What the Floor Designer Should Know_tcm45-345520

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Economical design of floors isonly possible to the extent that

the designer understands such as-pects as floor thickness, concre t es t rength, su rface hardness and ac-comm odation of shri n k a g e. Hemust be abl e to satisfy the perf o r-manc e re q u i rements of the floorf rom a realistic appraisal of thesefactors and any special other re-quirements.

T h e re are significant differe n c e sin matters of design for load capaci-ty between floors on grade andthose above grade (known as stru c-t u ral floors). These will be pointedout as needed .

Thick ness requirement s

The thickness re q u i rements of floors on grade are established onthe basis of what is needed to re s i s tthe bending loads to which they aresubjected with a practical minimumof cracking. Su b g rade support plays

an important role in the reaction toloading of slabs on gra d e. For many o rd i n a ry floors on grade thicknessescan be arbitra rily taken from re c o m-mendations based upon experi-e n c e. For example, residential floorsa re normally at least four inchesthick. Floors for offices, schools andhospitals shou Id be at least four tosix inches thick. Floors for re s i d e n-tial garages or light industry must beat least five inches thick. In d u s t ri a lf l o o r s, su bject to heavy loads and

we a r, should be six to 20 inches.Fu rther details are in ACI 302 St a n-d a rd, “Recommended Practice forCo n c rete Floor and Slab Co n s t ru c-t i o n .” *

The thickness re q u i red for slabson grade to be subjected to wheelloads can be read from a chart pub-lished in the ACI 302 St a n d a rd. Thisg i ves data for concretes of va ri o u s

s t re n g t h s, wheel loads up to 20,000pounds and wheel areas up to 80s q u a re inches. The chart is based onthe assumption that the subgra d esoil is of ave rage modulus and en-t i rely uniform, that the slab is ade-quately thickened at the edges ork e yed into the adjacent slab andthat impact factors have beenadded. For unusually heavy floorloads or for unusual soils the soilmodulus should be determined by asoils testing labora t o ry and a sepa-rate design calculation made.

W h e n e ver the data exceed thelimits of the chart in the ACI 302St a n d a rd the thickness may be de-t e rmined by re f e rence to “Design of Co n c rete Airport Pa ve m e n t ,” by Ro b e rt G. Pa c k a rd, (available fro mthe Po rtland Cement Association,Old Orc h a rd Road, Sk o k i e, I llinois60076 for $1.50). This method isuseful for situations that call forthicknesses greater than eight inch-

e s.Data for use when uniform loadsrather than wheel loads are the cri t-ical factors can be read from Table I.Guidance for design can be ob-tained from “Design of Co n c re t eFloors on Ground for Wa re h o u s eL o a d i n g s,” by Paul F. Rice, Jo u rnal of the American Co n c rete In s t i t u t e, August 1957, pages 105-113.

St ru c t u ral floors are designed asslabs re i n f o rced in more than oned i rection. They must conform to the

re q u i rements of the ACI Bu i l d i n g Code (ACI 318), and they must con-tain the amount of re i n f o rc e m e n tneeded and be properly placed fors t ru c t u ral adequacy. Until re c e n t l y the designer was not permitted toinclude the thickness of a floor top-ping, even if bonded to the base, asp a rt of the design thickness. Now,h owe ve r, it is permissible to include

it under some circumstances wherecomposite action is assured as setf o rth in Section 8.9 and Chapter 17of the Co d e.

Strength requirements

St ru c t u ral floors are designed, ac-c o rding to many pro c e d u re s, on thebasis of flexural strength. The com-p re s s i ve stresses in floors are smallc o m p a red to tensile stresses and inmany design pro c e d u res the tensiles t rength is used. The concrete mixesused for floors, howe ve r, are speci-fied partly in terms of compre s s i ves t rength. The reason for this is thatc o m p re s s i ve strength is widely usedas a measure of concrete quality andcan be used as a guide to the rate at which the body of concrete is in-c reasing in hardness during curi n g and later. For any given combina-tion of materi a l s, compre s s i ves t rength is directly related to flexur-al tensile strength and to wear re s i s-

t a n c e. Under normal conditionshigh compre s s i ve strength usually means long we a r.

Co m p re s s i ve strength by itself,h owe ve r, has not been considere dcompletely adequate as a guide tomix design. The concrete used infloors must not only be pro p o r-tioned to develop adequate stre n g t hbut it must contain the ri g h tamount of coarse and fine materi a lto provide good workability duri n g placing and finishing and to pro-

duce a hardened matrix that isdense and low in perm e a b i l i t y. Un-dersanding makes the mix difficultto work but oversanding re q u i re sm o re cement and more wat er andreduces the pro p o rtion of coarse ag-g regate part i c l e s, which are neededfor wear re s i s t a n c e. For this re a s o nrecommendations given in one of the mix pro p o rtioning methods of

What the floor designer should k now Besides load capacity the designer must provide for wear resistance,accommodation of changing length and width, and incorporation of auxiliary and special purpose materials



the ACI 302 St a n d a rd link theamount of c ement to be used andthe compre s s i ve strength together.Both re q u i rements must be satis-fied. In the altern a t i ve method inthe ACI 302 St a n d a rd it is possible touse concrete mixes with lower ce-ment content provided it can bes h own that they produce concre t e

with acceptable finishing pro p e r-t i e s, dura b i l i t y, surface hard n e s sand appearance as well as stre n g t h when evaluated by methods setd own in the St a n d a rd .

Surface hardness

The surface hardness needed inany floor depends on the kind of t raffic to which it will be subjected.The hardness of the surface will de-pend on the strength of the concre t emix, the timing of the finishing op-

e ra t i o n s, the number of steel trowe l-i n g s, the adequacy of the curing andh ow soon the curing is begun.

Residential floors and those to bec ove red with ti le (other than vinyl)re q u i re only a medium steel trowe lfinish free of trowel marks andlumps that might reflect through thet i l e. Exposed surfaces in offices,schools and hospitals re q u i re a goodsteel trowel finish but they may alsoneed a special colored, static-dis-seminating or nonslip finish. No n-

slip surf a c e s, often desirable onc o m m e rcial or residential gara g ef l o o r s, or on industrial floors subjectto pneumatic tire traffic, can be con-veniently produced by lightly b rooming the surface after trowe l-ing has been completed. Such floorsa re harder to clean than those withsmooth surf a c e s.

A sl ight swirl finish made by thet rowel can provide some re s i s t a n c eto slip. For highly abra s i ve we a rf rom steel or plastic wheel traffic onsingle-course floors it is desirable touse a special we a r- resistant aggre-gate or metallic or mineral powd e r s,as discussed in a previous art i c l e’and the ACI 302 St a n d a rd. For thee x t remely heavy duty floors whichre q u i re especially high resistance toa b rasion from steel wheel vehicles itis common to build two-course

f l o o r s. In these the bottom course isl e veled by screeding but left ro u g hto enhance the bond of the topa b ra s i o n - resistant course which isplaced later.

Accommodating shrinkage

The need to accommodates h rinkage and sometimes expan-

sion was explained in a previous ar-t i c l e.2 Floors on gra d e, if made withs h rinkage-compensating cementsor post-tensioned in both dire c-t i o n s, do not re q u i re joints. The de-sign of such floors is a special sub-

ject. In stru c t u ral floors joints arenot used except for expansion jointsb e t ween parts of a building that ares t ru c t u rally independent of eacho t h e r.

For all other floors joints must bep rovided to accommodate move-ment without producing unwantedc ra c k s. When cracks develop in

floors they are almost impossible torepair without some degree of dis-f i g u ration. Jo i n t s, howe ve r, usually a re not objectionable and can eve nbe esthetic, and they lend them-s e l ves to effective maintenance.

TABLE 1 Slab thicknesses and steel recommended for slabs onground for various uniform loads Adapted from CRSI DesignHandbook, page 14 -11 .

The design of slabs on ground to distribute either uniform or concen-trated loads involves the elastic properties of the subgrade and the

slab itself. An analysis can be made but it is quite involved.Uniform Loads, Slab

pounds Th ickness, Reinforcement* * “per squar e foot * i nc hes

One layer 6X6 10/ 10 welded wire fabricLess than 100 4 minimum for ideal conditions; 6X6

8/ 8 for average conditions

100—200 5 One layer 6X6 8/ 8 welded wire fabricor one layer 6X6 6/ 6

Not over 400—500 6 One layer 6X6 6/ 6 welded wire fabric

or one layer 6X6 4/ 4600—800 6 Two layers 6X6 6/ 6 welded wire fabric

or two layers 6X6 4/ 4

Two mats of bars lone top, one bottom)1500** 7 each of No.4 bars at 12 inches center

to center each way

Two mats of bars (one top, one bottoms2500** 8 each of No. 5 bars at 12 inches center

to center each way

Two mats of bars (one top, one bottom),3000—3500** 9 each of No. 5 bars at 8 to 12 inches center

to center each way

* Fill ma teria l a nd co mpa ction sho uld be eq uivalent to that of ordinary highw a ypractice. If lab oratory control of co mpa ction is a vailab le, the loa d c apa cities c anbe increased in the ratio of the a ctual co mpac tion c oefficient, k, to loo. For wheelloads calculate equivalent moments.

** For loa ds in exces s of 1500 pound s per sq uare foot the subs oil co nditionsshould be investiga ted w ith extra ca re.

*** P lace the first layer of reinforce ment tw o inches b elow top o f slab a nd se co ndlayer two inches up from bo ttom of slab.



Floors on grade seldom have toomany joints and often contain toof e w. Ca reful attention to the spac-ing of joints is of prime import a n c eto maintain the usefulness of thefloor and minimize maintenancec o s t s.

When we l d e d - w i re fabric is usedin floors its purpose is to hold the

c racks tightly together and to pro-vide better load transfer acro s sc ra c k s. It is necessary, howe ve r, thatit not be allowed to inhibit the open-ing of joints. For this reason caremust be taken to see that the we l d e d

w i re fabric is not continuous acro s sthe joints except at some dummy j o i n t s. When laid continuously itmust be cut at eve ry constru c t i o n

joint or else designed with a suffi-cient amount of steel to hold con-s t ruction joints together.

Design incident als

T h e re are a number of re l a t e dmatters that have some effect on thedesign. Co n c rete made from leanmixes may permit exc e s s i ve tra n s-mission of water vapor through any slab under which water has man-aged to accumulate. Usually the useof concrete containing a minimumof 570 pounds of cement per cubic y a rd, together with adequate curi n g , will avoid the pro b l e m .

Vapor barriers are often installedunder slabs on grade or as part of acontinuous wrapper which enclosesboth the basement floor and thefoundation walls to isolate themf rom the surrounding earth. Theirselection and use has been dis-c u s s e d . 3 z 4

When heating ducts are to be em-bedded in a concrete slab the slabmust be thickened as needed to givethem adequate cover and maintainthe integrity of the slab. Re i n f o rc i n g mesh should be placed above thed u c t s. Ducts may be made of as-besto s-cement, wax-impre g n a t e dp a p e r, or metal. If metal is usedc h l o rides must not be used in thec o n c rete mix because of the dangerof corrosion. Se rious corrosion canalso be caused by two or more dif-f e rent metals coming in contact

with the concre t e.In heated buildings it is necessary

to provide edge insulation such ascellular block material of glass, poly-s t y rene or urethane around slabs ong ra d e. It is recommended that thisinsulation be at least one inch thick and pre f e rably two inches and havea thermal conductance of not more

than 0.40 BTU per hour per squarefoot per degree Fa h renheit. On l y m a t e rials that are resistant to deteri-o ration from rotting, insects or fun-gus and are nonabsorbent shouldbe used.

Radiant heating pipes or otherpipes may be embedded in concre t eslabs if they are given cover of atleast an inch or two of concrete un-der the pipe and two to three inchesover the pipe. Ve rm i c u l i t e, perlite orcellular insulating concrete can be

used under a floor to minimize heatloss dow n w a rd. Piping should belaid out so that only the mains passt h rough joints and the pipes shouldbe provided with expansion joints atthese locations. The pipe should al-so be protected from the possibility of corrosion if chemicals can enterthe joint. With all the best of pre c a u-t i o n s, howe ve r, floors containing ra-diant heating pipes are likely toc rack more than others.

Special purpose floorsIt is recommended that aggre-

gates be chosen that have the lowe s ta vailable coefficients of thermal ex-pansion if they are to be used infloors that may be subjected to larg eand rapid changes in tempera t u re,as in walk -in re f ri g e rators or nearf u rn a c e s. Sometimes special con-c rete mixes containing aluminouscements are used adjacent to fur-n a c e s. Limestone has a low therm a lcoefficient, trap rock and granite arei n t e rm e d i a t e, and silica and quart za re high. If no special materials areused a sufficient number of jointsshould be provided to allow for fre eand adequate move m e n t .

In potentially explosive locations,such as paint spray booths or places w h e re even small explosions couldbe disastro u s, as in hospital opera t-

ing ro o m s, it is necessary to prov i d efloor surfaces that are static dissem-inating and abrasion spark re s i s t a n t .This is usually accomplished by means of carbon black in the mix orp owd e red iron troweled into thes u rf a c e. Pro c e d u res and re c o m-mended rates of application are giv-en by the Nationa l Fi re Pro t e c t i o n

Association (NFPA). It should benoted, howe ve r, that in constru c t i n g i n d u s t rial or heavy-duty surfaces itis virtually impossible to treat themso that they meet NFPA re q u i re-m e n t s.

Floors for resistance to chemicalsmust be designed for high stre n g t h ,density and low perm e a b i l i t y. Thechemicals to which concrete is re-sistant are re p o rted in the ACI 201Re p o rt , “Du rability of Co n c rete inSe rv i c e.” There are some chemicals,

notably acids, to which concrete hasinadequate re s i s t a n c e. Where suchm a t e rials are to be used floors mustbe protected by other materi a l s which do have adequate re s i s t a n c eto acids as well as wea r re s i s t a n c e.Recommendations are given in the ACI 515 “Guide for Protection of Co n c rete Against Chemical At t a c k by Means of Coatings and Ot h e rCo r ro s i o n - resistant Ma t e ri a l s.”

REFERENCES

All numbered referenc es a re to C ON-CR ETE C ONS TRUC TION, Novemb er 1973:

1. “Making Floor S urfac es Hard,”P.age 515.

2. “How Crac king is C ontrolled,”Page517.

3. “P reparing the S ite,” pa ge 523.

4. “S ea l Out Wate r Va por,” pa ge 550.

PUBLICATION #C730601Co py right © 1973, The Ab e rdeen Gro u p All rights re s e rve d

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