Fabricating a Watertight Precast Concrete Septic Tank

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Fabricating a Watertight PrecastConcrete Septic
Tank
Dan Wagner Milan Vault, Inc.
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SEPTIC TANK QUALITIES
What qualities do we want in a septic tank?
BENEFIT
COST
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DEPENDABLE
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ECONOMICAL
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Barney buys a concrete septic tank http//www.yout
ube.com/watch?vkW3JkVaAZEcfeaturerelated
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Components of Watertightness

• Engineering and Design
• Manufacturing
• Quality Assurance and Testing
• Installation

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Design Considerations

• The septic tank must
• Withstand all loading conditions
• Surface Surcharge
• Concentrated wheel loads
• Lateral Loads
• Soil Bearing Capacity

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NO TRAFFIC / NO LOADS
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ASTM C 1227 LOADS
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LIGHT TRUCK TRAFFIC
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Design Considerations

• The septic tank must not be buoyant it must not
  float

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Design Specifications

• Tanks should be designed at least to
• ASTM C1227
• Stdandard Specification for Precast Concrete
  Septic Tanks
• ASTM C890
• Standard Practice for Minimum Structural Design
  Loading for Monolithic or Sectional Precast
  Concrete Water and Wastewater Structures
• Local Codes, CSA B66, IAPMO/ANSI

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Manufacturing and Installation
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NPCA Design Recommendations

• Minimum concrete thickness for conditions
• Clean forms in good conditions
• Proper reinforcement placement
• Prepour and post pour inspections
• A well designed concrete mix
• Proper finishing and curing
• Lifting inserts designed with S.F. 4 min.
• Properly Sealed joints and fittings

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The Manufacturing Process
Raw Materials Mix Design Reinforcement Production
Practices (Seals, Fittings, and Joints) Post-Pour
Operations
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Raw Materials

• Use quality, certified materials
• Aggregates
• Cement
• Water
• Admixtures

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Mix Design for Watertightness

• Low Water/Cementitious Ratio!
• A maximum of 0.45
• (weight of water / weight of cement and
  pozzolans)
• Minimum 4,000 psi strength at 28 days
• Minimum strength when shipped?
• Quality materials, well graded aggregates
• Volumetric proportioning (ACI 211)
• Air entrainment

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WATER
STRENGTH
POROSITY
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Aggregates

• Comprise of about 70 of the total volume
• Conform to ASTM C 33
• Clean and well graded
• Free from deleterious substances
• Moisture content checked daily
• Properly handled and stored

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Air Content

• Entrained Air Desirable
• Improves freeze/thaw resistance, ASR resistance,
  sulfate resistance
• Improves workability
• Entrapped Air Non-desirable
• Bugholes
• Released through adequate vibration

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Purpose of Reinforcement

• Concrete is strong in compression but weak in
  tension
• Reinforcement supplies strength to withstand
  tensile and shear forces experienced by concrete
• Sometimes tanks experience greatest forces during
  stripping, handling, and transporting

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Non-reinforced Concrete Behavior
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Reinforced Concrete Behavior
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Reinforcement Requirements

• Meet ASTM requirements (A615 or A706)
• Designed to withstand all loading conditions
• Provide adequate cover to protect from corrosion
  (1 min. ASTM C1227)
• Spaced properly, not touching formwork

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Visible problems
REBAR SHOWING
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Rebar Touching Form
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What it should look like
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Pre-Pour Operations

• Pre-Pour activities include
• Setting of formwork
• Forms must be cleaned and inspected after each
  use
• Positioning of steel reinforcement
• Positioning of Blockouts
• Positioning of embedded items (i.e. lifting
  apparatus)

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Casting Concrete

• Placement of concrete is important
• Consolidating Concrete
• Insufficient vibration leads to poor
  consolidation, excessive vibration causes
  segregation
• Proper vibration techniques lead to smooth, dense
  surfaces that promote impermeability

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Curing

• Proper curing is important in developing
  strength, durability, and watertightness
• Two critical elements in curing
• Maintaining Correct Moisture
• Maintaining Concrete Temperature
• Note ACI 305 Hot Weather Concreting
• ACI 306 Cold Weather Concreting

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Post Pour Practices

• Sufficient Strength for stripping
• Handle to reduce or eliminate damage
• Perform a post poor inspection
• Mark the casting with all required information
• Store product to prevent damage in storage

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TANK SEAM / JOINT SEAL

• 3/8 maximum gap between two mating joint
  surfaces BEFORE sealant is applied.
• ASTM C 1227-05 Section 10.3

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PROOF TESTING

• ASTM C 1227-10a
• 9.1 Proof testing is used to demonstrate the
  strength of the tank to resist anticipated
  external and internal loads.
• 9.1.1 Proof testing, when required by the
  purchaser, shall be performed in such a way as to
  simulate the actual anticipated loads.

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PROOF TESTING

• Calculate for anticipated actual loads
• Internal and external
• Soil loads, lateral earth loads
• Weight of tank
• Use live load at the surface of 300 lb/ft2 unless
  heavier loads are anticipated.
• Proof test to 150 of the anticipated actual
  loads

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Proof Testing - Vacuum
With 300 psf Live Load from ASTM C-1227
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Proof Testing - Vacuum
With 300 psf Live Load from ASTM C-1227

• Top Loads
• (540 psf) x 150 810 psf
• Floor Loads
• (715 psf) x 150 1,073 psf
• Wall Loads
• (180 psf) x 150 270 psf

Since the floor load is the highest load force,
it can be used for vacuum testing. 1,073 psf
14.9 Mercury
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Alternative Loads

• Typical loads for a septic tank are
• Soils
• Internal fluids
• External soil / saturated soils
• Weight of tank
• Incidental loads from a vehicle

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Proof Testing - Vacuum
With a concentrated wheel load of 2,250 pounds
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Proof Testing - Vacuum
With a concentrated wheel load of 2,250 pounds

• Top Loads
• (325 psf) x 150 488 psf
• Floor Loads
• (500 psf) x 150 750 psf
• Wall Loads
• (220 psf) x 150 330 psf

Since the floor load is the highest load force,
it can be used for vacuum testing. 750 psf
10.4 Mercury (Better)
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Proof Testing - Alternative

• Place tank on a bed of sand (about 3-4)
• Vacuum test for the wall loads
• 330 psf or 4.6 of mercury
• Add weight to the top to simulate the additional
  live and soil loads
• (750 psf) (330 psf) 420 lbs/ft2
• Add 1,680 lbs. (4 ft2 x 420 lbs/ft2) on a 2 x
  2 area in center
• Voluntary proof testing in Ohio - approved by ODH
• 4 mercury 2,000 lbs. on a 2 x 2 area

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Area for Additional Weight
2 x 2 Footprint
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Testing in Action
Vacuum
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Testing in Action
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Watertight Testing
Vacuum Testing 4 of Mercury (Hg), for 5
minutes, must hold 100 to pass
certification. Per ASTM C 1227-10a
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Vacuum Testing
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Watertight Testing In-Plant
Vacuum Test
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Watertight Testing In Field
Vacuum Test
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Watertight Testing

• Water Test
• Fill the tank
• Let stand for 24 hours
• Refill the tank
• Tank passes if water level does not drop within 1
  hour
• Sealing a Watertight Septic Tankhttp//www.youtu
  be.com/watch?vMaMb4Epo_2A

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Watertight Testing In-Plant
Water Test
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Watertight Testing In Field
Water Test
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Installation

• Site and Excavation
• Locate buried utilities
• Safe and free of hazards
• Trees
• Overhead power lines
• Sloped or shored excavation if entry is required

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Installation

• Bedding
• Minimum of 4 of sand or granular bedding
• a firm uniform base
• Void of large boulders or rock edges
• Design to resist flotation in high water table

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Installation

• Placement of Tank
• Confirm orientation
• Inspect the bedding
• Check level of tank
• Slope should meet local plumbing and building
  codes
• Use safe lifting devices designed for the load

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Installation

• Sealing On-site
• Use a high quality joint sealant
• Should conform to ASTM C 990
• Should be compressible for the temperatures
  expected
• Molded into one continuous band
• Joint free of loose debris
• Remember to seal risers and lids also

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Installation

• Backfilling
• Uniform layers of less than 24 thick
• Free of large stones (gt3 in diameter)
• Dont damage the tank!

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Commitment to Excellence

• Adopt a good quality control program
• Have management committed to program
• Have trained QC personnel and production crew
• Design to standard specifications as minimum
• Plan for quality finished products

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