Chapter 2 Types of Shotcrete and Applications Corps of ...

1 EM 1110-2-200531 Jan 93 Chapter 2 Types of Shotcrete andApplications2-1. Why Shotcretea. Importance of proper Shotcrete is a structurally sound and durableconstruction material which exhibits excellent bondingcharacteristics to existing concrete, rock, steel, and manyother can have high strength, lowabsorption, good resistance to weathering, and resistanceto some forms of chemical attack. Many of the physicalproperties of sound Shotcrete are comparable or superiorto those of conventional concrete or mortar having thesame applied Shotcrete maycreate conditions much worse than the Advantages of is used in lieuof conventional concrete, in most instances, for reasonsof cost or is advantageous insituationswhenformworkiscostprohibitiv eorimpractical and where forms can be reduced oreliminated, access to the work area is difficult, thinlayers or variable thicknesses are required, or normalcasting techniques cannot be are possible because Shotcrete requires only asmall.

2 Portable plant for manufacture and operations can often be accomplished inareas of limited access to make repairs to Strength of excellent bonding ofshotcrete to other materials is often an important force of the impact of thispneumatically propelled material on the surface causescompaction of the Shotcrete paste matrix into the finesurface irregularities and results in good adhesion to limits, the material is capable ofsupporting itself in vertical or overhead ApplicationsThe selection of Shotcrete for a particular applicationshould be based on knowledge, experience, and a carefulstudy of required and achievable material success of the Shotcrete for that application iscontingent upon proper planning and supervision, plusthe skill and continuous attention provided by theshotcrete following paragraphs discussthe use of Shotcrete in several of the more commonapplications.

3 A number of Shotcrete Applications by theCorps of Engineers are listed in Appendix can be used to repair thedamaged surface of concrete, wood, or steel structuresprovided there is access to the surface needing following examples indicate a few ways in whichshotcrete can be used in repairs:(1) Bridges. Shotcrete repair can be used for bridgedeckrehabilitation,butithasgeneral lybeenuneconomical for major full-thickness repairs. It is veryuseful, however, for beam repairs of variable depths,caps, columns, abutments, wingwalls, and underdecksfrom the standpoint of technique and cost.(2) Buildings. Inbuildingrepairs,shotcreteiscommonly used for repair of fire and earthquake damageand deterioration, strengthening walls, and encasingstructural steel for fireproofing. The repair of structuralmembers such as beams, columns, and connections iscommon for structures damaged by an earthquake.

4 (3) Marine structures. Damage to marine structurescan result from deterioration of the concrete and of thereinforcement. Damaging conditions are corrosion of thesteel, freezing and thawing action, impact loading,structural distress, physical abrasion from the action ofwaves, sand, gravel, and floating ice, and chemical attackdue to problems can occur in mostmarine structures such as bridge decks, piles, pile caps,beams, piers, navigation locks, guide walls, dams,powerhouses, and discharge many cases, Shotcrete can be used to repair the deteriorated surfacesof these structures.(4) Spillway surfaces. Surfaces subject to high-velocity flows may be damaged by cavitation erosion orabrasion repairs are advantageousbecause of the relatively short outage necessary tocomplete the Underground the most part, Shotcrete is used in underground excavations in rock; buton occasion, it has been successfully used in theadvancement of tunnels through altered, cohesionless, andloose underground Shotcrete applicationsrange from supplementing or replacing conventionalsupport materials such as lagging and steel sets, sealingrock surfaces, channeling water flows, and installingtemporary support and permanent 1110-2-200531 Jan 93c.

5 Slope and surface is oftenused for temporary protection of exposed rock surfacesthat will deteriorate when exposed to isalso used to permanently cover slopes or cuts that mayerode in time or otherwise deteriorate. Slope protectionshould be properly drained to prevent damage fromexcessive uplift pressure. Application of Shotcrete to thesurface of landfills and other waste areas is beneficial toprevent surface water New is not necessarily thefastest method of placing concrete on all jobs, but wherethin sections and large areas are involved, shotcreting canbe used effectively to save followingparagraphs describe some of the Applications involvedwith construction of new structures.(1) Pools and tanks. Shotcrete has been usedextensively to construct concrete swimming pools. Morerecently, large aquariums have been constructed usingshotcrete.

6 (2) Shotcrete floors and walls. Shotcrete floors intanks and pools on well compacted subbase or onundisturbed earth have generally given excellent and overhead construction for walls, slabs,columns, andotherstructural membershasbeenfrequently shotcreted.(3) Shotcrete domes. Construction techniques usinginflatable air-forming systems have made the constructionof Shotcrete shells or domes largestructures have been used for residential housing,warehousing, bridge, and culvert Shotcrete ProcessesShotcrete can be applied by two distinct applicationtechniques, the dry-mix process and the wet-mix Dry-mix cementitious material andaggregate are thoroughly mixed and either bagged in adry condition, or mixed and delivered directly to the mixture is normally fed to a pneumatically operatedgun which delivers a continuous flow of material throughthe delivery hose to the interior of thenozzle is fitted with a water ring which uniformly injectswater into the mixture as it is being discharged from thenozzle and propelled against the receiving Wet-mix cementitious material.

7 Aggregate, water, and admixtures are thoroughly mixed aswould be done for conventional mixedmaterial is fed to the delivery equipment, such as aconcrete pump, which propels the mixture through thedelivery hose by positive displacement or by compressedair. Additional air is added at the nozzle to increase thenozzle discharge suitable for most requirements canbe produced by either the dry-mix or wet-mix , differences in the equipment cost, maintenancerequirements,operationalfeatu res,placementcharacteristics, and product quality may make one or theother more attractive for a particular of the processes is given in Table 2-1.(1) Bond strengths of new Shotcrete to existingmaterials are generally higher with dry-mix shotcretethan with wet-mix Shotcrete mixturesoften provide significantly higher bond strengths toexisting materials than does conventional concrete.

8 (2) Typically, dry-mix Shotcrete is applied at a muchslower rate than wet-mix Shotcrete . Dry-mix Shotcrete isoften applied at a rate of 1 or 2 cubic yards per hourcompared to wet-mix Shotcrete applied at a rate of up to7 or 8 cubic yards per hour. Depending on the appli-cation, the in-place production rate may be significantlylower because of obstacles, rebound, and other featureswhich may cause delays.(3) Rebound is the Shotcrete material that "bounces"off the shooting for conventionaldry-mix Shotcrete , in the best of conditions, can beexpected to be at least 20 percent of the total materialpassed through the nozzle. Wet-mix Shotcrete reboundssomewhat less than dry-mix Shotcrete .(4) The use of air-entraining admixtures (AEA) inshotcrete is practical only in wet-mix Shotcrete . Whenbatched properly, AEA forms an air-void system suitablefor providing frost resistance to wet-mix Shotcrete .

9 Theformation of an air-void system in dry-mix Shotcrete isnot possible. However, dry-mix Shotcrete , when properlyproportioned and applied, will have a compressivestrengthexceedingapproximatel y 7,000 poundsper2-2EM 1110-2-200531 Jan 932-3EM 1110-2-200531 Jan 93square inch (psi).It has performed well in moderateexposures to freezing and Fiber-Reinforced ,likeunreinforcedconventionalconcrete,is abrittlematerialthatexperiences cracking and displacement when subjected totensile stresses or strains. The addition of fibers to theshotcrete mixture adds ductility to the material as well asenergy absorption capacity and impact material is capable of sustaining postcrackloadings and often displays increased ultimate strength,particularly tensile strength. Fibers used in Shotcrete areavailable in three general forms:steel fibers, glassfibers, and other synthetic fibers.

10 Natural fiber, a fourthform, is not commonly used in Shotcrete and will not use of steel fibers has evolved rapidly sinceits inception in the late 1950 present third-generation steel fibers are greatly superior to the mixing and handling problems whichhampered uniform distribution of fibers in a mixture havebeen minimized by the manufacture of fibers with low-aspect ratios (ratio of length to diameter), surfacedeformations, and improved use of glass-fiber-reinforced Shotcrete (GFRS)is an adaptation of the technology of using chopped glassfibers and a resin binder. The equipment and process toapply glass-fiber Shotcrete is not a conventional shotcreteoperation, but requires a special gun and delivery process termed "spray-up" is used extensively in theconstruction of lightweight panels for building claddingand special architectural features and is usually applied ina plant production common onsiteapplicationistheconstructionofsimu latedrockstructures for animal exhibits at fibers aremade from a special zirconium alkali-resistant (AR) glassto resist deterioration in the highly alkaline portland-cement for the use of glass-fiber spray-up are provided by the Prestressed ConcreteInstitute (PCI) (1981).