Civil Engineering Reference
In-Depth Information
Stress
s
Fracture
F
IGURE
8.14
Stress-strain curve for a brittle
material
Strain
ε
F
IGURE
8.15
Failure of brittle
materials
(a)
(b)
(c)
In compression the stress-strain curve for a brittle material is very similar to that in
tension except that failure occurs at a much higher value of stress; for concrete the
ratio is of the order of 10 : 1. This is thought to be due to the presence of microscopic
cracks in the material, giving rise to high stress concentrations which are more likely
to have a greater effect in reducing tensile strength than compressive strength.
The form of the fracture of brittle materials under compression is clear and visible.
For example, a cast-iron cylinder cracks on a diagonal plane as shown in Fig. 8.15(a)
while failure of a concrete cube is shown in Fig. 8.15(b) where failure planes intersect
at approximately 45
◦
along each vertical face. Figure 8.15(c) shows a typical failure of
a rectangular block of timber in compression. Failure in all these cases is due primarily
to a breakdown in shear on planes inclined to the direction of compression.
Brittle materials can suffer deterioration in hostile environments although concrete is
very durable and generally requires no maintenance. Concrete also provides a protec-
tive cover for the steel reinforcement in beams where the amount of cover depends on
the diameter of the reinforcing bars and the degree of exposure of the beam. In some
situations, e.g. in foundations, concrete is prone to chemical attack from sulphates
contained in groundwater although if these are known to be present sulphate resisting
cement can be used in the concrete.
Brick and stone are durable materials and can survive for hundreds of years as evi-
denced by the many medieval churches and Jacobean houses which still exist. There
are, of course, wide variations in durability. For example, granite is extremely hard
whereas the much softer sandstone can be worn away over periods of time by the
