Workmanship Issues In Swimming Pool Wet-Mix Shotcrete .
Workmanship Issues inSwimming Pool Wet-MixShotcrete Constructionby Ron LacherSeveral months ago, I ran across a briefarticle in a swimming pool trade publi cation co-authored by two shotcretecontractors who presented what they believedto be proper wet-mix shotcrete application inswimming pool construction. In every constructiontrade, including swimming pool construction,it’s not uncommon for contractors to have differentopinions as to what’s the right way and wrongway to do things and often there is room forthese differing opinions in trade practices.But, when we get into the realm of the back boneof the structure, which in this case is the rein forcement and concrete that make up the pool’sstructural shell, we must look past contractors’opinions and focus instead on standards of practiceestablished by a recognized organi zation, suchas the American Concrete Institute (ACI), andadopted building codes.Although the previously referred to article wasmostly accurate, several statements were made thatencourage a common practice in swimming poolwet-mix application that, in my opinion, is not inconformance with proper ACI concrete placementpractices. The parts of the article I want to discussIn swimming pool wet-mix shotcrete application, it iscommon practice to shoot the pool walls first while leavingthe floor at the base of the wall open. Note the overspray,rebound, and voids under the reinforcing bar12are the statements “trimmings can be recycledif used in the right way” and “you can placetrimmings in the floor.” The authors made furtherclarification with the statement, “vibrate orproperly compact it for adequate steel encapsu lation.” Although these statements are technicallycorrect when discussing concrete placement ingeneral, the required procedures necessary topermit the recycling of trimmed material froma shot wall in swimming pool construction areseldom followed in practice.What gives me the greatest concern is thatthese statements could be construed to validatethe common practice in swimming pool wet-mixshotcrete application of shooting the pool wallsfirst while leaving the floor at the base of thewall open. After shooting the walls, the typicalpractice is to then cut the walls and allow thetrimmings to fall to the unshot floor at the baseof the wall where, according to the article coauthors, “trimmings can be recycled if used inthe right way.”Concrete consolidation and reinforcementencapsulation are critical elements of concreteplacement in structural concrete. Freshly placedHere’s the same wall (as in the first photo) after trimming.No joint preparation or removal of overspray or reboundwas doneShotcrete Spring 2007
unconsolidated concrete is in a honeycombedcondition and contains excessive and detrimentalentrapped air voids. If allowed to harden in thiscondition, the concrete will have low strength,high permeability, poor resistance to deterior ation,and a lack of proper bond to reinforcement.The major difference between shotcrete andconventional concrete in a structural applicationis the method by which consolidation and rein forcing bar encapsulation is obtained. Shotcreteis consolidated by the impact of the material,traveling at high velocity, on the applicationsurface. Trimmed shotcrete, because it’s beendisturbed, can no longer be considered consol idated, especially after falling up to 8 or 9 ft(2.4 to 2.7 m) from its shot location and landingon the pool floor. The slump of wet-mix shotcreteis normally between 1-1/2 and 3 in. (38 to 76 mm),classified as “stiff plastic” by ACI. Accordingto ACI 309R-05, “Guide for Consolidation ofConcrete,” manual methods of consolidation,that is, hand tamping, rodding, or spading,are not appropriate for stiff plastic concrete.According to ACI 301, “Specifications for Struc tural Concrete,” Section 5.3.2.5, Consolidating,states, “Consolidate concrete by vibration.”Simply stated, concrete placed in a structuralapplication must be vibrated by mechanicalmeans to obtain proper consolidation and rein forcing bar encapsulation.In my many years as a licensed engineer, whichincludes periods as a pool builder and deputyshotcrete inspector, I’ve observed shotcreteplacement in swimming pools all over thecountry. Throughout all of those observations, Ihave rarely seen trimmings properly processed,vibrated, or compacted to obtain the requiredconsolidation and encapsulation of rein forcement.A common practice is to hose down the piles oftrimmings with water and drag the “now overwetted” and re-tempered trimmed materialto envelop the floor reinforcement. The maincompaction method used is simply the shotcretecrew walking over the newly spread trimmings.The practice of incorrectly using trimmings inthe floor is especially widespread in swimmingpool wet-mix shotcrete construction. Because ofthis, it is my opinion that this practice should bediscouraged unless the shotcrete crew has beentrained in the requirements for proper structuralconcrete placement.A fact that I repeatedly stress in the seminars Igive to the pool industry on the shotcrete processis that shotcrete is not a special product; it issimply a method of placing concrete. Therefore,many of the recommended practices for concreteplacement by other methods apply. Trimmedmaterial dislodged from its shot location shouldbe treated as conventional concrete. Based on this,Shotcrete Spring 2007The typical practice is to cut the walls and allow thetrimmings to fall to the unshot floor at the base of the wallTrimmed shotcrete is no longer consolidated after fallingfrom its shot location and landing on the pool floorThe trimmed concrete typically lands at the base of the wallon top of rebound and overspray13
It’s common to hose down the piles of trimmings with water and dragthe “over wetted” and re-tempered trimmed material to envelop thefloor reinforcementThis trimmed material sat in a pile at the base of the wall forover 90 minutes. Remember, concrete consolidation andreinforcement encapsulation are critical elements of structuralconcrete placementI’ll offer eight reasons why the reuse of trimmingsin swimming pool construction should notbe encouraged.1. In swimming pool structural design, thelocation in the pool where stresses are thehighest is at the base of the wall where the wallmeets the floor. This is the most critical locationin the entire pool for proper shotcrete appli cation. The common wet-mix practice of usingtrimmings where they fall, that is, at the baseof the wall, places the lowest quality concreteat the location of highest stress. This is thevery worst place for the placement of recycledtrimmings that are not properly consolidated.2. In real-life swimming pool wet-mix appli cation, when walls are shot before the floor,14cut trimmings will fall the depth of the pool,often a vertical distance as much as 8 to 9 ft(2.4 to 2.7 m). The trimmed concrete doesnot fall freely to the floor but rolls or slidesdown the wall until it hits the uncoveredreinforcement at the base of the wall.Section 5.4.1 of ACI 304R-00, “Guide forMeasuring, Mixing, Transporting, and PlacingConcrete,” cautions:“Arrange equipment so that the concretehas an unrestricted vertical drop to thepoint of placement or into the containerreceiving it. The stream of concrete shouldnot be separated by falling freely overrods, spacers, reinforcement, or otherembedded materials. If forms are suffi ciently open and clear so that the concreteis not disturbed in a vertical fall into place,direct discharge without the use ofhoppers, trunks or chutes is favorable.”The concrete trimmed from the wall doesnot fall freely into final position but is disturbedas it slides and roles down the wall. Thispractice is clearly discouraged by ACI 304R.3. The trimmed concrete typically lands in a pileat the base of the wall, falling over and throughthe reinforcement. But, what has fallen first atthe base of the wall before the wall is trimmed?Rebound and overspray! Unless removedbefore the trimmed concrete falls, the reboundand overspray will be incorporated into thework. This is a clear violation of propershotcrete application. The picture used in theswimming pool trade publication actuallyshowed the overspray on the reinforcement andthe rebound at the base of the wall beingcovered by new shotcrete.4. After falling, the material trimmed from thewall typically sits in piles at the base of thewall. The piled recycled concrete is subse quently dragged, pushed, or in some mannermoved laterally and spread over the floor toenvelop the floor reinforcement. Section 5.4.1of ACI 304R-00, “Guide for Measuring,Mixing, Transporting, and Placing Concrete,”cautions “Concrete should be deposited ator near its final position because it tends tosegregate when it has to be flowed laterallyinto place.” The lateral movement of the pilesof trimmings is a practice clearly discouragedby ACI 304R. And, what about the likelihoodof poor reinforcing bar encapsulation resultingfrom this practice?5. Much of the concrete used for wet-mixshotcrete is delivered to the job site in readymix trucks. The American Society for TestingMaterials (ASTM) Standard Specificationfor Ready Mix Concrete C 94 requires thatconcrete be completely discharged withinShotcrete Spring 2007
1 1/2 hours of the introduction of water intothe cement and aggregate. This time limitis based on the fact that concrete that iskept agitated generally can be placed andcompacted within 1 1/2 hours after mixing.Further verification can be found in ACICCS-4, “Shotcrete for the Craftsman,” whichstates, “Wet-mix should typically be usedwithin 90 minutes of the first water addition.”In real life swimming pool wet-mix appli cation, is it realistic that concrete will betransported, shot, attain sufficient stiffness fortrimming, then be trimmed, relocated, andconsolidated (vibrated) all within 90 minutes?In my opinion, the answer is no. I recentlyobserved a wet-mix application in a swimmingpool where the trimmed material sat in a pileat the base of the wall, unconsolidated andenveloping the reinforcement for over90 minutes after trimming before the materialwas placed in the floor.6. What about the bottom of the shot wall wherea joint is created with the questionable trimmedmaterial placed in the floor? In Chapter 19,Shotcrete, Section 1924.7, Joints, of the Inter national Building Code (IBC), it states:“Except where permitted herein, unfinishedwork shall not be allowed to stand for morethan 30 minutes unless all edges are slopedto a thin edge. Before placing additionalmaterial adjacent to previously appliedwork, edges shall be cleaned and wetted.”The American Shotcrete Association (ASA)recommends that the slope referred to in IBCSection 1924.7 be no thinner then approxi mately 45 degrees to avoid the possibility ofpeeling-type delaminations.As with the other recommended concreteplacement practices I’ve discussed, I haverarely observed the base of a shot wallsloped, then cleaned and wetted before thematerial trimmed from the wall fell onto thejoint. Another important point is thatshotcrete is recognized for its superior bondingabilities brought about by its high impactvelocity on the receiving surface. Yet, ques tionable trimmed material is often placedagainst the joint at the base of the wallrather than shot material, leaving the integrityof the bond and the joint at this most criticallocation in question.7. In many areas of the country, a swimmingpool must be constructed in accordance witha structural plan prepared and stamped by alicensed engineer. Preparation of swimmingpool structural plans is the primary focusof my firm, and last year we provided thestructural plans for almost 16,000 swimmingpools. I’m also familiar with most of theShotcrete Spring 2007Joints standing for over 30 minutes must be cut to a45-degree angle then cleaned and wetted beforeplacing additional shotcreteHere’s a typical joint that stood for over 30 minutes—noticethe overspray and rebound. No cleaning and wetting tookplace before placing new materialShoot the cove and the floor at the base of the wall before thewall is shot to eliminate rebound, overspray, and trimmingsfrom becoming trapped at this critical location15
other engineers who provide this service.Generally, the specifications on these structuralplans require pneumatically placed concretebecause of the unique benefits and propertiesof concrete brought about by pneumaticplacement. As I’ve discussed, trimmings canno longer be considered as pneumaticallyplaced concrete. Therefore, permission of theengineer of record must be obtained beforevariations from the specified pneumaticallyplaced concrete could be used. This is justone more reason why trimmings should notbe reused except for non-structural elementsin the swimming pool.8. ACI 506R-90, “Guide to Shotcrete,” sums it allup where, in Chapter 7, Section.3, it states:“Reuse of discarded shotcrete materialshould not be tolerated.”What is the answer to the elimination of thesepoor practices that are so widely observed inthe swimming pool industry? The answer isobviously education, training, and certification.Should we encourage the use of trimmingseven when qualified with the statement “whenproperly processed” when the majority ofswimming pool shotcrete applicators don’t haveany idea what “properly processed” means?Remember the point made in No. 1: The locationin the pool where stresses are the highest is at thebase of the wall where the wall meets the floor.This is the most critical location in the entire poolfor proper shotcrete application. In my opinion,until a higher percentage of swimming poolwet-mix applicators catch up to the knowledgeof the rest of the industry, we should not beencouraging a practice that will likely result insubstandard workmanship.16In the meantime, the standard of practice forswimming pool wet-mix application should be:a) shoot the cove and the floor at the base ofthe wall before the wall is shot to eliminaterebound, overspray, and trimmings from becomingtrapped at this critical location; b) trimmingsmay be reused only if still fresh and plastic butwith out re-tempering or the addition of water; andc) trimmings may be reused only if vibrated bymechanical means for proper consolidation andreinforcing bar encapsulation.Ron Lacher, PE, CBP, is awell known speaker, edu cator, and author on propertrade practices and structuralengineering in swimmingpool construction. His firm,Pool Engineering, Inc.,provided the structuraldesigns for over 16,000 pools last year. Withexperience from prior ownership of a swimmingpool construction company, he has completedover 300 forensic investigations on problempools. He is a member of the Advisory Boardof the National Pool Industry Research Centerat California State Polytechnic University,San Luis Obispo, and is also a member of theAmerican Shotcrete Association where he ison the Pool & Spa Committee. Lacher graduated from California State Polytechnic University with a degree in civil engineering andis a Licensed Professional Engineer in theState of California.1/8 ad1/8 adCircle #x on reader response form—page xxCircle #x on reader response form—page xxShotcrete Spring 2007
of ACI 304R 00, “Guide for Measuring, Mixing, Transporting, and Placing Concrete,” cautions “Concrete should be deposited at or near its final position because it tends to segregate when it has to be flowed laterally into place.” The lateral movement of the piles of trimmings is a practice clearly discouraged by ACI 304R.
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