F. K. Kong MA, MSc, PhD, CEng, FICE, FIStructE, R. H. Evans CBE, DSc, D ès Sc, DTech, PhD, CEng, FICE, FIMechE, FIStructE (auth.)-Reinforced and Prestressed Concrete-Springer US (1987)

Durability of concrete 39
(a) an upper limit to the w/c ratio (see Table 2.5-7);
(b) a lower limit to the cement content (see Table 2.5-7);
(c) a lower limit to the concrete cover to reinforcement (see Table
2.5-7);
(d) good compaction; and
(e) adequate curing.
The durability of concrete is intimately related to its permeability, which
term refers to the ease with which water can pass through the concrete. A
low permeability makes the concrete better able to withstand the effects of
weathering, including the effects of driving rain and the disruptive action of
freezing and thawing. It is stated in Section 2.5(a), in connection with the
strength of concrete, that the evaporable water and the bubbles of trapped
air occupy spaces called voids. The permeability of concrete ( cf. its
strength) increases rapidly with the amount of voids. As expected,
therefore, the permeability increases rapidly with the w/c ratio and,
broadly speaking, factors which increase the strength of the concrete would
reduce its permeability and improve its durability: low w/c ratio, good
compaction and adequate curing. Indeed, the authors' own experience
with permeability is that when the absorption of water in a sample of
concrete exceeds 7%, there is bound to be corrosion of the reinforcement.
Corrosion of reinforcement can seriously affect the service life of a
concrete structure [20, 21 ]. The mechanisms of reinforcement corrosion
are explained in an ACI Committee report [23], which also gives guidance
on the protective measures for new concrete construction, on the
procedures for identifying corrosive environments and active corrosion in
concrete, and on remedial measures. Broadly speaking, the factors which
reduce the permeability of the concrete (and protect the reinforcement
from the ingress of external moisture) will help to inhibit reinforcement
corrosion: low w/c ratio, good compaction, adequate curing and an
adequate concrete cover. The chloride contents of the concrete must be
held down, marine aggregate must be washed and sea-water should not be
used in concrete making.
Experience has shown that too low a cement content makes it more
difficult to obtain a durable concrete. Hence it is often considered desirable
in practice to specify a minimum cement content (see Table 2.5-7). It
Table 2.5-7 Durability requirements (BS 8110: Clause 3.3.3)
Exposure
condition
Nominal cover
(mm)
Mild
Moderate
Severe
Very severe
25
20
35
20
30
40
50
20
20
25
30
Maximum free w/c ratio
Minimum cement content (kg/m3)
Concrete feu (N/mm 2 )
0.65
275
30
0.60
300
35
0.55
325
40
0.45
400
50