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M

ǘǰ ǻǹǺȀ

concrete

13

Figure 1: Headed reinforcing bars.

Concrete

Myths

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

Stuart Alexander

, now-retired but formerly of

WSP,

who

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

RichardBarnes

of

TheConcrete Society

.

T

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

calculations for:

shear, punching shear

bond and anchorage

evaluation of the cracking moment for

prestressed beams

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

f

ctm

= 0.3

f

ck

(2/3)

±30% (for

f

ck

≤C50/60)

Typically = 3.0MPa