Spec for Shortcrete

7/27/2019 Spec for Shortcrete

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506.2-1

This specification contains the construction requirements for the applica-

tion of shotcrete. Both wet-mix and dry-mix shotcrete are specified, and the

minimum standards for testing, materials, and execution are provided.

Keywords: dry-mix shotcrete; fiber reinforced shotcrete; nozzleman;

shooting; shotcrete; wet-mix shotcrete.

CONTENTS

Foreword, p. 506.2-2

Preface to specification checklist, p. 506.2-2

Mandatory requirements checklist, p. 506.2-2

Optional requirements checklist, p. 506.2-2

Submittals checklist, p. 506.2-3

Section 1General, p. 506.2-4

1.1Scope

1.2Definitions

1.3Reference organizations

1.4Reference standards

1.5Submittals

1.6Quality assurance

1.7Shotcrete core grades

1.8Evaluation of in-place shotcrete

1.9Acceptance

Section 2Materials, p. 506.2-6

2.1Cement

2.2Aggregate

2.3Reinforcement

2.4Water

2.5Admixtures2.6Curing materials

2.7Proportioning

2.8Fiber reinforced shotcrete

2.9Prebagged materials

2.10Delivery, storage, and handling

Section 3Execution, p. 506.2-6

3.1Examination

3.2Batching and mixing

3.3Surface preparation

3.4Joints

3.5Alignment control3.6Application

3.7Finishing

3.8Curing

Specification for Shotcrete (ACI 506.2-95)

Reported by ACI Committee 506

Steven H. GeblerChairman

Lars Balck, Jr.Secretary

Jon B. Ardahl Bruce K. Langson Ernest K. Schrader

Seymour A. Bortz Albert Litvin Vern Schultheis

Paul D. Carter Kristian Loevlie Raymond J. Schutz

Gary L. Chynoweth Dudley R. Morgan Philip T. Seabrook

John R. Fichter Dirk E. Nemegeer W. L. Snow, Sr.

I. Leon Glassgold H. Celik Ozyildirim Curt E. Straub

Jill E. Glassgold Harvey W. Parker Lawrence J. Totten

Warren L. Harrison Dale A. Pearcey Gary L. Vondran

Charles H. Henager John E. Perry, Jr. R. Curtis White, Jr.Merlyn Isaak Vankataswamy Ramakrishnan J. Wickman Zimmerman

Richard A. Kaden Thomas J. Reading

ACI 506.2-95 supercedes ACI 506.2-90 and became effective October 1, 1995.Copyright 1995, American Concrete Institute.All rights reserved including rights of reproduction and use in any form or by any

means, including the making of copies by any photo process, or by electronic ormechanical device, printed, written, or oral, or recording for sound or visual reproduc-tion or for use in any knowledge or retrieval system or device, unless permission inwriting is obtained from the copyright proprietors.


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506.2-2 ACI STANDARD

3.9Hot weather shotcreting

3.10Cold water shotcreting

3.11Protection

3.12Tolerances

FOREWORD

F1. This foreword is included for explanatory purposes

only; it does not form a part of Standard Specification ACI

506.2.

F2. Standard Specification ACI 506.2 is a reference stan-

dard which the Architect/Engineer may cite in the project

specifications for any project, together with supplementary

requirements for the specific project.

F3. Each technical section of Standard Specification ACI

506.2 is written in the three-part section format of the Con-

struction Specification Institute, as adapted by ACI and

modified to ACI requirements. The language is generally

imperative and terse.F4. Checklists do not form a part of Standard Specification

ACI 506.2. Checklists are to assist the Architect/Engineer in

properly choosing and specifying any necessary require-

ments for the project specifications.

PREFACE TO SPECIFICATIONS CHECKLIST

P1. Standard Specification ACI 506.2 is intended to be

used by reference or incorporation in its entirety in the

Project Specifications. Individual sections, articles, or para-

graphs shall not be copied into the Project Specifications,

since taking them out of context may change their meaning.

P2. If sections or parts of Standard Specification ACI

506.2 are edited into Project Specifications or any other doc-

ument, they shall not be referred to as ACI Standards, since

the Standard Specification has been altered.

P3. Building codes set minimum requirements necessary

to protect the public. These Standard Specifications may

stipulate requirements more restrictive than the minimum.

Adjustments to the needs of a particular project shall bemade by the Architect/Engineer by reviewing each of the

items in the Specifications Checklist and then including the

Architect/Engineer's decision on each item as a mandatory

requirement in the Project Specifications.

P4. These mandatory requirements designate the specific

qualities, procedures, materials, and performance criteria for

which alternatives are permitted or for which provisions

were not made in the Standard Specifications. Exceptions to

the Standard Specifications shall be made in the Project

Specifications, if required.

P5. A statement such as the following will serve to make

Standard Specifications ACI 506.2 a part of the ProjectSpecifications.

Work on (Project Title) shall conform to all the

requirements of ACI 506.2 Standard Specification for

Shotcrete, published by the American Concrete Insti-

tute, Detroit, Michigan, except as modified by the

requirements of these Contract Documents.

P6. The Standard Specifications Checklist identifies Ar-

chitect/Engineer choices and alternatives. The checklists

identify the sections, parts, and articles of the Standard Spec-

ifications and the action required by the Architect/Engineer.

MANDATORY REQUIREMENTS CHECKLIST

Section/Part/Article Notes to the Architect/Engineer

2.7Proportioning Specify design strength of shotcrete. Specify if other properties or a particular mix proportion is required.

3.3Surface preparation Specify results. Sometimes special surface preparation is required to assure bond. A rough surface provides the best

bond. Specifying a reveal is helpful. It may be difficult to measure bond strength. It also may be necessary to remove

existing concrete or shotcrete to allow for proper reinforcement encasement. In some cases, bond to an existing surface

may not be needed. If bond is not required, specify what result is required.

Coating material used on shotcrete forms must be used with caution. Shooting against forms removes surface materials

and mixes it with the shotcrete that may alter the shotcrete properties. Form coating materials may also interfere with

the bond of subsequent layer of shotcrete.

Specify if special surface preparation between layers of shotcrete is required. Preparation can be accomplished byscraping, brooming, high pressure water blasting or sand blasting.

3.12Tolerance Specify tolerance based on function and appearance. Shotcrete is typically not held to the same tolerance as cast-in-place concrete. Somtimes no tolerances are specified, while sometimes shotcrete tolerances are increased by a factor of2 times over the tolerances provided in ACI 117.

OPTIONAL REQUIREMENTS CHECKLIST


1.4Reference standards The following references are provided as guides. They are not legal documents. They cannot be referenced in theshotcrete specification. The references, however, provide excellent information, and can prove helpful in the design,the construction, and the inspection of shotcrete work.

ACI 506.4R Evaluation of In-Place Shotcrete

ACI 506.3R Guide to Certification of Shotcrete Nozzleman

ACI 506R Guide to Shotcrete

ACI 506.1RState-of-the-Art Report on Fiber Reinforced Shotcrete1.5Submittals Specify if other submittals are needed such as mix proportions and results of preconstruction tests when specified.


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SPECIFICATION FOR SHOTCRETE 506.2-3

OPTIONAL REQUIREMENTS CHECKLIST, continued


1.6Quality assurance Specify whether preconstruction testing is required. Specify compatibility for admixtures for wet-mix shotcrete.

Specify type and frequency of additional tests if needed.

Preconstruction testing is not always justified or practical. Examples are: noncritical applications or small jobs,

temporary construction, when job conditions do not allow enough time, and when the contractor has recently completed

similar acceptable work with similar materials, personnel, and equipment. The cost of testing should be weighed against

potential benefits.

If the contractor can show satisfactory results on previous jobs with the same materials, personnel and equipment, the

engineer should give consideration to eliminating the requirement of preconstruction testing.

Flexural strength in accordance with ASTM C 78 and toughness in accordance with ASTM C 1018 in addition to

compression tests are usually required as prequalification of fiber reinforced shotcrete. When prequalification testing is

required, a ratio of compressive to flexural strength is to be established for quality assurance purposes, then compressive

design strength would govern as acceptance. Specify the frequency of testing and acceptance criteria.

Concrete design strength is based on cast concrete cylinders. Shotcrete design strength, however, is based on cores or

sawed cubes taken from a sample panel. Core sample strength, however, is expected to be 0.85 of cast cylinder strength

(ACI 318 paragraph 5.6) since core samples are disturbed due to the coring or cutting process.

Quality shotcrete is highly dependent on the contractor and the workmanship of the nozzleman. The nozzleman should

demonstrate his ability and knowledge or have proven his ability through nozzleman testing. At the present time,

Nozzleman Certification ACI 506.3R is a report (not a standard) and may not be referenced in the ACI Shotcrete

Specification. The specifying authority, however, can specify nozzleman certification in accordance with 506.3R orother suitable requirement. Language, however, must be mandatory.

1.7Shotcrete core grades Specify core grade of shotcrete required. Grade 1 cores are optimum, however, Grade 1 cores cannot be consistently

shot. Grade 2 cores are used for most structural members.

1.8Evaluation of in-place shotcrete Specify acceptance/rejection criteria.

2.1Cement Specify if other type of cement is allowable If required, specify blended hydraulic cement in accordance with ASTMC 595. Base selection of cement type on sulfate exposure conditions or other conditions.

2.2Aggregate Specify if other grading, type or tests are required. Specify criteria for acceptance. Other gradings proposed by thecontractor may be acceptable if the contractor can demonstrate satisfactory performance under similar conditions ofuse.

2.3Reinforcement Specify type, size, configuration, minimum content, tests required, or required testing frequency and acceptance crite-ria.

2.5Admixtures A concrete mix doesn't become shotcrete until it is placed at a high velocity. Measuring air content at the pump hop-per, measures concrete air content not shotcrete air content. Shotcrete air content can be measured by taking samplesfrom in-place material. If air content is to be tested, specify where the sample is to be taken.

Wet-mix shotcrete exposed to moderate or severe freeze/thaw conditions should have air entrainment in the concrete

prior to shooting. Six to seven percent (6 - 7 percent) air content in the concrete is required prior to shooting. Entrainedair is lost during shotcreting. Obtaining greater than 4 percent entrained air in in-place wet-mix is difficult. If a greaterpercent is required, test practicality.

Specifying air entrainment for dry-mix is not recommended.Specify limits and types of other acceptable admixtures. Specify dosage if important.Calcium chloride is not recommended in reinforced shotcrete structures. Tables in ACI 201 or ACI 318 provide lim-

its. If specified, it shall conform to ASTM D 98. Specify if other admixtures are allowable.

2.8Fiber ACI 506.1RState-of-the-Art Report on Fiber Reinforced Shotcrete provides guidance for fiber shotcrete.

3.3.3Surface preparation It is recommended that steel surfaces should comply with SSPC-SP6 3.3.3.2.

3.3.5Forms High velocity air transporting the shotcrete mix, if not allowed to escape, may create voids at form corners.

3.4.1Joints Specify square joints for construction joints subject to compression loads.

3.5Alignment control For long spans (over 30 ft), devices that permit tightening of ground wires, such as springs or turnbuckles can be used.

3.6.1Application Do not place subsequent lifts until the previous lift has hardened enough to support the new shotcrete. Vertical wallsshould be gunned from the bottom up unless it can be demonstrated that another method will work successfully.

3.6.2Encasement of reinforcement Voids behind bars are not easily evident by inspection. Limiting the maximum size reinforcement bar to a No. 5 barincreases the likelihood of complete encasement, especially with limited experienced crews. Encasing larger barsrequires great care, along with high impact velocity and plasticity. Close inspection is also needed. See Guide to Shot-

crete, 506R-85 Section 5.4.1.2.3.7Finishing Specify type of finish. If other than a gun finish is required, specify finish types such as rod, broom, wood float, rubber

float, or steel trowel. Disturbance of the shotcrete surface by finishing may reduce the durability of the surface.

Material to correct small flaws may be added if the material is fresh and properly proportioned.

3.8Curing Caution must be exercised when using curing compounds. Curing compounds can interfere with the bond of

subsequent layers of shotcrete. If curing compounds are used and additional layers of shotcrete are to be applied, the

curing material must be removed by either water blasting or sand blasting.

Steam curing is acceptable provided the steam temperature is less than 150 F.

3.10Cold weather ACI 306.1 standard specification should be reviewed.

SUBMITTALS CHECKLIST


1.5Submittals Proposed shotcrete mix. Test panel results if required.


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SECTION 1GENERAL

1.1Scope

This Standard Specification describes the requirements for

materials, proportioning, and application of shotcrete.

1.2Definitions

AcceptedAccepted by or acceptable to the Architect/En-gineer.

Architect/EngineerThe architect, engineer, architectural

firm, engineering firm or architectural and engineering firm

issuing project drawings and specifications or administering

the work under the Contract Documents.

Contract DocumentsDocuments covering the required

work and including the project drawings and project specifi-

cations.

ContractorThe person, firm, or corporation that the

owner enters into an agreement with for construction of the

work.

Core gradeQuality ranking of a shotcrete core.Dry-mix shotcreteShotcrete in which most of the water

is added at the nozzle.

Fiber-reinforced shotcrete (FRS)Shotcrete mixed with

discrete discontinuous fibers commonly made of steel, plas-

tic, or alkali resistant (AR) glass.

Finish coatFinal thin coat of shotcrete preparatory to

hand finishing.

Flash coatThin shotcrete coat applied from a distance

greater than normal for use as a final coat or for finishing.

Ground wireWire used to establish line and grade for

shotcrete work.

GunDry-mix shotcrete delivery equipment.Gun finishUndisturbed final layer of shotcrete as ap-

plied from nozzle without hand finishing; also known as nat-

ural or nozzle finish.

NozzleAttachment at end of delivery hose from where

shotcrete is projected.

NozzlemanWorker on shotcrete crew who manipulates

the nozzle, controls consistency of dry-mix shotcrete, and

controls final disposition of the material.

OversprayShotcrete material deposited away from the

intended receiving surface by air blast.

OwnerThe corporation, association, partnership, indi-

vidual, or public body or authority with whom the contractorenters into agreement and for whom the work is provided.

ReboundShotcrete material that ricochets off the receiv-

ing surface.

Sand/rock pocketA porous zone in the shotcrete con-

taining aggregate with reduced mortar content.

ShootingAct of applying shotcrete.

ShotcreteMortar or concrete pneumatically projected

onto a surface at high velocity.

SloughingSubsidence of shotcrete, due generally to ex-

cessive water in mixture or to vibration of continuous rein-

forcement or formwork.

SubmittedSubmitted to the Architect/Engineer for re-

view.

Wet-mix shotcreteShotcrete in which plastic concrete

with cement, aggregate, and water are first mixed together

before introduction into the delivery hose.

WorkThe entire construction or separately identifiable

parts that are required to be furnished under the Contract

Documents. Work is the result of performing services, fur-

nishing labor, and providing and incorporating materials and

equipment into the construction, all as required by the Con-

tract Documents.

1.3Reference organizations

ACI

American Concrete Institute

P.O. Box 9094

Farmington Hills, Michigan 48333

ASTM

American Society for Testing and Materials

100 Bar Harbor Dr.

West Conshohocken, Pennsylvania 19428-2959

SSPC

Steel Structures Painting Council

4400 Fifth Avenue

Pittsburgh, Pennsylvania 15213

1.4Reference standards

The following Reference Standards are referred to in this

specification and declared to be a part of this Standard Spec-

ification:

1.4.1ACI Standards

ACI 301-89 Specifications for Structural Concrete for

Buildings, paragraph 5.7.1 and Chapter 9

1.4.2ASTM Standards

A 185 Specifications for Wire Fabric, Plain, Welded

Steel for Concrete Reinforcement

A 615 Specifications for Bars, Deformed and Plain, Bil-

let-Steel, for Concrete Reinforcement

A 820 Specifications for Steel Fibers for Fiber Rein-

forced Concrete

C 31 Practice for Making and Curing Concrete Test

Specimens in the Field

C 33 Specification for Concrete Aggregates

C 42 Test Methods for Obtaining and Testing Drilled

Cores and Sawed Beams of Concrete

C 94 Specification for Ready-Mixed Concrete

C 109 Test Method for Compressive Strength of Hydrau-

lic Cement Mortars

C 150 Specification for Portland Cement

C 171 Specification for Sheet Materials for Curing Con-

crete

C 309 Specification for Liquid Membrane-Forming

Compounds for Curing Concrete

C 330 Specification for Lightweight Aggregates for

Structural Concrete

C 595 Specification for Blended Hydraulic Cements

C 618 Specification for Fly Ash and Raw or Calcined

Natural Pozzolan for use as a Mineral Admixture

in Portland Cement Concrete


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C 642 Test Method for Specific Gravity, Absorption, and

Voids in Hardened Concrete

C 685 Specification for Concrete made by Volumetric

Batching and Continuous Mixing

C 989 Specification for Ground Granulated Blast-Fur-

nace Slag for use in Concrete and Mortars

C 1116 Specification for Fiber-Reinforced Concrete and

ShotcreteC 1140 Standard Practice for Preparing and Testing Spec-

imens from Shotcrete Test Panels

C 1141 Standard Specification for Admixtures for Shot-

crete

1.4.3SSPC Standards

SP6 Surface Preparation Specification No. 6, Commer-

cial Blast Cleaning

1.5Submittals

Submit proposed mix characteristics including:

Proportions by weights or volumes

Strength Water-cementitious materials ratio

Aggregate source and grading

Cement type and brand

Water source if other than potable

Proportions

Admixtures data sheets

Test results

Submit preconstruction test panel results when required.

1.6Quality assurance

1.6.1Preconstruction testing

1.6.1.1 Prepare preconstruction test panels for examina-tion by Architect/Engineer prior to job shotcrete placement.

Preparation and testing shall comply with ASTM C 1140.

1.6.1.2 Produce test panels for each proposed mix pro-

portion, each anticipated shooting orientation, and each pro-

posed nozzleman. Mixes shall meet requirements ofsection

2.7Proportioning. In half of the test panels provide rein-

forcement of the same size and spacing required for the

work. Obtain six test specimens from each panel, three non-

reinforced specimens and three with reinforcing steel.

1.6.1.3 Test the nonreinforced specimens for compli-

ance with the specified physical properties in accordance

with ASTM C 42.1.6.1.4 Visually grade the reinforced specimens for

compliance with specified core grade (Section 1.7Shot-

crete core grades).

1.6.1.5 Test admixtures for compatibility with cement in

accordance with ASTM C 1141.

1.6.1.6 Unless otherwise specified, only nozzlemen

with a test panel mean core grade less than or equal to 2.5

(section 1.7) shall be allowed to place job shotcrete. When

the prequalification test panel is rejected, a second panel may

be shot. When the nozzleman's second mean core grade is

greater than 2.5, the nozzleman shall not be permitted to

shoot on the project.1.6.2Construction testing

1.6.2.1 Produce a material test panel for each mix and

each work day or every 50 cubic yards placed, whichever is

less. Test panel shall be kept moist and at 70 F 10 F until

moved to test laboratory. Obtain test specimens either from

job site material test panel or from in-place shotcrete. Test

specimens from test panels in compliance with ASTM C

1140.

1.6.2.2 Test specimens from in-place shotcrete in com-pliance with ASTM C 42.

1.6.2.3 Grade cores that include reinforcement in accor-

dance withsection 1.7Shotcrete core grades.

1.6.2.4 The mean compressive strength of a set of three

cores shall equal or exceed 0.85 fc with no individual core

less than 0.75fc. The mean of a set of three cubes shall equal

or exceedfc with no individual cube less than 0.88fc.

1.7Shotcrete core grades

1.7.1 Grade 1: Shotcrete specimens are solid; there are no

laminations, sandy areas or voids. Small air voids with a

maximum diameter of 1/8 in. and maximum length of 1/4 in.are normal and acceptable. Sand pockets, or voids behind

continuous reinforcing steel are unacceptable. The surface

against the form or bond plane shall be sound, without a

sandy texture or voids.

1.7.2 Grade 2: Shotcrete specimens shall have no more

than two laminations or sandy areas with dimensions not to

exceed 1/8 in. thick by 1 in. long. The height, width and depth

of voids shall not exceed 3/8 in. Porous areas behind reinforc-

ing steel shall not exceed 1/2 in. in any direction except along

the length of the reinforcing steel. The surface against the

form or bond plane shall be sound, without a sandy texture

or voids.1.7.3 Grade 3: Shotcrete specimens shall have no more

than two laminations or sandy areas with dimensions ex-

Grade 1


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ceeding 3/16 in. thick by 11/4 in. long, or one major void, sand

pocket, or lamination containing loosely bonded sand not to

exceed 5/8 in. thick and 11/4 in. in width. The surface against

the form or bond plane may be sandy with voids containing

overspray to a depth of1/16 in.

1.7.4 Grade 4 core: The core shall meet in general the re-

quirements of Grade 3 cores, but may have two major flawssuch as described for Grade 3 or may have one flaw with a

maximum dimension of 1 in. (25 mm) perpendicular to the

face of the core with a maximum width of 11/2 in. The end of

the core that was shot against the form may be sandy and

with voids containing overspray to a depth of1/8 in.

1.7.5 Grade 5 core: A core that does not meet the criteria

of core grades 1 through 4, by being of poorer quality, shall

be classified as Grade 5.

1.7.6 Determination of grade shall be by computing the

mean of a minimum of three test specimens.1.7.7 A mean grade of 2.5 or less is acceptable unless oth-

erwise specified. Individual shotcrete cores with a grade

greater than 3 are unacceptable.

1.7.8 The above core grades are based on cores with a sur-

face area of 50 in.2 For cores with greater or lesser area than

50 in.2, adjust allowable flaws relative to 50 in.2

1.8Evaluation of in-place shotcrete

Remove and replace shotcrete that is delaminated, exhibits

laminations, voids, or sand pockets exceeding the limits for

the specified grade or shotcrete. Remove and replace shot-

crete that does not comply with the specified material prop-erties.

Repair core holes in accordance with Chapter 9 of ACI

301. Do not fill core holes by shooting.

1.9Acceptance

1.9.1 Shotcrete work that meets applicable requirements

will be accepted.

1.9.2 Shotcrete work that has previously failed to meet one

or more requirements, but which has been repaired to bring

it into compliance, will be accepted.

1.9.3 Shotcrete work that fails to meet one or more re-

quirements and which cannot be brought into compliance

may be accepted or rejected. Modifications may be required

Grade 2 Grade 3 Grade 4

Grade 5


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to assure that remaining work complies with the require-

ments.

SECTION 2MATERIALS

2.1Cement

Portland cement: ASTM C 150. Type I

2.2Aggregate

Normal weight aggregate: ASTM C 33. Aggregate not

meeting ASTM C 33 may be used provided preconstruction

tests demonstrate the shotcrete can meet specified require-

ments.

Lightweight aggregate: ASTM C 330

2.3Reinforcement

Deformed steel reinforcement: ASTM A 615

Welded wire fabric: ASTM A 185

Steel fibers: ASTM A 820 and C 1116

Glass fibers: ASTM C 1116

Synthetic fibers: ASTM C 1116

2.4Water

Clean and potable.

Mixing water for shotcrete shall meet requirements of

ASTM C 94.

Water shall be free of elements that cause stains when used

on architectural finishes.

2.5AdmixturesWater-reducing: ASTM C 1141

Retarding: ASTM C 1141

Accelerating: ASTM C 1141

Air-entraining: ASTM C 1141

Fly ash and natural pozzolans: ASTM C 618

Ground granulated blast-furnace slag: ASTM C 989

Silica fume: ASTM C 1240

2.6Curing materials

Curing materials that cause stains on architectural finishes

shall not be used.Sheet materials: ASTM C 171

Curing compounds: ASTM C 309, apply twice the cover-

age (double the application rate) recommended by manufac-

turers.

2.7Proportioning

Select shotcrete proportions to produce the specified ma-

terial properties.

Shotcrete mix proportions using normal weight aggregate

shall produce the following 28-day material property:

fc: ASTMC 42; ASTM C1140

Air content: ASTM C 173 or C 231

2.8Fiber-reinforced shotcrete

ASTM C 1116

2.9Prebagged materials

Are allowable with predampening.

2.10Delivery, storage, and handling

Deliver, store, and handle materials to prevent contamina-

tion, segregation, corrosion or damage. Store liquid admix-

tures in order to prevent evaporation and freezing.

SECTION 3EXECUTION

3.1Examination

Verify that surfaces are acceptable and are ready to re-

ceive work.

Verify that forms are to line and grade, braced against

vibration, and constructed to permit escape of air and

rebound.

Verify that placement and clearance around reinforce-

ment permits complete encasement.

Verify that surfaces to receive shotcrete have been

properly prepared according to the Contract Docu-

ments.

3.2Batching and mixing

Weight batching shall comply with the accuracy specified

in ASTM C 94.

Volume batching shall comply with the accuracy specified

in ASTM C 685. Volume batching shall be verified once a

week by a weight batching check.

Use batching and mixing equipment capable of propor-

tioning and mixing the required materials.

Shoot dry-mix shotcrete material within 45 min after

batching or predampening. This requirement does not in-clude dry, prebagged material unless prebagged material is

predampened.

Shoot wet-mix shotcrete material within 90 min after

batching.

3.3Surface preparation

3.3.1EarthPrepare surfaces to line and grade. Do not

apply shotcrete to frozen surface. Dampen surface immedi-

ately prior to shooting.

3.3.2Concrete, masonry, and shotcreteWhen bonding

is required, remove all deteriorated, loose unsound material

or contaminants that may inhibit bonding. Chip areas to berepaired to remove offsets causing abrupt changes in thick-

Table 2.7Total air content for frost resistant concrete*

Nominal maximum aggregate size Total air content, percent

Less than 3/8 7

Greater than 3/8 6

* Based on samples obtained from the concrete truck.

See ASTM C 33 for tolerances for various nominal maximum size designations.

A tolerance of 11/2 percent shall be allowed.


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ness. Taper edges to eliminate square shoulders at the perim-

eter of a cavity.

Surface shall be saturated surface dry immediately prior to

shooting.

3.3.3Structural/reinforcementThe surface shall be free

of deleterious materials that inhibit bonding. For new con-

struction, reinforcement laps shall be separated with a clear-

ance of at least three times the diameter of largest aggregate.Reinforcement shall be secured to prevent movement.

3.3.4RockRemove loose material, mud or other foreign

material that will prevent bonding. Clean surface. Prewet

surface immediately prior to shooting.

3.3.5 FormsUse form-release coating material on re-

movable forms. Secure forms to minimize the effects of vi-

bration. Construct forms to allow escape of placement air

and rebound.

3.4Joints

3.4.1Construction jointsTaper construction joints at a 1

to 1 slope where joint is not subject to compression loads.Surface preparation of joints shall comply with section 3.3.2.

Continue reinforcement through construction joint.

3.4.2Control jointsPlace as shown on drawings.

Discontinue reinforcement at control joints.

3.5Alignment control

Install taut ground wires or other means to establish thick-

ness and plane of required surface.

Install taut ground wires or other means at corners or off-

sets not established by forms.

3.6Application3.6.1Placement techniquesProvide a platform that per-

mits nozzleman unobstructed access to the receiving surface.

Place shotcrete first in corners, recesses, and other areas

where rebound or overspray cannot escape easily. Remove

rebound and overspray from previously prepared surfaces

prior to shotcrete placement.

Place shotcrete with nozzle held approximately perpendic-

ular to the receiving surface. In corners, direct nozzle at ap-

proximately 45 deg angle or bisect the corner angle. Apply

shotcrete so sags or sloughing do not occur. Discontinue

shooting or shield the nozzle stream if wind causes separa-

tion of ingredients during shooting.Do not reuse rebound or overspray. Remove laitance from

shotcrete surfaces to receive additional shotcrete layers. Sur-

face preparation after final set shall comply with section

3.3.2. Do not apply shotcrete on surfaces with standing water

or running water. Remove hardened overspray and rebound

from adjacent surfaces, including exposed reinforcement.

3.6.2Encasement of reinforcementPlace shotcrete to

completely encase reinforcing steel. Encase reinforcement

by shooting with sufficient velocity and plasticity so material

flows around and behind the reinforcement. Front face of re-

inforcement shall remain clean during encasement.

Place shotcrete to provide the cover over reinforcement re-

quired by ACI 301.

Minimum slump of wet-mix shotcrete is 1 in.

3.7Finishing

Gun finish or as specified.

Broom or roughen the surface of freshly placed shotcrete

after shotcrete has reached initial set when subsequent layersare to be applied.

3.8Curing

Immediately after finishing, cure shotcrete continuously

by maintaining in a moist condition for seven days or until

specified strength is attained or until succeeding shotcrete

layers are placed.

Cure by one of the following methods:

Ponding or continuous sprinkling

Covering with an absorptive mat or sand that is kept

continuously wet

Covering with impervious sheet material

Curing compounds

Natural curing shall be permitted if ambient relative hu-

midity is maintained above 95 percent.

3.9Hot weather shotcreting

Do not place shotcrete when material temperature is above

90 F for wet mix; 100 F for dry mix. Lower temperature of

reinforcement and receiving surfaces below 100 F prior to

shooting.

3.10Cold weather shotcreting

Shooting may proceed when ambient temperature is 40 F

and rising. 50 F for latex-modified shotcrete. Shooting shall

discontinue when ambient temperature is 40 F and falling

unless protective measures are taken to protect shotcrete.

Shotcrete material temperature, when shot, shall not be less

than 50 F or more than 90 F. Shotcrete shall not be placed

against frozen surfaces. Applicable procedures used for cold

weather concreting may be used for cold weather shot-cret-

ing.

3.11Protection

Protect surfaces not intended for shotcrete placement

against deposit of rebound and overspray or impact fromnozzle stream.

Remove rebound and hardened overspray from final shot-

crete surfaces and from areas not intended for shotcrete

placement.

3.12Tolerances

Tolerances of shotcrete shall comply as specified.

ACI 506.2-95 was submitted to letter ballot of the committee and approved in accor-dance with ACI standardization procedures.