Civil Engineering Reference
In-Depth Information
f
The type and condition of the cement paste
g
Any evidence of deleterious processes affecting the concrete.
Polished surfaces
A plate is cut, where possible, from each sample. This is typically about 20
mm thick and usually provides as large a section of the sample as is possible.
The plate is polished to give a high quality surface that can be examined
with a high quality binocular microscope or even with the petrological
microscope if necessary. The polished plate is used to assess the following:
• The size, shape and distribution of coarse and fine aggregate.
• The coherence, colour and porosity of the cement paste.
• The distribution, size, shape and content of voids.
• The composition of the concrete in terms of the volume proportions of
coarse aggregate, fine aggregate, paste and void.
• The distribution of fine cracks and microcracks. Often the surface
is stained with a penetrative dye, so that these cracks can be seen.
Microcrack frequency is measured along lines of traverse across the
surface.
•
The relative abundance of rock types in the coarse aggregate.
Thin sections
A thin section is prepared for each sample as appropriate. The 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 ×
70 mm.
In manufacturing the thin section, a plate some 10 mm thick is cut
from the sample. This is impregnated with a penetrative resin containing
a yellow fluorescent dye. The resin penetrates into cracks, microcracks
and capillary pores in the sample. One side of the impregnated plate is
then polished and the plate is mounted on to a glass slide. The surplus
sample is then removed and the plate is ground and polished to give a
final thickness of between 20 and 30 micrometres. At all stages the cutting
and grinding is carried out using an oil-based coolant in order to prevent
further hydrating of the cement and excessive heating of the section. The
thin section is covered and then examined with a high quality petrological
photomicroscope (
Figure 1.25).
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.
