Transfer and Development Lengths of Concentrically ... · = nominal diameter of prestress-ing strand f se = effective stress in prestressing steel (after allowance for all prestress

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M. A. Fernández-Prada, Ph.D.Civil Engineer

Department of Construction Engineering and Civil Engineering

ProjectsPolytechnic University of Valencia

Valencia, Spain

P. Serna-Ros, Ph.D.Civil EngineerDepartment of Construction Engineering and Civil Engineering ProjectsPolytechnic University of ValenciaValencia, Spain

C. A. Arbeláez, Ph.D.Civil Engineer



Valencia, Spain

J. R. Martí-Vargas, Ph.D.Civil EngineerDepartment of Construction Engineering and Civil Engineering ProjectsPolytechnic University of ValenciaValencia, Spain

P. F. Miguel-Sosa, Ph.D.Civil Engineer



Valencia, Spain

Transfer and Development Lengths of Concentrically Prestressed Concrete

An experimental investigation to develop a test to characterize the bond between prestressing strand and concrete has been carried out at the Polytechnic University of Valencia in Spain. The proposed test determines the transfer and development lengths by measuring the force in the strand in a series of test specimens with different embedment lengths. The test sequentially reproduces the transfer of prestress in a specimen and simulates service load through a pull-out test on the same specimen. With this technique, the Hoyer Effect is produced prior to the pull-out phase of testing. An experimental program determined the applicability of the test method and the feasibility of a standardized test for quality con-trol of prestressing strand bonding. Transfer- and development-length tests were conducted on seven-wire prestressing strand using 12 different concretes. Results show that the test is valid and practical. Test data indicate that ACI 318 equations are conserva-tive. Details of testing equipment and procedures are also presented in this paper.

The behavior of pretensioned concrete members depends on the bonding mechanism between the strand and the surrounding concrete; strand prestressing forces are main-

tained solely by this mechanism. Two fundamental requirements must be met to ensure bond integrity of prestressed concrete members: the transfer of prestressing forces at strand release and the ability of the strand to develop adequate stress forces when subjected to overload conditions.

Section 12.9 of ACI 318-05 defines the following lengths related to strand bond (Fig. 1):1

• Transfer length: the distance over which the strand should be bonded to the concrete to develop the effec-tive prestress in the prestressing steel fse.

• Flexural bond length: the additional length over which the strand should be bonded so that a stress in the prestressing steel at nominal strength of the member fps may develop.

• Development length: the sum of the transfer length and the flexural bond length.

The development length is represented by the following expression:1

ld=

fse

3000

d

b+

fps− f

se

1000

d

b

(1)

Development length

Transfer length Flexural bond length

Str

ess

inth

ep

res

tres

sin

gst

ran

d

At nominal strength of member

Prestressonly

fps

fse

Fig. 1. Idealized relationship between strand prestressing stress and development length.

September–October2006 75

76 PCIJOURNAL

where:ld = development length in tension of

pretensioned stranddb = nominal diameter of prestress-

ing strandfse = effective stress in prestressing

steel (after allowance for all prestress losses)

fps = stress in prestressing steel at nom-inal flexural strength of the member

The first term in Eq. (1) represents the transfer length of the strand, and the second term, the flexural bond length. This expression is based on test results for transfer length and flex-ural bond length.2,3 The ACI equation for ld first appeared in ACI 318-63 and has remained unchanged through ACI 318-05.1,4

The mechanical bond of steel pre-stressing strand to concrete is a function of numerous factors listed by Comité Euro-International du Béton (CEB).5 Analysis of the influence of these fac-tors was the objective of many past and recent studies.6–8 The impact of high-strength concrete in the fabrication and construction of prestressed concrete and the development of new (higher tensile strength, lower relaxation, and larger diameter) steel strands, and new materials for prestressing strands (epoxy-coated steel strands and fiber-reinforced polymers) have been the subjects of many research programs.

Significant experimental results were obtained by Cousins et al.,9 who ob-served transfer and development lengths of uncoated 0.5-in.-diameter (13 mm) seven-wire strand much greater than those calculated by ACI 318 equations. In response to these test results, the Fed-eral Highway Administration (FHWA) issued a memorandum that required the ACI development length expression to be multiplied by a factor of 1.6 and disallowed the use of 0.6-in.-diameter (15 mm) strand in pretensioned concrete for highways bridge applications.10 The FHWA memorandum generated subsequent research to study bond of pretensioned strand and to establish design recommendations for transfer and development lengths. Results from these research projects indicated wide variations in the measured transfer and development lengths; a review of these studies is presented by Buckner.11

Strand-concrete bond performance is essential for pretensioned concrete ap-plications. ACI 318-05 states that for bond applications, quality assurance procedures should be used to confirm that the strand is capable of adequate bond.1 Neither ASTM A416 nor ACI 318-05,1,12 however, include minimum requirements for bond performance of prestressing strands, and neither do other standards.13 In spite of the large number of experimental studies con-ducted to date, there is no consensus on a standard test method for bond qual-ity.14 According to Rose and Russell,15 a standard test for the strand bond in pretensioned concrete would benefit all parties involved in the precast/pre-stressed concrete industry: manufactur-ers, producers, designers, builders, and owners.

ObJeCTiVeS AnD SCOPe OF The ReSeARCh

The purpose of this study is to de-velop a test method for measuring the transfer and development lengths of prestressing strand. The most impor-tant condition for the test was that the strand release and the pull-out opera-tions be performed sequentially on the same specimen. In this way, the devel-opment length is determined for speci-mens that have been subject to prestress transfer with the same procedures used in the construction of precast, pre-stressed concrete in practice. In deter-mining the transfer and development lengths of seven-wire strand, the study included 12 different concretes in the experimental program to validate the test method.

exPeRiMenTAl DeTeRMinATiOn

OF TRAnSFeR AnD DeVelOPMenT lengTh

Various researchers use the follow-ing two methods to experimentally de-termine the transfer and development

lengths of prestressing strands:16

1. Measurement of the longitudinal concrete strains in the surface of a specimen.

2. Measurement of the strand-end slip of a prestressed member.

These two test methods are the basis of different standards to determine the transfer length of prestressing strands.17,18 These standards, however, do not include the following procedures to determine development length:

3. Breaking beams with concen-trated loads at various distances from the supports.

4. Pull-out tests.These four test methods are dis-

cussed in this paper.

Measurement of longitudinal Concrete Strain

Electrical or mechanical strain gauges are attached along the sides of a pretensioned concrete member prior to release of strand tension. When the prestressing force is transferred, the compressive strains in the concrete are registered at various distances from the ends of the member. Transfer length is determined directly from the data curve (concrete strains versus distance from the end of a member) by the slope-intercept method or the 95% av-erage maximum strain (AMS) method (Fig. 2).16,19,20 Some researchers apply a smooth fit to the data curve,21 while others use 100% of the average maxi-mum strain.22

Measurement of Strand-end Slip

Based on Guyon’s theory, this meth-od uses the amount of the strand-end slip δ during the release of tension on the casting bed.23 Guyon proposed an expression where the transfer length lt is linearly proportional to δ and in-versely proportional to the initial strand strain εsi.

lt=α

δ

εsi

(2)

The coefficient α represents the shape of the bond stress distribution along the transfer length and is typi-cally a value between 2 and 3. Several


researchers have proposed different values for α based on experimental re-sults and theoretical studies.24

beam breakage by Concentrated loading

After strand release, a series of preten-sioned beams are tested in a three-point or four-point flexural test. The variable in this beam series is the length from the end of the beam to the point of maximum moment. If the embedment length tested is greater than the required development length, the beam will reach the flexural crushing load without exhibiting bond failure. If the embedment length tested is less than the required development length,

the beam will exhibit bond failure with sig-nificant strand slip. The development length of the prestressing strand is determined as the embedment length at which flexural crushing without bond failure occurs; this method has been used in other research.21,25

Pull-Out Tests

Results of pull-out tests on specimens with different embedment lengths can be used to measure the tensile force reached by the prestressing strand in order to de-termine the development length. Mah-moud et al. provide a concrete specimen configuration for this method:22 a prism specimen comprising two concrete parts with a concentric strand, each connected by a hollow jack and a hollow load cell. After release, force is gradually applied with an internal hollow jack to separate the two concrete parts.

TeST MeThODS TO STuDy bASiC bOnD PROPeRTieS

Following are two testing techniques often used to study the basic bond prop-erties of prestressing strand:14

• Pull-out tests.• Push-in tests.

Pull-Out Tests

Several experimental studies of un-tensioned prestressing strand have

been conducted with this method.26,27 The influence of the embedment length on the maximum and average values of the bond stress are documented.14

The Moustafa pull-out test uses a concrete block and a number of unten-sioned prestressing strand samples with long embedment lengths.28 In this test, the strands are individually pulled out. This test has been used by researchers to correlate the results of the pull-out force of prestressing strand with the results of transfer and development lengths obtained in beams.6,15,25 Pull-out tests with tensioned prestressing strands were conducted to simulate an-chorage bond behavior,29 and with the exception of work done by Mahmoud et al.,22 these tests do not reproduce the transverse expansion (Hoyer Effect30) of the prestressing strand.

Push-in Tests

The influence of prestressing strand expansion can be analyzed by means of a push-in test as follows (Fig. 3):

• A strand is pretensioned be-tween two plates (A and B).

• A concrete specimen is cast around the strand near the third plate (C).

• The prestressing force is re-leased at plate A, originating movement of the strand toward plate B and creating bond stresses because the concrete is fixed at plate C.

The push-in technique has been used by researchers to simulate bond behav-ior along the transfer length.29,32 For greater control during the strand stress, a testing frame with longer free strand lengths was patterned by Rose and Russell.15 A simulation of the transfer length and the flexural bond length is simultaneously performed in the test proposed by Cousin et al.,33 wherein a hydraulic actuator at the reaction plate C moves the concrete specimen toward plate A (Fig. 3). Strand stress on the top side (toward plate A) of the concrete specimen will decrease (push in) and the strand stress on the bottom side (at plate C) will increase (pull out). Sophisticated measurement procedures that do not disturb the bond phenomenon (photoelasticity or the ul-

Fig. 2. Idealized relationship between concrete strain and specimen length.

Strand

Concrete

A

C

B

Fig. 3. The setup of plates A, B, and C shows a push-in test based on Keuning et al.31

Transferlength

Transferlength

95%Averagemaximumstrain

Specimenlength

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trasonic waves34,35) have not yet been sufficiently developed.

PROPOSeD TeST

A test to measure the transfer and de-velopment lengths of prestressing strand has been developed.36 Figure 4 shows the design of this test (called ECADA, the Spanish acronym for “Ensayo para Caracterizar la Adherencia mediante Destesado y Arrancamiento,” which translates to “Test to Characterize the Bond by Release and Pull Out” in English).

The test equipment has been de-signed to introduce some variations from the Keuning test (Fig. 3):31

• The length between frame plates A and C allows measurement of the transfer and development lengths of prestressing strand in addition to providing data on basic strand bond properties.

• Plates C and B, the two separa-tors between these plates, and the sleeve on the bottom end of the strand specimen are de-signed to simulate the sectional stiffness of the specimen during the transfer of the prestress-ing force when the specimen embedment length is equal to or greater than the transfer length. This test assembly is called the anchorage measurement access (AMA) system and is designed

to prevent the confining influ-ence of plate C, measure the force in the strand by means of a hollow-load cell, and increase the strand stress at plate B with a pull-out operation.

Test Procedure

The test procedure for a specimen is as follows:A. Preparation stage

1. Strand is placed through the holes between plates D and B.

2. Anchorage devices are put into place, and a hydraulic actuator is placed at plate A.

3. The strand is tensioned between plates D and B using the hydrau-lic actuator to pull the anchorage devices separating plate D from plate A.

4. Adjustable strand anchor-age is engaged until it makes contact with the plate D. The initial force in the strand PI is anchored. Then, the hydraulic actuator is unloaded and stress is relieved (Fig. 5).

5. Concrete is mixed and placed into the prepared form in the frame near plate C and consoli-dated. After concrete placement, the specimen is cured to achieve the desired concrete properties for testing.

B. Testing stage B1. Prestress transfer phase

1. The hydraulic actuator is placed at the frame plate A and is

A BC

Adjustable strandanchorage

Hydraulic

StrandEmbedment length

AMAsystem

Anchorage

Hollow load cell

Support separatorSleeveConcretespecimen

Frameactuator

D

Fig. 4. Design of anchorage-measurement-access (AMA) test equipment setup.

Fig. 5. Adjustable prestressing strand anchorage.


loaded to recover the force P0 (force in the strand just before release) at the adjustable strand anchorage at plate A.

2. The prestressing force P0 is released at plate D by means of the hydraulic actuator. At this moment, the transfer of the pre-stressing force to the concrete is complete. The concrete speci-men is supported at the reaction frame plate C.

3. Stabilization period. B2. Pull-out phase

1. The hydraulic actuator is posi-tioned at the frame plate C.

2. Strand stress is increased using the hydraulic actuator to pull the anchorage device to separate plate B from plate C (Fig. 6).

Using the same specimen, the ECADA test sequentially reproduces the transfer of the prestress (push-in) and anchorage (pull-out) forces in the strand in the same manner as in-service pretensioned concrete members (re-lease and overloading). In this way, the Hoyer Effect is reproduced prior to the

pull-out operation.The force in the strand is recorded

during all test stages (tensioning, re-lease, and pull-out) by a hollow-load cell placed between plate B and the an-chorage device at plate B. A pressure sensor controls the force exerted by the hydraulic actuator. The instrumentation used in the test is simple and reusable. To prevent interference with the bond phenomenon, an internal measuring device was not used in the specimens.

If the AMA system stiffness and the sectional stiffness of the specimen are equal, the value of the measured force in the strand at end B (after release) is a measure of the strand force inside the concrete specimen. System com-ponents are manipulated so the AMA system stiffness matches the specimen sectional stiffness. From the charac-teristics of the AMA system and those of the specimens, one may obtain the theoretical estimation of loss of force ΔPest registered in the strand during the transfer of prestress for specimen embedment lengths equal to or greater than the transfer length.36

interpretation of Specimen Test

Figure 7 shows the force in the strand and the force exerted by the hydraulic actuator versus time. The represented force in the strand P0 corresponds to the variation registered at the onset of the testing. Two possible test results are presented depending on the speci-men embedment length:

• If the specimen embedment length is equal to or greater than the transfer length, after the stabilization period, the force in the strand at the AMA system shows a drop ΔP equal to ΔPest (Fig. 7). This drop is due to compatibility of strains between concrete and strand. The force in the strand after the stabilization period PT (prestressing force transferred [PT = P0 - ΔP] to the concrete) will be the effective prestressing force after elastic shortening loss (PE = P0 - ΔPest).

• If the specimen embedment length is less than the transfer length, the force in the strand at the AMA

Fig. 6. Pull-out phase of testing.

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Pest

P

P

Fig. 7. Specimen test results show strand force and hydraulic actuator force versus time: a prestressed concrete specimen where embedment length > transfer length and a specimen with embedment length < transfer length.


system shows a drop ΔP greater than ΔPest after the stabilization period (Fig. 7). The prestressing force transferred to the concrete PT will be less than PE.

After the stabilization period, if the transfer of prestress force is complete (PT ≈ PE), the force in the strand is in-creased by the pull-out operation. The maximum force reached in the strand PR will depend on the specimen em-bedment length. Related to the nominal development force PD, the following conditions are noted:

• If the specimen embedment length is less than the devel-opment length, PD cannot be reached in the strand (PR < PD).

• If the specimen embedment length is equal to or greater than the development length, PD is reached in the strand during the pull-out operation.

In addition to the definition of the development length in ACI,1 Buck-ner indicates that the stress fps must be developed without strand-end slip.11

Thus, the development length will cor-respond to the lesser embedment length for which PD is reached without strand slip at the free end of the specimen. In light of this, a displacement transducer is used as additional instrumentation at the free end of the specimen during the pull-out process.

interpretation of a Complete Test

With the ECADA method, the trans-fer and development lengths are deter-mined by testing a series of specimens with different embedment lengths. The transfer length will be the lesser em-bedment length of the test specimens where the effective prestressing force PE in the strand is reached after the stabilization period. The development length will correspond to the lesser em-bedment length of the test specimens in which the nominal development force PD in the strand is reached in the pull-out operation and in the absence of strand slip at the free end of the speci-men. The resulting determination of the

transfer and development lengths will depend on the sequences of lengths of the specimens tested.

exPeRiMenTAl PROgRAM

An experimental program to verify the feasibility of the ECADA method for determining the transfer and de-velopment lengths of prestressing strand was conducted. Twelve differ-ent concretes were tested with a range of water-cement ratios (w/c) of 0.3 to 0.5, cement quantities C of 590 lb/yd3 to 843 lb/yd3 (350 kg/m3 to 500 kg/m3), and compressive strengths at time of testing f ci ranging from 3481 psi to 7977 psi (24 MPa to 55 MPa).

The concrete components are CEM I 52.5 R type cement,37 crushed lime-stone aggregate, washed rolled lime-stone sand, and ISOCRON FM-211 high-range water-reducing admixture. Table 1 shows the different concretes tested. The prestressing strand is a

Table 1. Test Program

Cement C (lb/yd3) w/cf ci

(psi)

Tested Embedment Lengths, in.

Series A Series B

843

0.30 7977 2.0, 5.9, 9.8, 13.8, 17.7, 21.7 3.9, 7.9, 11.8, 15.7, 19.7, 23.6

0.35 6817 13.8, 15.7, 17.7, 19.7, 21.7, 23.6

0.40 4496 13.8, 17.7, 21.7, 23.6, 25.6, 29.5 19.7, 23.6, 27.6, 31.5, 35.4, 39.4

7580.35 6817 13.8, 17.7, 21.7, 25.6, 29.5, 33.5 15.7, 19.7, 23.6, 27.6, 31.5, 35.4

0.40 5221 13.8, 17.7, 21.7, 25.6, 29.5, 33.5 15.7, 19.7, 23.6, 27.6, 31.5, 35.4

674

0.35 6526 13.8, 17.7, 21.7, 25.6, 29.5, 33.5 15.7, 19.7, 23.6, 27.6, 31.5, 35.4

0.40 5511 9.8, 15.7, 21.7, 27.6, 33.5, 53.1 13.8, 15.7, 17.7, 19.7, 21.7, 23.6

0.45 4061 17.7, 19.7, 21.7, 23.6, 25.6, 27.6

0.50 3481 19.7, 23.6, 27.6, 31.5, 35.4, 39.4 21.7, 25.6, 29.5, 33.5, 37.4, 41.3

590

0.40 6672 17.7, 19.7, 21.7, 23.6, 25.6, 27.6

0.45 5366 17.7, 19.7, 21.7, 23.6, 25.6, 27.6

0.50 4061 13.8, 17.7, 21.7, 25.6, 29.5, 33.5 15.7, 19.7, 23.6, 27.6, 31.5, 35.4

843 0.30 7977

Additional seriesC: 13.8, 31.5, 37.4, 53.1D: 11.8, 13.8, 15.7, 17.7, 19.7, 21.7E: 15.7, 15.7, 17.7, 17.7F: 17.7, 17.7, 19.7, 39.4

Notes: 1 lb/yd3 = 0.5932 kg/m3; 1 psi = 6.895 kPa; 1 in. = 25.4 mm.

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low-relaxation, seven-wire strand as specified in UNE 36094:97 Y 1860 S7 13.0,13 with diameter of 0.5 in. (13 mm) and a guaranteed ultimate strength of 270 ksi (1860 MPa).

The concrete specimens are 4 in. × 4 in. (100 mm × 100 mm) in cross section and are prestressed with a concentri-cally located single strand at a prestress level of 75% of the ultimate tensile strength. All specimens were subjected to the same concrete consolidation and curing conditions. The release and pull-out processes were gradual.

For each concrete type, the transfer and development lengths are deter-mined from a series of 6 or 12 speci-mens. Table 1 lists the embedment lengths that were necessary to deter-mine the transfer and development lengths in each concrete type. Addi-tional tests on the C 843 (w/c = 0.3) concrete had a double objective: to ver-ify the operation of the designed AMA system and to verify the applicability and effectiveness of the test method.

For each specimen, the release and pull-out operations of the strand with the ECADA test method were sequen-tially performed. Strand release oc-curred 24 hours after concrete place-ment, and the pull-out operation took place subsequent to the stabilization period, typically two hours after re-lease. The nominal force to attain PD in the strand was established at 35.6 kip (158 kN), corresponding to 0.1% of the nominal yield strength of strand.13

TeST ReSulTS AnD AnAlySiS

Figure 8 shows the prestressing force transferred to the concrete PT ver-sus the embedment length for the C 843 (w/c = 0.3) concrete. Test specimens with embedment lengths equal to or greater than 17.7 in. (450 mm) present identical values of PT, with PT equal to

the effective prestressing force after elastic shortening loss PE. In contrast, all the test specimens with embedment length less than 15.7 in. (400 mm) pre-sented values of PT less than PE. The test specimens with embedment lengths equal to 15.7 in. (400 mm) present data that border on values for complete transfer of prestress. Therefore, for the embedment lengths tested, it may be concluded that the transfer length de-termined by the ECADA test method is 17.7 in. (450 mm).

Figure 9 shows the maximum force reached in the strand without strand slip at the free end of the specimen dur-ing the pull-out operation. Data in Fig. 9 show that all test specimens that at-tained the effective prestressing force PE present values of PR greater than PE. Specimens of embedment length equal to or greater than 23.6 in. (600 mm) present values of force PR greater than the nominal force to develop PD (35.6 kip [158 kN]). Breakage occurred in the anchorage device without strand

0

5

10

15

20

25

30

35

40

45

0 5 10 15 20 25 30 35 40

Embedment length (in.)

Fo

rce

(kip

)

0

20

40

60

80

100

120

140

160

180

200

(kN

)

Development length

Transfer length

Cement 590 lb/yd3

w/c 0,45

0

5

10

15

20

25

30

35

40

45

0 5 10 15 20 25 30 35 40


Fo

rce

(kip

)

0

20

40

60

80

100

120

140

160

180

200

(kN

)

Development length

Transfer length

Cement 674 lb/yd3

w/c 0,35

0

5

10

15

20

25

30

35

40

45

0 5 10 15 20 25 30 35 40


Fo

rce

(kip

)

0

20

40

60

80

100

120

140

160

180

200

(kN

)

Development length

Transfer length

Cement 758 lb/yd3

w/c 0,40

0

5

10

15

20

25

30

35

40

45

0 5 10 15 20 25 30 35 40


Fo

rce

(kip

)

0

20

40

60

80

100

120

140

160

180

200

(kN

)

Development length

Transfer length

Cement 843 lb/yd3

w/c 0,35

P0 PT PR

Fig.10. Test results of prestressing force versus embedment length for various concretes. Note: 1 in. = 25.4 mm.


slip at the free end of the specimen. With the embedment lengths tested, it may be concluded that the development length determined by the ECADA test method is 23.6 in. (600 mm).

The recorded prestressing force transferred and the force developed for the different embedment lengths (Fig. 8 and 9) indicate a similar rela-tionship to that of the idealized dia-gram represented in Fig. 1 but with a slight discontinuity at the point of slope change. This result agrees with data re-ported by Mahmoud et al.22

From the results obtained in the se-ries of six specimens made with C 843 (w/c = 0.3), it is concluded that the test is systematic and reliable. For the other concretes, a series of 6 or 12 specimens with different embedment lengths were tested under the same conditions as concrete C 843 (w/c = 0.3). Results are shown in Fig. 10.

Figures 11 and 12 show results for the transfer and development lengths, respectively, for all tested concretes versus the compressive strength of concrete at the time of testing. A de-crease in the transfer and development lengths with an increase of compres-sive strength of concrete can be ob-served. Results for lengths are less than those of ACI 318-051 (where fse is ap-plied to the corresponding strand stress for the average force PE for all tested concretes and where fps corresponds to strand force PD). For transfer lengths, test results are between 50% and 80% of those calculated by ACI 318-05 equations.1 For development length, test results are between 45% and 65% of those calculated by ACI 318-05 equations.1

For the materials used and the estab-lished test parameters, good correlation to experimental results is shown in the following expressions:

lt = 29.85 - 0.0016 f ci (3)

ld = 35.08- 0.0014 f ci (4)

where:lt = transfer length (in.)ld = development length (in.)f ci = compressive strength of

concrete at time of testing (psi)The application feasibility of the

0

5

10

15

20

25

30

35

40

45

50

0 5 10 15 20 25 30 35 40 45 50 55


Fo

rce

in t

he

stra

nd

(ki

p)

0

20

40

60

80

100

120

140

160

180

200

220

Series A Series B Series C Series D

Series E Series F Average P0 Calculated value PE

(kN

)

Fig. 8. Results of prestressing force transferred versus embedment length for concrete C 843 (w/c = 0.3). Note: 1 in. = 25.4 mm.

Fig. 9. Results of maximum prestressing strand force reached without free-end slip versus embedment length for concrete C 843 (w/c = 0.3). Note: 1 in. = 25.4 mm.

Fig. 11. Transfer length versus concrete compressive strength. Note: 1 psi = 6.895 kPa.

84 PCIJOURNAL

ECADA test method for the determi-nation of the transfer and development length of prestressing strands has been verified. Moreover, studies on basic bond properties of prestressing strand can be made using this test, and the test can be applied to any type of pre-stressing tendons and wires. Therefore, the proposed test is valid as a basis for standardization of bond quality in the pretensioning industry.

COnCluSiOnS

Based on the results of this research, the following conclusions can be made:

• A valid test method for measur-ing the transfer and development lengths of prestressing strands is proposed.

• Sequential testing operations on the same specimen are repre-sentative of procedures used in actual precast/prestressed construction.

• The test is systematic and reliable.• The instrumentation used in the

test is simple and reusable. Inter-nal devices that might distort the bond phenomenon are not used.

• An increase in concrete com-pressive strength at the time of testing results in a decrease of strand transfer and development lengths.

• For concretes with compres-sive strengths from 3481 psi to

7977 psi (24 MPa to 55 MPa) at the time of testing, the transfer length results are between 50% and 80% of those calculated by ACI 318-05 equations, and the test development lengths are between 45% and 65% of those calculated by ACI 318-05 equa-tions.

• Expressions to predict the transfer and development lengths from concrete compressive strength (at time of testing and for test conditions) are presented.

ReCOMMenDATiOnS

The proposed test method can be standardized for different purposes:

• For quality control of prestress-ing strand: Under standard conditions, the transfer and development lengths can be determined. Standard conditions must include concrete materials specifications, concrete dosage, and methodology for making specimens.

• To evaluate the bond quality of a prestressing strand in construc-tion site conditions: The mate-rial characteristics (prestressing strand and concrete) must be taken into consideration.

• As an acceptance-rejection test of a prestressing strand in specific working conditions:

The test would be limited to checking whether the effective prestressing force and/or the nominal force to develop in the strand are achieved.

• To determine the effects of manufacturing procedures on bond features of a prestressing strand.

ACknOwleDgMenTS

The contents of this paper are part of the research programs being carried out by the Concrete Technology and Sci-ence Institute of the Polytechnic Uni-versity of Valencia. The authors grate-fully acknowledge the collaboration of the companies PREVALESA and ISO-CRON. Financial support provided by IMPIVA (Project IMTEIE/2000/85), the Ministry of Education and Culture (Project MAT2000-0346-P4-03), the Ministry of Science and Technology, and FEDER funds (Project MAT2003-07157) made it possible to conduct this research. The authors also express their gratitude to the technical staff of the Concrete Structures Laboratory at the Polytechnic University of Valencia for their assistance in the preparation and testing of the specimens.

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Documents

Transfer and Development Lengths of Concentrically ... · = nominal diameter of prestress-ing strand f se = effective stress in prestressing steel (after allowance for all prestress