Research Note RN-1982-1 search Note - CRSI

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Bending and Straightening Grade 60 Reinforcing Bars

Research NoteRN-1982-1

ABSTRACT

A series of bending and straightening tests on Grade 60 reinforcing bars was conducted for the CRSI Technical Committee of the Associ-ated Reinforcing Bar Producers. Field bending and straightening for various bend diameters and bend angles was simulated in the laboratory. The effect on the bars was evaluated by observation and by subsequent tensile testing.

Testing was performed on three sizes of re-inforcing bars, #5, #8 and #11. One common source of supply was used for all three sizes, plus two additional sources for the #11 bars. Bend-ing and straightening was done with the bar at a temperature of 30° F and 70° F. Two types of bends were tested, a bend diameter specified in the American Concrete Institute’s “Building Code Requirements for Reinforced Concrete” (ACI 318-77) and a bend made to as small a diameter as possible. Bending was done around both the weak and strong axis.

Some bars were flame heated to approximate-ly 1500° F to reduce breakage during bending and straightening.

Tensile tests were conducted on straightened bars to obtain the yield and tensile strength.

These results were compared to tensile tests of unbent control bars.

INTRODUCTION

A frequently encountered construction prob-lem is that a partially embedded reinforcing bar requires bending or straightening. A bar may need to be bent because of incorrect fabrica-tion, inaccurate placement, or to provide access. A bent bar may need to be straightened for any of the same reasons or because it was bent ac-cidentally during construction. Field bending or straightening can be done by placing a steel pipe over the bar and pulling on the bar. Smaller bars can be pulled with manual effort.

Accidentally bent bars could be at various bend angles and bend diameters. The direction of the bending is unpredictable so the bar could have its longitudinal ribs at the neutral axis of the bend (weak axis bending), at the extreme fiber (strong axis bending), or at any intermediate lo-cation.

Bars that are intentionally field bent for access may later require straightening. It may be expect-ed that a deliberately field bent bar would have a bend diameter at least equal to that specified in Paragraph 7.2.1 of the American Concrete Insti-tute “Building Code Requirements for Reinforced Concrete” (ACI 318-77)1. However, the bend may have been made to a smaller diameter inadver-tently or deliberately to allow better access. The bend may be around the weak or strong axis of the bar.

ACI 318 in Paragraph 7.3.2 allows field bend-ing as follows:

“Reinforcement partially embedded in con-crete shall not be field bent, except as shown on the design drawings or permit-ted by the Engineer.”

The Commentary to ACI-772 places the au-thority to perform field bending with the inspect-ing engineer and provides guidance on the use of heat.

Note: This Research Note was originally printed by CRSI in 1982 under the title Bending and Straighten-ing Grade 60 Reinforcing Bars. The report described a series of bending and straightening tests on Grade 60 reinforcing bars performed by Wiss, Janney, El-stner & Associates, Inc. (WJE No. 78282, April 6, 1982). Among the test variables were mill sources (3), bar sizes (#5, #8 and #11), bend diameters (ACI 318 and half ACI 318), orientation of bending axis (weak and strong), temperature (30° F, room tem-perature, and 1500° F), and bend angles (15, 30, 45, 60, 75 and 90 degrees). The CRSI report has been out of stock for several years, but the research represented therein is still valid and of worth in the reinforced concrete community, so it has been re-printed here in this Research Note, verbatim.

2 Bending and Straightening Grade 60 Reinforcing Bars [RN-1982-1]

“7.3.2 - Construction conditions may make it nec-essary to bend bars that have been embedded in concrete, and it usually is not possible to provide a pin of the minimum diameter specified in the code at the point of bend. Such field bending should not be done without authorization of the inspecting en-gineer. The inspecting engineer must determine whether the bars can be bent cold without damage, or if heating is necessary. If heating is required it must be controlled to avoid splitting of the concrete or damage to the bars.

Partially embedded reinforcing bars can be suc-cessfully re-bent (or bent for the first time, which should be less critical) if they are first preheated to 1100 – 1200F and then bent as gently and in as gradual an arc as possible. If there is no failure at the bend area, the reworked bars should perform as originally intended. Heating must be performed in a manner that will avoid damage to the concrete. If the bend area is within 6 in. or so of the concrete, some protective insulation may have to be applied. The heating operation should be done in such a manner that the temperature of the steel does not exceed 1200F as measured by temperature indi-cating crayons or other suitable means. The heated bars should not be artificially cooled (such as by water or forced air) until after cooling to at least 600F.”

The recommendation of preheating to between 1100° F and 1200° F is a revision from the previous edition of the ACI Code, which called for heating to between 600° F and 800° F."

The change was based on testing by Black reported in a paper entitled “Field Corrections to Partially Embed-ded Reinforcing Bars”3. The testing examined field bend-ing and straightening of #10 and #11, Grades 40 and 60 reinforcing bars, that were heated to 1100° F. His study shows that straightening of Grade 60 reinforcing bars can be improved without a loss of tensile strength by heating to 1100° F. Some bars did break during straight-ening. However, heating to higher temperature was not tested at that time because of concern over loss of ten-sile strength due to the heating.

Later unpublished tests on #11 bars reported that field bending and straightening could be improved by heating the bars to between 1400 and 1500° F. These later tests confirmed that a loss of strength did occur as a result of heating. However, it appeared that the yield and tensile strength after heating were within acceptable limits.

A paper entitled “Cold Straightening of Partially Em-bedded Reinforcing Bars”4 presented the results of cold field straightening of #8, Grade 60, reinforcing bars. Re-inforcing bars were field bent 45 or 90 degrees to ap-proximately an 8 in. diameter and straightened cold. This bend diameter is less severe than the 6 in. allowed by

ACI 318 for #8 bars. No breakage occurred during bend-ing or straightening. The straightened bars had a yield and tensile strength comparable to corresponding con-trol bars.

A paper5 published in June, 1981, discussed the ef-fect of aging on bent and straightened bars. According to Erasmus brittle fracture during straightening becomes more likely with:

1. Small bend diameter2. Large bar diameter3. Low temperature4. Impact load during straightening5. Aging of the bent bar

The testing program described in this report was initiated in late 1979 to examine a wider range of vari-ables than those previously reported. The purpose of the program is to determine the feasibility of unheated field bending and straightening of reinforcing bars ranging in size from #5 to #11 and to evaluate the effect of 1500° F flame heat on straightening and strength.

TEST VARIABLES

Variables that were evaluated in this research are:

• Bend diameter • Bend axis• Bar size • Bar source• Temperature

Bend Diameter

Bend diameter is an important variable in the bend-ing and straightening of reinforcing bars. Tensile strains are induced on the outside of a bend and compressive strains are induced on the inside. The strain varies in-versely with bend diameter and is approximately equal to

d / (D + d)

where: D = inside diameter d = bar diameter

Two bend diameters were examined for each bar size, the ACI bend diameter and approximately one-half of the ACI diameter. ACI 318-77 in Paragraph 7.2.1 specifies a minimum inside bend diameter of 8 times bar diameter (8d) for #11 bars and 6 times the bar diameter (6d) for #8 and #5 bars.

Shop fabricated bends are typically made to the ACI diameter. It is reasonable to expect that a supervised field bend would be made to the ACI specified diameter. This, then, was selected as the larger of the two bend diameters for each bar size.

It was anticipated that bending and straightening would be successful at the ACI diameter. The lower bound for bend diameter was established as the sharp-est bend that could be made using a pipe placed over

CRSI Research Note 3

the bar and as close to the embedment as possible. This simulates the condition of a bar accidentally knocked over or a bar that is tightly bent to allow access. The tightest bend that was achieved was approximately 4d for the #11 bars and 3d for the #8 and #5 bars. These values are half of the ACI specified diameter.

Bar Size

Bar size affects the ability to make or straighten a bend in two ways. At a given bend diameter, a smaller diameter will have less strain, and a smaller bar can ac-commodate greater strain.

The American Society of Testing Materials (ASTM) specification for A615 Grade 60 reinforcing bars requires that the smaller reinforcing bars have greater elongation than the larger bars. The elongation, as determined by tensile testing, is 7 percent for #11 bars, 8 percent for #8 bars and 9 percent for #5 bars. In addition, both ASTM and ACI 318 allow smaller bars to be bent to a sharper radius than large bars.

Bar Source

Bars from three sources referred to as A, B and C, were tested to examine the effect of deformation geom-etry and steel chemistry. Each supplier used a different deformation type as shown in Fig. 1. The transverse de-formations cause a stress concentration in the bar. Pre-vious research on fatigue in reinforcing bars6, 7 shows that the stress concentration depends on the sharpness of the lug base radius, the lug height, the lug width, and the flank angle of the lug and the diameter of the barrel at the bar.

The lug geometry for #11 bars was obtained by cut-ting a section through the bar, enlarging a photograph of the cut section, and measuring the geometry. Steel chemistry was provided by the mill for the reinforcing bars used in this program.

Bending Axis

Bends were made with the longitudinal rib at the neu-tral axis (weak axis bending) for all three bar sizes. #11 bars were also bent with the longitudinal rib at the ex-treme fiber (strong axis bending). Strong axis bending affects two variables, depth of bar and transverse defor-mation geometry. Depth of the #11 bars was about ¼ in. greater in the strong axis than in the weak axis, resulting in a 15 percent increase in bending strain.

However, a more important effect is the change in lug geometry. The stress concentration caused by the trans-verse lug is different at the longitudinal rib and at the body of the bar. The transverse rib on the bar from Sup-plier A tapers before meeting the longitudinal rib which eliminates the stress concentration caused by the trans-verse lug for strong axis bends. The lugs on B and C bars run into the longitudinal rib and the intersection of the lug with the longitudinal rib is sharper than the inter-section of the lug to the body of the bar.

Temperature

Most bars were bent and straightened at laboratory temperatures of 60° F to 80° F. Reinforcing bar tempera-tures in this range would occur at a construction site typi-cally during the summer months or with minimal heating of the bars. To examine the effect of colder temperatures, #11 bars from one supplier were bent and straightened at cold temperature (25° F to 35° F).

Flame heating to a temperature of 1500° F was also used in order to study a means of improving formability and to evaluate loss of strength caused by heating to this temperature.

DESCRIPTION OF TEST PROGRAM

Field bending and straightening of Grade 60 reinforc-ing bars was simulated in the laboratory. The bending and straightening was done in four phases:

1. Room temperature bending and straightening.

2. Room temperature bending fol-lowed by heated straightening.

3. Heated bending and straighten-ing.

4. Cold temperature bending and straightening.

The straightened bars from all four phases, in addition to a set of control bars, were tested to obtain yield point and tensile strength.

Fig. 1 - Reinforcing bars


Room Temperature Bending and Straightening

The first phase of the testing program examined bending and straightening of #5, #8 and #11 reinforcing bars at ambient laboratory temperature. Bars from three suppliers were examined. Bends were made around both the weak and strong axis to the ACI specified di-ameter and to the smallest diameter possible. Sixteen combinations of variables were examined in this phase, as follows:

Each “X” in the above table represents a test sequence of bend angles from 15 degrees to 90 degrees that was performed for a particular combination of variables. For a typical test sequence, a reinforcing bar was bent to a 15 degree angle and then straightened. A second bar was bent to 15 degrees and straightened. Additional pairs were bent to increasing angles and straightened. The bend angle was increased in 15 degree increments to a maximum of 90 degrees or until breakage occurred in a pair of specimens.

Room Temperature Bending and Heated Straightening

The next phase of the testing program studied im-provement in straightening obtained by heating the bent bar to 1500° F. Heated straightening was used on any combination of variables that caused cracking or break-age during room temperature straightening in the previ-ous phase. The combinations of variables that were ex-amined in this phase are summarized as follows:

Each test sequence, represented by an X in the above table, consisted of bending a pair of bars in in-creasing angle increments. Two bars were bent to the angle that first caused cracking or breakage during the room temperature straightening phase. Each bent bar was then flame heated and straightened. This was re-peated on additional pairs of bars at increasing angles until two 90 degree bends were straightened, or until the heated bend phase was initiated.

Heated Bending and Straightening

Another phase was conducted to study the improve-ment obtained by field bending with heat. This testing was done on variable combinations that caused cracking or breakage during the room temperature straightening from the first or second phase, as follows:

For each of the four above combinations of variables a test sequence was used in which a pair of bars was flame heated and then bent to the angle at which crack-ing or breakage had previously occurred at room tem-perature. Each bar was then straightened. This was re-peated, and it was possible to continue until a pair of bars was bent to 90 degrees and straightened.

Cold Temperature Bending and Straightening

Two sequences of bending and straightening were performed to examine the effect of winter temperatures on the deformability of reinforcing bars. The two combi-nations of variables that were examined are:

All four phases of the bending and straightening tests are presented in a combined format in Table 1 (see page 5).

Tensile Testing

Tensile tests were made on all successfully straight-ened specimens. “Successfully straightened” is defined as having an offset or kink in the bend region less than one bar diameter and not having transverse cracks. A bar with excessive offset was considered to be an invalid specimen and replaced with a more carefully straight-ened bar.

A bar that developed transverse cracks due to bend-ing or straightening was classified as an unsuccessful test and, in general, was not tensile tested. However, a supplemental set of tensile tests was conducted on eight cracked bars to observe the loss of strength caused by the cracks.

Bar

Siz

e

Supp

lier Weak Axis Strong Axis

ACI Diameter

Minimum Diameter

ACI Diameter

Minimum Diameter

#11 BC

XX

XX

XX

Bar

Siz

e

Supp

lier Weak Axis

ACI Diameter

Minimum Diameter

#11 B X X

Bar

Siz

e

Supp


ACI Diameter

Minimum Diameter

ACI Diameter

Minimum Diameter

#11BCA

XXX

XXX

XXX

XXX

#8 B X X

#5 B X X

Bar

Siz

e

Supp


ACI Diameter

Minimum Diameter

ACI Diameter

Minimum Diameter

#11BCA

XXX

XXX

XX

XX


TEST PROCEDUREBending and Straightening Procedures

The reinforcing bars were bent and straightened in a machine that was built for this test program. A photo-graph of the machine is shown in Fig. 2. The machine consists of a horizontal steel tube that holds the reinforc-ing bar, and the C-shaped frame that is pinned on the horizontal tube. The frame rotates about the pin, pow-ered by a two-way hydraulic ram.

A reinforcing bar was encased in three oak blocks, as shown in Fig. 3, and then inserted into the hori-zontal tube. The oak blocks were cut so that the bar could bear against the end grain of the wood during bending. Oak loaded in this direction has compressive strength of approximately 3,000 psi, which will simu-late 3,000 psi concrete. Half of the bar is embedded by the blocks and tube. The remaining half projects out of the tube.

A round steel pipe with an inside diameter slightly greater than the large diameter of the bar was placed

on the bar. A chain was wrapped around this pipe and around the top arm of the movable C-shaped frame, as shown in Fig. 4.

Bending was accomplished by pulling the frame backward with the hydraulic ram, as shown in Fig. 5. This operation must be repeated sev-eral times, because of limited ram stroke. When the bend angle reached approximately 45 degrees, the chain was taken off the top arm of the frame

and placed onto the vertical arm as shown in Fig. 6. By following this procedure, the chain pull on the bar re-mained at approximately a right angle. This procedure probably caused more severe stresses than the usual field bending.

For a test sequence using minimum diameter bend the pipe was placed as close as possible to the end oak

Fig. 3 - #11 reinforcing bar encased in oak blocks

Fig. 4 - Bending machine prepared to start bend

Fig. 5 - Simulated field bending of #11 reinforcing bar

TABLE 1 -TEST PROGRAM – BENDING AND STRAIGHTENING

ROOM AND COLD TEMPERATUREB

ar S

ize

Supp

lier 70º F 30º F

Weak Axis Strong Axis Weak Axis

ACI Diameter

Minimum Diameter

ACI Diameter

Minimum Diameter

ACI Diameter

Minimum Diameter

#11

B X X X X X X

C X X X X

A X X X X

#8 B X X

#5 B X X

FLAME HEATED

Bar

Siz

e

Supp

lier Bent at 70º F, Straightened at 1500º F Bent at 1500º F, Straightened at 1500º F

Weak Axis Strong Axis Weak Axis Strong Axis

ACI Diameter

Minimum Diameter

ACI Diameter

Minimum Diameter

ACI Diameter

Minimum Diameter

ACI Diameter

Minimum Diameter

#11

B X X X X X X X

C X X X X

A X

Fig. 2 - Bar bending machine


block. Bend diameter decreased with increasing bend angle. For 15 degree bends, the bend diameter was ap-proximately 20 in. Diameter continued to decrease with increasing bend angle until a minimum was reached at 90 degrees. However, at 75 degrees, the bend diameter was almost as small as at 90 degrees.

For test sequences that evaluated the ACI diameter, bend diameter was controlled by placement of the pipe. For bend angles of 15 or 30 degrees the bend diameter was larger than the ACI specified diameter even though the steel pipe was placed flush with the face of the oak block. The ACI diameter was reached at a bend angle of 45 degrees. At larger bend angles, the end of the pipe was moved away from the oak block embedment to maintain the desired bend diameter.

Bars were bent to the desired angle by sighting against a set of guidelines that were placed behind the bar. After completion of bending, the diameter was mea-sured using a set of cardboard templates. Diameter was measured to the inside of the lugs, as shown in Fig. 7, or against the longitudinal rib for strong axis bends.

The bend area was then examined for cracks. The bar was lightly wire-brushed, to remove loose rust and mill scale. The area was illuminated with a flashlight and observed with the naked eye.

Straightening of the bar was accomplished by placing the rotating frame in its rear-most position, moving the chain to the bottom arm, as shown in Fig. 8, and then powering the rotating arm forward. This process was re-peated until the bar is brought to a straight condition, as shown in Fig. 9. This procedure probably causes more severe stresses than usual field straightening.

At the final stage of bending or straightening, it was necessary to pull the bar slightly beyond the desired an-gle to allow for elastic rebound.

Following straightening, most of the bars had a kink such that the two parallel lengths of the bar would be offset as seen in Fig. 10. The offset was measured by placing a straightedge along the longitudinal rib on one of the straight portions of the bar, and measuring the dis-tance between the straightedge and other longitudinal rib with a carpenter’s scale. The specimens were then again examined for cracks. A typical transverse crack is shown in Fig. 11.

Fig. 7 - Measurement of bend diameter

Fig. 8 - Simulated field straightening of #11 reinforcing bar

Fig. 9 - Final straightening of #11 bar

Fig. 10 - Offset in straightened bar

Fig. 6 - Bending a reinforcing bar past 45 degrees


Temperature Control

Bars that were bent and straightened at cold temper-atures (30° F ± 5° F) were placed into a 25° F cooler at the end of the working day and allowed to cool overnight. The next morning, specimens were wrapped in insula-tion, to reduce heat gain, and taken out of the cooler one at a time. Bending and straightening was done within ap-proximately 10 minutes.

To ensure that the temperature of the test specimens did not rise above 35° F, temperature increase of a com-panion bar was monitored periodically. A control bar with a thermocouple installed in a hole drilled into the interior was placed in the cooler overnight. The bar was wrapped in insulation and removed. Temperature rise was moni-tored. The time required for the control bar to reach 35° F was found to be between 10 and 15 minutes. Since the test bars were bent and straightened within 10 minutes, it was concluded that they were tested while within the specified temperature range.

Several methods of heated straightening were used. Temperature of the flame heated specimens was moni-tored with 1400° F and 1500° F crayons. Attempts were made to improve straightening performance by uniformly heating the bar to 1400° F. At this temperature the color of the bar was black with a faint orange glow. Several tri-als with bars heated to this temperature did not show an improvement in the straightening.

Heating to a higher temperature was then tried. Bars were heated until the 1500° F mark was at the verge of melting. At this temperature, the bar had a bright orange glow as shown in Fig. 12.

Heat was applied over the entire bend area and over 1-1/2 in. lengths in the bend area. Both of these methods allowed the bent bars to be straightened without break-age. Heating the full bend area resulted in more rapid progress. Heating the incremental length allowed for bet-ter control of offset.

Based on this work, a heating method was used that would minimize the time required to heat straighten a bar in the field. The full bend length was heated until the 1500° F crayon just started to melt and the bar had a glowing orange color. If an excessive offset was ob-served prior to completing the straightening, a shorter increment was heated to reduce the offset.

For the bend specimens heated prior to bending, a very small bend diameter could be achieved by heating a short length of bar. However, the bend diameter was so small that the bar would break during bending, even at the 1500° F temperature. To prevent breakage due to this extreme condition, minimum diameter bends at 1500° F were made by heating approximately 9 inches of bar so that the resulting bend diameter would be equal to the minimum diameter bend that was obtained at room temperature.

Heating was done with a Torchweld No. 36 welding tip using an oxygen-acetylene mixture. In later stages of testing, a Victor No. 8-MFA Heating Nozzle using an ox-ygen-propane mixture was used. Heating took approxi-mately 5 to 10 minutes. The portion of the bar beyond the embedment was encased in damp sand to simulate the thermal effect of fresh concrete.

Test Specimens

All of the test specimens were ASTM A615 Grade 60 reinforcing bar. Chemical analysis and physical test re-sults were provided by each steel mill for each of the heats. These properties are given in Table 2.

Most bars from a particular size and supplier were taken from one heat. However, the #11 bars from Sup-plier A were taken from two separate heats. Test speci-mens were taken from one bundle until the bundle was exhausted. By doing this, all but six of the Supplier A specimens were taken from the same heat. There does not appear to be a significant difference in the chemistry of the two heats. To account for a potential difference in

Fig. 11 - #11 reinforcing bar with transverse crack Fig. 12 - Reinforcing bar heated to 1500° F


physical properties one control bar for tensile testing was selected from each heat.

To measure the geometry of transverse deformations a reinforcing bar from each supplier was sectioned and photographed. Rust and mill scale were removed from the surface by placing the sample in a 50 percent solu-tion of hydrochloric acid and water at room temperature for 30 minutes. Next, the sample was immersed in a 5 percent neutralizer solution of sodium carbonate and water, and then thoroughly rinsed in cold water. This was followed by drying in an oven at 120° F for 20 minutes. The sample was gently wire-brushed. The prepared sample bar was lathed to a radial plane.

Photographs of the sectioned bar surface were ob-tained through a microscope. Measurements of the criti-cal lug dimensions were made from photographic prints.

Lug base radii, flank angles, height, and width were measured. The radii were determined by comparison with several circles on a template. Flank angles were determined by drawing the lug base line and using a protractor to establish the angle to the most representative slopes on the sides of the lug. Height of the lug was deter-mined as the greatest height from the lug base line. Width of the lug was determined as the dis-tance along the lug base line between the points of intersection of the tangent lines used to de-termine the flank angles. Dimensionless values were obtained by determination of the lug base radius-to-height ratio and the lug height-to-width ratio. These ratios are presented in Table 3.

Tensile Testing

Yield point, tensile strength and location of break were obtained for each of the success-fully straightened specimens. Testing was done essentially in accordance with ASTM A370 and ASTM E8. However, a load-strain curve and per-manent elongation were not obtained, because the offset due to straightening results in a load-strain curve in air that is not representative of

the behavior when encased in concrete. Yield point was obtained by observation of pause in the load indicator needle. Approximately 60 percent of the straightened bars had a distinct yield point that could be observed by a pause. Yield was not obtained on the remaining 40 percent. Tensile strength was obtained by observing the trailing needle of the test machine dial for maximum load.

Two control bars of each size and grade were tensile tested in accordance with ASTM A370 and ASTM E8. A load-strain curve, yield and tensile strength and total elongation at failure were obtained.

Prior to testing, the control bars were marked with a series of punch marks spaced at 8/3 in. on opposite sides along the length of the bar. This established a se-ries of overlapping 8 in. gage lengths with the middle-third of each gage length clearly defined. The distance between these marks was measured using dividers and a machinist’s scale.

The specimen was positioned in the testing machine and an extensometer was attached to the specimen. The extensometer consists of two DG-LVDT’s mounted in a spring-loaded extendable frame. The extensometer gage length was 8 in. The universal test machine produced an electrical signal indicating the applied load. While load-ing the specimens, an autographic load-strain curve was obtained by driving an X-Y plotter with the electrical out-put of the extensometer and test machine signals.

At 2 percent strain, the extensometer was removed and the specimen was loaded to failure. Yield point was obtained by noting a pause in the load indicator needle.

TABLE 2 – PROPERTIES OF REINFORCING BARS

Supplier A Supplier B Supplier C

#11, Heat S10392

#11, Heat S25927

#11, Heat 218H067

#8, Heat 431H2041

#5, Heat 208J103

#11, Heat T00096

C (%) 0.39 0.36 0.42 0.39 0.43 0.37

Mn (%) 1.01 0.91 1.29 1.19 1.27 1.06

P (%) 0.016 0.009 0.008 0.010 0.012 0.020

S (%) 0.033 0.035 0.041 0.021 0.035 0.019

Si (%) 0.27 0.24 0.095 0.04 0.079 0.23

Ni (%) 0.10 0.10 0.11 0.02 0.08 0.09

Cr (%) 0.30 0.29 0.10 0.03 0.09 0.10

Mo (%) 0.03 0.02 0.026 0.005 0.017 NR

Cu (%) 0.42 0.30 0.35 0.02 0.44 0.28

V (%) NR NR 0.013 0.040 0.003 0.011

Sn (%) NR NR 0.024 NR 0.023 0.005

Cb (%) 0.017 0.015 NR NR NR NR

Yield (ksi) 73.1 68.0 70.5 67.3 66.1 62.7

Tensile (ksi) 113.0 107.0 109.0 95.3 106.5 101.4

Elongation (%) 11.4 9.8 10.0 17.0 16.0 8.5

Note: NR indicates not reported

TABLE 3 - DEFORMATION GEOMETRY

Bar r/h h/w Flank Angle (Degrees) Ɵ

#11A 0.1 0.4 30

#11B 0.3 0.4 45

#11C 0.3 0.2 60

#8B 0.3 0.3 30

#5B 0.2 0.3 10

h r

w

Ɵ


This yield compared well with the yield load indicated on the load-strain curve, and verified that the yield point ob-served by pause in the load indicator on the straightened specimens is accurate.

The maximum load of the specimens was observed with the trailing needle of the test machine dial. The total elongation at failure was measured using a machinist’s scale with the failure surface included in the middle-third of the gage length. Elongation measurements were made on both sides of the specimen, and the results were averaged to exclude the effects of curvature.

BENDING AND STRAIGHTENING TEST RESULTS

Field Bending at Room Temperature

Fig. 13 has four sets of bar graphs showing the results of field bending of #11 bars at 70° F. Each set consists of three bars, one for each of three suppliers with each

one representing a testing sequence of bending pairs of reinforcing bars from 15 degrees to 90 degrees. The up-per two sets represent results of ACI diameter bends; the lower two represent the results of bending to a minimum diameter. The decrease in bend diameter with increasing bend angle is shown by presenting average bend diam-eter obtained in all bars beside the bend angle.

Bars from all three suppliers were bent to the ACI specified diameter or larger around the weak axis at angles increasing to 90 degrees without breakage or cracking. When bending was around the strong axis one bar from Supplier B in the 45 degrees angle pair broke during bending. Remaining specimens in that se-quence were bent to angles up to 90 degrees without further breakage. Transverse cracking was observed in two specimens after bending to the ACI diameter around the strong axis.

As shown in the lower two sets of graphs in Fig. 13, a bend diameter smaller than the ACI specified diameter

was reached at a bend angle of approximately 45 to 60 degrees.

When bent around the weak axis, one bar from Supplier B broke while attempting a 5.5d, 60 degree angle bend; the other bar of that pair was successfully bent. Bars from Supplier A and C were bent up to a 3d, 90 degree angle without breakage.

For bends around the strong axis, one bar from Supplier B broke at an 8d, 45 degree angle bend, the second specimen was bent without breakage. One bar from Supplier C broke at a 5.5d, 60 degree angle bend. The second specimen was bent without breakage. Reinforcing bars from Supplier A could be bent around the strong axis to ends at tight as 4d and 90 degrees.

All of the #5 and #8 reinforcing bars were bent around the weak axis to a diameter as small as 3d and to an angle as large as 90 de-grees without breakage. This is summarized in Fig. 14.

Field Straightening at Room Temperature

Fig. 15 shows the results of 70° F field straightening of the bent #11 bars represented in Fig. 13. Each bar on the graphs represents straightening of a sequence of bent specimens. The upper limit of each bar indicates that one or both specimens was straightened without breakage. At the next larger angle increment, either both bars broke during straightening or one bar broke during straightening and the oth-er bar had broken during bending.

Fig. 13 - Bending of #11 bars at room temperature

Fig. 14 - Bending and straightening of Supplier B bars around weak axis at room temperature


All of the bars that had an ACI bend around the weak axis at angles up to 90 degrees could be straightened without breakage. Ninety degree ACI bends around the strong axis could be straightened for bars from Suppliers A and C. However, for Supplier B, one 45 degree bend specimen broke during straightening. Since the other specimen had broken during bending, the bar graph is terminated at 30 degrees.

The results of field straightening of #11 bars that were bent around the weak axis to a diameter smaller than that specified by ACI 318 are shown in the lower left hand graph of Fig. 15. Bends varying from 6-inch to 12-inch diameter, depending on the supplier, could be straightened.

Two #11 bars from Supplier A that were bent 45 de-grees around the weak axis to a 10-inch diameter were straightened without breakage. At 60 degrees, one spec-imen had a 7-inch bend; the other had an 8-inch bend. The 8-inch diameter bend was successfully straight-

ened; the 7-inch diameter bent bar broke dur-ing straightening. At 75 degrees, both speci-mens had a 6-inch diameter bend and both broke during straightening.

For Supplier B, two specimens with a 45 de-gree, 12-inch diameter bend around the weak axis were straightened. However, both had transverse cracks. At an angle of 60 degrees, both specimens had an 8-inch diameter. One specimen broke during straightening. The other had broken during the initial bend.

For Supplier C, two specimens with a 75 degree angle bend around the weak axis were straightened but had transverse cracks. One bar had a 6-inch diameter bend; the other had an 8-inch diameter bend. At 90 degrees, both bars had a 6-inch diameter bend and both bars broke during straightening.

The three test sequences of straightening minimum diameter bends around the strong axis are shown in the lower right hand graph of Fig. 15. For Supplier B and C, breakage oc-curred in smaller angle, larger diameter bends as compared to weak axis bending. However, for Supplier A, straightening of bends around the strong axis was more successful. Bends as sharp as 90 degrees and 6-inch diameter were straightened without breakage or cracking.

The #5 and #8 bars that had been bent around the weak axis, as tight as 3d and to an angle as large as 90 degrees, could be straight-ened without breakage or cracking. This is summarized in Fig. 14.

Heated Bending and Straightening

The results of heated bending and straightening of #11 are presented in Fig. 16. Bars that were bent at room temperature and straightened at 1500° F are rep-resented with cross-hatching. Bars that were bent and straightened at 1500° F are shown with a solid shading. The cross-hatching starts at the angle at which heat was first used during straightening. The shading starts at the angle at which flame heat was used during bending and straightening. The graph is extended down to indicate that, although tests were not performed, it is expected that a reinforcing bar could be straightened at bend an-gles smaller than those tested.

Heating the bar to 1500° F allowed bars with ACI or minimum diameter bends around the weak or strong axis to be straightened without breakage. The heating elimi-nated cracking in most cases. There were two excep-tions and they are shown in Fig. 16.

Fig. 16 - Heated bending and straightening of #11 bars

Fig. 15 - Straightening of #11 bars at room temperature


Heated straightening was not used on #5 or #8 bars because no breakage occurred during the room temper-ature straightening sequences.

Cold Temperature Bending and Straightening

The results of the 30° F bending and straightening of #11 bars around the weak axis are presented in Fig. 17 and are compared to the results of room temperature bending and straightening.

Straightening of the ACI diameter bend sequence was halted at a bend angle of 60 degrees. Transverse cracks developed in both specimens while straightening the bend. Straightening of the minimum diameter test se-quence was halted at a bend angle of 45 degrees. One specimen broke during straightening; the other devel-oped a transverse crack.

TENSILE TEST RESULTS

The results of tensile testing of the uncracked re-straightened bent bars and control bars for each manu-facturer are summarized in Table 4. The average yield strength for the bent bars was obtained for bars with a distinct yield point. Approximately 60 percent of the un-heated bars and 80 percent of the heated bars had a distinct yield point.

Average yield strength of the straightened bars, whether heated or not, is virtually the same as the yield strength of the control bars. The average tensile strength of the unheated straightened bars is essentially the same as that of the control bars. Approximately 35 percent of the cold straightened bars fractured within the bend region. The remaining 65 percent broke outside of the bend area. Tensile strength of the heated bars is 7 to 14 percent less than that of the control bars. However, the strength is above the minimum requirement for Grade 60 reinforcing bars. Almost all of the heated bars fractured in the bent and heated area.

The range of yield and tensile strengths of the un-cracked straightened specimens is presented in Table 5. For unheated bars, yield strength varied from a low of 57 ksi to a high of 67 ksi. Only one specimen had a yield strength less than 60 ksi. Tensile strength varied from 79 to 108 ksi. Three specimens had a strength below 90 ksi. Of a total of 143 specimens straightened without heat, only 5 failed to meet both the yield and tensile strength

requirements for Grade 60 reinforcing bars.

The yield strength of the specimens that were bent or straightened using flame heat varied from 59 to 66 ksi. One bar had a yield strength less than 60 ksi. Tensile strength varied from 80 to 102 ksi. Three specimens had a strength less than 90 ksi. Of the 58 specimens straightened using flame heat, 5 had a yield or tensile strength that did not meet Grade 60 strength requirements.

Fig. 17 - Cold temperature, weak axis straightening of #11 bars from Supplier B

TABLE 4 - TENSILE TEST RESULTS – UNCRACKED SPECIMENS

Bar

Siz

e

Supp

lier Control Bars Unheated Bent Bars Heated Bent Bars

Average Yield Point,

ksi

Average Tensile Strength,

ksi


ksi


ksi


ksi


ksi

#11

A 64.4 106.4 63.8 103.7 61.7 91.8

B 65.1 105.1 64.7 103.9 65.1 93.5

C 64.8 105.1 64.6 103.6 65.1 97.5

#8 B 68.2 96.5 66.2 95.6 --- ---

#5 B 66.1* 106.5* 64.5 104.5 --- ---*Taken from mill test report

TABLE 5 - VARIATION IN TENSILE TEST RESULTS FOR UNCRACKED SPECIMENS

Bending and Straightening Temperature

Bar Size Supplier

Yield Point Tensile Strength

Average, ksi

Min., ksi

Max., ksi

Number of Yield Points

Number Below 60 ksi

Average, ksi

Min., ksi

Max., ksi

Number of Yield Points

Number Below 90 ksi

70º F and 30º F

#11

A 63.8 56.7 67.3 19 1 103.7 78.8 108.0 44 3

B 64.7 62.5 65.7 20 0 103.9 84.8 106.3 29 1

C 64.6 60.9 65.7 22 0 103.6 102.9 105.0 25 0

#8 B 66.2 63.9 68.4 22 0 95.6 93.0 98.7 24 0

#5 B 64.5 62.9 66.1 21 0 104.5 102.6 105.5 21 0

1500º F #11

A 61.7 59.0 65.7 5 2 91.8 89.9 93.9 6 1

B 65.1 63.8 65.7 19 0 93.5 79.8 97.9 29 2

C 65.1 64.3 66.0 22 0 97.5 94.4 101.8 23 0


Supplemental Tensile Tests

Yield and tensile strength of six Supplier C bars that developed transverse cracks during straightening are given in Table 6. Crack sizes were categorized as small, medium and large. Two bars from each category were tested. The tensile test results are compared to the av-erage yield and tensile strength of the uncracked bars from Supplier C. Cracks described as small had a length of 0.10 in. and were hairline wide. The two cracks de-scribed as medium were 0.20 in. long by 0.01 in. wide and 0.15 in. long and 0.005 in. wide respectively. Large cracks were 0.25 in. long and 0.015 in. to 0.02 in. deep.

The specimens with small and medium sized cracks were tensile tested shortly after straightening. Average yield strength of the cracked specimen was virtually the same as the average yield strength of the uncracked straightened bars. Tensile strength was slightly reduced when compared to the uncracked bars. The reduction was more pronounced in the medium sized cracks. How-

ever, the strength remains above the minimum tensile strength requirement of Grade 60. Fracture of the bar occurred inside the bend region and initiated at a crack caused during straightening.

The specimens with large cracks were tested one year after straightening. Yield and tensile strength increased to above the strength of the uncracked specimens, and was essentially equal to the strength of the control bars. Fracture of the bar occurred outside of the bend area, so the tensile strength was not reduced by the preexist-ing cracks. The increase in strength is most likely due to strain age hardening effects and will be more fully dis-cussed in the next section of this report.

EVALUATION AND DISCUSSION OF RESULTS

Effect of Bend Diameter, Angle and Orientation

Bend diameter and orientation had a strong effect on the ability to bend or straighten a #11 bar. This is dem-onstrated in Figs. 18 and 19 in which the percentage of bars that were straightened without breakage is plotted as a function of bend diameter. The percentage of bars that were successfully bent and straightened is shown in Fig. 18 for bends made around the weak axis and in Fig. 19 for bends around the strong axis. The probability of straightening without breakage increases with increas-ing bend diameter. For weak axis bends, 100 percent of the bars were straightened at a bend diameter of 12 in or greater. For strong axis bends, the bend diameter required for straightening of all the bars was 18 in.

When the bend was oriented with the longitudinal rib at the extreme fiber of bending, breakage during straightening occurred at a larger bend diameter for bars obtained from Supplier B and C. However, straightening of bars from Supplier A was more successful.

It appeared that the improved performance of A bars about the strong axis was due to a minimization of the notch caused by transverse deformation at the longitu-dinal rib.

However, the angle to which a bar is bent at a given bend diameter does not appear to have a strong influ-ence on bending and straightening. As shown previous-ly, the bending strain is dependent on the bar diameter and bend diameter. For example, in the test sequence of making ACI bends around the strong axis, breakage occurred in one #11 specimen at an angle of 45 degrees. The same bend diameter was maintained by moving the pipe sleeve outward, and bending was continued up to an angle of 90 degrees. No further breakage occurred. In straightening these bends, breakage occurred in approx-imately half the specimens. Both specimens bent initially to 75 degrees were straightened even though breakage occurred in one of the 60 degree specimens.

Fig. 18 - Straightening as a function of bend diameter

Fig. 19 - Straightening as a function of bend diameter

TABLE 6 - SUPPLEMENTAL TENSILE TESTS

Crack Size Yield Point, ksi

Tensile Strength, ksi Break Location

None 64.6 103.6 Varies

Small 64.3 102.1 In bend area

Medium 64.4 96.5 In bend area

Large* 65.5 104.7 Outside of bend area*These specimens were tensile tested one year after being straightened.


Effect of Bar Size

The #5 and #8 bars were bent to a diameter as small as three times the bar diameter without breakage. This is well below the minimum bend diameter that is required in the ASTM A615, Grade 60 Specification. By comparison, the bend diameter at which 100 percent of the #11 bars could be straightened was eight times the bar diameter.

The steel chemistry of the #5 and #8 bars does not appear to be significantly different than that of the #11 bars. The deformation geometry of the #8 bar is approxi-mately the same as that of the #11 B bar. The lug base radius of the #5 bar is more acute than that of the #11 bar. However, this apparently did not reduce the bend-ability of the #5 bar.

One significant difference of the #5 and #8 bars was that their elongation, as reported by the steel mills, was 16 to 17 percent as compared to approximately 10 per-cent for the #11 bars.

Effect of Source of Bars

There was a distinct difference in the bendability of the #11 reinforcing bars produced by each of Suppliers A, B and C. Bars obtained from Supplier A could be bent to the most severe angle. Supplier B bars had the least bendability.

Chemistry of the bars from all three suppliers ap-pears to be quite similar. Supplier A Bars had a sharper lug base radius than either the B or C Bars. The higher stress concentration associated with the smaller base was therefore not related to the greater bendability of the A bars.

There may be other factors that affect the perfor-mance of bars from different suppliers. However, this research effort has not identified these factors.

Effect of Cold Temperature and Flame Heating

The sequence of cold temperature bending and straightening of #11 bars to a minimum diameter can be compared to the sequence at room temperature. When the straightening was done at room temperature, two 12 in. diameter bends were straightened without breakage. Breakage occurred when the bend diameter was de-creased to 8 in. When the temperature was decreased to 30° F, one of the 12 in. diameter bends broke during straightening. This comparison was generally confirmed in the sequence of bends made to the ACI diameter at 70° F and 30° F. Transverse cracks occurred at a smaller bend angle with straightening done at a temperature of 30° F. Therefore, it appears that cold temperatures slightly increase the probability of breakage during straightening.

Flame heating significantly improved the bending and straightening performance. #11 bars could be bent to a diameter as small as 4d and straightened without break-

age. Cracking occurred in a few of the heated speci-mens. However, the incidence of cracking was minimum.

Heating temperature was important. Early trials of heating to 1400° F did not noticeably improve bending or straightening. However, when the bar temperature was raised to 1500° F a marked improvement was noted. Heating both the full bend area and 1-1/2 to 2 in. incre-ments resulted in good bendability. Heating the full area resulted in a more rapid operation. Heating increments of the bar allowed a greater control of offset. In most cases, offset could be controlled within one bar diam-eter by heating the entire bend. This method was used because it was more rapid, which would be preferred in field applications.

Tensile Properties of Straightening Bars

The yield and tensile strength of the bent and straight-ened bars is compared to the strength of the unbent control bars in Table 4. Both average yield and tensile strength of the uncracked specimens are virtually the same as in the control bars. The variations in the proper-ties are given in Table 5. Of all the bars tested, only one had a yield point less than 60 ksi. The yield strength was 57 ksi, a 5 percent reduction in strength.

The tensile properties of specimens that cracked dur-ing straightening are compared to the properties of the uncracked specimens in Table 6. The bars that were tested soon after straightening had a slight reduction in yield and tensile strength when compared to the un-cracked straightened bars.

The cracked bars that were allowed to sit at room temperature for one year prior to tensile testing exhibited an apparent increase in yield and tensile strength when compared to the uncracked specimens. This is consis-tent with the increased yield and tensile strength due to strain age hardening as described in Erasmus’ paper. Apparently, the tensile strength in the bend region was raised sufficiently by the aging process to cause failure to occur outside of the bend region.

The #11 from two of the suppliers that were flame heated for bending and straightening had essentially the same yield point as the control bar. The bars from the third supplier had a 4 percent reduction in yield strength when compared to the control bar, however the average yield strength remained above 60 ksi.

There was a reduction in tensile strength of approxi-mately 10 percent. However, the bars still met the re-quirements for tensile strength of the A615 Specifica-tions.

Method of Simulating Field Bending and Straightening

Field bending and straightening was done by placing a steel pipe over the bar and pulling on it. The hydraulic ram was used to obtain sufficient force to bend the larger


bars (#11 and #8). The #5 bars were bent with manual effort. We believe that this is a good simulation of meth-ods of bending reinforcing bars in the field. However, oth-ers believe that the incremental method of bending and straightening the bars produces a more severe condition than that obtained by pulling the bar without stopping.

In either case, the results obtained in this program will be at least as severe as field bending performed by plac-ing a long tube over the bar and continuously pulling until the reinforcing bar reaches its desired position.

CONCLUSIONS

Field bending, field straightening and tensile tests were performed on 254 Grade 60 reinforcing bars. #5, #8 and #11 were tested. One common source of supply was used for all three sizes, plus two additional sources for the #11 bars. The major variables were bend diam-eter, bend angle, bend orientation, bar source and tem-perature.

Based on this testing, the following conclusions are drawn:

1. The #5 and #8 reinforcing bars exhibited better bending and straightening performance than the #11 bars. They were bent at a nominal temperature of 70° F to a diameter as small as three times the bar diam-eter and straightened with no breakage or cracking.

2. #11 bars from all three suppliers could be bent at a temperature of 70° F to the ACI specified diam-eter around the weak axis without breakage. Some of these bars broke when they were bent to the ACI specified diameter around the strong axis or when bent to a smaller diameter either around the weak or strong axis. The breakage increased with decreasing bend diameter.

3. #11 bars that had been bent as much as 90 de-grees at the ACI diameter around the weak axis could be field straightened at a temperature of 70° F without breakage. Breakage during straightening occurred in some bars for ACI bends around the strong axis and for smaller diameter bends around either axis. The breakage increased with decreasing bend diameter.

4. It appears that breakage may occur at a slightly smaller angle when straightening is done at 30° F.

5. Bending or straightening #11 bars around the strong axis resulted in a greater incidence of breakage in the two deformation patterns in which the transverse lugs run into the longitudinal rib. Breakage was less likely in bends around the strong axis for the deformation pat-tern that tapered out before meeting the longitudinal rib.

6. Heating the bars to 1500° F significantly improved the ability to bend and straighten #11 reinforcing bars. Bends as small as 5 in. diameter around the strong axis could be straightened without breakage.

7. The tensile properties of uncracked reinforcing bars that were straightened without flame heat were virtual-ly the same as that of the unbent control bars. Fracture during tensile testing occurred both inside and outside the bent area.

8. The yield strength of #11 reinforcing bars that were flame heated during bending or straightening was the same as that of the unheated bars. Tensile strength was approximately 10 percent less than that of the cor-responding control bar. Most of the tensile fractures occurred within the bent and heated area.

9. The yield and tensile strength of #11 straightened reinforcing bars with transverse cracks, tested shortly after straightening, was slightly less than that of un-cracked specimens. Fracture occurred at an existing crack.

10. The yield and tensile strength of #11 cracked, straightened, bars that were allowed to age for one year after straightening was higher than for the un-cracked specimens. The fracture surface was outside the bend region.

It is evident from this investigation that field bending and straightening of reinforcing bars up to #11 should generally be permitted. Although variability must be an-ticipated in the performance of bars from different sourc-es, it is expected that most bars can be bent about the weak axis up to 90 degrees at the ACI specified diam-eters and straightened at normal temperatures. Under other more severe conditions, use of flame heat to 1500° F may be desirable. The very minor reduction in strength from local heating to this temperature, compared to the present limit recommended by ACI of 1200° F, is more than offset by the minimization of breakage and the abil-ity to restore the bar to a straight or desired alignment.

REFERENCES

ACI Committee 318, “Building Code Requirements for Reinforced Concrete” (ACI 318-77), American Concrete Institute, (1977), par. 7.3.2. p 20.ACI Committee 318, “Commentary on Building Code Requirements for Reinforced Concrete (ACI 318-77)”, American Concrete Institute, par. 7.3.2, pp. 24-25.Black, William C., “Field Corrections to Partially Em-bedded Reinforcing Bars,” ACI Journal, Proceedings V.70, No. 10. Oct. 1973, pp. 690-691.Lalik, J. R. and Cusick, R. L., “Cold Straightening of Partially Embedded Reinforcing Bars”, Concrete Interna-tional V. 1 No. 7, July 1979, pp. 26-30.


Erasmus, L.A., “Cold Straightening of Partially Embed-ded Reinforcing Bars - A Different View”, Concrete Inter-national V. 3 No.5, June 1981, pp. 47-52.Helgason, Thorsteinn; Hanson, J. M.; Somes, N. F.; Corley, W. G.; and Hognestad, Eivind, “Fatigue Strength of High-Yield Reinforcing Bars”, National Coop-erative Highway Research Program, Report 164, Part 1, Portland Cement Association, 1976, p. 31.Derecho, A. T., and Munse, W. H., “Stress Concentra-tion at External Notches in Members Subjected to Axial Loadings”, Engineering Experiment Station Bulletin 494, University of Illinois, Urbana, January, 1968, pp. 51.

Contributors: The principal authors of this publication are Jack P. Stecich and John M. Hanson, Ph.D., P.E., NAE, M.ASCE, of Wiss, Janney, Elstner & Associates, Inc. This document represents a summary of their CRSI research project on the subject topic; the final report should be referenced for more information on the research.

Keywords: bending, deformed bars, reinforced concrete, reinforcement, straightening.

Reference: Concrete Reinforcing Steel Institute - CRSI [2019], “Bending and Straightening Grade 60 Reinforcing Bars,” CRSI Research Note RN 1982-1, Schaumburg, Illinois, 16 pp.

Note: This publication is intended for the use of professionals competent to evaluate the signifi-cance and limitations of its contents and who will accept responsibility for the application of the material it contains. The Concrete Reinforcing Steel Institute reports the foregoing material as a matter of information and, therefore, disclaims any and all responsibility for application of the stated principles or for the accuracy of the sources other than material developed by the Institute.

The opinions and findings expressed in this Research Note are those of the researchers and do not necessarily reflect the opinions or recommendations of the Concrete Reinforcing Steel Institute.

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