Bursting and Spalling in Pretensioned U-Beams - Ferguson ...

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Most beams observed to crack did so near the section centroid (in this case, middepth),at which height maximum spalling strains were measured at the beam end.Interestingly, a number of beams cracked at top flange/web junction (40% of the beamswith observed cracking, 7 of 18), perhaps due to bursting effects from the prestressingforce applied to the top flange.The maximum spalling strain in each end region was reported by the researchers.It can be observed that the mean of the maximum spalling strains in “uncracked” beams(370 microstrain) was strikingly similar to that for the “cracked” beams (380microstrain). The spalling strains for beams without observed cracking ranged from 170to 900 microstrain; all were above the nominal 150-microstrain cracking strain. AsArthur and Ganguli stated, “Some of these strains are so high that it is suspected thatcracking had in fact occurred, although there was no evidence of it” (p. 95).Unlike other studies, Arthur and Ganguli also reported time-dependent behaviorof their measured strains. From measurements made immediately after prestress transfer,24 hours after transfer and 48 hours after, the researchers reported that strains had atendency to decrease (100 microstrain over 48 hours) except at locations of web cracking.Where gages spanned cracks, strains increased with time.2.4.1.6 Krishnamurthy, 1970Krishnamurthy performed a similar study to that Arthur and Ganguli, also atGlasgow. Krishnamurthy’s study investigated the effect of detensioning method (i.e. timespent detensioning) on transverse tensile strains and transfer length in 12-in. I-beams.Strains were measured on the concrete surface with a small mechanical gage, whosetarget points covered the spalling zone: the middle portion of beam, within h/2 of thebeam end (Figure 2.25).38
Figure 2.25 Mechanical gage points on I-beam specimen (after Krishnamurthy, 1970)The finding from this study was that the more sudden prestress was transferred,the longer the transfer length (by 15% on average) and the higher the spalling strains (by40%). While greater transverse tension resulting from more sudden transfer is notsurprising, the correlation of longer transfer lengths and higher spalling strains is—as itcontradicted the prevailing view (Guyon, 1955; Base, 1958; Marshall & Mattock, 1962)that shorter transfer lengths should lead to increased bursting and spalling effects.2.4.1.7 Uijl, 1983In addition to his analytical work, Uijl completed tests measuring concrete surfacestrains in 11 pretensioned I-shaped members representing the ribs of hollow-core slabs.Per standard practice in the hollow-core industry, the specimens had no transversereinforcement. Uijl noted that for most beams of other section geometries, in whichsufficient transverse reinforcement can be easily provided to resist end-region cracking,“it is of minor interest at what stress level cracks occur. In hollow-core slab, on thecontrary, it is of vital importance to know whether the occurring tensile stresses remainsmaller than the tensile strength of the concrete” (p. 21).39
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CopyrightbyDavid Andrew Dunkman2009
Page 3 and 4: Bursting and Spalling in Pretension
Page 5 and 6: Bursting and Spalling in Pretension
Page 7 and 8: 2.4.2.3 Itani & Galbraith, 1986 ...
Page 9 and 10: 4.2.2.4 Beam 1: Lateral-Bar Burstin
Page 11 and 12: List of TablesTable 2.1 Range of st
Page 13 and 14: Figure 2.25 Mechanical gage points
Page 15 and 16: Figure 3.30 Thermocouple placement
Page 17: CHAPTER 1Introduction1.1 IMPETUS FO
Page 20: At present, current end-region deta
Page 23 and 24: Figure 2.1 Severe cracking in a pre
Page 25 and 26: through use of tensioned, high-stre
Page 27 and 28: As splitting stress is bond-related
Page 29 and 30: Figure 2.6 Effect of bursting & spa
Page 31 and 32: magnitude of the end-region transve
Page 33 and 34: FEA can readily output color maps o
Page 35 and 36: 2.3.1.3 Plastic Analysis of Beam En
Page 38 and 39: force can increase by 400% (Yettram
Page 40 and 41: 2.4 EXPERIMENTAL STUDIES OF BURSTIN
Page 42 and 43: The trial specimens included one 40
Page 44 and 45: mechanical gage (2 in. gage length)
Page 46 and 47: Bursting stress(normalized by prest
Page 48 and 49: proprietary post-tensioning anchors
Page 50 and 51: Though Marshall and Mattock placed
Page 52 and 53: ectangular beam” (p. 21). At high
Page 56 and 57: The behavior of these beams was dom
Page 58 and 59: 2.4.2 Studies of End-Region Transve
Page 60 and 61: Figure 2.28 I-beams studied by Mars
Page 62 and 63: transverse forces compared to those
Page 64 and 65: (a) Actual spalling stress variatio
Page 66 and 67: stirrups in Gergely, Sozen and Sies
Page 68 and 69: 50% more reinforcement than require
Page 70 and 71: strength concrete, increasingly hig
Page 72 and 73: the reinforcement located within th
Page 74 and 75: Typical crack patterns for the beam
Page 76 and 77: “Very common”crack locationPred
Page 78 and 79: Figure 2.38 Typical gage locations
Page 80 and 81: 2.4.2.7 Smith et al., 2008Most rece
Page 82 and 83: the provided transverse reinforceme
Page 84 and 85: Itani and Galbraith (1986) reported
Page 86 and 87: Figure 2.45 Confining reinforcement
Page 88 and 89: cracks located along lines of prest
Page 90 and 91: odies relevant to pretensioned conc
Page 92 and 93: than that in another shape. For thi
Page 94 and 95: Figure 2.47 Required splitting rein
Page 96 and 97: No attempt was made in the Tentativ
Page 98 and 99: No citation is provided in the PCI
Page 100 and 101: Section Propertiesy b22.4 in.A 1120
Page 102 and 103: minimum for greater skews. For beam
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treatment (counting the transverse
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Pretensioned/post-tensioned beams h
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Figure 2.57 Harped strands in webs
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transversereinforcementwithin voidp
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2.8 SUMMARYIn this chapter, literat
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3.2.1.1 Linear and Nonlinear Stress
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Empirical expressions can be used t
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where: 0.021 = spalling force,
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3.3 CONCRETE PROPERTIES3.3.1 Concre
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It was reasoned that the dimensiona
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Splitting tensile-strength testing
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For temperature-match curing, therm
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3.4.1 Reinforcing-Bar Mechanical Te
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Grade 60 rebar yielded at 65 ksi an
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Equation 3.10where:ε1,2 = gage str
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Figure 3.17 Typical strand failure
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SouthNorth North120NorthSouthFigure
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Beam 1 also included an additional
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Figure 3.22 Strand strain-gage inst
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Figure 3.25 Interface board used to
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eam has a “hot spot” (measureme
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3.6 BEAM FABRICATION3.6.1 Beam Cast
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Figure 3.33 Installing the interior
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In order to use a pretensioning bed
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standardside-formprofilespecialtape
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Figure 3.41 Strands installed, tens
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Correspondence between the values o
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Figures 3.45 to 3.50 show the fabri
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Figure 3.49 Casting end block (and
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Figure 3.52 Detensioning by gradual
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CHAPTER 4Results & Discussion4.1 OV
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The strain time history for each ga
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1DNorthSouth1E18 ksi1C1C,E1D1521B1B
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Figure 4.4 Beam 1 prestress losses
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crack adjacent to this location was
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The temperatures in the end blocks
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Table 4.4 Beam 2 applied prestressi
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Figure 4.10 Beam 2 prestress losses
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terms of force distribution seems t
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limits were only exceeded at locati
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spalling stresses were measured in
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transverse reinforcement contained
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Based on Figure 4.18, the 4% Pi des
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Uniform and linearly-decreasing tra
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4.3.5 Contributing Factors to High
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stress ratio multiplied by a typica
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Figure 4.25 Relation between total
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4.4.1 Recommended Texas U-Beam Rein
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Figure 4.28 Proposed end-region det
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At present, AASHTO LRFD (2009) lack
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CHAPTER 5Conclusions5.1 SUMMARY OF
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tested at transfer and under shear
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APPENDIX ADatabase of Transverse-Ba
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194Figure A.1 U-beam with skewed en
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196Figure A.3 Shallow Texas bulb te
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198Figure A.5 Lightweight-concrete
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200Figure A.7 Washington bulb tee (
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202Figure A.9 Nebraska bulb tees an
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204Figure A.11 Nebraska bulb tees a
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206Figure A.13 Shallow I-beams
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where:= lower-bound concrete tensil
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B.3.4 Transmission Length (Clause 6
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Equation B.11where:= bursting force
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50High Web gage4030Max Moment heigh
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C.2 CONCRETE RELEASE STRENGTHA bott
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Table C.1 Specimen Design for Beams
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Table D.1 Beam 1 match-cured cylind
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D.1.2 Beam 28Release-Point Cylinder
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Cylinder Strength (ksi)131197Ambien
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D.2 REINFORCING-BAR MODULUS & STREN
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APPENDIX EEnd-Block Temperature Pro
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E.2 BEAM 2T max = 142°F∆T = 30°
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REFERENCESREFERENCED STANDARDSAmeri
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Fédération Internationale du Bét
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Davis, R.T.; Buckner, C.D. & Ozyild
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Lin, T.Y. & Burns, N.H. (1981), Des
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Stone, W.C. & Breen, J.E. (1984),
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VITADavid Andrew Dunkman was born 2
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