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14
CHAPTER 2
STATE OF PRACTICE
2.1 INTRODUCTION bands, and alternatives follows. Since the aggregate crushing
process affects the resulting shape and texture of the aggre-
The Superpave mix design method, a product of SHRP, gate particles, a brief discussion on crushing is also included.
was introduced in 1994 (1). The Superpave method included
binder, aggregate, and mixture specifications. Many of these
specifications used new test methods. One goal of the Super- 2.2 COARSE AGGREGATE ANGULARITY
pave method was to provide uniform test methods and spec-
ifications to be used across the United States. 2.2.1 Background
Aggregates were addressed through gradation, consensus
Prior to the development of Superpave, several studies
aggregate properties, and source aggregate properties. Defi-
indicated increased resistance to permanent deformation with
nitions were provided for NMAS and maximum aggregate
increasing fractured faces in coarse aggregate (412). The
size. A limited number of gradation control points were estab-
fourth questionnaire used in the Delphi process to identify the
lished for each NMAS. A restricted zone was recommended consensus aggregate properties ranked coarse aggregate angu-
along the maximum density line to prevent the use of large larity second to gradation limits in terms of importance (1).
quantities of natural sand and to help ensure adequate VMA. However, no test method was identified. FHWA's Office of
Four consensus aggregate properties were specified: coarse Technology Applications recommended Pennsylvania DOT
aggregate angularity, flat and elongated particles, uncom- Test Method 621 (13). ASTM D5821 was based on the Penn-
pacted voids content in fine aggregate, and sand equivalent. sylvania test method and was later adopted as the method for
Specification levels for the consensus property tests depend measuring coarse aggregate angularity within the Superpave
on design traffic level and depth in the pavement structure mix design method (14, 15).
(for tests related to permanent deformation). Specifications The fractured face count of a representative sample of
for consensus aggregate properties are based on the blend of coarse aggregate is determined by visual inspection. ASTM
materials used in a given HMA mix and not on individual D5821 (16) defines a fractured face as "an angular, rough, or
aggregates. The consensus aggregate properties were to be broken surface of an aggregate particle created by crushing, by
uniformly implemented across the United States regardless other artificial means or by nature." A fractured face is only
of local geology. Source aggregate properties include LA counted if its area is greater than 25% of the largest projection
abrasion, soundness, and deleterious materials. It was felt that (cross-sectional area) of the particle. A fractured particle is "a
these properties would need to be adjusted depending on the particle of aggregate having at least the minimum number of
local geology; therefore, agencies were to set the specifica- fractured faces specified (usually one or two)" (16).
tion levels for source aggregate properties for Superpave- To run the test, a representative sample is washed over the
designed mixes. 4.75-mm sieve and dried to a constant mass. The size of the
There has been controversy regarding some of the consen- sample is dependent on the nominal maximum aggregate size
sus aggregate property tests and specification levels. All four of the aggregate. The aggregate particles are visually inspected
tests are empirical in nature, and there is very little data avail- and divided into piles of particles with no fractured faces and
able to support the establishment of specification require- one or more fractured faces. Prior to 2001 when the ASTM
ments. Their relationship to performance has been questioned. D5821 was revised, the separation included a questionable
In some cases, the implementation of the consensus aggregate pile for particles where the tester was uncertain whether a
properties and Superpave gradation bands have prevented the fractured face met the definitions (14). The questionable pile
use of materials or mixes that have been historically used was eliminated from the ASTM standard in 2001 (16); how-
to provide good-performing mixes. This has led to research ever, the use of a questionable pile is still included in a pro-
relating the consensus aggregate properties and aggregate gra- visional AASHTO Standard TP-61 (15). After all of the par-
dations to the performance of HMA and to efforts to develop ticles are sorted, the mass of each pile is determined. The
or use alternative tests. A discussion of research related to percent fractured particles are expressed as the mass of par-
each of the Superpave aggregate tests, Superpave gradation ticles having a given number of fractured faces divided by
