Civil Engineering Reference
In-Depth Information
Micro-silica and other additives can block pores or reduce pore sizes, thus
reducing carbonation. However, such materials can also use up some of the
reservoir of calcium hydroxide and reduce the possible buffer to carbonation.
Carbonation is common in old structures, poorly-built structures and
reconstituted stone elements containing reinforcement that often have a low
cement content and are very porous. Conversely, well-built structures such
as bridges are often much less prone to carbonation (although unfortunately,
historically, not to chloride ingress).
Carbonation is easy to detect and measure. A pH indicator, usually
phenolphthalein in a solution of water and alcohol, will detect the change
in pH across a freshly exposed concrete face. Phenolphthalein changes from
colourless at low pH (carbonated zone) to pink at high pH (uncarbonated
concrete). Measurements can be taken on concrete cores, fragments and by
breaking the bridge between twin drilled holes used for chloride sampling.
Care must be taken to prevent dust or water from contaminating the surface
to be measured, but the test, with the indicator sprayed on to the surface, is
cheap and simple. Petrographic examination can provide further information
and is capable of detecting carbonation down cracks that phenolphthalein
may not detect. A detailed report on petrographic examination is available
from the Concrete Society (Concrete Society, 2010). As a cautionary
note, phenolphthalein is now regarded as carcinogenic, and appropriate
precautions to avoid skin contact and inhalation of spray must be taken.
Chlorides in concrete
Chloride salts, when present in reinforced concrete, can cause very severe
corrosion of the steel reinforcement. Chlorides can originate from two main
sources:
a
'Internal' chloride, i.e. chloride added to the concrete at the time of
mixing. In this category, calcium chloride accelerating admixtures,
contamination of aggregates and the use of sea water or other saline
contaminated water are included.
b
'External' chloride, i.e. chloride ingressing into the concrete post-
hardening. This category includes both de-icing salt as applied to many
highway structures and marine salt, either directly from sea water in
structures such as piers or in the form of air-borne salt spray in structures
adjacent to the coast.
The effect of chloride salts depends to some extent on the method
of addition. If the chloride is present at the time of mixing, the calcium
aluminate (C3A) phase of the cement will react with the chloride to some
extent, chemically binding it as calcium chloroaluminate. In this form, the
chloride is insoluble in the pore fluid and is not available to take part in
damaging corrosion reactions. The ability of the cement to complex the
