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)

40 Properties of structural concrete
should also be pointed out that, while a higher cement content usually
improves the quality of the concrete, this is no longer so if the cement
content exceeds a certain limit. Too high a cement content increases the
risk of cracking due to drying shrinkage in thin sections or to thermal
stresses in thicker sections. It is therefore necessary to specify a maximum
cement content; BS 8110: Clause 6.2.4.1 recommends that the cement
content should not exceed 550 kg/m 3 .
Comments
(a) In Table 2.5-7, nominal cover is the term used by BS 8110 to mean
the design depth of cover, measured from the concrete surface to the
outermost surface of ALL steel reinforcement, including links. It is
the dimension used in the design and indicated on the drawings. The
nominal cover should not be less than:
(1) the amount shown in Table 2.5-7;
(2) the nominal maximum size of the aggregate; and
(3) (for main bars) the bar size.
(b) The exposure conditions in Table 2.5-7 are defined in BS 8110:
Clause 3.3.4.1. Broadly, the environment condition is:
(1) Mild: for concrete surfaces protected against weather, e.g.
indoors.
(2) Moderate: for concrete surfaces sheltered from severe rain or
freezing while wet, and for concrete surfaces in contact with
non-aggressive soil.
(3) Severe: for concrete surfaces exposed to severe rain or
alternating wetting and drying or occasional freezing.
(4) Very severe: for concrete surfaces exposed to sea-water spray,
de-icing salts or freezing conditions while wet.
2.5(f) Failure criteria for concrete
The concept of failure under a multiaxial stress state was introduced earlier
when the difference between cube and cylinder strengths was discussed. A
three-dimensional stress state, with any combination of normal and shear
stresses, can always be reduced to an equivalent triaxial stress state of three
principal stresses
fl; fz; h
as explained, for example, in Section 11.5 of Reference 17. A particular
case of the general triaxial state is the biaxial stress state
fl; fz(f3 = 0)
in which only two of the three principal stresses are non-zero. Another
particular case is the uniaxial stress state
ft(fz = h = 0)
with only one non-zero principal stress. A universal failure criterion for
concrete which allows for all possible states of triaxial stresses has not yet
been found, though some proposals have been made for design purposes,