A Simple Method of Gripping Prestressing Strands for Tension Tests

A Simple Method of

Gripping Prestressing

Strands for Tension Tests

Chandu V. Shenoy

Structural Engineer Blakeslee Prestress, Inc. Branford, Connecticut

Gregory C. Frantz

Associate Professor

Department of Civil Engineering University of Connecticut Storrs, Connecticut

Presents a simple technique for gripping a prestressing strand

to measure its breaking strength, modulus

of elasticity

or

stress-strain behavior. A combination of prestressing chucks

and strand splicing sleeves are used to anchor

the

strand

in

an ordinary universal testing machine with

V-grips.

Test

results from using this method are compared

with those

employing a special strand testing machine.

A

tension test is, conceptually at least, perhaps the simplest of mechanical tests. However, performing a tension test on a length of prestressing strand is very difficult. Due to the nature of both the strand and the common universal testing machine, substantial effort and/or expense is needed to produce reliable test results.

This paper presents a simple way of gripping a prestressing strand to per-form a tension test of the strand. The method can be used with a relatively short length of strand and can be

uti-lized with any universal testing machine.

CURRENTLY USED

TEST METHODS

ASTM A3701 states, "the true mechanical properties of the strand are determined by a test in which fracture of the specimen occurs in the free span

between the jaws of the testing machine." However, the serrated teeth of the gripping devices of ordinary testing machines, such as those used to test solid steel tensile coupons, will bite deeply into the outer surface of the wires and will usually result in a premature failure of one of the wires in the grips. This same ASTM Stan-dard lists several procedures that can be used for testing strand.

These methods consist of using a cushioning material such as aluminum foil between the grips and the strand, encasing the gripped portion of strand in tin, using epoxy to bond the strand inside metal tubing, using smooth grips with an abrasive slurry, using sockets with zinc to anchor the strands, or using dead end eye splices. They caution that prestressing chuck-ing devices should not be used because the sharp teeth of the chuck will also nick the wires and lead to premature failure just as with standard

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24" PLP GRIP MIN CLEAR STRAND- 28" 24" PLP GRIP

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USE TESTING MACHINE "v" GRIPS FOR 3/4" DIA. TO 1" DIA. BARS.

THESE DETAILS APPLY TO 3/e" AND 71t6" DIA. STRANDS.

Fig. 1. Details of the PLP splice sleeve (1 in. = 25.4 mm) (from Ref. 2).

test machine grips. A paper by Preston2 _{evaluated several of these}

methods. The following techniques produced the best results:

1. Sand grip - The strand is gripped by specially fabricated long, smooth U-grips with a sand slurry

coating applied to the strand to help prevent slipping. This method yields only about 50 percent clear breaks and

is not preferred over the following three methods, which yield about 90 percent clear breaks.

2. Aluminum insert - Aluminum angles grip and cushion the strand in machines that use tapered mechanical or wedge grips.

3. Tinius-Olsen grip - These are long, toothless U-grips that have been developed and fabricated by the Tinius-Olsen Testing Machine Com-pany. The grips fit only special kinds of test machines. This method is usu-ally used only by strand producers or large testing companies.

4. PLP grip - These grips are made from devices for splicing seven-wire

strand that are manufactured by the

Preformed Line Products Company.

The splice is composed of two sets of

three wires and one set of four wires that are helically preformed so that they will fit tightly around the strand.

The inside surfaces of the splice wires are coated with a grit which also helps to prevent slippage. The PLP splice is then gripped in a testing machine. About 23 in. (580 mm) of splice at

each grip will develop the full ultimate

strength and elongation of a~ in. (12.7

mm) Grade 270 strand. Fig. 1 shows

details for the PLP grip for % and Yl6 in.

(9.5 and 11.1 mm) strands.

METHOD DEVELOPED

BY THIS RESEARCH

As part of a research projectl.4

study-ing the strength of old prestressed con-crete bridge beams, it was necessary to measure the stress-strain curve of

some 'X6 in. ( 11.1 mm) prestressing

strands that were removed from the

beams. Some preliminary testing was done using standard 4 in. (102 mm) V-grips or using standard chucking

devices to anchor the strand in a

uni-versal testing machine. Premature

fail-ures resulted with both methods due to fracture of wires in the gripping

devices.

The PLP splices recommended by Preston are no longer being manufac-tured. However, Florida Wire and Cable Company manufactures the FLO-LOC strand grip system, which is very similar to the PLP splice and commonly used in cross member

brac-ing systems of buildings. This

FLO-LOC strand grip system, which is

sized for various strand diameters,

uses a helical winding of two groups of five wires interwoven around the

strand to encase it. The wires are also

coated with a grit to help prevent strand slippage.

Tests were conducted on Yl6 in. ( 11.1 mm) Grade 250 and ~ in. (12.7 mm) Grade 270 strands using the

appropri-ate sizes of the FLO-LOC gripping

devices. Testing was done in a Young

universal testing machine equipped with standard 4 in. (102 mm) long V-grips typically used for testing % to 1

in. (19.0 to 25.4 mm) diameter

coupons. The strands with the sleeves

had outside diameters of about 0.69 and 0.80 in. (18 and 20 mm) for the Yl6

and ~in. (11.1 and 12.7 mm) strands,

respectively. Fig. 2 shows a sketch of the test setup.

Preliminary testing showed that the

strand slipped in the sleeve at a load of

about 7 kips (31 kN) for the~ in. (12.7

mm) strand in 14 in. (356 mm) long

splice sleeves, and at about 23 kips

(102 kN) for the 'X6 in. (11.1 mm)

strand in 17 in. ( 430 mm) long splice

sleeves. It appeared that the V -grips

could bear on a larger outside portion

of the smaller diameter sleeves of the 'X6 in. (11.1 mm) strand than they

could on the larger diameter sleeves of

the ~ in. (12.7 mm) strand. This

prob-ably accounted for the much higher load at slipping for the 'X6 in. (11.1

mm) strand. Tests were not conducted

to determine how much length of

sleeve, if any, would prevent slipping

and yield clear center breaks of the

strand as reported using the PLP

splices.

Various methods to increase the frictional resistance between the

V - GRIPS ( 4") MACHINE HEAD SLEEVE .... STRAND CHUCK V- GRIPS (4") MACHINE HEAD SLEEVE STRAND

Fig. 2. Original gripping method using sleeves and V-grips

(1 in. = 25.4 mm).

Fig. 3. Modified gripping method using sleeves, V-grips and

chucks (1 in. = 25.4 mm).

strand and the sleeves were tried, including inserting carborundum cloth between the sleeves and the strand.

However, none of the methods pre-vented slipping of the strands at rela-tively low loads.

MODIFIED TEST METHOD

It was clear that using these lengths

of sleeves with our particular test

machine would not produce

satisfacto-ry results. Nor would using prestress-ing chucks as gripping devices be suc-cessful. Therefore, the test method

Fig. 4. Modified test setup (unloaded).

using the FLO-LOC sleeves was mod-ified (Fig. 3).

The sleeves were attached to the

strand so that about 4 in. (102 mm) of

strand extended beyond the ends of

the sleeves. As before, the sleeves

were gripped using the 4 in. (102 mm) long V -grips. The specimen was

care-fully aligned so that the ends of the

sleeves did not extend beyond the

out-side ends of the V -grips.

A load of about 5 kips (22 kN), which was below the slipping load, was applied to seat the sleeves in the

grips and to seat the wedge-shaped

Fig. 5. Detail at end of specimen.

grips in the machine head. The machine was then held at constant load while standard prestressing

chucks were attached to each end of

the strand with the chucks bearing on the V -grips. The test was continued by

slowly increasing the load.

Figs. 4 and 5 show details of the modified test setup. The steel plate below the chuck was attached to the test machine for safety in case of a

specimen rebounding out of the

machine after failure (none did). The sleeves prevent the grips from nicking the strand and resist part of the

Table 1. Description of tests.

Strand gripping method Breaking

Specimen Area With With With First slip Seat chuck strength

No. sq in. chucks V-grips sleeves kips kips (kips) Comments on failure

li in. diameter Grade 270 strands

1 NA X NS 39.2 At top at first notch in grip

2 NA X NS 35.0 At top at first notch in chuck jaw

3 NA X X NS 34.5 At top in chuck jaw

4 0.1526 X X 12 in. 7* 4 39.75 At bottom, inside sleeve, 2 in. outside grips

5 NA X X 14 in. 7* 5 40.8 Two wires fail inside top sleeve, li in.

inside grips, remaining five wires fail in center

6 0.1513 X X 20 in. NS 5 41.0 Clean break at center

y., in. diameter Grade 250 strands

7 0.1071 X X 20 in. NS 5 28.5 Clean break at center 8 NA X X 17 in. 23.5* 5 28.35 Clean break at center

9 0.1067 X X 14.5 in. NS 5 28.65 Clean break at center ..

10 0.1116 Tested by Florida Wire and Cable 28.55

II 0.1104 Tested by Rorida Wire and Cable 28.16

-12 0.1095 Tested by Rorida Wire and Cable 28.50

Metric (SI) conversion factors: I in. = 25.4 mm; I kip= 4.448 leN. Notes:

"NA" means not available.

"NS" means no slip was observed; for Specimens 6, 7 and 9 the chucks were installed before slipping occurred.

• These specimens were first installed without chucks and were loaded to first slipping; then the specimen was unloaded, chucks were installed and seated, and then loading continued to failure.

tension force. The chucks prevent the strand from slipping through the sleeves and provide the additional resistance needed to properly test the strand. Furthermore, the bearing of the chuck on the V -grips wedges the V-grips even tighter against the sleeve and strand to help prevent further slip-page. The length of the sleeve needs to be only long enough so that the sum of the chuck strength and the sleeve strength exceeds the strand capacity.

TEST PROGRAM

A series of twelve tests were done on both ;.{6 in. ( 11.1 mm) Grade 250 and ~ in. (12. 7 mm) Grade 270 strands (see Table 1). Various methods of gripping arrangements, as well as vari-ous lengths of sleeves, were tried. Only a small number of tests were per-formed due to the limited number of available ;.{6 in. (11.1 mm) strands (which had been removed from an old bridge beam) and a limited supply of FLO-LOC grip systems.

To check our results, Florida Wire and Cable Company tested three pieces of the ;.{6 in. (11.1 mm) Grade 250 strands using a Tinius-Olson

test-ing machine specially designed for testing seven-wire strands and using an extensometer.

INSTRUMENTATION

A suitable extensometer was not available for our tests. On several strands, strains were measured in the center region between the sleeves using either three or four SR-4 electri-cal resistance strain gages of Ys in. (3.18 mm) gage length bonded to dif-ferent wires of the seven-wire strand.

The clear length between sleeves var-ied from 5 to 24 in. (127 to 610 mm). Load was applied with a 60 kip (270 kN) hydraulic universal testing machine. The machine was held at constant load for about 15 seconds while the gages were read. Readings were taken at load increments of 2 kips (9.0 kN) except near the failure load where the increment was 1 kip (4.5 kN).

TEST RESULTS

Table 1 summarizes the results. Specimens 1 to 6 were Y, in. (12.7 mm) Grade 270 strands used to

devel-op the test method. When these strands were gripped using only machine V -grips (Specimen 1) or only prestressing chucks (Specimen 2), no strand slippage was observed; howev-er, the strands failed prematurely at the location of the first nick caused by the V -grip or the chuck jaw. Specimen 2, using only chucks, carried about 85 percent of the full strand strength.

Using a combination of chucks and machine V-grips (Specimen 3) pro-duced the same mode of failure as using only chucks, probably because the chuck jaw teeth were much sharp

-er than the worn V -grip teeth.

When Specimens 4 and 5 were encased with 12 and 14 in. (305 and 356 mm) lengths of sleeves, respec-tively, and then gripped with only the machine V -grips, the strands slipped in the sleeves at very low loads. When using these same specimens with the modified procedure with ·a chuck attached, no slippage was observed,

but the break occurred inside the sleeve at a load slightly below the full strand strength. Specimen 6 with chucks and sleeve lengths of 20 in. (508 mm) produced a clear break in the center region of the specimen.

Specimens 4 and 5 were first tested without chucks to determine their slip-ping loads. This initial slippage may have worn off some of the grit coating on the sleeves and may have lowered the effectiveness of the sleeves when these same specimens were subse-quently tested to ultimate capacity. It

is possible that even these shorter sleeve lengths may have been effec-tive for this modified method if slip-ping tests had not been done first on these specimens.

Specimens 7 to 12 were Y.6 in. (11.1 mm) Grade 250 strands that had been removed from the same bridge beam. Specimens 7 to 9, tested by the modi-fled test method, and Specimens 10 to 12, tested by Florida Wire and Cable

Fig. 6. Typical break in center region.

Fig. 7. Closeup view of strand showing pure tensile failure.

300

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Strain (in/in)

Fig. 8. Stress-strain curve for Specimen 7 (1 ksi = 6.895 MPa).

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Fig. 9. Stress-strain curve for Specimen 9 (1 ksi = 6.895 MPa).

Company with a special test machine, were randomly selected from the six available samples. Specimens 7 to 9 had measured areas less than the nomi -nal area, while Specimens 10 to 12 had measured areas greater than the nomi-nal area (Table 1). To eliminate the effect of variations in the strand area measurements, it is probably best to interpret the test results using the mea -sured load or the stress and modulus calculated using the nominal strand area (as used in the figures in this paper).

Specimens 7, 8 and 9 used the modi-fied procedure with sleeve lengths of 20, 17 and 14.5 in. (508, 432 and 368 mm), respectively. Overall strand lengths and the corresponding clear spans between sleeves were 66 and 18 in. (1680 and 457 mm), 48 and 6 in. (1220 and 279 mm), and 48 and 11 in. (1220 and 279 mm), for Specimens 7,

8 and 9, respectively. Each specimen failed with a clear break in the center region (Figs. 6 and 7) and showed the characteristic pure tensile failure sur-faces. It is possible that even shorter sleeve lengths may be satisfactory; however, this minimum length was not determined.

Specimens 7 and 9 were instrument-ed with strain gages. The test results,

using nominal strand area, are shown in Figs. 8, 9 and 10. These curves are similar to the load-deformation curves typically recorded for strands. The curves are best fit curves through the average strain values. The two tests compare well with each other. The last data point in each figure corresponds to the last load stage for which a strain reading was available, not the maxi-mum load or strain sustained by the specimen. The total strain at failure was not recorded.

Data were recorded past the elastic and yield points well into the inelastic region of behavior. By using only one strain indicator to monitor all of the strain gages, it was not possible to get reliable strain readings past the points shown. Although the readings from the individual strain gages varied, using several gages and the best fit curve through all of the data seemed to give a reliable stress-strain curve. Of course, an extensometer rather than strain gages could be used with this

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Fig. 10. Comparison of stress-strain curves for Specimens 7 and 9

(1 ksi = 6.895 MPa).

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Specimen 11

Specimen 12

0.01

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Strain (in/in)

Fig. 11. Stress-strain curves for Florida Wire and Cable Company Specimens 10,

300

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to....

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50

Modified Specimen 7

FWC Specimen 12

0

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Fig. 12. Comparison of stress-strain curves for Specimens 7 and 12

(1 ksi = 6.895 MPa).

Table 2. Comparison of test results for V.s in. strands.

Nominal* Actual t Actual Breaking Breaking Modulus of Breaking Modulus of

Specimen area load strength elasticity strength elasticity

No. sq in. (kip) (ksi) (ksi) (ksi) (ksi)

Using modified test procedure

7 0.1071 28.50 264 26.1 266 26.3

8 - 28.35 262 - -

-9 0.1067 28.65 265 26.5 269 26.8

Average 28.50 264 26.3 267.5 26.6

Using special strand testing machine

lO 0.1116 28.55 264 I 25.6 256 24.8

II 0.1104 28.16 261 25.5 255 24.9

12 0.1095 28.50 264 25.3 260 24.9

Average 28.40 263 25.5 257 24.9

Metric (Sl) conversion factors: I sq in.= 6.451 sq em; I kip= 4.448 kN; I ksi = 6.895 MPa.

• Using nominal area of0.108 sq in.

t Using measured strand area.

modified gripping method.

Fig. 11 shows the stress-strain

curves for Specimens 10 to 12, tested

by the Florida Wire and Cable

Com-pany using their special testing

machine. Fig. 12 compares the stre

ss-strain curves of Specimen 12 and our

Specimen 7. The stress values in Figs.

11 and 12 are based on nominal strand

area. Table 2 compares the essential

results from all of these tests using

both the actual and nominal strand

areas. There is good agreement

between the two methods, especially

when using the nominal strand area.

The two test methods gave

essen-tially identical breaking strengths. As

expected, because of the tendency of

the strand to straighten under loading,

the modulus of elasticity determined

by using strain gages applied directly to the individual wires was higher than the modulus determined by using an

extensometer attached to the entire strand.

Generally, it is very difficult to

accurately measure the stress-strain

curve for strands without special

extensometers. If strain gages are

used, experienced personnel are required to install the very small strain

gages. It is best to verify the testing

technique by comparing some test

results with those from a strand

fabri-cator, as was done in this study.

ADVANTAGES OF THE

MODIFIED TEST METHOD

These tests have demonstrated a rel-atively simple procedure for gripping a prestressing strand for performing an

accurate tension test of the strand.

This gripping method is a significant

improvement over the previous

grip-ping methods. Some of these methods

require fabrication of special machine

grips or the use of special test

machines.

When gripping uses only sleeves,

such as the PLP or FLO-LOC sleeves,

the required sleeve length is affected

by many variables, such as (1) the

type of sleeve, (2) how well the sleeve

fits around the strand, (3) the direction

of the helical twist pattern of the

strand and sleeve, (4) how well the

V-grips fit around the sleeve, and (5) the

length of the V -grips.

In this modified method, different

types of commercially available

sleeves and common universal testing machines can be used. The length of

the sleeve is not nearly as critical since

the chucks can carry most of the load.

The strand gripping technique consists

of a combination of encasing the ends

of the strand with splicing sleeves

(such as the FLO-LOC strand gripping

system or a similar system) and using

standard prestressing chucks attached

to the strands and bearing on the ends

of the V -grips. The sleeves prevent the

grips from nicking the strand and

resist part of the tension force.

The chucks prevent the strand from

slipping through the sleeves and

pro-vide the additional resistance needed

to properly test the strand. The chucks

also wedge the V -grips even tighter against the sleeves. The length of the sleeve needs to be only long enough so that the sum of the chuck strength and the sleeve strength exceeds the strand capacity. Since the sleeve does not carry the full tension force, the sleeve length can be relatively short,

permitting tests of shorter strand lengths in shorter testing machines.

With this technique, laboratory per-sonnel can easily solve the difficult problem of how to grip a strand by using a common universal testing machine and readily available sleeve gripping devices.

RECOMMENDATIONS

The performance of this gripping technique depends on many variables, such as the type of strand, splicing sleeve and test machine used. It is rec-ommended that initial tests be done

with a universal testing machine using standard V -grips, with sleeve lengths of about 15 in. (380 mm) long, and without chucks. If slipping does occur,

chucks can be added to provide posi-tive anchorage of the strand.

ACKNOWLEDGMENTS

This work was performed as part of the research project, "Tests of Pre-stressed Concrete Bridge Beams," sponsored jointly by the Connecticut Department of Transportation and the Precast/Prestressed Concrete Institute. The authors gratefully acknowledge the assistance provided by Blakeslee Prestress, Inc., in Branford, Connecti-cut, and, Florida Wire and Cable Company in Jacksonville, Florida, for donating the FLO-LOC grips and for performing stress-strain tests, H. Kent Preston, and graduate_student Valerie Murray.

REFERENCES

1. ASTM, "A370, Methods and Defini-tions for Mechanical Testing of Steel Products," Annual Book of ASTM Stan-dards, V. 01.04, American Society for Testing and Materials, Philadelphia, P A,

1989.

2. Preston, H. K., "Testing 7-Wire Strand for Prestressed Concrete - The State of the Art," PCI JOURNAL, V. 30, No. 3,

May-June 1985, pp. 134-155.

3. Shenoy, C. V., and Frantz, G. C., "Tests on Prestressed Concrete Bridge Beams, Part I - Structural Tests of the Bridge Beams," Research Report JHR 91-l98a, Project 87-3, Department of Civil Engi-neering, University of Connecticut, Jan-uary 1991, 120 pp.

4. Murray, V. E., and Frantz, G. C., "Tests on Prestressed Concrete Bridge Beams, Part II - Chloride Penetration in the Bridge Beams," Research Report JHR 91-198b, Project 87-3, Department of Civil Engineering, University of Con-necticut, January 1991, 125 pp.