Fabricating a Watertight Precast
Concrete Septic Tank
Dan Wagner – Milan Vault, Inc.
SEPTIC TANK QUALITIES
What qualities do we want in a septic tank?
COST
BENEFIT
DEPENDABLE
ECONOMICAL
Barney buys a concrete septic tank
http://www.youtube.com/watch?v=kW3JkVaAZEc&feature=related
Components of Watertightness
•
Engineering and Design
•
Manufacturing
•
Quality Assurance and Testing
•
Installation
Design Considerations
The septic tank must:
•
Withstand all loading conditions
–
Surface Surcharge
–
Concentrated wheel loads
–
Lateral Loads
–
Soil Bearing Capacity
NO TRAFFIC / NO LOADS
240 psf
2' Cover
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text format
 Weight)
(Earth

180 psf avg.
5'
Second Outline
Level
4' x 8' Tank

180 psf avg.
Third Outline
Level
(Assume 7,000 lbs.)

Assume soil
at 120 lb/ft3
415 psf
(Earth + Tank Weight)
Fourth Outline
Level
 Fifth
Outline
Level
ASTM C 1227 LOADS
300 psf
(Live Loads)
240 psf
(Earth Weight)
2' Cover
180 psf avg.
5'
4' x 8' Tank
(Assume 7,000 lbs.)
Assume soil
at 120 lb/ft3
715 psf
(Earth, Tank Weight, Live Loads)
180 psf avg.
LIGHT TRUCK TRAFFIC
325 psf
(Earth + LL Weight)
2' Cover
220 psf avg.
5'
4' x 8' Tank
(Assume 7,000 lbs.)
Assume soil
at 120 lb/ft3
500 psf
(Earth + LL + Tank Weight)
2,250 lbs.
wheel load
LL = 85 lb/ft2
220 psf avg.
Design Considerations
to edit the outline
The septic tank must not Click
be buoyant;
it
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must not float


Second Outline
Level

Third Outline
Level

Fourth Outline
Level
 Fifth
Outline
Level
Design Specifications
Tanks should be designed at least to:
•
ASTM C1227
–
•
ASTM C890
–
•
Stdandard Specification for Precast Concrete Septic Tanks
Standard Practice for Minimum Structural Design Loading for
Monolithic or Sectional Precast Concrete Water and Wastewater
Structures
Local Codes, CSA B66, IAPMO/ANSI
Manufacturing and Installation
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
The Manufacturing Process
Raw Materials
Mix Design
Reinforcement
Production Practices
(Seals, Fittings, and Joints)
Post-Pour Operations
Raw Materials
Use quality, certified materials
•
Aggregates
•
Cement
•
Water
•
Admixtures
Mix Design for Watertightness
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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
WATER
STRENGTH
POROSITY
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
Air Content
Entrained Air = Desirable


Improves freeze/thaw resistance, ASR resistance, sulfate
resistance
Improves workability
Entrapped Air = Non-desirable

Bugholes

Released through adequate vibration
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
Non-reinforced Concrete Behavior
Reinforced Concrete Behavior
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)
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Spaced properly, not touching formwork
Visible problems
REBAR SHOWING
Rebar Touching Form

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text format

Second Outline
Level

Third Outline
Level

Fourth Outline
Level
 Fifth
Outline
Level
What it should look like

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text format

Second Outline
Level

Third Outline
Level

Fourth Outline
Level
 Fifth
Outline
Level
Pre-Pour Operations
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Pre-Pour activities include:
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Setting of formwork
•
Forms must be cleaned and inspected after each use
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Positioning of steel reinforcement
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Positioning of Blockouts
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Positioning of embedded items (i.e. lifting
apparatus)
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
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
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
TANK SEAM / JOINT SEAL
•
•
3/8” maximum gap
between two mating
joint surfaces BEFORE
sealant is applied.
ASTM C 1227-05 Section 10.3
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.
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
Proof Testing - Vacuum
With 300 psf Live Load from ASTM C-1227
300 psf
(Live Loads)
240 psf
(Earth Weight)
2' Cover
180 psf avg.
5'
4' x 8' Tank
(Assume 7,000 lbs.)
Assume soil
at 120 lb/ft3
715 psf
(Earth, Tank Weight, Live Loads)
180 psf avg.
Proof Testing - Vacuum
With 300 psf Live Load from ASTM C-1227
Top Loads
•
240 psf
(Earth Weight)
2' Cover
180 psf avg.
4' x 8' Tank
5'
180 psf avg.
(Assume 7,000 lbs.)
(715 psf) x 150%
= 1,073 psf
Wall Loads
•
(Live Loads)
(540 psf) x 150%
= 810 psf
Floor Loads
•
300 psf
(180 psf) x 150%
= 270 psf
Assume soil
at 120 lb/ft3
715 psf
(Earth, Tank Weight, Live Loads)
Since the floor load is the highest load force,
it can be used for vacuum testing.
1,073 psf = 14.9” Mercury
Alternative Loads
•
Typical loads for a septic tank are:
–
Soils
–
Internal fluids
–
External soil / saturated soils
–
Weight of tank
–
Incidental loads from a vehicle
Proof Testing - Vacuum
With a concentrated wheel load of 2,250 pounds
325 psf
(Earth + LL Weight)
2' Cover
220 psf avg.
5'
4' x 8' Tank
(Assume 7,000 lbs.)
Assume soil
at 120 lb/ft3
500 psf
(Earth + LL + Tank Weight)
2,250 lbs.
wheel load
LL = 85 lb/ft2
220 psf avg.
Proof Testing - Vacuum
With a concentrated wheel load of 2,250 pounds
325 psf
Top Loads
•
(500 psf) x 150%
= 750 psf
Wall Loads
•
2' Cover
2,250 lbs.
wheel load
LL = 85 lb/ft2
(325 psf) x 150%
= 488 psf
Floor Loads
•
(Earth + LL Weight)
(220 psf) x 150%
= 330 psf
220 psf avg.
5'
4' x 8' Tank
220 psf avg.
(Assume 7,000 lbs.)
Assume soil
at 120 lb/ft3
500 psf
(Earth + LL + Tank Weight)
Since the floor load is the highest load force,
it can be used for vacuum testing.
750 psf = 10.4” Mercury (Better)
Proof Testing - Alternative
•
Place tank on a bed of sand (about 3”-4”)
•
Vacuum test for the wall loads
–
•
Add weight to the top to simulate the additional live
and soil loads
–
•
•
330 psf or 4.6” of mercury
(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
Area for Additional Weight
2’ x 2’ Footprint
2' x 2' Footprint
Testing in Action
Vacuum
Testing in Action
Watertight Testing
Vacuum Testing
4” of Mercury (Hg), for 5 minutes,
must hold 100% to pass certification.
Per ASTM C 1227-10a
Vacuum Testing
Watertight Testing: In-Plant
Vacuum Test
Watertight Testing: In Field
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 Tank
http://www.youtube.com/watch?v=MaMb4Epo_2A
Watertight Testing: In-Plant
Water Test
Watertight Testing: In Field
Installation
•
Site and Excavation
–
Locate buried utilities
–
Safe and free of hazards
–
•
Trees
•
Overhead power lines
Sloped or shored excavation if entry is required
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
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
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
Installation
•
Backfilling
–
Uniform layers of less than 24” thick
–
Free of large stones (>3” in diameter)
Don’t damage the tank!
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