Civil Engineering Reference
In-Depth Information
designer must understand if he is to make durable structures, safely and economically.
The principal problem introduced by reinforcement is the corrosion of the steel leading
to the deterioration of concrete structures, and to their need for maintenance.
Modern designers appear to have forgotten that concrete in compression does not
need to be reinforced, and that leaving out the steel not only saves money but improves
durability. The concrete side walls of the Byker tunnel, for instance, are not reinforced,
Figures 17.6 and 17.7. However, there is no doubt a great future for reinforcement
that does not corrode.
3.3 General principles of reinforced concrete
3.3.1
The composite material
Concrete is an artifi cial stone. Like stone, it is strong in compression, but weak and
brittle in tension. Also like stone, its strength in tension may be further compromised
by fi ssures or cracks. Thus, unreinforced concrete can only be used in circumstances
when the material is in compression, or when the tensile stresses are very low.
Embedding steel bars in the concrete creates a composite material, where the concrete
provides the strength in compression, while the bars carry any tensile forces.
3.3.2
Concrete
The strength of concrete in compression is measured by crushing standard samples.
In the UK these samples are in the form of 150 mm cubes, while on the continent of
Europe and in the USA they are in the form of cylinders of 150 mm diameter and
300 mm height. In general, the stress at which a cylinder fails is approximately 85 per
cent of the strength of a cube made of the same material, although the difference is
less the stronger the concrete [2]. As the strength of concrete increases with time,
it is conventionally determined at an age of 28 days after casting, although the 90
day strength is also relevant. The degree to which the strength increases with time
depends on the nature of the cement. For instance, concrete made with blended cement
including a proportion of pulverised fuel ash (PFA), increases in strength more than
that of unblended Portland cement. This enhanced strength is signifi cant in establishing
the true factor of safety of a structure.
The strength of a concrete test specimen depends on the rate at which it is loaded,
the faster the application of the load the higher the strength. Thus tests for concrete
need to specify a rate of loading [3].
Concrete used in bridges typically has a 28 day cube strength of between 40 MPa and
60 MPa. Strengths up to 100 MPa have been used exceptionally while even stronger
concretes, up to 150 MPa and beyond, are still the subject of research involving the
construction of some trial structures.
Concrete is not an elastic material. As the stress/strain curve does not have a straight
portion, one cannot defi ne a unique Young's modulus for concrete and different codes
of practice use different defi nitions. The most common defi nitions are the tangent
modulus, which is the slope of the curve at one point and the secant modulus which is
the slope of the line that connects the origin to a point on the curve, Figure 3.2
(a) [4].
If the concrete is loaded by a pulse, it will exhibit a dynamic modulus that is similar
to the initial tangent modulus [5]. The Young's modulus of concrete increases with its
