Cover Image

Not for Sale



View/Hide Left Panel
Click for next page ( 50


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 49
49 TABLE 11 AASHTO T103 Procedures A, B, and C Procedure Solution Immersion Number of Cycles A Water 24 hours 50 B 0.5% solution of Vacuum Saturation 16 ethyl alcohol and (25.4 mm Mercury) water C Water Vacuum Saturation 25 (25.4 mm Mercury) better discrimination than the sulfate test and is more precise." for high-volume roadways. Woodward (109), who has per- Freeze-thaw losses of less than 6% for high-volume roads and formed extensive skid testing, notes a quandary for British of less than 30% for low-volume roads are recommended. aggregate sources between desirable levels of polish stone value to maintain adequate skid resistance and desirable lev- els of micro-deval loss to ensure durability against raveling 2.7.3 Aggregate Properties Related to and popouts. Polishing and Frictional Resistance The friction of a pavement surface is a function of the sur- 2.7.4 Summary of Tests Related to Aggregate face textures that include the wavelength ranges described by Durability microtexture and macrotexture. Microtexture provides a gritty surface to penetrate thin water films and produce good fric- The following is a summary of the tests related to aggre- tional resistance between the tire and the pavement. Macrotex- gate durability. ture provides drainage channels for water expulsion between the tire and the pavement, thus allowing better tire contact Aggregates are subject to breakdown during stock- with the pavement to improve frictional resistance and to pre- piling, mixing and compaction. Excessive aggregate vent hydroplaning. Initial macrotexture is a function of gra- breakdown can alter in-place gradations and can affect dation, although tests for abrasion resistance--such as the the volumetric properties of the HMA. In the United aggregate abrasion value and micro-deval abrasion loss--are States, the LA abrasion test (AASHTO T96) is the most related to the change in macrotexture with time (119). commonly used test to assess aggregate breakdown dur- Polishing is the loss of microtexture with traffic wear. Pol- ing construction. It is correlated to other tests for aggre- ishing is common in many carbonate aggregates. No test meth- gate breakdown. There is no evidence to suggest that the ods were included in the Superpave mix design system to LA abrasion test should be replaced by another impact assess aggregate polishing. Agencies with aggregate sources test for the purpose of assessing aggregate breakdown prone to polishing have developed a number of means for during construction. qualifying or grading aggregate sources for various traffic Aggregates in HMA are subject to weathering and levels. Polish Stone Value (BS 812) may be the most widely abrasion in situ. Although originally intended to assess used test. Coarse aggregate particles having certain dimen- degradation from freezing and thawing, sulfate sound- sions are first cemented into a curved mold. A group of the ness tests (AASHTO T104) have been widely used to curved molds are attached to the outside of a steel wheel. The assess aggregates' resistance to weathering. Several steel wheel rotates bringing the samples into contact with a studies indicated good correlation between magnesium rubber wheel treated with coarse and then with fine emery sulfate soundness loss and micro-deval abrasion loss powders, which abrade the aggregate samples. Water is con- (AASHTO TP58). Several studies have also indicated tinually applied to the aggregate surface. Upon completion of that the strength of some aggregates is significantly the polishing, the mold with cemented aggregate particles is lower when wet. The micro-deval test offers improved mounted on a British Pendulum Tester. A pendulum arm with precision over sulfate soundness. The micro-deval test an attached rubber pad swings across the mold so that the also indicates abrasion resistance. This suggests that the rubber pad drags across the samples. A scale measures the micro-deval test may be more suitable to predicting height of the pendulum swing after contact with the sample. aggregates performance in relation to weathering and Lower numbers represent greater microtexture. abrasion than is sulfate soundness. However, data sug- Some aggregates are resistant to polishing. Other aggre- gests specifications for micro-deval loss may have to be gate types maintain their microtexture by the continual abra- based on aggregate type. sion of mineral grains exposing fresh nonpolished grains. There is debate as to whether aggregates used in This renewal is desirable as long as the loss is not so great HMA need to be resistant to freezing and thawing. as to represent an aggregate that is not durable. Senior and Historically, sulfate soundness tests have been used to Rogers (115) recommend polish stone values in excess of 50 assess aggregates ability to resist weathering. A limited