Geoscience Reference
In-Depth Information
Aggregates
4
INTRODUCTION
potential alkali-silica reactivity and this would normally
be conducted following BS 812: Part 104 (British
Standards Institution, 1994). In America, ASTM C295
(ASTM International, 2008a) is the applicable standard
and this method is often specified in the Middle East. An
in-depth review of the different aggregate petrography
methods used around the world is provided by Jensen
(2007).
The simple EN 932-3 method involves examination of
a relatively small number of aggregate particles in hand
specimen, using a low-power microscope. The more
thorough BS 812: part 104 and ASTM C295 methods
involve hand separation of a large number of coarse
aggregate particles, and/or point-counting of fine
aggregate to determine the proportions of each aggregate
constituent. In addition, for the more thorough methods,
thin sections of aggregate particles are prepared for
detailed high-power microscopical examination to
confirm the hand specimen identifications. Aggregate
particles are separated out into constituent groups based
on differences in geological and engineering properties
such as rock type, degree of alteration, weathering grade,
particle shape, and surface texture.
If the microscopical examination proves insufficient to
classify aggregate constituents, further investigation may
be necessary. Mineralogical analysis by XRD is particularly
useful for determining the composition of fines that are
beyond the resolution of the optical microscope.
The term 'aggregate' covers a variety of materials used in
the construction industry. Aggregates can be defined as
'particles of rock, manufactured or recycled material which,
when brought together in a bound or unbound condition,
form part or whole of an engineering or built structure'.
The vast majority of aggregates used in construction work
are for roads or concrete structures, where they are used
unbound in the lower layers of road pavements, bitumen-
bound in the upper road layers or bound by cement in
concrete. Other applications for aggregates include mortar
sand, railway ballast, and filter media. Each end use
requires aggregate with specific properties in terms of
particle size distribution (grading), shape and surface
texture, strength, and resistance to degradation.
The principal applications of petrographic
examination to investigation of aggregates are:
• Initial suitability assessment of aggregates from new
sources.
• Routine quality assurance of aggregate from the
production run.
• Assessing the potential for alkali-aggregate
reactivity in concrete.
• Diagnosing the causes of in-service
deterioration/failure.
• Matching aggregate types for restoration of mortar
in historic buildings.
PETROGRAPHIC EXAMINATION AND
COMPLEMENTARY TECHNIQUES
AGGREGATE TYPE
The 'aggregate type' depends on the type of resource it is
won from and the processing that the material undergoes
to make the aggregate product. Aggregate type should
be described as follows (adapted from Fookes et al .,
2001):
1. Whether natural or artificial.
2. If natural, whether crushed rock, gravel, or sand.
3. If a gravel or sand, whether uncrushed, partly
crushed, or crushed.
4. If a gravel or sand, whether land won or marine.
5. If recycled, this should be stated.
Petrographic examination of aggregate is usually
conducted following methods intended for the
petrographic examination of concrete aggregates. In
Europe, the standard for concrete aggregate (EN 12620)
requires that petrographic description be conducted at
least once every 3 years in accordance with the EN 932-3
method (British Standards Institution, 1997). EN 932-3
is a simplified method intended only for general
classification of aggregate. However, a more thorough
assessment is often required, for example, to determine
 
 
 
 
 
 
 
Search WWH ::




Custom Search