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Review, NPCA "Precast Concrete On-Site Wastewater Tank Best Practices Manual" (HG News)

Posted by Hill and Griffith Company on Thu, Jul 14, 2016 @ 05:12 PM

Today we'll review the "Precast Concrete On-Site Wastewater Tank Best Practices Manual" provided by the National Precast Concrete Association.

Best_Practices_Concrete_Form_Release_Agents.jpgTable of Contents highlights
Structural Design ................................................4
Materials.............................................................4
Concrete Mix Proportioning..............................10
Lifting Inserts.................................................... 11
Coatings............................................................12
Production Practices.........................................12
Pre-Pour Checklist.............................................16
Casting Concrete..............................................17
Curing ...............................................................18
Post-Pour Operations........................................19
Post-Pour Checklist...........................................20
Finishing and Repairing Concrete.....................21
Seals, Fittings and Joints ..................................22
Tank Installation................................................24
Testing ..............................................................26 

Introduction
This manual provides guidance on material selection, manufacturing techniques, testing and installation to attain structurally sound, watertight precast concrete septic tanks and related components for on-site wastewater treatment systems. It is not intended for use as a regulatory code or minimum design standard, but rather as an aid to manufacturers, engineers, contractors and owners.

Chemical Admixtures
Although not mentioned specifically, this is good advice for concrete form releases as well, "Store admixtures as recommended by manufacturer and in a manner that avoids contamination, evaporation and damage. Protect liquid admixtures from freezing and extreme temperature changes, which could adversely affect their performance."

Concrete_Pre-Pour_Checklist_copy.jpgConcrete Mixture Proportioning
We liked this description of the "shock absorber" effect of air-entraining, "Air-entraining admixtures are designed to disperse microscopic air bubbles throughout the concrete’s matrix to function as small “shock absorbers” during freeze-thaw cycles. The required air content for frost resistant concrete is determined by the maximum aggregate size and severity of in-service exposure conditions (ACI 318, “Building Code Requirements for Structural Concrete and Commentary”) In addition, air entrainment improves workability and reduces bleeding and segregation of fresh concrete while greatly improving the durability and permeability of hardened concrete."

Production Practices
Excerpt from the part about form releases, "Apply form release agents in a thin, uniform layer on clean forms. Do not apply form release agents to reinforcing steel or other embedded items, as it can compromise the bond between the steel and the concrete. Do not allow the form release agent to puddle in the bottom of forms. Remove excess form release agent prior to casting."

Stripping and Handling Products
For the best release it's important not to remove the forms too quickly, "Concrete must gain sufficient strength before stripping it from the forms. Due to the nature of the precast business, the American Concrete Institute recognizes that forms will usually be stripped the next workday. Under normal conditions (concrete temperature greater than 50° F [10˚C]), a properly designed concrete can reach the minimum compressive strength for stripping within this time period. Periodic compressive strength testing of one-day strength or stripping strength cylinders is recommended to confirm that proper concrete strength is attained."

The manual concludes with a comprehensive glossary. Thanks to the NPCA for a great reference guide.


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