Figure 1: Headed reinforcing bars.
is a series of short articles intended to shed light on some myths about concrete. A myth is something
that is widely held to be true but in fact is not; it is surprising how many have perpetuated about concrete.
The series is based on a one-hour talk by
, now-retired but formerly of
supported the firm’s structural engineering team with in-house publications, graduate training lectures, industry
liaison and a helpdesk service. It is co-authored by
hat concrete has no tensile strength
is a myth that is often perpetuated
when explaining the use of steel
reinforcement in concrete.
In fact, concrete does have a tensile strength,
which is taken as 0.3 characteristic compressive
strength, to the power of two-thirds, ±30%.
There is quite a lot of variability depending
on the aggregate type; for example, smooth,
rounded aggregates give poorer bond and
hence a lower tensile strength.
Typically, for general concrete with a
compressive strength of 30MPa, tensile
strength is about 3MPa, ie, about 10%.
When it is pointed out that concrete does
in fact have a tensile strength, a second myth
is often expressed – that the tensile strength
of concrete plays no part in design. While it is
true that the tensile strength of concrete is not
taken into account in bending calculations, it
is used in both serviceability and limit state
shear, punching shear
bond and anchorage
evaluation of the cracking moment for
the design of reinforcement to control
crack width and spacing resulting from
restrained early-age thermal contraction
the design of plain (unreinforced) sections
such as ground-bearing concrete floors,
pavements and footings
the design of fibre-reinforced concrete
the design of expanding anchors.
Tensile strength is not explicitly mentioned
in the design codes for things such as shear,
but it is used. It is ‘hidden’ in the design
codes, often by making values depend on
the characteristic strength, which helps to
perpetuate this common myth. For example,
the tensile strength of concrete needs to be
used when designing for headed reinforcing
bars (see Figure 1).
Concrete has no tensile strength –
and it plays no part in design
What is the tensile
strength of concrete?
From BS EN 1992-1-1, Table 3.1
Typically = 3.0MPa