Draft proposal of standards for five kinds of high strength re-bars, USD685A, USD685B, USD980, USD785 and USD1275, were presented to the Reinforce- ment Committee in the last year of the 5-year project. The full text of these proposed standards was not published for five years after the conclusion of the project conforming to the cooperatives research contract, that is, until March, 1998.
The specified values in these proposed standards were all confirmed to be sufficiently manufacturable by trial manufacture during five years of the New RC project. Thus these standards are accompanied with good amount of practical experience. At present these new brands of re-bars do not necessarily circulate in the market, but they should be available to order to the steel manufactures who participated in the New RC project.
Table 3.1 summarizes the required mechanical properties of five kinds of new re-bars. First three columns, USD685A, USD685B and USD980, are re- bars that can be used for axial reinforcement of beams and columns. They are included in the Proposed Standard for High Strength Deformed Bars for Reinforced Concrete, and cover the diameter range from D10 to D51 as shown in Table 3.2. Last two columns of Table 3.1, USD785 and USD1275 are to be used exclusively for lateral reinforcement such as lateral confinement or shear reinforcement. They are included in the Proposed Standard for High Strength Deformed Bars for Lateral Reinforcement, and Table 3.3 shows the diameter ranges for bars of these two grades.
The new name of USD was adopted to clarify that the nature of these Standards is not an official standard of general nature but is a kind of self- imposed standard with the strength range exceeding that of current JIS, although it follows the JIS requirements in regard to shape and size of bars.
Current JIS G 3112 specifies SD490 as the strongest re-bar for reinforced con- crete, and USD685A is the weakest re-bar specified in the new Standards, leaving a large gap of yield strength in between to which no standard exists.
This is the consequence of concentrated and efficient effort for trial manufac- ture of new re-bars in the New RC project, where manufacture of USD685 was one of the most immediate target.
As shown partly in Table 3.2, re-bar diameters and other dimensions, and shape of surface deformation of USD685 and USD980 follow the specifications in the current JIS G 3112, Deformed Bars for Reinforced Concrete. On the other hand, specifications for USD785 and USD1275 are made to match those of PC tendon manufacturing companies who have already acquired special permission of Construction Minister to practically manufacture re-bars of corresponding strength for lateral reinforcement. The requirement for surface deformation is more liberal compared to JIS G 3112, because it is generally accepted that bond requirement for lateral reinforcement need not be as strict as for axial reinforcement.
Table 3.1. Required mechanical properties of high strength re-bars.
Yield stress*1
(MPa)
Tensile strength (MPa)
Strain at yield plateau*2
Fracture strain Yield ratio Inner radius for 90° bending*3
Range of diameter Surface deformation Major use
Grade of steel USD685A
685-785
USD685B 685-755
USD980 980 and above not specified
1.4% and above 10% and above 85% and
below
80% and below 2d
USD785 785 and above 930 and
above
USD1275 1275 and above 1420 and
above not specified 7% and
above 90% and
below id D10-D51 similar to JIS G 3112 axial reinforcement for
beams and columns
8% and above
7% and above not specified
1.5d S6-S13
2.5d H6-H13 indent or groove lateral reinforcement
%1 Yield stress is taken as 0.2% offset stress in case clear yielding is not observed.
*2See Fig. 3.28 for definition of strain at yield plateau.
*3 "d" iindicates nominal diameter of the bar.
Table 3.2. Dimensions and unit mass of USD685 and USD980.
Grade
USD685A USD685B USD980
Mark D10 D13 D16 D19 D22 D25 D29 D32 D35 D38 D41 D51
Nominal diameter m m 9.53 12.7 15.9 19.1 22.2 25.4 28.6 31.8 34.9 38.1 41.3 50.8
Nominal area m m2
71.33 126.7 198.6 286.5 387.1 506.7 642.4 794.2 955.6 1140 1340 2027
Nominal perimeter
mm 30 40 50 60 70 80 90 100 110 120 130 160
Unit mass k g / m 0.560 0.995 1.56 2.25 3.04 3.98 5.04 6.23 7.51 8.95 10.5 15.9
Table 3.3. Dimensions and unit mass of USD785 and USD1275.
Grade
USD785
USD1275 Mark S6 S8 S10 S13 H6 H7 H9 H l l H13
Nominal diameter
mm 6.35 7.94 9.53 12.7 6.4 7.4 9.2 11.0 13.0
Nominal area m m2
31.67 49.51 71.33 126.7 30.0 40.0 64.0 90.0 125.0
Nominal perimeter
m m 20 25 30 40 20 23 29 35 41
Unit mass k g / m 0.249 0.389 0.560 0.995 0.236 0.314 0.502 0.707 0.981
3.2.4.2. Specified Yield Strength
A clear yield plateau is the most desirable feature of axial reinforcement in the yield hinges. As USD685 is to be used in this situation, both upper and lower bounds of yield stress are specified in the proposed Standard. The difference between upper and lower bounds is 100 MPa for USD685A, and 70 MPa for USD685B. The narrower interval allows the structural engineer more accurate estimation of flexural yield strength leading to smaller magnification of required strength of nonyielding members. On the other hand, manufacture of USD685B would require more stringent quality control, possibly resulting in higher cost, compared with USD685A. It is up to the decision of structural engineers in future which one of USD685A or USD685B would be favored.
USD980, ultrahigh strength bars to be used in nonyielding members, and USD785 and USD1275 both for lateral reinforcement, have the specified values of lower bound of yield stress only. These kinds of steel usually show no distinct yield plateau, and yield stress is defined by 0.2 percent offset stress as in the current Standard.
3.2.4.3. Strain at Yield Plateau
A new concept of strain at yield plateau is introduced in the specifications for USD685A and USD685B. It is the strain at the end of yield plateau, or in other words, strain at the start of strain hardening. As shown in Fig. 3.28, it is defined as the strain at which upper bound of yield stress is exceeded. As shown
Stress Tensile Strength
Upper bound yield stress Lower bound yield stress
0 ,"- : •
M Yield Stain at Strain H 0.2% s t r a i n yield plateau
(2 1.4%)
Fig. 3.28. Stress-strain relationship of USD685.
in Table 3.1, this value is specified not to be smaller than 1.4 percent for both USD685A and USD685B. This requirement is expected to ensure prescribed amount of yield plateau in the stress-strain relationship, by avoiding the onset of strain hardening and accompanied strength increase of structural members within certain range of deformation after re-bar yielding. This type of behavior is believed to be useful for the structural engineers to ensure the occurrence of intended collapse mechanism.
3.2.4.4. Yield Ratio
Yield ratio of steel is defined as the ratio of measured yield stress to measured tensile strength. Lower the yield ratio, larger is the increase of stress after yielding due to strain hardening. As may be read in Fig. 3.27 where yield ratios are shown in parentheses, yield ratio of ordinary re-bars such as SD295 or SD345 is low, around 0.7, but it increases as the yield strength gets higher.
It was found in the process of trial manufacture that yield ratio of high strength re-bars could go up to almost 1.0 depending on the manufacture method.
Yield ratio has been an important consideration for steel structures since long ago, but it has received little attention in case of reinforced concrete as a potential source of inferior behavior. However in the New RC project structural tests were conducted to demonstrate the possibility of strain concentration and fracture of bars when steel with yield ratio as high as almost 1.0 is used. Hence provisions for upper limit of yield ratio were introduced to re-bars that are
Strain at yield plateau is taken as strain at y. . . which upper bound
l e yield stress is exceeded /, Point ' '
expected to be used as axial reinforcement. As shown in Table 3.1, values of limiting yield ratio for USD685A and USD685B are 85 percent and 80 percent, respectively, while that for USD980 is 95 percent. These requirements of yield ratio serve to specify the minimum value of tensile strength in effect. Hence tensile strength is not specified in Table 3.1.
3.2.4.5. Elongation and Bendability
Elongation of re-bars at fracture is desirable to be as large as possible, for easier bending process without bar fracture, but loss in elongation capacity is unavoidable as tensile strength increases. As shown in Table 3.1, 10 percent for USD685A and USD685B and 7 percent for USD980 are the specified minimum values of fracture strain. Bendability of deformed re-bars is affected by the shape of surface deformation, and in general re-bars with screw shaped lugs are unfavorable to ordinary lateral lugs in bending process. Table 3.1 specifies inner radius of 90 degree bend to be twice bar diameter for USD685A and USD685B, and four times bar diameter for USD980.
For lateral reinforcement of USD785 and USD1275, elongation of 8 percent and 7 percent, and inner radius at 90 degree bend of 1.5 times and 2.5 times bar diameter, are insured respectively.
Strain age hardening, which means that a bar tends to harden and becomes susceptible to fracture with age after receiving processing strain, is sometimes observed depending on the type of steel. To see whether this effect is observed in case of high strength re-bars, tensile tests were conducted of specimens of D32 screw-deformed bars of USD685 manufactured by component adjustment and hot rolling (as roll), first subjected to tensile prestrain of 10 percent, then subjected to accelerated ageing of one hour at 100 degree Celsius in the electric furnace. It was confirmed no strain age hardening was observed in case of D32 bars of USD685 manufactured by component adjustment and hot rolling (as roll).