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CENTRAL 8UILDING RESEARCH INSTITUTE, ROORKEE-247667 (INDIA)

E, R. N. 5THIN R.C. RIBBED SLAB FOR FLOORS AND ROOFS

~ pr r d.,;qn ovtf suppor I s

I .Fig. 1 Thin R.C. Ribbed Slab

Thin R.C . ribbed slab consists of precastR.C. ribs 110 x 200 mm spaced at 1200 mm c/cwi th 50 mm thick cast-in-situ R.C. flange above(Fig. 1) . It can be used for floors and fl at aswe l l as sloping roofs in sing le and multi-storeyed residential and other types of I ghtty loadedbuildings. In case of hea vil y loaded floors androofs, the size and reinforcemen t of the ribs andflanges will have to be increased . Conventionalfloor/ roof finish has to be used above the ribbedslab, a9 the case may be. Cei ling plaster canbe omitted in low cost constructions. In situations, where very good f inish is ca ll ed for,cei l ing should be plastered.

STRUCTURAL DESIGNThe structural design may be carried out

as per lim it state method given in 15:456-1978,'Code of Practice for Plain and ReinforcedConcrete'. The ribs are designed as rectangularbeams to support the weight of concrete in theflange, the shuttering and the live load ofworkmen and eQuipments during construction.They act as Tee beams for full design loads,after the concrete in the flanges has attainedstrength . The flange is designed as a continuou9slab spanning the ribs . To keep the deflectionswithin permissible lim i ts, the span/depth ratios

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for the flange and the rib have been kept as perrelevant provisions of IS : 456-1978. To controlflexural cracki ng, the spacing requirements forreinforcement as given in the code have beencomp l ied with. To ensure monolith ic actio n ofprecast ribs with cast in-situ flange, stirrupsin the ribs are projected i nto the cast- in-situconcrete of the flange. Relevapt provisions ofIS : 3935-1966, 'Code of Pracfice for CompositeConstruction, have been satisfied. A ready-madedesign table is given in Table 1.CONSTRUCTIONPrecasting and Erection of Ribs:

The ribs are precast, oreferab ly in stee lmoulds (Fig . 2) over a casting pla tform. A lternatively, these can be cast in timber mou lds.Mould oi l is applied on the casting pla tformand inner sides of the mou ld and reinforcementcage is kept in position with th e required cover.To provide holes in the ribs, 25mm square M .S.hollow box sections are inserted through squareho les in the longitudinal members of the mo ul d.Concrete used is grade M 15 with 20 mm anddo wn, graded coarse aggregate. The concreteshall be thoroughly compacted by Of'edle vibrator having 25mm dia pin and th e top surfacesha ll be finished rough. Whe n ord inary port landcement is used, the ribs sha ll be demou lded by

sliding the longitudinal pieces of the mouldaway from the precast rib , t hree to four hOllrsafter being cast . In case, portland pozzolanacement or any other slow setting cement is usedthe time of demou lding shall be increased.The ribs shalt be kept covered with we t gunnybags fo r 72 hours Afterwards t hey shall beslid horizolltal ly on the casting platform to breakthe bond with platform and transpor ted to thecuring yard or ta nk. After two weeks of watercuring, the units sha ll be al lowed to air dry fo ranother tw o weeks. before being used in anyco nstruction. The precast ribs shall be placedin 115mm w ide recesses left in the wa l l atspecified spacings.Assembl y of Shuttering Pane ls:

Plane si mp le shutte ri ng panels made oftimber fram ing and plywood panelling coveredwit h G.!. sheets (Fig. 3) are used for casti ng th eflange portion. At least 20 reuses can beexpected from G.!. shee t lined shuttering.Alternately. steel sh uttering panels may be usedin w hich case, ' the number of reuses shall bemore. Wid th of the sh utt ering panel is kept asthe clear distance between ribs wit h a clearanceof 5 mm on both sides and the length is preferably kept in modules of 30 0 mm i . e., 1200,1500, 1800 mm etc . A combi nation of thesecan be used to suit the span of the room. Even

Table 1 : Moment and Shear Resistance of Ribs with Different Re in forcementsNo. and Dia . Moment of Resistance (Nm ) Shearof Bar W ith Mild Stee l With High Strength Deformed Resistance

(f y = 250 N/ mm2) Bars (f y =41 5 N/mm2) (N)At Mid At At mid AtSpan Support Span Support

2- 8mm 781 2 7071 281552 - 10 mm 7281 6525 12207 10273 301582 - 12 mm 10297 8729 17198 10917 318992 - 14 mm 13883 11281 23000 331682 - 16 mm 17923 14889 29518 343172-18 mm 22362 18926 344092 - 20 mm 27464 34250

Note: The moment and shear resistance values given above are based on l imitstate method gi ven in IS 456-1978.

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1000 (m a l ) 1000 (mcu ) XJ 1000 {m ax ) 1000 ( mOl )L"""cll .",., a t c - - + . - - ~ " - - - - - + m.s. flat 50.5

: : I : ; 3 1 1 5 0 l l 5 0 1 ~--10 , m.s. boll

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Fig. 2Steel Mould for precast Ribsec lIon X.X

Section AA

r--.. ~allen 5 oISO

: .A ;

-til

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Wooden batten50 x 50

~ N ! M _ wooden battensSOxSOsquare m.S bar insquare ho

G. I . sheCi!t lining

Shu tteri ng in position

1.,,

f ig . 3. Timber Shuttering for Thin R.C. Ribbed Slab

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number of panels shall be used in each bay, sothat they can be placed symmetrical withrespect to midspan of the rib . The shutteringpanels are kept supported on 20 mm squareM .S. bars projecting out of holes left in precastribs as shown in Fig. 3. As the ribs are designedas rectangular beams to take the load of insituconcrete, sh uttering, workmen and equipmentsduring concreting, no props are required . Theshuttering panels are then kept in position andlevelled with the help of wedges with the topsurface of the precast ribs. Gaps, if any,between shuttering panels or between thepanels or between the panels and the precastribs are to be filled with mud mortar or leancement mortar and leve rted flush with the topsurface of shuttering.INSITU CONCRETE IN f lANGE:

Reinforcement in the flanges is placed overthe shuttering as shown in Fig. 1. The specifiedCOVN to the main reinforcement is ensured bytying the reinforcement to the projecting stirrupsof the precast ribs at supports and by keepingcement mortar cover blocks below the mainreinforcement bars at mid-span. The topsurface of precast ribs shall be given a coat ofcement slurry with 0.25 Kg of cement per squaremetre of area, just before concreting. Concreteof grade M 15 with 12 mm and down, grad edcoarse aggregate is then laid over the shut teringand ribs and compacted to a thickness of 50 mmby a plate vibrator and the top surface, finishedwit h wooden float. The concrete shall be watercured for tw o weeks by ponding water over theflange and then allowed to air cure. Dependingupon the weather, the shuttering panels can beremoved after about three days. In caseportland pozzolana cement is used, the shuttering panels shall be removed after 5 days.FITTINGS AND FIXTURES

Fan hooks shall preferably be provid ed inthe precast ribs at the time of casting. In casethe fan is to be located in the f lange portion,the fan hook shall extend over the adjacent ribsand rest over them. Position of electricaljunction boxes and other fittings shall be preplanned and provided i n the precast ribs or in

the cast-In-situ flange, as the case may be, atthe time of casting. If concealed wiring is tobe done, conduits of diameter not exceeding20 mm may be provided above the precast ribsalong their length. In the oth er di rect ion,conduits shall be provided al ong recess cut inthe w all .BA LCONVjCHAJJA PROJECTIONS

Short ca nti levers, such as roof or cha jjaprojections upto 500 mm can be provided in theribbed slab along or across the direction of tharibs by keeping the flange projected out. Thecantilever has to be designed as per norm alpractice. For larger projections along thedirection of the ribs, th e ribs have to be designed suitably . They can be precast and keptprojecting out and the can tileve r portion of thethe ribs can be cast - in-situ along with theflange . For large cantilever projections acrossthe direction of the ribs, cant i lever beams haveto be provided in that direction.flOOR/ROOF TREATM ENT

In case of ribbed slab f loor, conventionalf loor finish such as Indian Patent Stone can beprovided above the slab. In case of flat roofs,lime concrete terracing or mudphuska treatmentwit h t i le above or any other proven treatmentcan be provided over the slab, after applyingbitu men coating A sl ope of at least 1 in 50sha ll be provided on the roof top.PRECAUTIONS IN CO NSTRUCTION1. Concrete used for precast ribs sha ll be

grade M 15 with 20 mm and dow n, gradedcoarse aggregate. Concrete shall be tho roughly com pacted by a nee :Ue vi brator andthe top and ends of the ribs sha ll be finished rough . Specified projection of stirrupsabove the ri b concrete shall be ensured.Cement mortar cover blocks shall be tied tothe reinforcementcover (25mm). to ensure spec ified

2. The ribs sha ll be t ransported wit h the mainreinforcement at the bottom, i.e ., in thenatura l position in w hich t hey wi l be usedin structure. While transporting, they

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shall be supported at tha two ends (a t adistance not more t han 0.15 time th elength from the ends) and midspan . The ribsshall not be transported or kept supportedat or near midspan onl y, at any stage. Incase tan hooks ha ve been provided nearmidspan, the ribs sha ll not be kept suppor ted on the fa n hooks at any stage, as thismay cause cracks in them.

3. Only ribs cured for at least 28 days shall beused. The age difference between twoadjacent ribs in the same floor or root of abuilding shall preferably be not more thantwo weeks. This is necessary to ensu rethat the deflections of adjacent ribs arepracticall y the same.

4 . Free movement of the ribbed slab oversupporting walls/beams shall be ensured byplastering the beari ng area of the flangeand ribs with 1:3 cement : sand mortar,finishing it level and sm ooth and providinga thick coat of lime w ash or polythene sheetover the bearings and vertical sides of theribs.

5. The ribs shall be provid ed w it h a m in imumbeari ng of 75 mm over the wa ll s/beams, incase the ribs are designed as continuous .In case of simply supported ribs, the bearingshall be 100 mm min im um.

7. The shu ttering panels shall be kept suppor-ted so tha t they do not move horizo nta ll y orvertically during concret in g. Gaps, if any,betwee n the panels and the ribs shall bethoroughly pa cked before con creti ng.Mou ld oi l shall be applied to the panelsbefore concreti ng.

8. Cement : sand mortar cover blocks sha ll betied to the reinforcement to maintain th erequ ired cover to flang e re inforcement.

9 . When thin R C. ribbed slab construction isdone in coastal areas or In any corrosiveenv ironment, the concrete cover to there inforcement has to be increased assuggested in IS : 456-1978 , dnd / or thereinforcement shall be coated wi th anticorrosive paints.

10. Concrete for the flange shall be grade M15 with 12 mm and down, graded coarseaggregate and shall be compacted using aplate vibrator.

'1 Proper curing of the slab shall be ensuredby ponding water over it or by continuousspray.

12. It is advisable to provide roof projectionaround the bui lding to prevent leakage atthe junction of walls with roof. In caseparapet wa ll is to provided just above thewall below, the junction between the rooffinish and the parapet wall shall be treatedas per re levant code provisions.

13 . I n case no roo f treatment is provided overth e slab, expansion joints shall be providedin the roof slab at a closer spac ing asrecommended for thin unprotected slabs inIS : 3414-1968, 'Code of Practice for Designand Installation of Join ts in Bui ldings'.

ALTERNATE CONSTRUCTION METHOD

In case, the construction of R.C. ribbedslab f loor /roof with precast ri bs and ready madeshuttering panels is not possible due to somereason or the other, the con str uction may bedone w ith traditional sh utter ing. In such casesthe ri bs need not be designed as rectangularbeams to carry the load at the constructionstage. They shall be designed as Tee beamsfo r t he tinal stage of loading. In this case thedepth/ re inforcement in t he ribs may work outto be less than that of precast rib . Also thelon gitudinal reinforcement bars provided insidethe precast ribs at top to take handling stressescan b3 avoided the cost of concrete/ reinforcemen tin the R.C. ribbed sl ab is l ikely to be less incase of cast-in-s i tu const ruction . However, thecost of shuttering w ill be more in this case. Theoverall saving is likely to be sli ghtly less, whentradi tion al sh uttering is used . Hence, only inexceptional cases, w here due to some reasonor the other, the use of precast r ibs and readymade shutter ing panels is not feasible, use oftraditio nal shu tter ing is recommended. l .

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STRUCTURAL TESTS produced by foot wear, movement of furniture,activ ities in the kitchen etc., is a source ofDeflection recovery test was carried out on nuisance and ca ll s for insulation against impactthe thin R.C. ribbed slab as per IS : 456-1978 noise in the intermediate floors. Impact Noiseand it passed the test. The maximum measured Rating (I N.R.) number is an indication of thedeflection was only 1 in 2164 the span. The degree of impact noise insu lation provided by aslab was also subjected to ultimate load test. floor For comparison, the ra t ings are g ivenThe loading was stopped at 1.25 times the below:design ultimate load, due to prob lems of keeping

the loading b locks further. Slab I.N,R.Though codes of practice do not give any 50 mm thick R.C. slab -17 dbprocecure fo r testing slabs against impact, tests 120 mOl th ick R.C slab -16 dbwere carried ou t to check the impact resistance The thin ribbed slab and the conventionalof the slab. A gunny bag filled wit h 400 N slab have practica lly the sa me impact noiseof sand was dropped from a height of 1500 mm. rating .No damage was observed. A 50 N weight was

dropped from a height of I 200 mm over an area Ponding tests and observations during rai ns,of 700 mm2 Though indentation of about 2mm carried out on the ribbed slab without anywas noticed at the top of the slab, where the treatment above show that it is leakproof.weight struck it, no other damage was observed .The slab was su bjected to further impact test 50 mm thick R C.C. slab of size 1080 xby pounding 'Haldi'. one of the hardest materials 1080 mm, with 30 mm th ick Indian Patent Stoneused at home in a 'Hamam Dasta'. No cracks floor ing over it was subjected to fire resi tan ceor other signs of weakness developed during test as per IS: 3809-1979 'Specif ication forthe test. Hence this slab is safe against impact Fire Resistance Test o f Structures'. The fi reand vibrations normally expected in residential resistance of thin R.C. ribbed slab in respect ofand office buildings. thermal insulation criteria was found to be

1 hour and 40 minutes, whereas the fire rat ingFUNCTIONAL PERFORMANCE required for a residential floor is 1 hr.Themal performance indices (for T.P 1., re f r I n thin R.C. ribbed slab, the fla nge portion

C.B .R I. Building Digest No . 94) of the ribbed being insitu w ith the design reinforcement proslab and the conventional slab with the same vided continuous in the two perpendicularroof treatment above are given below : directions, the slab t ies the longitudinal anderos wa lls. together and behaves similar to castSlab with treatment above T.P.1. in -situ R.C . slab to res ist forces due toa) 50 mm R.C, ribbed slab with 90 mm thick earthquakes and wind loads. Strengtheninglime concrete above 167 measures such as lintel bands, vertical steel atb) 50 mm R.C. ribbed slab with 75 mOl t hick corners and junctions of walls etc ., sha ll bemudphuska and 50 mm thick brick tiles provided as in the case of cast- in-situ slab.above 134

c) 100 mm thick R.C. slab with 90 mm thick MATERIAL AND LABOURlime concrete above 134 REQUIREMENTd) 100 mm thick R.C. slab w ith 75 mm thickmudphuska and 50 mm thick brick tiles Based on the experience gained from experiabove 110 mental construction of t hin R.C. ribbed slab

sc heme, the basic data of labour and materialsWith the same treatment above, ribbed slab required fo r the construction was worked out - ~,-is only slightly inferior to 100 mm th ick conven and is given below, Using t his data and know -ional slab. ing the prevai ling rates of materials and labour In multistoreyed bu i ld ings, impact noise at a place, the cost of construct ion can be '.

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worked out.BASIC DATA OF MATERIAL AND LABOURREQUIREMENT1. Fabrication of Shuttering Panels ( for

10 sq.m. of area)(a) Materials

i) Timber (sal wood) battens of50 x 50 mm . .. 0.1 53 m'

ii ) Shuttering plywood 10 mmthick 8.9 m2

i ii ) G.I. sheet 28 gauge 13 .5 m2iv) Sa l ballies average 75 mm

dia and 3000 mm long . . ' 1.5 Nos.v) Screws. na il s, etc . LS . . . . .. Rs . 25 .00(b) Labour

i) Carpenter ... 1.5 mandayi i) Mazdoor . 1.5 manday

Note : (i) One set of shuttering with G I. sheetlining can be used at least 20 times.

(ii) Wastage w' 1 has been added inthe quantities of materials g ivenabove.

II. Fabrication of mould for precast ri b of3600mm span (1 No.)(a) Materials

i) M.S. ChannelISJC 200(3600mm long, 2 Nos) 100 kg.

ii ) M S. angle iron 50 x 50 x 5mm(50mm long, 8 Nos.) 6 kg.

ii i ) M .S. plate 3mm thi ck200 x 240mm size, 2 Nos.) 2. 4 kg .

iv ) 1Omm dia . m.s. bolts withnuts (50mm long) 8 Nos .

v) Welding charges L.S. Rs.7 .00vi) 20mm sq . M .S. bars 300mmlong 5 Nos. 1.5 m

(b) Labouri) Fitter/ Mechanic .. . 0.5 mandayii ) Mazdoor .. . 1.0 manday

Note: A steel mou Id can be reused for 100castings at least

III. Precasting of rib (110mmx200mm and3600mm long, 1 No.)

Quanti ty of concr'3te 0 .0792 ma(a) Materials

Cement .,. 0.503 bagCoarse aggregate(20mm & down, grad6d) ... 0 .07 maSand 0.036 m'High strength deformedbars 1084 kg,Mi ld steel rei nforcemeet .. . 6.45 kg.25mm sq. M .S. conduitpipes 100mm long. 5 Nos ... 0.5 mSundries and bindingwire L.S. Rs . 2.00

( b) labourMason 0.05 mandayBar bender 0.50 mandayMazdoor 0.40 manday

(c) MiscellaneousCost of mould per use .. As per item IICost of mould oi l L S. ... Rs . 0.50Vibrator and mixercharges L.S. Rs . 0.50Electricity charges(for vibrator and mixer) ... Rs . 0.50Cost of platformcharges per casting Rs . 050Water charges forconcreting and curing Rs . 030

(d) Total cost of on eprecast ri b (a \- b + c)

Note : (i ) The number of precast ribs requ ired for10 m2 area will depend upon the planan d is to be taken as per drawing .

( ii ) The quantity of steel required wi l l varydepending upon load ing, span andcontinu ity condition . The reinforce-ment given here is for a roof havingcontinuous spans of 3600 mm clc withprecast ribs spaced @ 1200 mm c/c.Two ribs can serve an area of 12.96 m2

IV. Transportation . erection and assembly of precast rib (1 No.) (a) Labour

Mason 0.05 mandayM azdoor 0.2 mandayT & P L S. Rs 2.00

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V. Erection an d removal of shuttering (for10 sq. m. area)(a) Labour

Carpenter 0.44 mandayMazdoor 0.88 mandayT & P and sundriesincluding wedges,cement slurry, etc. L.S. Rs. 4 .00

VI. Providing and laying of reinforcement

scheme with conventional R.C. slabs is givenbelow . In case of A, the comparison has beengiven for a 3600 x 3600 mm size two wayspanning. continuous slab and in case of B, fora one way spanning . simp ly supported slab. Theyhave been designed as per li mit state methodand mild steel has been used as reinforcement.A. Co mp ari son in case of two way spanning

continuous slab.(for 10 sq.m. area)(a ) Materials

Mild Steel barsSundries. bindi ngwire etc. L.S.

37.70 kg.Rs.2 .00

(b ) LabourBar benderMazdoor

0.6 manday0.6 manday

VII. Insitu concreting and curing (for 10sq m. area)

Quantity of concrete .. 0.53 m3(8) Materials

Cement 3.5 bagCoars aggreate (12 mmand down) 0.445 m3Sand 0.223 m3

(b) LabourMason 0.15 mandayMazdoor 1.00 manday

(c) MiscellaneousMould oil. T & P etc. L.S . Rs 5.00Vibrator and electricitycharges L S. Re.1.00

SAVINGS IN MATERIALS AND COSTComparison of consumption of materia ls

and cost of construction of th in R.C. ribbed slab

Printed at :Anubhav Printers & Packers, Civil Lines. RoorkeeCopies : 2000

Item Cement Steel Cost(kgJm2 ) (kgjm2 )

Conventional slab1OOmm thick 32.0 7 .82Ribbed slab, 68mmav. thickn ess 23 .7 7.51Saving 26% 4" 22%B. Compar ison in case of one way span ningsimply supported slab.

Item Cement(kg/m2)Conventional slab140mm thick 44 .8 8.77Ribbed slab, 68mmavo thicknessSaving

23.747%

7.5214 0/. , 30%

It ca n be seen that the adoption of R.C.ribbed slab scheme in place of conventi onal R.C.slab will result in a saving of 26 to 47% insteel and 22 to 30% in cost. Apa rt from this,there is a saving of 30 to 45% in aggregates.The scheme eliminates the use of props to agreat extent. Being sim ilar to the conventionalconstruction, it wi ll be easy for the site engineerto adopt this type of construction and hence, itw i ll be read il y acceptable to him. Thus thi nR C. ribbed slab sc heme is ideally sui ted forrepet it ive type of construction of floors androofs for low cost houses and other buildings.

Prepared by : M .P. Jaisingh. Suresh Chand,Lathika Ja i singh

Pub lished by : Centra l Bui ld ing Re search Institute. Roorkee (U .P.) INDIA ,"First Pr inted: May, 1982 (as Tech. Note 11) Revised : Jan. 1985. Jan. 1987 and Feb. 1995