Geoscience Reference
In-Depth Information
50,000 buldings in the United Kingdom alone (Rushton,
2006). These must all be subject to investigations and
monitoring to ensure satisfactory performance. In general
terms, HAC concrete investigations comprise identification
of members containing HAC, strength appraisal, and
durability assessment. This has created a demand for
petrographic examination of lump samples taken from HAC
concrete beams (typically sampling 5% of all beams
present) to provide definitive identification of cement type
and assess a number of parameters affecting durability.
Optical microscopy is the most popular definitive
method of identifying the presence of HAC in concrete.
Unhydrated HAC clinker consists mainly of calcium
aluminates with much smaller amounts of ferrite and
silicate phases. Once hydrated, HAC consists largely of
calcium aluminate hydrates. In cross-polarized light in thin
section, the appearance of hydrated HAC is largely
isotropic, if uncarbonated, and light brown birefringence,
if carbonated.
232
shows a comparison of the general
microscopical appearance of HAC concrete and Portland
cement concrete from a structure where one was cast
directly against the other. In plane-polarized light many
residual grains of unhydrated and partially hydrated clinker
are usually seen, which are often opaque or red (
233
).
These phases cannot be identified with certainty in thin
section, although ferrites are usually assumed to be one of
the constituents (Hewlett, 1998). One characteristic
constituent that can be identified is 'pleochroite', which is
fibrous and has distinctive blue to white pleochroism (
234
).
In hand specimen, the HAC used in British structural
concrete was usually dark greyish-brown in colour.
Although colour may suggest that HAC is present it should
not be used as a basis for identification as certain Portland-
type cements may also exhibit dark colours. Calcium
232
232
Precast HAC concrete (left) with a carbonated
HAC matrix (brown) and in situPortland cement
(right) with an uncarbonated Portland cement matrix
(black); XPT, ×35.
233
233
Close view of HAC concrete illustrating the
appearance of the cement matrix; PPT, ×150.
