Natural aggregates can be
naturally occurring graded material or mechanically crushed from mineral soils
or rock mass. Mineral soils consists of mainly inorganic stone types which over
the years under the influence of water, weather and pressure have been reduced
in size and formed in shape. In Finland repeated glacial periods have
significantly affected the development of soils, and different types of soils
have separated and formed layers. Because of this aggregate fractions suitable
for use in concrete production are readily available. The most common soil type
in Finland is moraine. The consumption of aggregates in concrete production is
very high, which means that sources need to be readily available. When
aggregates need to be transported from further costs will raise and options may
be limited. Especially in southern Finland it is difficult to find suitable
naturally graded fine aggregates (Suomen Betoniyhdistys, 2005a, p.32). Good
sources of suitable naturally graded aggregates are diminishing since removal
of aggregates has begun to endanger the ground water sources; for this reason
the possibility of underwater aggregate sources has been studied since 1980s
(Uudenmaan ympäristökeskus, 2006).

In Finland mainly granite
based aggregates are used and more and more often they are mechanically crushed.
Rock masses in Finland are mainly hard enough for concrete production, but
weathering of rock mass can be a problem in particular areas. Naturally graded
and crushed aggregates have slightly different properties, mainly caused by
different grain shape and surface textures. Using crushed aggregates the voids
content and the combined aggregate friction are greater than when naturally
graded aggregates with same grain size distribution are used. To achieve the
same consistency with crushed aggregates higher percentage of finer part of
combined aggregates and increase of water is usually needed. Increase of water
leads to increase of cement to achieve the same water-cement ratio. On the
other hand, the bonding between cement stone and crushed aggregates is stronger
than the one between cement stone and naturally graded aggregates; also the
impact resistance of crushed rock is greater. At least in theory this allows
higher watercement ratios to reach certain strength with crushed aggregates.
However it has to be noted that shrinkage and creep of hardened concrete depend
on the volume percentage of cement stone and water-cement ratio; these
deformations increase with higher water-cement ratios. (Suomen Betoniyhdistys,
2005a, 46.)

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