The Effects Of As-Cast Depth And Concrete Fluidity On .
The Effects of As-Cast Depth andConcrete Fluidity on Strand BondRobert J. Peterman, Ph.D., P.E.Martin K. Eby Distinguished Professorin EngineeringKansas State UniversityManhattan, Kans.This paper presents the results from strand end-slip measurements andload tests of members that were fabricated at six different precast concrete plants over the past 2½ years. All of the work reported hereinis based on specimens that were produced using standard concretemixtures and placement techniques. As such, the data presented arebelieved to be representative of current industry practice.This study revealed that the occurrence of the so-called top-bar effect(top-strand effect) for pretensioned strands is primarily a function of theamount of concrete above the strand rather than the amount of concrete below it. Accordingly, the results of this investigation indicatethat the current design assumptions for bond in pretensioned membersare unconservative for members with strands near the top (as-cast) surface. This phenomenon can result in extremely large transfer lengthsfor strands located within a few inches of the top surface, includingthose in thin members. In addition, the top-bar effect typically becomesmore pronounced in members as concrete fluidity increases.However, these same findings also revealed that the current designassumptions for bond were generally accurate when strands werelocated deeper in the members. This was true for members made witheither flowable concrete or self-consolidating concrete.72In recent years, the use of selfconsolidating concrete (SCC) hasbeen increasing steadily amongprestressed concrete producers in theUnited States. SCC is defined as ahighly workable concrete that can flowthrough densely reinforced or geometrically complex structural elementsunder its own weight. It adequately fillsvoids without segregation or excessivebleeding and without the need for vibration.1In 2004, the Precast/PrestressedConcrete Institute (PCI) co-funded anextensive investigation to evaluate thebond between SCC and prestressingsteel in pretensioned concrete members. The PCI study had the followingthree objectives: Determine the ability of six currently used SCCs made with admixtures from each of four majoradmixture suppliers in the United States to meet current ACI2and AASHTO3 requirements forPCI JOURNAL
In 2001, detailed information aboutthe bond behavior of pretensionedstrands in SCC was essentially absentfrom the literature. Because SCC doesnot require any external vibration during placement, many design engineershave questioned the ability of SCC toachieve adequate bond with smoothprestressing steel.In 2002, the Kansas Department ofTransportation (KDOT) funded aninitial investigation to evaluate thebond between seven-wire pretensionedstrand and SCC. In this initial study,large block pullout tests18 (LBPTs)were performed at Kansas State University (KSU) using two different concrete mixtures. The first mixture wasthe standard concrete recommended byLogan,18 and the second was an SCCthat had been proposed for use in thestate of Kansas (Table 1).Concrete compressive strengthsat the time of testing were 5600 psi(38.6 MPa) for the standard concreteand 6800 psi (49.6 MPa) for the proposed SCC. LBPT results with SCChad significantly lower first-slip andultimate-load values compared withMay–June 2007Logan MixSCC MixCement (type 3)660750Fine aggregate11001500Crushed gravel19000Limestone01360Type D water reducer & retarder260Type F HRWRA070Air-entraining admixture05Water292225Water–cementitious materials ratio0.440.30Note: All values given in kilograms except for admixtures, which are shown in milliliters. HRWRA high-range waterreducing admixture; SCC self-consolidating concrete. 1 mL 0.0338 oz; 1 kg 2.2 lb.those values from the standard concrete tests (Fig. 1).Logan recommended that 0.5-in.-diameter (13 mm) strand has an averageminimum pullout capacity of 36 kip(160 kN) with a maximum coefficientof variation of 10% for a six-samplegroup. Logan has since recommendedthat the minimum average value offirst-observed slip of 0.5-in.-diameterstrand be 16 kip (71 kN).Furthermore, the results from theLBPTs with SCC in 2002 were belowthe values of 16 kip (71 kN) and 36 kip(160 kN) for first-observed slip andmaximum pullout force, respectively.Both LBPTs used strand from the sameunweathered reel that had exhibitedsatisfactory bond performance in flexural beam tests. This strand is referredto as the control strand.Based on the results from the LBPTsand similar findings from pullout testsconducted by other prestressed concrete producers in the United States(including High Concrete StructuresInc. and Stresscon Corp.), it appearedthat the use of SCC can significantly reduce the bond capacity of untensionedstrand cast in the vertical position,which would have direct design implications for all lifting devices that relyon friction. However, limited pretensioned (flexural) testing using SCCs atboth Stresscon Corp. and KSU yieldedsatisfactory results. Thus, the ability of200454039.6Maximum Pullout Force1st Observed Movement (slip)351503026.22522.510020Average Pullout Force (kN)BackgroundTable 1. Mixture Proportions for Concrete Used in Large Block Pullout TestsAverage Pullout Force (kip)transfer and development length(see “Code Provisions for Bondin Pretensioned Members” onp. 76); Quantify the top-strand effectfor pretensioned members madewith SCC; and Develop a simple, industrystandard bond test for membersmade with SCC that precasterscan readily conduct (this test willbe described in detail in a futurePCI Journal article).This paper presents the findingsof this study, which indicate that, ingeneral, the assumption of a transfer length equal to 50db or (fse/3)db islargely unconservative for memberswith pretensioned strands located nearthe top (as-cast) surface, where db isthe diameter of the strand and fse is theeffective stress in prestressed strandafter all losses. The use of highly fluidconcrete seems to exacerbate this effect.1511.85010500Logan Concrete with Control StrandSCC with Control StrandFig. 1. Results from pullout tests with conventional concrete and self-consolidatingconcrete (SCC).73
Table 2. Six SCC Mixes Used in This Study, yd3ContinuousmaterialsMix 1Type 1 cementMix 3Mix 4Mix 5Mix 6700602700584750600Type 3 cementSlagMix 2200Class C fly ash198120AggregatesClass F fly usmaterials r 01380124813751422Sand-aggregrate ratio0.490.400.490.590.470.48AdmixturesType A waterreducing admixture15Type D waterreducer and retarder43Type F HRWRA5634AE85.5Corrosion inhibitor256Admixture supplierA503255715.535.1384BCBDDNote: All values are given in pounds except for admixtures, which are shown in ounces; AE air-entraining admixture; HRWRA high-range water-reducing admixture; SCC self-consolidating concrete. 1 lb 0.45 kg; 1 oz 29.6 mL.pretensioned members to meet currentdesign assumptions for strand bondwas uncertain.Experimental ProgramAt the outset of this study, the project steering committee decided that theevaluated SCCs should represent thecurrent mixtures used at PCI ProducerMember plants. No mixtures were developed as part of this investigation.Thus, to achieve the objective of representing each of four different admixture suppliers, work was conductedat different precast concrete facilitiesthroughout the United States.This decision to fabricate specimensat different precast concrete facilitiesacross the United States meant thatthere would be numerous variablesintroduced at each plant, includingaggregate source, admixture typesand dosages, mixer type, delivery74vehicles, placement techniques, andcuring methods. Evaluating the effectof individual producer components orprocedures on bond was not a goal ofthe PCI study.To maximize consistency amongplants, the author and a doctoral student were present at each facility during casting to measure rheologicalproperties and to perform all initialmeasurements of strand end slip. In addition, each plant produced identicallysized specimens and used prestressingstrand from the same reel.Unweathered strand was set asidefrom a reel, and the required amountwas sent to each of the precast concrete producers prior to casting. Thisstrand was first prequalified for itsbonding capability using the LBPT. Inaddition, a strand with lower bondingcharacteristics was purposely introduced to the study during the castingat one plant (mix 4) in order to determine the sensitivity of the specimengeometry used in this study.As part of the experimental program,transfer length measurements weretaken via the strand end-slip measurements of the bottom- and top-strandspecimens. Due to an observed trendbetween transfer length and strandcasting position, the project steeringcommittee authorized more testing todetermine the relationship betweenstrand bond and casting position.In an independently funded study,additional testing was performed on4-in.-thick (100 mm) panels in twoprecast concrete plants, and end-slipmeasurements were gathered. Finally,a design approximation was developedfrom the gathered data.Specimen NomenclatureSpecimens in this study were designated by a mix number, a letter,and a numerical description. Theletter B is used to refer to the 10PCI JOURNAL
Table 3. Material Properties for the Six SCCsPropertiesMix 1Mix 2Mix 3Mix 4Mix 5Mix 6Air content, %6.44.01.06.07.93.5L-box differential, H2/H10.930.760.950.760.931.00Slump flow, in.2627271/2193/4231/226J-ring, in.26262718221/2241/2Visual stability index00.50.50.500Column segregation, B/T1.000.580.660.931.080.91Compressive strength at release, psi35004560370036005150583028-day compressive strength, psi8440597010,1706610923010,76028-day split tensile strength, psi45341057639549055528-day modulus of elasticity, ksi525523005200310043005350Note: B bottom; T top. 1 in. 25.4 mm; 1 psi 6.894 kPa; 1 ksi 6.894 MPa.in.-wide 15-in.-deep (250 mm 380 mm) specimens with a singlestrand cast 2 in. (50 mm) from thebottom of the specimen. Letter T refers to the 10-in.-wide 15-in.-deepspecimens with a single strand cast 2in. from the top of the specimen. SB isused to refer to the 8-in.-wide 6-in.deep (200 mm 150 mm) specimens.The designation 10 means that thespecimen had an embedment lengthequal to the calculated ACI development length Ld, while the designation08 implies that the specimen had anembedment length of only 0.80Ld.This is the reason for the two 10in.-wide beams of different lengths.All of the 8-in.-wide 6-in.-deep SBspecimens had embedment lengthsequal to 0.80Ld.Evaluated SCCs, MaterialPropertiesTable 2 shows the mixture proportions for each of the six SCCs evaluated in this study. It can be seen fromthis table that there is a large difference in the constituents and proportions of the different mixtures. Mixtures were developed by each of theprecasting plants with their admixturesuppliers. The precasting plants andmixtures chosen for evaluation in thisstudy were selected by the admixturesuppliers. Chosen plants had beenproducing SCC for more than one yearand were believed (by the admixturesuppliers) to best represent the currentstate of practice in the industry.In Table 2, the four admixture supMay–June 2007pliers are represented by the letters A,B, C, and D. In addition, each mixtureis denoted by a number, rather thanby the identity of the producer plant,to maintain the confidentiality of theparticipating organizations. One of theprecasters produced both a normalweight mixture and a lightweight mixture (mix 4). Thus, there was a total ofsix mixtures produced at five plants.Table 3 lists the properties determined for each concrete. Duringplacement, the following fresh concrete properties were measured andrecorded: air content, slump flow(spread), visual stability index (VSI),J-ring, static (column) segregation,and L-box differential. In addition tothe fresh concrete properties, the following hardened concrete propertieswere also determined: concrete compressive strength at release and at 28days, modulus of elasticity (MOE),and split tensile strength.From Table 3, it is clear that mix 4was not truly an SCC because it hada spread of only 19 3 4 in. (500 mm).However, this was the standard mixture used by a precaster that was selected by its admixture supplier forinclusion in the SCC study.Bottom-Strand SpecimensSix flexural (beam) specimens werecast with each of the six mixtures inorder to evaluate the correspondingtransfer and development lengths. Asnoted previously, the strand sourcewas identical for all beams in thisstudy except for the lower-bondingstrand that was purposely introducedduring the casting of mix 4 to determine the sensitivity of the specimengeometry.These six specimens had two different rectangular cross sections. Five ofthe specimens had cross sections thatwere 10 in. (255 mm) wide 15 in.(380 mm) deep. These beams werecast in two lengths, 11 ft 10 in. (3.6 m)and 9 ft 6 in. (2.9 m). The top (compression) width of 10 in. was selectedso that the tensile strain in the strandsat nominal moment capacity fps wouldexceed 3.5%, as recommended byBuckner.11 A rectangular shape provided a relatively large width, therebyreducing the likelihood of shear failures in these specimens (they did notcontain any stirrups). Both the projectsteering committee and external technical review committee did not wantthe strand bond in these members tobe influenced by the presence of shearreinforcement. Thus, the specimensin this study contained only a single1/2-in.-diameter (13 mm), 270 ksi(1860 MPa) prestressing strand.In addition to the 10 in. 15 in.(255 mm 380 mm) rectangular specimens, one 8-in.-wide 6-in.-deep(205 mm 150 mm) specimen with anoverall length of 9 ft 6 in. (2.9 m) wascast with each mixture. These specimens were tested as part of ongoingwork to develop a simple strand bondtest that can be conducted by precasters at their facilities.(continued on page 78)75
Code Provisions for Bond in Pretensioned MembersTransfer length is the distance required to transfer thefully effective prestressing force from the strand tothe concrete. Development length is the bond lengthrequired to anchor the strand as it resists external loads on amember.4 As external loads are applied to a flexural member, the member resists the increased moment demandthrough increased internal tensile and compressive forces.This increased tension in the strand is achieved throughbond with the surrounding concrete.5Neither the American Concrete Institute’s (ACI’s)Building Codes for Structural Concrete (ACI 318-05) andCommentary (ACI 318R-05) 2 nor the fourth edition of theAmerican Association of State Highway and TransporationOfficials’ AASHTO LRFD Bridge Design Specifications3require the use of a specific transfer length. However, ACI318 suggests a transfer length of 50 strand diameters (50db)in section 11.4.4, while AASHTO suggests a value of 60dbin section 5.11.4.The expression for development length is found in ACI318 section 12.9.1 and is shown in Eq. (1):Development length Ld fsedb/3 (fps - fse)db(1)wheredb diameter of strand (in.)fse effective stress in prestressing strand after allowance of prestress losses (ksi)fps stress in prestressing strand at calculated ultimatecapacity of section (ksi)This expression suggests a transfer length Ltr fsedb/3.The AASHTO specifications have a similar expressionfor development length but require an additional 1.6 multiplier to Eq. (1) for precast, prestressed concrete beamswith a depth greater than 24 in. (600 mm). However, instead of a suggested transfer length of fsedb/3, AASHTOFig. C5.11.4.2-1 explicitly shows the transfer length tobe 60db in the idealized bilinear depiction of strand stressvariation.Neither of the current ACI or AASHTO expressions fortransfer or development length consider the casting positionof the strand as a factor that influences the bond in pretensioned members. However, both of these documents consider the casting position of an untensioned deformed bar to becritical to the bond and corresponding development length.ACI requires a 1.3 multiplier on development lengthof deformed bars for “horizontal reinforcement so placedthat more than 12 inches (305 mm) of fresh concrete iscast in the member below the development length orsplice.”AASHTO requires a 1.4 multiplier for “top horizontal, or nearly horizontal reinforcement, so placed thatmore than 12.0 in. of fresh concrete is placed below thereinforcement.”The multipliers of 1.3 in ACI and 1.4 in AASHTO areused to address what is commonly referred to as the topbar effect. Many researchers (Clark,6,7 Menzel,8 Fergusonand Thompson,9 and Jirsa and Breen10) have documented76that bars cast near the tops of deep members can have significantly longer development lengths than those bars castnear the bottoms of identical members. This effect hasbeen attributed to the combined effects of bleed water andsettlement.When fresh concrete is placed, the excess (bleed) waterand air tend to migrate upward toward the surface, therebyallowing the remaining concrete to settle (move downward). Because reinforcing bars are typically held in position by chairs or other supports, the settlement of the surrounding concrete acts to pull it away from the horizontalbars and the effect becomes more pronounced with increasing amounts of fresh concrete placed below the bars.In addition, the bottom surface of the solid bars providesa place for bleed water and air to become trapped, therebycausing a higher water–cementitious materials ratio andpoorer consolidation of the paste in the immediate vicinityof the top bars.In 1995, Buckner11 recommended that that the development length be multiplied by a similar factor of 1.3 for anystrands (straight or draped) that end in the upper one-thirdof the member depth and have 12 in. (305 mm) or more ofconcrete cast beneath them.In 1996, Petrou and Joiner12 reported on excessiveslip that was occurring in strands located near the topsof pretensioned piles in South Carolina. The researchers experimentally determined that the strand transferlengths were directly proportional to the extremely longend slips. In many instances, the amount of end slipfor top strands was found to be two to three times theamount of end slip occurring for corresponding bottomstrands in the same section. Additional documentationof this top-strand effect in piles was presented by Petrouet al.13 and Wan et al.14,15Wan et al.14 noted that, in general, “as concrete slumpincreases, strand end slip increases.” They recommendedthat “wherever practical, the slump of concrete mixturesused for prestressed concrete pile construction should belimited to 4 inches.” The researchers also recommendedthat the use of a retarder should be avoided in pretensionedpile construction because “the presence of a retarder increases the top strand end slip while having little effect onthe bottom strand slip.” They further noted that the current practice of de-tensioning a member from the top downtended to increase the top-strand effect and recommendedthat the strands be de-tensioned in a symmetric manner,starting at the bottom of the pile.In 1998, Lane16 proposed a new development lengthequation that incorporated a 1.3 multiplier for strands thathave 12 in. (300 mm) or more of concrete cast beneaththem. A form of this equation was later adopted by theAASHTO LRFD Bridge Design Specifications17 in section5.11.4.2 with a 1.3 multiplier for top strand. However, thisequation and the corresponding top-strand multiplier wasremoved in the next edition of the specifications.3PCI JOURNAL
Mix 6Mix 5Mix 4Mix 3Mix 2Mix 1Table 4. Results from Load Tests to FailureBeam No.Age at Test,daysSpan, ftLoad atFailure, kipSlip atFailure, in.Moment atFailure Mexp,kip-ftCalculatedMn, kip-ftMexp / MnB10R2711.516.33 0.0149.5344.691.11T10R2811.516.32 0.0149.5044.271.12B10F3611.516.48 0.0149.9444.051.13SB318.837.53 0.0117.1213.701.25B08R349.1720.98 21B10R2511.516.38 0.0149.6243.731.13T10R2711.516.05 0.0148.7143.481.12B10F3411.516.94 B08R329.1721.23 22B10R2611.516.30 12B10F3511.516.31 0.0149.4843.931.13SB329.176.16 0.0114.6412.201.20B08R359.1721.20 0.0150.2539.201.28T08R379.1720.99 0.0149.7139.401.26B10R2511.515.99 0.0148.0142.471.13T10R2711.516.13 0.0148.4043.311.12B10F3211.516.56 0.0149.6943.341.15SB309.176.28 0.0114.8211.901.25*B08R329.1720.93 719.020.08744.8937.901.18*B10R2611.516.36 0.0149.5844.571.11T10R2711.516.35 0.0147.7943.931.09B10F3311.516.59 0.0150.1444.581.12SB279.176.35 0.0116.1512.601.28B08R309.1720.50 0.0147.0038.601.22T08R329.1720.72 0.0147.5038.901.22B10R2711.516.41 0.0149.6844.141.13T10R2811.516.62 0.0148.5843.291.12B10F3211.516.33 B08R329.1720.87 19** Indicates failure in shear.Note: 1 in. 25.4 mm; 1 ft 0.3048 m; 1 kip 4.45 kN.May–June 200777
(continued from page 75)60Implied Transfer Length (in.)55ReleaseTop-Strand Specimens21-Day504540(fse /3)*db 30.4"3530252050db 25"151050Mix 1Mix 2Mix 3Mix 4Mix 4Mix 5Mix 6Low BondFig. 2. Implied transfer lengths for 10 in. 15 in. bottom strand beams. Note: '' inch. 1 in. 25.4 mm.60Implied Transfer Length (in.)55Release21-Day504540(fse /3)*db 30.4"3530252050db 25"151050Mix 1Mix 2Mix 3Mix 4Mix 5Mix 6Fig. 3. Implied transfer lengths for 10 in. 15 in. top strand beams. Note: '' inch.1 in. 25.4 mm.60Implied Transfer Length (in.)55Release21-Day504540(fse /3)*db 28.5"3530252050db 25"151050Mix 1Mix 2Mix 3Mix 4Mix 4Low BondMix 5Mix 6Fig. 4. Implied transfer lengths for 8 in. 6 in. beams. Note: '' inch. 1 in. 25.4 mm.78In three of these specimens, thestrand was centered 2 in. (50 mm) fromthe bottom and had a correspondingdepth from the top of 13 in. (330 mm).In the other two specimens, the strandwas cast with a strand depth from thetop surface of only 2 in. and a corresponding distance from the bottom of13 in. These two companion specimenswere used to quantify the top-strandeffect for strand in SCC, where 12 in.(300 mm) or more of concrete was castbelow the strand.Load Test ResultsEach of the 38 specimens (six specimens per each of six SCCs with theproject strand plus two specimenswith mix 4 and a low-bonding strand)was able to withstand the calculatedACI nominal moment capacity whenloaded to failure in center-point bending. These specimens were tested afterthey achieved their design compressive strength fc' and within 38 daysof casting. This was done to investigate the perceived worst-cast scenarioin which the precast concrete productwould be erected and loaded at an earlyage. Nominal moment capacities werecalculated using strain compatibility,considering the as-built dimensions ofeach specimen and the actual concretecompressive strength.During specimen loading, the load,midspan deflection, and strand slip ateach end were continuously monitoredand recorded using a Keithley 20-bitdata acquisition system. Table 4 liststhe results from each load test, including the strand end slip occurring at themaximum load and the correspondingmoment achieved. In each case, the experimental moment exceeded the calculated ACI nominal moment capacity.In most cases, the ultimate failure wasby rupture of the strand in tension. Allother specimens failed in bond/shear,except mix 4 SB, which failed in shear.These failure modes are indicated by anasterisk in the column titled Mexp/Mn.The two specimens that incorporatedthe lower-bonding strand, and whichhad considerably higher strand endPCI JOURNAL
2.52.1Top Strand Ltr / Bottom Strand Ltrslip values, were still able to achievethe ACI nominal moment capacity.Both of these specimens failed in bond/shear. Because all of the specimens inthis study were tested at an early age, itis uncertain whether the strand in thesetwo specimens would have undergoneadditional slip with age, resulting in areduced long-term capacity. Furthermore, while several of the members inthis study had 21-day implied transferlengths that were longer than those assumed by the ACI code, these members were still able to withstand theACI nominal moment capacity whenloaded to failure.1.8May–June 20071.81.71.51.31.2 1.21.21.21.00.50.0M ix 2M ix 3M ix 4M ix 5M ix 6Fig. 5. Effect of strand casting position on implied transfer lengths.2"41/2"13"10" 15"Top-StrandBeams10" 15"Top-StrandBeams8" 6"BeamsFig. 6. As-cast strand depth of each member type. Note: '' inch. 1 in. 25.4 mm.504540Transfer Length (in.)Figures 2 through 4 show the transfer lengths, implied from strand endslip measurements, for the three different specimen types (see “EstimatingTransfer Length from Strand End-SlipValues” on p. 87). In each of these figures, the average implied transfer lengthat release is denoted by a square, witha vertical bar representing the range ofvalues obtained for the different specimen ends measured. Transfer-lengthvalues measured after 21 days are denoted in a similar manner but with acircle representing the average value.Figure 2 shows that the averagetransfer length for the 10-in.-wide 15-in.-deep (255 mm 380 mm) bottom-strand specimens was shorter than(fse/3)db for all six SCCs (when usingthe project strand that was prequalifiedfor bond using the LBPT procedure18).When the lower-bonding strand wasintroduced, strand end-slip values andthe corresponding transfer lengths morethan doubled. In these cases, the transfer lengths at release and at 21 dayswere clearly longer than the values assumed by ACI methods. This indicatesthat the sizes of the specimens used inthis study made the specimens sensitive enough to detect potential bondingproblems.Figure 3 shows the implied transfer-length results for the 10-in.-wide 15-in.-deep (255 mm 380 mm) topstrand specimens. For the top-strand21-D ay2.02.0M ix 1Results from OriginalTransfer-LengthMeasurements in SCCR elease2.22.13530252015105Mix 1Mix 2Mix 3Mix 4Mix 5Mix 6002468101214Distance from Top of Beam to Center of Strand (in.)Fig. 7. Implied transfer length versus as-cast strand depth. Note: 1 in. 25.4 mm.79
12'-0"36.3% for the bottom-strand beams,15.8% for the top-strand beams, and26.7% for the smaller, 8 in. 6 in.(205 mm 150 mm) beams. These values are somewhat larger than the 10% to20% typical increase reported by Barneset al.22 and Oh and Kim23 for pretensioned concrete members.12'-0"AB2 Per Mix2 Per MixABObserved Dependence of TransferLength on Strand Casting Position4"2"6"4"6"2"28"6"6"16"6"6"2"2"Section ASection B(Five 1/2" Special Strands)(Three 1/2" Special Strands)Fig. 8. Specimens used to de-couple the effect of strand placement on transfer length.Note: '' inch; ' foot. 1 in. 25.4 mm; 1 ft 0.3048 m.specimens, the average transfer lengthsat 21 days for four of the six mixtures(mix 3 through 6) were longer than theACI-assumed value of (fse/3)db when theprequalified project strand was used. Inthe top-strand beams, the average 21day transfer length of all six mixtureswas nearly 30% longer than the assumed value of 50db. The lower-bonding strand was not used in any of thetop-strand beams.Mix 3 had the highest implied transfer lengths for the top-strand beams.The average 21-day transfer length forthis mixture was nearly 70% longerthan 50db and nearly 40% longer than(fse/3)db. Mix 3 was the only one thatcontained a retarding admixture (Table2). Wan et al.14 noted that the use of aretarding admixture increased the topbar effect in pretensioned piles.Mix 3 also had the second largestamount of column segregation (B/T 0.66) (Table 3). Mix 2, however, hadthe largest amount of column segregation (B/T 0.58) yet had the secondlowest average implied transfer lengthsat 21 days of all top-strand beams. Byexamining Table 3 and Fig. 2, 3, and 5,it is evident that none of the rheologi80cal properties measured during the casting of the six mixtures showed consistent correlation with the correspondingstrand end-slip measurements and implied transfer lengths.Figure 5 compares the average transfer lengths Ltr of the top-strand beamswith those of the bottom-strand beamsfor each mixture. From this figure, asignificant reduction in bond associatedwith the strand casting position for allsix mixtures is evident, with the largestinitial ratio occurring for mix 3. In thetop-strand beams, there was only 2 in.(50 mm) of concrete above the center ofthe strand at the time of casting. However, this figure can be somewhat misleading if viewed independently fromFig. 2 and 3. Figure 5 indicates that mix1 has a large 2.1 top-strand ratio (theratio of implied transfer lengths for topcast versus bottom-cast strands), yet thismixture had the lowest total 21-day average transfer lengths of the top-strandbeams (Fig. 3).Figures 2 through 4 indicate that therewas typically an increase in strand endslip and, therefore, implied transferlengths during the first 21 days afterde-tensioning. This increase averagedWhile it is apparent (from Fig. 5) thatthe strand casting position can have asignificant effect on pretensioned strandbond, it is not clear from the data presented whether the effect is primarilydue to the amount of concrete below thestrand (as implied by the ACI requirement of a 1.3 multiplier for deformedbars with more than 12 in. [300 mm] ofconcrete below the bar), the lack of concrete above the strand, or a combinationof both. In Fig. 2 through 4, the authornoted that the implied transfer lengthsfor the 8 in. 6 in. (205 mm 150 mm)rectangular beams (Fig. 4) were generally between the corresponding values forthe 10 in. 15 in. (
The Effects of As-Cast Depth and Concrete Fluidity on Strand Bond I n recent years, the use of self-consolidating concrete (SCC) has . the top (as-cast) surface, where d b is the diameter of the strand and f se . reducer and retarder
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