Structural Materials - Concrete

Saturday, December 18, 2010 , Posted by HB at 2:02 AM

 

Concrete, which is a composite of stone fragments (aggregate) and cement binder, may be regarded as a kind of artificial masonry because it has similar properties to stone and brick (high density, moderate compressive strength, minimal tensile strength). It is made by mixing together dry cement and aggregate in suitable proportions and then adding water, which causes the cement to hydrolyse and subsequently the whole mixture to set and harden to form a substance with stone-like qualities.

 

Plain, unreinforced concrete has similar properties to masonry and so the constraints on its use are the same as those which apply to masonry. The most spectacular plain concrete structures are also the earliest – the massive vaulted buildings of Roman antiquity.

 

Concrete has one considerable advantage over stone, which is that it is available in semi-liquid form during the building process and this has three important consequences. Firstly,it means that other materials can be incorporated into it easily to augment its properties. The most important of these is steel in the form of thin reinforcing bars which give the resulting composite material (reinforced concrete) (Pic. 1) tensile and therefore bending strength as well as compressive strength. Secondly, the availability of concrete in liquid form allows it to be cast into a wide variety of shapes. Thirdly, the casting process allows very effective connections to be provided between elements and the resulting structural continuity greatly enhances the efficiency of the structure.

 

 

steel reinforcing bars

Pic. 1. In reinforced concrete, steel reinforcing bars are
positioned in locations where tensile stress occurs

 

 

Reinforced concrete possesses tensile as well as compressive strength and is therefore suitable for all types of structural element including those which carry bending-type loads. It is also a reasonably strong material.Concrete can therefore be used in structural configurations such as the skeleton frame for which a strong material is required and the resulting elements are reasonably slender. It can also be used to make long-span structures and high, multi-storey structures.

 

Although concrete can be moulded into complicated shapes, relatively simple shapes are normally favoured for reasons of economy in construction (Pic. 2). The majority of reinforced concrete structures are therefore post-and-beam arrangements of straight beams and columns, with simple solid rectangular or circular cross-sections,supporting plane slabs of constant thickness.The formwork in which such structures are cast is simple to make and assemble and therefore inexpensive, and can be re-used repeatedly in the same building. These non-form-active arrangements are relatively inefficient but are satisfactory where the spans are short (up to 6 m). Where longer spans are required more efficient ‘improved’ types of cross-section and profile are adopted. The range of possibilities is large due to the mould ability of the material. Commonly used examples are coffered slabs and tapered beam profiles.

 

 

Despite the mouldability of the material

Pic. 2. Despite the mouldability of the material,
reinforced concrete structures normally have a relatively
simple form so as to economise on construction costs. A
typical arrangement for a multi-storey framework is shown

 

 

The mouldability of concrete also makes possible the use of complex shapes and the inherent properties of the material are such that practically any shape is possible.Reinforced concrete has therefore been used for a very wide range of structural geometries.Examples of structures in which this has been exploited are the Willis, Faber and Dumas building, where the mouldability of concrete and the level of structural continuity which it makes possible were used to produce a multi-storey structure of irregularly curved plan with floors which cantilevered beyond the perimeter columns,and the Lloyd’s Building, in London, in which an exposed concrete frame was given great prominence and detailed to express the structural nature of its function. The buildings of Richard Meier and Peter Eisenman are also examples of structures in which the innate properties of reinforced concrete have been well exploited.

 

Sometimes the geometries which are adopted for concrete structures are selected for their high efficiency. Form-active shells for which reinforced concrete is ideally suited are examples of this (see Fig. 1.4). The efficiency of these is very high and spans of 100m and more have been achieved with shells a few tens of millimetres in thickness. In other cases the high levels of structural continuity have made possible the creation of sculptured building forms which, though they may be expressive of architectural meanings, are not particularly sensible from a structural point of view. A well-known example of this is the roof of the chapel at Ronchamp by Le Corbusier, in which a highly individual and inefficient structural form is executed in reinforced concrete. Another example is the Vitra Design Museum by Frank Gehry. It would have been impossible to make these forms in any other structural material.

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