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Petrographic examination opens up the possibility of a much more detailed assessment of
concrete quality than can be obtained by any other method. It involves preparing thin
sections from samples, usually from cores.
A thin section is usually made from a plate cut at right angles to the external surface
of the concrete, so that the outer 70 mm or so of the concrete are included in the section.
Sometimes it is more appropriate to make the section from inner parts of the concrete.
This might be appropriate where specific problems are being investigated for example.
The section normally measures about 50 x 70 mm.
The thin section supplies the following types of information.
- Details of the rock types present in the coarse and fine aggregate and in particular
structures seen within those rocks.
- Details of the aggregate properties are measured such as the degree of strain in quartz.
- The size, distribution and abundance of phases in the cement paste are assessed including,
for example, the occurrence of calcium hydroxide and the amount of residual unhydrated clinker.
- The presence of cement replacement phases such as slag or PFA can usually be recognised
(though the amount of these phases cannot be judged accurately). The presence of high alumina
cement can be detected and the type of cement clinker can often be assessed.
- Any products of processes of deterioration of either the cement paste or the aggregate can be recognized.
We perform these microscopic examinations to identify materials and to determine the cause and extent of deterioration of concrete.
Such deterioration may occur because of normal weathering or wear of buildings, bridges, floors, dams, etc.; because of fires; or because of chemical spills.
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