface characteristics may not be selected to such as the micro-deval test and tests for polishing-- obtain VMA. Conversely, VMA of a mixture is essentially because some aggregates with high polish stone values obtained by default. If additional VMA is required, changes may not be durable. are usually made to the aggregate gradation. In some cases, natural sands, which are predominantly -600m sieve mate- rial, are added. 2.8 EFFECT OF AGGREGATE GRADING Natural sands have been identified as a cause of decreased ON HMA PROPERTIES resistance to permanent deformation and of tender mix Gradation is perhaps the most important property of an problems during construction (121). As a result, limits have aggregate. The link between aggregate gradation and asphalt been placed on sand content and increases in VMA must be mixture performance was recognized early in the develop- achieved by overall adjustment of gradation. Unlike natural ment of mix design methods (121). Gradation affects almost sand addition, gradation adjustment can sometimes produce all the important properties of HMA, including stiffness, sta- confusing results. Moving away from the maximum density bility, durability, permeability, workability, fatigue resistance, line sometimes causes decreases rather than increases in VMA. frictional resistance, and resistance to moisture damage. The In the recent years, the Bailey method was developed mixture volumetric properties including asphalt content, VMA, to select aggregate gradations for HMA mixture design. The and VFA have been identified as important parameters for Bailey method was originally developed by the Illinois DOT durability and performance. However, the VMA is consid- and has become a systematic approach to aggregate blending ered the most important parameter and is used in the Super- that is applicable to all dense-graded asphalt mixtures, regard- pave mixture design specifications to eliminate use of poten- less of the maximum size aggregate in the mixture (124126). tially poor-performing mixtures. The Bailey method uses two principles that are the basis of the relationship between aggregate gradation and mixture volumetrics: aggregate packing and definition of coarse and 2.8.1 Methods for Analyzing Gradations fine aggregate. In the Bailey method, aggregate interlock is selected as a Prior to the establishment of formal mixture design meth- design input. Gradation selection considers the packing char- ods, gradation was evaluated to determine asphalt demand. acteristics of aggregates. The parameters in the method are Formulas were applied to the gradation, and asphalt require- related directly to VMA, air voids, and compaction properties. ments were calculated to provide satisfactory durability with The definition of "coarse" and "fine" is more specific in order minimum amount of asphalt binder (121). By the 1920s, the to determine the packing and aggregate interlock provided by HubbardField method of mix design recognized the impor- the combination of aggregates in various sized mixtures: tance of air voids as a key parameter controlling field perfor- mance of mixtures (122). The HubbardField mix design is Coarse Aggregate: Large aggregate particles that when based on the need for air voids and for a minimum amount of placed in a unit volume create voids. asphalt binder. Voids in total mix and voids in aggregate Fine Aggregate: Aggregate particles that can fill the mass were both specified. Early mixture design methods voids created by the coarse aggregate in the mixture. were based on a belief that a "gradation law" existed that controlled asphalt mixture properties. Considerable research The primary steps in the Bailey Method are (1) compare effort was expended to discover this law that controlled aggregates by volume and (2) analyze the combined blend. aggregate packing. Associated with the gradation law was Aggregate is blended by volume. The combined blend is bro- the belief that an "ideal" gradation existed that would provide ken down into three distinct portions: coarse aggregate, coarse adequate space for minimum amount of asphalt and air voids portion of fine aggregate, and fine portion of fine aggregate. and adequate stability under traffic. Each portion is evaluated individually. Figure 16 shows a Today, aggregate gradations are commonly evaluated using schematic of how the gradation is divided into the three por- a "0.45 power chart" (123). Despite the chart's usefulness, tions. A factor of 0.22 was used to determine a primary con-