An investigation of high-strength, deformed steel bars for concrete reinforcement

INTRODUCTION High strength steel as reinforcement for concrete has received some serious attention by researchers and by the construction industries as long ago as the early 1900\u27 s. Recent developments abroad, notably in Sweden, Germany and Austria, have caused renewed interest in the use of this material in concrete structures. The term high-strength is somewhat ambiguous since it usually has been employed to describe the type of steel used in cables and strands for prestressed concrete or suspension bridge structures. Such steels commonly have proof stresses of the order of 200,000 psi. However, in the case of ordinary reinforced concrete structures the term high-strength steel is used to designate steels having yield stresses in excess of 50,000 psi. In this report the latter definition will be used. There are two general methods for producing a high-strength steel. One is by metallurgical means such as alloying of the steel with small amounts of such elements as: Silicon, Phosphorous, Nickel, Chromium, Manganese or Molybdenum and/or raising the carbon content. The other method of achieving a high-strength steel is by cold working an ordinary grade of steel. The usual methods of cold working reinforcing bars are by cold stretching or by cold twisting between two fixed chucks. The cold-twisted bar is at present the more common of the two types of cold-worked bars. Several companies both in the U. S. and abroad manufacture cold-twisted bars which appear under such commercial trade names as: Isteg Webrib , Torstahl , and Tentor to mention some of the better known ones. A metallurgically-produced, high-strength steel or alloy steel shows several differences as compared to cold-worked steel. Firstly, alloy steels can be produced which have stress-strain curves with sharp, definite yield points whereas cold-worked steels are almost always of the gradual-yielding type. Secondly, alloy steels can be produced which maintain a customary relation of yield stress to ultirnate stress (i.e. yield stress approximately sixty percent of ultimate stress); whereas, cold-working a steel simply raises the ratio of yield stress to ultimate stress without affecting the ultimate stress significantly. Thirdly, some alloy steels have fairly low ductilities thus making them difficult to cold bend. Finally, they may be difficult to weld by ordinary techniques under field conditions. Considerable information is available, particularly in the foreign literature, on test behavior of members reinforced with cold-worked, high-strength steel bars but little data is available on metallurgically-produced high-strength steel reinforcement. In view of the differences between these two types of high-strength bars, a sizeable investigation utilizing metallurgically-produced, high-strength bars as concrete reinforcement appeared advisable