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.
This specification contains the construction requirements for the applica-tion of shotcrete. Both wet-mix and dry-mix shotcrete are specified, and theminimum standards for testing, materials, and execution are provided.
Section 1—General, p. 506.2-41.1—Scope1.2—Definitions1.3—Reference organizations1.4—Reference standards1.5—Submittals1.6—Quality assurance1.7—Shotcrete core grades1.8—Evaluation of in-place shotcrete1.9—Acceptance
Section 2—Materials, p. 506.2-72.1—Cement
2.2—Aggregate2.3—Reinforcement2.4—Water2.5—Admixtures2.6—Curing materials2.7—Proportioning2.8—Fiber reinforced shotcrete2.9—Prebagged materials2.10—Delivery, storage, and handling
Section 3—Execution, p. 506.2-73.1—Examination3.2—Batching and mixing3.3—Surface preparation3.4—Joints3.5—Alignment control3.6—Application3.7—Finishing3.8—Curing3.9—Hot weather shotcreting3.10—Cold weather shotcreting3.11—Protection3.12—Tolerances
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 SchultheisPaul 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 ZimmermanRichard A. Kaden Thomas J. Reading
ACI 506.2-95
506.2-2 ACI STANDARD
FOREWORDF1. This foreword is included for explanatory purposes
only; it does not form a part of Standard Specification ACI506.2.
F2. Standard Specification ACI 506.2 is a reference stan-dard which the Architect/Engineer may cite in the projectspecifications for any project, together with supplementaryrequirements for the specific project.
F3. Each technical section of Standard Specification ACI506.2 is written in the three-part section format of the Con-struction Specification Institute, as adapted by ACI andmodified to ACI requirements. The language is generallyimperative and terse.
F4. Checklists do not form a part of Standard SpecificationACI 506.2. Checklists are to assist the Architect/Engineer inproperly choosing and specifying any necessary require-ments for the project specifications.
PREFACE TO SPECIFICATIONS CHECKLISTP1. Standard Specification ACI 506.2 is intended to be
used by reference or incorporation in its entirety in theProject 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 ACI506.2 are edited into Project Specifications or any other doc-ument, they shall not be referred to as ACI Standards, sincethe Standard Specification has been altered.
P3. Building codes set minimum requirements necessaryto protect the public. These Standard Specifications maystipulate requirements more restrictive than the minimum.Adjustments to the needs of a particular project shall bemade by the Architect/Engineer by reviewing each of theitems in the Specifications Checklist and then including theArchitect/Engineer's decision on each item as a mandatoryrequirement in the Project Specifications.
P4. These mandatory requirements designate the specificqualities, procedures, materials, and performance criteria forwhich alternatives are permitted or for which provisionswere not made in the Standard Specifications. Exceptions tothe Standard Specifications shall be made in the ProjectSpecifications, if required.
P5. A statement such as the following will serve to makeStandard Specifications ACI 506.2 a part of the ProjectSpecifications.
“Work on (Project Title) shall conform to all therequirements of ACI 506.2 Standard Specification forShotcrete, published by the American Concrete Insti-tute, Detroit, Michigan, except as modified by therequirements of these Contract Documents.”
P6. The Standard Specifications Checklist identifiesArchitect/Engineer choices and alternatives. The checklistsidentify the sections, parts, and articles of the Standard Spec-ifications and the action required by the Architect/Engineer.
MANDATORY REQUIREMENTS CHECKLISTSection/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. Sometimes 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/Engineer1.4—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 ShotcreteACI 506.3R Guide to Certification of Shotcrete NozzlemanACI 506R Guide to ShotcreteACI 506.1R State-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.
SPECIFICATION FOR SHOTCRETE 506.2-3
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 ofuse.
2.3—Reinforcement Specify type, size, configuration, minimum content, tests required, or required testing frequency and acceptancecriteria.
2.5—Admixtures A concrete mix doesn't become shotcrete until it is placed at a high velocity. Measuring air content at the pumphopper, measures concrete air content not shotcrete air content. Shotcrete air content can be measured by takingsamples 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 concreteprior 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
limits. 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 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.
3.7—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.
OPTIONAL REQUIREMENTS CHECKLIST
Section/Part/Article Notes to the Architect/Engineer
SUBMITTALS CHECKLIST
Section/Part/Article Notes to the Architect/Engineer1.5—Submittals Proposed shotcrete mix. Test panel results if required.
OPTIONAL REQUIREMENTS CHECKLIST, continued
506.2-4 ACI STANDARD
SECTION 1—GENERAL1.1—Scope
This Standard Specification describes the requirements formaterials, proportioning, and application of shotcrete.
1.2—DefinitionsAccepted—Accepted by or acceptable to the Archi-
firm, engineering firm or architectural and engineering firmissuing project drawings and specifications or administeringthe work under the Contract Documents.
Contract Documents—Documents covering the requiredwork and including the project drawings and project specifi-cations.
Contractor—The person, firm, or corporation that the ownerenters 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,plastic, or alkali resistant (AR) glass.
Finish coat—Final thin coat of shotcrete preparatory tohand finishing.
Flash coat—Thin shotcrete coat applied from a distancegreater than normal for use as a final coat or for finishing.
Ground wire—Wire used to establish line and grade forshotcrete work.
Gun—Dry-mix shotcrete delivery equipment.Gun finish—Undisturbed final layer of shotcrete as
applied from nozzle without hand finishing; also known asnatural or nozzle finish.
Nozzle—Attachment at end of delivery hose from whereshotcrete is projected.
Nozzleman—Worker on shotcrete crew who manipulatesthe nozzle, controls consistency of dry-mix shotcrete, andcontrols final disposition of the material.
Overspray—Shotcrete material deposited away from theintended receiving surface by air blast.
Owner—The corporation, association, partnership, indi-vidual, or public body or authority with whom the contractorenters into agreement and for whom the work is provided.
Rebound—Shotcrete material that ricochets off thereceiving 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
excessive water in mixture or to vibration of continuous rein-forcement or formwork.
Submitted—Submitted to the Architect/Engineer forreview.
Wet-mix shotcrete—Shotcrete in which plastic concretewith cement, aggregate, and water are first mixed togetherbefore introduction into the delivery hose.
Work—The entire construction or separately identifiableparts that are required to be furnished under the ContractDocuments. Work is the result of performing services,furnishing labor, and providing and incorporating materialsand equipment into the construction, all as required by theContract Documents.
1.3—Reference organizationsAmerican Concrete InstituteP.O. Box 9094Farmington Hills, Mich. 48333-9094
1.5—SubmittalsSubmit 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 resultsSubmit preconstruction test panel results when required.
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 mixproportion, each anticipated shooting orientation, and eachproposed nozzleman. Mixes shall meet requirements ofSection 2.7—Proportioning. In half of the test panels providereinforcement of the same size and spacing required for thework. Obtain six test specimens from each panel, threenonreinforced specimens and three with reinforcing steel.
1.6.1.3 Test the nonreinforced specimens for compliancewith the specified physical properties in accordance withASTM C 42.
1.6.1.4 Visually grade the reinforced specimens for com-pliance with specified core grade (Section 1.7—Shotcretecore grades).
1.6.1.5 Test admixtures for compatibility with cement inaccordance with ASTM C 1141.
1.6.1.6 Unless otherwise specified, only nozzlemen witha test panel mean core grade less than or equal to 2.5 (Section1.7) shall be allowed to place job shotcrete. When theprequalification test panel is rejected, a second panel may beshot. When the nozzleman's second mean core grade isgreater than 2.5, the nozzleman shall not be permitted toshoot on the project.
1.6.2 Construction testing1.6.2.1 Produce a material test panel for each mix and
each work day or every 50 cubic yards placed, whichever isless. Test panel shall be kept moist and at 70 F ± 10 F untilmoved to test laboratory. Obtain test specimens either fromjob site material test panel or from in-place shotcrete. Testspecimens from test panels in compliance with ASTM C1140.
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 threecores shall equal or exceed 0.85 fc′ with no individual coreless than 0.75 fc′ . The mean of a set of three cubes shall equalor exceed fc′ with no individual cube less than 0.88 fc′ .
1.7—Shotcrete core grades1.7.1 Grade 1: Shotcrete specimens are solid; there are no
laminations, sandy areas or voids. Small air voids with amaximum diameter of 1/8 in. and maximum length of 1/4 in.are normal and acceptable. Sand pockets, or voids behindcontinuous reinforcing steel are unacceptable. The surfaceagainst the form or bond plane shall be sound, without asandy texture or voids.
1.7.2 Grade 2: Shotcrete specimens shall have no morethan two laminations or sandy areas with dimensions not toexceed 1/8 in. thick by 1 in. long. The height, width anddepth of voids shall not exceed 3/8 in. Porous areas behindreinforcing steel shall not exceed 1/2 in. in any direction ex-cept along the length of the reinforcing steel. The surfaceagainst the form or bond plane shall be sound, without asandy texture or voids.
1.7.3 Grade 3: Shotcrete specimens shall have no morethan two laminations or sandy areas with dimensionsexceeding 3/16 in. thick by 1-1/4 in. long, or one major void,sand pocket, or lamination containing loosely bonded sandnot to exceed 5/8 in. thick and 1-1/4 in. in width. The surfaceagainst the form or bond plane may be sandy with voidscontaining overspray to a depth of 1/16 in.
1.7.4 Grade 4 core: The core shall meet in general therequirements of Grade 3 cores, but may have two major
Grade 1
506.2-6 ACI STANDARD
flaws such as described for Grade 3 or may have one flawwith a maximum dimension of 1 in. (25 mm) perpendicularto the face of the core with a maximum width of 1-1/2 in. Theend of the core that was shot against the form may be sandyand with voids containing overspray to a depth of 1/8 in.
1.7.5 Grade 5 core: A core that does not meet the criteriaof core grades 1 through 4, by being of poorer quality, shallbe classified as Grade 5.
1.7.6 Determination of grade shall be by computing themean of a minimum of three test specimens.
1.7.7 A mean grade of 2.5 or less is acceptable unlessotherwise specified. Individual shotcrete cores with a gradegreater than 3 are unacceptable.
1.7.8 The above core grades are based on cores with asurface area of 50 in.2 For cores with greater or lesser areathan 50 in.2, adjust allowable flaws relative to 50 in.2
1.8—Evaluation of in-place shotcreteRemove and replace shotcrete that is delaminated, exhibits
laminations, voids, or sand pockets exceeding the limits forthe specified grade or shotcrete. Remove and replace shotcretethat does not comply with the specified material properties.
Repair core holes in accordance with Chapter 9 of ACI301. Do not fill core holes by shooting.
1.9—Acceptance1.9.1 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 bringit into compliance, will be accepted.
1.9.3 Shotcrete work that fails to meet one or morerequirements and which cannot be brought intocompliance may be accepted or rejected. Modificationsmay be required to assure that remaining work complieswith the requirements.
Grade 2 Grade 3 Grade 4
Grade 5
SPECIFICATION FOR SHOTCRETE 506.2-7
SECTION 2—MATERIALS2.1—Cement
Portland cement: ASTM C 150. Type I
2.2—AggregateNormal weight aggregate: ASTM C 33. Aggregate not
meeting ASTM C 33 may be used provided preconstructiontests demonstrate the shotcrete can meet specified require-ments.
Lightweight aggregate: ASTM C 330
2.3—ReinforcementDeformed steel reinforcement: ASTM A 615Welded wire fabric: ASTM A 185Steel fibers: ASTM A 820 and C 1116Glass fibers: ASTM C 1116Synthetic fibers: ASTM C 1116
2.4—WaterClean 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—AdmixturesWater-reducing: ASTM C 1141Retarding: ASTM C 1141Accelerating: ASTM C 1141Air-entraining: ASTM C 1141Fly ash and natural pozzolans: ASTM C 618Ground granulated blast-furnace slag: ASTM C 989Silica fume: ASTM C 1240
2.6—Curing materialsCuring materials that cause stains on architectural finishes
shall not be used.Sheet materials: ASTM C 171Curing compounds: ASTM C 309, apply twice the coverage
(double the application rate) recommended by manufacturers.
2.7—ProportioningSelect 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 C1140Air content: ASTM C 173 or C 231
2.8—Fiber-reinforced shotcreteASTM C 1116
2.9—Prebagged materialsAre allowable with predampening.
2.10—Delivery, storage, and handlingDeliver, store, and handle materials to prevent contamina-
tion, segregation, corrosion or damage. Store liquid admix-tures in order to prevent evaporation and freezing.
SECTION 3—EXECUTION3.1—Examination
• Verify that surfaces are acceptable and are ready toreceive work.
• Verify that forms are to line and grade, braced againstvibration, and constructed to permit escape of air andrebound.
• Verify that placement and clearance around reinforce-ment permits complete encasement.
• Verify that surfaces to receive shotcrete have beenproperly prepared according to the Contract Documents.
3.2—Batching and mixingWeight 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 aweek 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 afterbatching or predampening. This requirement does notinclude dry, prebagged material unless prebagged material ispredampened.
Shoot wet-mix shotcrete material within 90 min afterbatching.
3.3—Surface preparation3.3.1 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 bondingis required, remove all deteriorated, loose unsound materialor contaminants that may inhibit bonding. Chip areas to berepaired to remove offsets causing abrupt changes in thick-ness. Taper edges to eliminate square shoulders at the perim-eter of a cavity.
Surface shall be saturated surface dry immediately prior toshooting.
3.3.3 Structural/reinforcement—The surface shall be freeof deleterious materials that inhibit bonding. For newconstruction, reinforcement laps shall be separated with aclearance of at least three times the diameter of largest aggre-gate. Reinforcement shall be secured to prevent movement.
3.3.4 Rock—Remove loose material, mud or other foreignmaterial that will prevent bonding. Clean surface. Prewetsurface immediately prior to shooting.
3.3.5 Forms—Use form-release coating material onremovable forms. Secure forms to minimize the effects of
Table 2.7—Total air content for frost-resistant concrete*
Nominal maximum aggregate size† Total air content, percent‡
Less than 3/8 in. 7
Greater than 3/8 in. 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.
506.2-8 ACI STANDARD
vibration. Construct forms to allow escape of placement airand rebound.
3.4—Joints3.4.1 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 controlInstall 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
offsets not established by forms.
3.6—Application3.6.1 Placement techniques—Provide a platform that
permits nozzleman unobstructed access to the receivingsurface. Place shotcrete first in corners, recesses, and otherareas where rebound or overspray cannot escape easily.Remove rebound and overspray from previously preparedsurfaces prior to shotcrete placement.
Place shotcrete with nozzle held approximately perpendic-ular to the receiving surface. In corners, direct nozzle atapproximately 45 deg angle or bisect the corner angle. Applyshotcrete so sags or sloughing do not occur. Discontinueshooting or shield the nozzle stream if wind causes separa-tion of ingredients during shooting.
Do not reuse rebound or overspray. Remove laitance fromshotcrete surfaces to receive additional shotcrete layers.Surface preparation after final set shall comply with Section3.3.2. Do not apply shotcrete on surfaces with standing wateror running water. Remove hardened overspray and reboundfrom adjacent surfaces, including exposed reinforcement.
3.6.2 Encasement of reinforcement—Place shotcrete tocompletely encase reinforcing steel. Encase reinforcementby shooting with sufficient velocity and plasticity so materialflows around and behind the reinforcement. Front face ofreinforcement shall remain clean during encasement.
Place shotcrete to provide the cover over reinforcementrequired by ACI 301.
Minimum slump of wet-mix shotcrete is 1 in.
3.7—FinishingGun finish or as specified.
Broom or roughen the surface of freshly placed shotcreteafter shotcrete has reached initial set when subsequent layersare to be applied.
3.8—CuringImmediately after finishing, cure shotcrete continuously
by maintaining in a moist condition for seven days or untilspecified strength is attained or until succeeding shotcretelayers 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 compoundsNatural curing shall be permitted if ambient relative
humidity is maintained above 95 percent.
3.9—Hot weather shotcretingDo not place shotcrete when material temperature is above
90 F for wet mix; 100 F for dry mix. Lower temperature ofreinforcement and receiving surfaces below 100 F prior toshooting.
3.10—Cold weather shotcretingShooting may proceed when ambient temperature is 40 F
and rising. 50 F for latex-modified shotcrete. Shooting shalldiscontinue when ambient temperature is 40 F and fallingunless protective measures are taken to protect shotcrete.Shotcrete material temperature, when shot, shall not be lessthan 50 F or more than 90 F. Shotcrete shall not be placedagainst frozen surfaces. Applicable procedures used for coldweather concreting may be used for cold weather shotcreting.
3.11—ProtectionProtect 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 shotcreteplacement.
3.12—TolerancesTolerances 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.
