Concrete
Structural Efficiency, Space, and Form
Concrete is a unique material in that it combines significant structural strength with its process of construction. That is by pouring a relatively cheap liquid into a mould. It also has the properties of high durability and fire resistance. The compressive strength of concrete is typically one fifth that of steel and is controlled by the type and quantity of cement added to the mix. Its tensile strength is very low; however in combination with steel, the composite material can resist significant tensile and flexural forces. The construction technique of combining the wet concrete with steel reinforcing bars allows almost limitless forms to be created. Shuttering formwork complexity is the only real limit to form.
The simplicity of the construction process allows structural continuity to be easily attained. For example, the reinforced concrete flat slab forms very efficient structure, producing very high span to depth ratios, and a wonderfully elegant diffusion of the forces. There is a direct relationship between the distribution of the body forces within a structural element, for example compression, bending, etc. and the pattern and density of reinforcement. In fact, the relationship can be carefully controlled by varying the reinforcement pattern and so marshalling the body forces. Conceptually, forces will always 'flow' in the direction of the stiffest component. The stiffness of a concrete structure is determined by the proportion and position of steel relative to concrete. This tool allows the engineer to powerfully manipulate form.
Pre-stressing the element manipulates this balance further by creating initial body forces within the structural component that counteract the those induced by externally applied loads. Pre-stressing creates what might be termed a 'virtual section' when compared to a simply reinforced component.
Concrete construction can be used to create impressive self finished components; however this requires a good understanding of the construction techniques and careful control of the process. Prototyping in the form of test panels is an essential part of the process, as it ensures that the design team and contractor have an understanding of what can be achieved, and the necessary methods of construction.
