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 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. 506.2-2 ACI STANDARD 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. SPECIFICATION FOR SHOTCRETE 506.2-3 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. 506.2-4 ACI STANDARD 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 SPECIFICATION FOR SHOTCRETE 506.2-5 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 pro- 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 accordance 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 1 506.2-6 ACI STANDARD ceeding 3 / 16 in. thick by 1 1 / 4 in. long, or one major void, sand pocket, or lamination containing loosely bonded sand not to exceed 5 / 8 in. thick and 1 1 / 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 1 1 / 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 2 Grade 3 Grade 4 Grade 5 SPECIFICATION FOR SHOTCRETE 506.2-7 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, segregation, 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 encasement. • 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 ‡ 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 1 1 / 2 percent shall be allowed. 506.2-8 ACI STANDARD 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 shotcrete 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. . 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. 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. 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