CEMENT: MANS MIRACLE MIX
1.One of man's oldest building materials is finding its way into a lot of new places these days. Concrete, first discovered by the Romans, is now more widely used in construction than all other materials together.
2. The magic ingredient that makes concrete possible is cement, about which, according to one expert, more has been learnt in the past three decades than in the preceding 2,000 years. Concrete is a synthetic stone which can be formed while soft into practically any shape the builder wants. Portland cement mixed with water is the paste that binds sand, gravel, clinker into an artificial rock that becomes harder as the years pass. Portland cement does not come from a place of that name; it was called Portland because Joseph Aspdin, the English builder who invented the first dependable, scientifically made cement about 1824, thought it resembled the rock excavated on the Isle of Portland on the Dorset Coast.
3. What's so new about cement after all these years? Several things. One item is squeezed concrete, known technically as pre-stressed concrete. By giving concrete a big squeeze after it has hardened, builders can increase its elasticity ten times, so that it will bend under a heavy load without breaking. This is important in building bridges, viaducts, and floors of large buildings. The simplest way to pre-stress concrete is to put steel wires or rods in the concrete when it is poured.
4. An unusual American use of reinforced concrete is the floating highway bridge across Lake Washington. The depth of the lake made piers too expensive, so engineers built the bridge on hollow concrete pontoons anchored in place by steel cables. It is the longest pontoon bridge structure on earth.
5. Until recently, the aim of engineers was to make concrete with as few bubbles as possible. Now they have come up with a new concrete that has millions of microscopic bubbles per cubic foot. It is made by adding an agent which foams to form the bubbles when the concrete is mixed. This concrete doesn't crack when freezing. The first "air bubble" roads were built many years ago. They have stood up under winter freezes so well that today this concrete is used for new road construction.
6. Another discovery is "soil cement". Several years ago road builders lacking funds found that they could mix cement with soil on the site of the road, wet it and compact it, then cover it with a bitumen. The first road they built is still carrying traffic. There are miles of soil-cement secondary roads and streets in the US today. Construction goes so fast that with modern equipment a road builder can complete a mile of soil cement road in one day.
7. Scientists are working on research into the behaviour or cement and concrete under all kinds of conditions. Collaborating with engineers they are developing new ways of using concrete. Cement is changing the face of the Earth.
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How is the cement with microscopic bubbles made?
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PRESTRESSED CONCRETE
1. It was not until the beginning of the twentieth century and the beginning of the present century that reinforced concrete really came into its own as a structural material with a great variety of uses. Great advances have been made in the knowledge of its behaviour in structures and much experience has been gained in its use so that today it is deservedly a material of importance.
2. The outstanding characteristic of concrete is that it can be manufactured readily and cast in a mould to any desired shape. It can be said to combine strength with plasticity. The addition of reinforcement or the application of the more recent technique of prestressing adds to its strength and does not decrease its plasticity.
3. Concrete has a much higher strength when it is under compression, than it has when it is subjected to tension. In fact its tensile strength is only one-tenth of its compressive strength. Its ability to sustain shearing forces is also frequently inadequate. The purpose of reinforcing is to provide a material with a high tensile strength which can be cast in the concrete in such a way that it is able to take the tensile and shear stresses which would otherwise have to be taken by the concrete, and at the same time allows full advantage to be taken of the concrete's high compressive strength.
4. Prestressed concrete has captured the imagination of many designers for several reasons. It enables lighter construction than ordinary reinforced work. This means that not only is less material required in prestressed members, but that reduced weight of the members makes for lighter columns and foundations. Less steel is required than in ordinary reinforced concrete. The lightness of prestressed concrete also makes it particularly useful for precast work. It is not subject to cracking, so that there is little danger of corrosion of prestressing wires, and the concrete will not weather easily.
5.Prestressed concrete depends for its reliability on first-class control of the concrete aggregates and mix proportions, the placing of the concrete, the quality of the prestressing wires or bars and the application of the precise degree of pre-stress to the wires. These requirements make it particularly suitable for factory production.
6. The idea of prestressing is an extremely simple one. If one has a material that has no tensile strength it will fracture immediately it is called on to take tension. If, however, the material is given an initial compression, then, when the tension force is applied the material will be able to withstand the tension provided it does not exceed the initial compression.
7. The great merit of prestressed concrete is that, in giving to the concrete itself an effective tensile strength, it enables all the concrete to contribute to the strength of the member, unlike reinforced work where only the concrete in compression is contributing.
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On what does prestressed concrete depend for its reliability?
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