Tailor Made Concrete Structures – Walraven & Stoelhorst (eds)© 2008 Taylor & Francis Group, London, ISBN 978-0-415-47535-8

Structural performance of pretensioned member with Ultra High StrengthFiber Reinforced Concrete

T. Ichinomiya, N. Sogabe, Y. Taira & Y. HishikiKajima Technical Research Institute, Kajima Corporation, Tokyo, Japan

ABSTRACT: The authors have developed a kind of Ultra High Strength Fiber Reinforced Concrete (UFC)with high compressive strength and high tensile ductility and have been studying for practical use. UFC makesit possible to reduce the weight of prestressed concrete structures. Particularly in case that UFC is appliedto pretensioned members, high bond strength can also be expected to reduce the transfer length. Autogenousshrinkage and creep in early age of UFC, however, reduces effective prestress. In this study, its basic mechanicalproperties concerning effective prestress such as shrinkage and creep in early age were investigated to get designvalues for the material. Prestressing tests were also conducted to determine transfer length and effective stress.Furthermore, flexural and shear tests using beam members were conducted and it was revealed that flexural andshear capacity could be estimated using the formula shown in “Recommendations for Design and Constructionof Ultra High Strength Fiber Reinforced Concrete Structures, -Draft” by Japan Society of Civil Engineers.

1 INTRODUCTION

Ultra high strength fiber reinforced concrete (UFC),which has high compressive strength and bendingtoughness, has come into practical use. The design andconstruction methods of the concrete are suggested by“Recommendations for Design and Construction ofUltra High Strength Fiber Reinforced Concrete Struc-tures (Draft)” (referred to as the UFC Guidelines) [1]of the Japan Society of Civil Engineers. When the UFCis used for prestressed concrete members, advantagessuch as simplicity of construction owing to the light-ness of the structure as well as the reduced size ofthe substructure can be expected. Because reinforcingbars are not required in UFC members, restrictionson construction can be removed and the thickness ofmembers can be reduced effectively.

It is expected that UFC members, which are gen-erally manufactured at factories, will be often usedas pretensioned members. In this case, prestress maybe introduced before steam curing, so that shrinkageof the UFC during the curing will be restrained bysteel members and cracking will be suppressed. How-ever, the transmission length and effective prestressof pretensioned UFC members are not clearly known.Although prestressing steel strands with a nominaldiameter of 15.2 mm or less are generally used in pre-tensioned members with ordinary concrete, it may bepossible to realize an efficient structure utilizing thecompressive strength of the UFC by adopting strandswith larger diameters.

In this study prestressing steel strands with a diam-eter of 19.3 mm are used in pretensioned membersto conduct physical property tests that investigate theshrinkage and creep of the material as well as a pre-tensioning test, so as to discuss transmission lengthand effective prestress. Flexural and shear failure testson pretensioned members are also conducted, andmethods to evaluate capacity are discussed.

2 OUTLINE OF EXPERIMENT

2.1 Mix proportion and curing of UFC

The mix proportion of the UFC used in the experi-ment is shown in Table 1. Compressive strengths ofthe order of 200 N/mm2 are achieved in the UFC, inwhich ettringite is formed. By adding a mixture of steelfibers having lengths of 22 and 15 mm to a volume

Table 1. Standard Mix proportion of the UFC used.

Mass for unit volume (kg/m3)

Pre- SteelAir mixed Fine Super- fiber(%) Water powder aggregate plasticizer (kg)

2.0 195∗ 1287 905 32.2 137.4(1.75 Vol.%)

∗Contains water in superplasticizer.

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Table 2. Prestressing test cases.

Prestressing Yield Prestressing Plannedsteel load Py load 0.9Py prestress

Specimen strand (kN) (kN) (N/mm 2)

S19 1S19.3 387 348 27S15 1S15.2 222 200 17

ratio of 1.75%, tensile strengths of about 15 N/mm2

can be achieved. In principle, wet curing at from 5 to40 deg C for 24 hours (primary curing) and steam cur-ing at 85 deg C for 24 hours (secondary curing) wereconducted.

2.2 Physical property tests

(1) ShrinkageA shrinkage test was conducted to investigate theshrinkage of the UFC during and after the curing. Alow-rigidity strain gauge was embedded in the centerof an UFC specimen of 100 mm × 100 mm × 400 mmin dimensions to measure the progression of strain withtime from the initial set.

(2) CreepA creep test was conducted by using a cylindrical spec-imen of 75 mm in diameter and 150 mm in height.The test was conducted on three stacked specimensafter the secondary curing, and the specimens wereloaded at a constant load by using a hydraulic jack.Three cases of loading at stresses of 36, 72, and100 N/mm2 were tested. Strain was measured withstrain gauges attached to the specimens, and creepstrain was obtained by deducting free shrinkage strainobserved under the same condition from the measuredstrain values.

2.3 Prestressing test

A prestressing test was conducted to investigate thetransmission length and effective prestress of preten-sioned UFC members. The test parameters are listedin Table 2, and the specimen is schematically outlinedin Figure 1.

Each UFC specimen is 100 mm × 100 mm in crosssection and 2000 mm in length, and a prestressing steelstrand is embedded at the center of the specimen. Twospecimens were used in each run of test. Prestressingsteel strands having diameters of 19.3 and 15.2 mmwere used in specimens S19 and S15, respectively.Because transmission length could be more than halfthe length of specimen S19, one end of the specimenwas mechanically secured with a grip. Each speci-men was manufactured by securing a tensioned steelstrand horizontally in a steel frame and placing UFC.After the primary curing, the specimen was prestressed

800

400

200@5=1000

200 200@2

100 200@2 300

200@2 200

200@2300 100

200@2300 300

Strain gauge

200@5=1000100

Anchor plate

400@2=800 100

S19-1

Grip

100

100

S19-2

100

100

100

100

100

100

S15-1

S15-2

Figure 1. Specimens of prestressing test.

by releasing the tension of the strand, and the sec-ondary curing followed immediately. The compressivestrength of the UFC at the time of prestressing wasabout 40 N/mm2.

Strain gauges were attached to prestressing steelstrands to measure the strain of the strands duringthe tensioning, immediately after the prestressing, andafter the secondary curing. The strain gauges werearranged at a minimum interval of 200 mm in con-sideration of the effect of their bond strength to thestrands. Strain was measured at different points of twospecimens in each run of test.

2.4 Flexural and shear tests

The test conditions are listed in Table 3, and thespecimens are schematically outlined in Figure 2. Pre-stressed beam specimens were prepared for the tests.One specimen (MPC) was used in the flexural test, andtwo specimens (SPC1 and SPC2) with different axialcompressive stress were used in the shear test. Spec-imen MPC has a T-shaped cross section with a webwidth of 100 mm and a height of 305 mm. SpecimensSPC1 and SPC2 have I-shaped cross section with aweb width of 50 mm and a height of 280 mm. A pre-stressing steel strand of 19.3 mm in diameter was used,and it was subjected to a tensile force of 348 kN, whichis 0.9 times the specified yield load. Among the threesteel strands arranged in specimen MPC, the centralstrand was untensioned and used as reinforcement. Inspecimens SPC1 and SPC2, two deformed prestress-ing steel reinforcing bars of 23 mm in diameter werearranged without tensioning, so that flexural yieldingwould not proceed in ahead of shear failure.As with theprestressing test, the specimens were prestressed after

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Table 3. Flexural and shear test cases.

Specimens

MPC SPC1 SPC2Flexural

Failure mode failure Shear failure

UFC Compressive strength (N/mm2) 201.2 201.2 196.4Flexural strength (N/mm2) 28.4 28.4 27.1Tensile strength (N/mm2) 13.4 13.4 12.9First cracking strength (N/mm2) 9.1 9.1 9.2Young’s modulus (kN/mm2) 44.9 44.9 43.5Compressive stress (N/mm2) 6.0 7.0 4.5

Prestressing Specification 1S19.3steel strand Yield load (kN) 427

Tensile load (kN) 480Young’s modulus (kN/mm2) 193

(b)Specimen MPC.

(b)Specimens SPC1 and SPC2.

400

2500

900 900100 100100

1S19.3

5812

971

47

7523

0

1S19.3

Deformed prestressing bar

400

1800

200

240

280

40

50

200550 550

Figure 2. Specimens of prestressing test.

the primary curing, and the secondary curing followedimmediately.

The specimen was simply supported, and load at twopoints of the specimen was increased monotonously.During the period from the start to the end of the load-ing, the load on the specimen was measured with aload cell, and deflection at the center of the specimenwas measured with a displacement gauge.

3 TEST RESULTS AND DISCUSSION

3.1 Physical property tests

(1) ShrinkageThe progression of shrinkage with time from the ini-tial set is shown in Figure 3. The shrinkage strainsduring the primary and secondary curing were about320 × 10−6 and 380 × 10−6, respectively, and the totalshrinkage strain was about 700 × 10−6. The shrinkage

Figure 3. Shrinkage strain after initial set.

strain after the secondary curing was very small (lessthan 50 × 10−6), probably because the entire shrinkagewas mostly caused by autogenous shrinkage associatedwith hydration.

(2) CreepFigure 4 shows the experimental results of creep strainper unit stress as well as estimates based on an equationconsidering the effect of compressive strength in Stan-dard Specifications for Concrete Structures (referredto as the Standard Specifications) [2] of the JapanSociety of Civil Engineers. The applicable range ofcompressive strength in the Standard Specifications is80 N/mm2 or less.

The measured creep strain of the specimens sub-jected to stresses of 72 and 100 N/mm2 is about80% of the estimates obtained by substituting themeasured compressive strength (206 N/mm2) into theequation of the Standard Specifications. Creep strainwas slightly smaller in the specimen subjected to astress of 36 N/mm2.

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The measured values of creep strain are verysmall (about one fifth), compared with the estimatesobtained from the equation of the Standard Specifica-tions for concrete with normal compressive strength(40 N/mm2). A study [3] reported that, in a spec-imen of ultra high strength concrete without steelfiber reinforcement (compressive strength of about150 N/mm2), creep strain observed when the specimenwas subjected to a stress of 50 N/mm2 was about 80%of the value estimated from the Standard Specifica-tions. This observation agrees with the trend observedin the present study.

3.2 Prestressing test

(1) Transmission lengthThe progression of measured strain distribution of steelstrands with time in prestressing is shown in Figure 5.Whereas two specimens were tested for each of spec-imens S15 and S19, the strain values measured atdifferent points of the two specimens are plotted ina superimposed manner in Figure 5.

The transmission lengths measured at the timeof prestressing were in the range between 500 and600 mm regardless of the diameter of the prestress-ing steel strand (about 25 and 35 times diameterin specimens S19 and S15, respectively). Althoughsufficient data could not be obtained in specimenS19 because the strain gauges became incapable ofmeasurement, transmission length remained about thesame in specimen S15 after the secondary curing.

The conservative transmission lengths used fordesigning pretensioned normal-strength concretemembers are 65 and 90 times steel strand diameterfor diameters of 15.2 and 19.3 mm, respectively. Asimilar test was conducted by using a specimen ofultra high strength concrete without steel fiber rein-forcement (compressive strength of about 150 N/mm2)at the material age of two days when compressivestrength reached 75 N/mm2, and a transmission lengthof 30 times steel strand diameter was reported for adiameter of 15.2 mm [3]. Although the compressive

Figure 4. Creep strain after secondary curing.

strength of the UFC at the time of prestressing wasabout 40 N/mm2, the transmission length of the steelstrand in the UFC was smaller than that in the normal-strength concrete and was about the same as that in theultra high strength concrete.

(2) Effective prestressBecause the shrinkage and creep after the secondarycuring are very small, the effective prestress only untilthe end of the secondary curing is considered here.Relaxation of the prestressing steel is ignored becauseof the short duration of the test.

Regarding the loss of prestress, the measured andpredicted values of steel strain reduction are shownin Table 4. The measured values are the average val-ues at the center of the specimen where strain becameinvariable. The predicted values were obtained fromthe predicted change of strain in the UFC in consider-ation of stress balance between the steel strand and theUFC as well as the compatibility conditions of strain.When predicting the change of strain in the UFC, themeasured values of shrinkage and creep strain (Figure6) were used. The compressive strength at the start ofloading was assumed to be 40 N/mm2.AYoung’s mod-ulus of 20 kN/mm2 was used for calculating elastic

Figure 5. Measured strain distribution.

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shrinkage and shrinkage during the primary curing,on the basis of the relationship between compressivestrength and Young’s modulus obtained from a sep-arate test. As for the Young’s modulus of prestressingsteel strands, 215 and 227 kN/mm2 were used for spec-imens S19 and S15, respectively, on the basis of therelationship between the strain of strand and the stressof steel obtained from a separate tensile test.

The loss of prestress until the end of prestressing((A)+(B) in Table 4) is due to the elastic deformationof the UFC and the shrinkage during the primary cur-ing. The predicted value was close to the measuredvalue in specimen S19 and S15. The loss of prestressduring the secondary curing ((C)+(D) in Table 4) isdue to the creep strain and shrinkage during the cur-ing. Although the predicted value was close to themeasured value in specimen S19, overestimation wasobserved in specimen S15.The estimated effective pre-stress factor, given by the ratio of steel strain at the timeof tensioning to steel strain at the end of the secondary

Figure 6. Creep strain during primary curing.

Table 4. Estimation of effective prestress. Unit of strain: 10−6.

S19 S15

Specimen Experiment Pridiction Experiment Pridiction

(A) Shrinkage – 252 – 276(Primari curing)

(B) Elastic deformation (1318)∗ 1384 (792)∗ 864(A) + (B) 1661 1636 1212 1140(C) Creep strain∗∗ – 953 – 618(D) Shrinkage – 300 – 328

(Secondary curing)(C) + (D) 1164 1253 563 946(A) + (B) + (C) + (D) 2825 2889 1775 2086Steel strain 6791 6555

at prestressingEffective prestress 0.58 0.57 0.73 0.68

factor

∗Value measured on UFC surface.∗∗Value estimated measured creep strain from prestressing to start of secondary curing.

curing, was somewhat close to the measured value inspecimen S19 and underestimated in specimen S15.

The observed differences between specimens S19and S15 may be explained by difference in bond con-dition before secondary curing, difference betweenpredicted and measured creep values owing to largeprestress values, and the effect of slippage in the 19.3-mm-diameter prestressing steel strand, which consistsof three layers. Further studies are required in thefuture to evaluate these effects quantitatively.

3.3 Flexural test

The relationship between load on specimen MPC anddisplacement at the center of the specimen is shown inFigure 7, and a failure state of the specimen is shownin Photo 1.

Flexural cracks occurred when a load of 120 kN wasapplied to the specimen, and the number of fine cracksincreased with load. After reaching the maximum loadof 416 kN, the upper edge of the UFC failed and loaddecreased gradually.

The results of analysis using stress-strain curvesmodeled according to the UFC Guidelines are shownFigure 7. Here, measured values of compressivestrength were used, and tensile strength was convertedfrom flexural strength on the basis of the relationshipbetween tensile strength and flexural strength obtainedin a separate test. The material factor was assumed tobe 1.0. For the 19.3-mm-diameter prestressing steelstrand, a bilinear curve having a bend at 0.7 timesthe specified tensile strength was used for the mod-eling. In the analysis, a cross section was dividedinto fiber elements, and equilibrium of force withinthe cross section based on Bernoulli-Euler theory was

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Figure 7. Results of flexural loading test for specimen MPC.

Photo 1. Specimen MPC after loading.

Figure 8. Results of shear loading tests.

considered. Because the initial strain of the prestress-ing steel strands right before the loading could not bemeasured, strain values obtained from the prestressingtest at the same tensile force were used.

The flexural capacity obtained by the analysis is425 kN, which is a precise prediction. As for the load-displacement curves, it appears necessary to reevaluatethe stress-strain curves of the UFC or steel becausethe stiffness before reaching the maximum load wasslightly overestimated.

3.4 Shear test

Figure 8 shows, for specimens SPC1 and SPC2, therelationship between load and displacement at the cen-ter of the specimen. A failure state of specimen SPC1is shown in Photo 2.

Photo 2. Specimen SPC1 after loading.

Table 5. Comparison of Shear Capacity.

Experiment PredictionSpecimen (kN) (kN) Exp./Pre.

SPC1 306 261 1.17SPC2 293 246 1.19

Diagonal cracks occurred in specimen SPC1 at aload of 200 kN, and the number of diagonal cracksincreased with load. At the maximum load of 611 kN,one of the diagonal cracks developed widely and loaddecreased abruptly. A similar trend was also observedin specimen SPC2; diagonal cracks started to occur ata load of 175 kN, and the maximum load achieved was586 kN.

Table 5 compares the measured values of shearcapacity with the calculated values based on the UFCGuidelines. Here, measured values of compressivestrength were used, tensile strength was convertedfrom flexural strength, and the material factor wasassumed to be 1.0.

Shear strength was evaluated rather conservativelyby the guideline equation. It has been confirmed thatthe shear strength of pretensioned members using pre-stressing steel strands can be evaluated by using theequations of the UFC Guidelines, even when the stranddiameter is as large as 19.3 mm.

4 SUMMARY

The findings obtained in this study are summarized asfollows:

(1) The shrinkage strains after the start of set-ting obtained by the shrinkage test are about320 × 10−6 and 380 × 10−6 for the primary andsecondary curing, respectively. The total shrink-age factor is about 700 × 10−6, and the shrinkagefactor after the secondary curing is very small (lessthan 50 × 10−6).

(2) The values of creep strain per unit stress measuredin the creep test are about 80% of the estimatesobtained from the Standard Specifications andabout one fifth of the estimates for normal-strengthconcrete.

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(3) The transmission lengths obtained by the prestress-ing test are about 25 to 35 times the nominaldiameters of prestressing steel strands and aboutone third to one half of those in the normal-strengthconcrete.

(4) Although effective prestress can be predictedbroadly by using the existing calculation meth-ods, further studies are required to investigate theeffects of bond condition before secondary curing,slippage in strands, and creep before the secondarycuring.

(5) The flexural capacity of pretensioned members canbe estimated by using an equation given in the UFCGuidelines.

(6) The shear strength of pretensioned members canbe evaluated conservatively with the equation ofthe UFC Guidelines.

REFERENCES

[1] Japan Society of Civil Engineers. 2006. Recommen-dations for Design and Construction of Ultra HighStrength Fiber Reinforced Concrete Structures (Draft).JSCE Guidelines for Concrete No.9.

[2] Japan Society of Civil Engineers. 2005. Standard Spec-ifications for Concrete Structures – 2002. JSCE Guide-lines for Concrete No.3.

[3] Hishiki, Y. et al. 2004. Basic Study on PretensionedMembers with Ultra-High Strength Concrete. Journalof the Society of Materials Science, Vol. 53, No. 6:678–685.

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