Posted by Manuel Oliveros on April 14, 2003 at 11:04:52:
In Reply to: Steel fixing ties. posted by Gareth Guiver on April 10, 2003 at 12:39:28:
Well, Gareth, if I understand you well you are talking about the bond between reinforcement bars and concrete, and how the steel bars are made to be suitable for such purpose.
If such is yoru question, twisting bars to babricate reinforcement is almost a forgotten practice in most developed countries. In fact most reinforcement bars, usually abbreviated as rebar, are now what is called deformed rebar, that is, the protrusions are imprinted upon the fabrication process.
The twisted bars were more likely than not of square section, and this combined with the quite "flat" spiral surfaces even after twisting were not conducive to good bond, which is an important requirement of reinforced concrete, since it is bond ultimately assured by the mechanical interlock that the protrusions generate what warrants good transfer of the stresses from the concrete to the steel reinforcement, and hence a behaviour that is conform to the intent of reinforced concrete practice.
Since the protrusions are imprinted the steel is not dependent in being cold drawn or twisted to attain its wanted behaviour, what is considered preferable, since it preserves the natural ductility that corresponds to the particular alloy of steel used in the fabrication, and hence its ductility.
Drawing the bars (or more commonly wires) is notwithstanding a practice very common to attain the biggest stresses available from steels, but this is normally only for wire, strand or tendon to be used with prestressed concrete. The called High Strength bars that are used with reinforced concrete are lately just fabricated but not drawn. Other high strength bars (of maybe bigger strength) are used as well as prestressing material, normally for specific applications such strirrups or to ensure proper connection at prestressed joints, or in memebers of not too big dimensions (say 40 ft maximum) since this the usual length in stock, and more requires joints.
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