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Reinforced Concrete

Reinforced Concrete

The reinforced concrete is a wonder material used extensively throughout the world. Reinforced concrete must be well designed and contructed to provide structural strength and particularly to withstand earthquakes and other natural disasters. Here we look at the placement of the concrete and steel and how these give structural strength. We also look at simple steps to make sure it is well designed and constructed.

Cement, Concrete and Reinforced Concrete

We start with cement, we mix it with sand, small stones and water to give us concrete and finally we with combine this with steel bars which gives us the most useful and widely used building material in the world - reinforced concrete.

Reinforced concrete combines the rigidity of concrete with the strength of steel.

Reinforced concrete is pretty clever stuff. It was invented by a bloke from Newcastle Upon Tyne in the 1850's. It allows us to do all sorts of things in a constructional sort of way that without it we couldn't.

Concrete on it's own is hard and brittle, you can't crush or bend it but it does have a tendency to crack. It has very high compressive strength but low shear strength. It doesn't cope very well when you try to stretch it. It is very stable it does no rot or rust.

Steel on the other hand has high shear and tensile strength, Unfortunately in thin sections it bends relatively easily and it rusts.

The combination of steel and concrete however gives us a material that is far stronger and more useful than the sum of the individual components. By embedding steel rods into concrete the concrete and steel support each other. The concrete prevents the rods of steel from bending while the steel stops the concrete from cracking or shearing off. More than this the steel holds the concrete in its strongest compressed state while the concrete stops the steel from rusting.

This very useful material has revolutionised building and, incidentally, the creation of mindboggling statues we see at roundabouts and road junction.

Reinforced concrete must be designed and constructed correctly.

Unfortunately there are two fundamental drawbacks in the use of reinforced concrete. Firstly the combination of steel and concrete must be properly designed by structural engineers if it is to be used for the important structural members in a building or a bridge. Secondly construction has to be carried out to certain standards if it is to be expected to perform correctly.

Unfortunately in a country where construction standards are neither known nor understood by the majority of people working in the building industry building a house can be a bit of a hit and miss affair. The problem is not assisted by unethical or technically inadequate contractors who wish to save a bit of money on cement and steel with little understanding of the consequences.

Thios was evident in the people of the Yogyakarta area who built their houses with cement using ten parts of sand to one of cement when it should have been two parts of sand to one of the magic grey powder. “Steel in columns? Oh we don't need to bother with that.” We know the rest, 400,000 houses fell down, many on people's heads!

Anyone building any sort of serious structure needs to keep a watchful eye on the process and get some independent advice if in doubt.

Structural engineering design makes rigid structures

The structure of most buildings consists of vertical columns and horizontal beams that form a rigid, boxlike framework for the building. This framework provides the fundamental strength of the building. Walls, roofs, floors, etc. are all supported on this structure.

It is important to get this right. The structure should be designed by a civil engineer who will consider the shape and size of the building and very importantly the ground that it is on. Rock is generally a good solid foundation but if the land is unstable then the structure must be designed to withstand movement.

Earthquake resistance

Here in Bali it is only a matter of time before there is a sizeable earthquake. Solid rock will transfer sharp jolts to a building while the people of Bantul near Yogyakarta told me of seeing 50cm high waves coming across the soft pliable ground of paddy fields.

The engineer must design accordingly and the structural drawings should clearly indicate the dimensions of the concrete sections and the size and spacing of the steel reinforcing bars. The drawings should be signed by the engineer.

Construction must be supervised to make sure it is carried out properly

The steel used in the actual construction should match the drawing specifications. There are few tricks to watch for here, steel is expensive in Indonesia and contractors will work hard at reducing the amount of steel used. Remember that once the concrete is in place you can't see the steel, you don't know what is in there.

Reading the drawings, checking the structure and seeing through the games played by dodgy builders you are far better to leave to engineers, get a good one and make sure he performs. I recently came across a project where the clients had engaged a structural engineer they trusted, he has a good reputation and they left him to it. Unfortunately he obviously was not present when a bunch of cowboys cast the concrete around steel rods that were less than half the diameter specified.

To protect against dodgy practice it is standard convention in Britain and Australia that the engineer is present when the concrete is being poured to make sure the steel is properly installed. I don't know about America but let us hope their building standards are better than their mortgage lending processes.

Concrete must be thick enough to stop the steel rusting

Remember that the concrete must provide protection to the steel by insulating it from the atmosphere to stop it rusting. To do this the steel should have a covering of 50mms of dense concrete to prevent oxygen penetration this means that the steel reinforcing must be correctly placed to ensure sufficient cover.

Cast in one piece to avoid joints in concrete

Finally the concrete must be cast all in one piece to make sure there are no cracks or weaknesses in the structure. Joints between different batches of concrete can allow oxygen to enter and corrode the steel and can also provide lines of weakness where the structure can crack.

Well designed and built reinforced concrete is important for building structures and is in fact the most important component of any building. Most other things can be fixed fairly easily but structural weakness can be a serious matter. and render a building dangerous. Don't take risks and if you are in doubt get some independent advice.

Reinforced concrete is particularly important for swimming pools but we'll look at that another day.

Copyright © Phil Wilson October 2008
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