*www.concrete.org.uk*

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