Because concrete is weak in tension, it is reinforced with steel bars or wires that resist the tensile stresses. The most common types of reinforcement for nonprestressed members are hot-rolled deformed bars and wire fabric. In this book, only the former will be used in examples, although the design principles apply with very few exceptions to members rein- forced with welded wire mesh or cold-worked deformed bars.
The ACI Code requires that reinforcement be steel bars or steel wires. Significant modifications to the design process are required if materials such as fiber-reinforced-plastic (FRP) rods are used for reinforcement because such materials are brittle and do not have the ductility assumed in the derivation of design procedures for concrete reinforced with steel bars. In addition, special attention must be given to the anchorage of FRP reinforcement.
Hot-Rolled Deformed Bars Grades, Types, and Sizes
Steel reinforcing bars are basically round in cross section, with lugs or deformations rolled into the surface to aid in anchoring the bars in the concrete (Fig.3-30). They are produced according to the following ASTM specifications, which specify certain dimensions and certain chemical and mechanical properties.
1. ASTM A 615:Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement.This specification covers the most commonly used rein- forcing bars. They are available in sizes 3 to 18 in Grade 60 (yield strength of 60 ksi) plus sizes 3 to 6 in Grade 40 and sizes 6 to 18 in Grade 75. The specified mechanical properties are summarized in Table3-3. The diameters, areas, and weights are listed in Table A-1 in Appendix A. The phosphorus content is limited to percent.
2. ASTM A 706:Standard Specification for Low-Alloy Steel Deformed and Plain Bars for Concrete Reinforcement.This specification covers bars intended for special appli- cations where weldability, bendability, or ductility is important. As indicated in Table 3-3, the A 706 specification requires a larger elongation at failure and a more stringent bend test than A 615. ACI Code Section 21.2.5.1 requires the use of A 615 bars meeting special re- quirements or A 706 bars in seismic applications. There is both a lower and an upper limit on the yield strength. A 706 limits the amounts of carbon, manganese, phosphorus, sulfur,
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TABLE 3-3 Summary of Mechanical Properties of Reinforcing Bars from ASTM A 615 and ASTM A 706
Billet-Steel Low-Alloy Steel,
A 615 A 706
Grade 40 Grade 60 Grade 75 Grade 60 Minimum tensile strength, psi 70,000 90,000 100,000 80,000
Minimum yield strength, psi 40,000 60,000 75,000 60,000
Maximum yield strength, psi — — — 78,000
Minimum elongation in 8-in.
gauge length, percent
No. 3 11 9 — 14
No. 4 and 5 12 9 — 14
No. 6 12 9 7 14
No. 7 and 8 — 8 7 12
No. 9, 10, and 11 — 7 6 12
No. 14 and 18 — 7 6 10
Pin diameter for bend test, where
No. 3, 4, and 5 3.5d 3.5d — 3d
No. 6 5d 5d 5d 4d
No. 7 and 8 — 5d 5d 4d
No. 9, 10, and 11 — 7d 7d 6d
No. 14 and 18 — 9d 9d 8d
But not more than 1.25 times the actual yield.
Bend tests are 180°, except that 90° bends are permitted for No. 14 and 18 A 615 bars.
b a
d = nominal bar diameter
b
a Main ribs
Letter or symbol for producing mill Bar size #11
Type steel*
S Billet-steel (A 615) I Rail-steel (A 996) R Rail-steel (A 996) A Axle-steel (A 996) W Low-Alloy steel (A 706)
Grade mark Grade line (one line only)
*Bars marked with an S and W meet both A 615 and A 706
H 11
S
H 11
S 60
(a) Grade 60
Main ribs Letter or symbol for producing mill Bar size #36
Type steel*
S Billet-steel (A 615M) I Rail-steel (A 996M) R Rail-steel (A 996M) A Axle-steel (A 996M) W Low-Alloy steel (A 706M)
Grade mark Grade line (one line only)
*Bars marked with an S and W meet both A 615 and A 706
H
S 36
H
S 4 36
(b) Grade 420 Fig. 3-30
Standard reinforcing-bar markings. (Courtesy of Concrete Reinforcing Steel Institute.)
and silicon and limits the carbon equivalent to percent. These bars are available in sizes 3 through 18 in Grade 60.
3. ASTM A 996: Standard Specification for Rail-Steel and Axle-Steel Deformed Bars for Concrete Reinforcement.This specification covers bars rolled from discarded rail- road rails or from discarded train car axles. It is less ductile and less bendable than A 615
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steel. Only Type R rail-steel bars with R rolled into the bar are permitted by the ACI Code.
These bars are not widely available.
Reinforcing bars are available in four grades, with yield strengths at 40, 50, 60, and 75 ksi, referred to as Grades 40, 50, 60, and 75, respectively. Grade 60 is the steel most commonly used in buildings and bridges. Other grades may not be available in some areas.
Grade 75 is used in large columns. Grade 40 is the most ductile, followed by Grades 60, 75, and 50, in that order.
Grade-60 deformed reinforcing bars are available in the 11 sizes listed in Table A-1.
The sizes are referred to by their nominal diameter expressed in eighths of an inch. Thus, a No. 4 bar has a diameter of (or ). The nominal cross-sectional area can be comput- ed directly from the nominal diameter, except for that of the No. 10 and larger bars, which have diameters slightly larger than and so on. Size and grade marks are rolled into the bars for identification purposes, as shown in Fig.3-30. Grade-40 bars are available only in sizes 3 through 6. Grade-75 steel is available only in sizes 6 to 18.
ASTM A 615 and A 706 also specify metric (SI) bar sizes. They are available in 11 sizes. Each is the same as an existing inch–pound bar size but is referred to by its nominal diameter in whole millimeters. The sizes are #10, #13, #16, #19, #22, #25, #29, #32, #36,
#43, and #57, corresponding to the nominal diameters 10 mm, 13 mm, 16 mm, and so on.
The nominal diameters of metric reinforcement are the traditional U.S. Customary unit diameters— in. (9.5 mm), in. (12.7 mm), in. (15.9 mm), and so on—rounded to the nearest whole millimeter. The bar size designation will often include an “M ” to denote a metric size bar. The diameters, areas, and weights of SI bar sizes are listed in Table A-1M in Appendix A.
ASTM A 615 defines three grades of metric reinforcing bars: Grades 300, 420, and 520, having specified yield strengths of 300, 420, and 520 MPa, respectively.
For the review of the strength of existing buildings the yield strength of the bars must be known. Prior to the late 1960s, reinforcing bars were available in structural, intermediate, and hardgrades with specified yield strengths of 33 ksi, 40 ksi, and 50 ksi (228 MPa, 276 MPa, and 345 MPa), respectively. Reinforcing bars were available in inch–pound sizes 3 to 11, 14, and 18. For sizes 3 to 8, the size number was the nominal diameter of the bar in eighths of an inch, and the cross-sectional areas were computed directly from this diameter. For sizes 9 to 18, the diameters were selected to give the same areas as previously used square bars, and the size numbers were approximately equal to the diameter in eighths of an inch. In the 1970s, the 33-ksi and 50-ksi bars were dropped, and a new 60-ksi yield strength was introduced.
Mechanical Properties
Idealized stress–strain relationships are given in Fig. 3-31 for Grade-40, and rein- forcing bars, and for welded-wire fabric. The initial tangent modulus of elasticity, for all reinforcing bars can be taken as Grade-40 bars display a pronounced yield plateau, as shown in Fig. 3-31. Although this plateau is generally present for Grade-60 bars, it is typically much shorter. High-strength bars generally do not have a well-defined yield point.
Figure3-32 is a histogram of mill-test yield strengths of Grade-60 reinforcement having a nominal yield strength of 60 ksi. As shown in this figure, there is a considerable variation in yield strength, with about 10 percent of the tests having a yield strength equal to or greater than 80 ksi—133 percent of the nominal yield strength. The coefficient of variation of the yield strengths plotted in Fig. 3-32 is 9.3 percent.
ASTM specifications base the yield strength on mill teststhat are carried out at a high rate of loading. For the slow loading rates associated with dead loads or for many live
29 * 106 psi.
Es, -75 -60,
5 8 4
8 3
8
10
8 in.,118 in.,
1 2in.
4 8in.
Fig. 3-31
Stress–strain curves for reinforcement.
Fig. 3-32
Distribution of mill-test yield strengths for Grade-60 steel.
(From [3-72].)
loads, the static yield strengthis applicable. This is roughly 4 ksi less than the mill-test yield strength [3-72].
Fatigue Strength
Some reinforced concrete elements, such as bridge decks, are subjected to a large number of loading cycles. In such cases, the reinforcement may fail in fatigue. Fatigue failures of the reinforcement will occur only if one or both of the extreme stresses in the stress cycle is tensile. The relationship between the range of stress, and the number of cycles is shown in Fig.3-33. For practical purposes, there is a fatigue threshold or endurance limit below which fatigue failures will normally not occur. For straight ASTM A 615 bars, this is about 24 ksi and is essentially the same for Grade-40 and Grade-60 bars. If there are
Sr,
fewer than 20,000 cycles, fatigue will not be a problem with deformed-bar reinforcement.
The fatigue strength of deformed bars decreases:
(a) as the stress range (the maximum tensile stress in a cycle minus the algebraic minimum stress) increases,
(b) as the level of the lower (less tensile) stress in the cycle is reduced, and (c) as the ratio of the radius of the fillet at the base of the deformation lugs to the height of the lugs is decreased. The fatigue strength is essentially independent of the yield strength.
(d) In the vicinity of welds or bends, fatigue failures may occur if the stress range exceeds 10 ksi. Further guidance is given in [3-74].
For design, the following rules can be applied: If the deformed reinforcement in a particular member is subjected to 1 million or more cycles involving tensile stresses, or a combination of tension and compression stresses, fatigue failures may occur if the differ- ence between the maximum and minimum stresses under the repeated loading exceeds 20 ksi.
Strength at High Temperatures Deformed-steel reinforcement subjected to high tem- peratures in fires tends to lose some of its strength, as shown in Fig.3-34 [3-54]. When the temperature of the reinforcement exceeds about 850°F, both the yield and ultimate strengths drop significantly. One of the functions of concrete cover on reinforcement is to prevent the reinforcement from getting hot enough to lose strength.
Welded-Wire Reinforcement
Welded-wire reinforcement is a prefabricated reinforcement consisting of smooth or deformed wires welded together in square or rectangular grids. Sheets of wires are welded in electric-resistance welding machines in a production line. This type of reinforcement is Fig. 3-33
Test data on fatigue of deformed bars from a single U.S. manufacturer.
(From [3-73].)
used in pavements, walls or slabs where relatively regular reinforcement patterns are pos- sible. The ability to place a large amount of reinforcement with a minimum of work makes welded-wire fabric economical.
The wire for welded-wire fabric is produced in accordance with the following speci- fications: ASTM A82 Standard Specification for Steel Wire, Plain, for Concrete Rein- forcement, and ASTM A496 Standard Specification for Steel Wire, Deformed, for Concrete Reinforcement. The deformations are typically two or more lines of indentations of about 4 to 5 percent of the bar diameter, rolled into the wire surface. As a result, the de- formations on wires are less pronounced than on deformed bars. Wire sizes range from about 0.125 in. diameter to 0.625 in. diameter and are referred to as W or D, for plain or deformed wires, respectively, followed by a number that corresponds to the cross-section- al area of the wire in approximately increments. Thus a W2 wire is a smooth wire with a cross-sectional area of ACI Code Section 3.5.3.5 does not allow wires smaller than size D4. Diameters and areas of typical wire sizes are given in Table A-2a.
Welded-wire fabric satisfies the following specifications: ASTM A185 Standard Specification for Steel Welded Wire Reinforcement, Plain, for Concrete, and ASTM A497 Standard Specification for Steel Welded Wire Reinforcement, Deformed, for Concrete.
Deformed welded-wire fabric may contain some smooth wires in either direction. Welded- wire fabric is available in standard or custom patterns, referred to by a style designation (such as ). The numbers in the style designation refer to: spacing of longitu- dinal of transverse wires—size of longitudinal of transverse wires. Thus a fabric has W4 wires at 6 in. on centers each way. Areas and weights of common welded-wire fabric patterns are given in Table A-2b.
Welded smooth-wire fabric depends on the crosswires to provide a mechanical anchorage with the concrete, while welded deformed-wire fabric utilizes both the wire deformations and the crosswires for bond and anchorage. In smooth wires, two crosswires are needed to mechanically anchor the bar for its yield strength.
The minimum yield and tensile strength of smooth wire for wire fabric is 65 ksi and 75ksi. For deformed wires, the minimum yield and tensile strengths are 70 ksi and 80 ksi.
According to ASTM A497, these yield strengths are measured at a strain of 0.5 percent.
6 * 6 – W4 * W4
wires * size wires * spacing
6 * 6 — W4 * W4
0.06 in.2 0.03-in.2 Fig. 3-34
Strength of reinforcing steels at high temperatures.
(From [3-54].)
ACI Code Sections 3.5.3.6 and 3.5.3.7 define the yield strength of both smooth and deformed wires as 60 ksi, except that if the yield strength at a strain of 0.35 percent has been measured, that value can be used.
The elongation at failure decreases as the wire size decreases, because the cold-working process used in drawing the small-diameter wires strain-hardens the steel. Reference [3-75]
quotes tests indicating that the mean elongation at failure ranges from about 1.25 percent for W1.4 wires (0.133 in. diameter) to about 6 percent for W31 wires (0.628 in. diameter). These are smaller than the elongations at failure of reinforcing bars, given in Table 3-3, which range from 6 to 14 percent. There is no ACI Code limitation on minimum elongation at failure in tension tests. If it is assumed that 3 percent elongation is adequate for moment redistribution in structures reinforced with A 615 bars, wires of sizes W8.5 or D8.5 (0.328 in. diameter) or larg- er should have adequate ductility. References [3-76] and [3-77] describe tests in which weld- ed-wire fabric showed adequate ductility for use as stirrups or joint ties in members tested under cyclic loads.