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Copyright 2006 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

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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

Precast concrete is ideally suited for utility, industrial andcommunications structures. A properly manufactured andinstalled precast concrete structure can last almostindefinitely. Precast concrete is inherently durable, highlyimpermeable and corrosion-resistant.

Utility, industrial and communications structures are animportant part of the manufactured concrete productsindustry, and we must continue to provide project ownersand end users with the best possible products atcompetitive prices. It is also important that thoseproducts contribute to the quality, timeliness and ease ofinstallation of construction projects. The best practicesoutlined in this manual are intended to helpmanufacturers achieve these goals.

When properly designed and manufactured, precastconcrete is capable of maintenance-free performancewithout the need for protective coatings, except in certaincircumstances such as highly-corrosive environments. The“precast advantage” is further solidified through precast’sease of installation and strength. Because precastconcrete products typically are produced in a controlledenvironment, they exhibit high quality and uniformity.Adverse factors affecting quality which are typicallyfound on job sites – variable temperature, uneven curingconditions, inconsistent material quality andcraftsmanship – are significantly reduced in a plantenvironment.

High-quality concrete products are important for manyapplications, most notably for underground structuresthat must resist soil pressures and sustain surface loads.The controlled manufacturing environment in a precastconcrete plant is ideal for the production of high-quality,durable concrete products.

This manual is intended to guide manufacturers in qualityproduction processes for manufacturing utility vaults. Themanual is not intended to be all inclusive and therecommendations are not intended to exclude anymaterials or techniques that would help achieve the goalof providing structurally sound, high quality products.Attention to detail, quality materials, proper training anda workforce dedicated to quality control will ensure thatthe utility vault meets or exceeds the expectations ofcontractors and end users.

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UTILITY VAULT MANUFACTURING BEST PRACTICES MANUAL

STRUCTURAL DESIGN

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Loading ConditionsA properly designed precast concrete utility vault mustwithstand a variety of loading conditions, which varyduring manufacturing, installation, testing and service.These structures are designed to withstand loadingconditions through rational mathematical designcalculations, by proof-of-load testing or in accordancewith ASTM C 857 “Standard Practice for MinimumStructural Design Loading for Underground PrecastConcrete Utility Structures.”

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

Precast concrete utility vaults should be designedaccording to one or more of the following industry codesand standards and as required by any regulatory groupswith jurisdiction:• ACI 318 – Building Code Requirements for Structural

Concrete• AASHTO – Specification for Highway Bridges• ASTM C 857 – Standard Practice for Minimum

Structural Design Loading for Underground PrecastConcrete Utility Structures

• ASTM C 858 – Standard Specification for UndergroundPrecast Concrete Utility Structures

The loading conditions illustrated in the diagram(s)should be analyzed and considered in the design of autility structure.

The following design characteristics have a critical impacton the performance of utility structures.

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

Concrete Mix Design Concrete must have a minimum compressive strength of4,000 psi at 28 days. Consider methods to reducepermeability, improve durability and increase strength.Maintain a low water-cementitious ratio at or below 0.45.

Reinforcement Proper design and placement is critical to withstand thesignificant loads applied to utility vaults. Reinforcementmust be sufficient for required strength and serviceconditions, including temperature and shrinkage effects.Control cracking by using multiple small-diameter steelbars rather than fewer large-diameter steel bars. Fiberreinforcement also helps to reduce cracking and may addsome strength, but should not replace primaryreinforcement. Welded-wire reinforcement can also beused, but particular attention should be paid to designand location. All reinforcement should meet applicableASTM specifications.

Loading Diagram*Manufacturer to specify the maximum depth of cover

Top SeamVault

Grade line

Groundwater Level

+ +

Mid-SeamVault

Residential Soil

Residential Soil

Depth ofCover

Hydrostatic Loading

Soil Loading

Hydrostatic Loading

Soil 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.

Cement The 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 willprevent exposure to moisture or other contamination.Bulk cement should be stored in watertight bins or silos.If different types of cement are used at a facility, storeeach type in a separate bin or silo. Clearly identifydelivery 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 baggedcement on pallets to permit proper air circulation and toavoid undesirable moisture and condensation. On a short-term basis (less than 90 days), stack the bags no morethan 14 high. For long-term storage, do not exceed sevenbags in height (or per manufacturer’s recommendations).Use the oldest stock first. Discard any cement with lumpsthat cannot be reduced by finger pressure.

Aggregates Ensure 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 utility vault or75 percent of the clear cover between reinforcement andthe surface of the vault. Larger maximum sizes ofaggregate may be used if evidence shows thatsatisfactory concrete products can be manufactured.

Aggregates are an important constituent of precastconcrete utility vaults. Nearly 75 percent of a precastconcrete utility vault’s structure consist of coarse andfine aggregates.

The selection of appropriate aggregates is largely aregional concern. The selection process is based on usingthe best available clean, hard, durable aggregates andproper gradations. Aggregates should conform to ASTMC 33 – Standard Specification for Concrete Aggregatesand ACI 318 – Building Code Requirements for StructuralConcrete.

Quality of Aggregates Concrete is exposed to continuous moist and potentiallycorrosive conditions in underground applications.Consequently, two important selection parameters are theaggregate’s expansive properties when exposed tomoisture and its porosity. It is important to specify awell-graded, sound, nonporous aggregate in accordancewith ASTM C33 “Standard Specification for ConcreteAggregates.” This should be executed with limitedamounts of materials that are deleteriously reactive withthe alkalis in cement. When receiving aggregate from anarea known to have potential problems with alkali-aggregate reaction, alkali-silica reaction or alkali-carbonate reaction, the aggregate supplier should providesufficient laboratory data on the aggregate’s potentialreactivity.

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If the only available aggregates in your area have beenfound to cause excessive expansion of mortar orconcrete, appropriate precautionary measures should betaken during the mix design process. The use of mineraladmixtures, a low water-cement ratio or a low-alkalicement can all aid in controlling such expansivereactions. When using mineral admixtures or a low-alkalicement, trial batches should be tested to establish theirviability in controlling the expansive reactions.

Gradation of Aggregates Aggregate gradation influences both the economy andstrength of a finished utility vault. The purpose of propergradation is to produce concrete with a maximum densityalong with good workability to achieve sufficientstrength.

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 coarse 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.

Aggregate Deleterious SubstancesEnsure all aggregates are free of deleterious substances,including:

• Substances that cause an adverse chemical reaction infresh 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 Aggregate The measurement of aggregate moisture content isimportant in the control of concrete workability, strengthand quality. Aggregates – particularly fine aggregates(sands) – can collect considerable amounts of moistureon their surfaces. Fine aggregates can hold up to 10percent moisture by weight; coarse aggregates can holdup to 3 percent.

Water on the surface of an aggregate that is notcalculated into 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 need to add additional cement tooffset excess water. This will maintain high-qualitystandards and save on expensive raw materials.

The plant should have a program in place that managessurface moisture content or accounts for moisturevariation during batching.

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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.

Handling and Storage of AggregateAggregate handling is an important operation. Handle andstore aggregates to prevent contamination and minimizesegregation and degradation. Accurately graded coarseaggregate can segregate during a single improperstockpiling operation, so handling should be minimized 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.

• 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. Avoid takingaggregate from the exact same location each time.

WaterWater used in mixing concrete should meet therequirements of ASTM C 1602, “Standard Specification forMixing Water Used in the Production of HydraulicCement Concrete.” Avoid water containing deleteriousamounts of oils, acids, alkalis, salts, organic material orother substances that may adversely affect the propertiesof fresh or hardened concrete.

Chemical AdmixturesCommonly used chemical admixtures in precast concretemanufacturing 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 and

temperature extremes. Ensure they are accessible for visualobservation and periodic maintenance. Perform periodicrecalibration of the batching system as recommended bythe manufacturer or as required by local 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 are includedin ACI 212.3, “Guide for Use of Admixtures in Concrete.”Avoid accelerating admixtures that contain chlorides inorder to prevent possible corrosion of reinforcing steelelements and other embedded metal objects.

Supplementary CementitiousMaterials (SCMs)Supplementary Cementitious Materials are oftenincorporated into a concrete mix to reduce cementcontents, improve workability, increase strength andenhance durability. The most common types of SCMs arefly ash, silica fume and ground granulated blast furnaceslag (GGBFS) as know simply as slag. More than oneSCM may be incorporated into a mix while various typesof blended cements are also available. The followingstandards establish the minimum requirements for someof the common SCMs and blended cements.• ASTM C595: Standard Specification for Blended

Hydraulic Cements• ASTM C618: Standard Specification for Coal Fly Ash and

Raw or Calcined Natural Pozzolan for Use in Concrete• ASTM C989: Standard Specification for Ground

Granulated Blast-Furnace Slag for Use in Concrete andMortars

• ASTM C1240: Standard Specification for Use of SilicaFume Used in Cementitious Mixtures

Ready-Mixed Concrete Verify that the ready-mixed concrete supplier is operatingin accordance with ASTM C94, “Standard Specificationfor Ready-Mixed Concrete.” Perform plastic concrete tests(slump, temperature, air content and density) at the plantprior to casting products. Record any added water on thedelivery batch ticket for each truck and keep it on file.

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Concrete mix design, also known as mix proportioning, isa broad subject – one that is specific to concrete ingeneral and not necessarily to utility vaults in particular.This discussion will focus only on critical factors thatpertain to high-quality precast concrete that isrecommended when casting utility vaults.

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

readily placed by traditional methods; this is typicallymeasured by slump)

High water-cement ratios lead to undesirable increasedcapillary porosity within the concrete. Capillary pores arevoids resulting from the consumption and evaporation ofwater during the hydration or curing process. Enlargedand interconnected capillary voids serve as pathways forwater and other contaminates to infiltrate the concretesystem. Lower water-cement ratios result in smaller andfewer pores, reducing the concrete’s permeability.

Aggregate quality and gradation have a tremendousimpact on the concrete’s overall quality. Mix designs withwell-graded aggregates and sufficient quantities of fineaggregate have reduced water demands (i.e., lowerwater-cement ratios) while maintaining adequateworkability and ease of placement.

Proper consistency of fresh concrete is a critical elementin producing high-quality precast concrete. Fresh

concrete must be sufficiently plastic (flowable ordeformable) to be properly placed, consolidated andfinished. The size, shape and grading of aggregates,cement content, water-cement ratio and admixtures affectthe workability of a mix.

Water-reducing admixtures and superplasticizers cangreatly increase the workability of fresh concrete withoutchanging the water-cement ratio. Experience has shownthat concrete with low water-cement ratios (less than0.45) can be properly placed and consolidated with theproper use of admixtures. However, air entrainmentshould be limited to keep the air content at a maximumof 7 percent.

The use of chemical admixtures for wet-cast concrete(such as air-entraining admixtures, water-reducingadmixtures and superplasticizers) helps to attainworkable concrete with a low water-cement ratio. Theiruse is particularly important, since most utility vaultsrequire heavier reinforcing and have large penetrationsthat require special attention to ensure full consolidation.In certain circumstances, and where local regulationsallow, properly designed and tested self-consolidatingconcrete (SCC) can be used to improve the likelihood ofproper bond between concrete and reinforcement.

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).Air entrainment improves workability and reducesbleeding and segregation of fresh concrete while greatlyimproving the durability of hardened concrete.

CONCRETE MIXTUREPROPORTIONING

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LIFTING INSERTS

Commercially manufactured lifting devices comefurnished with documented and tested load ratings. Usethe devices as prescribed by the manufacturer’sspecification sheets. Lifting devices should meet theminimum requirements of ASTM C857, “Standard Practicefor Minimum Structural Design Loading for UndergroundPrecast Concrete Utility Structures.

Lifting devices designed in the plant and notcommercially manufactured must be load tested orevaluated by a professional engineer. In general, industryspecifications require that lifting inserts have a minimumsafety factor of three (ASTM C857) or four (29 CFR1926.704, ANSI A10.9).

According to the “NPCA Quality Control Manual forPrecast Concrete Products”: “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 C 857.

A factor of safety of at least 4 is recommended for liftingdevices. Manufacturers of standard lifting devices shouldprovide test data to allow selection of appropriateloading.

Because of their brittle nature, reinforcing bars shouldnot be used as lifting devices. Instead, smooth bars madeof steel conforming to ASTM A36 can be used.

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Good concrete with a water-cement ratio less than 0.45and a compressive strength greater than 4,000 psi issufficient for utility vaults. Under normal conditions,there is no need for additional applications of asphalt,bituminous, epoxy or cementitious coatings. However, aprotective exterior coating may be specified when a soilanalysis indicates a potential for chemical attack.

COATINGS

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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 quality 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 Products” as the basis fordeveloping a strong quality control program.

Forms Forms 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.

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.

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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 reinforcementshould meet the requirements set forth in ACI 318, asdefined in Table 1. Make all bends while thereinforcement is cold. The minimum bend diameter forconcrete reinforcing welded 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 as defined by ASTM A706, “Specificationfor Low-Alloy Steel Deformed and Plain Bars for ConcreteReinforcement,” for any welding of reinforcing steel,including tack welds. Take special care to avoidundercutting or burning through the reinforcing steel.

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. A maximumrecommended placement tolerance for the depth ofreinforcement is +1/4 inch (ACI 318). As a general rule thevariation in spacing between bars should not exceedmore than 1/12 of the designed bar spacing nor exceed1.5 inch in variation, except for welded wire meshconforming to Specifications A185 or A497. Therecommended minimum concrete cover over reinforcingsteel is 3/4 inch (ASTM C858).

REINFORCEMENT

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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

Fiber ReinforcementData must be available to show conclusively that thetype, brand, quality and quantity of fibers to be includedin the concrete mix are not detrimental to the concrete orto the precast concrete product. Fiber reinforced concretemust conform to ASTM C1116, “Standard Specification forFiber-Reinforced Concrete and Shotcrete” (Type I or TypeIII). The two most popular types of fibers are syntheticand steel fibers. Steel fibers must conform to ASTMA820, “Specification for Steel Fibers for Fiber-ReinforcedConcrete.” Do not use fibers as a replacement forprimary structural reinforcing steel. In general, fibers willnot increase the compressive or flexural strength ofconcrete.

Synthetic microfibers in concrete typically reduce plasticshrinkage 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 with weldedwire reinforcement and light-gauge steel reinforcement.Typical dosage rates for synthetic macrofibers vary from3.0 to 20 lbs/yd3. Steel fiber dosage rates may vary from20 to 60 lbs/yd3. Fibers must be approved by aregulatory agency or specifying engineer prior toconcrete 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 ItemsItems such as plates and inserts must be held rigidly inplace during casting. All embedded items should beresistant to corrosion. Special consideration should begiven when welding operations are required.

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

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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 areprevalent in the precast industry:

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 two 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, ensure that the field of vibration overlaps withanother insertion to best consolidate the concrete andminimize defects.

Some external vibrators are mounted on a piece of steelattached to the form. Position them to allow for overlapof vibration areas. Continue the vibration process untilthe product is completely consolidated. Vibration isconsidered complete when large bubbles (3/8” diameteror greater) no longer appear at the surface. Care shouldbe taken to avoid overvibration, because segregation ofthe aggregate from the cement paste can result inlowering the concrete 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.

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Two critical elements in curing concrete are maintainingcorrect moisture content and maintaining correct concretetemperature. Proper curing is important in developingstrength, durability and chemical resistance andwatertightness – all important considerations forunderground utility vaults.

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 three methods of curing:

1. Maintaining water moisture by wetting (fogging,spraying, wet coverings, etc.)

2. Preventing the loss of water by sealing (plasticcoverings or applying curing compounds)

3. Applying heat (often in conjunction with moisture, withheaters 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 utility vaults. Maintaining moisturerequires constant wetting, which is labor intensive.Alternate wetting and drying can lead to problems withcracking. Steam curing can also be effective. Concretetemperatures should never exceed 150 F. Both of thesetechniques are described in ACI 308, “Standard Practicefor Curing Concrete,” and the PCA publication “Designand Control of Concrete Mixtures.” Plastic coverings ormembrane-forming curing compounds require less laborefforts and allow form stripping the next work day. Thereare some special considerations for both:

1. Plastic sheeting must comply with ASTM C171,“Standard Specification for Sheet Materials for CuringConcrete,” which specifies a minimum thickness of 4mm and be either white or opaque in color. PCA statesthat 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.

CURING PROCEDURES

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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 reach 500psi. 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.

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.

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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 handlingequipment are necessary. Qualified personnel shouldmake periodic maintenance and repairs as warranted. Tagall chains and slings 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 productimmediately after stripping to check for damage.

Post-Pour Inspections A post-pour inspection checklist, as illustrated, provides amethod of identifying and communicating qualityproblems as they occur. It serves as a method ofgathering data for identification of any trends that maybe evident. After a utility vault is stripped from the form,it should be inspected for conformance with thefabrication drawings and any necessary repairs should bemade. All products should be clearly labeled with thedate of manufacturing and marked in accordance withASTM C 858.

POST-POUR OPERATIONS

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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 thesecondary pour is to be made should be free of laitance,dirt, dust, grease or any other material that will tend toweaken the bond between the original and newconcretes. If the surface is very smooth, roughen it tohelp promote a good bond. As a minimum, use a high-quality water stop, keyway and continuation ofreinforcing between pours to ensure 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.

Final Product Inspection Utility vaults should be visually checked for requiredsupplementary items, embedded items and quality at theplant prior to shipping.

Product Shipment All vehicles used to transport products must be in goodcondition and capable of handling the product withoutcausing damage. Utility vaults should be adequatelycured as specified prior to shipment to a job site ordistant storage areas. All products must be properlysecured with appropriate blockage and either nylonstraps or chains with guards as to avoid product damageduring shipment. NPCA’s publication Cargo Securementfor the Precast Concrete Industry outlines propermethods for securing product. It is recommended that thefinal inspection include checking these items.

FINISHING ANDREPAIRING CONCRETE

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SEALS, FITTINGS AND JOINTSCareful attention to joint details, sealing materials andpenetration fittings are important to ensure quality utilityvaults. Systems in areas of high water tables may requirespecial methods for joint and penetration seal designs.

Joint DesignsThe most common joint designs are tongue-and-grooveor lap joints.

For the manufacture of utility vaults, it is recommendedthat only interlocking joints be used. In cases ofpotentially significant frost heave, differential settlementand groundwater exposure, greater attention to jointdesign detail is needed. Mechanical fasteners orsecondary pours for lids on bases may be necessary inareas with severe site conditions. The key to preventingmost differential settlement is proper bedding preparation(especially compaction) of the site.

Sealing MaterialsHigh-quality, preformed flexible joint sealants can beused to achieve a dependable joint. Use only sealantsthat contain less than 3 percent volatiles as defined inASTM D6 – Standard Test Method for Loss on Heating ofOil and Asphaltic Compounds.

The characteristics of a high-quality sealant include:• Workability over a wide temperature range• Adhesion to clean, dry surfaces• Good performance over time (must not shrink, harden

or oxidize)

It is important that all joints be properly cleaned andprepared, according to the sealant manufacturer’srecommendations. Preformed flexible joint sealants mustbe sufficiently pliable to compress a minimum of 50percent at the temperature during assembly. Utility vaultsections sealed on site should not be backfilled until thesealant has settled.

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 in parallel and carefully mold the

two pieces together

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.

Connections and HardwareThe functionality of a utility vault is increased throughthe use of high quality ancillary components such ascable racking assemblies, pulling irons, conduitterminators and pipe supports.

Cable racking assemblies should be specified to matchthe characteristics of the type of cable to be supported.The assemblies and their hardware for attachment to thewall of the vault should be engineered to carry theanticipated weight of the supported cables.

Pulling irons should be structurally engineered to resistthe anticipated cable pulling loads. Additionally, thepulling load placed upon the wall of the utility vaultshould be considered in the structural design calculationsfor the vault. Pulling irons should not be used for liftingand handling product.

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Lap Joint Shiplap JointTongue &

Groove Joint

Cable industry standards require a smooth or rolled edgefor the cable to pass over as it leaves the conduit toenter the utility vault. End bells and conduit terminatorsthat are cast into the wall of the precast concrete vault atthe time of manufacture provide a quality labor savingmethod to terminate conduit. These items should be sizedand located on the wall of the vault to match the ductbank configuration.

Utility vaults may be configured to accommodate thepiping systems used for fluid conveyance. Integralconcrete sleepers, thrust blocks and pipe supporthardware may be cast into the vault. Boots and gasketsmay be incorporated at pipe entrances to ensurewatertightness and should be sized and located for thepipe being used.

Hardware should always meet specification requirementsof the design engineer and the customer.

Access, Risers and Manholes All access risers and manholes must be structurallysound and watertight.

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INSTALLATION

Proper installation is absolutely critical for maintainingthe inherent quality of plant-manufactured concreteproducts and should be performed in accordance withASTM C 891 – Standard Practice for Installation ofUnderground Precast Concrete Utility Structures. Many ofthe problems experienced with troublesome utility vaultscan be attributed to incorrect procedures duringinstallation. In addition to damaging the structure,improper installation techniques can lead to safetyhazards.

Site ConditionsThe installation site must be accessible to large, heavytrucks or cranes weighing up to 80,000 pounds. Theconstruction area should be free of trees, branches,overhead wires or parts of buildings that could interferewith the delivery and installation of the utility vault. Mosttrucks require access within 3 to 8 feet of the excavationto be unloaded.

ExcavationPrior to excavation, all buried utilities should beidentified and located. OSHA regulations governingexcavation work should be followed at all times; 29 CFR,Part 1926.650-652.

Excavations should be made with approximately 18 inchesof clearance around the installed structure to allow roomfor adequate compaction. More space should beprovided, as needed, if work other than installation isrequired. Excavations should be sloped to comply with allconstruction safety requirements.

BeddingProper use of bedding material is important to ensure along service life of the utility vault. Engineered beddingmaterial should be used as necessary to provide auniform bearing surface. A good base should ensure thatthe structure would not be subjected to adversesettlement. A minimum 4-inch-thick sand or granular bedoverlaying a firm and uniform base is recommendedunless otherwise specified. Utility vaults should not bearon large boulders or massive 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 flotation.

Proper compaction of the underlying soil and bedding iscritical to ensure there is no differential settlement.

PlacementPrior to placement in the excavation, the structure’sorientation should be confirmed. Inlet penetrations shouldbe aligned in the proper direction and the beddingmaterial should be checked. After placement, check toensure that the structure is level.

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Not lessthan 60˚

4" Sand or Granular Bedding (Minimum)

Properly Compacted Base to Prevent Settling

Underground Utilities to be Located Prior to Excavation in Accordance to OSHA Regulations

Excavation Slope to Comply withConstruction Safety Requirements

Product Orientation to beDetermined Prior to Setting

and Checked Prior toBackfilling

Site to be Accessible to Heavy Trucks & Cranes (Up to 80,000 lbs.) in an Area Free of Trees and Overhead Obstructions

Product Notes:1) After Placement, Check That the Structure is Level

2) Backfill Shall be Placed in Uniform, Properly

Compacted Layers Not to Exceed 24" Thick.

1'-6"Min.

1'-6"Min.

Figure 3

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, C890 andC913.

A factor of safety of at least 4 is recommended for liftingdevices. Manufacturers of standard lifting devices shouldprovide test data to allow selection of appropriateloading.

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.

BackfillingBackfill should be placed in uniform, mechanicallycompacted layers less than 24 inches thick. This fillshould be equally and uniformly placed around the vault.Backfill should be free of any large stones (greater than3 inches in diameter) or other debris. Each layer shouldbe adequately compacted.

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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 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 Blended

Hydraulic Cements”ASTM C618, “Standard Specification for Coal Fly Ash and

Raw or Calcined Natural Pozzolan for Use in Concrete”ASTM C857-95, “Practice for Minimum Structural Design

Loading for Underground Precast Concrete UtilityStructures”

ASTM C858-83, “Specification for Underground PrecastConcrete Utility Structures”

ASTM C891-90, “Practice for Installation of UndergroundPrecast Concrete Utility Structures”

ASTM C989: Standard Specification for GroundGranulated Blast-Furnace Slag for Use in Concrete andMotars

ASTM C1037-85, “Practice for Inspection of UndergroundPrecast Concrete Utility Structures”

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

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

ASTM C1240: Standard Specification of Use of SilicaFume Used in Cementitious Mixtures

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)

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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. Typically are liquid incomposition.

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, mineral – finely divided, powdered orpulverized materials added to concrete to improve oralter the properties of the plastic or hardened concrete.

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 – general 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 (75 mm).

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.

air void – a space in cement paste, mortar or concretefilled with air; an entrapped air void ischaracteristically 1 mm or more in width and irregularin shape; an entrained air void is typically between 10Ìm and 1,000 Ìm in diameter and spherical in shape.

alkali-aggregate reactivity (AAR) – a chemical reactionthat occurs between the alkalies (sodium andpotassium) from portland cement or other sources andcertain constituents of some aggregates; under certainconditions resulting in deleterious expansion ofconcrete or mortar; often known as alkali-silicareaction (ASR). Cement manufacturers often testaggregates for AAR as a service to their customers.

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 concrete,caused 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 – a 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.

GLOSSARY

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concrete – a composite material that consists essentiallyof a binding medium within which are embeddedparticles of fragments of aggregate, usually acombination of fine aggregate and coarse aggregate; inportland-cement concrete, the binder is a mixture ofportland cement and water.

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 devlop.

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 so as toprovide an opportunity for the concrete to develop itsproperties in a favorable temperature and moistureenvironment.

dead load – a constant load that in structures is due tothe mass of the members, the supported structure, andpermanent attachments or accessories.

delayed ettringite formation (DEF) – occurs at laterages (months to years) and the related heterogeneousexpansion in hardened concrete can produce crackingand spalling. DEF is related to environmental orinternal sulfate attack.

deleterious substances – materials present within or onaggregates that are harmful to hardened concrete,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 in mortar or concrete duringmixing, typically between 10 Ìm and 1,000 Ìm (1 mm) indiameter and spherical or nearly so.

entrapped air – see air void; air voids in concrete thatare not purposely entrained and that are larger, mainlyirregular in shape, and less useful than those ofentrained air; typically greater than 1 mm in diameterand may be of various shapes.

ettringite – a mineral, high-sulfate calciumsulfoaluminate occurring in nature or formed by sulfateattack on mortar and concrete.

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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.

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 (i.e., ASTM C 136); 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.

initial set – a degree of stiffening of a mixture of cementand water less than final set, generally stated as anempirical value indicating the time in hours andminutes required for cement paste to stiffen sufficientlyto resist to an established degree, the penetration of aweighted test needle; often performed on a sample ofmortar sieved from a concrete sample.

live load – any load that is not permanently applied to astructure; including transitory loading such as water,vehicles and people.

organic impurities (re: aggregate) – extraneous andunwanted organic materials (twigs, soil, leaves, otherdebris) that are mixed in aggregates; these materialsmay have detrimental effects on concrete producedfrom such aggregates.

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

plastic concrete – see concrete, fresh.

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

portland cement clinker – a partially fused ceramicmaterial consisting primarily of hydraulic calciumsilicates and calcium aluminates.

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.

psf – pounds per square foot

psi – pounds per square inch

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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 non-crystalline 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.

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,and thus to be inversely related to concrete strength.Under field conditions, however, such a strengthrelationship is not clearly and consistently shown.Care should therefore be taken in relating slumpresults obtained under field conditions 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.

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

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

water-cement ratio – the ratio of the mass of water,exclusive only of that absorbed by the aggregates, tothe mass of portland cement in concrete, mortar orgrout; stated as a decimal and abbreviated 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.

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

Special thanks are given to the Utility Vault ProductCommittee for updating/compiling this manual.

Steve Truax, Wieser Concrete Products Inc.Jay Behney, By-CreteDouglas Bowen, Bowco Industries Inc.Todd Ebbert, San Diego Precast Concrete Inc.Paul Heidt, Garden State Precast Inc.Donald McNutt, Spillman CompanyMichael Menard, Firebaugh Precast Inc.Brian Rhees, Oldcastle Precast Inc.Robert Thornton, Hughes Concrete Products

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