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

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3.3 CONCRETE PROPERTIES3.3.1 Concrete Mixture DesignEfficient precast concrete fabrication demands high early-strength concretes.Typically, large amounts of Type III cement (6 to 7 sacks, or 570 to 670 lb/cy) is mixedwith small amounts of water (water-cement ratio, w/c < 0.40) so that beams can reachtheir release strengths (4 to 7 ksi) within 18 to 24 hours. Beams can then be released andmoved from the pretensioning bed the day after casting.Precast concrete mixes are highly engineered: much high-range water-reducer isneeded for workable concrete with the low w/c used. The balance of coarse aggregate tofine aggregate may be lower than in conventional concrete (1 to 1.5:1, compared to 1.5 to2:1) to avoid segregation while the concrete is fresh. Lastly chemical retarders are oftenused to prevent flash setting, especially during hot Texas summers.Concrete made with Type III cement is not commercially available as ready-mixin the Austin area at present, likely due to insufficient demand and concerns over flashsetting. A local precast concrete fabricator, Coreslab Structures, was able to provide helpdesigning the concrete mixture proportions for application on this project, and donatedthe concrete, batched at their facility 15 miles north of the Ferguson Laboratory. Theconcrete mixture design used for both beams is shown in Table 3.1. This design is similarto high-early-strength mixtures used by Texas precast highway-beam fabricators.The design used is a “straight-cement” concrete mixture, containing no fly ashreplacement. Within the first year of this project, TxDOT mandated the use of Class F flyash in precast concrete mixes at 25% replacement of cementitious material (Marek,2008). The purpose of this new prescription was control of alkali-silica reaction (ASR), aslow-acting chemical reaction in which alkalis from cement and other sources interactwith reactive (glassy) silica in aggregate to form an expansive gel. The provision of fly104
ash changes the composition of the calcium-silicate-hydrate gel (C-S-H), the “glue” inconcrete, so that the gel absorbs alkalis and prevents them from reacting with the silica(Folliard et al., 2006).Table 3.1 Concrete mixture proportionsMaterial Detail QuantityCement Alamo Grey Type III 611 lb/cyCoarse Aggregate ¾ in. Crushed Limestone 1600 lb/cyFine Aggregate River Sand 1379 lb/cyWaterWater-Cement Ratio (w/c)High-Range Water-Reducer—Sika Viscocrete 2100Sikament 686202 lb/cy0.3313 oz/Cwt25 oz/CwtRetarder Sika Plastiment 5 oz/CwtSlump — 9 in.lb/cy = pounds per cubic yard of concreteoz/Cwt = fluid ounces per hundred-weight of cementWith shear testing for each beam to be completed within two months of casting,beam long-term behavior is not under study; the reasoning behind the TxDOT fly-ashprescription does not apply. A straight-cement mixture was used in an effort to maximizethe internal curing temperature. Cement reacts quickly with water and gives off muchheat during early hydration. The reaction of Class F ash, on the other hand, cannot beginuntil cement hydration has progressed sufficiently, so is slower-acting and contributesnegligibly to early heat and strength gains (Schindler & Folliard, 2005; Wang, Lee &Park, 2009).105
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CopyrightbyDavid Andrew Dunkman2009
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Bursting and Spalling in Pretension
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Bursting and Spalling in Pretension
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2.4.2.3 Itani & Galbraith, 1986 ...
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4.2.2.4 Beam 1: Lateral-Bar Burstin
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List of TablesTable 2.1 Range of st
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Figure 2.25 Mechanical gage points
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Figure 3.30 Thermocouple placement
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CHAPTER 1Introduction1.1 IMPETUS FO
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At present, current end-region deta
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Figure 2.1 Severe cracking in a pre
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through use of tensioned, high-stre
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As splitting stress is bond-related
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Figure 2.6 Effect of bursting & spa
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magnitude of the end-region transve
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FEA can readily output color maps o
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2.3.1.3 Plastic Analysis of Beam En
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force can increase by 400% (Yettram
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2.4 EXPERIMENTAL STUDIES OF BURSTIN
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The trial specimens included one 40
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mechanical gage (2 in. gage length)
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Bursting stress(normalized by prest
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proprietary post-tensioning anchors
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Though Marshall and Mattock placed
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ectangular beam” (p. 21). At high
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Most beams observed to crack did so
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The behavior of these beams was dom
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2.4.2 Studies of End-Region Transve
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Figure 2.28 I-beams studied by Mars
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transverse forces compared to those
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(a) Actual spalling stress variatio
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stirrups in Gergely, Sozen and Sies
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50% more reinforcement than require
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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
Page 104 and 105: treatment (counting the transverse
Page 106 and 107: Pretensioned/post-tensioned beams h
Page 108 and 109: Figure 2.57 Harped strands in webs
Page 110 and 111: transversereinforcementwithin voidp
Page 112 and 113: 2.8 SUMMARYIn this chapter, literat
Page 114 and 115: 3.2.1.1 Linear and Nonlinear Stress
Page 116 and 117: Empirical expressions can be used t
Page 118 and 119: where: 0.021 = spalling force,
Page 122 and 123: It was reasoned that the dimensiona
Page 124 and 125: Splitting tensile-strength testing
Page 126 and 127: For temperature-match curing, therm
Page 128 and 129: 3.4.1 Reinforcing-Bar Mechanical Te
Page 130 and 131: Grade 60 rebar yielded at 65 ksi an
Page 132 and 133: Equation 3.10where:ε1,2 = gage str
Page 134 and 135: Figure 3.17 Typical strand failure
Page 136 and 137: SouthNorth North120NorthSouthFigure
Page 138 and 139: Beam 1 also included an additional
Page 140 and 141: Figure 3.22 Strand strain-gage inst
Page 142 and 143: Figure 3.25 Interface board used to
Page 144 and 145: eam has a “hot spot” (measureme
Page 146 and 147: 3.6 BEAM FABRICATION3.6.1 Beam Cast
Page 148 and 149: Figure 3.33 Installing the interior
Page 150 and 151: In order to use a pretensioning bed
Page 152 and 153: standardside-formprofilespecialtape
Page 154 and 155: Figure 3.41 Strands installed, tens
Page 156 and 157: Correspondence between the values o
Page 158 and 159: Figures 3.45 to 3.50 show the fabri
Page 160 and 161: Figure 3.49 Casting end block (and
Page 162 and 163: Figure 3.52 Detensioning by gradual
Page 164 and 165: CHAPTER 4Results & Discussion4.1 OV
Page 166 and 167: The strain time history for each ga
Page 168 and 169: 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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