NATIONAL PRECAST CONCRETE ASSOCATION - …mowra.org/BestPracticesManual-PrecastConcreteOn... · as a regulatory code or minimum design standard, but ... Design precast concrete tanks

Copyright 2005 by National Precast Concrete Association (NPCA)Second Edition, 2005 All rights reserved.

No part of this manual may be reproduced In any form without permission in writing fromthe National Precast Concrete Association.

The association of the manufactured concrete products industry.10333 North Meridian Street, Suite 272 | Indianapolis, Indiana 46290800-366-7731 | 317-571-0041 (fax) | www.precast.org

2

NATIONAL PRECAST CONCRETE ASSOCATION

NOTES1. This manual does not claim or imply that it

addresses all safety-related issues, if any,associated with its use. Manufacture ofconcrete products may involve the use ofhazardous materials, operations andequipment. It is the user’s responsibility todetermine appropriate safety, health andenvironmental practices and applicableregulatory requirements associated withthe use of this manual and the manufactureof concrete products.

2. Use of this manual does not guarantee theproper function or performance of anyproduct manufactured in accordance withthe requirements contained in the manual.Routine conformance to the requirementsof this manual should result in products ofan acceptable quality according to currentindustry standards.

INTRODUCTION

This manual provides guidance on material selection,manufacturing techniques, testing and installation toattain structurally sound, watertight precast concreteseptic tanks and related components for on-sitewastewater treatment systems. It is not intended for useas a regulatory code or minimum design standard, butrather as an aid to manufacturers, engineers, contractorsand owners.

This manual will be most effective when used inconjunction with a complete review of local codes anddesign considerations before designing or manufacturingany on-site wastewater treatment tank or system.

It is impossible for a manual of this type to be allinclusive, and the recommendations are not intended toexclude any materials or techniques that will help achievethe goal of producing structurally sound and watertightprecast concrete tanks. Attention to detail, appropriatematerials, proper training of workers and quality controlthroughout a repeatable process will ensure that theconcrete tanks meet the needs of specifiers, contractorsand owners while protecting the environment.

The on-site wastewater treatment industry is rapidlyevolving, with greater demand for product quality andperformance. In the past, on-site wastewater systemsoften were considered temporary structures with limitedperformance expectations, because, as a communitymatured, centralized collection and treatment systemsreplaced the on-site systems that helped to develop thearea. However, this is no longer the trend.

Today, on-site wastewater treatment and disposalsystems have become the first choice of developers andpublic health officials in many parts of North America.The growing dependence on these systems places agreater emphasis on system performance and componentstructural integrity. As such, protection from waterinfiltration and exfiltration is a critical element in thedesign of tanks used in on-site systems. Regulatorycodes and project specifications requiring structurallysound and watertight tanks are becoming the rule ratherthan the exception, as they should be.

Precast concrete on-site wastewater treatment systems,when manufactured and installed properly, willoutperform and outlast systems consisting of competingmaterials. With the increasing regulatory demands forstructurally sound and watertight tanks, it is critical forprecast manufacturers to continually raise the bar onquality. It is with this in mind that many industryspecialists have come together to create this manual.

3

SEPTIC TANK MANUFACTURING BEST PRACTICES MANUAL

STRUCTURAL DESIGN

4


Loading ConditionsA properly designed precast concrete on-site wastewatertreatment tank must be able to withstand a variety ofloading conditions, which vary during manufacture,installation, testing and service. Tanks must be designedto withstand these loading conditions with rationalmathematical design calculations performed by a qualifiedprofessional engineer or by proof load testing inaccordance with ASTM C1227 “Standard Specification forPrecast Concrete Septic Tanks.”

Consider the following in the design:• Surface surcharge• Concentrated wheel loads• Lateral loads• Presumptive soil bearing capacity• Buoyant forces • Connections and penetrations• Point loads

Design precast concrete tanks for on-site applications inaccordance with one or more of the following applicableindustry codes and standards or as required by theauthority or authorities having jurisdiction:• ASTM C890, “Standard Practice for Minimum Structural

Design Loading for Monolithic or Sectional PrecastConcrete Water and Wastewater Structures”

• ASTM C1227, “Standard Specification for PrecastConcrete Septic Tanks”

• IAPMO/ANSI• CSA B66

The loading conditions illustrated in the these diagramsshould be analyzed and considered in the design of anon-site wastewater tank.

The following design characteristics have a critical impacton the performance of on-site wastewater tanks.

Concrete ThicknessThe concrete thickness must be sufficient to meetminimum reinforcement cover requirements and withstanddesign loading conditions.

Concrete Mix DesignConcrete must have a minimum compressive strength of4,000 psi at 28 days. Consider methods to reducepermeability, improve durability and increase strength.Maintaining a low water-cementitious ratio is one way toachieve this and must not exceed 0.45.

ReinforcementProper reinforcement is critical to withstand the loadsapplied to an on-site wastewater tank. Reinforcementmust be sufficient to provide adequate strength duringearly-age handling, installation and service, includingtemperature and shrinkage effects. All reinforcement mustmeet applicable ASTM International specifications.

External Soil and Water Loading: Tank Empty

Top SeamTank

Grade Line

Groundwater Level

Hydrostatic Loading

+

Soil loading

Grade Line

Groundwater Level

Hydrostatic Loading

+

Soil Loading

Mid-SeamTank

Top Seam Mid-Seam

Top Seam Mid-Seam

Internal Hydrostaic Loading – Tank Full (on grade or without backfill)

Optional Loading Where Appropriate*Manufacturer to specify the maximum depth of cover

Top SeamTank

Grade line

Groundwater Level

+ +

Mid-SeamTank

Residential Soil

Residential Soil

Depth ofCover

Hydrostatic LoadingSoil Loading Hydrostatic LoadingSoil Loading

SurchargeSurcharge

MATERIALSThe primary constituents of precast concrete are cement,fine and coarse aggregates, water and admixtures. Thefollowing discussion covers relevant factors in theselection and use of these fundamental materials.

CementThe majority of cement used in the manufacturedconcrete products industry is governed by ASTM C 150,“Standard Specification for Portland Cement.”

The five primary types of ASTM C 150 cement are:Type I NormalType II Moderate Sulfate ResistanceType III High Early StrengthType IV Low Heat of HydrationType V High Sulfate Resistance

Select the cement type based on project specifications orindividual characteristics which best fit the operation andregional conditions of each manufacturer. Note thatcertain types of cement may not be readily available incertain regions.

Store cement in a manner that will prevent exposure tomoisture or other contamination. Bulk cement should bestored in watertight bins or silos. If different types ofcement are used at a facility, store each type in aseparate bin or silo. Clearly identify delivery locations.

Design and maintain bin and silo compartments todischarge freely and independently into the weighinghopper. Cement in storage should be drawn downfrequently to prevent undesirable caking.

Stack bagged cement on pallets to permit propercirculation of air and avoid exposure to undesirablemoisture or condensation. On a short-term basis (lessthan 90 days), stack the bags no more than 14 high. Forlong-term storage, do not exceed seven bags in height(or per manufacturer’s recommendations).

Use the oldest stock first. Discard any cement with lumpsthat cannot be reduced by finger pressure.

AggregatesEnsure aggregates conform to the requirements of ASTMC33, “Standard Specification for Concrete Aggregates.”Evaluate the aggregates and maintain documention at theplant for potential deleterious expansion due to alkalireactivity, unless the aggregates come from a statedepartment of transportation-approved source. Themaximum size of coarse aggregate should be as large aspractical, but should not exceed 20 percent of theminimum thickness of the precast concrete tank or 75percent of the clear cover between reinforcement and thesurface of the tank. Larger maximum sizes of aggregatemay be used if evidence shows that satisfactory concreteproducts can be produced.

Quality of AggregatesConcrete is exposed to continuous moist and corrosiveconditions in wastewater applications. It is important tospecify a well-graded, sound, nonporous aggregate inaccordance with ASTM C33 “Standard Specification forConcrete Aggregates.”

Gradation of AggregatesAggregate gradation influences both the economy andstrength of a finished on-site wastewater tank. Thepurpose of proper gradation is to produce concrete witha maximum density along with good workability toachieve sufficient strength.

Well-graded aggregates help improve workability,durability and strength of the concrete. Poorly graded orgap-graded aggregates rely on the use of excess mortarto fill voids between course aggregates, leading topotential durability problems. Concrete mixes containingrounded coarse aggregates tend to be easier to place andconsolidate. However, crushed aggregates clearly areacceptable. The use of elongated, flat and flakyaggregates is discouraged. Gap-graded aggregateslacking intermediate sizes are also discouraged.

5


Experience has shown either very fine or very coarsesand or aggregate having a large deficiency or excess ofany size is undesirable. Sand gradation should beuniform and have a fineness modulus of not less than 2.3nor more than 3.1. A variation in base fineness modulusgreater than 0.2 may call for an adjustment to the mixdesign as suggested in ASTM C 33, “StandardSpecification for Concrete Aggregates.”

Aggregate Deleterious SubstancesEnsure all aggregates are free of deleterious substances.Deleterious substances include:• Substances that cause an adverse chemical reaction in

fresh or hardened concrete• Clay, dust and other surface-coating contaminants• Structurally soft or weak particles

For good bond development, ensure aggregate surfacesare clean and free from excessive dust or clay particles.Excessive dust or clay particles typically are defined asmaterial passing a #200 sieve, the limit of which is nomore than 3 percent. Friable aggregates may fracture inthe mixing and placement process, compromising theintegrity of the hardened concrete product.

Moisture Content of AggregateThe measurement of aggregate moisture content isimportant in the control of concrete workability, strengthand quality. Aggregates, particularly fine aggregates(sands), can collect considerable amounts of moisture ontheir surfaces. Fine aggregates can hold up to 10 percentmoisture by weight; coarse aggregates can hold up to 3 percent.

Water on the surface of an aggregate that is notaccounted in the mixture proportions will increase thewater-cementitious ratio. The moisture content ofaggregates will vary throughout a stockpile and will beaffected by changes in weather conditions. Therefore,adjust mixture proportions as necessary throughout theproduction day to compensate for moisture contentchanges in the aggregate.

The following methods will increase the likelihood ofuniform moisture content:• Enclose storage of daily production quantities• Store aggregates in horizontal layers• Have at least two stockpiles• Allow aggregate piles to drain before use• Avoid the use of the bottom 12 inches of a stockpile• Continuously sprinkle aggregate stock piles (climate

dependent)• Store entire stockpile indoors or under cover

Careful monitoring of aggregate moisture content duringbatching will reduce the necessity of using additionalcement to offset excess water. This will maintain high-quality standards and save on expensive raw materials.The plant should have a program in place that managessurface moisture content or accounts for moisturevariation during batching.

Handling and Storage of AggregateHandle and store aggregates in a way that preventscontamination and minimizes segregation anddegradation.

Aggregate handling is an important operation. Accuratelygraded coarse aggregate can segregate during a singleimproper stockpiling operation, so minimize handling toreduce the risk of particle size segregation. Alsominimize the number of handling operations and materialdrop heights to avoid breakage.

The following methods can prevent segregation:• Store aggregates on a clean, hard, well-drained base to

prevent contamination. Bin separation walls shouldextend high enough to prevent overlapping and cross-contamination of different-sized aggregates.

6


Figure 2 — Poorly graded mix vs. Well-graded mix design

Porous concrete resulting fromabsence of fine materials. Arrowsindicate water infiltration.

Inclusion of fine materials providesfilling for spaces between coarseaggregates.

• Avoid steep slopes in fine aggregate stockpiles. Fineaggregate stockpiles should not have slopes greaterthan the sand’s angle of repose (i.e., natural slope,typically 1:1.5) to prevent unwanted segregation.

• Remove aggregates from a stockpile by workinghorizontally across the face of the pile. If possible,avoid taking aggregate from the exact same locationeach time.

WaterWater used in mixing concrete should meet therequirements of ASTM C 1602, “Standard Specification forMixing Water Used in the Production of Hydraulic CementConcrete.” Avoid water containing deleterious amounts ofoils, acids, alkalis, salts, organic material or othersubstances that may aversely affect the properties offresh or hardened concrete.

Chemical AdmixturesCommonly used chemical admixtures in precastmanufacturing include:• Accelerating admixtures (ASTM C494, “Specification for

Chemical Admixtures for Concrete”)• Air entrainment admixtures (ASTM C260, “Specification

for Air-Entraining Admixtures for Concrete”)• Water reducing admixtures (ASTM C494, “Specification

for Chemical Admixtures for Concrete”)• High-range water-reducing admixtures or

superplasticizers (ASTM C1017, “Chemical Admixturesfor Use in Producing Flowing Concrete”)

Store admixtures in a manner that avoids contamination,evaporation and damage. Protect liquid admixtures fromfreezing and extreme temperature changes, which couldadversely affect their performance. It is also important toprotect admixture batching components from dust andtemperature extremes; ensure they are accessible forvisual observation and periodic maintenance. Performperiodic recalibration of the batching system asrecommended by the manufacturer or as required bylocal regulations.

Chemical admixture performance can vary; exercisecaution, especially when using new products. Test sometrial batches and document the results before using anew admixture for production. Follow manufacturers’recommendations exactly. Carefully check admixtures forcompatibility with the cement and any other admixturesused. Do not mix similar admixtures from differentmanufacturers without the manufacturer’s agreement ortesting to verify compatibility.

Additional guidelines for the use of admixtures areincluded in ACI 212.3, “Guide for Use of Admixtures inConcrete.”

Avoid accelerating admixtures that contain chlorides inorder to prevent possible corrosion of reinforcing steelelements and other embedded metal objects.

Supplementary CementitiousMaterials (SCMs)SCMs have three classifications:1. Cementitious materials – blast furnace slag (ASTM

C595, “Standard Specification for Blended Cements”)2. Pozzolanic materials – fly ash, silica fume, metakaolin

(ASTM C618, “Standard Specification for Coal Fly Ash,and Raw or Calcined Natural Pozzolan for Use inConcrete”)

3. Pozzolanic and cementitious materials – Class C flyashes (ASTM C618)

SCMs have a varying impact on the amount of water andair entrainment admixture required. Some SCMs,particularly fly ash, silica fume and blast furnace slag,could lead to significant improvements in permeabilityand resistance to sulfate attack, which are importantconsiderations in on-site wastewater tank design andperformance.

Ready-Mixed ConcreteVerify that the ready-mixed concrete supplier is operatingin accordance with ASTM C94, “Standard Specificationfor Ready-Mixed Concrete.”

Perform plastic concrete tests (slump, temperature, aircontent and density) at the plant prior to castingproducts. Record any added water on the delivery batchticket for each truck and keep it on file.

7


The design of concrete mixtures is a broad and extensivesubject, one that is specific to concrete in general but notnecessarily to on-site wastewater tanks. This discussionwill focus only on critical factors that pertain to theseproducts.

Mix designs are selected based upon several necessaryfactors including permeability, consistency, workability,strength and durability. The elements necessary toachieve high-quality watertight precast concrete include:• Low water-cementitious ratio (less than 0.45)• Minimum compressive strength of 4,000 psi at 28 days• Use of good quality and properly graded aggregate• Proper concrete consistency (concrete that can be

placed readily by traditional methods.

For concrete to be watertight, water must not be able toflow through its hardened pore structure. Low water-cementitious ratios are critical for increased concretestrength, watertightness and decreased permeability.

High water-cementitious ratios yield undesirableincreased capillary porosity within the concrete. Capillarypores are voids resulting from the consumption andevaporation of water during the hydration or curingprocess. Enlarged and interconnected capillary voidsserve as pathways that allow water and othercontaminants to either infiltrate or exfiltrate through theconcrete. Lower water-cementitious ratios result insmaller and fewer pores, reducing the permeability of theconcrete. ACI 350, “Code Requirements for EnvironmentalEngineering Concrete Structures,” recommends amaximum water-cementitious ratio of 0.45.

Proper consistency of fresh concrete is a critical elementin producing high-quality, watertight concrete. Freshconcrete must be sufficiently plastic (flowable ordeformable) to be properly placed, consolidated and

finished. The size, shape and grading of aggregates,cement content, water-cementitious ratio and admixturesaffect the workability of a mix.

Water-reducing admixtures and superplasticizers cangreatly increase the workability of fresh concrete withoutchanging the water-cementitious ratio. Experience hasshown that concrete with low water-cementitious ratios(less than 0.45), can be properly placed and consolidatedwith the aid and proper use of admixtures. Concreteshould be air-entrained in accordance with ACI 318.Their use is particularly important, since most on-sitewastewater tanks are relatively thin-walled and requirespecial attention to ensure full concrete consolidationduring casting. In certain circumstances, and where localregulations allow it, a properly designed and tested self-consolidating concrete (SCC) mix can reduce thenecessary effort to achieve proper consolidation of theconcrete.

Air-entraining admixtures are designed to dispersemicroscopic air bubbles throughout the concrete’s matrixto function as small “shock absorbers” during freeze-thaw cycles. The required air content for frost-resistantconcrete is determined by the maximum aggregate sizeand severity of in-service exposure conditions (ACI 318).In addition, air entrainment improves workability andreduces bleeding and segregation of fresh concrete whilegreatly improving the durability and permeability ofhardened concrete.

CONCRETE MIXTUREPROPORTIONING

8


LIFTING INSERTS

LIFTING INSERTS:Commercially manufactured lifting devices comefurnished with documented and tested load ratings. Usethe devices as prescribed by the manufacturer’sspecification sheets.

If lifting devices are homemade, have them load tested orevaluated by a professional engineer. Note that ASTMC1227, “Standard Specification for Precast Concrete SepticTanks,” calls for the following:

6.1.6 Inserts embedded in the concrete shall bedesigned for an ultimate load that is four times theworking load (Factor of Safety = 4).

9


COATINGS:Good concrete (water-cementitious ratio less than 0.45and compressive strength greater than 4,000 psi) issufficiently watertight for on-site wastewater tanks.Under normal in-service conditions, there is no need foradditional applications of asphalt, bituminous, epoxy orcementitious coatings. Coatings serve only to add costbut offer no benefit to concrete tanks that are alreadywatertight. Additionally, coatings can be difficult to applyproperly and may peel away from the concrete surface.Dislodged particles of coatings will move through theseptic system and may clog the drain field. However, aprotective exterior coating may be specified when a soilanalysis indicates a potential for chemical attack.

COATINGS

10


PRODUCTION PRACTICES

Quality ControlAll plants must have a quality control program andmanual, including but not limited to the following:• Documented mix designs• Pre-pour inspection reports• Form maintenance logs• Post-pour inspection reports• Performance and documentation of structural and

watertightness testing discussed in this manual• Plant qualty control procedures• Raw materials• Production practices• Concrete mixes• Reinforcement fabrication and placement• Concrete testing• Storage and handling

Records of the above listed items should be available forreview by appropriate agencies upon request.

Participation in the NPCA Plant Certification Program andfuture programs is recommended as an excellent way toensure product quality. Use the NPCA “Quality ControlManual for Precast Concrete Plants” as the basis fordeveloping a strong quality control program.

FormsForms must be in good condition. Frequent inspectionintervals and regular maintenance ensure that forms arefree of any damage that could cause concrete placementdifficulties or dimensional problems with the finishedproduct. Uniform concrete surfaces are less permeableand consequently enhance the watertightness ofcompleted tanks.

Use forms that prevent leakage of cement paste and aresufficiently rigid to withstand the vibrations encounteredin the production process. Maintain forms properly,including cleaning after each use and inspection prior toeach use, to ensure uniform concrete surfaces. Ensureforms are level and on a solid base.

Apply form release agents in a thin, uniform layer onclean forms. Do not apply form release agents toreinforcing steel or other embedded items, as it cancompromise the bond between the steel and the concrete.Do not allow the form release agent to puddle in thebottom of forms. Remove excess form release agent priorto casting.

Forms must be designed to meet individual productspecifications. Select forming materials, configuration,hardware and accessories that help the stripping processand minimize stripping damage. Inspect and maintainjoint areas to ensure proper tolerances on concrete jointsand keyways.

11


Fabrication drawings must be a part of every QCprogram. Fabrication drawings should detail thereinforcement requirements and all necessary informationpertaining to the product prior to casting.

Conventional ReinforcementFabricate reinforcing steel cages by tying or welding thebars, wires or welded wire reinforcement into rigidstructures. The reinforcing steel cages should conform tothe tolerances defined on the fabrication drawings. If notstated, minimum bend diameters on reinforcement shouldmeet the requirements set forth in ACI 318, as defined inTable 1. Make all bends while the reinforcement is cold.The minimum bend diameter for concrete reinforcingwelded wire reinforcement is 4db.

Table 1: Concrete Reinforcing Steel

db = nominal diameter (inch or mm) of bar

Weld reinforcement (including tack welding) inaccordance with AWS D1.4, “Structural Welding Code,Reinforcing Steel.” This code requires either specialpreheat requirements (when required) or weldable gradereinforcement according as defined by ASTM A706,“Specification for Low-Alloy Steel Deformed and PlainBars for Concrete Reinforcement,” for any welding ofreinforcing steel, including tack welds. Take special careto avoid undercutting or burning through the reinforcingsteel.

Conventional reinforcement (ASTM A615, “Specificationfor Deformed and Plain Billet-Steel Bars for ConcreteReinforcement”) is produced from recycled metals thathave higher carbon contents and are likely to becomebrittle if improperly welded. A brittle weld is a weak link,which can compromise the structural integrity of thefinished product. ASTM A615/615M states: “Welding ofmaterial in this specification should be approached withcaution since no specific provisions have been includedto enhance the weldability. When the reinforcing steel isto be welded, a welding procedure suitable for thechemical composition and intended use or service shouldbe used.”

Ensure lap splices for steel reinforcement (rebar andwelded-wire reinforcement) meet the requirements of ACI318. Adequate development length is required to developthe design strength of the reinforcement at a criticalsection. A qualified engineer should determinedevelopment length and clearly indicate it on shopdrawings.

Reinforcement steel should be free of loose rust, dirt andform release agent. Cut, bend and splice reinforcing steelin accordance with fabrication drawings and applicableindustry standards. Inspect reinforcing cages for size,spacing, proper bends and length. Secure the reinforcingcage in the form so that shifting will not occur duringcasting. Use only chairs, wheels and spacers made ofnoncorrosive materials.

It is important to place and hold reinforcement in positionas shown in the fabrication drawings. Due to therelatively thin walls of some on-site wastewater tanks, amaximum recommended placement tolerance for thedepth of reinforcement is +1/4 inch (ACI 318). As ageneral rule, the variation in spacing between barsshould not exceed 1 inch, except where inserts mayrequire some shifting of bars.

REINFORCEMENT

12


ASTM A615 and A706

Inch-Pound Bar Sizes

# 3 through # 8

# 9, # 10 and # 11

# 14 and # 18

Minimum Bend

Diameter

6db

8db

10db

ASTM A615 and A706

Soft Metric Bar Sizes

# 10 through # 25

# 29, # 32 and # 36

# 43 and # 57

The recommended minimum cover is 1 inch (ASTM C1227,“Standard Specification for Precast Concrete SepticTanks.”

Fiber ReinforcementData must be available to show conclusively that thetype, brand, quality and quantity of fibers to beincluded in the concrete mix are not detrimental to theconcrete or to the precast concrete product.

Fiber reinforced concrete must conform to ASTM C1116,“Standard Specification for Fiber-Reinforced Concreteand Shotcrete” (Type I or Type III).

The two most popular types of fibers are synthetic andsteel fibers. Steel fibers must conform to ASTM A820,“Specification for Steel Fibers for Fiber-ReinforcedConcrete.” Do not use fibers as a replacement forprimary structural reinforcing steel. In general, fiberswill not increase the compressive or flexural strengthof concrete.

Synthetic microfibers in concrete typically reduceplastic shrinkage cracks and improve impact resistance.They can help reduce chipping when products arestripped. Typical dosage rates will vary from 0.5 to 2.0lbs/yd3. Synthetic macrofibers and steel fibers mayreplace secondary reinforcement to provide equivalentbending stress and strength when compared withwelded wire reinforcement and light-gauge steelreinforcement. Typical dosage rates for syntheticmacrofibers vary from 3.0 to 20 lbs/yd3. Steel fiberdosage rates may vary from 20 to 60 lbs/yd3. Fibersmust be approved by a regulatory agency or specifyingengineer prior to concrete placement.

Design the concrete mix so that the mix is workable andthe fibers are evenly distributed. Chemical admixtures oradjustments to the concrete mixture design may benecessary to achieve proper consolidation andworkability. It is important to adhere to themanufacturer’s safety precautions and to followinstructions when introducing the fibers into the mix.

Embedded ItemsEmbedded items such as plates, inserts, connectors andcast-in seals must be held rigidly in place during casting.

Pre-Pour InspectionA typical pre-pour checklist, as illustrated on the nextpage, provides a means of documenting the requiredquality checks. A qualified individual should makeinspections prior to each pour. Correct any deviationsprior to the start of placement activities.

Pre-Pour Operations Include:

• Preparing and setting forms• Positioning steel reinforcement according to structural

design• Placing blockouts• Positioning embedded items

13


14


PRE-POUR CHECKLIST

PRODUCT: ________________________________________________ Job No. ____________

Casting Date Sun Mon Tues Wed Thurs Fri Sat

Form Condition

Form Cleanliness

Form Joints

Release Agent/Retarder

Design Length (ft/in)

Set-Up Length (ft/in)

Design Width (ft/in)

Set-Up Width (ft/in)

Design Depth (ft/in)

Set-Up Depth (ft/in)

Blockouts

Squareness

End and Edge Details

Reinforcing Steel

Size of Reinforcement

Spacing

Rustification

Plates and Inserts

Lifting Devices

Top Finish (wet)

REMARKS: ____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

QC Supervisor ________________ Date _______ Inspector _______________ _______________

CASTING CONCRETE

Transporting ConcreteWhen transporting concrete from mixer to form, use anymethod that does not contaminate the concrete, causedelay in placing, or segregation. Concrete can bedischarged directly from the mixer into the forms - ACI304, “Guide for Measuring, Mixing, Transporting andPlacing,” is a valuable reference.

Placing Concrete

Conventional Concrete

Keep the free fall of the concrete to a minimum anddeposit as near to its final location as possible. Donot use vibration equipment to move fresh concretelaterally in the forms.

Fiber Reinforced Concrete (FRC)Follow the same practices as described forconventional concrete, but note that the workabilityof the FRC may be slightly reduced.

Self-Consolidating Concrete (SCC)Place self-consolidating concrete into itself at aconstant pressure head from one end of the form,allowing air to escape as the concrete flows into andaround steel reinforcement. Avoid placementpractices that add additional energy to the mix andcause unwanted segregation, such as excessivevibration, increased pour heights or increaseddischarge rates.

Consolidating ConcreteSCC generally requires minimal consolidation efforts.However, when using conventional concrete, consolidationoperations are required to minimize segregation andhoneycombing. Consolidation can be improved onparticular molds by using vibrators with variablefrequency and amplitude.

Three types of vibration are prevalent in the precastindustry:Internal – stick vibratorExternal – vibrator mounted on forms or set on a

vibrating tableSurface – vibrator can be moved across the surface

Lower internal vibrators vertically and systematically intothe concrete without force until the tip of the vibratorreaches the bottom of the form. When using internalvibrators, concrete should be placed in wall sectionsusing lifts not exceeding 2 feet. Do not drag internalvibrators horizontally. Once consolidation is complete inone area, remove the vibrator vertically and move thevibrator to the next area. Regardless of the vibrationmethod, insure that the field of vibration overlaps withanother insertion to best consolidate the concrete andminimize defects. Some external vibrators are mountedon a piece of steel attached to the form. Position them toallow for overlap of vibration areas.

Continue the vibration process until the product iscompletely consolidated. Vibration is considered completewhen large bubbles (3/8” diameter or greater) no longerappear at the surface.

Also take care to not overvibrate, because segregation ofthe aggregate from the cement paste can result loweringconcrete quality and strength.

Finishing Unformed SurfacesEach product is to be finished according to its individualspecifications. If finishing techniques are not specified,take care to avoid floating either too early or for toolong. Premature finishing can trap bleed water below thefinished surface, creating a weak layer of concretesusceptible to freeze-thaw cycles and chemical attack.Finishing with a wood or magnesium float isrecommended.

15


Two critical elements in curing concrete are maintainingcorrect moisture content and maintaining correct concretetemperature. Proper curing is important in developingwatertightness, chemical resistance, and strength anddurability, all important considerations in on-sitewastewater tank construction.

Note: Concrete temperature discussed in this manualrefers to the temperature of the concrete itself, not theambient temperature.

The nature of precast operations poses unique challengesto proper curing. To ensure cost-effective use of forms,precasters often strip the forms at the beginning of thenext workday. That is an acceptable standard, accordingto ACI 308, “Standard Practice for Curing Concrete.” Thetime necessary to develop enough strength to strip theforms is highly dependent on ambient temperature in thecasting area.

The Portland Concrete Association (PCA) lists threemethods of curing:1. Maintaining water moisture by wetting (fogging,

spraying, wet coverings, etc.)2. Preventing the loss of water by sealing (plastic

coverings or applying curing compounds)3. Applying heat (often in conjunction with moisture, with

heaters or live steam)

Choose the method(s) that best suit the particularproduction operation. All three are permissible, butpreventing the loss of water (method 2) may be thesimplest choice for on-site wastewater tanks. Maintainingmoisture requires constant wetting, which is manpower-intensive. Alternate wetting and drying can lead toproblems with cracking. Steam curing can also beeffective. Concrete temperatures should never exceed150 F. Both of these techniques are described in ACI 308,“Standard Practice for Curing Concrete,” and the PCApublication “Design and Control of Concrete Mixtures.”

Plastic coverings or membrane-forming curingcompounds require less manpower and allow formstripping the next work day. There are some specialconsiderations for both:

1. Plastic sheeting must comply with ASTM C171,“Standard Specification for Sheet Materials for CuringConcrete,” which specifies a minimum thickness of 4 mm and be either white or opaque in color. PCAstates that other colors can be used depending on sunconditions and temperature. When using multiplesheets, overlap them by approximately 18 inches toprevent moisture loss.

2. Curing compounds can be applied when bleed water isno longer present on the surface. As with plastic,white-colored compounds might reflect sunlight betterand limit temperature gain. Follow the manufacturer’srecommendations.

Cold and Hot Weather ConcretingIn hot and cold weather, special precautions arenecessary.

Cold Weather – Hydration rates are slower during coldweather. Concrete temperatures below 50 F areconsidered unfavorable for pouring due to the extendedtime required for strength gain and the possibility offreezing. However, once concrete reaches a minimumstrength of 500 psi (usually within 24 hours) freezing hasa limited impact. Ideally, precast concrete operationsshould be performed in heated enclosures that willprovide uniform heat to the products until they reach500 psi. If necessary, heating the mixing water and/oraggregates can increase the concrete temperature. Do notheat water above 140 F, and do not use clumps of frozenaggregate and ice. ACI 306, “Cold Weather Concreting,”contains further recommendations on cold-weatherconcreting.

CURING PROCEDURES

16


Hot Weather – High temperatures accelerate hydration.Do not allow fresh concrete temperature to exceed 90 Fat time of placement. Keep the temperature of theconcrete mix as low as possible using a variety of means,including:• Shading the aggregate piles• Wetting the aggregates (mix design must be adjusted

to account for the additional water)• Using chilled water

Note: During the curing process, ensure that the concretetemperature does not exceed 150 F. In all cases, protectfreshly cast products from direct sunlight and dryingwind. ACI 305, “Hot Weather Concreting,” containsfurther recommendations on hot-weather concreting.

17


Handling EquipmentCranes, forklifts, hoists, chains, slings and other liftingequipment must be able to handle the weight of theproduct with ease and comply with federal and localsafety requirements.

Routine inspections of all handling equipment arenecessary. Qualified personnel should make periodicmaintenance and repairs as warranted. Tag all chains andslings with individual load capacity ratings. For U.S.plants, refer to the specific requirements of theOccupational Safety and Health Administration (OSHA).For Canadian plants, refer to the specific requirements ofthe Canadian Centre for Occupational Health and Safety(CCOHS).

Stripping and Handling ProductsMinimum Strength Requirement – Concrete must gainsufficient strength before stripping it from the forms. Dueto the nature of the precast business, the AmericanConcrete Institute recognizes that forms will usually bestripped the next workday. Under normal conditions(concrete temperature greater than 50° F), properlydesigned concrete can reach the minimum compressivestrength for stripping within this time period. Periodiccompressive strength testing of one-day or strippingstrength cylinders is recommended to confirm that properconcrete strength is attained.

Handle recently poured and stripped products with care.Perform lifting and handling carefully and slowly toensure that dynamic loads do not damage the tank.Always follow recognized safety guidelines.

Product Damage During Stripping – Inspect the tankimmediately after stripping to check for damage.

Post-Pour InspectionsA post-pour inspection checklist, as illustrated on thefollowing page, provides a method of identifying andcommunicating quality problems as they occur and toidentify any trends. After stripping a tank from its form,inspect the tank for conformance with the fabricationdrawings. Clearly label all products with the date ofmanufacturing and mark these in accordance with ASTMC1227, “Standard Specification for Precast Concrete SepticTanks.”

POST-POUR OPERATIONS

18


19


POST-POUR CHECKLIST

PRODUCT: ________________________________________________ Job No. ____________

Casting Date:_________ Sun Mon Tues Wed Thurs Fri Sat

Inspection Date:______

Mark Number

Stripping Strength

Top Finish

Bottom Finish

Surface Texture

As Cast Length (ft/in)

As Cast Width (ft/in)

As Cast Depth (ft/in)

Cracks or Spalls

Squareness

Chamfers

Honeycomb / Grout Leak

Bowing

Exposed Reinforcement

Exposed Chairs

Plates and Inserts

Chamfer & Radius Quality

Openings / Blockouts

Lifting Devices

REMARKS: ____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

QC Supervisor ________________ Date _______ Inspector _______________ _______________

Finished Formed Surfaces – Formed surfaces must berelatively smooth and free of significant honeycombedareas, air voids and “bugholes.”

Repairing Minor Defects – Defects that do not impair theuse or life of the product are considered minor orcosmetic and may be repaired in any manner that doesnot impair the product.

Repairing Honeycombed Areas – Remove all loosematerial from the damaged area. Cut back the damagedzone in horizontal or vertical planes deep enough toremove the damaged concrete. Coarse aggregate particlesshould break rather than merely dislodge when chipped.Use only materials that are specifically developed forconcrete repair, and make repairs according to themanufacturers’ specifications.

Repairing Major Defects – Major defects are defined asthose that impair the intended use or structural integrityof the product. If possible, repair products with majordefects by using established repair and curingprocedures only after a qualified person evaluates thefeasibility of the repair.

Secondary PoursFor products that require secondary pours, establishprocedures to assure that the new concrete bondsadequately to the product and becomes an integral partof it.

The surfaces of the product against which the secondarypour is to be made should be free of laitance, dirt, dust,grease or any other material that will tend to weaken thebond between the original and new concretes. If thesurface is very smooth, roughen it to help promote agood bond. As a minimum, use a high-quality water stop,keyway and continuation of reinforcing between pours toensure a watertight joint.

Cold JointsCold joints require special care and, as a minimum,should include a high-quality water stop, bonding agentand continuation of reinforcing between pours.

Storage of ProductsStorage areas must be flat and strong enough to supportthe product without causing damage. Store the product ina manner that will not damage it in any way whilestacking, moving or handling and in a manner that willfacilitate rotation of inventory.

Marking of ProductsUnless otherwise specified by project specifications orauthority having jurisdiction, mark tanks and tank lids inaccordance with ASTM C1227, “Standard Specification forPrecast Concrete Septic Tanks.”

Final Product InspectionCheck the tanks visually at the plant prior to shipping,preferably after being loaded and secured on the deliveryvehicle.

Product ShipmentAll vehicles used to transport products must be in goodcondition and capable of handling the product withoutcausing damage. Allow tanks to adequately cure prior toshipment to a job site or distant storage areas. Secure allproducts properly with appropriate blockage and eithernylon straps or chains with guards in order to avoidproduct damage during shipment. NPCA’s publication“Cargo Securement for the Precast Concrete Industry”outlines proper methods for securing product. The finalinspection should include a check of these items.

FINISHING ANDREPAIRING CONCRETE

20


SEALS, FITTINGS AND JOINTS

Joint details, sealing materials and pipe connectors areimportant to the watertight integrity of on-sitewastewater tanks. Systems to be installed in areas withhigh water tables require special consideration for jointand connection seal designs.

Joint Designs To ensure structurally sound mid-seam tank withwatertight joints, use only interlocking joints. Interlockingjoints have the effect of increasing the overall structuralstrength of mid-seam tanks. The most common types ofinterlocking joints used in tank construction are tongue-and-groove and lap joints. Non-interlocking joints areacceptable on top-seam tanks provided that the top piece(lid) is properly secured to the structure with appropriatesealing materials.

Examples of tank joints are illustrated in the followingdiagrams.

Sealing MaterialsUse high-quality, preformed, flexible joint sealants toachieve a watertight seal in multiple-piece tanks.Characteristics of high-quality joint sealants include:• Compressibility in ambient temperatures below 40 F• Adhesion to clean, dry surfaces• Resistance to degradation caused by aging (shrinkage,

hardening or oxidizing)• Resistance to degradation caused by exposure to

sewage materials

Sealants conforming to ASTM C990 typically meet thecriteria listed above.

Sealant SizeA critical factor when evaluating the sealing potential of asealant is cross-sectional area. Cross-sectional area isdefined as the geometric shape of the sealant (i.e., 0.75inches high by 1.0 inches wide). Industry experience hasshown that a sealant’s cross-sectional height must becompressed a minimum of 30 percent to create a goodseal; 50 percent compression is desirable.

Apply sealants to form a continuous length of seal.Properly splice the sealant by one of the followingmethods:• Overlap splice – Place one piece on top of the other

and carefully mold together• Side-by-side – Place ends in a parallel position and

carefully mold the two pieces together

Pipe to Tank SealsThe connection between the pipe and tank must beaccomplished with a watertight, resilient connector. Thisconnector must be the sole means of sealing between thepipe and tank and cannot permit the infiltration of fluidsor loss of vacuum around installed pipes (or pipe stubs)when tested in accordance with the requirements of thisspecification. Connectors conforming to ASTM C923 fulfillthe requirement of this section. In the event that anASTM International specification for pipe-to-tankconnectors becomes available, connectors in accordancewith this new specification are also acceptable for use inthis application.

Access Risers and ManholesAll access risers and manholes must be structurallysound and watertight.

21


Slab Joint Lap Joint Shiplap JointTongue &

Groove Joint

Proper installation of the tank is absolutely critical formaintaining structural integrity and watertightness. Manyof the problems experienced with leakage can beattributed to incorrect installation procedures. In additionto damage to the tank, improper installation techniquescould be a safety hazard.

Site ConditionsThe installation site must be accessible to large, heavytrucks weighing up to 80,000 pounds. The constructionarea should be free of trees, branches, overhead wires orparts of buildings that could interfere with the deliveryand installation of the on-site wastewater tank. Mosttrucks will need to get within 3 to 8 feet of theexcavation to be unloaded.

ExcavationPrior to excavation, identify and locate all buried utilities.Follow OSHA regulations governing excavation work at alltimes. Excavations should be sloped to comply with allconstruction safety requirements.

BeddingProper use of bedding material is important to ensure along service life of an on-site wastewater treatmentsystem. Use imported bedding material as necessary toprovide a uniform bearing surface. A good base shouldensure that the tank would not be subjected to adversesettlement. Use a minimum of 4 inches thickness of sandor granular bed overlying a firm and uniform base unlessotherwise specified. Tanks should not bear on largeboulders or rock edges.

Sites with silty soils, high water tables or other “poor”bearing characteristics must have specially designedbedding and bearing surfaces. In the presence of highwater tables, structures should be properly designed toresist floatation.

Proper compaction of the underlying soils and beddingmaterial is critical to eliminate later settlement, which canultimately occur in all tank installations regardless of thetank material. Potential tank settlement is measurable,predictable and preventable. Proper evaluation of theoriginal soil, bedding materials, water table, backfillmaterials and potential soil bearing stresses reduces thelikelihood of later tank settlement. Set the tanks level toprovide the proper elevation drop from the inlet to theoutlet.

Worker safety is of primary importance. If it is necessaryto have a worker enter the excavation to check elevationor compact bedding materials, use proper excavationmethods that will prevent the sidewalls from collapsing.Alternatively, trench boxes may also be used ifnecessary.

Tank PlacementPrior to placement in the excavation, confirm the tank’sorientation. Check the bedding material and ensure thatinlet penetrations face the residence. After placement,check that the tank is level. The slope of the sewer lineand tank elevation should meet local plumbing andbuilding codes.

Lifting DevicesVerify lifting apparatus such as slings, lift bars, chainsand hooks for capacity, and ensure an adequate safetyfactor for lifting and handling products. The capacity ofcommercial lifting devices must be marked on thedevices.

All lifting devices and apparatus should meet OSHArequirements documented in “Code of FederalRegulations” Title 29 Part 1926. Other applicable codesand standards are ANSI A10.9 and ASTM C857, C890and C913.

TANK INSTALLATION

22


A factor of safety of at least 4 is recommended forlifting devices. Manufacturers of standard liftingdevices should provide test data to allow selection ofappropriate loading.

Because of their brittle nature, do not use reinforcingbars as lifting devices. Use smooth bars made of steelconforming to ASTM A36 instead.

A factor of safety of at least 5 is recommended for liftingapparatus, such as chains, slings, spreader beams, hooksand shackles.

Joint Seals For two-piece tanks, use high-quality preformed jointseals. Surfaces should be clean. Ensure seals meetminimum compression and other installation requirementsas prescribed by the seal manufacturer and detailedherein. During the time of installation, ambienttemperatures below 50 F sometimes affect thecompressibility of the sealant. Care must be taken todetermine that tank sections installed on site have beenproperly sealed. Inspecting the joint area to determinethat the tank sections have been properly seated helpsprevent soil materials from entering the joint area duringbackfilling operation. Properly seal manholes and risersto prevent infiltration.

BackfillingPlace backfill in uniform layers less than 24 inches thick.Backfill should be free of any large stones (greater than3 inches in diameter) or other debris.

23


4 Inches Minimum

Backfill Lift

24 inches

18 Inches Minimum

Figure 3 — Basic principles of bedding,tank placement and backfilling

Stripping Strength TestingPerform periodic compressive strength testing at oneday, or test stripping strength cylinders. The frequency isat the judgment of the precaster and based on the plant’squality control manual. Cure these cylinders in a similarmanner as the finished product.

Tank TestingEach plant must develop a quality control program thatincludes testing. Define the test procedures clearly priorto tank fabrication and installation. Consider whethertests will be conducted in the plant or on the job site(prior to or after backfilling). Use one of the followingtwo primary testing methods:

Vacuum TestingThe recommended procedure is to introduce a minimumvacuum of 4 inches of mercury and hold this pressurefor five minutes.

Depending upon the tank configuration, it may take sometime to stabilize the vacuum level due to various factors(compression of sealant, temperature variations, etc.). Itis permissible to apply vacuum until the vacuum levelstabilizes at 4 inches. Once vacuum is stabilized, removethe vacuum source and hold the vacuum for 5 minutes. Ifthe tank fails the test, it may be repaired and retested.The suggested range of the gauge is 0-10 inches ofmercury (maximum).

WARNING: Testing with negative pressure involvespotentially hazardous conditions. It is recommendedthat the negative air pressure testing of concrete tanksnot exceed 7 inches of mercury, which is therecognized maximum requirement for structuralstrength proof testing. Take precautions to minimizepotential risks by incorporating safety devices that willprevent excessive vacuum levels (safety release valves,etc).

Water TestingFill the tank with water to 2 inches above the top of thecover inside the riser and allow it to stand for 24 hours.If there is visible leakage (water flowing or dripping in asteady stream), repair the tank, refill it and allow it tostand for one hour. No visible leakage is allowed. Do notreject the tanks for damp spots on the exterior. If wateris dripping or flowing in a steady stream, repair the tankand retest. Condensation on the exterior of the tank dueto temperature variation is not considered a failure.

Select the method and location of testing in order toensure watertightness and to ensure that the actual testload condition does not exceed engineered design.

Frequency of TestingPerform testing on one tank per form per year at aminimum or every 250 tanks per form, whichever isgreater. Forms producing tanks that fail this test mustundergo additional testing commencing with the nextproduction of tanks from the form and continuing until 10consecutive tanks pass the test.

TESTING

24


REFERENCES

SpecificationsAmerican Concrete Institute (ACI)ACI 116R, “Cement and Concrete Terminology”ACI 211.1, “Standard Practice for Selecting Proportions for

Normal, Heavyweight and Mass Concrete”ACI 211.3, “Standard Practice for Selecting Proportions for

No-Slump Concrete”ACI 212.3, “Chemical Admixtures for Concrete”ACI 304R, “Guide for Measuring, Mixing, Transporting and

Placing Concrete”ACI 305R, “Guide for Hot Weather Concreting”ACI 306R, “Guide for Cold Weather Concreting”ACI 308R, “Guide to Curing Concrete”ACI 318, “Building Code Requirements for Structural

Concrete and Commentary”ACI 350R, “Code Requirements for Environmental

Engineering Concrete Structures and Commentary”ACI 544, “State-of-the-Art Report on Fiber Reinforced

Concrete”

ASTM InternationalASTM A185, “Standard Specification for Steel Welded

Wire Reinforcement, Plain, for Concrete Reinforcement”ASTM A496, “Standard Specification for Steel Wire,

Deformed for Concrete Reinforcement”ASTM A497, “Standard Specification for Steel Welded

Wire Reinforcement, Deformed, for ConcreteReinforcement”

ASTM A615, “Standard Specification for Deformed andPlain Carbon Steel Bars for Concrete Reinforcement”

ASTM A706, “Standard Specification for Low Alloy SteelDeformed Bars and Plain for Concrete Reinforcement”

ASTM A820, “Specification for Steel Fibers for ReinforcedConcrete”

ASTM C33, “Standard Specification for ConcreteAggregates”

ASTM C125, “Standard Terminology Relating to Concreteand Concrete Aggregates”

ASTM C150, “Standard Specification for Portland Cement”ASTM C260, “Standard Specification for Air-Entraining

Admixtures for Concrete”ASTM C494, “Standard Specification for Chemical

Admixtures for Concrete”

ASTM C595, “Standard Specification for BlendedHydraulic Cements”

ASTM C618, “Standard Specification for Coal Fly Ash andRaw or Calcined Natural Pozzolan for Use in Concrete”

ASTM C890, “Standard Practice for Minimum StructuralDesign Loading for Monolithic or Sectional PrecastConcrete Water and Wastewater Structures.”

ASTM C913, “Standard Specification for Precast ConcreteWater and Wastewater Structures”

ASTM C1017, “Chemical Admixtures for Use in ProducingFlowing Concrete”

ASTM C1116, “Standard Specification for Fiber ReinforcedConcrete and Shotcrete”

ASTM C1227, “Standard Specifications for PrecastConcrete Tanks”

ASTM C1602, “Standard Specification for Mixing WaterUsed in the Production of Hydraulic Cement Concrete “

ASTM D6, “Standard Test Method for Loss on Heating ofOil and Asphaltic Compounds”

American Welding Society (AWS)AWS D1.4, “Structural Welding Code - Reinforcing Steel”

Occupational Safety and HealthAdministration (OSHA)29 CFR 1910.184 (Slings)29 CFR 1926.650-652 (Excavation)

25


Periodicals and TextbooksMunshi, J.A., “Rectangular Concrete Tanks,” Revised 5th

Edition, Portland Cement Association, Skokie, IL, 1998Gowripalan, N., et al, “Effect of Curing on Durability”,

ACI, Durable Concrete, 1994Hunter, L.E., “Concrete Waterproofing,” Pitman & Sons,

London, 1947Kosmatka, Steven H. and Panarese, William C., “Design

and Control of Concrete Mixtures,” 13th edition,Portland Cement Association, Skokie, Illinois, 1988

Levett, M. , “Precast Concrete,” Applied SciencePublishers, London, 1982

Mindess, Sidney and Young, Francis T., “Concrete,”Prentice Hall, Englewood Cliffs, N.J. 1981

Neville, A.M., “Properties of Concrete,” 4th edition, Wiley& Sons, New York, 1996

U.S. Department of Interior, “Concrete Manual,” 8thedition, Denver, 1981

American Concrete Institute, ACI 350-89, “EnvironmentalConcrete Structures,” Farmington Hills, MI

Portland Cement Association, “Circular Concrete Tankswithout Prestressing,” 1993, Skokie, IL

Portland Cement Association, “Design and Control ofConcrete Mixtures,” Fourteenth Edition, 2002

Portland Cement Association, “Rectangular ConcreteTanks,” 1994, Skokie, IL

U.S. Department of Interior, “Concrete Manual,” 8thedition, Denver, 1981

Waddell, Joseph, “Fundamentals of Quality PrecastConcrete,” National Precast Concrete Association,Indianapolis, IN

26


GLOSSARY

admixture – a material other than water, aggregates,cement and fiber reinforcement used as an ingredientof concrete and added to the batch immediately beforeor during its mixing.

admixture, accelerating – an admixture that acceleratesthe setting and early strength development of concrete.

admixture, air-entraining – an admixture that causes thedevelopment of a system of microscopic air bubbles inconcrete, mortar or cement paste during mixing.

admixture, water-reducing – admixture that eitherincreases the slump of freshly mixed concrete withoutincreasing the water content or that maintains theslump with a reduced amount of water due to factorsother than air entrainment.

aggregate – granular material, such as sand, gravel,crushed stone or iron blast-furnace slag used with acement medium to form hydraulic-cement concrete ormortar.

aggregate, coarse – generally pea-sized to 2 inches;aggregate of sufficient size to be predominatelyretained on a No. 4 sieve (4.75 mm).

aggregate, fine – generally coarse sand to very fine;aggregate passing the 3/8 inch sieve (9.5 mm) andalmost entirely passing a No. 4 sieve (4.75 mm) andpredominately retained on the No. 200 sieve (0.75mm).

air content – the volume of air voids in cement paste,mortar or concrete, exclusive of pore space inaggregate particles; usually expressed as a percentageof total volume of the paste, mortar or concrete.

ASTM – ASTM International is a not-for-profitorganization that provides a forum for producers,users, ultimate consumers and those having a generalinterest (government and academia) to meet and writestandards for materials, products, systems andservices.

bedding material – gravel, soil, sand or other materialthat serves as a bearing surface on which a structurerests and which carries the load transmitted to it.

bleeding – the separation of mixing water or itsemergence from the surface of newly placed concretecaused by the settlement of the solid materials.

bonding agent – a substance applied to a suitablesubstrate to create a bond between it and a succeedinglayer, such as between a layer of hardened concreteand a layer of fresh concrete.

cement, hydraulic – cement that sets and hardens bychemical interaction with water and is capable of doingso under water.

cementitious material – an inorganic material or mixtureof inorganic materials that set and develop strength bychemical reaction with water by formation of hydrates.

cold joint – a joint or discontinuity formed when aconcrete surface hardens before the next batch isplaced against it.

concrete – a composite material that consists essentiallyof a binding medium within which are embeddedparticles of aggregate fragments, usually a combinationof fine aggregate and coarse aggregate; in portlandcement concrete, the binder is a mixture of portlandcement and water.

27


concrete, fresh – concrete that possesses enough of itsoriginal workability so that it can be placed andconsolidated by the intended methods.

compressive strength – measured maximum resistanceof a concrete or mortar specimen to axial compressiveloading; expressed as a force per unit cross-sectionalarea; or the specified resistance used in designcalculations.

consistency – the relative mobility or ability of freshlymixed concrete to flow; it is usually measured by theslump test.

consolidation – the process of inducing a closerarrangement of the solid particles in freshly mixedconcrete during placement by the reduction of voids,usually accomplished by vibration, centrifugation,rodding, tamping or some combination of these actions.Consolidation facilitates the release of entrapped air; asconcrete subsides, large air voids between coarseaggregate particles are filled with mortar.

curing – action taken to maintain moisture andtemperature conditions in a freshly placed cementitiousmixture to allow hydraulic cement hydration and, ifapplicable, pozzolanic reactions to occur so that thepotential properties of the mixture may develop.

curing compound – a liquid that can be applied as acoating to the surface of newly placed concrete toretard the loss of water or to reflect heat in order toprovide an opportunity for the concrete to develop itsproperties in a favorable temperature and moistureenvironment.

deleterious substances – materials present within or onaggregates that are harmful to fresh or hardenedconcrete, often in a subtle or unexpected way. Morespecifically, this may refer to one or more of thefollowing: materials that may be detrimentally reactivewith the alkalis in the cement (see alkali aggregatereactivity) clay lumps and friable particles, coal andlignite, etc.

dry-cast (no-slump concrete) – concrete of stiff orextremely dry consistency showing no measurableslump after removal of the slump cone.

differential settlement – the uneven sinking of material(usually gravel or sand) after placement.

elongated aggregate – a particle of aggregate where itslength is significantly greater than its width.

entrained air – see air void; microscopic air bubblesintentionally incorporated into mortar or concreteduring mixing, typically between 10 µm and 1,000 µm (1 mm) in diameter and spherical or nearly so.

exfiltration - to cause (as a liquid) to flow outwardthrough something by penetrating its pores orinterstices.

fiber reinforcement – discontinuous tensile filaments ofsteel or synthetic materials designed to providesecondary reinforcement of concrete structures and tohelp mitigate the formation of plastic shrinkage cracks.

float – a tool, usually of wood, aluminum or magnesium,used in finishing operations to impart a relatively evenbut still open texture to an unformed fresh concretesurface.

floating – the operation of finishing a fresh concrete ormortar surface by use of a float, preceding trowelingwhen that is to be the final finish.

28


fly ash – the finely divided residue transported by fluegases from the combustion of ground or powderedcoal; often used as a supplementary cementitiousmaterial in concrete.

forms (molds) – a structure for the support of concretewhile it is setting and gaining sufficient strength to beself-supporting.

friable – easily crumbled or pulverized, as it refers toaggregates.

gap grading – aggregate graded so that certainintermediate sizes are substantially absent (i.e.,aggregate containing large and small particles withmedium-size particles missing).

gradation – the particle-size distribution as determinedby a sieve analysis (ASTM C 136, etc.); usuallyexpressed in terms of cumulative percentages larger orsmaller than each of a series of sizes (sieve openings)or the percentages between certain ranges of sizes(sieve openings).

hydration – formation of a compound by the combiningof water with some other substance; in concrete, thechemical process between hydraulic cement and water.

infiltration – to cause (as a liquid) to permeatesomething by penetrating its pores or interstices.

organic impurities (re: aggregate) – extraneous andunwanted organic materials (twigs, soil, leaves andother debris) that are mixed in aggregates; thesematerials may have detrimental effects on concreteproduced from such aggregates.

OSHA – Occupational Safety and Health Administration,U.S. Department of Labor.

plastic concrete – see concrete, fresh.

portland cement – hydraulic cement produced bypulverizing portland cement clinker, usually incombination with calcium sulfate.

pozzolan – a siliceous or siliceous and aluminousmaterial that in itself possesses little or nocementitious value but will, in finely divided form andin the presence of moisture, chemically react withcalcium hydroxide at ordinary temperatures to formcompounds possessing cementitious properties.

psi – pounds per square inch

secondary pour – a situation when a succeeding layer ofconcrete is placed on previously placed hardenedconcrete.

segregation – the unintentional separation of theconstituents of concrete or particles of an aggregate,resulting in nonuniform proportions in the mass.

set – the condition reached by a cement paste, mortar orconcrete when it has lost plasticity to an arbitrarydegree, usually measured in terms of resistance topenetration or deformation; initial set refers to firststiffening; final set refers to attainment of significantrigidity.

silica fume – very fine noncrystalline silica produced inelectric arc furnaces as a byproduct of the productionof elemental silicon or alloys containing silicon; alsoknown as condensed silica fume and micro silica. It isoften used as an additive to concrete and can greatlyincrease the strength of a concrete mix.

29


slump – a measurement indicative of the consistency offresh concrete. A sample of freshly mixed concrete isplaced and compacted by rodding in a mold shaped asthe frustum of a cone. The mold is raised, and theconcrete is allowed to subside. The distance betweenthe original and displaced position of the center of thetop surface of the concrete is measured and reportedas the slump of the concrete. Under laboratoryconditions, with strict control of all concrete materials,the slump is generally found to increase proportionallywith the water content of a given concrete mixture andthus to be inversely related to concrete strength(unless water-reducing admixtures are used). Underfield conditions, however, such a strength relationshipis not clearly and consistently shown. Therefore, takecare when relating slump results obtained under fieldconditions to strength (ASTM C 143).

specification – an explicit set of requirements to besatisfied by a material, product, system or service thatalso indicates the procedures for determining whethereach of the requirements is satisfied.

standard – as defined by ASTM, a document that hasbeen developed and established within the consensusprinciples of the Society.

superplasticizer – see admixture, water-reducing.Superplasticizers are also known as high-range water-reducing admixtures.

Supplementary Cementitious Materials (SCMs) – finelydivided, powdered or pulverized materials added toconcrete to improve or alter the properties of theplastic or hardened concrete.

surcharge – a surface load applied to the structure,transferred through the surrounding soil.

troweling – smoothing and compacting the unformedsurface of fresh concrete by strokes of a trowel.

water-cementitious ratio – the ratio of the mass ofwater, exclusive only of that absorbed by theaggregates, to the mass of portland cement inconcrete, mortar or grout; stated as a decimal andabbreviated as w/c.

waterstop – a thin sheet of metal, rubber, plastic orother material inserted across a joint to obstruct theseepage of water through the joint.

water table – the upper limit of the portion of theground wholly saturated with water.

workability of concrete – that property of freshly mixedconcrete or mortar that determines the ease with whichit can be mixed, placed, consolidated and finished to ahomogenous condition.

30


This Best Practices Manual is subject to revision at anytime by the NPCA Septic Tank Product Committee, whichmust review it at least every three years.

Special thanks are given to the Septic Tank ProductCommittee for updating/compiling this manual.

Mark Wieser, Wieser Concrete ProductsSeptic Tank Product Committee Chairman

Jeffrey Hoffman, Flemington PrecastMike Kistner, Kistner Concrete Products Inc.Michael Mahoney, Euclid Chemical Co.Brian McQuestion, Lake Shore Burial VaultMichael Miller, Press-Seal Gasket Corp.Rick Oliver, Jensen PrecastMichael Price, Norweco Inc.Michael Stidham, E–Z Set Co. Inc.Michael Tidwell, Bartow Precast Inc.Daniel Wagner, Milan Vault Inc.Howard Wingert, Concrete Sealants

31


Boost your profitability with the quality control,

credibility and promotional advantages of NPCA’s

Plant Certification.

When you enroll, you are matched with a mentor to

guide you through the process and an independent

engineering firm will audit your operations. Their cost-

effective recommendations will maximize your

productivity and efficiency.

Achieving certification demonstrates your commitment

to first-class products and services and when specifiers

require certification, you will be prepared.

Call NPCA today for a free video and program details.

The association of the manufactured concrete products industry

10333 N. Meridian St., Suite 272 | Indianapolis, IN 46290

317.571.9500 | 800.366.7731 | www.precast.org

Plant Certificationmeans business for you

Documents

NATIONAL PRECAST CONCRETE ASSOCATION - …mowra.org/BestPracticesManual-PrecastConcreteOn... · as a regulatory code or minimum design standard, but ... Design precast concrete tanks