Welding of Reinforcing Bars

Concrete reinforcing bars, or as they are more technically known, deformed steel reinforcing bars, are used in reinforced concrete construction. This includes buildings, bridges, highways, locks, dams, docks, piers, etc.
Welding is finding increasing importance for splicing concrete reinforcing bars. Three welding processes are used for the majority of welding splices; however, several of the other processes can be used.

Concrete reinforcing bars, or as they are more technically known, deformed steel reinforcing bars, are used in reinforced concrete construction. This includes buildings, bridges, highways, locks, dams, docks, piers, etc. The principle applications of reinforcing bars include reinforcement of columns, girders, beams, slabs, pavements, as well as precast and prestressed concrete structures.

Concrete is strong in compression and shear but is weak in tension. By using deformed steel reinforcing bars imbedded in the concrete, tensile stresses can be accommodated, thus reinforced concrete provides compression strength of concrete and tensile strength of steel.

It is necessary, however, that the concrete and steel work together. This is accomplished by means of a bond between the bar and the concrete which is achieved by means of deformations which are rolled into the bars. These deformations keep the bars from slipping through the concrete.

Concrete reinforcing bars come in different sizes which are designated by a series of numbers. The numbers assigned to bars are based on the number of 1/8 inches included in the nominal diameter. The nominal diameter of a deformed reinforcement bar is equivalent to the diameter of a plain steel bar having the same weight per foot as the deformed bar.

There are three ASTM specifications for reinforcing bars:

  • A 615, plain billet steel bars;
  • A 616, rail steel reinforcing bars; and
  • A 617, axle steel reinforcement bars.
  • All of the reinforcing bars produced in the USA are identified by markings rolled into the bar. These markings will show, by means of raised letters, the code for the manufacturer of the steel bar. This is then followed by the letter identifying the specific steel mill where the bar was produced, based on standard designations. The next symbol indicates the bar size by the bar number. The next symbol indicates the type of steel as follows: N indicates new billet steel, A indicates axle steel, and the third symbol which is a cross section of a railroad rail indicates that the bar was rerolled from used railroad rails.

    The next identification symbol is a number indicating the grade of the steel. If there is no number it normally means it is the minimum grade within the specification. Grades are also identified by a single or double continuous longitudinal line through at least five spaces offset from the center of the bar. A single line indicates the middle-strength grade and a double line indicates the highest-strength grade. It is important to determine the type of steel and the grade since this will be valuable information in establishing the welding procedure.

    The specifications do not include chemical requirements for the different classes; however, when bars are purchased from the mill, the mill will provide a chemical analysis report of the bars, if requested. The grade number is the indication of the strength of the bars and the numbers indicate the yield point in thousand pounds per square inch minimum.

    It is necessary to splice concrete reinforcing bars in all but the most simple concrete structures. In the past, splicing was done by overlapping the bars from 20 to 40 diameters and wiring them together and relying on the surrounding concrete to transmit the load from one bar to the other. This method is wasteful of the steel and is sometimes impractical. An example of this occurs when splices must be made in heavily reinforced columns. The close spacing of the bars makes lap splicing particularly difficult and often requires a larger column diameter to provide sufficient concrete between bars and covering the bars.

    Welding is thus finding increasing importance for splicing concrete reinforcing bars. Three welding processes are used for the majority of welding splices; however, several of the other processes can be used. There is a mechanical splice similar to welding which utilizes medium-strength metal cast metallic grout around the ends of the bars enclosed within a steel sleeve having internal grooves.

    The welding processes most commonly used are the shielded metal arc welding process, the gas metal arc welding process, and the thermit welding process.

    There are no chemistry requirements for the three ASTM specifications. However, the reinforcing bars of specification A 616 rail steel are produced from used railroad rails which were originally made to specification A 1. Old railroad rails are salvaged and heated and cut into three parts, the flange, the web, and the head. The heads are then rolled into the deformed reinforcing bars. ASTM specification A 1 has chemical requirements for steel rails and they contain relatively high amounts of carbon and manganese.

    The reinforcing bars made to specification A 617 are made from salvaged carbon steel axles used for railroad cars. These axles when originally produced were made to specification A 21. In this ASTM specification the carbon and manganese is relatively high. These are both considered in the hard-to-weld category of steels.

    The bars produced to A 615 have only a maximum for phosphorous content; however, based on the strength level of steels the alloy content should not be too high. For quality welding it is best to assume that they, too, are in the hard-to-weld category. If at all possible, the analysis of the reinforcing bars should be determined from mill reports. If this is not possible the bars could be analyzed for exact composition. It is recommended that the bars be considered to have a carbon equivalent of 0.75, thus in the hard-to-weld category.

    The American Welding Society has provided a specification entitled, "Reinforcing Steel Welding Code". This code provides a table of carbon equivalents which relates to the bar size and then presents recommended preheat and interpass temperature. The standard formula for the determination of carbon equivalent is used. There are six carbon equivalents which can only be calculated if the analysis of the reinforcing bars are known.

    The code also provides joint design information for making direct butt splices, for making indirect butt splices, and for making lap splices. A butt splice is a direct end-to-end splice of bars with their axis approximately in line and of approximately the same size. An indirect butt splice is one in which an intermediary piece such as a steel plate or rolled angle is used with each reinforcing bar welded directly to the same piece.

    The lap welded splice is made by overlapping the two bars alongside each other and welding together. For butt splices when the bars are in the horizontal position the single groove weld is most often used with a 45° to 60° included angle. Double groove welds can be made in the larger size bars. When the bars are to be welded with the axis vertical a single or double bevel groove weld is used with the flat side or horizontal side on the lower bar. On occasion, the reinforcing bar may need to be welded to other steel members and a variety of weld joints can be used.

    This code provides filler metal selection information based on the grade number of the steel. When welding using the shielded metal arc welding process, Grade 40, the AWS E-7018 would be recommended, for Grade 50 the AWS E-8018 would be recommended, for Grade 60 and the low-alloy A706 the AWS E-9018 electrode is recommended, and for the Grade 75 the AWS E-10018 electrode would be recommended.

    If the XXI8 is not available the XX15 or XX16 can be used. In the case of gas metal arc welding, the E-70S electrode would be used and for flux-cored arc welding the E70T type would be used when welding Grade 40 bars. There are no filler metal specifications for the higher levels of gas metal arc welding electrodes or flux-cored arc welding electrodes at this time. If these processes are to be used the filler metal would need to meet the same mechanical properties as the equivalent shielded metal arc welding electrode mentioned.

    The code also provides minimum preheat and inner-pass temperatures based on the carbon equivalent of the reinforcing bars. It also relates to the size of the bar. It is therefore important to determine the composition of the bar so that carbon equivalent can be determined. In the case of large bars and if the carbon equivalent is not known the 500° preheat would be recommended.

    The code further requires that joint welding procedures should be established based on the welding process, filler metal type and size, and welding technique, which involves position, joint detail, etc.

    Welders must be qualified. A direct butt splice or indirect butt splice specimen is used. The gas metal arc welding process will make the weld in approximately one-half the time required for shielded metal arc covered electrodes. In either case, however, or with the flux-cored arc welding process, the welds will develop strengths equal or exceeding the specified yield strength of the reinforcing steel bars.

    Welding is highly recommended as the way to splice reinforcing bars since the concrete will fail at values substantially below the yield strength of the reinforcing bars. This means that the strength of the welds will exceed the requirements for most applications. In any case, the welded splices will exceed the strength of lapped and wired splices. It will also exceed a strength level of the cast metal splices which are sufficiently strong to withstand the strength level of the reinforced concrete composite structure.

    November, 2006
    Risolvete le vostre sfide di materiali.
    Scopri come possiamo aiutarti.