AN INVESTIGATION ON THE STRENGTH OF FERROCEMENT ROOF-ING ELEMENTS FOR LOW-l NCOME HOUSING

Z~JAHKALITA, M.K.C. NAMBIAR, B.C. BORTHAKUR and P.

Regional Research Laboratory, Council of Scientific 6 IndustrialResearch

AbstractResearch and development work on ferrocement roofing elements wasundertaken to develop a housing technique sultable for the low-Income group. Load tests were carried out on elements havingdlfferent shapes, spans and reinforcements. The load factors werecalculated based on limiting deflectlon and llmltlng crack widthcrlterla, and they were found to be adequate. Fer rocement rooflng,when adopted for low-cost housing, Is cheaper than corrugatedgalvanized Iron sheet or asbestos cement sheet roofing by 30% andcheaper than relnforced cement concrete roof by 56%.KSF

words: Ferrocement, Rooflng , Housing, Prefabrlcatbn, Cost,raw-rack, Deflectkm.

1. I ntroductlon

In Asla and the Paclflc region, where 70-66% of the population live Invillages, there Is an alarming houslng shortage. In lndla alone, atleast 17 million addltlonal houses will be required during the 7thFive-Year Plan. In general, an economical and a slmple roofingsystem will contrlbute greatly In solving the problem of housing. ItIs In this context that research has been undertaken In the ReglonalResearch Laboratory, Jo&at, to develop simple roofing elementssultable for low-cost housing. Ferrocement, a cheap and durablemater/al gaining popularity In the last threa decades, was selected asa sultable Intermediate technology for Immedlate adoption. Segmentaland trough-shaped roofing elemants of dlfferent spans and types ofrelnforcemant were made and tested. The load factors for theelemants were calculated based on the criteria of llmltlng deflectlonand limiting crack wldth. It was found that the ferrocement roofingIs Ideally suitable for low-cost houslng.

2. Materlals

The msterlals used were conventional relnforcemants, coarse sandwlth a fineness modulus of 2.67, and ordinary Portland cemsntconformlng to Indian Standard Speclflcatlon, lS:269-1976. Therelnforcemsnts were 4 mm to 6 mm diameter galvanized and

19

6

r48A

2

0 4 8 I2 16 20

Contra1 def Iection 1 mm)

Fig. 3. Load-deflection characteristics of elements E,, E2 and E3.

2

tI I I I I I

0 0.2 0.4 0.6 0.8 Ix)Maximum crack width (mm)

Fig. 4. Plot of load versus maximum crack width for elementsE2 and 5. El’

rods in the transverse dlrectkm with two layers of hexagonal wiremesh on each side of the skeletal steel. The reinforcement byvoluma was 1.68%. Elements E and E trough-shaped, 4.00 mlong, and 0.40 m wlda used 5-6 km dlama!& rods In the longltudlnaldlrectlon and 11-6 mm rods In the transverse dlrectlon wlth twolayers of hexagonal W/M mesh on each side of the skeletalreinforcement. The flnlshed thickness of these elemants was 20 mmwith 1.77% reinforcement by volume (Fig. 2).

Flg. 2. Details of reinforcement for elements E4, E5 and E6.

4. Load test

Load tests were carried out on the elements after 28 days of curingby placing sand bags to simulate uniformly dlstrlbuted load. Thecentral deflectkm was measured uslng load Increments. Theload-deflectlon characterlstlcs of the first three elemants are shownIn Fig. 3, and the maxlmum crack wldth at dlfferent stages ofloadlng was measured and pFyfd 1; Flg. 4. Load tests were alsocarried out on elements E Since In practlm either sideof the trough-shaped e&at&s can’be used as bearing surface,separate bearing tests were conducted for each side. Load-deflectioncurves are shown In Flg. 5.

non-galvanized mild steel rods; 20-24 gauge hexagonal wire meshwith 12 mm opening, and 1.36 mm diameter galvanized square wovenmesh wlth 12 mm opening.

3. Deslgn of elements

Tha 2.50 m long, 0.60 m wide and 18 mm thick element, designatedas E was reinforced wlth 6 mm dlameter bars, 6 bars In thelong&dlnal dlrectlon and 10 bars In the transverse dlrectkm, wlthtwo layers of hexagonal galvanlzed mesh on both sides of th skeletalreinforcement. The amount of reinforcement was 1.48% by volume(Kallta e t a l . 1985a, 1985b). The average cube strength of themortar was 22 N/mm2 at 28 days using 1:2 cement-sand ratlo bywelght, and 0.42 water-cement ratlo. These proportlons were usedfor all elements. The 4 mm diameter skeletal reinforcement of elementE has the same arrangement as E . The flnlshed thickness of thee&ment was 15 mm and the amour!t of reinforcement was 1 .ll% byvolume. Element E dld not contain any skeletal reinforcement. Theonly relntircement &as a single layer square mesh. In this element,the thickness was maintained at 15 mm wlth 1.55% reinforcement byvolume. The detalls of reinforcement for elements El, E2, and E3 areshown In Flg. 1.

( Dlmenslons In m 1

Fig. 1. Details of reinforcement for elements El, E2, and E3.

Element Em wlde and

In the form of segmental shell, was 4.00 m long, 0.65s IIMII thick. The skeletal relnk f-cement conslsted of 6d

mm dlameter rods In the longitudinal dlrection. and 136 mm dlamater20

0 6 I6 24 32 40

Contra1 dofloction (mm)

*Fig. 5. Load-deflection characteristics of elements E4, Es and E8.

5. Losd factors

Load factors for elements Edefiection and iimiting

E and E were caiariated from iimiting

1985).c&k %idth &nsiderations (Desayi e t a l .

5.1 Limiting deflection considerationFoiiawinIndian B

the practice of reinforced concrete structures as pertandard Specification, iS:458-1978, the iimiting deflection

could be taken as L/250, where L is the eftixtlve span.total deflection is given by

Then, tha

A, + A1 = At = & (1)

where AiAt

= short-time deflection,= total deflection (A, + Al).

Al = long-tima deflection and

23

The long-time deflection, A1 may be taken from ACI 318-71 as

A1 = (2 - l.+) A,

with A1 L 0.6/$ , where A is the area of compression steel,and A Is the area of tensioiSc steel.approx&ely equal to 1, Eq. (2) becomes

Assumln

At = 1.8Ai.A1 = 8

that ASc//+is.8 Ai.

Therefore from Eq.(l), the limiting value ifinstantaneous deflection is

(Ai),lm = 1.8 k 256=&

From the load-deflection cuwe, if P is the load corresponding todeflection given by Eq.(3), the ldd factor fl using llmitlngdeflection consideration Is

where P = ultimate load.i n Table”l.

Values of f, for all elements are shown

Table 1. Values of load factors.___-_---------~~~~~~~~~

Relnforce- Cracking Ultimate Load factorsment by l o a d l o a d - - - - - - - - - - - - - - - - - - - - - - - -

Specimen voluma Deflection Crack widthllmitatlon limitation

(%I Ml NJ) t f,

E1 1.48 3.50 7.83 1.70 1.77E2 1.11 3.40 8.51 1.71 1.50E3 1.55 3.40 5.88 1.78 1.43E4 1.88 - 8.79 2.70E5 1.77 - 7.52 2.09E8 1.77 - 10.50 1.82------------------------------________I___________---------

5.2 Limltlng crack width considerationSimilarly, following the practice of reInforced concrete structures,the maximum allowable crack width could be taken as 0.3 mm. Notingthat width of crack under long-time loading would Increase to 80% inexcess of that under short-tima loading, the magnitude of limitingcrack width (w) would be equal to 0.30/1.80 = 0.19 mm. From thegraph of load &us crack width In Fig. 4, If P Is the loadcorresponding to 0.19 mm, the load factor f2 from ~lmitlng crackwidth consideration Is

Values of fa for all elements are shown In Table 1.

A load factor of 1.25 [Paul and Pama 1978) Is assumed for llmltlngstate of collapse when losd factors are determlned from limit states ofdeflection and crack wldth. From Table 1 It Is seen that, for all theelements, load factors am mom then 1.25.

6. Cost analysis

The cost analysis of the elements was computed based from the costof materials and labour In Jorhat (Table 2).

Table 2. Cost analysls for segmental shell 4.00 m x 0.65 m.--------------------_________I____---- ---B-B-Mild steel rod 6 mm diameter 7.57 kg Rs. 49.21Wire mesh 0.88 kg Rs. 20.24Sand Rs. 3.60Cement

60.00 kg30.00 kg RI. 48.60

Form work lump sum Rs. 15.00

Add 35% labour

Cost per ma = Rs. 71 (US$5.5) Rs.184.48--------------------______1___1___11____----

To study the long-tlms environmental behavlwr, especiallyagainst water, weatherlng, etc., two experlmental sheds have beenconstructed In the Regional Research Laboratory campus at Jorhst:one using segmental roofing elements (Fig. 6) and the other usingtrough-shaped rwflng elements. F l g . 7 shows the layout of atypical low-income house using ferrocement roof.

7. Conclusions

(a) From the load tests, It Is seen that the stmngth of ferro-cement rooflng elements Is structurally acceptable and can beadopted for low-cost housing. The load factors obtained for all theelements were adequate.

(b) The percentage of relnfot-cement required may vary from l-2%by volume depending upon span and shape of the elements.

(c1 The segmentel elements appear to be more economkzal ascompared to trough-shaped ones.

(d1 Ferrocement construction should preferably be prefabricatedand the elements can be lifted manually or by light equlpments.

Fig. 6. An experimental cycle shed with ferracement roofing.

m

Elevation

I - -3.90 I

Kitchen

--------------------__L J

t6.30

--IP lan ( Dimensions in m)

Fig. 7. One raam law-income house with ferrocement roofing.

Section A- A0.15

I 13 : 6 concretew

8. Acknowledgement

This paper is being published wlth the kind permisslon of Dr. J.N.Baruah, Director, Regional Research Laboratory, Jorhat.

References

Desayl, P., Nathan, A.S. and Reddy, Y.M. (1985) Appllcatlon ofWrocement to roofing joists. Journal of ths Instltutlon of Englneers ( lndla

Kalita.. .,4;,65&:‘51;r22K( .C - - -c

Borthakur, B.C.. and Baruah. P.(1985a) Experlmentai s&d&’ ‘on ferrocement roofing elemsnts. InProceedIn s of the 2nd Internatlonal Symposlum on Ferrocement(cd.L.s-Audriaco e t I J I nternational FGocement I n -formation Center, Asian Instl& if Technology, pp. 405-413.

Kallta, U.C., Namblar, M.K.C., Borthakur, B.C. and Baruah P.

rocemant. Internatlonal Ferro-cement Information Center, Bangkok.