CHAPTER 1

Properties of
reinforced
concrete

CHAPTER INTRODUCTION

Reinforced concrete is a strong durable building material that can be formed into
many varied shapes and sizes ranging from a simple rectangular column, to a slender
curved dome or she" . Its utility and versatility are achieved by combining the best
features of concrete and steel. Consider some of the widely differing properties of
these two materials that are listed below.

Concrete Steel
strength in tension poor good
strength in compression good good, but slender bars will buckle
strength in shear fair good
durability good corrodes if unprotected
fire resistance good poor - suffers rapid loss of strength at
high temperiltures

It can be seen from this list that the materials are more or less complementary. Thus,
when they are combined, the steel is able to provide the tensile strength and probably
some of the shear strength while the concrete, strong in compression, protects the
steel to give durability and fire resistance. This chapter can present only a brief
introduction to the basic properties of concrete and its steel reinforcement. For a more
comprehensive study, it is recommended that reference should be made to the
specialised texts listed in Further Reading at the end of the book.

W. H. Mosley et al., Reinforced Concrete Design

1.1 Composite action

The tensile strength of concrete is only about 10 per cent of the compressive strength.
Because of this, nearly all reinforced concrete structures are designed on the assumption
that the concrete does not resist any tensile forces. Reinforcement is designed to carry
these tensile forces, which are transferred by bond between the interface of the two
materials. If this bond is not adequate, the reinforcing bars will just slip within the
concrete and there will not be a composite action. Thus members should be detailed
so that the concrete can be well compacted around the reinforcement during
construction. In addition, some bars are ribbed or twisted so that there is an extra
mechanical grip.
In the analysis and design of the composite reinforced concrete section, it is assumed
that there is a perfect bond, so that the strain in the reinforcement is identical to the
strain in the adjacent concrete. This ensures that there is what is known as 'compatibility
of strains' across the cross-section of the member.
The coefficients of thermal expansion for steel and for concrete are of the order of
10 x 10-6 per °C and 7-12 x 10-6 per °C respectively. These values are sufficiently
close that problems with bond seldom arise from differential expansion between the two
materials over normal temperature ranges.
Figure 1.1 illustrates the behaviour of a simply supported beam subjected to bending
and shows the position of steel reinforcement to resist the tensile forces, while the
compression forces in the top of the beam are carried by the concrete.

A
Figure 1.1 load
Composite action

Compression

Tension
LfD
Strain
Distribution
Section A·A

Reinforcement
A

Wherever tension occurs it is likely that cracking of the concrete will take place. This
cracking, however, does not detract from the safety of the structure provided there is
good reinforcement bond to ensure that the cracks are restrained from opening so that
the embedded steel continues to be protected from corrosion.
When the compressive or shearing forces exceed the strength of the concrete, then
steel reinforcement must again be provided, but in these cases it is only required to
supplement the load-carrying capacity of the concrete. For example, compression
reinforcement is generally required in a column, where it takes the form of vertical bars
spaced near the perimeter. To prevent these bars buckling, steel binders are used to
assist the restraint provided by the surrounding concrete.