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Engineering, Technology & Applied Science Research Vol. 11, No. 4, 2021, 7452-7457 7452

www.etasr.com Dalaf & Mohammed: The Impact of Hybrid Fibers on Punching Shear Strength of Concrete Flat Plates …

The Impact of Hybrid Fibers on Punching Shear

Strength of Concrete Flat Plates Exposed to Fire

Ahmed Naji Dalaf

Department of Civil Engineering
University of Baghdad

Baghdad, Iraq

ahmed_naji.2007@yahoo.com

Shatha Dheyaa Mohammed

Department of Civil Engineering
University of Baghdad

Baghdad, Iraq

shatha.dh@coeng.uobaghdad.edu.iq

Abstract-This study presents an investigation about the effect of

fire flame on the punching shear strength of hybrid fiber

reinforced concrete flat plates. The main considered parameters

are the fiber type (steel or glass) and the burning steady-state
temperatures (500 and 600°C). A total of 9 half-scale flat plate

specimens of dimensions 1500mm×1500mm×100mm and 1.5%

fiber volume fraction were cast and divided into 3 groups. Each

group consisted of 3 specimens that were identical to those in the

other groups. The specimens of the second and the third groups

were subjected to fire flame influence for 1 hour and steady-state

temperature of 500 and 600°C respectively. Regarding the

cooling process, water sprinkling was applied directly after the

burning stage to represent the sudden cooling process. Generally,

the obtained results exhibited a significant increase in the

punching shear capacity of the fiber-reinforced slabs as

compared to the corresponding no fiber-reinforced slabs even at

elevated burning temperatures 600°C. The ultimate load was
increased by about 16.6, 19, and 21.5% at temperatures of 25,

500, and 600°C respectively, for steel fiber reinforced slabs and

by about 13.9, 27.2, and 34.6% for slabs containing two mixed

types of fibers (steel and glass), as compared with the reference

specimen at the same temperatures respectively. In addition, the

results indicated that fibers' presence in concrete resulted in

gradually punching failure with more ductile mode, whereas the
failure was sudden with a brittle mode in the slabs that did not
contain fibers.

Keywords-steel fibers; hybrid fibers; punching shear

I. INTRODUCTION

Flat plates are reinforced concrete slabs supported directly
on columns without beams or girder system. Such slabs have a
low capacity to transfer the shear loads to the columns due to
their relatively small depth (thickness). As a result, most
probable failure cases of flat plates are caused by higher shear
stresses in the column connection. These failures are described
as punching shear failures [1]. A punching shear failure takes
place suddenly around the column in a particular collapse
mechanism when a plug of concrete is pushed out from the slab
immediately above the column [2]. Punching failure may occur
due to slab overloading, un-conservative design for the slab-
column connections, and deterioration in the strength of
concrete and steel reinforcement resulted from the exposure of
the slabs to fire [3]. At elevated temperatures, both steel and

concrete exhibit a significant reduction in their strength,
stiffness, and physical properties [4-7]. Some of these variables
are not recoverable after sudden cooling, therefore, the
punching strength of flat slabs subjected to fire is expected to
be significantly affected [8]. Recently, a new technique using
different types of fibers to improve the performance of concrete
has been proven to give advanced outcomes [9-15]. The main
objective of this study is to investigate experimentally the
effect of fire on the punching shear strength of flat plates and
the effect of adding fibers to the concrete mix as a solution to
increase the resistance and ductility of slabs and to control
cracking during fire exposure.

II. LITERATURE REVIEW

An experimental study about the effect of fire on some
mechanical properties of concrete was carried out in [16]. The
compressive strength of concrete was measured for 150mm
cubes whereas the flexural strength was measured for
100×100×400mm prisms. The specimens were subjected to
fire with temperatures between 25 and 700°C. Three
temperature levels of 400, 500, and 700°C were chosen with 4
different exposure durations (0.5, 1.0, 1.5, and 2.0h). The
specimens were heated and cooled under the same regime and
tested after exposure to fire. The results showed that the
residual compressive strength ranged between 70% and 85% at
400°C, 59% and 78% at 500°C, and 43% and 62% at 700°C.
The flexural strength was found to be more sensitive to flame
exposure than the compressive strength. The residual flexural
strength ranged between 67% and 78% at 400°C, 40% and
67% at 500°C, and 20% and 45% at 700°C. Authors in [17]
studied the behavior and capacity of Steel Fiber Reinforced
Concrete (SFRC) flat slabs under punching shear force. The
results showed that steel fiber improved the punching shear
resistance of the slabs considerably. Using steel fiber (30 to
60kg/m

3
) increased punching shear resistance of the slabs from

9.0% to 39.8% and this increase was directly proportional to
fiber volume fraction. Steel fiber reduced significantly the
average crack width of the slabs up to approximately 70.8%.

III. EXPERIMENTAL WORK

The experimental work comprised 9 half-scale two-way
specimens of 1500×1500×100mm that were designed
according to the ACI-318M-2019 [18] requirements as a

Corresponding author: Ahmed Naji Dalaf

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simply supported specimen over an effective span of 1400mm.
The specimens were fiber reinforced by 1.5% volume fraction
to investigate the effect of fire on the punching shear strength
of fiber reinforced concrete flat plates. The geometrical layout
and the reinforcement details are shown in Figure 1.

(a)

(b)

Fig. 1. Details of the tested specimens.

The 9 specimens were divided according to the steady state
temperature into 3 groups, where each group consisted of 3
specimens, as can be seen in Table I, where NFC means No
Fiber Content, SF1.5 means Steel Fiber with volume fraction of
1.5%, GF0.5-SF1 means a mix of Glass 0.5% and Steel Fiber
1% and F500 or F600 represented the fire exposure at
temperatures of 500 or 600°C respectively.

TABLE I. DETAILS OF THE SPECIMENS GRO UPS

Group No. Slab designation Steady state fire temperature (
o
C)

Group 1

NFC-NF No fire effect

SF1.5-NF No fire effect

GF0.5-SF1-NF No fire effect

Group 2

NFC-F500 500

SF1.5-F500 500

GF0.5-SF1-F500 500

Group 3

NFC-F600 600

SF1.5-F600 600

GF0.5-SF1-F600 600

The specimens in Groups 2 and 3 were tested in punching
conditions after being exposed to fire for 1h in order to study
the effect of steady state temperature simultaneously with a
uniform equivalent dead load (8kN/m

2
) that represented 40% of

the service design load (Figure 2). After the burning process,
sudden cooling by water sprinkling took place to reduce the
temperature.

Fig. 2. Burning setup.

IV. MATERIALS AND METHODS

The concrete mix was made from cement (Type I), coarse
aggregates of 12mm maximum size and Zone 2 fine
aggregates. Steel and glass fibers were added to the concrete
mix. The specimens were covered with polythene sheets for
24h to avoid cracks caused by moisture loss, then they were
cured by covering with damp canvases soaked with water
continuously for 28 days to ensure good curing treatment. The
characteristics of the considered fibers are reported in Table II.
All the specimens were reinforced using deformed steel bars of
10mm diameter at 70mm c/c for each direction regarding the
tension zone and at 150mm c/c for each direction in the
compression zone. The steel yield strength was 470MPa. For
each specimen, the compressive strength of concrete was
measured for 3 standard 150×300mm cylinders subjected to the
same burning conditions of the corresponding specimen. Table
III shows the adopted concrete mixes proportions.

TABLE II. PROPERTIES OF THE STEEL AND GLASS FIBERS.

Fiber type
Length

(mm)

Diameter

(mm)

Tensile

strength (MPa)

Aspect ratio

(Lf/Df)

Steel fiber 30 0.375 1700 80

Glass fiber 30 0.6 1200 50

TABLE III. CONCRETE MIXES' PROPORTIONS.

Mix Type NFC MSF (1.5) MGS (0.5&1)

Cement (kg/m3) 420 420 420

Sand (kg/m3) 490 490 490

Gravel (kg/m3) 1042 1042 1042

w/c 0.45 0.45 0.45

Super plasticizer (L/m3) 3 3 3

Steel fiber (% ) --- 1.5 1

Glass fiber (% ) --- --- 0.5

V. TESTED SPECIMENS AND INSTRUMENTATIONS

The specimens of Groups 2 and 3 were burned before the
punching test, in a furnace with inner dimensions of
3500×2000×900mm used for the purpose. The fire source
consisted of 20 methane burner nozzles distributed at the
bottom of the furnace sides and the temperature was monitored
by a digital thermometer reader of ATP DT-612 model, and a
thermocouple sensor wire type K as shown in Figure 3. All the
specimens were tested in punching test conditions as simply
supported and loaded using one concentrated load applied at
the center of the column. The vertical deflection at each

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loading stage was measured using mechanical dial gauges of
0.01mm sensitivity allocated at the center and at the quarter
span in each direction. Figure 4 shows the punching test setup.

Fig. 3. Furnace and burning procedure.

Fig. 4. Test setup.

VI. RESULTS AND DISCUSSION

A. Initial Crack Load (Pcr) and Ultimate Punching Load (Pu)

Table IV shows the initial crack (Pcr) and the ultimate
punching (Pu) loads. The results proved that adding fibers to
the concrete mix increases the initial crack load and the
ultimate load for all the tested specimens even at high elevated
temperatures (600°C) since the fibers bridge the tensile cracks
and allows the transferring of the stresses through the opposite
sides of the cracks. This means that slab tension zone is still
able to resist additional stresses. From the results of Group 1
(Table IV) it can be concluded that specimen SF1.5-NF of
1.5% steel fiber volume fraction presented the optimum
improvement in the initial crack load and the ultimate load by
about 40% and 16.6% respectively, as compared with the
reference specimen NFC-NF. The highest improvement in the
initial crack load and the ultimate load for the burned
specimens was recorded in the models GF0.5-SF1-F500 and
GF0.5- SF1-F600 which contained two types of fibers (steel
and glass fiber). The modification percentages reached 63.1%

and 27.2% and 96.3% and 34.6% for steady state temperatures
of 500 and 600°C respectively, as compared with the reference
specimen of each group. The disadvantage behavior for the
burned specimens SF1.5-F500 and SF1.5-F600 belongs to the
considerable thermal expansion level of steel material
(10×10

-6
/°C) compared with those of concrete and glass

materials (10×10
-6
/°C and 4.9×10

-6
/°C respectively). The ratio

(1.5%) of steel fibers resulted in a substantial reduction in the
concrete compressive strength in comparison with the ratio
(1%) due to the expansion damage effect of the steel fiber
which was greater in the 1.5% specimen at the adopted burning
temperatures (500 and 600°C). Moreover, hybrid fibers of
different sizes and styles provide different restriction
conditions, and these conditions can be the result of the
mechanical bond strength modification since each type can
delay micro-crack formation and prevent their propagation
[19].

TABLE IV. INITIAL CRACK AND ULTIMATE LOAD RESULTS

Group Specimens
f 'c

(MPa)

Pcr

(kN)

Increase in

Pcr %

(kN)

Increase in

Pu %

NFC-NF 35.52 50 --- 180 ---

SF1.5-NF 40.32 70 40 210 16.6

GF0.5-SF1-

NF
39.2 68 36 205 13.9

NFC-F500 21.81 38 --- 147 ---

SF1.5-F500 26.37 53 39.5 175 19

GF0.5-SF1-

F500
28.24 62 63.1 187 27.2

NFC–F600 18.36 27 --- 130 ---

SF1.5-F600 21.83 40 48.1 158 21.5

GF0.5-SF1-

F600
25.85 53 96.3 175 34.6

B. Load-Deflection Relationships

Figure 5 shows the load-deflection relationships for the
unburned and burned specimens at 500 and 600°C respectively.
Two zones were recognized in the unburned specimens’
curves. The first refers to the behavior up to the initial crack
load generation where the responses of all the tested specimens
were similar and approximately linear. The second zone
extended beyond the initial crack load generation during which
other cracks were developed and the slope of the load-
deflection curves was decreased rapidly as a result of specimen
stiffness reduction. Different behavior in the burned specimens
at 500°C was detected, i.e. the linear stage limit was reduced.
Moreover, in the case of 600°C, the behavior was completely
nonlinear due to the increasing fire-induced damage. In Figure
5(b)-(c), a strain hardening case can be observed in the
reference specimens resulting from the highly fire damaged
concrete which led to an increase of the steel reinforcement
stresses. This case was prevented in the fiber reinforced
specimens, since the steel fiber bridges the concrete parts and
softens their tensile strength behavior. Moreover, it can be
observed that, at the same level of loading, fiber reinforced
specimens (unburned and burned) had less deflection than that
of the reference specimens (without fiber) since fiber enhances
the stiffness of the specimens due to its bridging role between
the two sides of cracks and this can be noticed more vividly
beyond the cracking stage. For the same level of loading, the

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specimen of 1.5% steel fiber had the minimum deflection of
Group 1 (Figure 5 (a)), whereas the specimens that contained
two types of fibers showed less deflection (Figure 5(b)-(c)).
This usually reflects specimen stiffness modification.

(a)

(b)

(c)

Fig. 5. Load- deflection curves: (a) Group 1, (b) Group 2, (c) Group 3.

Table V symmetrizes the outcomes of the tested specimens
including the ultimate punching loads and their corresponding
ultimate deflections. The specimens were grouped according to
the fiber’s type (no fiber, steel fiber, and mixed fibers). In
general, there is a significant reduction in the punching shear
resistance with an increase in the peak deflection of the burned
specimens compared with the corresponding unburned
specimens. The maximum drop (27.78%) in the punching load
was detected at 600°C burning temperature by the specimen
NFC-F600. This was improved to 24.76% when steel fiber was
added whereas the optimum modification was presented by the
specimen GF0.5-SF1-F600 in which the reduction percentage
was 14.63%. The main reason for this is that the fire damage
caused a reduction in the stiffness of the burned slabs.

TABLE V. VARIATION IN THE ULTIMATE LOAD AND DEFLECTION
AT DIFFERENT BURNING TEMPERATURES

Specimen
Temperature

(
o
C)

(kN)

Reduction

in Pu%

Ultimate

deflection ∆∆∆∆u

(mm)

Increase

in ∆∆∆∆u %

NFC–NF 25 180 ---- 15.50 ----

NFC-F500 500 147 18.33 16.76 8.13

NFC-F600 600 130 27.78 17.42 12.39

SF1.5-NF 25 210 ---- 17.22 ----

SF1.5-F500 500 175 16.67 18.86 9.52

SF1.5-F600 600 158 24.76 19.65 14.11

GF0.5-SF1-

NF
25 205 ---- 16.95 ----

GF0.5-SF1-

F500
500 187 8.78 19.33 14.04

GF0.5-SF1-

F600
600 175 14.63 20.28 19.65

C. Crack Patterns

After the burning stage, there were hair cracks distributed
randomly on the burned surface of the specimens as shown in
Figure 6. These hair cracks were more in number and more
clearly observed in the burned specimens at 600°C. For all the
tested specimens, additional cracks appeared around the
column at the tension face of the tested specimen, during the
loading stage. When the applied load was further increased,
these cracks' were width and number increased and they were
extended diagonally towards the specimens' edges causing
failure in punching mode as shown in Figure 7.

(a) (b)

Fig. 6. Hair cracks in the burned specimens at (a) 500°C, (b) 600°C.

Actually, fibers decreased the crack width for both
unburned and burned specimens. The average maximum crack
width in Group 1 was 0.65mm for no fiber specimens and
0.50mm for fiber reinforced specimens. In Group 2, it was
0.80mm for specimens without fibers and 0.60mm for fiber
reinforced specimens, whereas in Group 3, it was 0.95 and
0.70mm respectively. It was also noticed that the punching
failure in the reference specimens (without fiber) was sudden
with brittle mode, whereas the fiber reinforced specimens
failed in a gradual way with a more ductile mode since the fiber
presence in the concrete mix improved the specimens' ductility
by increasing the ultimate deflection. The burned reference
specimens NFC-F500 and NFC-F600 exhibited extensive cover
spalling in the tension side of the specimens at failure stage
resulting from the strength deterioration of concrete caused by
burning. In the fiber reinforced specimens, the spalling in the
concrete cover was more controlled. This indicates that fiber
presence in the concrete mix contributes to the connection
between the damaged concrete parts in the tension side of the
specimens.

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(a)

(b)

(c)

Fig. 7. Crack patterns in (a) Group 1,(b) Group 2, (c) Group 3.

D. Area, Perimeter, and Angle of Failure

For all the tested specimens, the punching failure mode had
approximately the shape of a pyramid. Table VI illustrates the
perimeter, the area, and the failure angle of the punching failure
zone for the unburned and burned specimens. The failure angle
was measured considering the dimensions of the crushed zone
at the center line passing through the loaded area. Several
studies indicated that fiber reinforced concrete slabs had failure
angle higher than that in the normal reinforced concrete slabs
where the smaller value of failure angle means wider base for
the failure pyramid zone that’s pushed out [20]. It can be
concluded that, for both unburned and burned specimens, the
failure angle for the fiber reinforced specimens was larger than
that in the reference specimen in each group which means that
the failure punching perimeter was smaller in the fiber
reinforced specimens in comparison with the reference
specimens. This usually belongs to the fiber contribution that
prevents the disintegration in the concrete cover below the
flexural steel reinforcement and helps to integrate the whole
section. The results obtained in Group 1 indicate that the
specimens with 1.5% steel fiber volume fraction had the
highest decrease in the punching failure perimeter by about
20.7% in comparison with the reference specimen (NSC-NF).
On the other hand, specimens containing two mixed types of
fibers (steel and glass) showed the optimum modification in the
perimeter of the failure punching by about 22% and 21.3% for
Groups 2 and 3 respectively.

TABLE VI. PERIMETER, AREA AND FAILURE ANGLE

Group Specimen
Perimeter

(mm)

Perimeter

decrease (%)

Area

(mm
2
)

Failure

angle (Ø°)

NFC-NF 3080 --- 592900 17.9

SF1.5-NF 2440 20.7 372100 23.5

GF0.5-SF1-

NF
2600 15.6 422500 21.8

NFC-F500 3640 --- 828100 14.7

SF1.5-F500 3160 13.2 624100 17.3

GF0.5-SF1-

F500
2840 22 504100 19.6

NFC–F600 4120 --- 1060900 12.8

SF1.5-F600 3560 13.6 792100 15.2

GF0.5-SF1-

F600
3240 21.3 656100 16.8

VII. CONCLUSIONS

The following conclusions can be drawn from the
experimental work outcome:

• There is a reduction in the initial load crack and the ultimate
load of the burned slabs compared with the corresponding
unburned slabs. This reduction was improved by adding
fiber to the specimens.

• After burning, random hair cracks were observed on each
slab. These hair cracks were multiplied and more clearly
observed in the specimens burned at 600°C than in the
specimens burned at 500°C.

• Adding fibers to the concrete has a significant effect on the
punching failure perimeter zone, crack width, and concrete
spalling for all the tested slabs (unburned and burned).

• Fiber reinforced slabs failed gradually in a more ductile
mode. While the failure was sudden with brittle mode in
slabs that contained no fibers.

• Steel fiber ratio 1.5% by volume had more effect in
improving the response than the two mixed fibers (steel and
glass) regarding the unburned specimens. For burned
specimens, the two mixed fiber specimens had more
improved response for both considered burning
temperatures.

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