specification for shotcrete

Keywords: dry-mix shotcrete; fiber reinforced shotcrete; nozzleman; shooting; shotcrete; wet-mix shotcrete.. “Work on Project Title shall conform to all the requirements of ACI 506.2 Sta

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 1—General, p 506.2-4

1.1—Scope

1.2—Definitions

1.3—Reference organizations

1.4—Reference standards

1.5—Submittals

1.6—Quality assurance

1.7—Shotcrete core grades

1.8—Evaluation of in-place shotcrete

1.9—Acceptance

Section 2—Materials, p 506.2-6

2.1—Cement 2.2—Aggregate 2.3—Reinforcement 2.4—Water

2.5—Admixtures 2.6—Curing materials 2.7—Proportioning 2.8—Fiber reinforced shotcrete 2.9—Prebagged materials 2.10—Delivery, storage, and handling

Section 3—Execution, p 506.2-6

3.1—Examination 3.2—Batching and mixing 3.3—Surface preparation 3.4—Joints

3.5—Alignment control 3.6—Application 3.7—Finishing 3.8—Curing

Specification for Shotcrete (ACI 506.2-95)

Reported by ACI Committee 506

Steven H Gebler Chairman

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 or mechanical 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 in writing is obtained from the copyright proprietors.

3.9—Hot weather shotcreting

3.10—Cold water shotcreting

3.11—Protection

3.12—Tolerances

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 be made 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 Project Specifications

“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

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

3.3—Surface 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 by scraping, brooming, high pressure water blasting or sand blasting.

3.12—Tolerance 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 of

2 times over the tolerances provided in ACI 117.

OPTIONAL REQUIREMENTS CHECKLIST

Section/Part/Article Notes to the Architect/Engineer

1.4—Reference standards The following references are provided as guides They are not legal documents They cannot be referenced in the

shotcrete 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 Shotcrete 1.5—Submittals Specify if other submittals are needed such as mix proportions and results of preconstruction tests when specified.

OPTIONAL REQUIREMENTS CHECKLIST, continued

Section/Part/Article Notes to the Architect/Engineer

1.6—Quality 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 or other suitable requirement Language, however, must be mandatory.

1.7—Shotcrete 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.

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

2.1—Cement Specify if other type of cement is allowable If required, specify blended hydraulic cement in accordance with ASTM

C 595 Base selection of cement type on sulfate exposure conditions or other conditions.

2.2—Aggregate Specify if other grading, type or tests are required Specify criteria for acceptance Other gradings proposed by the

contractor may be acceptable if the contractor can demonstrate satisfactory performance under similar conditions of use.

2.3—Reinforcement Specify type, size, configuration, minimum content, tests required, or required testing frequency and acceptance

crite-ria.

2.5—Admixtures 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 samples from 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 Entrained air is lost during shotcreting Obtaining greater than 4 percent entrained air in in-place wet-mix is difficult If a greater percent 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.

2.8—Fiber ACI 506.1R—“State-of-the-Art Report on Fiber Reinforced Shotcrete” provides guidance for fiber shotcrete 3.3.3—Surface preparation It is recommended that steel surfaces should comply with SSPC-SP6 3.3.3.2.

3.3.5—Forms High velocity air transporting the shotcrete mix, if not allowed to escape, may create voids at form corners 3.4.1—Joints Specify square joints for construction joints subject to compression loads.

3.5—Alignment control For long spans (over 30 ft), devices that permit tightening of ground wires, such as springs or turnbuckles can be used 3.6.1—Application Do not place subsequent lifts until the previous lift has hardened enough to support the new shotcrete Vertical walls

should be gunned from the bottom up unless it can be demonstrated that another method will work successfully 3.6.2—Encasement of reinforcement Voids behind bars are not easily evident by inspection Limiting the maximum size reinforcement bar to a No 5 bar

increases the likelihood of complete encasement, especially with limited experienced crews Encasing larger bars requires 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.

3.7—Finishing 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.

3.8—Curing 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.

3.10—Cold weather ACI 306.1 standard specification should be reviewed.

SUBMITTALS CHECKLIST

Section/Part/Article Notes to the Architect/Engineer

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

SECTION 1—GENERAL

1.1—Scope

This Standard Specification describes the requirements for

materials, proportioning, and application of shotcrete

1.2—Definitions

Accepted—Accepted by or acceptable to the

Architect/En-gineer

Architect/Engineer—The 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 Documents—Documents covering the required

work and including the project drawings and project

specifi-cations

Contractor—The person, firm, or corporation that the

owner enters into an agreement with for construction of the

work

Core grade—Quality ranking of a shotcrete core.

Dry-mix shotcrete—Shotcrete 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 coat—Final thin coat of shotcrete preparatory to

hand finishing

Flash coat—Thin shotcrete coat applied from a distance

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

Ground wire—Wire used to establish line and grade for

shotcrete work

Gun—Dry-mix shotcrete delivery equipment.

Gun finish—Undisturbed final layer of shotcrete as

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

nat-ural or nozzle finish

Nozzle—Attachment at end of delivery hose from where

shotcrete is projected

Nozzleman—Worker on shotcrete crew who manipulates

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

controls final disposition of the material

Overspray—Shotcrete material deposited away from the

intended receiving surface by air blast

Owner—The corporation, association, partnership,

indi-vidual, or public body or authority with whom the contractor

enters into agreement and for whom the work is provided

Rebound—Shotcrete material that ricochets off the

receiv-ing surface

Sand/rock pocket—A porous zone in the shotcrete

con-taining aggregate with reduced mortar content

Shooting—Act of applying shotcrete.

Shotcrete—Mortar or concrete pneumatically projected

onto a surface at high velocity

Sloughing—Subsidence of shotcrete, due generally to

ex-cessive water in mixture or to vibration of continuous

rein-forcement or formwork

Submitted—Submitted to the Architect/Engineer for

re-view

Wet-mix shotcrete—Shotcrete in which plastic concrete

with cement, aggregate, and water are first mixed together before introduction into the delivery hose

Work—The 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

1.3—Reference 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

1.4—Reference standards

The following Reference Standards are referred to in this specification and declared to be a part of this Standard Spec-ification:

1.4.1 ACI Standards

ACI 301-89 Specifications for Structural Concrete for

Buildings, paragraph 5.7.1 and Chapter 9

1.4.2 ASTM 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

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

Shotcrete

C 1140 Standard Practice for Preparing and Testing

Spec-imens from Shotcrete Test Panels

C 1141 Standard Specification for Admixtures for

Shot-crete

1.4.3 SSPC Standards

SP6 Surface Preparation Specification No 6,

Commer-cial Blast Cleaning

1.5—Submittals

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

1.6—Quality assurance

1.6.1 Preconstruction 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

portion, each anticipated shooting orientation, and each

pro-posed nozzleman Mixes shall meet requirements of section

2.7—Proportioning 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 accordcompli-ance

with ASTM C 42

1.6.1.4 Visually grade the reinforced specimens for

compliance with specified core grade (Section 1.7

—Shot-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

1.6.2 Construction 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 with section 1.7—Shotcrete core grades

1.6.2.4 The mean compressive strength of a set of three

cores shall equal or exceed 0.85 f c′ with no individual core

less than 0.75 f c′ The mean of a set of three cubes shall equal

or exceed f c′ with no individual cube less than 0.88 f c′

1.7—Shotcrete 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

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 of 1/16 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 flaws

such 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 of 1/8 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

1.8—Evaluation 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

1.9—Acceptance 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 5

to assure that remaining work complies with the

require-ments

SECTION 2—MATERIALS

2.1—Cement

Portland cement: ASTM C 150 Type I

2.2—Aggregate

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

2.3—Reinforcement

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

2.4—Water

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

2.5—Admixtures

Water-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

2.6—Curing 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

2.7—Proportioning

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:

f c′: ASTMC 42; ASTM C1140

Air content: ASTM C 173 or C 231

2.8—Fiber-reinforced shotcrete

ASTM C 1116

2.9—Prebagged materials

Are allowable with predampening

2.10—Delivery, storage, and handling

Deliver, store, and handle materials to prevent contamina-tion, segregacontamina-tion, corrosion or damage Store liquid admix-tures in order to prevent evaporation and freezing

SECTION 3—EXECUTION 3.1—Examination

• 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 encasereinforce-ment

• Verify that surfaces to receive shotcrete have been properly prepared according to the Contract Docu-ments

3.2—Batching 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

3.3—Surface preparation

3.3.1 Earth—Prepare surfaces to line and grade Do not

apply shotcrete to frozen surface Dampen surface immedi-ately prior to shooting

3.3.2 Concrete, masonry, and shotcrete—When bonding

is required, remove all deteriorated, loose unsound material

or contaminants that may inhibit bonding Chip areas to be repaired to remove offsets causing abrupt changes in

thick-Table 2.7—Total air content for frost resistant concrete*

Nominal maximum aggregate size† Total air content, percent‡

* Based on samples obtained from the concrete truck.

† See ASTM C 33 for tolerances for various nominal maximum size designations ‡ A tolerance of 1 1 / 2 percent shall be allowed.

ness Taper edges to eliminate square shoulders at the

perim-eter of a cavity

Surface shall be saturated surface dry immediately prior to

shooting

3.3.3 Structural/reinforcement—The 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

3.3.4 Rock—Remove loose material, mud or other foreign

material that will prevent bonding Clean surface Prewet

surface immediately prior to shooting

3.3.5 Forms—Use 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

3.4—Joints

3.4.1 Construction joints—Taper 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

3.4.2 Control joints—Place as shown on drawings.

Discontinue reinforcement at control joints

3.5—Alignment 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

3.6—Application

3.6.1 Placement techniques—Provide 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

3.6.2 Encasement of reinforcement—Place 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

3.7—Finishing

Gun finish or as specified

Broom or roughen the surface of freshly placed shotcrete after shotcrete has reached initial set when subsequent layers are to be applied

3.8—Curing

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

3.9—Hot 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

3.10—Cold 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

3.11—Protection

Protect surfaces not intended for shotcrete placement against deposit of rebound and overspray or impact from nozzle stream

Remove rebound and hardened overspray from final shot-crete surfaces and from areas not intended for shotshot-crete placement

3.12—Tolerances

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.