Residential Swimming Pools_tcm45-345775

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O

nce a luxury enjoyed only by the we a l t h y, the at-home

swimming pool has become an in-c reasingly common backyard facili-ty in the past decade. Ho m e ow n e r sh a ve found that a swimming poolis as easily attainable as a secondcar. In-the-ground pools are readily financed by banks and other lend-ing institutions because a modernc o n c rete pool adds considerably toproperty value. Not only is the poola place for beneficial, healthful ex-e rcise but also it often becomes a

focus of family entertainment andsocial activity.Co n c rete is the material of choice

for a properly designed and con-s t ructed in-ground pool. No otherm a t e rial offers the same desira b l ecombination of plasticity ands t rength together with needed dura-b i l i t y.

T h e re are seve ral ways to buildc o n c rete pools: cast-in-place con-c re t e, shotcrete (also called gunite),p recast concrete and concrete ma-

s o n ry. The two most popular meth-ods are cast-in-place concrete ands h o t c re t e. Each method has cert a i na d vantages and experienced pooldesigners and builders can deter-mine the method best suited for lo-cal site conditions. This article out-lines some of the basic factors thatmust be considered by the builderand owner if the pool is to be a con-tinuing source of pride and satisfac-t i o n .

Planning the poolThe first step in planning a pool

is selecting a site and then deciding on the shape and size. Local zo n i n g and code re q u i rements should bechecked since they may have an im-p o rtant bearing on pool locationand total cost of facilities.

Provision must be made for the

floor and walls of the pool and ap a ved area around it. For the latter,a minimum width of 3 feet (5 feet isp re f e rable) is needed to provide ac-cess to the water and pre ve n tsplashes from waterlogging the sur-rounding lawn or ground. The posi-tion of existing water lines, sewe r sand drains must be established sothat they can be avoided or re l o c a t-ed when exc a vation begins.

Most swimmers prefer a sunny a rea sheltered from the wind butsome shade may also be desired to

p rotect nonswimmers from the sun.If a natural windbreak is not ava i l-a b l e, a screen wall of concrete ma-s o n ry can be used to provide shel-ter and pri va c y. The same wall may also serve to meet code re q u i re-ments for fencing the pool.

A level site is pre f e r red to a slop-ing one in order to avoid having tobackfill sloping ground or to sup-p o rt the floor and walls of the poolin some other way. A site withg round that offers uniform beari n g

s u p p o rt is also desira b l e. Sandy soilsa re most satisfactory because they a re easy to exc a vate and prov i d egood support. Although rock offerse xcellent support it is costly to ex-c a va t e.

Heavy clays or waterloggedg round call for special design tre a t-ment. Clays tend to expand when wet and can exert pre s s u re on poolsides and floor. Groundwater alsoe x e rts pre s s u re and may tend to up-lift or float the pool when it is emp-

t y.Pool shape and size are matters of

personal pre f e rence for selection within the constraints imposed by the site, construction method andbudget. Some common shapes ares h own in the box with brief notes ontheir problems and adva n t a g e s.

In general, 10 by 25 feet is consid-

Residential swimm ing poolsHow to design and build them


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e red a minimum pool size. Howe v-e r, a size of 20 by 40 feet—the sizechosen for Fi g u re 1—is popularsince it allows ample room for semi-s e rious swimming and is not ove r l y l a rg e.

Pools va ry in depth from a few inches to 9 or 10 feet. A depth of atleast 2 1/2 to 3 feet is necessary (top e rmit swimming from one end tothe other) but most swimmers pre-fer 4 to 5 feet. For divers 8 or 9 feet isre q u i red. Di ve r s, swimmers andnonswimmers can all be accommo-dated in one pool by sloping thefloor of the pool gently at first andthen sharply at one end, as shown inFi g u re 1.

Design considerations

Fundamentally the pool must be

both watertight and stru c t u ra l l y sound. These conditions will existonly if the concrete is of good quali-ty and the pool has been pro p e r l y designed for the unique set of localsite conditions and load. A pool en-t i rely below ground must be capableof resisting internal water pre s s u re when full and external earth pre s-s u re when empty. Ge n e rally the

e a rth pre s s u re should not be con-s i d e red as aiding in resisting the wa-ter pre s s u re, except perhaps for as h o t c rete lining placed againstundisturbed eart h .

Ex t e rnal pre s s u re depends on thetype and condition of the surro u n d-ing soil. If groundwater is close tothe surface it may be necessary toconsider external water pre s s u re as well as soil pre s s u re. Gro u n d w a t e routside the pool not only demandsg reater wall and floor thickness butmay also create pre s s u re sufficientto lift the pool out of the gro u n d when empty. This possibility andva rious methods of ove rcoming itmust be considered by the designer.Se ve re conditions may dictate locat-ing the pool above gro u n d .

In large expanses of concre t e,

t e m p e ra t u re changes and dry i n g s h rinkage may cause cracking. Thisp roblem is ove rcome by prov i d i n g sufficient re i n f o rcing steel to keepc rack openings negligible and by us-ing carefully designed and pro p e r l y spaced joints. Use of a high-quality c o n c rete with good moist curing al-so reduces the tendency to cra c k .

Since loss of water by leakage is

one of the pri m a ry concerns of swimming pool owners it is part i c-ularly important to provide jointsthat will retain water under pre s-s u re. The best method is to use wa-terstops cast in the concrete and joint sealants located on the waterside so that the head of water tendsto force the sealants into the jointinstead of out of it. For more infor-mation on joint location, types andinstallation methods see Re f e re n c e1 .

Some smaller pools are designedto give good service without joints.The pool i llustrated in Fi g u re 1 hasno expansion or contraction jointsbut does have construction joints atthe base of the wall. The layo u ts h own, one of an infinite number of p o s s i b i l i t i e s, was chosen as a pre-

l i m i n a ry guide to the designer fors e ve ral re a s o n s :

• The size of 20 by 40 feet is ample,although many residential poolsa re even smaller. Larger pools re-q u i re special joints in walls andfloor as well as more elabora t e water filtration and circ u l a t i o ne q u i p m e n t

Figure 1. One of many possible layouts to serve divers, swimmers andnonswimmers with one moderate-size pool. The slope of the floor may beadjusted to fit individual requirements.


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• The hopper-shaped floor at thedeep end effectively reduces themaximum wall height. Even on aslope like this, floor slabs are usu-ally cheaper to build than ve rt i c a l w a l l s. Low walls are subjected tol ower pre s s u res and thus can besmaller in cross section than high w a l Is.

• A rectangular shape and pri s m a t-ic slab arrangement generally af-f o rd economy in layout, designand construction. Howe ve r,rounded corners and gently c u rved surfaces are better fors h o t c rete pool constru c t i o n .

Concrete quality

An early decision should be madeon quality concrete specificationsfor the pool. Recommendations of ACI Committee 350 for watert i g h t ,c h e m i c a l - resistant concrete (Re f e r-ence 2) offer some good guidelines.The 28day compre s s i ve stre n g t hshould be at least 3500 psi wherec o n c rete is not exposed to seve reand frequent fre ezing and thawing or 4000 psi where seve re and fre-quent fre ezing and thawing are an-ticipated. St ru c t u ral considera t i o n smay re q u i re greater strength. The w a t e r-cement ratio should not beg reater than 0.45 and slump should

be between 1 and 4 inches. A i r- e n t rained concrete is re c o m-mended for all p ools. Air entra i n-ment improves the workability of f resh concrete and the watert i g h t-ness of hardened concrete as well asits resistance to fre ezing and thaw-ing. Air content should be betwe e n4 and 7 percent, the optimumamount depending on grading of the coarse aggre g a t e.

Im p e rmeability of the concrete isi m p roved by slow moist curing. This

should be specified to obtain bestq u a l i t y.

Excavation and subgradepreparation

Once the necessary permits havebeen secured or the plans approve dl o c a l l y, construction can pro c e e d .L a yout for rectangular pools is sim-p l e, re q u i ring only stakes at corn e r s

and perhaps along the pool centerl i n e. Oval or irregular pools will re-q u i re stakes at frequent interva l sa round the peri m e t e r. There shouldbe some bench marks outside thee xc a vation to serve as perm a n e n tre f e rence points from which loca-tions and exc a vation depths can bem e a s u re d .

Exc a vation in rock is costly but, if rock exists over the entire site, thefoundation will be extremely solidand satisfactory. If rock exists ove ronly part of the pool site, the siteshould be exc a vated to at least 6inches below the desired pool bot-tom level and then backfilled with atleast 6 inches of we l l - c o m p a c t e dg ranular material to act as a cush-ion. This is necessary to reduce thepossibility of a crack occurring inthe floor slab due to uneven settle-

m e n t .O ve re xc a vation of pool sides does

not present a problem when wallsa re cast in place. It must definitely be avoided when they are shotcre t-ed since the exc a vated surface willnot only support the freshly placeds h o t c rete but also determine thepool contours. Since ove re xc a va t i o nof the pool bottom is undesirable re-g a rdless of the type of constru c t i o n(because the bottom slab can beplaced best on solid, undisturbed

soil), final exc a vation and shaping of the inve rt by hand are re c o m m e n d-ed. In the event of ove re xc a va t i o n ,the cavity should be backfilled withc o n t rolied density fill (Re f e rence 3),a well-compacted granular materi a lor lean concre t e.

Adequate subsurface drainage isessential, particularly in clay subjectto a seasonally fluctuating gro u n d- water level. Where subsoil dra i n sve ry slowly it is advisable to placethe pool on a compacted bed of c rushed rock or sand and gra ve l .

Minimum subsoil drainage wouldbe a line of drain tile around theoutside of the wall footing and a lineunder the deepest portion of thepool. Larger pools re q u i re lines laidunder other parts of the slab. Co n-s t ruction for these lines is similar tothat for drain tile laid around the

foundation of a house—with open j o i n t s, a minimum slope of 1 in 50and trenches backfilled withc rushed stone or gra vel. Thisd rainage is re l a t i vely easy and eco-nomical to install early in constru c-tion. Ex p e rienced designers prov i d eit even in pools which do not de-mand it because it provides low -

cost insurance against futurebuildup of water pre s s u res and con-sequent permanent damage to thep o o l .

Floor construct ion

Re g a rdless of wall type, the firststep in building the pool is t o castthe floor slab together with any re-q u i red wall or buttress foundation.Re i n f o rcing steel is set according tothe engineer’s drawings and sup-p o rted the correct distance above

g round on small concrete pads,pieces of concrete block or conve n-tional bar support s. It is necessary to maintain the position of the steelc a refully to ensure the designeds t ru c t u ral action and the necessary amount of concrete cover (over thebars) pre venting corrosion. Ve rt i c a ld owels which connect walls or but-t resses to the foundation should beset at the same time as the steel forthe slab.

Co n c rete should be worked into

position around the re i n f o rc e m e n tand thoroughly compacted. Itshould then be floated and trowe l e das re q u i red to obtain a good finishin much the same way as for a re g u-lar floor slab. The type of f inish useddepends on whether the pool is tobe painted or otherwise coated.Moist curing for 7 days is re c o m-mended. Co n s t ruction of the pool walls can begin after the floor hash a rdened and before final curing isc o m p l e t e.

Cast-in-place w alls

Ca n t i l e ve red cast-in-place wallsa re commonly used for small pools.Fi g u re 2 illustrates such a wall.

Co n s t ructi on of cast-in-place walls begins with setting the outsidef o rm s. Then the ve rtical steel is tiedto the dowels projecting from the


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footing and the hori zontal steel tiedto the ve rtical bars. Just as for thes l a b, correct positioning of the steelis important for stru c t u ral actionand corrosion protection. Pl a s t i cspacer devices should be slippedover the bars at sufficient interva l sto keep the bars at the re q u i red dis-tance from the form s.

Next, inside wall forms are ere c t-ed and either tied to the outsidef o rms or braced externally on bothf o rm faces. The form w o rk must bestiff and strong because plastic con-c rete exerts considerable pre s s u re,p a rticularly at the base of the wall,and if the forms move during con-c rete placement there is virtually no way of forcing them back to the cor-rect position. The result will be anunsightly bulge in the swimming pool wall.

If form ties are used they shouldbe of a pre f a b ricated type with wa-t e r- s t o p s. Ties should not be pulledf rom the hardened wall but bro k e nback or disconnected so that theends of tie metal remaining in the wall are at least 1 1/2 inches back f rom the wall face. Also, a good, well-bonded and carefu lly cure dpatch over the tie ends is essential to

maintaining the watertightness andd u rability of the pool.

Co n c rete should be placed in the wall fo rms at or near its final posi-tion so that it does not have to flow l a t e rally into place, causing segre g a-tion. Ac c o rding to ACI re c o m m e n-dations (Re f e rence 4) the best re-sults will be obtained by placing the

c o n c rete in hori zontal layers not ex-ceeding 2 feet in depth to avoid in-clined layers and cold joints. Fo rmonolithic construction each laye rshould be placed while the underly-ing layer is still re s p o n s i ve to vibra-tion; layers should be shallow enough to knit together when pro p-erly vibra t e d .

Ca reful compaction by intern a lv i b ration, spading or external vibra-tion is essential to obtaining a good-q u a l i t y, impervious wall. Ca re f u l

c o n t rol of the placement rate is alsoi m p o rtant to avoid overloading thef o rm s.

Shotcreting

Sh o t c rete swimming pools mustbe built by contractors who special-

i ze in this pneumatic application of c o n c re t e. Skilled equipment opera-tors are needed to produce a shot-c rete l ining of uniform thickness with proper cover of re i n f o rc e m e n t . A l s o, since the re i n f o rcement is sup-p o rted on the exc a vated surf a c e

which establishes the desired shapeof the pool, the final exc a vation re-

q u i res accura c y. Where the soil canbe shaped readily to the desire dcontour and retain its shape untilthe concrete has hardened, shot-c rete pool construction is satisfacto-ry and economical.

Sh o t c rete equipment consists of ac o m p re s s o r, a mechanical feeder,one or more long hoses and a noz-z l e. In the dry mix process the ce-ment and aggregate (usually sand)a re mixed dry and put in a pre s s u rec h a m b e r; there the mix is forc e d

t h rough a hose to a special nozzle in which water is introduced. The re-sulting high-velocity jet of fre s hm o rtar is directed onto the surf a c eto be concreted. The operator mustk n ow how to locate and direct then ozzle to achieve complete cove r-

Figure 2. One common way of buildinga cast -in-place concret e pool makesuse of a cantilevered wall whose baseis cast internally with the pool floorslab. The same design can beadapted for continuous placement,

eliminating t he construction joint.

Shotcrete pool construction. Mort ar isgunned in a thick coat over previouslypositioned steel reinforcement.

Excavati ng must be shaped carefully tosupport the reinforcement and mortarto the desired contours but little or noformwork is needed.


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age of the re i n f o rcement. He mustalso clear away all rebound (aggre-gate particles that ricochet off thes u rface) before it is cove red withm o rtar to pre vent undesirable sandp o c k e t s.

In the wet mix process water ismixed with the cement and aggre-gate before the mix is introduced in-to the pre s s u re chamber. Co m-p ressed air is used to force the mix t h rough the hose and additionalc o m p ressed air is introduced at then ozzle to boost ve l o c i t y.

Typical shotcrete pool constru c-tion is shown in Fi g u re 3 and the

p h o t o. Ge n e rally the wall thicknessis built up by shotcreting seve ral lay-e r s. Also, shotcrete linings have aro u g h t e x t u red surface that usually must be smoothed with a hand floatb e f o re the concrete hard e n s. Co m-

p re h e n s i ve recommendations re-g a rding good materials and work-manship for shotcreting will befound in Re f e rence 5.

Pool finishes

Paint, portland cement plasterand tile are the three most commonfinishes for swimming pools, eacho f f e ring the owner a selection of vi-

b rant colors for the poolscape.Painting is the most economical ini-tially but invo l ves periodic re n e w a l . Vinyl, chlorinated ru b b e r, epox y and portland cement paint as we l las other coatings can be used.

Po rtland cement plaster is anoth-er common pool finish which, if p roperly applied, will result in a

smooth, waterproof, nonskid sur-face finish with practically unlimit-ed life. Although white portland ce-ment plaster is ve ry popular, a widerange of permanent colors is ava i l-a b l e, using either colored cementsor added pigments in the plasterm i x .

Since tile is costly its use in re s i-dential pools is frequently limited toa band around the pool above the water line with either paint or plas-ter below. Tile joints must be care-

fully sealed to keep water from get-ting between the tile and the wall.Both tile and plaster re q u i re aroughened wall surface for goodb o n d .

Pool surroundings andother aspect s

For full enjoyment, since peopleusually spend as much time aro u n dthe pool as in the water, the sur-rounding area should be paved andlandscaped. Cast-in-place concre t e

paving can be given decora t i ve ex-p o s e d - a g g regate or textured finish-es such as those described in Re f e r-ence 6.

The coping around the top of thepool is usually precast from white orsuitably colored concrete andshaped so that water will flow away f rom the edge of the pool. The tops u rface should be nonskid.

A nonskid flatwork strip 3 to 5 feet wide around the edge of the pool isn e c e s s a ry to keep the pool cleanand serve for walking and diving. Al-s o, flagstones, patio block or what-e ver materials are used to pave thes u r rounding deck may be carried tothe pool edge with an overlap of about 3/4 inch, thus eliminating theneed for a standard coping.

The systems necessary for filter-ing, purifying, re c i rculating and, insome locations, heating the pool

The popular kidney-shaped pool can be modified to make good use of availablespace.

This small free-form pool was built wit hin a modest budget. The surroundingexposed-aggregate concret e deck was c arried to t he edge, eliminat ing the needfor precast coping.


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water are beyond the scope of thisa rt i c l e. Howe ve r, Re f e rence 7 offersindepth information on all aspectsof layout, design and constru c t i o n ,including water treatment and oth-er serv i c e s, for both large and smallp o o l s. Re f e rence 8 is also ve ry use-ful—and was a pri m a ry source of in-f o rmation for this article along with

the re f e renced ACI documents.

REFERENCES

1. “Leakproof J oints in Sw immingP ools,” Co ncrete Co nstruction, J uly1972, pages 329331 .

2. ACI Committee 350, “ConcreteSanitary Engineering Structures,”J ournal of the America n Co ncrete In-stitute, August 1971, pages 560577;reprinted in ACI Manua l of P rac tice ,Part 2, 1974.

3. B rewe r, Willia m E., “ The End of th eBa ckfill Problem,” C oncrete Co nstruc-tion, October 1975, page 448.

4. Recommended Practice for Mea-suring, Mixing, Trans porting a nd P la c -ing C onc rete (ACI 304-73), America nCo ncrete Institute, Detroit, Michiga n,1973, 40 pages .

5. Recommended P rac tice for Shot-c reting (ACI 506-66; reaffirmed 1972),America n C onc rete Institute, De troit,

Michigan, 1972, 26 pag es.

6. P ana rese, William C., Ceme nt Ma-so n’s Guide to Building Co ncreteWalks, Drives, P atios a nd S teps, P ort-land C ement Ass oc ia tion Skokie, Illi-nois, 1973, 64 pag es.

7. Perkins, Philip H., Swimming Pools,Applied Sc ience P ublishers Limited,London, England , 1971, 358 pag es.

8. Sma ll Sw imming P ools, Cementand Concrete Association of Australia,

S ydne y, New S outh Wales , Australia ,1963, 16 pages .

PUBLICATION #C770313Co py right © 1977, The Ab e rdeen Gro u p

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