Geoscience Reference
In-Depth Information
H
IGH
-
ALUMINA CEMENT
(HAC)
CONCRETE
HAC is a type of calcium aluminate cement, which is
manufactured by fusing limestone and bauxite, rather
than the limestone and clay/shale used for Portland
cement. HAC was used in structural concrete mainly from
the 1950s to the early 1970s, most commonly in the
manufacture of precast, prestressed concrete beams.
Although more expensive than Portland cement, HAC was
popular in the precast industry for its high early strength
that provided higher production rates from moulds.
Unfortunately, in the early 1970s several structural
failures occurred and it was determined that HAC
undergoes a natural and inevitable reaction known as
'conversion'. Conversion comprises the mineralogical
change of the HAC paste from metastable hydrates to
their stable form over a period of years/decades. This
results in HAC concrete with increased porosity and
significantly lower strength. Consequently, by 1976 HAC
was effectively banned for use in structural concrete and
it remains so today. Calcium aluminate cements are still
used for specialist products such as grouts, repair
mortars, and refractory products (Concrete Society, 1997).
A considerable amount of the existing building stock
contains structural HAC concrete elements, for example
discolouration of aggregate), 500°C (cement matrix
becomes wholly birefringent) (
230
), 600°C (
-
quartz transition), 800°C (calcination of limestone), and
1200°C (first signs of melting).
Figure
231
shows some microscopical features that
may be observed in fire-damaged concrete (example
adapted from Smart, 1999). Some aggregate particles
have been reddened indicating that the concrete has
reached at least 300°C at that point. Particles of flint have
been calcined and so have been heated to 250-450°C.
The cement matrix is bisected by numerous fine cracks,
some of which radiate from quartz grains in the fine
aggregate fraction. This deep cracking and cracking
associated with quartz suggest that the concrete has
reached 550-575°C. Overall we can deduce that the
concrete has been heated to approximately 600°C in the
area represented by the sample.
By determining the position of thermal contours
through the cross-section of a concrete element, an
estimate can also be made of the likely condition of
reinforcement bars. At 200-400°C prestressed steel shows
considerable loss of strength, at >450°C cold-worked
steel loses residual strength, and at >600°C hot-rolled
steel loses residual strength.
α
- to
β
230
231
230
Fire-damaged concrete showing anisotropic
properties and yellow-beige colour of the cement
matrix, indicating heating to 500°C; XPT, ×35.
231
Fire-damaged concrete showing both reddened
(red) and calcined (brown mottled) flint aggregate
particles. There are numerous fine cracks (white)
within the cement matrix (dark), some of which
radiate from quartz fine aggregate (white); PPT, ×35.
