Gunite Versus Shotcrete In Swimming Pool Construction

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Gunite Versus Shotcrete inSwimming Pool ConstructionBy Mason Guarino and Ryan OakesThe term “gunite pools” has been used for decades inthe swimming pool industry and is commonly used todescribe a pool using shotcrete placement, whether itis the wet-mix process or the dry-mix process. Pool buildersand designers have often debated whether a pool should be“made from gunite or from shotcrete.” In this article, we clarify the terminology that leads to the debate and then providearguments for both sides of the discussion.Gunite and shotcrete are not materials, but rather areplacement methods for pneumatically placing (shooting)concrete materials at high velocity. In the history of shotcrete,the term “Gunite” was a tradename used for the dry sandcement mixture pneumatically shot from the Cement GunCompany’s equipment and hydrated at the nozzle. When ACIstarted writing standards for pneumatically placed mortar in1951, it adopted the term “shotcrete,” as proprietary tradenames were frowned upon in technical standards. When reliable concrete pumping equipment allowed pneumatic placement of ready mixed concrete, the terminology was modifiedto include “wet-mix shotcrete,” while the original dry processbecame termed “dry-mix shotcrete.” Many companies stilluse the original term “gunite” to refer to dry-mix shotcrete.Thus, the term “shotcrete” can really be applied to either thedry-mix or wet-mix process.It has long been considered that shotcrete is the bestway to place quality concrete for a swimming pool structure. Shotcrete requires less formwork, fewer touchups afterform stripping, provides excellent strength and durability,and can be installed far more quickly than form-and-pourpool construction. Along with many other benefits of shotcrete, there is no question that shotcrete is how concretepools should be constructed. However, with two shotcrete processes, which one is the best for swimming poolconstruction? The short answer is that both processes workexceptionally well when correctly placed with a well-trainedcrew using the proper materials, equipment, and placementtechniques employed by experienced shotcrete companies.THE DRY-MIX VERSUS WET-MIX DEBATECommon points of debate are whether one method is stronger than the other and whether one method cracks morethan the other. There are many variables that influence thesepoints, including the mixture design and the placement ofthat mixture. If dry-mix shotcrete has a 5000 psi (35 MPa)compressive strength and wet-mix shotcrete has a 5000 psi12 Shotcrete Summer 2018compressive strength, they are indeed the same strength.This seems elementary, but actual compressive strengthas measured by compressive values is often ignored duringarguments of one method being stronger than the other.Though many pool builders feel a 2500 psi (17 MPa)28-day compressive strength is adequate for pool construction, ASA’s position is that shotcrete must have a minimum4000 psi (28 MPa) to allow proper encasement of reinforcement, low permeability, and long-term durability. This isespecially important in shotcreted pool shells that areexpected to be watertight and provide decades of troublefree service. This topic is covered in more detail in the“Overview of ASA Position Statements” article on p. 28 ofthis issue and online in Pool and Recreational ShotcreteCommittee Position Statement #1 (www.shotcrete.org/media/pdf/ASAPositionPaper PoolRec 1.pdf).When it comes to cracking, the debate becomes more difficult. There are many reasons why pool shells crack. Commonreasons include inadequate reinforcing bars, constructionon poor soil, poor curing practices, seasonal temperaturechanges, concrete shrinkage, and even whether rebound isleft in the pool rather than being removed during the installation. Aside from not removing the rebound, most of the poolcracks that occur have nothing to do with the shotcrete placement. Shrinkage (autogenous, early-age plastic, and long-termdrying) takes place in all concrete whether cast or shotcreted.We will only be covering shrinkage cracking in this article.The wet-mix process typically uses a water-cementitiousmaterials ratio (w/cm) range of 0.40 to 0.45 and the dry-mixprocess typically uses a lower w/cm range of 0.35 to 0.40.This lower w/cm reduces water in the concrete mixture andcan reduce shrinkage cracking. Moreover, when wet-mixconcrete leaves the plant or arrives to the jobsite with a 0.40w/cm, it may have more water added if it has been aging andneeds more fluidity to be pumped down the line. Unfortunately, adding additional water to the concrete mixture afterthe mixture has stiffened is a common practice in the poolindustry, resulting in concrete placed in many pools havinga w/cm higher than 0.50. This higher water content not onlyreduces the strength of the shell and increases the permeability but also creates the potential for more shrinkage.The negative effects of a too-high w/cm cannot be overemphasized in creating watertight pool shells. ASA Position Statement #4, “Watertight Shotcrete for SwimmingPools,” explains that ACI 350, “Code Requirements forwww.shotcrete.org

Environmental Engineering Concrete Structures,” mandatesa maximum w/cm of 0.45 with a minimum 4000 psi forconcrete intended to have low permeability when exposedto water. If concrete water-containing structures are exposedto freezing and thawing while saturated (a common occurrence in northern climates), an even lower maximum w/cmof 0.42, with a minimum 4500 psi (31 MPa) is required.A well-trained crew can mitigate this by using waterreducing admixtures rather than adding water. Strength isnot an argument for one shotcrete process over the other,as both, with the proper concrete mixture designs, canproduce strong, functionally impermeable concrete shells.With proper attention to shrinkage, cracking is typically notan argument for one process over the other.Fig. 1: Volumetric mixer truck on a pool shootWHAT IS THE DIFFERENCE BETWEENDRY-MIX AND WET-MIX?Dry-mix is the process of conveying dry concrete materials,often just a coarse sand and cement mixture, through adelivery hose, injecting the majority of water at the nozzle,then shooting the newly hydrated concrete at high velocityonto the receiving surface. The wet-mix process pumpspremixed concrete (usually a coarse sand, 3/8 in. [10 mm] orless coarse aggregate, and cement mixture) down the delivery line and adds air at the nozzle to accelerate the concreteto a high velocity.Benefits of Dry-Mix ShotcreteThe benefits of dry-mix shotcrete make it an exceptionalmethod for all pool construction. Dry-mix really excels inpools with less than 1000 ft 2 (90 m2) of water surface areaand pools with a high level of detail such as perimeter overflow pools or vanishing-edge pools. Pools in this size rangetypically can be finished in a day barring any extravagantfeatures that could delay the crew into a second or third day.Pools this size are often found in backyards where thereis very little area to work in and limited space for a concretetruck to clean out. Dry-mix shotcrete creates minimal messand can be easily cleaned up, and that helps get crews inand out in a day. A wet-mix crew must provide a way forconcrete delivery trucks to clean their chutes while a drymix crew simply shovels up any leftover materials at the endof the day. Additionally, these size pools often have only onecurtain of reinforcing bars, making the shooting process andmanaging rebound easier for the nozzlemen.The cleanup process after shooting dry-mix is easier thanwet-mix. There are only dry concrete materials to scoop orvacuum up—no concrete wash water or leftover concreteto worry about, especially when having to shoot from citystreets (Fig. 1, such as Commonwealth Ave. in Boston). Wetmix would have added hours of cleanup to the job; even agood plastic washout pan can fail and then you are cleaningup wet concrete with nowhere to wash it to.Residential pools commonly have a lot of intricate detail.This detail work is typically easier to place and finish witha mixture that only contains cement and concrete sandwithout a coarse aggregate. Many dry-mix shotcrete poolwww.shotcrete.orgFig. 2: The crew is able to walk on recently shot dry-mix materialwithout sinking into the surfacecontractors use volumetric mixer trucks to produce theirconcrete mixtures. This allows the crew to stop and startas necessary to ensure a quality placement and finish, whileeliminating the concern of set time of premixed concretematerial not yet placed. The concrete materials in the volumetric mixer truck are dry and can sit as long as neededuntil the crew is ready to start shotcreting again. A greatadvantage to dry-mix shotcrete is that with the correctmixture and placement, the in-place material can handlefoot traffic immediately, leaving little more than a footprint(Fig. 2).Unless a wet-mix concrete design uses a retarder orhydration control admixture, the batched concrete needsto be placed within 90 minutes of batching. By the time theconcrete truck drives to the jobsite (hopefully not stuck inSummer 2018 Shotcrete 13

traffic), waits their turn to pump, and then is shotcreted, itis often difficult to meet the 90-minute time frame.Using volumetric mixer trucks for creating wet-mixconcrete on site can help mitigate this problem. However,it still leaves the problem of walking on the freshly placed,relatively fluid concrete mixture and disrupting the surface.Benefits of Wet-Mix ShotcreteWet-mix shotcrete has significant benefits for pools largerthan 1000 ft 2 (90 m2) of water surface area and when morethan 1 day is required to complete a pool structure. Wet-mixshotcrete has substantial production advantages over drymix. At South Shore Gunite (SSG), a volumetric mixer truckis used to produce both dry-mix and wet-mix shotcrete.With dry-mix production, the mixer truck runs at about 8 to12 rpm, whereas in producing wet-mix concrete, the truckruns between 20 and 25 rpm—both with similar output perrpm. Thus, using wet-mix can double the production rateof concrete as compared to dry-mix when the job allows.Wet-mix shotcrete can be easier to place, especially whenjobs have high amounts of reinforcing bars with multiplecurtains of reinforcing bars to shoot through (Fig. 3 and4). The wet-mix process typically has less rebound thandry-mix, making areas with complicated layouts easier tomanage with an experienced nozzleman and blow pipeFig. 3: Wet-mix shotcrete was used on this pool because of thecongested reinforcing bar layout, as well as requiring about300 yd 3 (230 m 3)Fig. 4: The same pool shown in Fig. 3 contained over 55 tons(50 tonnes) of reinforcing bars. With an extremely tight schedule,wet-mix easily allowed placement of concrete in 5 days14 Shotcrete Summer 2018operator. In similar circumstances with dry-mix shotcrete,the speed that rebound collects can be more challengingfor the blow pipe operator to control.Wet-mix shotcrete also allows the use of more complicated concrete mixtures than dry-mix. Wet-mix shotcreteis readily available from ready mixed plants with fibers,air entrainment, high-range water-reducing admixtures,accelerators, and other forms of concrete admixtures. Drymix can include supplemental materials or admixtures, butit involves a more advanced setup, or use of prebagged,plant-produced materials. Revolution Gunite (RG) has usedsupplementary cementitious materials (SCMs) such as silicafume and has added color or fiber to its mixture design whenrequired for the job. However, it takes more planning andsetup on the job, and is more difficult than simply orderingready mixed concrete from a concrete plant.SELECTING A PROCESSSSG installs pools smaller than 1000 ft 2 (90 m2) of surfacearea with dry-mix shotcrete (Fig. 5) and pools over 1000 ft 2(90 m2) with either method. SSG does not use wet-mix shotcrete for pools under 1000 ft 2 (90 m2) because these poolscan be done in 1 day, and thus require crews to walk onfreshly placed concrete in the floor. With dry-mix shotcrete,the placed material is very stiff and walking on freshly shotmaterial disturbs just the top surface (minor footprints)without disturbing the embedded reinforcing bars. Wet-mixshotcrete tends to be more plastic when shot and walking prematurely on the surface can disturb and displacethe embedded reinforcing steel in the floor.Some finishing takes a lot of time, meaning that the shotcrete crew often chooses between one of two methods:1. Shooting the walls first and cutting the excess shotcretedwall material into the floor/wall cove and then attemptingto clean this undesirable material out from under the reinforcing steel; or2. Shooting the cove in first and then walking through thewet, freshly placed material, potentially disturbing theconcrete well into its initial set time.However, neither method is advised. It is especially difficult to properly remove ALL the shavings, rebound, andexcess concrete from under the reinforcing steel in the floor/wall cove. For strength and watertightness of the concretepool shell, it is essential that all concrete is well placed withhigh velocity from the nozzle, providing thorough compaction of the material. This is equally important for both wetmix and dry-mix. Tight spaces with a lot of congestion informwork, embeds, pipe penetrations, or reinforcing steel—such as a spa—can create excessive rebound and in mostcases the floor should be shot or cast first.SSG mostly uses its wet-mix equipment for larger jobsstarting at around 200 yd3 (150 m3) and going up to 600 yd3(460 m3) (Fig. 6). SSG typically works under tight schedules,so the ability to install a pool structure a week faster thanwith dry-mix is a huge advantage. However, before SSGstarted using the wet-mix process, it shot many large pools,including a 50 m (160 ft) pool for Brown University withwww.shotcrete.org

Fig. 5: SSG used dry-mix shotcrete for this smaller pool withabout 50 yd 3 (38 m 3) of concrete and easily completed in 1 daywith dry mix. Additionally, the steel schedule was light and witha tight jobsite it is easier to clean up a dry-mix setup than awet-mix setupFig. 6: Wet-mix shotcrete pump and on-site volumetric mixerallowed for faster production, as there was no swapping out ofconcrete trucksdry-mix equipment. This pool had multiple layers ofreinforcing bars and tricky areas. Having an experiencedACI-certified shotcrete nozzleman and an experiencedblow pipe operator, SSG was able to build an exceptionalstructure that easily passed the watertightness test.RG shoots every pool, large or small, with the dry-mixprocess. RG found that when working on multiple-dayshoots, using the dry-mix process has the added advantage of having a delivery hose that, when not conveyingconcrete material, can provide high-volume air and water.www.shotcrete.orgSummer 2018 Shotcrete 15

This high-volume air-water blast allows crews to easily pre pare the previous days’ placement, creating an optimumclean, saturated surface-dry (SSD) surface before applyingthe new concrete. RG uses on-site volumetric mixers forcontinuous batching of dry concrete materials. RG hasachieved rates as high as 110 yd3 (84 m3) per day with onegun. Though admittedly not as high a production rate aswet-mix, it is fast enough for one crew to keep up. WhenTable 1: Dry-Mix ProcessProsa higher placement rate is needed, RG simply adds anothergun and finish crew to double the production. High production is important in large commercial pools; however, mostresidential pools average 50 to 60 yd3 (38 to 46 m3) in NorthCarolina and 25 yd 3 (19 m 3) in Florida, so the region maydictate the installation method as well.RG recently transported its volumetric mixer, compressor truck, and a cement silo along with multiple loads ofaggregate and cement by barge to a remote island, off theConsLow w/cm ratio (high strength and low shrinkage)High investment cost to get started or add experienced crewsFinishers can walk on material without disturbing theplaced materialHigher level of experience needed from the nozzleman and crewFinishers can work more easily with the materialVolumetric on-site batching facilitates logisticsHigh level of maintenance for equipment and trucks when usingvolumetric mixersRequires specialized equipment to meter the material deliveries andwater must be available on site or trucked inHydration at the nozzle ensures that the concretemixture is freshLower production rates compared to wet-mixHoses are lighter and easier to manage for the crewProcuring dry aggregate and cement in remote areas where batchingfacilities are not owned by the contractor can be extremely difficultHoses do not surge, making the delivery hose layoutmore flexibleRunning long distance (up to 500 ft [150 m]) does notrequire additional procedures or concernsDry materials are easier to clean up and dispose of thanwet-mix concreteAll job logistics are typically handled by the ShotcreteContractorTable 2: Wet-Mix ProcessProsConsHigher production rates compared to dry-mixCan be more difficult to coordinate timely deliveries to place materialwithin allowed timelineEasier to get material in most localesIf problems arise on the job, stopping batching of subsequent trucksis difficultEasier to use specialized admixtures in the mixtureIf lines plug, they are difficult to clear and provide an added risk toproperty and crewMixture comes ready to pump; no on-site water neededContractors are typically at the mercy of the concrete supplier andtheir schedulesCan also use on-site volumetric mixer rather thanready mixedTends to have a higher w/cm for pumpabilityLow start-up cost relative to dry-mix process usingvolumetric mixersAdding water to the mixture to facilitate pumping is often a problemWalking on previously placed material can disturb the concrete andembedded reinforcementContractor must provide a place for cleanout of concrete trucksHoses are heavy and require more strength and endurance from thenozzlemen and crewCoarse stone in the mixture makes it more difficult on the finishers toachieve required finishesDistances from the pump to nozzle are limited by line type and size,often requiring steel pipe or larger lines to pump as far as a dry-mixhose can deliver with ease16 Shotcrete Summer 2018www.shotcrete.org

coast of North Carolina, to shoot three commercial pools.The winds that could result in barge closings, and ability tocoordinate placement on the jobsite made dry-mix shotcreteideal, even though these were larger pools with congestedreinforcing steel layouts.Another example where dry-mix excelled for RG was on aresidential project where the delivery lines had to be routedthrough the home—and the nearly completed home was worthapproximately 20 million! Because dry-mix delivery hosesare relatively low pressure and not surging from the stroke ofthe concrete pump swing tube, the crew felt more comfortableusing dry-mix. Water hoses ran through the home too, requiring RG to take special care to assure the connections were notleaking. This job could theoretically have been done with a wetmix shotcrete line; however, it would be precarious to say theleast just to protect the floors from the surging lines and in theevent of a plug and the hose break or a simple cleanout, thingscould quickly become scary and expensive.Past arguments against dry-mix shotcrete for a pool haveincluded the fact that the w/cm is not precisely measured,and injecting water with the concrete materials so soonbefore placement cannot adequately mix the concrete.However, injecting water at high pressure while the drymaterials pass through the water body completely saturatesthe materials. RG assures a constant flow of high-pressurewater to our dry-mix nozzles by using on-board hydraulicwater pumps pulling from our water tanks to get uniformwater pressure. Although RG uses flow meters to determineactual w/cm, those concerned with the precise w/cm willfind if you shoot dry-mix too wet, it simply won’t stand up.Generally, d

Unless a wet-mix concrete design uses a retarder or hydration control admixture, the batched concrete needs to be placed within 90 minutes of batching. By the time the concrete truck drives to the jobsite (hopefully not stuck in Fig. 1: Volumetric mixer truck on a pool shoot Fig. 2: T