fabricating a watertight precast concrete septic tank

Fabricating a Watertight PrecastConcrete 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 tankhttp://www.youtube.com/watch?v=kW3JkVaAZEc&feature=related

Components of Watertightness

• Engineering and Design

• Manufacturing

• Quality Assurance and Testing

• Installation

The septic tank must:• Withstand all loading conditions

– Surface Surcharge

– Concentrated wheel loads

– Lateral Loads

– Soil Bearing Capacity

Design Considerations

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NO TRAFFIC / NO LOADS

240 psf

180 psf avg. 180 psf avg.

415 psf

2' Cover

5' 4' x 8' Tank

(Earth + Tank Weight)

(Earth Weight)

Assume soil at 120 lb/ft3

(Assume 7,000 lbs.)

ASTM C 1227 LOADS

240 psf

180 psf avg . 180 psf avg .

715 psf

2 ' C o ver

5 ' 4 ' x 8 ' Tank

(Earth , T an k W eig h t, L ive L o ad s)

(Earth W e ig h t)

A ssu m e so il a t 120 lb /ft3

(A ssu m e 7 ,000 lb s .)

(L ive L o ad s)300 psf

LIGHT TRUCK TRAFFIC

325 psf

220 psf avg. 220 psf avg.

500 psf

2' Cover

5' 4' x 8' Tank

(Earth + LL + Tank Weight)

(Earth + LL Weight)

Assume soil at 120 lb/ft3

(Assume 7,000 lbs.)

2,250 lbs. wheel load

LL = 85 lb/ft2

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

Design Considerations

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

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• 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

Entrained Air = Desirable Improves freeze/thaw resistance, ASR resistance, sulfate

resistance Improves workability

Entrapped Air = Non-desirable Bugholes Released through adequate vibration

Air Content

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)

• Spaced properly, not touching formwork

REBAR SHOWING

Visible problems

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Rebar Touching Form

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What it should look like

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)

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

CuringProper 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 TESTINGASTM 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 - VacuumWith 300 psf Live Load from ASTM C-1227

240 psf

180 psf avg. 180 psf avg.

715 psf

2' Cover

5' 4 ' x 8 ' Tank

(Earth, Tank W eight, Live Loads)

(Earth W eight)

Assum e soil at 120 lb /ft3

(Assum e 7,000 lbs.)

(Live Loads)300 psf

Proof Testing - Vacuum

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

With 300 psf Live Load from ASTM C-1227

240 psf

180 psf avg. 180 psf avg.

715 psf

2' C over

5' 4' x 8' Tank

(Earth , Tank W eight, L ive Loads)

(Earth W eight)

A ssum e so il a t 120 lb /ft3

(Assum e 7 ,000 lbs .)

(L ive Loads)300 psf

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 - VacuumWith a concentrated wheel load of 2,250 pounds

325 psf

220 psf avg. 220 psf avg.

500 psf

2' Cover

5' 4' x 8' Tank

(Earth + L L + T an k W eig h t)

(Earth + L L W eig h t)

A ssu m e so il at 120 lb /ft3

(A ssu m e 7,000 lb s.)

2,250 lb s. w h eel lo ad

L L = 85 lb /ft2

Proof Testing - Vacuum

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)

With a concentrated wheel load of 2,250 pounds

325 psf

220 psf avg. 220 psf avg.

500 psf

2' C over

5' 4' x 8' Tank

(Earth + LL + Tank W eight)

(Earth + LL W eight)

Assum e soil at 120 lb/ft3

(Assum e 7,000 lbs.)

2,250 lbs. wheel load

LL = 85 lb/ft2

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

Area for Additional Weight

2' x 2' Footprint2’ 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

Vacuum Test

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

Water Test

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