Civil Engineering Reference
In-Depth Information
1
Understanding defects, testing
and inspection
1.1 Introduction
Concrete, when it first came into use, was hailed as being long-lasting
and largely maintenance free. The need for extensive repairs to concrete
structures was hardly considered. And, with appropriate design of the
concrete, with due recognition of the exposure conditions and a few simple
rules in designing the mix and some simple pre-service testing, there is no
reason why extended lifetimes for RC structures cannot be enjoyed. However,
experience tells us that reinforced concrete is often not the maintenance-free
material that some people expect and early failure sometimes ensues. It is
worth exploring the reasons for this before proceeding.
Far too often, the basic design of the concrete and attention to detail in its
placement are lacking. In the author's experience, many concrete mixes are
not correctly designed and suffer a tendency to bleed and segregate, all of
which renders the protection that should be afforded to the steel reinforcement
compromised from the start.
Add to that incorrect placement of the steel, with inadequate cover and you
have a recipe for early failure, whatever the exposure conditions. In severe
conditions, with exposure to marine or de-icing salt, for example, failure can
be very rapid indeed. Concrete Society Advice Note 17 (Roberts, 2006) lists
some simple rules for good-quality surface finishes for cast-insitu concrete (see
next section). Not surprisingly, those same rules will help to provide durable
structures when combined with due attention to selection of an appropriate
concrete and cover for the exposure conditions it will experience. The author
once worked on the old Severn crossing, now replaced by the new bridge. The
concrete in this bridge was of such high quality that it had not carbonated at all
in 35 years and attempts to demonstrate a Windsor Probe device for strength
estimation, caused the pin to simply bounce off, as the concrete was so hard!
There are other reasons why premature failure can occur: in the 1960s,
for example, following pressure from the industry to improve formwork
stripping times, the cement industry responded by grinding the cement finer
and increasing the tricalcium silicate content (C3S) in the cement. Readymix
producers were quick to spot that it was now possible to achieve the 28-day
