1. Fabricating a Watertight Precast Concrete Septic Tank1
Fabricating a Watertight Precast Concrete Septic Tank
Dan Wagner – Milan Vault, Inc.

2. SEPTIC TANK QUALITIES What qualities do we want in a septic tank?2
A variety of loading conditions: manufacture, transportation and installation, testing, service (full or empty)
SEPTIC TANK QUALITIES
What qualities do we want in a septic tank?
BENEFIT
COST

5. DEPENDABLE

6. ECONOMICAL

7. Barney buys a concrete septic tank

8. Components of Watertightness8
Components of Watertightness
Engineering and Design
Manufacturing
Quality Assurance and Testing
Installation

9. Design Considerations
The septic tank must:
Withstand all loading conditions
Surface Surcharge
Concentrated wheel loads
Lateral Loads
Soil Bearing Capacity

10. NO TRAFFIC / NO LOADS

11. ASTM C 1227 LOADS

12. LIGHT TRUCK TRAFFIC

13. Design Considerations
The septic tank must not be buoyant; it must not float

14. 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

15. Manufacturing and Installation

16. NPCA Design Recommendations
Concrete thickness must be sufficient to meet minimum reinforcement cover requirements and withstand loading conditions
w/c < 0.45 reduce permeability, improve durability, increase strength
1616
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

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

19. Raw Materials Use quality, certified materials Aggregates Cement Water
Admixtures

20. 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

21. WATER
STRENGTH
POROSITY

23. 2323

24. 2424

25. 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

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

30. Purpose of Reinforcement
3030
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

31. Non-reinforced Concrete Behavior

32. Reinforced Concrete Behavior

33. Reinforcement Requirements
3333
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

34. Visible problems
REBAR SHOWING

35. Rebar Touching Form

36. What it should look like

38. 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)

39. 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

42. 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

43. Post Pour Practices Sufficient Strength for stripping
4343
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

44. TANK SEAM / JOINT SEAL
3/8” maximum gap between two mating joint surfaces BEFORE sealant is applied.
ASTM C Section 10.3

47. 4747
PROOF TESTING
ASTM C a
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.

48. 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

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

50. Proof Testing - Vacuum Top Loads Floor Loads Wall Loads
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

51. Alternative Loads Typical loads for a septic tank are: Soils
Internal fluids
External soil / saturated soils
Weight of tank
Incidental loads from a vehicle

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

53. With a concentrated wheel load of 2,250 pounds
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)

54. 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

55. Area for Additional Weight
2’ x 2’ Footprint

56. Testing in Action
Vacuum

57. Testing in Action

58. Watertight Testing Vacuum Testing
4” of Mercury (Hg), for 5 minutes, must hold 100% to pass certification.
Per ASTM C a

59. Vacuum Testing

60. Watertight Testing: In-Plant
Vacuum Test

61. Watertight Testing: In Field
Vacuum Test

62. 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

63. Watertight Testing: In-Plant
Water Test

64. Watertight Testing: In Field
Water Test

65. Installation Site and Excavation Locate buried utilities
6565
Installation
Site and Excavation
Locate buried utilities
Safe and free of hazards
Trees
Overhead power lines
Sloped or shored excavation if entry is required

66. Installation Bedding Minimum of 4” of sand or granular bedding
6666
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

67. Installation Placement of Tank Confirm orientation Inspect the bedding
6767
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

69. Installation Sealing On-site Use a high quality joint sealant
6969
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

70. Installation Backfilling Uniform layers of less than 24” thick
7070
Installation
Backfilling
Uniform layers of less than 24” thick
Free of large stones (>3” in diameter)
Don’t damage the tank!

71. Commitment to Excellence
7171
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