Performance of Precast Ferrocement Panel for ...

ICCBT2008

Performance of Precast Ferrocement Panel for Composite Masonry Slab System Y. Yardim, Universiti Putra Malaysia, MALAYSIA W. Thanoon, Universiti Putra Malaysia, MALAYSIA M.S. Jafaar, Universiti Putra Malaysia, MALAYSIA J. Noorzaei, Universiti Putra Malaysia, MALAYSIA Shibli R. M. Khan*, Universiti Putra Malaysia, MALAYSIA Kamal N.M, Universiti Tenaga Nasional, MALAYSIA

ABSTRACT This study investigates the performance of inverted two-way ribs precast ferrocement thin panel as permanent formwork. The two-way inverted ribs in the ferrocement panel enhanced its flexural stiffness, as well as providing link between the precast layer and the in situ elements without shear reinforcement. Flexural behaviors of two precast panels and two composite slabs are investigated under two line load and distributed load. Test results indicate that the thin panel with suitable ribs layout and support distance can be used as permanent formwork. Typical load from construction worker and in situ elements could be sustained by the panel. The panel also acts as good composite component with in situ brick and concrete. Composite full slab can sustain typical design loads for residential buildings and until ultimate load and no separation or any horizontal cracks between the layers were observed. Keywords: Precast ferrocement rib slab, Composite full slab, Design load, Line load.

*Corresponding Author: Dr. Shibli R.M. Khan, Universiti Tenaga Nasional, Malaysia. Tel: +6038921 2020, Fax: +60389212116. E-mail: [email protected]

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Performance of Precast Ferrocement Panel for Composite Masonry Slab System

1.

INTRODUCTION

Composite structures combine two or more materials in a unit structure to provide tangible benefits and a versatile solution to suite different applications. A composite system reduces the unnecessary and unwanted material properties, such as weight and cost, without sacrificing required capacity. There are five different capacity criteria to be considered for the full precast flooring units; (1) bearing capacity, (2) shear capacity, (3) flexure capacity, (4) deflection limits, (5) handling restriction (Kim S. Elliott, 2002). The concept is more or less the same with composite precast slab system. Generally, usage priority will be taken by the composite system which is light, stiff, thin and able to joint well to the cast in situ layer. Precast system take place in the construction with temporary prop for longer span. Most precast companies eliminate the props during construction to speed up the construction. However, it causes heavier structure, more steel to support the wet concrete before the composite action take place. In addition, heavier precast units are more expensive to handle, transport and erect. They also require large and more expensive foundation and column to support them. Temporary supports for precast slab reduce temporary bending moments in the element due to wet weight of the cast in situ concrete thus resulting of a substantial reduction in the quantities of concrete and steel required in the precast unit. (Alfred A. 2001). Key elements for precast system are to be stiff structure at the same time light. Ferrocement with its versatile properties is the most efficient system available to achieve a light, thin, and stiff structure. Many investigations have been carried out about ferrocement as permanent formwork to be used in composite slab system (M. Sulaiman 1985). Research on the use of ferrocement as alternative to steel deck has also show some good results. Ferrocement panel have been proposed as an alternative material for profiled decks in composite slab construction M.A. Mansur (1986). System connection behavior was observed in the study and it is concluded that system connection is as good as steel deck slab connection. Moreover permanent ferrocement formwork is cheaper and fire resistance is more then steel deck. However, same as steel deck system, some separation was observed before ultimate load. M.A. Al-Kubaisy (2000) used a ferrocement in the tension zone of a reinforced concrete slab as shown in Figure 1. Considerable reduction in cracks and their spacing (64%-84%) was observed. Addition of ferrocement enhances the ductility and energy absorption properties.

500mm 60 mm 20 mm

Ferrocement cover with 3 layer of wire mesh reinforcement

Figure 1: Reinforced concrete slap with ferrocement in the tension

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Shibli R.M. Khan et. al.

Existing half precast slab systems are widely used all over the world. Although the systems are one of the widely used systems among the precast slab systems, very limited studies have been found. The half precast system is supported by steel lattice girder. Lattice girder is responsible for ensuring that the panels will span the required distance in the temporary condition, a chair for supporting the top mat of reinforcement and as lifting points during erection. The lattice also acts as a physical link between the precast panel and the in-situ portion after construction. Lattice girder is mainly working as temporary construction element and it requires about 8 m steel for every two dimensional and more than 10 m steel for every three dimensional lattice girders for 3 m span. Furthermore the system needs temporary support after certain span length (after 2.5 m) (Hanson 2004). However this connection system has been questioned by many researchers (Sittichai, 2003; Alfred, 2001; Engstrom, 1982). Moreover ability of the shear trusses to be shear connectors for composite two layers are yet to be explored. Studies show that, the structure responds of composite slab sound behavior till service load (Sittichai 2003). Efficiency of connection between two layers of the composite system under ultimate loads remains a major concern for such system. Therefore the composite ferrocement precast panel’s development and its capacity on ultimate load still need further investigation.

2.

PROPOSED SYSTEM

This system is developed with the aim of gaining as many benefits as possible during and after construction. As a cumulative effect, the system can reduce the duration of construction with having at least the same ability to perform satisfactorily as what can be achieved by a conventional concrete slab. Composite Ferrocement Masonry Slab “CFMS” system as the name suggests, utilizes ferrocement technology in producing composite slab consisting of precast ferrocement layer (Figure 2). Thin precast layer provides lighter and strong element therefore easy to carry transport, handle and less dependent on heaver equipment on job site. The main purpose of using hollow clay brick at core of slab is getting optimum benefit of every part of composite system. Solid material decreases the weight, amount of in situ concrete, which is difficult to control its exact strength, and construction time, provides effective insulation, at the same time work as interlocking element between two wythes of slab.

Precast layer cast in the form of ribbed slabs but ribs are engaged differently from other ribbed slabs encountered in practice. This precast element has their ribs above the plates, which is ferrocement, when they are in place in the system. The ribs have versatile function in the system; it is concerned both flexural soundness of precast layer and shear soundness for composite system.

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In situ layer Bricks layer Precast layer, Ferrocement panel

Composite slab Figure 2: Layout of Composite Ferrocement Masonry Slab

3.

PREPARATION OF TEST SPECIMENS AND TEST SETUP

Ordinary Portland cement and natural sand were used for concrete in the ratio 1:3 with water/cement ratio of 0.5. The mortar mix was designed to give 28-day cube strength of 30 N/mm². The ferrocement reinforcement used in all slabs consisted of high tensile steel bars and galvanized welded square wire mesh of 0.9 mm diameter and 12 mm openings. The tensile strength of the mesh was found 321 N/mm². Four one way ferrocement panels were cast. Two of them (FP1, FP2) tested as ferrocement panel other two were used for casting of composite slab (CS1, CS2) and tested as composite slab. The slab specimens tested under two line loads and distributed load. The specimens were arranged in two groups FP (ferrocement panel) and CS (composite slab). Details of specimens are given in Table 1.

No FP1 FP2 CS1 CS2

Table 1: Details of specimens’ dimension and loading type. Dimensions Precast layer Load type dimensions 3mx1.3mx75mm N.A. Two line load 1.75mx0.78mx75mm N.A. Distributed 3mx1.3mx125mm 3mx1.3mx75mm Two line load 3mx1.3mx125mm 3mx1.3mx75mm Distributed

Four formworks with the dimension of the slab specimens were prepared. In order to shape the ferrocement decking with main and secondary ribs as specified in design, blocks of polystyrene was made and pasted in the formwork (Figure 3.a). The wire mesh is cut according to the required dimension. Then, the steel bars were tied on top of the mesh with a 400

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specified spacing using spacers. Strain gauges were welded in the middle of each steel bar (Figure 3.b). Then, the mesh together with the steel bars was placed in the formwork on top of the polystyrene with a specified spacing using spacers. The mortar was poured into the formwork to fill the voids between polystyrene blocks (Figure 3.c). The specimens were cured for six days. On the seventh day, the specimens were demoulded. The polystyrene blocks were removed leaving empty spaces for bricks (Figure 3.d). Bricks were then placed in the empty spaces on the ferrocement deck. Concrete was poured in the voids between formwork and bricks. The level of concrete topping will be the same as the level of bricks (Figure 3.e). Sequences of casting are shown in Figure 3.

a

c

b

e

d

Figure 3: Sequences of specimens casting All four specimens were tested as simply supported slabs with two-line load and distributed load. Typical set-up of two-line load test is shown in Figure 4. Precast layer (FP2) of the composite system and composite slab (CS1) tested under uniformly distributed load by using 50 kg cement bags. Second specimens (FP1 and CS2) tested under two line loads. Deflection, strain on steel bars and strain tension and compression zone of concrete monitored by dial gage, strain gage and demec points respectively. First crack and cracks width was monitored by crack measurement instrument. Cement bags are added on top of slab as uniformly disrupted load, bags are placed one by one and symmetrically to avoid uneven loading as shown in Figure 5.

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401


Figure 4: Schematic view set-up of test, Two line load test (actual picture)

Figure 5: Precast layer test set-up for uniformly distributed load

4.

RESULTS AND DISCUSSION

Structure’s behaviors were observed and recorded carefully for every steps of load increment. Composite slabs (CS1 and CS2) show similar behavior during entire testing. Same magnitude of deflection were observed for CS1 and CS2 until first crack load and load deflection curves goes parallel with very minor difference during nonlinear stage of structure as shown in Figure 6. Similar to most of the ferrocement slab structure (A.W.Hago 2005, S.K. Kaushik1991); three different behaviors were recorded during entire loading history. Therefore load deflection curves have different slope at three different zones. The first zone represents the load deflection relationship for uncracked stage of testing and the curve is linear. Along this zone, both the concrete matrix and reinforcement behaved elastically, whereby the load is mainly carried by the concrete matrix. The zone ends at the load of 6.4kN/m2, for CS1 and 6.35kN/m2 for CS2 which are the first crack load of the specimens with 0.1mm crack width. The second zone starts after the first crack occurred and called as multiple crack zones. Curves changed its slope but still positioned as almost linear curve. As the load increases, number and length of cracks increased but the width of cracks does not increase much. This zone ends at the load of 14 kN/m2. At the end of multiple crack zones it is observed that, maximum crack width is not more than 1.5 mm and more than 10 cracks counted.

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The third zone is indicated by the intensification of existing cracks with the increase of load. Here the load is carried mainly by the reinforcement. The specimen undergoes rapid increase in deflection. The failure loads of the specimens are 18.6kN/m2 and 17.93 kN/m2 for CS1 and CS2 respectively. Full composite action observed for all entire loading stages, no single horizontal crack or any separation sign was recorded. The results of each panel were tabulated in Table 2. Table 2: Summary of Theoretical and Experimental results First Crack Load (kNm2) Spec. CS1 CS2 FP1 FP2

Theor.

Exper.

4.6 4.6 2.2 5.8

6.4 6.35 2.9 7.5

Ultimate load (PULT) (kN/m2)

Ultimate Moment, MU (kNm/m)

Theor.

Theor.

Exper.

16.05 16.05 5.4 10.8

18.6 17.93 6.5 13.7

16.3 16.3 5.5 3.7

M U (Expt) M U (Theor)

Exper.

18.9 18.21 6.6 4.5

1.16 1.12 1.20 1.21

Load kN/m2

The ferrocement plates had considerable ductility and behaved in composite manner throughout the entire loading process. The results of Precast (FP) specimens were also tabulated in Table 2 and the load deflection characteristics are shown in Figure 7. Three stages load deflection curve were also observed for those specimens. Load deflection curve show linear attempt until first crack load which is around 2.8kN/m2 for FP1. The plate shows large deflection of 8 mm (deflection /L = 0.00285) with load of 2.8kN/m2 which is less then total topping and construction load. Systems behave in ductile manner and first crack appear at the load of 2.9kN/m2. Crack numbers are increased with increasing of load and large deformation is observed. At the deflection of 55 mm system fails by crashing of ribs.

17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

Full Slab CS1 Full Slab CS2

0

5

10

15

20

25

30

35

40

45

50

55

Deflection mm

Figure 6: Load Deflection Curves for Composite Slabs

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403

Performance of Precast Ferrocement Panel for Composite Masonry Slab System 15 14 13

Precast Layer 3 m FP2

12

Precast Layer 1.75m FP1

11 Load kN/m2

10 9 8 7 6 5 4 3 2 1 0 0

5

10

15

20

25

30

35

40

45

50

55

Deflection mm

Figure 7: Load Deflection Curves for Precast Layers FP2 specimen is representing non propped structure with it is short span. Every loading stages of the specimen are carefully investigated. Total load during construction is considered as 4.5kN/m2 which includes self weight of precast panel, topping load, construction load of 1 kN/m2 and safety factor of 1.4. Deflection was observed less than 0.5 mm from 0 to 2kN/m2 loading. Average deflection until the 4.5 kN/m2 load is 1 mm (Deflection/L =0.0006). Variation of strain along the depth of slab is shown in Figure 12, which is representing tension and compression strain of bottom and top part of slab. Curve was linearly increased until applied load of 6.8kN/m2; Strain in tension and compression zone was observed as 0 at the load of 1.5kN/m2. First recorded strain at tension zone is 0.0004 at the load of 3.0kN/m2. Until the first crack load 7.5kN/m2, average strain increment is 0.0003 for every 0.7-0.8 kN/m2 load. Total strain observed before first observed crack is 0.0023 at tension zone and 0.001at compression zone. Compression and tension strain points was jointed by strain line which is passed from center line of slab and between 1.5kN/m2 to 7.5kN/m2 natural axis was observed at the range of 46mm to 50mm from bottom of slab. First observed crack was measured at 0.15mm width and it extended until 15 mm from the bottom part of slab. The crack was appeared at left side of slab. Possibly some other hair cracks was occurred at bottom part of slab but because of access difficulties, that much small crack could not monitored at bottom part of slab. At the load of 7.9kN/m2 the crack extended with same width towards bottom of slab and second crack appeared at right side of the slab. Increment on the steel strain is accelerated with last 0.4kN/m2 the load and stain along the depth graph show sudden increase of 0.002 strains. Furthermore, sudden deflection was observed 1 mm, on the load deflection cure at this stage of load. This increment is the biggest deflection increment so far for 0.4 kN/m2 load increment. Cracks are remained as it is at the loading of 12 kN/m2, however, maximum crack width was observed as 3 mm.

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Variation Of Strain Along The Depth

Depth (mm)

1.5kN/m2 3.0kN/m2 3.7kN/m2 4.5kN/m2 5.2kN/m2 6.0kN/m2 6.8kN/m2 7.5kN/m2 7.9kN/m2 8.3kN/m2 9.0kN/m2 10.5kN/m2 11.3kN/m2 12.0kN/m2 13.0kN/m2 13.7kN/m2

75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0

-7

-6.5

-6

-5.5

-5

-4.5

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

3

Strain (10e-3)

Figure 8: Variation of Strain along the Depth of Slab FP1 At the load of 13kN/m2 cracks got wider and branches’ numbers and length of side cracks were increased. Some crushing symptoms (like local spoiling on concrete surface) were observed at top surface of side ribs. Largest deflection was recorded at this load as 18 mm (Deflection /L= 0.011). Strain was recorded as 0.0023 for compression and 0.00645 for tension on concrete surface. Steel strain was exceeding the limit of 0.002 strains. All the result was showed that next loading will be the fail load for system. Three stages load deflection curve were also observed for this specimen. At the load of 18.25 kN the system fail by flexural fail (Figure 9).

Part I

Part II

Figure 9: Failure of Ferrocement Precast Layer (failure load17.25 kN)

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As a result, Composite Ferrocement Masonry Slab (CFMS) showed satisfactory structural characteristic in terms of ductility, deflection, load carrying capacity and full composite behavior until end of ultimate load. No horizontal or separation cracks were recorded at any stages of two line load and distributed load tests. Therefore connection capabilities of ferrocement precast inverted rib panel was found to be satisfactory. Behaviors of the FB2 ferrocement panel structure shows that, system enable to carry topping plus construction load safely without any mid support.

5.

CONCLUSIONS

This study introduces a new invert ribbed precast ferrocement slab system for composite slab. Tests were complied for precast layer of composite slab system and composite slab system under two line loads and uniformly distributed loads. System shows excellent connection between two layers of slab until ultimate load. Integrity of structure was sustained until ultimate load and no separation observed. The results show that integrity of slab is well established with interlocking mechanism. Precast layer achieves its required tasks which are working as formwork, forming interlocking connection and carrying temporary loads such as toping and construction loads. System was carried out the load which was more then theoretical estimation. Test results shows that the precast layer is able to carry temporary loads and light enough to handle with simple crane. Ferrocement layer creates high modulus of rupture so that first crack load was observed at 40% of ultimate loading, provides strong protection against spalling and most of others surface distress and Eliminate cost of formwork.

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[7]. Sittichai Seangatith, 2003, “ The Ninth East Asia-Pacific Conference on Structural Engineering and Construction” Thailand, Page MMR 54- MMR 59. [8]. S.K. Kaushik, V.K. Grupta and M.K. Rahman, 1987, “Efficiency of Mesh Overlaps of Ferrocement Elements”, Journal of Ferrocement, Vol. 17, No. 4, Page 329-336. [9]. A.W. Hago, K.S. Al- Jabri, A.S. Alnuaimi, H. Al- Moqbali, M.A. Al-Kubaisy, 2005, Construction and Building Materials 19, Page 31-37. [10]. S.K. Kaushik, V.K. Grupta and K.T. Singh, 1991, “Behavior of Composite Slabs with lost Formwork”, journal of Ferrocement, vol.21, No.3, Page 203-213.

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