Geoscience Reference
In-Depth Information
aluminate cements presently manufactured exhibit colours
ranging from white, grey, buff or black (Hewlett, 1998).
The strength of HAC concrete is normally determined
directly by compressive strength testing of core samples.
As all of structural HAC concrete beams in the UK are
more than 30 years old, they are now assumed to be fully
converted and to have reached their stable minimum
strength. Therefore, assessment of the degree of
conversion is rarely required; if necessary this is best
estimated by differential thermal analysis (DTA).
The main durability issues affecting HAC concrete
beams are reinforcement corrosion and chemical attack
(Dunster, 2002b). Typically these would be investigated by
optical microscopical examination in thin section. The
likelihood of carbonation-induced reinforcement corrosion
is determined by measuring the depth of carbonation
microscopically (and determining whether carbonation has
reached the prestresssing wires of the beam). The
carbonation reaction of HAC is different to that of Portland
cement, with calcium aluminate hydrates reacting with
carbon dioxide from the atmosphere to form a mixture of
calcium carbonate (chiefly calcite) and aluminium
hydroxide (chiefly gibbsite). Figure
235
illustrates the
contrasting appearance of carbonated and uncarbonated
HAC matrix. For the pre-1976 HAC concrete found in the
UK, the shallow depth of cover concrete (typically 20 mm)
is usually fully carbonated with alkaline protection of the
prestressing wires being lost.
Where HAC concrete has been exposed to prolonged
wet conditions it may be subjected to deleterious
chemical attack. The two forms of chemical attack
applicable to HAC concrete are sulfate attack and alkaline
hydrolysis. These are detected by a combination of
deterioration seen with optical microscopy and high
234
234
Very close view of HAC concrete showing grains
of pleochorite (centre right, blue/black striped and
centre left, white/black striped); PPT, ×600.
235
235
HAC concrete with the left half appearing
carbonated (light brown cement matrix) and the right
half appearing uncarbonated (dark brown cement
matrix); XPT, ×150.
