Civil Engineering Reference
In-Depth Information
Curing, if carried out, would have been by damp sand or hes-
sian. Mixes of 1:2:4 achieving an allowable strength in com-
pression of 3000 psi were the order of the day.
Today a cubic metre of concrete might contain a wide variety of
cements of which there are now 90 types manufactured by British
Cement Association (BCA) companies. Cements may be blended
with pulverised fuel ash (PFA) and/or ground granulated blast
furnace slag (GGBS) and/or microsilica. There should be suffi-
cient water for hydration. The reinforcement might be a mix of
mild steel, high tensile with plain or deformed cross-sections. The
reinforcement might also be stainless steel or possibly epoxy-
coated or galvanised. Alternatively, reinforcement may be by way
of carefully controlled doses of steel and/or polymer fibres evenly
distributed throughout the mix. Various additives to improve
workability and/or to accelerate/retard strength gain might be pre-
sent and the mix might be air-entrained (too much air entrainment
might lower the strength of the concrete). Most mix materials
would be weigh-batched. It is also likely that the concrete would
be delivered to the site ready-mixed and possibly pumped into
position. Compaction could be achieved by sophisticated vibra-
tion techniques. Curing would most likely be by use of a sprayed
chemical membrane. Characteristic strengths can vary between
2 N/mm
2
(for no-fines concrete) to well in excess of 100 N/mm
2
.
The increased complexity brings with it many benefits but also
more chance of error and loss of long-term durability.
Concrete is specified in grades to BS 8500 and BS EN206.
Standard grades vary from C25/30 to C50/60 where the numeric
symbol is the cylinder strength and cube strength respectively in
N/mm
2
. Nominal cover to reinforcement usually varies between
25 mm and 60 mm. Recommended values are available for
C40/50 concrete made with ordinary Portland cement (OPC)
as being satisfactory for a 50 year life. Steel bar reinforcement
is usually high tensile deformed bar to BS 4449:2005 with a
characteristic yield strength of 500 N/mm
2
. Bars are classified
H6 to H40 being 28 mm
2
to 1257 mm
2
in cross-sectional area
respectively. For slab reinforcement high tensile fabric (to BS
4483:2005) is also readily available in sheet or roll format.
Dr George Somerville in his 1986 IStructE award-winning
paper has argued that the four essentials for good reinforced
concrete are special attention to the four Cs: Constituents,
Compaction, Cover and Curing.
A range of special cements is available; these include sul-
phate resisting cement (SCPC) in which the tri-calcium alu-
minate content is controlled to a low level. However, it has
been shown that the resistance to the thaumasite form of sul-
phate attack may not be sufficiently controlled by SCPC in
cool ground conditions. For further advice, readers are referred
to DETR (1999).
In some countries, there is a variety of hydraulic cements
available for other special purposes such as those used to off-
set cracking due to shrinkage, those used for work in high
temperatures and those that are finely ground in which the
constituents are selected to react early with water, and those
for specialist use in rendering, plastering and masonry work.
In 1973, the roof of the assembly hall at the Camden School
for Girls collapsed. In 1974, a similar collapse occurred over
the swimming pool at Sir John Cass's Foundation & Redcoat
School in Stepney, East London. Investigations revealed that
the use of high alumina cement (HAC) in the precast pre-
stressed concrete beams to these roof structures was the prin-
cipal cause of collapse. Concrete made using HAC may be
subject to conversion causing a large loss of strength. The use
of this cement in concrete was of considerable advantage to
manufacturers because it gained high early strength thus enab-
ling formwork to be struck early and immediately reused. As a
result of these disasters the use of HAC for structural purposes
is now banned under the Building Regulations.
Concrete, including reinforced concrete, subjected to atmos-
pheric conditions also incurs carbonation. When carbon diox-
ide in the air combines with rainwater it forms carbonic acid.
The alkalinity of the protective concrete of cover to reinforcing
steel is reduced by the carbonic acid so that water and oxygen
attack and corrode the steel. This neutralisation is known as
carbonation. The rate at which it proceeds from the surface
depends on a number of factors such as porosity and type of
cement. One authority has quoted that carbonation proceeds at
a rate of 5-10 mm every 10 years.
Other potential defects include alkali silica reaction (ASR).
This reaction requires the presence of
a high alkali cement,
■
a reactive aggregate and
■
moisture.
■
This problem has been identified in at least 50 countries around
the world.
When damaging ASR is present, the concrete cracks (often
with an Isle of Man symbolic three-legged appearance) and,
in the most severe cases, will require demolition and replace-
ment of the structure. In less severe cases it may be possible
to lengthen the life of the structure by removing the source of
the water. Such structures should then be subjected to regular
monitoring to check on the efficacy of the remedial measures.
It is important that those involved in repair and refurbish-
ment of structures recognise the many changes that have taken
place in the development of concrete and the need to under-
stand the contemporary environment in which the structure
under consideration was designed and constructed.
14.6.1 Performance in fire
Well designed and constructed reinforced concrete has good
inherent resistance to fire. BS 8110-1 states that
A structure or element required to have fire resistance should
be designed to possess an appropriate degree of resistance to
flame penetration, heat transmission and collapse.
BS 8110-2 gives recommendations for cover to reinforcement
based on element shape and mix constituents. It also allows
