Properties of concrete - Concrete Mix Design, Quality Control and Specification - page 94

The theory is that water vapour from the interior will be unable to escape
and will cause explosive spalling. This may seem unlikely considering the
self-desiccation referred to earlier, but in fact chemically combined water
can be driven off. Nylon or polypylene fibres introduced to the mix melt
and provide an escape path for moisture. Generally, structures fail in a fire
more due to a failure to protect the steel than from deterioration of the
concrete, so lightweight aggregate concrete, providing better thermal insu-
lation, will show an improved result.
5.2.2 Tensile strength
Concrete is relatively weak in tension, which is the reason for the use of
reinforcement in most concrete. Cracking in concrete will occur when the
stress exceeds the tensile strength and therefore this property is specified
in different applications. Indirect tensile tests or “Brazil splitting test”
is the most common procedure and can give quite consistent results in a
good laboratory. Direct tensile tests, on the other hand, are difficult to
conduct without causing eccentric stresses, which results in high variability
and unrealistically low values. As with many other test procedures, the
frequency of tensile strength testing is generally significantly less than com-
pressive strength testing and may not be conducted by the premix company
for in-house quality control. Therefore production quality control should
be based on an established relationship between the compressive strength
and tensile strength using the proposed materials.
5.2.3 Flexural strength
Flexural strength or modulus of rupture is an important property,
particularly in pavements where it is often specified. Flexural strength is
generally measured by three-point loading of a beam. Flexural strength
can be more affected by changes in aggregate properties than compressive
strength. A good laboratory can achieve low variability results that can be
used directly for quality control but often its relationship with compressive
strength would be used for quality control by the premix company.
5.2.4 Modulus of elasticity
Modulus of elasticity gives an indication of the stress-strain behaviour
of concrete. The Young's modulus of elasticity is calculated from the
linear part of the stress-strain curve or the initial tangent (Neville, 2011).
There are some situations where it is good for the concrete to have a
low modulus such as roads or dams where strains caused by settlement
can be accommodated without cracking. Many super tall structures are