1

Performance of Precast Composite Slab with Steel Fibre Reinforced Concrete

Topping

Nurul Nadia Hasbullah1*

and Izni Syahrizal Ibrahim2

1

Student, Faculty of Civil Engineering, Universiti Teknologi Malaysia, 81310 Johor Bahru, Johor.

2

Lecturer, Faculty of Civil Engineerin,g Universiti Teknologi Malaysia, 81310 Johor Bahru, Johor.

*Corresponding Author: nadiahasbullah@gmail.com

Abstract- This study covers flexural behavior of composite slab reinforced with steel fiber concrete topping. The

steel fiber introduced in concrete topping to replace conventional method which is using welded wire reinforcement.

The combined bending and shear test with two-point loads gives improvement in terms of energy absorption and

ductility of the concrete. It is shown herein, ultimate resistance of composite slabs increase due to the increasing in

volume fraction of steel fiber (Vf). Besides, addition of steel fiber also influence in ductility performance due to high

deflection at mid-span of the composite slab compared with the conventional (plain) one. Positive development of

strength was obtained in both concrete cube compressive test and cylinder splitting tensile test with Vf range 0.25%

to 1.0%.Transverse roughened surface was applied at all precast surface to increase its interface capacity.

Keywords: Steel fibre reinforced concrete; In-situ concrete topping; Composite action; Ultimate bending capacity

1.0 Introduction

Precast concrete structure basically consists of finite size of precast concrete elements that joined

to form a complete structure. Use of precast composite slab enhances its bending strength

meanwhile in-situ topping will reduce vibration and improve thermal performance. Advantages

of precast concrete slab include time-saving, inherent fire properties, economic in terms of

formwork, scaffolding and reducing use of wet concrete, etc. Furthermore, Malaysia government

also encouraged the industry to implement prefabricated system in construction works. This

concept is called Industrialized Building System (IBS). Purpose of IBS concept is to improve

level of construction industry and mass production by ensuring good quality of work not

excluding safety and health. High quality of structure also can be obtained because of controlled

conditions in factory.

Concrete topping is one of the important factor contributes in producing excellent slab system.

Therefore, to ensure the function of concrete topping is fully utilized, welded wire reinforcement

used to increase slab strength. This conventional reinforced method is being practices since

behavior of the concrete is little in tensile resistance. As known, concrete is in its weakest

condition for first six hours of life, it can cracks itself due to tremendous tensile forces as the

plastic mix sets up, shrinks and attempts to draw in upon itself. Slight shifts or movement of the

2

sub-base slab will create tensile forces and generate cracks. As steel behavior is high in resisting

tensile force, the stress generated will be distribute evenly over the large area and hold tightly the

concrete bond hence minimize crack propagation. The concrete matrix bond reduces gradually as

crack width increase due to loading (Paine 1998 and Li et al. 1993).

Steel fibre reinforced concrete (SFRC) becomes common alternative in industrial flooring

nowadays to prevent opening of micro cracks. In concrete structures, crack growth due to the

loading and shrinkage that occurred during fresh state. Therefore, short steel fibre will function

as a bridge by transferring tensile forces across the crack hence lower stress concentration at the

crack-end (Figure 1). It is useful especially at the contraction joint of the precast panels. Steel

fibres may also reduce the amount of steel reinforcement used and accelerate the construction

time.

Unreinforced Fibre reinforced

Figure 1: Stress Distribution

2.0 Background Problems and Related Works

The application of wire mesh in traditional slab construction caused many problems such as

concrete spalling, debonding and long installation period (Figure 2). Substitution of steel fibres

in concrete is expected to overcome these problems. One of the major criteria that influence

flexural performance of SFRC is the volume fraction (Vf). It is necessary to determine the

optimum amount of steel fibre in concrete to achieve its ultimate flexural strength. This paper

investigated potential of steel fibres in slab topping with Vf varied from 0% to 1% with an

increment of 0.25% for each series. Altun et al. (2007) claimed the optimum Vf of steel fibre is

between 0.75% to 2% because there will be difficulty during compaction process later if the

amount of steel fibre exceed 2%.

As fibre topping placed on the top surface of the precast slab, hence shear will be generated

between the interfaces. In practical, shear key usually used to increase adhesion between both

surfaces but this technique is high in cost. Other alternative can enhance adhesion of the layers is

by increasing the surface roughness. This technique is much more economical and it is applied

before the placement of concrete topping. There are various method of surface roughening which

could be used for example longitudinal, transverse, wire brushing, etc.. In this experiment,

transverse direction applied at all precast slab surface to increase the interface capacity between

precast slab and SFRC topping.

3

\ (a) Placement of wire mesh reinforcement (b) Concrete casting

Figure 2: Conventional method

Based on the experimental results done by Ackermann and Schnell (2007), within the hogging

area, the slabs performed good rotation capacity. This proved the ability of SFRC in well

distribute crack stress. Besides, they also found that moment capacity after cracking occurred

could be maintained over large range of rotation. On the other hand, experiment on continuous

composite slabs shown that high moment redistribution also could be achieved. As steel fibre

become one of the important materials used in structural engineering, therefore further studies on

the effects of replacing conventional reinforced concrete member to fiber reinforced need to be

carried out (Byung Hwan Oh 1991; Craig 1987; Swamy and Bahia 1985; Kormeling et al. 1981;

Henager and Doherty 1976). Byung Hwan Oh (1991) stated the compressive strengths of test

cylinders increase by 17% as fibres added to the concrete until 2% by Vf. The objectives of this

research are:

a) To investigate the performance of precast composite slab with steel fibre reinforced

concrete as topping.

b) To compare the ultimate load of composite slab between SFRC and conventional

reinforcement as concrete topping.

c) To determine the most preferable volume fraction of steel fibres (Vf) in concrete topping.

3.0 Research Methodology

Experimental process basically divided into two stages; (i) specimens preparation including

casting and curing and (ii) flexural test. Thickness of precast slab and topping was fixed as

100mm and 75mm, respectively. As a result, neutral axis of the specimen is 148 mm from base

of the slab which means 48 mm upward from the interface. Mix design standard for this study is

according to BS 5328; 1981: Methods of Specifying Concrete.

4

Design concrete for the precast slab and concrete topping is 60N/mm2 and 40N/mm

2,

respectively. For precast slab layer, ready mix was used in order to ensure constant strength for

all specimens were achieved. During casting, the precast surface was roughened in transverse

direction by using stiff brush. After finish casting for precast slab and achieved required strength,

second stage of casting was carried out. The surface should be clean and free from dust or debris

before applying SFRC topping on top of the precast slab. Air compressor used to remove any

unwanted particles that could disturb bonding between concrete topping and precast slab.

Furthermore, the surface needs to wet by water to increase the bonding between fresh and

hardened concrete.

While for SFRC topping layer, common method of concreting was used. For fine aggregate, the

percentage passing 600 m is 46.25% while for coarse aggregate the maximum size used is

10mm. Ordinary Portland Cement (OPC) and 0.53 of water cement ratio also used for the

concrete mix. In order to increase concrete workability (range of 30 – 60 mm), small amount of

admixture (Super plasticizer) was added. Table 1 shows mix proportions design for plain

concrete and SFRC mixes while Table 2 shows mix proportions for composite slab specimens.

Table 1: Mix proportions design for plain concrete and SFRC mixes

Con

cret

e B

atc

h

Con

cret

e

Com

pre

ssiv

e

Str

ength

at

28

days

(N/m

m2)

Cem

ent

(kg)

Fin

e A

ggre

gate

(kg)

Wate

r (k

g o

r L

)

Coars

e

Aggre

gate

(k

g)

Ste

el F

ibre

s (%

)

Su

per

-

pla

stic

izer

(L

)

Batch 1

40 23.24 40.42 76.99 102.15

0.0

0.0465

Batch 2 0.25

Batch 3 0.50

Batch 4 0.75

Batch 5 1.0

Table 2: Mix proportions for composite slab specimens

Sp

ecim

en T

yp

e

Con

cret

e

Com

pre

ssiv

e

Str

ength

at

28

days

(N/m

m2)

Cem

ent

(kg)

Fin

e A

ggre

gate

(kg)

Wate

r (k

g o

r L

)

Coars

e

Aggre

gate

(k

g)

Ste

el F

ibre

s (%

)

Su

per

-

pla

stic

izer

(L

)

Precast 60 480 822 150 927 - 3.8

5

Hooked-end type of steel fibres was used in this experiment (density of 7955 kg/m3). The fibres

were in uncollated or loosed form and dimension of steel fibre used is 33mm length with

0.55mm diameter that gives aspect ratio (length/diameter) of 60 as in Figure 3. Based on the

experiment done by Ackermann (1998), anchorage is one of the essential parts in fibre-matrix

bond. By conducting pull-out tests, he concludes hooked-end type is among the most sufficient

anchorage. The statement also supported by Paine and Peaston (1997).

(a) Loosed form

(b) Dimension

Figure 3: Hooked-end steel fiber

Common method of concrete mix was conducted and the steel fibres were added into the rotary

mixer after the concrete mixed evenly. The mixer should be in slow mode when steel fibres are

added to ensure uniform distribution of steel fibres. For controlled specimen, wire mesh was

used instead of steel fibre. 8mm diameter with 300mm spacing of wire mesh was installed for

plain concrete topping.

For SFRC topping, six mould cubes of 150mm x 150mm x 150mm and three cylinders with

300mm height and 150mm diameter were used for each concrete batch to determine small slab

properties. After 24 hours, the composite slabs, cubes and cylinders concrete were cured using

wet burlap until the test day. However, the cube concrete should be tested at 7 days and 28 days

to measure compressive strength. Meanwhile, cylinders concrete undergoes tensile splitting test

at its 28 days. Steel fibres amount with Vf of 0%, 0.25%, 0.50%, 0.75% and 1.0% shown in

Table 3.

33 mm

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Table 3: Amount of steel fibres

Concrete Batch 1 2 3 4 5

Volume Fraction of

Fiber, Vf (%) 0 0.25 0.50 0.75 1.0

Fibre Amount in

Specimen (kg) 0 2.0 4.0 6.0 8.0

The test procedure for this experiment basically has three stages; (i) prior cracking, (ii) first

cracking and (iii) ultimate failure. Prior cracking, the test controlled by loading with increment of

10 kN. After cracking, the test was controlled by deflection at a constant rate until the specimen

failed. This was to ensure as much information as possible could be recorded (Ackermann and

Shnell, 2008). The ultimate failure may cause of shear or interface. The combined bending and

shear test setup is shown in Figure 4.

Figure 4: Combined bending and shear test setup (all dimensions in mm)

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3.0 Results and Discussion

3.1 Concrete Properties of Precast Member and SFRC Topping

Concrete properties of precast slab and SFRC topping are shown in Table 4. All the properties

were achieved the strength which is 40 N/mm2 for concrete topping and 60 N/mm

2 for precast

slab. In general, area under load versus deflection graph represents toughness of the concrete.

Table 4: Concrete properties of precast member and SFRC topping

Precast Member SFRC Topping

Concrete

Batch

Concrete Cube

Compressive

Strength

(N/mm2)*

Splitting

Tensile

Strength

(N/mm2)*

Concrete Cube

Compressive

Strength (N/mm2)

Splitting

Tensile

Strength

(N/mm2) *

7 days 28 days 28 days 7 days 28 days† 28 days

Batch 1 (Plain

concrete, Vf =

0%)

62.57 81.79 5.90

22.27 41.28 3.68

27.88

Batch 2 (Vf =

0.25%)

27.07 43.15 4.08

28.38

Batch 3 (Vf =

0.50%)

28.41 44.39 5.08

28.38

Batch 4 (Vf =

0.75%)

29.81 46.33 5.87

31.37

Batch 5 (Vf =

1.0%)

28.43 44.71 6.53

29.08

* Average of (3) samples † Average of (4) samples

3.2 Composite Slab

For combined bending and shear test, all specimens were placed in flexural test frame. A single-

span system was setup. The clear span of the specimen is 450 mm, 350 mm width and slab depth

175 mm (precast: 100 mm and concrete topping: 75 mm). Two point loads were setup linearly by

hydraulic jack at the middle of the span. Distance of each point loads was 100 mm apart. Data

such as ultimate strength, deflection at mid-span and strain value of the specimens could be

obtained through the experiments. In order to measure mid-span deflection, one LVDT was

placed at the bottom of the specimen. Apart from that, the strain at concrete surface was obtained

by using Demec gauge. Table 5 shows experimental results of composite slab reinforced with

steel fibre and plain concrete topping.

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Table 5: Experimental results of composite slab reinforced with steel fibre and plain concrete topping.

Sample

Volume

Fraction, Vf

(%)

Ultimate

Load, Pu

(kN)

Ultimate

Shear

Capacity, Vu

(kN)

Ultimate

Moment

Capacity, Mu

(kN.m)

Maximum

Deflection at

Ultimate, δmax

(mm)

First

Crack

Load, Pc

(kN)

Deflection at

First Crack

Load, δc (mm)

0-S1 0

(Control)

214.8 107.4 18.80 3.62 90.2 2.37

0-S2 176.5 88.25 15.44 2.85 80.5 1.71

0-S3 177 88.5 15.49 3.2 70.2 1.82

0.25-S1

0.25

185.5 92.75 16.23 2.96 75.1 2.34

0.25-S2 214.3 107.15 18.75 3.79 70.3 2.67

0.25-S3 182 91 15.93 3.66 81.5 2.16

0.50-S1

0.5

205.9 102.95 18.02 3.64 90.2 2.3

0.50-S2 192.2 96.1 16.82 3.39 70.6 2.14

0.50-S3 186.2 93.1 16.29 3.98 71.6 2.73

0.75-S1

0.75

251.1 125.55 21.97 3.96 100.5 2.31

0.75-S2 199 99.5 17.41 3.15 80 1.98

0.75-S3 182.3 91.15 15.95 3.43 70.2 2.23

1.0-S1

1

178.3 89.15 15.60 3.27 70.1 1.99

1.0-S2 208.5 104.25 18.24 4.19 69.8 2.46

1.0-S3 167 83.5 14.61 3.44 59.8 2.25

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3.2 Loads versus Deflection

Almost all of the specimens’ ultimate loads obtained were above the calculated load except 1.0-

S3. Figure 5 demonstrate relationship of applied loads and mid-span deflection for Vf of 0%,

0.25%, 0.50%, 0.75% and 1.0%. The best sample was chosen for each of concrete batch to

compare with one of the control specimen (plain). The relationship of load and deflection

showed similar trend for various amount of fibers including plain concrete. The mid-span

deflection increased gradually with increment of loads. It can be seen 0-S1 gives the highest load

which is 214.8 kN while ultimate load of 0.75-S2 was recorded as second highest with 199 kN of

load. By comparing between SFRC composite slabs, it shows that the ultimate load increase as

steel fibres amount increase. However, at Vf of 1.0% the ultimate load start to reduce. This may

due to insufficient amount of steel fibres that cause low bonding between concrete matrix and

steel fibers.

Overall, 0.50-S2, 0.75-S2 and 1.0-S1 show lower deflection than 0-S. The maximum deflection

at ultimate load was occurred in 0-S1 with 3.62mm while the minimum deflection at ultimate

load is 2.96mm (0.25-S1). For applied load and mid-span relationship, the deflection basically

reduces with increasing amount of steel fiber. While for plain concrete the deflection show

slightly higher than other specimens. This shown SFRC topping gives good performances in

terms of ductility of concrete. It is also proved that SFRC specimens could sustain the applied

load for slightly long time than plain (conventional) one and slightly delay the failures. The steel

fibres actually act as a bridge that transferred the tensile forces across the crack. Therefore when

applied load increased, the micro-crack is being grip by the steel fibers.

Figure 5: Load versus mid-span deflection relationship

Pcal = 174 kN

0

20

40

60

80

100

120

140

160

180

200

220

0 2 4 6 8 10 12 14 16 18

Ap

plie

d L

oad

(kN

)

Mid-span Deflection (mm)

0-S1 0.25-S1 0.50-S2 0.75-S2 1.0-S1

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3.4 Strain Profile

Strain is defined as deformation of the specimen and can be measured as changes of specimen

length over the entire specimen length. The compressive region will gives negative value of

strain while tension region will result as positive value. Figure 6 demonstrate theoretical strain

distribution diagram of the specimen with Demec gauge installed at point A, B, C and D. As in

Figure 6(a), the strain distribution is in full-bond which means there is no slippage. However,

when the interface was distressed, the strain changes between point B and C (Figure 6(b)).

Figure 6(c) illustrated measurement of strains changes. The interface slip could be obtained when

gradient of A’B’ (apparent strain gradient) equal to gradient of C’D’ (true strain gradient).

Figure 6: Strain distribution diagram (Ibrahim I.S et. al, 2008)

Strain distribution of the selected specimens are shown in Figure 7 (a), (b), (c), (d) and (e) for Vf

of 0%, 0.25%, 0.50%, 0.75% and 1.0%, respectively. According to the results, it was illustrated

that point B, C and D as in Figure 6 of the specimens were in tension as loads increased.

Meanwhile, top of the specimens, point A (Figure 6) showed compressive behavior. Besides that,

the specimen interface was observed to start detached and does not act as a monolithic structure

when the load achieved at certain value for instance 50 kN. This means both topping and

substrate were behave as a different structure. Overall structure basically experience positive

deflection (sagging) because of negative strain value at the upper side of the composite and

positive strain value at the bottom. However for 0.50-S2, the strain distribution demonstrates

large slip at the interface of the specimen. This may due to poor bonding between concrete

topping and precast slab.

11

SFR

C t

op

pin

g (m

m)

Pre

cast

Sla

b (

mm

) SF

RC

to

pp

ing

(mm

) P

reca

st S

lab

(m

m)

(a) 0-S1 (Vf = 0%)

(b) 0.25-S1 (Vf = 0.25%)

N.A

-100

-75

-50

-25

0

25

50

75

-2000 -1500 -1000 -500 0 500 1000 1500 2000

Strain (x 10-6)

20

50

100

130

N.A

-100

-75

-50

-25

0

25

50

75

-2000 -1500 -1000 -500 0 500 1000 1500 2000

Strain (x 10-6)

20

50

100

130

12

Pre

cast

Sla

b (

mm

) SF

RC

to

pp

ing

(mm

) P

reca

st S

lab

(m

m)

SFR

C t

op

pin

g (m

m)

(c ) 0.50-S2 (Vf = 0.50%)

(d ) 0.75-S2 (Vf = 0.75%)

N.A

-100

-75

-50

-25

0

25

50

75

-2000 -1500 -1000 -500 0 500 1000 1500 2000

Strain (x 10-6)

20

50

100

120

N.A

-100

-75

-50

-25

0

25

50

75

-2000 -1500 -1000 -500 0 500 1000 1500 2000

Strain (x 10-6)

20

50

100

120

13

SFR

C t

op

pin

g (m

m)

Pre

cast

sla

b (

mm

)

(e) 1.0-S1 (Vf = 1.0%)

Figure 7: Strain distribution

5.0 Conclusion

Based on the experiment conducted to determine flexural performance of composite slab

reinforced with steel fiber concrete topping, there are several significant conclusions could be

obtained:

i) Energy absorption and ductility concrete enhanced when steel fibers were introduced

to the composite slab. The maximum ultimate load of SFRC topping is Vf of 0.75%.

ii) Overall, 0.50-S2, 0.75-S2 and 1.0-S1 show least deflection than 0-S. The maximum

deflection at ultimate load was occurred in 0-S1 with 3.62mm while the minimum

deflection at ultimate load is 2.96mm (0.25-S1). Hence, the SFRC topping results in

good performance in terms of flexural or ductility of the concrete and Vf of 0.75% is

the preferable amount of steel fibre in concrete topping.

iii) Both concrete cube and cylinder gives positive improvement in terms of compressive

strength and splitting tensile strength, respectively. By conducting both tests,

effective Vf for splitting tensile strength is 1.0% while for compressive strength is

0.75%.

iv) Based on the strain distribution diagram, interface slip was occurred at all specimens.

N.A

-100

-75

-50

-25

0

25

50

75

-2000 -1500 -1000 -500 0 500 1000 1500 2000

Strain (x 10-6)

20

50

100

130

14

6.0 Acknowledgements

Specially thanks to Dr Izni Syahrizal Ibrahim for his devoted assistance and Noor Nabilah

Sarbini for her guidance in completing this project. Also thanks to technicians at Structural

Laboratory, Faculty of Civil Engineering, Universiti Teknologi Malaysia.

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REFERENCES

1) Ackermann, F.P. and Schnell, J. (2008). Steel Fibre Reinforced Continuous Composite

Slabs. Proceedings of the 2008 Composite Construction in Steel and Concrete

Conference VI.

2) Paine, K. A. (September 1998). Steel Fibre Reinforced Concrete for Pre-stressed Hollow

Core Slabs. Department of Civil Engineering, University of Nottingham.

3) Byung, Hwan Oh (October 1992). Flexural Analysis of Reinforced Concrete Beams

Containing Steel Fibers. Journal of Structural Engineering, Vol. 118, No. 10.

4) Altun F., Haktanir T., and Ari K. (2007). Effects of Steel Fibre Addition on Mechanical

Properties of Concrete and RC Beams. Construction and Building Materials.

5) Ibrahim I.S., Elliot K.S and Copeland S. (2008). Bending Capacity of Precast Prestressed

Hollow Core Slabs with Concrete Topping. Malaysia Journal Civil Engineering, Vol. 20,

No.2, pp. 260-283.