VSL

D115 prestressed concrete tendon tank

AWWA Tank Comparison Chart

AWWA D115 prestressed concrete tendon tankBelow is a comparison of  AWWA D110-04 (Wrapped Prestressed) and AWWA D115-06 (Tendon Prestressed) tanks:

Floor Slab TOP

AWWA D110-04 Code 

Wrapped Prestressed Tank

AWWA D115-06 Code

Tendon Prestressed Tank

Minimum Thickness

3.8.2

four inch for non-prestressed, no prestressed option - ACI 350 allows three inch thickness if reinforced with WWF 

3.6

5" if prestressed, 6" if non-prestressed

Concrete Placement

3.8.2

multiple placements - shrinkage cracking and joints that are a potential for leakage and maintenance

 

single placement is standard practice - no joints or penetrations

Reinforcement

 

mild steel only

3.6

prestressed with 200 psi residual each direction

Minimum Reinforcement Ratio

 

0.005 - mild reinforced only - # 5 at 15.5" maximum

3.6

.0015 in addition to prestressing

Concrete Cover on Reinforcement

3.8

none specified - ACI 350 H.4 allows 1.5" top and 1.5" bottom - less for 3" slabs

3.11

1.5" top, 2" bottom

Wall TOP

AWWA D110-04 Code 

Wrapped Prestressed Tank

AWWA D115-06 Code

Tendon Prestressed Tank

Minimum Thickness

3.5.4

as low as 3.5" - depending on wall type I, II, III or IV

3.7.3

9", however 11" -12" is most common 

Required minimum horizontal residual compression from prestressing

3.4.2.2

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

3.7.4.1

200 psi for wall above grade, 100 psi for wall below grade - entire wall thickness is prestressed with minimum residual compression

Required minimum vertical residual compression from prestressing

3.5.3

200 psi only on concrete core wall within the wrapping is prestressed - shotcrete coatings on prestressing steel has no residual compression

3.7.5

200 psi for above grade, 125 psi for below grade - entire wall thickness is prestressed with minimum residual compression

Sheet steel diaphragms provide watertightness?

3.5.6

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.

 

steel diaphragms not used or required as quality concrete is impermeable 

Steel sheet diaphragms provide flexural reinforcement?

2.5.1.3, 2.9.1

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 

 

steel diaphragms not used or required as quality concrete is impermeable 

Inside face concrete cover on vertical mild reinforcement

3.5.4.2, 3.5.6.2

1" on steel sheet diaphragm and rebar

3.11

1.5" on rebar if not two-way prestressed, 1" on rebar if two-way prestressed

Precast panel misalignment?

5.2.7

up to 3/8" misalignment allowed for precast wall segments, 

 

wall segments are cast-in-place, no misalignment possible

Vertical mild reinforcement ratio

3.5.6

0.0025 each face (steel diaphragm is included in ratio for type II, III and IV walls)

 

no minimum may be required if prestressed and depending on concrete stresses

Vertical prestressing protection

 

"unbonded grouted" system - grade 150 bar in a PVC duct with epoxy grout, vertical prestressing only used on type I tanks

1.3

5" minimum cover of bi-axially precompressed concrete, HDPE duct and coating in accordance with PTI and ACI standards

Horizontal prestressing protection

5.3.3.3.4

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.

1.3

3" minimum cover of bi-axially precompressed concrete, HDPP duct, high-performance grouting in accordance with PTI and ACI standards

Wall construction methods

1.1.1

four different wall types that can result in sole sourcing of a particular type

 

cast-in-place construction is non-proprietary

Shotcete cover on metal diaphragms 

3.5.4.2, 3.5.6.2

for diaphragms, 1" inside face cover and 0.5" cover on outside prior to wrapping 

 

 

Roof Slab TOP

AWWA D110-04 Code 

Wrapped Prestressed Tank

AWWA D115-06 Code

Tendon Prestressed Tank

Concrete placement

 

multiple placements - shrinkage cracking and joints with horizontal waterstops that are a potential for contamination and maintenance

 

single placement is standard practice, no joints,

Method of reinforcement

 

mild steel only

3.8.1

prestressed with 150 psi minimum residual precompression in each direction

Membrane required to prevent leakage into tank?

 

possibly - depending on roof slope and if leakage needs to be prevented

3.8.4

no, concrete surface is crack-free and sloped 1% minimum for drainage 

Materials TOP

AWWA D110-04 Code 

Wrapped Prestressed Tank

AWWA D115-06 Code

Tendon Prestressed Tank

Prestressing steel for hoop reinforcement

 

270 ksi "prior to galvanizing" (high temperature of galvanizing process reduces favorable mechanical properties of strand) 

 

270 ksi, low relaxation - no degradation due to hot dip galvanizing

Galvanizing of hoop prestressing steel

2.5.2.2

galvanizing may be used for primary corrosion protection, 40% loss of zinc is allowed for die-drawn stressing 

 

galvanizing of strand not required for corrosion protection

Concrete strength and durability

2.2.2

no minimum strength or maximum w/c specified - may be modified per "purchaser's specifications" that are usually provided by the wrapped tank supplier 

2.2.2.1

maximum w/c is 0.42 resulting in concrete strengths over 5,000 psi - no allowance for modifications in "purchaser's specifications"

Shotcrete quality control

5.3.3.6

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)

 

not applicable - no shotcreting on tendon tanks

Shotcrete quality control

6.2.2.8.2, 6.4.1

"sound testing" with hammer required to identify debonded shotcrete during initial construction and routine maintenance - not possible where wall is backfilled

 

not applicable - no shotcreting on tendon tanks

Vertical post-tensioning duct

2.7.1

brittle pvc pipe used for vertical duct material - "grouted unbonded" system, no minimum thickness specified, PVC not recommended by PTI

 

all ducts are non-corrosive, corrugated HDPP

Structure geometry

Forward II.A.

can only provide circular tanks

Forward V.2.

can provide circular, rectangular, oval, egg-shaped, etc. 

Schedule and weather limitations

5.3

shotcrete operations weather-dependent and must be moist cured, limited quantities of shotcreting and wrapping equipment available

 

not as weather dependent, concrete is protected during winter conditions

Verification of stress in prestressing steel

5.6.1.7

procedure defined by wrapped tank manufacturers, not recognized by AWWA, ACI, or ASTM codes

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

Sandblasting 

5.3.3.2

sandblasting of core wall required for bonding of shotcrete

 

no sandblasting required

Qualifications of prestressing field workers

 

defined by wrapped tank manufacturers - no industry guidelines

5.2.4.2, 5.2.4.3

five years experience with systems being used, PTI certified placers

Certification and quality control of prestressing materials

 

n/a

5.2.4.1

PTI Field Procedures Manual for Unbonded Single Strand Tendons, PTI Specification for Grouting of Post-Tensioned Structures, PTI Plant Certification Program required

Miscellaneous TOP

 

 

 

 

"Cement Seeding" for sealing leaks through floor joints

 

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

 

not required - no joints in floors

Specialized equipment

 

proprietary shotcrete , tensioning, and curing methods and equipment 

 

non-propriety stressing and grouting equipment

Hydrogen embrittlement of galvanized wire/strand

 

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

Applications of prestressing methods

 

circular tanks and concrete water pipe only

 

tanks, silos, bridges, concrete building frames, parking structures, slab foundations, etc.

Contact us about DuraStor Tanks

© 2013 VStructural, LLC - Exclusive licensee of VSL International LTD for the USA