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Floor Slab TOP
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AWWA D110-04 Code
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Wrapped Prestressed Tank
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AWWA D115-06 Code
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Tendon Prestressed Tank
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Minimum Thickness
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3.8.2
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four inch for non-prestressed, no prestressed option - ACI 350 allows three inch thickness if reinforced with WWF
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3.6
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5" if prestressed, 6" if non-prestressed
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Concrete Placement
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3.8.2
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multiple placements - shrinkage cracking and joints that are a potential for leakage and maintenance
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single placement is standard practice - no joints or penetrations
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Reinforcement
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mild steel only
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3.6
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prestressed with 200 psi residual each direction
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Minimum Reinforcement Ratio
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0.005 - mild reinforced only - # 5 at 15.5" maximum
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3.6
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.0015 in addition to prestressing
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Concrete Cover on Reinforcement
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3.8
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none specified - ACI 350 H.4 allows 1.5" top and 1.5" bottom - less for 3" slabs
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3.11
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1.5" top, 2" bottom
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Wall TOP
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AWWA D110-04 Code
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Wrapped Prestressed Tank
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AWWA D115-06 Code
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Tendon Prestressed Tank
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Minimum Thickness
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3.5.4
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as low as 3.5" - depending on wall type I, II, III or IV
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3.7.3
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9", however 11" -12" is most common
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Required minimum horizontal residual compression from prestressing
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3.4.2.2
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200 psi for wall above grade, 50 psi for wall below grade - only concrete core wall within the wrapping is prestressed - shotcrete coatings on prestressing steel has no residual compression and is usually in tension
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3.7.4.1
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200 psi for wall above grade, 100 psi for wall below grade - entire wall thickness is prestressed with minimum residual compression
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Required minimum vertical residual compression from prestressing
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3.5.3
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200 psi only on concrete core wall within the wrapping is prestressed - shotcrete coatings on prestressing steel has no residual compression
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3.7.5
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200 psi for above grade, 125 psi for below grade - entire wall thickness is prestressed with minimum residual compression
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Sheet steel diaphragms provide watertightness?
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3.5.6
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yes - if wall type II, III or IV. If not vertically post-tensioned wall must have a steel diaphragm. Vertical rebar may be eliminated depending on design.
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steel diaphragms not used or required as quality concrete is impermeable
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Steel sheet diaphragms provide flexural reinforcement?
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2.5.1.3, 2.9.1
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26 gauge steel sheets relied on for watertightness and vertical flexural reinforcement - field joints require field-applied sealants, type IV walls require field-applied sealants at form tie holes
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steel diaphragms not used or required as quality concrete is impermeable
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Inside face concrete cover on vertical mild reinforcement
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3.5.4.2, 3.5.6.2
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1" on steel sheet diaphragm and rebar
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3.11
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1.5" on rebar if not two-way prestressed, 1" on rebar if two-way prestressed
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Precast panel misalignment?
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5.2.7
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up to 3/8" misalignment allowed for precast wall segments,
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wall segments are cast-in-place, no misalignment possible
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Vertical mild reinforcement ratio
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3.5.6
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0.0025 each face (steel diaphragm is included in ratio for type II, III and IV walls)
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no minimum may be required if prestressed and depending on concrete stresses
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Vertical prestressing protection
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"unbonded grouted" system - grade 150 bar in a PVC duct with epoxy grout, vertical prestressing only used on type I tanks
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1.3
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5" minimum cover of bi-axially precompressed concrete, HDPE duct and coating in accordance with PTI and ACI standards
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Horizontal prestressing protection
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5.3.3.3.4
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Sacrificial galvanizing on wire with one inch of non-prestressed shotcrete cover. Shotcrete cover goes into tension as it shrinks and when tank is filled, cover can delaminate, leakage source difficult to locate - water migrates beneath the shotcrete.
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1.3
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3" minimum cover of bi-axially precompressed concrete, HDPP duct, high-performance grouting in accordance with PTI and ACI standards
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Wall construction methods
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1.1.1
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four different wall types that can result in sole sourcing of a particular type
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cast-in-place construction is non-proprietary
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Shotcete cover on metal diaphragms
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3.5.4.2, 3.5.6.2
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for diaphragms, 1" inside face cover and 0.5" cover on outside prior to wrapping
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Roof Slab TOP
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AWWA D110-04 Code
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Wrapped Prestressed Tank
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AWWA D115-06 Code
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Tendon Prestressed Tank
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Concrete placement
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multiple placements - shrinkage cracking and joints with horizontal waterstops that are a potential for contamination and maintenance
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single placement is standard practice, no joints,
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Method of reinforcement
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mild steel only
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3.8.1
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prestressed with 150 psi minimum residual precompression in each direction
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Membrane required to prevent leakage into tank?
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possibly - depending on roof slope and if leakage needs to be prevented
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3.8.4
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no, concrete surface is crack-free and sloped 1% minimum for drainage
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Materials TOP
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AWWA D110-04 Code
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Wrapped Prestressed Tank
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AWWA D115-06 Code
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Tendon Prestressed Tank
|
|
Prestressing steel for hoop reinforcement
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|
270 ksi "prior to galvanizing" (high temperature of galvanizing process reduces favorable mechanical properties of strand)
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270 ksi, low relaxation - no degradation due to hot dip galvanizing
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Galvanizing of hoop prestressing steel
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2.5.2.2
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galvanizing may be used for primary corrosion protection, 40% loss of zinc is allowed for die-drawn stressing
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galvanizing of strand not required for corrosion protection
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Concrete strength and durability
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2.2.2
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no minimum strength or maximum w/c specified - may be modified per "purchaser's specifications" that are usually provided by the wrapped tank supplier
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2.2.2.1
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maximum w/c is 0.42 resulting in concrete strengths over 5,000 psi - no allowance for modifications in "purchaser's specifications"
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Shotcrete quality control
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5.3.3.6
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highly dependent on skill of nozzleman, mix design, proper equipment operation, and ambient temperature; shotcrete cover is not structural and must be moist-cured (not possible during freezing temperatures)
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not applicable - no shotcreting on tendon tanks
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Shotcrete quality control
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6.2.2.8.2, 6.4.1
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"sound testing" with hammer required to identify debonded shotcrete during initial construction and routine maintenance - not possible where wall is backfilled
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not applicable - no shotcreting on tendon tanks
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Vertical post-tensioning duct
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2.7.1
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brittle pvc pipe used for vertical duct material - "grouted unbonded" system, no minimum thickness specified, PVC not recommended by PTI
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all ducts are non-corrosive, corrugated HDPP
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Structure geometry
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Forward II.A.
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can only provide circular tanks
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Forward V.2.
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can provide circular, rectangular, oval, egg-shaped, etc.
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Schedule and weather limitations
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5.3
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shotcrete operations weather-dependent and must be moist cured, limited quantities of shotcreting and wrapping equipment available
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not as weather dependent, concrete is protected during winter conditions
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Verification of stress in prestressing steel
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5.6.1.7
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procedure defined by wrapped tank manufacturers, not recognized by AWWA, ACI, or ASTM codes
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5.2.4.4
|
all tendons tensioned with calibrated hydraulic rams, elongations measured and checked against calculated values, procedures and acceptance in accordance with AWWA and ACI codes
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Sandblasting
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5.3.3.2
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sandblasting of core wall required for bonding of shotcrete
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no sandblasting required
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Qualifications of prestressing field workers
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defined by wrapped tank manufacturers - no industry guidelines
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5.2.4.2, 5.2.4.3
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five years experience with systems being used, PTI certified placers
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Certification and quality control of prestressing materials
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n/a
|
5.2.4.1
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PTI Field Procedures Manual for Unbonded Single Strand Tendons, PTI Specification for Grouting of Post-Tensioned Structures, PTI Plant Certification Program required
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Miscellaneous TOP
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|
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"Cement Seeding" for sealing leaks through floor joints
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specified method to repair floor leakage is to fill tank with 2" of water, spread cement at a rate of one sack every 1,000 sq. ft. of floor area, do not allow floor to dry, inject cracks with epoxy if "cement seeding" is not successful in sealing leaks
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not required - no joints in floors
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|
Specialized equipment
|
|
proprietary shotcrete , tensioning, and curing methods and equipment
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non-propriety stressing and grouting equipment
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Hydrogen embrittlement of galvanized wire/strand
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|
hydrogen embrittlement of hot dipped galvanized steel is a concern when ultimate tensile strength is 160 ksi or greater per ASTM A143
|
|
galvanizing of strand not required for corrosion protection
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Applications of prestressing methods
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circular tanks and concrete water pipe only
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tanks, silos, bridges, concrete building frames, parking structures, slab foundations, etc.
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