Geoscience Reference
In-Depth Information
acts as a cathode.
To estimate the risk of reinforcement corrosion in a
concrete element, one approach would be to determine
chloride content (by chemical analysis of dust or lump
samples) and the depth of carbonation (by indicator
solution or petrography). Then if the exposure
conditions are known (dry, >60% relative humidity, or
damp, 70-80% relative humidity), the charts in BRE
Digest 444 (Building Research Establishment, 2000) can
be used to estimate the risk on a scale that runs from
negligible to extremely high. For example, uncarbon-
ated concrete in a dry environment that has a chloride
content of <0.4% (by mass of cement) would be
regarded in general terms as being at negligible risk of
reinforcement corrosion.
The resulting rust occupies a larger volume than the
parent material and the expansion resulting from rusting
of reinforcement causes the surrounding concrete to
crack and spall, the bond between the concrete and
reinforcement is lost and steel reinforcement is weakened
through loss of section. Cracks caused by reinforcement
corrosion may be seen over the bar position as linear
cracks or surface spalling, or connect between bars to
delaminate the cover concrete. The cracks may be filled
by rust deposits (
195
).
Differences in electrical potential that may cause
corrosion can arise from differences in the environment
along a reinforced concrete element. These include
variations of exposure to moisture, oxygen, and salts,
differences in the depth of cover concrete, or where two
dissimilar metals are connected. Carbonation is a
common cause of reinforcement corrosion for the reasons
discussed earlier in this section. Another common cause
of corrosion is exposure to chloride ions and in concrete
these can originate from various internal or external
sources. Potential internal sources include marine
aggregates and calcium chloride accelerator (CaCl
2
). Use
of calcium chloride as an accelerator ceased in the United
Kingdom in 1997, following imposition of limits on total
chloride content in reinforced and prestressed concrete.
External sources of chlorides include sea water (and sea
spray), airborne chlorides in coastal areas, and de-icing
salts used on highways. Chloride ions cause corrosion
cells to form by activating surface locations of the steel
to form an anode, while the remaining passivated surface
C
RACKING
When reinforced concrete is designed it is assumed that
the concrete will crack owing to thermal and humidity
cycles; however, by careful design and detailing, cracks
can be controlled and crack widths limited (Metha &
Monteiro, 2006). Concrete is liable to crack for a variety
of other reasons that may affect the durability, structural
integrity, watertightness, sound transmission, and
aesthetics of the structure. Unexplained cracking is a
common reason for engineering investigations of
concrete structures.
The main causes of concrete cracking are listed in
Table 22
and further details can be found in Concrete
Society Technical Report No. 22 (1992). It is not easy to
distinguish between different crack formations. Often, a
number of laboratory tests and compilation of the
complete history of the project, including concrete
mixture design, placement condition, curing methods,
formwork removal, and loading history, is required
(Metha & Monteiro, 2006).
Petrographic examination of concrete slices and thin
sections can aid the diagnosis of the cause of cracking
and the determination of its severity. Examination (in
ultraviolet light) of samples impregnated with fluorescent
resin is used to highlight crack systems, which aids
observation of crack morphology. Crack properties are
usually described manually but may also be determined
using image analysis techniques (Litorowicz, 2005). As a
minimum the petrographer would normally describe such
crack features as width (and changes in width),
orientation, and distribution, whether the cracks occur in
aggregate or cement paste, details of the adjacent cement
paste, and the presence/identity of cracking infillings.
Cracks types may be classified based on their nominal
width (
Table 23
).
195
195
Concrete suffering from reinforcement corrosion
exhibiting a crack (yellow) lined with rust deposits (dark
brown). Fine aggregate particles appear white and the
cement matrix is shown light brown; PPT, ×35.
