mechanical properties of a new prestressing strand with ultimate strength of 2160 mpa

KSCE Journal of Civil Engineering (2014) 18(2):607-615

Copyright ⓒ2014 Korean Society of Civil Engineers

DOI 10.1007/s12205-014-0065-6

− 607 −

pISSN 1226-7988, eISSN 1976-3808

www.springer.com/12205

Structural Engineering

Mechanical Properties of a New Prestressing Strand with

Ultimate Strength of 2160 MPa

Jin Kook Kim*, Jeong-Su Kim**, and Seung Hee Kwon***

Received January 29, 2013/Revised March 24, 2013/Accepted May 15, 2013

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Abstract

A new prestressing strand was recently developed. The objective of this study is to investigate and provide the mechanicalproperties of the new strand, as these properties are essential in the design and construction of prestressed concrete structures. Theexperimental program includes a tensile test, a fatigue test, a relaxation test, and a stress corrosion test. In the tensile test, an optimalgripping method was initially determined and the measured ultimate strength was found to be higher the nominal strength of the newstrand. The new strand does not exhibit any degradation in its mechanical performance after fatigue loading. Compared to theexisting strand with the strength of 1,860 MPa, the new strand also showed equal or enhanced performance in the stress relaxationand the stress corrosion tests.

Keywords: high strength steel strand, 2160 MPa, tensile test, fatigue, relaxation, stress corrosion

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1. Introduction

Concrete is vulnerable in tension. Tensile cracking, which

negatively influences the safety, serviceability, and durability of

concrete structures, is inevitable in RC (reinforced concrete)

structures even while they are in service. To control cracking

proactively or to avoid cracks altogether, compressive pre-

stressing is generally introduced over the potential tensile region

of concrete members. This is done by applying tensile force to

strand wire at a high strength. In addition, the prestressing also

reduces the size of the structural members by making them more

slender and longer. Typical structures to which prestressing force

is applied are the containments in nuclear power plants and in

LNG storage tanks, floating structures, cross-beams of pylons in

suspension and cable-stayed bridges, and long-span concrete

girders.

One of the key elements in the prestressing technique is the

strand wire. A prestressing strand with the ultimate strength of

1,860 MPa has been in common use around the world since the

use of this type of strand was first commercialized in early

1980s. In the early 2000s, a prestressing strand with the ultimate

strength of 2,230 MPa was introduced in Japan, with the new

strand later applied to a footbridge in Tokyo. In Korea, a new

strand with the strength of 2,160 MPa was developed in 2010,

the anchorage system for the new strand was devised in 2011,

and these strands were subsequently used in two prestressed

concrete bridges (Kim et al., 2010).

Although strands with higher strengths have been developed

since, they are not broadly applied to real structures due to the

insufficient experimental verification of the mechanical perform-

ance of the new strands, a lack of studies that have investigated

the structural behavior of the structures adopting these strands,

and the lack of the completion of the design specifications.

The structural performance measures, that is, the safety, ser-

viceability, and durability, strongly depend on the mechanical

properties of the construction material. The use of a construction

material with a higher strength can make it possible to reduce the

size of the structural members and the total amount of required

materials, but it may also decrease the safety margin for the

structures when they become more slender with the use of the

higher strength material. When a new construction material is

given to the construction field, therefore, the mechanical prop-

erties of the new material should be fully experimentally verified.

The objective of this study is experimentally to investigate the

mechanical properties of the newly prestressing strand with the

ultimate strength of 2,160 MPa, as these are the essential par-

ameters in the design and construction of prestressed concrete

structures. The experimental program performed in this study

includes a tensile test, fatigue test, relaxation test, and a stress

corrosion test. The tensile behavior, especially the tensile failure

TECHNICAL NOTE

*Member, Senior Researcher, Energy Infrastructure Research Dept., Steel Structure Research Division, Research Institute of Industrial Science and Tech-

nology, POSCO Global R&D Center, Incheon 406-840, Korea (E-mail: [email protected])

**Senior Researcher, Hanwha Research Institute of Technology, Hanwha E&C, Daejeon 305-804, Korea (E-mail: [email protected])

***Member, Associate Professor, Dept. of Civil and Environmental Engineering, Myongji University, Yongin 449-728, Korea (Corresponding Author, E-

mail: [email protected])

Jin Kook Kim, Jeong-Su Kim, and Seung Hee Kwon

− 608 − KSCE Journal of Civil Engineering

of the strand during the tensile test, strongly depends on the

gripping method owing to the strand’s vulnerability to form a

notch in the vicinity of the grip. Several gripping methods were

considered in the tensile test and the optimal gripping method

was determined. The fatigue test and the relaxation test were

performed with the optimal gripping method. First, an explan-

ation of the factors influencing the failure behavior of the strand

during the tensile test will be given. Second, the experimental

program will be explained in detail. The test results and a

discussion follow, after which conclusions are drawn.

2. Factors Influencing the Failure Behavior of theStrand in the Tensile Test

2.1 Friction between the Center Wire and the Circumfer-

ential Wires

The new strand with the ultimate strength of 2,160 MPa con-

sists of seven wires. One wire is located in the center, and the

other six wires are twisted around the center wire. Fig. 1 shows

the twisting process during the manufacturing of the strand and

the six circumferential twisted wires.

When the strand is subjected to tension force in the tensile test,

the force or the stress should be uniformly distributed over all of

the wires to obtain the accurate tensile behavior. If a uniform

distribution is not attained, the wire on which the stress is more

concentrated may fail or fracture first. In this case, the accurate

strength of the strand cannot be measured in the test.

In the test, while the gripping force is directly transmitted to

the outer six wires, the center wire is pulled by friction force at

the interface between the center wire and the other six wires. The

friction force is generated by confinement force, specifically the

normal force acting on the side face of the center wire, as shown

in Fig. 2. As tensile force is applied to the strand, the twisted

wires tend to straighten, having a confinement effect on the

center wire. If the friction coefficient between the wires is not

large enough, the center wire would remain mostly intact, even

after a rupture takes place in the circumferential wire. Therefore,

a sufficient amount of friction between the center and the

circumferential wires must exist in the tensile test.

2.2 Protection Method for the End Parts of the Strand

In the tensile test, two end sides of the strand are held by the

grip. If the grip wedge comes into direct contact with the strand,

a notch appearing in the form of a small crack may form at the

end part of the strand due to the stress concentration induced by

the gripping force. Such a notch would lead to the early rupture

of the wire at the end part of the strand prior to the failure of the

other wires.

There are two typical methods to protect against the formation

of a notch, as shown in Fig. 3. The first method is to wrap the end

part of the strand with silica-coated aluminum foil. The second is

to use a round aluminum tube. The end part is inserted into the

tube and epoxy is injected into the space between the strand and

the tube. Earlier studies reported that an aluminum tube is more

efficient when seeking to prevent the formation of a notch

(Godfrey, 1956; Hill, 2006; Podolney, 1967; Preston, 1985; Preston,

1990).

2.3 Shape of Grip Wedge and Gripping Force

It is certain that the shape of the grip wedge and the griping

force are the main factors that influence the formation of a notch

Fig. 1. Manufacturing of the Prestressing Strand: (a) Twisting Pro-

cess of Seven Wire Rods, (b) Seven Wire Prestressing

Strand Fig. 2. Confinement Stress Acting the Center Wire under Tension

Mechanical Properties of a New Prestressing Strand with Ultimate Strength of 2160 MPa

Vol. 18, No. 2 / March 2014 − 609 −

in the vicinity of the grip. These factors also affect the uniform

distribution of the applied tension force to each wire. Fig. 4 shows

a schematic illustration of the shape of the grip and the gripping

force. The extent of the stress concentration in the grip and the

position of the highest stress concentration depend on the shape

of the grip wedge. In addition, the magnitude of the gripping

force should be appropriate to fix the end part of the strand to the

grip. If the force is too small, slippage between the grip and the

strand would occur and a uniform distribution of the tensile force

to the wires would likely not be accomplished. Contrarily, if the

force is too large, an excessive stress concentration would be

induced and would lead to an early rupture of the wire.

3. Experimental Programs

3.1 Materials

Two types of strands were prepared for the tests: a strand with

the ultimate strength of 2,160 MPa and a strand with the ultimate

strength of 1,860 MPa. The strands consist of seven wires and all

have the same dimensions: a diameter of 15.2 mm and a nominal

cross-sectional area of 138.7 mm2.

The tensile test, relaxation test and the stress corrosion test were

performed with both strands (the 2,160 MPa strand and the 1,860

MPa strand). The 1860 MPa strand is supposed to satisfy the fati-

gue performance requirement because the strand has been widely

used in the practice. This study only concerned about the fatigue

performance of the new strand. Therefore, the fatigue test was

carried out only with the strand of 2,160 MPa. The test param-

eters and the test method for each test are given below. The test

methods for all of the tests follow the standard test specifications

(ASTM A416, 2012; Fib, 2005; ISO 15630-3, 2010; JIS G3536,

1999; KS B 0802, 2003; KS D 7002, 2011; prEN 10138-3, 2009).

3.2 Tensile Test

Three test variables were considered: the nominal strength, the

type of grip, the protection method for the end part and the fric-

tion condition. Five types of grips with different shapes and dif-

ferent gripping forces were adopted in the tests, as listed in Table

1. Also, the notations for the wedge angle (α), wedge length (lG)

and the wedge height (hG) are illustrated in Fig. 4. The length of

the grip holding the test specimen was 100 mm. Table 2 shows

the test parameters. Two protection methods, the silica-coated

Aluminum Foil (AF) protection and the epoxy filled Aluminum

Fig. 3. Typical Protection Method for the End Parts of the Strand:

(a) Silica-coated Aluminum Foil, (b) Epoxy-filled Aluminum Tube

Fig. 4. Shape of the Grip and the Gripping Force

Fig. 5. Test Setup for the Tensile Tests

Table 1. Types of Grip used in the Tests

Grip Type PG (kN)Dimensions of the Grip

α (°) lG (mm) hG (mm)

G149.0

124 27.0 6.00

G2 110 22.0 7.05

G3 56.0

120 15.0 3.68G4 70.0

G5 91.0



Tube (AT) were used, as mentioned above. There are two condi-

tions for the friction between wires; first case involves a lubri-

cating agent spread onto the surface of the strand (NB), whereas

in the second no treatment is applied to the strand (B).

Figure 5 shows the test setup for the tensile test. A universal

testing machine with a capacity of 1,000 kN was used, and the

elongation of the strand was measured at the cross-head of the

actuator. Fig. 6 shows the grips for the specimens, S1-G2-AF-B

and S1-G3-AT-B.

3.3 Fatigue Test

The fatigue test followed the method specified in the ISO

15630-3 specification (2010). A cyclic load was applied to two

strands with the ultimate strength of 2,160 MPa. The same testing

machine used in the tensile test was used here as well, and the

test setup was identical to that of specimen S1-G4-AT-B, as shown

in Table 3.

The stress range for the fatigue test of the post-tensioned strand

specified in prEN 10138-3 was adopted in the test. The max-

imum load was 210 kN, which corresponds to 70% of the nom-

inal ultimate strength, i.e., 1,512 MPa. The range between the

maximum and the minimum stress was 190 MPa. The frequency

and the number of the cyclic loadings were 6 Hz and 2 million,

respectively. After applying the cyclic load, the tensile test was

performed with one of the two strands.

3.4 Relaxation Test

The test setup used in the relaxation test is the same as the G4-

AT-B specimen shown in Table 3. At a room temperature of 20oC,

tensile force was applied to a strand at a loading speed of 2005±0

MPa per minute and up to 70% of the actual strength. After

loading, it was confirmed that the maximum load is constantly

sustained for 120±2 seconds. Next, as the elongation of the strand

at the maximum stress was fixed for 1,000 hours, the variation of

the load was measured. In the ISO 15630-3 specification (2010),

the low relaxation strand should meet the condition for which the

variation of the load after 1,000 hours is less than 2.5% of the

maximum load such that the strand can be specified as a low-

relaxation strand. The relaxation tests were performed with two

Table 2. Test Program for the Tensile Tests

Nominal Strength(MPa)

Grip typeProtection of

end partFriction

between wiresInitial distance

between grips (l0)The number of

companion specimenDesignation of

specimen

2160 (S1)

G1

AF*

B+

540

3

S1-G1-AF-B-#1

S1-G1-AF-B-#2

S1-G1-AF-B-#3

NB++ 3

S1-G1-AF-NB-#1

S1-G1-AF-NB-#2

S1-G1-AF-NB-#3

G2 B 2S1-G2-AF-B-#1

S1-G2-AF-B-#2

G3

AT** B 800

2S1-G3-AT-B-#1

S1-G3-AT-B-#2

G4 2S1-G4-AT-B-#1

S1-G4-AT-B-#2

G5 1 S1-G5-AT-B-#1

1860 (S2) G4 AT B 800 3

S2-G4-AT-B-#1

S2-G4-AT-B-#2

S2-G4-AT-B-#3

*AF: Aluminum Foil, **AT: Aluminum Tube, +B: Bond, ++NB: No Bond

Fig. 6. Gripping Methods used in the Tests: (a) Specimen S1-G2-

AF-B, (b) Specimen S1-G3-AT-B


Vol. 18, No. 2 / March 2014 − 611 −

strands with different nominal strengths of 2,160 MPa and 1,860

MPa and the results were compared.

3.5 Stress Corrosion Test

The stress corrosion tests were performed with the center wire

extracted from the strand because the loading capacity of the test

instrument is limited and because stress corrosion is a character-

istic related to chemical reactions in the raw material of the

strand. Fig. 7(a) shows the apparatus used to apply the sustained

load, and Fig. 7(b) represents the details of the corrosion test cell.

During the test, the wire was immersed in a solution that accel-

erated the corrosion process. The solution was composed of 800

ml of distilled water and 200 g of ammonium thiocyanate. The

temperature of the solution inside the test cell was set to 50±1oC

and was held constant over time by external heating. The immer-

sion length of the specimen was 240 mm. The tension force cor-

responding to 80% of the ultimate strength was applied to the

immersed wire and was sustained over time. The preliminary

tests were performed to measure the ultimate strength of the wire

with three wires. The measured ultimate strength of the wire

extracted from the 2,160 MPa strand was 47.7 kN and the applied

load was 38.1 kN. Thirteen wires in all for the 2,160 MPa strand

were tested. The same test for the 1,860 MPa strand were per-

formed for a comparison with the results from the 2,160 MPa

strand. The specification, ISO 15630-3 (2010), specifies that the

minimum and the median elapsed times until the rupturing of the

wire should exceed 2 and 5 hours, respectively.

4. Results and Discussion

4.1 Tensile Test

All of the specimens were tested up the ultimate strength, as

determined by the rupturing of the wires. Three failure modes

Table 3. Results of the Tensile Tests

Specimen

Test Results

Failure ModePeak Load (kN) Maximum Displacement (mm)

Measured Averaged Measured Averaged

S1-G1-AF-B-#1

Mode 1

282

288

59.8

65.9S1-G1-AF-B-#2 294 71.6

S1-G1-AF-B-#3 287 66.2

S1-G1-AF-NB-#1

Mode 1

253

250

57.5

61.4S1-G1-AF-NB-#2 270 67.8

S1-G1-AF-NB-#3 228 59.0

S1-G2-AF-B-#1Mode 3

303306

33.541.3

S1-G2-AF-B-#2 310 49.1

S1-G3-AT-B-#1 Mode 1 263 263 10.8 10.8

S1-G3-AT-B-#2 Mode 3 310 310 44.4 44.4

S1-G4-AT-B-#1Mode 3

309310

42.047.9

S1-G4-AT-B-#2 311 53.7

S1-G5-AT-B-#1 Mode 2 264 264 18.2 18.2

S2-G4-AT-B-#1

Mode 3

273

272

51.2

51.5S2-G4-AT-B-#2 271 52.8

S2-G4-AT-B-#3 273 50.4

Fig. 7. Corrosion Test Setup: (a) Apparatus, (b) Test Cell



were observed in the snapped wires, as shown in Fig. 8. In Mode

1, the cutting plane appeared to be torn, creating an angle of 30 to

45 degrees along the longitudinal axis. In Mode 2, the cutting

plane was very rough, and its angle was larger than that of Mode

1. In Mode 3, the cross-sectional area of the wire near the cutting

plane was reduced. In other words, necking occurred.

The test results are summarized in Table 3, in which the failure

mode, peak load, and maximum displacement for each specimen

are listed. All of the specimens with the G1 grip have the failure

mode of Mode 1. The specimens with the G2 and G4 grips ex-

hibit the Mode 3 failure. In the case of G3, one Mode 1 failure

was observed and one Mode 3 failure was observed. The Mode 2

failure occurred only in the G5 grip. The ultimate loads are com-

pared according to the failure modes in Fig. 9(a). The load corres-

ponding to the nominal strength of 2,160 MPa is 300 kN. The

ultimate loads of the only specimens (G2 and G4) showed the

Mode 3 failure exceeded 300 kN, whereas the others did not

reach the nominal strength level.

As shown in Fig. 9(b), the ultimate loads were different for the

G1 and G2 grips, for which the gripping forces were identical but

the shape of the wedge differed. This indicates that the ultimate

strength depends on the shape of the wedge, most likely because

the stress distribution or the stress concentration near the grip is

influenced by the shape of the wedge.

Regarding cases G3, G4, and G5, the shapes of the wedge are

identical and the gripping forces increase in the order of G3, G4

and G5. As depicted in Fig. 9(c), the ultimate loads also depend

on the gripping force, which should be appropriate to avoid slip-

page between the grip and the strand due to the small gripping

force and to prevent the formation of a notch near the grip via the

large gripping force. It was noted that G4 grip provides an op-

timal gripping condition.

The effects of the friction conditions between the wires on the

tensile behavior were compared, as shown in Fig. 10(a). The

strength and the increasing slope of the specimens for case NB

Fig. 8. Failure Modes of the Strands: (a) Mode 1, (b) Mode 2,

(c) Mode 3

Fig. 9. Comparison of the Ultimate Loads: (a) Failure Mode,

(b) Grips G1 and G2, (c) Grips G1, G2 and G3


Vol. 18, No. 2 / March 2014 − 613 −

were definitely lower than those for case B. This finding indi-

cates that slippage between the center and the circumferential

wires occurred for case NB and that the slippage led to a reduc-

tion of the strength and the stiffness of the strand. To avoid slip-

page in the tensile tests, a lubricating substance or any foreign

object that can hinder the friction between the wires should be

removed.

Figure 10(b) shows the relationships between the load and the

strain for the specimens with the aluminum tube protections, AT.

The initial slopes for the AT case are much higher than those of

the specimens with the aluminum foil protection, AF. The lower

initial slopes for the AF case are most likely due to slippage

between the grip and the strand. From a comparison between

Figs. 10(a) and (b), it can be seen that the aluminum tube protec-

tion is more efficient for preventing slippage between the strand

and the grip.

As a result, the optimal gripping condition was found to be the

G4-AT-B case, in which the grip type was G4, the aluminum pro-

tection AT was used, and the friction condition B was applied.

The maximum load and the elongation of the new strand were

310 kN and 6.0%, respectively. The maximum load corresponds

to the strength of 2,230 MPa.

The strands with the nominal strength of 1,860 MPa were tested

with the optimal grip condition, G4-AT-B, and the results were

also shown in Table 3 and Fig. 10(b). The averaged ultimate

strength was 272 kN which is equivalent to the strength of 1,960

MPa, and the elongation was 6.4%.

In the test with the same grip condition, the actual strengths of

both strands were higher than the nominal strengths, and the

elongations were also greater than 3.5% which is the limit spe-

cified in the standard test specification (ASTM A 416, 2012). It

was turned out from the comparison that the new strand satisfies

the mechanical performance targeted in terms of the ultimate

strength and the elongation.

4.2 Fatigue Test

The fatigue tests were performed with two strands under the

same gripping condition as G4-AT-B. After two million cyclic

loadings, no failure occurred in the two strands. Although the

steel experiences fatigue loading, the strength of carbon steel

does not degrade as long as any damage does not occur. If the

strands are tested under a wedge grip, a small damage may

propagate near the wedge tip and affect a tensile property after

fatigue loading. Therefore, the static tensile test was carried out

with one of the two strands, and the result was compared to that

of the G4-AT-B#2 specimen, as shown in Fig. 11. Although the

maximum strain of the strand experiencing the cyclic loading

was slightly less, the increasing slope and the ultimate strength

were nearly identical. It was found that the new strand does not

exhibit any degradation in the mechanical performance after the

cyclic loading.

4.3 Relaxation Test

In the relaxation tests, the initial loads applied to the strands

with the nominal strengths of 1,860 MPa and 2,160 MPa were

190 kN and 220 kN, repectively. The levels of the initial loads

were determined as 70% of the actual strengths for the both

strands. Fig. 12 shows a comparison of the relaxation test results

for both strands. None of the strands exceeded the maximum

Fig. 10. Relationships between the Load and the Strain: (a) G1-AF-

B and G1-AF-NB, (b) G3, G4 and G5-AT-B

Fig. 11. Fatigue Test Result



allowable limit of 2.5%. The amount of stress relaxation of the

new strand was slightly lower than that of the 1,860 MPa strand.

The new strand showed at least equal performance to the 1,860

MPa strand in the stress relaxation test. It was observed that the

relaxation of the 2,160 MPa strand was reduced in the last data

point. The reduction of the last data point at 1,000 hours com-

pared to the very former point at 700 hours was 0.064% of the

initial load. It seems that the reduction is not the real relaxation

behavior but a small inevitable experimental error.

4.4 Stress Corrosion Test

Thirteen specimens extracted from the 1,860 MPa and 2,160

MPa strands were tested under the same conditions. Fig. 13 shows

the cutting positions of the thirteen specimens and the cutting

plane, which is usually made upon the propagation of a small

notch that forms by corrosion. The time elapsed until the rupture

of the specimen was measured and was plotted over the fracture

probability, in which 100% corresponds to the specimen with the

maximum elapsed time; that is, the point at which all of the

specimens failed. The probability was assumed to linearly in-

crease with the order of the ruptures.

For the specimens extracted from the 1,860 MPa strand, the

minimum elapsed time and the median time were 2.9 and 7.9

hours, respectively, which satisfies the required performance

specified in prEN 10138-3 (2009). For the specimens from the

2,160 MPA strand, the minimum elapsed time and the median

time were 3.3 and 12.2 hours, respectively. The new strand

exhibited enhanced performance in terms of stress corrosion.

5. Conclusions

From the experiments performed in this study, the following

conclusions can be drawn.

1. The measured ultimate strength and elongation of the new

strand were 2,230 MPa and 6.0%, respectively.

2. The tensile behavior of the strand in the tensile test depends

on the gripping conditions, specifically the friction between

the wires, the protection method for the end parts of the

strand, the shape of the grip wedge, and the gripping force.

An optimal gripping method for the new strand was deter-

mined.

3. The new strand does not exhibit any degradation in its

mechanical performance after fatigue loading.

Fig. 12. Relaxation Test Result

Fig. 13. Stress Corrosion Test Result: (a) Cutting Positions, (b) Cut-

ting Plane

Fig. 14. Stress Corrosion Test Result


Vol. 18, No. 2 / March 2014 − 615 −

4. The new strand shows at least equal performance to the

1,860 MPa strand in terms of stress relaxation.

5. Compared to the 1,860 MPa strand, the new strand exhibits

enhanced stress corrosion performance.

Acknowledgement

This research was supported by a grant from the Construction

Technology Innovation Program (08CTIPE01 - Super-Long-Span

Bridge R&D Center) funded by the Ministry of Land, Transporta-

tion and Maritime Affairs (MLTM) of the Korean government.

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mechanical properties of a new prestressing strand with ultimate strength of 2160 mpa

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