Aggregates - Geomaterials: Under the Microscope - page 58

Recycled aggregates are being used increasingly in the
current drive for sustainable construction. It is possible to
use demolition waste as engineering fill, road aggregate,
or concrete aggregate (Building Research Establishment,
1998). Recycled aggregates typically comprise crushed
concrete and/or brick masonry. Special care is required to
ensure that they do not also contain potentially
deleterious contaminants such as glass, bituminous
material, timber, gypsum, metal, plastic, and clay lumps.
The approximate composition of recycled aggregate can
be determined by hand separation. Detailed investi-
gations to determine the nature of the constituents
should be conducted using petrographic examination.
One of the key quality parameters in assessing the likely
performance of recycled concrete aggregate is the
quantity of cement mortar (cement and fine aggregate)
that remains on the surface of the coarse aggregate.
Abbas et al . (2007) have developed a petrographic/image
analysis method to measure this.

and fine aggregate <4.75 mm size. An 'all-in' aggregate
consists of a mixture of coarse and fine aggregates.
The particle size distribution of aggregate (also known
as aggregate 'grading') has an important influence on its
properties as a construction material. The grading is
definitively classified by sieve testing but the
petrographer will need to estimate the grading as part of
petrographic examination. Aggregate grading may be
estimated by visual comparison with the chart provided
in Figure 130 . It should be noted that the standard
terminology used for aggregate grading in the
construction industry differs from geological
classification of sediments, with a well graded aggregate
corresponding to moderately sorted sediment. Figure 131
shows a well graded fine aggregate, while Figure 132
shows a uniformly graded fine aggregate.
Aggregate particles within a particular size fraction
may exhibit a variety of different shapes, which affects
the workability and particle interlock of aggregate.
Surface textures of aggregate particles are important in
determining workability and adhesion to binders.
Aggregate particle shape and surface texture may be
classified using the chart in Figure 133 . In addition to
the terms used in Figure 133 , other terms appropriate for
use when describing surface texture include glassy,
smooth, granular, rough, crystalline, honeycombed, and
porous.
Good concrete aggregate has rounded particles, to
slide over each other and make concrete more workable

AGGREGATE GRADING, SHAPE, AND
SURFACE TEXTURE

Aggregates are divided into coarse aggregate and fine
aggregate on the basis of particle size. The European
standard (British Standards Institution, 2002a) defines
coarse aggregate being >4 mm size and fine aggregate
<4 mm size. The American standard (ASTM International,
2008b) defines coarse aggregate being >4.75 mm size

130

130 Comparison
chart for
classification of
aggregate particle
grading by visual
assessment
(Ingham, 2005).

CONSTRUCTION CLASSIFICATION

Uniformly graded Moderately well graded Well graded Poorly graded

Very well sorted Well sorted Moderately sorted Poorly sorted

GEOLOGICAL CLASSIFICATION