A material can be subjected to many mechanical stresses: shearing, bending, torsion, and others. But two of them, tension and compression, are fundamental, for the others can often be expressed in terms of these two.
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For example, when you walk over a beam forming a bridge, it will bend under your weight
¾as shown exaggeratedly in the figure below. But when you look at the beam more closely, it is obvious that this bending stress is resolvable into the two fundamental stresses: The top layers of the beam are under compression as their grains press against each other to squeeze onto the shorter curve forming the inside of the bend; the bottom layers, to the contrary, have their grains pulled apart as they are drawn out along the longer outside curve of the bend. Somewhere between these two layers of extreme tension or compression lies the neutral layer (it goes though the cross section's center of gravity, actually), which is under no stress at all.The reason why this is interesting is that most materials have far higher strengths in compression than in tension. (The strength of a material is the stress under which it is deformed beyond its range of elasticity, that is, beyond the range within which it returns to its original dimensions when the stress is removed.) And though one may not be able to change the strength of a material directly (it depends on its chemical composition and on its physical microstructure), one can arrange for it to be stressed under the type of stress for which its strength is greatest.
Concrete is a material that is enormously strong under compression, but relatively weak under tension. The statement holds even for reinforced concrete, which contains the metal skeleton over which it was poured before it set. However, you can arrange for concrete to be stressed exclusively by compression by "pre-stressing" it.
To see this, consider another "material" that has great strength in compression, but none in tension: a stack of books (piled vertically on the table). You would need a hydraulic press to deform it permanently by pressure; yet by tension it is ruined immediately: the top book comes off with no effort at all. But you can make the stack firm
¾ firm enough to hold together horizontally¾by taking it between your hands and pressing, or "prestressing" it. Any stress in tension will have to overcome the compression by your hands before it becomes effective.With concrete, it works like this: The concrete is compressed by metal bars or wires called "tendons" that pull it together. The tendons are stretched, for example, by heating them (electrically) and anchoring them to the ends of the concrete beam while the tendons are hot. As they cool and try to shrink, they will compress the concrete.
For a beam subjected to bending (as in our bridge example), this will add compression to all of its layers, so that the top layer is more compressed than before, but the stress on the bottom layer is now under the pre-stress of compression minus the original stress in tension: a net result of compression for reasonable loads. The neutral layer gets pushed out into thin air by simply adding a hefty plus to what used to go from plus (compression) to minus (tension) as you go down through the beam's cross section (see upper figure on p. 1).
It is also essentially tensile stresses that make concrete crack; prestressed concrete will not crack under normal loads, and if it does crack under overloads, the cracks will usually close when the load is removed.
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Vol. 11, No. 3
Newsletter: Access to Energy Newsletter Archive Volume: Issues Issue/No.: Vol. 11, No. 3 Date: November 29, 2004 11:13 AM Title: The racists recoil
Copyright © 2004 - Access to Energy Newsletter Archive
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