Civil Engineering Reference
In-Depth Information
or protection in these areas). Here, the drip groove is placed underneath a
horizontal bar, with typically 5-10 mm cover at the top of the groove! The
author cannot count the number of times this simple error has been observed
on structures during his career!
Another wonderful example of poor durability design is the use of so-
called 'reconstituted stone' for window mullions and sills. These are made
from a semi-dry mortar mix rammed into moulds and usually contain
one or more steel rods for handling purposes. An alternative, sometimes
found, comprises a rather poorly compacted concrete core, containing the
steel rod, with a well-formed attractive stone-like, sandy coloured, mortar
facing. Both types have a tendency to carbonate rapidly and corrosion of
the steel then ensues with splitting and spalling of the units. Since these are
often in use on high-rise structures and offices, the risk of falling material
causing injury is high. Indeed, when tackled on this issue, one company,
who shall remain nameless, advised the author that carbonation was no
longer an issue, because they carbonate the units at the factory! He was of
course referring to the other problem that reconstituted stone can suffer -
flaking and crumbling of edges and corners, due to inadequate water and
compaction in the mix. Carbonating the concrete hardens the surface and
thus helps to avoid damaged edges, but compromises the durability of any
units containing steel reinforcement, unless this is galvanised or otherwise
protected.
Billions of pounds are spent each year on the repair of structures. In the
US, alone, for example, annual repair costs are estimated at 18-21 billion
USD (source: American Concrete Institute). Getting it right is therefore of
critical importance.
In the remainder of this chapter we will explore some simple rules for
avoiding problems in the first place and how non-destructive testing (NDT)
and semi-destructive tests like coring and lab analysis can pay dividends in
avoiding or diagnosing defects.
1.2 Get the concrete right
To ensure good-quality, well-finished and inherently durable concrete in the
first place, a few simple steps need to be followed.
1
The correct quality of concrete, which has been designed to achieve
an appropriate strength, durability for the exposure conditions it will
experience and 'finishability'.
2
The use of the correct type and quality of form-face material and release
agent suitable for the finish specified.
3
Workmanship, both in producing the formwork and mixing, placing,
compacting and finishing the concrete.
The concrete itself should meet the criteria given in
Table 1.1.
