Civil Engineering Reference
In-Depth Information
High-alumina cement (HAC)
HAC achieved some notoriety during the 1970s following the collapse of
several buildings in which it had been used (Building Research Establishment,
1974). This was due to a conversion of the cement from one crystalline
form into another, weaker, form. At normal temperatures, the hydration
of HAC results in the formation of hydrated calcium monoaluminate
(CAH10). Smaller amounts of C2AH8 and hydrous alumina are also
formed. However, these hydrated calcium aluminates are metastable and
can, at higher temperatures and in the presence of moisture, change to give
the stable hydrated calcium aluminate C3AH6. This phenomenon is known
as 'conversion', and the amount of the change occurring, 'the degree of
conversion'.
At normal temperatures, conversion may take many years but at
temperatures in excess of 40°C a considerable amount of conversion can
occur within a few months. Structures built with HAC are likely to have fully
converted now and petrography and X-ray diffraction analysis are considered
to be the best means of examination, with the degree of conversion test
having fallen into disuse. Petrography should be accompanied by some
means of strength evaluation and/or load testing.
Conversion results in a loss of strength, increased porosity and reduced
resistance to chemical attack. There has been increasing concern regarding
carbonation of high-alumina cement concrete. Following conversion, the
increased porosity may permit rapid carbonation of the concrete, removing
alkaline protection to the steel reinforcement, which may then suffer from
corrosion.
T
EST
METHODS
A test was devised by the Building Research Station to show whether HAC
was likely to be present in a concrete (BRE, 1974). It essentially tests for a
significant content of soluble aluminium in solution, following extraction
with dilute sodium hydroxide solution.
1.5 Petrographic examination (see also
Chapter 2)
Preliminary examination
The samples are examined with the binocular microscope as received and
their dimensions and main features are recorded. The features observed
include the following:
a
The presence and position of reinforcement
b
The extent to which reinforcement is corroded
c
The nature of the external surfaces of the concrete
d
The features and distribution of macro and fine cracks
e
The distribution and size range and type of the aggregate
