Introduction
Modern cementitious concretes have been produced since the 18th century. Concrete is our most widely used man-made material at 1.5m3/person annually. As a result, the production of concrete accounts for more than 5% of all CO2 produced.
Concrete’s widespread availability, ease of use, formability and low cost have ensured its enduring use for a great variety of structures. Its surface finish and texture may be varied widely by the use of appropriate formwork, making it amenable to an extensive range of architectural styles.
In general, it is concrete’s compressive strength which is used in structural calculations; the tensile strength of concrete is typically only around 1/10th of its compressive strength and is often ignored in calculations. This lack of tensile capacity is overcome through the use of tensile reinforcement within the concrete member, allowing the development of elegant long span concrete structures, thin shell structures and robust retaining walls.
Components of concrete
Concrete is a heterogeneous material formed from combining cement, fine and coarse aggregates and water. Admixtures may also be added to enhance properties either during placing or curing. The quality of each ingredient affects the properties of both the fresh and cured concrete mixtures.
Cement
Within the UK, cement is classed as either a pure Portland cement (CEM-1) manufactured from limestone and clay, or a blended cement (CEM-2) manufactured from Portland cements and a number of additional ‘pozzolans’. These are generally by-products from coal-fired power stations, such as pulverized fuel ash (PFA), or by-products from steelmaking, such as ground granulated blast-furnace slag (GGBS). The inclusion of pozzolans reduces the cost and embodied energy of cement. As global cement production accounts for more than 5% of all CO2 emissions, there is considerable research into the development of low carbon alternatives.
Aggregate
The shape and texture of aggregate has significant impact on its bond strength with the cement paste. Crushed angular aggregate is preferred over rounded smooth aggregate when strength is required. In some circumstances, round smooth aggregate may be required to improve workability at low water–cement (W:C) ratios. Both coarse and fine aggregates are essential to minimise the volume of cement needed to fill the voids. Recycled aggregates, such as crushed brick and concrete, have received much interest in recent years as a means of lowering the embodied energy of concrete. However, care should be taken when using such aggregates as they can absorb much of the water required to hydrate the cement.
Water
The chemical make-up of the water used during the hydration process can have a significant effect on the final cured concrete. In general the water should be potable and free of suspended particulates. The use of sea water and highly saline water will lead to enhanced degradation of any ferrous rebar, as well as reducing the durability of the cured concrete.
Admixtures
There are a great many admixtures which can be mixed with fresh concrete in order to change particular properties of either wet or cured concrete. Admixtures should only be used in small, carefully controlled quantities. Typical admixtures and their effects include:
• Air entraining agents. These form microscopic air bubbles in the wet concrete helping it to flow, and reducing density.
• Retarding agents slow the hydration process.
• Accelerating agents speed up the hydration process, but may cause excess heat build-up and thermal cracking.
• Pigments allow the colour of the cured concrete to be altered.
• Water reducers allow for a reduction in the W:C ratio, providing a high-strength and fully hydrated concrete.
• Plasticisers can greatly increase the workability of low W:C ratio mixtures, allowing such mixtures to be pumped with ease.
Mechanical properties
Due to its heterogeneous nature, concrete can exhibit a wide range of properties depending on its constituents. For design purposes, concrete specification is based on its strength class, which directly relates to its characteristic compressive strength after 28 days’ curing. For example, a C30/37 concrete has a characteristic cylinder strength of 30MPa and a characteristic cube strength of 37MPa. (These results are obtained from standard test procedures). The response of a typical concrete, say C30/37, under uniaxial compressive stress. The stress-strain relationship is initially linear, with the material behaving elastically. Upon further increase in strain, the material develops nonlinear behaviour, reaching a peak strength at a strain of 0.2%. Failure occurs at a strain of around 0.35%. It should be noted, that as the compressive strength of concrete increases, the failure becomes more brittle. As the cementitious matrix is prone to creep, the exact shape of the curve is dependent on the rate of loading.
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