Cement &Concrete Composites 13 (1991) 21-28 • ~ ' ~ , ~ ' / I

Flexural Behavior of Ferrocement Sandwich Panels I. A. Basunbul, Mohammed Saleem & G. J. A1-Sulaimani

King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia

(Received 16 July 1990; accepted 7 January 1991)

Abstract

The flexural behavior of ferrocement sandwich panels has been studied. The parameters con- sidered in the experimental investigation were the number of wire mesh layers, the skeletal steel, the web mesh reinforcement and the number of webs.

Ultimate moment capacities were computed analytically using conventional reinforced concrete theory. The analytical results are compared with the experimental results by tests on 12 sandwich panels. Cracking behavior and failure patterns for all panels were also obtained and compared.

Keywords: Ferrocement, sandwich panels, flexural strength tests, flexural behavior, ultimate strength, cracking (fracturing), bending moments, bending stress, deflection, strength of materials.

Nw

tw T

Ts Twin

Ts Vf

Number of wire mesh layers in each web Number of webs or ribs in the panel Thickness of plates Thickness of ribs Tensile force in cross-section of panel Tension contribution from bottom plate mesh Tension contribution from skeletal steel Tension contribution from wire mesh in ribs Tension contribution from skeletal steel Volume fraction of wire mesh in tension face Volume fraction of skeletal steel in tension face

INTRODUCTION

NOTATION

a

Ab Aw bp C

Depth of compression block Area of each skeletal steel bar Area of wire in the wire mesh Width of the plate or panel

fym ks h mult mwm n 1

n2

n3

N1

Compressive force in cross-section of panel Compressive strength of mortar Yield stress of wire mesh Yield stress of skeletal steel Depth of panel Ultimate moment Moment contribution from web mesh Number of wires in one wire mesh layer in plates of the panel Number of wires in tension in one layer of web mesh reinforcement Number of skeletal steel bars in bottom or tension plate Number of wire mesh layers in bottom or tension plate

21

Ferrocement (FC) sandwich panels are attracting attention in the production of building com- ponents. However, ferrocement skin elements are very thin and therefore offer not only less moment resistance but also less thermal insulation. Sand- wich panels provide an economic method of pro- viding structural requirements and thermal insulation. 1-4

The faces of the sandwich panel provide pro- tection to the core material and withstand the imposed loads acting as tension and compression elements. The ribs of the panel provide the neces- sary shear transfer and enable the panel to act as a single unit.

The successful and wider acceptance of these panels is greatly influenced by their strength, serviceability, durability and availability of rational design procedures. One of the early investigations on the flexural strength of ferroce- ment sandwich panels was due to Narmi & Chang/ who tested single core sandwich panels having a span of 1800 mm and width 457 mm with two different depths, 122 mm and 96 mm.

Cement & Concrete Composites 0958-9465/91/$3.50 © 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain

22 l .A . Basunbul, M. Saleem, G. J. Al-Sulaimani

The panels were tested under three- and four- point loads. Test results showed that instability of the facing governed the ultimate capacity of the sandwich panels. The presence of unreinforced lateral solid mortar ribs and a polystyrene core did not contribute to restrain outward displace- ment of the facing.

Trikha et aL 6 reported an empirical expression for estimating the modulus of rupture for lightly reinforced cored sections (circular) based on Logan and Shah's expression for the first crack stress in bending. 7 Also, expressions for the effec- tive moment of inertia were proposed. The pre- dicted values agreed reasonably well with the results from tests on six cored panels.

Saed s investigated the flexural behavior of sandwich panels. The panels were made of cement mortar reinforced with multiple layers of wire fabric. Four panels were tested as two equal- span continuous beams.

Kaushik et aL 9 modified the empirical rela- tionship suggested by Trikha et aL 6 for the modulus of rupture and effective moment of iner- tia. The experimental results show that the relia- bility of the proposed analytical procedure is 85-95%.

Lee et al. 3 investigated sandwich panels in pure bending to study their cracking behavior and ultimate strength. The major parameters of the study were the amount of dispersion of reinforce- ment, and the method of shear transfer. It was found that a finer distribution of reinforcement gives a larger number of narrow cracks.

The purpose of this investigation was to study the flexural behavior of FC sandwich panels and to contribute to the structural design procedure. The traditional reinforced concrete approach was used to predict the ultimate moment capacity. Theoretical results were then compared with experimental results on 12 sandwich panels. Also, cracking behavior and failure patterns were obtained for all the panels.

From the literature review, the strength of fer- rocement sandwich panels was found to be con- trolled by the following four variables:

(1) the mesh type and volume fraction of re- inforcement;

(2) the skeletal steel; (3) the number of ribs; (4) the rib reinforcement.

The mortar strength is variable, in so far as it is a statistical quantity. However, since the water/ cement ratio should be kept below or near 0-4,

mortar strengths should be based on a constant target strength. In the panels tested, there was a considerable variation in cylinder strength from apparently the same concrete mix. This was due to the fact that testing took place at various time intervals after casting, and steam curing, which was assumed to be adequate, gave only the 7-10- day strength. It is not considered to be an import- ant parameter in ultimate strength calculations.

The variables studied in this investigation include the number of wire mesh layers, the skeletal steel, the web mesh reinforcement and the number of ribs.

EXPERIMENTAL PROCEDURE

The experimental investigation consisted of cast- ing 12 ferrocement sandwich panels with variable parameters as discussed earlier.

Test specimens As shown in Fig. 1, the cross-section consisted of two ferrocement plates of 20 mm thickness re- inforced with a varying number of wire mesh layers (0, 1,2, 3, 4) with and without skeletal steel. Shear transfer between two plates was achieved by 20 mm thick mortar ribs. The ribs were re- inforced with shear connectors and varied between three and five in number, as shown in Fig. 1. Shear connectors were made out of 4 mm diameter bars and spaced 250 mm apart longi- tudinally, as shown in Fig. l(a). Apart from shear connectors, the ribs were also reinforced with woven wire mesh in some panels, as shown in Fig. l(b). The core of the sandwich was made of hard polystyrene 110 mm deep running throughout the length. The total cross-section of the panel was always 1200 mm × 150 mm with an overall length of 2400 mm.

Twelve specimens were designated according to the number of wire mesh layers and the pre- sence of skeletal steel in plates, the number of cores in the section and the presence of wire mesh reinforcement in the webs. This information is presented in Table 1. Two or three groups of panels with two or three panels in each group were used to study each variable.

Materials

Cement Sand

Ordinary Portland cement Beach dune sand with fineness modulus of 1.6

Flexural behavior of ferrocement sandwich panels 23

Mesh

Skeletal steel

Galvanized woven square wire mesh:

Diameter of wire = 0"9 mm Mesh opening = 6-35 mm Yield strength = 414 MPa

Diameter of bars = 4.0 mm Eight bars placed longitudinally and ten bars in transverse direc- tion providing a mesh opening of

SHEAR CONNECTORS

P OLcYSR~Y R E NE . . . . . . . WIREMESH

RIB 20

l

20ram

(a/

~ WIRtEMESI'I REINFORCEMENT,

J~ 4MM I~ BARS

Mix

150minx 250 mm Yield strength = 620 and 707 MPa Cement/sand = 0"5 Water/cement ratio = 0"45

Casting of specimen The bottom plate reinforcement with shear con- nectors and web mesh reinforcement, if any, were placed in the mold and the bottom plate was cast and vibrated. Polystyrene cores were then placed and ribs were filled with mortar. Lastly, top plate reinforcement was placed, fixed, cast and vibrated.

Testing of specimens All the test specimens were tested under four- point loading applied at the third points after 28 days of curing. Deflections along the centerline and the third points of the panels were recorded automatically using a data logging system for each 2 kN load increment. The position of linear variable displacement transducers (LVDTs) and strain gauges is shown in Fig. 2.

(b)

.) I 41L'xX~ I I

(c)

Fig. 1. Cross-section of three typical sandwich panels: (a) shear connectors with bars; (b) shear connectors with bars and mesh; (c) faces with skeletal steel and mesh. Dimensions are in mm.

ANALYSIS FOR ULTIMATE MOMENT CAPACITY

Since FC is a special kind of reinforced concrete, the flexural strength of FC sandwich panels may be predicted on the basis of the conventional procedure adopted for the analysis of reinforced concrete flexural members using ACI Code ultimate strength theory. Ferrocement sections are more likely to be over-reinforced, and reinforce- ment is evenly dispersed throughout the sections.

Table 1. Panel designation details

No. Reference No. of Skeletal designation mesh layers steel

in each face

No. of Web mesh Vf V= cores reinforcement (× 10-3) ( x 10-3)

1 2L3C 2 Absent 2 3L3C 3 Absent 3 4L3C 4 Absent 4 2LS2C 2 Present 5 0LS3C 0 Present 6 2LS3C 2 Present 7 4LS3C 4 Present 8 4LS4C 4 Present 9 4LS2CW 4 Present

10 4LS3CW 4 Present 11 4LS4CW 4 Present 12 4L3CW 4 Absent

3 Absent 9-967 1 0 3 Absent 14.951 0 3 Absent 19.934 0 2 Absent 9.967 1 4-19 3 Absent 0 4.19 3 Absent 9-967 1 4.19 3 Absent 19.934 4.19 4 Absent 19.934 4.19 2 Present 19.934 4.19 3 Present 19.934 4.19 4 Present 19-934 4.19 3 Present 19.934 0

24 L A. Basunbul, M. Saleem, G. J. Al-Sulaimani

All wires below the neutral axis are assumed to be yielding due to the large ultimate deflection the panel has undergone. The theoretical moment capacity (Mul t values have been calculated (Table 2) using the following procedure with reference to Fig. 3.

Assuming that the top plate is partially under compressive stress as shown in Fig. 3(a),

Compressive force C = 0"85 f'cabp

Tensile force T = Tbm + T s + Twm

where

// y

~d/// / / I ~ -O ~770 L i n ] = 150 _ Stroin G0uol

Tbm = contribution of the bottom plate

= n I N I fymAw

T~ = contribution of the skeletal steel

= rt3 fysA b

Twin = contribution of the web reinforcement

='2X2Xwf mAw Equating C to T, the value of a is evaluated, and if a < tp (Fig. 3(a)) the value of mul t is given by:

Mult= Tbm(h -tp-al + Ts( h 2] 2 2) A l l d i m e n s i o n s o re in m m

Fig. 2. Schematic showing the positions of LVDTs and strain gauge on panel.

t Polystyrene

Section

? 4=

C

TFIm

Stresses and Forces

,o 4 :

- • C

Twm

tp (a)

(b) Section Stresses ond Forces

Fig. 3. Stress diagram of ferrocement sandwich pane| section using reLnforced concrete approach: (a) compression block within top flange; (b) compression block below top flange.

Flexural behavior of ferrocement sandwich panels

Table 2. Cracking moments and experimental and theoretical moment capacities

25

Specimen Cracking moment

(10 2 kN mm)

Ultimate moment capacity (102 kN mm)

Experimental Theoretical Difference

2L3C 65.17 148-35 137-50 3L3C a 99-67 195"50 206" 15 4L3C ~ 103"5 203"17 274"06 2LS2C a 88"17 184"00 235"51 0LS3C 19-16 123"43 86"84 2LS3C a 103"50 215"40 235"60 4LS3C a 99.67 210.83 357.72 4LS4C ~ 105.42 257.60 370.08 4LS2CW 95.83 347.68 388.72 4LS3CW 93.92 354.20 394.40 4LS4CW 99.67 364-93 400.10 4L3CW 105.42 302.83 299.04

-7.31

- 29.6

11.8 11.3 9.64

- 0 . 0 1

~Panels failing in shear without developing full moment capacity.

).00 g.O0

Fig. 4 .

• 21.3¢ • OLI~IC + 4LS;L~U X 4i.S3CV

• 41.30W

18.00 27.00 31.OO 4S.00 54.00 83.00 DEFLECTION IN MM

Load-deflection curves of panels failing in flexure.

I f a > tp (Fig. 3(b)) the c o m p r e s s ] o n b lock lies out- side the c o m p r e s s i o n pla te a n d ins ide the web:

C o m p r e s s i v e fo rce C = 0-85 f'cbptp

+ Nw[0-85 f'ctw(a - tp)]

Tensi le f o r c e T--- Tbm "[- T s -t- Twin

Here Twin--- n 3 N 2 N w f y m

T h e va lue o f a is reca lcu la ted a n d Mul t is approxi- ma te ly given by:

26 1. A. Basunbul, M. Saleem, G. J. Al-Sulaimani

A A

To simplify the above expression for Mwm, it is assumed that the center of gravity of the web mesh wires lies at the center of the web in tension.

RESULTS AND DISCUSSION

(c)

A

I I A

/ l/ II A

Fig. 5. Typical cracking patterns for panels failing in flexure: (a) panel OLS3C; (b) panel 2L3C; (c) panel 4LS4CW.

The load versus deflection and moment versus curvature plots obtained for all ferrocement sand- wich panels failing in flexure exhibited three dif- ferent stages corresponding to uncracked, cracked and post-yield conditions, as shown in Fig. 4. These panels have shown multiple cracks concentrated on the central zone of the panel and the spacing between cracks is closer and the cracks are finer for panels containing a higher number of wire mesh layers, as shown in Fig. 5. Panels failing in shear exhibited in most cases two stages of flexural behavior, one uncracked stage and a part of the second stage. This can be seen in Fig. 6 and is due to the fact that these panels cotdd not develop theft full moment capacities, either because of absence of web mesh or because of a smaller number of ribs. In fact these panels had shown diagonal cracks initiated over the support,

8

it / c;

° - -

i t , ~.ao

- 1 J i x et~m

%.00 9.00 I0.00 :~7.00 3S.O0 4s.oo 1 1 4 . 0 0 S3.00 DEFLECTION IN I'll't

Fig. 6. Load-deflection curves of panels failing in shear.

Flexural behavior of ferrocement sandwich panels 27

lal 1

A A

(b)

Fig. 7. Typical crack patterns for panels failing in shear: (a) panel 2LS2C; (b) panel 4LS3C.

and panels with four wire mesh layers had experi- enced separation along the ribs and displacement in the facing as shown in Fig. 7. Flexural charac- teristics exhibited by panels with web mesh are shown in Fig. 8. Increasing the number of ribs between three andfive does not seem to improve the flexural strength and/or the ductility much. This is obvious from the comparison of load- deflection curves of panels 4LS2CW, 4LS3CW and 4LS4CW in Fig. 4.

Comparison of load-deflection curves of panels with two layers of wire mesh and four layers of wire mesh shows an increase in ductility and stiffness in the second stage of the flexural behavior. Panels with wire mesh reinforcement along the ribs had exhibited multiple cracks in the central zone and spacing depended on the num- ber of wire mesh layers, but the final failure took place when diagonal cracks developed at the sup- port toward the loading line.

In this investigation cracking moment is taken as the moment at which the moment-curvature curve deviates from linearity. 1° Table 2 shows the cracking moments for all the panels. The cracking moment seemed to increase with increasing number of wire mesh layers.

Use of skeletal steel bars also seemed to improve the elastic as well as the post-cracking behavior of the panel.

Ult imate m o m e n t s Ultimate moments obtained experimentally were compared with theoretical ultimate moments

0 0

~ 8 • 4L3C • 4L3CV

° £ o o ' 131so ' 2 r l ~ ' 4 0 : . ' ' 54 ,00 (a) DEFLECTION IN PtH

67.50

g

JL

=g

"J ~ h l r Q 4L$3C • 4LS3CV

~oTo, ' ,~:,o ' 2r:oo ' 4 0 : . ' (b) DEFLECTION IN HH

1

< 8 Q 4LS4C • 4LS4CV

**T** ' i ~ s o ' 2r:** ' 40:50 ' 54:00 (c) DEFLECTION IN HH

Fig. 8.

J I 54.00 67.50

67'. 50

Behavior of panels with and without web mesh.

computed using the conventional reinforced con- crete approach (see Table 2). In this approach wire mesh in webs were assumed to yield because of large ultimate defections obtained in panels failing in flexure. For panels failing in flexure, the conventional reinforced concrete approach seems to predict the ultimate flexural capacity with reasonable accuracy.

CONCLUSIONS

Based on the results of the research undertaken, the following conclusions may be drawn.

28 1. A. Basunbul, M. Saleem, G. J. Al-Sulaimani

1. The number of ribs and the presence of web mesh reinforcement play an important role in developing full moment capacities.

2. Increasing the number of wire mesh layers and using skeletal steel in the tension plate improves the ductility in the working range and also the ultimate strength.

3. The number of wire mesh layers has no significant contribution to stiffness and rigidity in the uncracked stage, but improves stiffness and rigidity in the cracked stage.

4. Increasing the number of ribs beyond the minimum number of ribs required to pro- duce full moment capacity does not improve flexural behavior much in all three stages.

5. Conventional reinforced concrete theory predicts the ultimate moment capacities within reasonable limits.

6. Failure pattern and cracking behavior depend on the volume of the reinforcement and the shear connectors between the facings.

ACKNOWLEDGEMENTS

This study was made possible by a grant from the King Abdul Aziz City for Science and Tech- nology. The authors also acknowledge the sup- port given by the King Fahd University of Petro- leum and Minerals.

REFERENCES

1. Sun, N. C., Austriaco, L. R. & Pama, R. P., Development of a prefabricated ferrocement housing system. Proceed- ings of the Second International Symposium on Ferroce- ment, Bangkok, Thailand, 14-16 Jan. 1985, pp. 415-30.

2. Wang, M. L., The building system of ferrocement light panel elements. Proceedings of the Second International Symposium on Ferrocement, Bangkok, Thailand, 14-16 Jan. 1985, pp. 457-68.

3. Lee, S. L., Mansur, M. A., Paramasivam, E, Ong, K. C. G. & Tam, C. T., A study of sandwich wall panels. Journal of Ferrocement, 16 (3)(July 1986) 295-313.

4. Austriaco, L. R., Sun, N. C. & Pama, R. P., Thermal cap- ability of ferrocement panels. Journal of Ferrocement, 17 (3) (July 1987) 241-51.

5. Nanni, A. & Chang, W. F., Ferrocement sandwich panels under bending and edgewise compression. Journal of Ferrocement, 16 (2)(Apr. 1986) 129-40.

6. Trikha, D. N., Kaushik, S. K., Gupta, V. K. & Mini, S., Behavior of ferrocement cored slabs. Proceedings of the Second International Symposium on Ferrocement, Bang- kok, Thailand, 14-16 Jan. 1985, pp. 122-35.

7. Logan, D. & Shah, S. P., Moment capacity and cracking behavior of ferrocement in flexure. AC1 Journal Proc., 70 (12)(Dec. 1973) 799-804.

8. Saed, A., Inelastic analysis of continuous ferrocement sandwich beams under static load. Thesis submitted to the Faculty of the University of Miami in partial fulfill- ment of the requirements for the Master of Science, 1970.

9. Kaushik, S. K., Gupta, V. K., Trikha, D. N. & Mini, S. M., Investigations on ferrocement cored slabs. Journal of Ferrocement, 16 (3)(July 1986) 227-37.

10. Gordon, B. B., Gajanam, M. S. & Naaman, A. E., Survey of mechanical properties of ferrocement as a structural material. Ferrocement, Material and Applications, American Concrete Institute, SP-61, 1979, pp. 9-24.