Experimental Evaluation of the Effects of Steel Fiber

Jordan Journal of Civil Engineering, Volume 12, No. 3, 2018

- 484 - © 2018 JUST. All Rights Reserved.

Experimental Evaluation of the Effects of Steel Fiber Percentage on

the Mechanical Behavior of Reinforced SCC Beams Subjected to

ATC-24 Loading Protocol

H. R. Tavakoli 1), Pedram Jalali 2) and Sina Mahmoudi 3)

1),2),3) Department of Civil Engineering, Babol University of Technology, Babol, Iran. * Corresponding Authoor. E-Mail: [email protected]

2) E-Mail: [email protected] 3) E-Mail: [email protected]

ABSTRACT

Beams containing steel fiber with different fractions (0.1%, 0.2% and 0.3% of volume ratio), as well as without

fiber as reference beam, were fabricated and tested in the laboratory. Monotonic and cyclic loadings were

applied to 12 specimens with different mix designs. Reinforcement details and constituents except for fraction

of fiber were constant in all specimens. The loading protocol which specimens were subjected to was adapted

from ATC-24 guideline. The results achieved from this investigation indicated that adding more fiber in

concrete would improve load bearing capacity in the monotonic and cyclic behavior of the specimens. By

increasing fraction of fiber in concrete, the amount of dissipated energy would increase.

KEYWORDS: Steel fiber, SCC beam, Cyclic loading, ATC-24 loading protocol, Dissipated energy.

INTRODUCTION

Self-compacting concrete (SCC) was presented for

the first time in 1988 in order to achieve a concrete with

stable structure. The early studies on the efficiency of

SCC were conducted by Ozawa and Okamura at Tokyo

University (Okamura, 1997; Okamura and Ozawa,

1994; Ozawa et al., 1996). According to ACI-237R-07

definition, SCC is a highly flowable, non-segregating

concrete that can spread into place, fill the form work

and encapsulate the reinforcement without any

mechanical consolidation (Tavakoli et al., 2017).

Due to the brittle nature of concrete and its limitation

in strain capacity, fiber-reinforced concrete (FRC) has

been pronounced as a construction material to improve

the behavior of concrete in tension, in addition to other

aspects. The reason for using discrete fiber in concrete

is to improve the results after cracking with controlling

the opening and distribution of cracks (Tavakoli et al.,

2018). The mechanical behavior of FRC highly depends

on material, geometry, shape and the physical and

chemical properties of the fiber, but generally, using

fiber is considered as an important step in avoiding

cracks and micro-crack distribution and compensation

of tensile strength weakness (Balaguru and Shah, 1992).

In the past few decades, using fiber as an admixture

in concrete and reinforced concrete members was the

main topic of many investigations (Masud and

Chorzepa, 2015; Song et al., 2004; Valipour et al., 2015;

Wang et al., 2015). Studies have been conducted on

many aspects of using fiber in cementitious materials,

but only a few of them have been carried out on the

mechanical behavior of structures under seismic loads,

Received on 27/11/2017. Accepted for Publication on 14/4/2018.


- 485 -

which have been modeled via cyclic loading (Shaheen

et al., 2003; Yun et al., 2008). The results of past studies

indicated that adding fiber to RC members enhances the

energy dissipation capacity and strength.

For evaluating the behavior of structures under

major earthquakes in experimental and analytical

investigations, a series of loading regimes based on the

world's most reliable codes and standards was used. The

main aim was to present an appropriate loading history,

which is statistically representative of the full range of

characteristics. This issue becomes possible by adopting

an increasing magnitude loading history. In most

practical cases (except for near-fault ground motions),

the actual sequence of cycles (excursions) occurring in

an earthquake has to be rearranged into cycles of

increasing amplitude in order to avoid commitment to a

single maximum amplitude that may only have meaning

for a specific combination of ground motion and

structural configuration. There is no unique and best

loading history, because no two earthquakes are alike.

The main issue is to account for cumulative damage

effects through cyclic loading. These protocols may not

be completely representative of the earthquakes, but

they evaluate the mechanical behavior of a structure

under cyclic or dynamic loading.

Type of material and its behavior under excitation

are so effective in selecting a loading history for

application to the specimens. As an example, for a

material such as wood, due to different nature of

behavior than that of steel or reinforced concrete, a

specific loading history is more suitable for evaluating

its cyclic behavior. Because a parameter such as yielding

point, neither can be calculated by theoretical

formulations (due to lack of behavioral theories’

improvement) nor can be specified or observed by

laboratory tests, another parameter should be used to

control loading amplitudes. For this reason, loading

histories corresponding to this kind of material are

offered by different guidelines. However, in reinforced

concrete beams, similar to steel, due to the ability of

determination of yielding point and reliable behavior

prediction, it would be a proper choice to use this

parameter to control the amplitudes. Therefore, ATC-24

loading history is employed in this study, in which

amplitudes in different steps are defined by means of

yielding point. ATC-24 recommendations for

experimental work are intended to produce reliable

information that can serve as a basis for analytical

modeling or the development of rational design criteria

or that will demonstrate the integrity and safety of

specific structural configurations under various levels of

ground motion (ATC-24, 1992).

In the present study, the specimens are subjected to

ATC-24 loading protocol and the effects of fiber

percentage (1%, 2% and 3% of volume fraction) are

studied on the mechanical properties of the beams.

Hysteresis loops, envelope curves and energy

dissipation capacity are analyzed and some valuable

results are achieved.

Experimental Program

Materials

For this investigation, hooked steel fibers with 36

mm length, 0.7 mm diameter, aspect ratio of 50 and

yielding strength of 2100 Mpa were added to the

concrete in different volume ratios. P10-3R super-

plasticizer based on modified polycarboxylate ether

with bulk density of 1.1 ⁄ was used. Coarse

aggregate and fine aggregate with maximum dimensions

of 12.5 and 4.75 mm were adopted for this test. Type 2

Portland cement and limestone powder with bulk

density of 2.6 ⁄ were also used for this study.

Mixture Proportions and Casting

In this study, 4 mixture proportions including one

mix design without fiber as reference concrete and 3 mix

designs with different steel fiber volume fractions (0.1,

0.2 and 0.3%) were fabricated and tested. Table 1

demonstrates details of mixture proportions. In all four

mix designs, the amounts of concrete components are

constant, except the fiber volume percentage. Water to

cement ratio was 0.39.

Experimental Evaluation of.... H. R. Tavakoli, Pedram Jalali and Sina Mahmoudi

- 486 -

Table 1. Mixing proportions ( )

Mix design name

(%)

Coarse

aggregate

Fine

aggregate

Limestone

powder Cement Water

Super-

plasticizer

Cont --- 722 826 288.9 413.1 162 7

ST10 0.1 722 826 288.9 413.1 162 7

ST20 0.2 722 826 288.9 413.1 162 7

ST30 0.3 722 826 288.9 413.1 162 7

In Table 1, refers to the fiber volume percentage.

A conventional rotary concrete mixer was used for

constructing concrete. Dry aggregate, limestone powder

and cement were mixed at first and after that water was

added gradually. Finally, the fiber was added to prevent

balling. After the mixing procedure, the concrete was

put into design frameworks and kept for 24 hours in

laboratory situation. After that, the specimens were put

into water for 28 days.

In the process of making, it should be considered that

adding fiber with different volume percentages to SCC

can decrease its performance (Fernandes et al., 2013).

To be sure about this subject, L-box, slump-flow and T50

tests can be carried out and the results are compared with

the European guideline for self-compacting concrete

specifications.

Details of Specimens

Twelve rectangular SCC reinforced beams with

similar geometry (cross-section of 200 × 150 mm and

height of 1300 mm) and reinforcement detailing were

fabricated in the laboratory for this test. The dimensions

and distributed steel bars of specimens are demonstrated

in Figure 1. Two ribbed bars with 8mm diameter were

applied for longitudinal reinforcement of top and bottom

of the specimens. Yielding stress and ultimate stress of

longitudinal reinforcement are 405 Mpa and 600 Mpa,

respectively. The dimensions of the samples were

selected based on the maximum dimensions and

capacity of the testing machine.

Test Setup and Apparatus

A UTM (Universal Testing Machine) device was

used in this study for applying cyclic load and is

demonstrated in Figure 2. As is shown, the supports at

the end of beams are hinged, so that no bending moment

is formed at supports and the beam can easily rotate.

This machine contains a loading cell with a loading

capacity of 120 kN and a displacement capacity of ±500

mm. In the process of loading, mid-span displacement

of the specimen was measured via LVDT (Linear

Variable Differential Transducer) and two data per

second were recorded. To accurately apply the load to

the specimen, steel plates are fixed on top and bottom of

the beam via bolts and the load is applied through 180

mm steel plate in the middle of the beam to avoid local

bearing crush failure caused by point load pressure.

Figure (1): Dimensions and distributed steel bars


- 487 -

Figure (2): Testing apparatus

Loading Program As demonstrated in Figure 3, loading cycle

amplitudes in ATC-24 loading protocol are based on ∆ . Therefore, for this study, because of using fiber

with different ratios in concrete mixture and as a result

changing the concrete's properties, ∆ is obtained from

monotonic loading of the beam specimens for each

design proportion. For this purpose, two specimens from

each mix design were tested monotonically and from the

average results, the yielding point for each of them was

determined separately. Afterwards, ∆ ∆⁄ ratio was

calculated and given to the loading device for

application to the specimens. ATC-24 loading protocol

consists of 7 steps of loading amplitudes, in which for

the first five steps, 3 cycles with repetitive amplitudes

and for the last two, 2 cycles with repetitive amplitudes

are applied.

A low and constant loading rate of 2 mm/min was

adopted for this test to minimize the dynamic effects of

high loading rate and to allow observing the events and

variations during the tests.

Figure (3): ATC-24 loading protocol

RESULTS AND DISCUSSION

Monotonic Response

To study the monotonic behavior of the beams with

different mix designs, one specimen from each design

was tested monotonically. To prove the similarity of the

specimens (similarity in fabrication, casting and

loading), another specimen was tested for each mix

design (repeatability test). The average monotonic

behavior of two specimens of each mix design is

demonstrated in Figure 4 as representative of that

design. The displacement and force values

corresponding to yielding points for all mix designs are

listed in Table 2. Furthermore, displacement amplitudes

for each mix design and every step of ATC-24 are

calculated and presented in Table 3.

Cyclic Response

During major earthquakes, it is expected that the

structure enters into elasto-plastic zone and hysteresis

loops can provide a proper understanding for elasto-

plastic response analysis (Xue et al., 2008). Force-

displacement hysteresis response demonstrates the

energy dissipation capacity of the structure by

considering the area enclosed by the hysteresis loops.

Figure 5 shows the force-displacement hysteresis curves

for all four mix designs.

-600

-400

-200

0

200

400

600

Δ/ Δ

yiel

d(%

)

Number of cycles

5 75200

300400

500

100

3 6 9 12 15 17 19

50


- 488 -

Table 2. Force and displacement values of yielding points

Yielding Displacement Yielding Force

CONT ST10 ST20 ST30 CONT ST10 ST20 ST30

Mon1 1.4359 1.8499 2.2973 2.6606 23797.8 30598.79 36360.80 37983

Mon2 1.3829 1.8125 2.2977 2.5582 23252.3 30164.40 36309.20 37574.8

Average 1.4094 1.8312 2.2975 2.6094 23525.05 30381.60 36335 37778.9

Table 3. Loading amplitudes of ATC-24

Mix Design

Yield Displacement

(mm)

Loading Amplitude (Percentage of ∆ ) (mm)

50% 75% 100% 200% 300% 400% 500%

Cont 1.45 0.725 1.0875 1.45 2.9 4.35 5.8 7.25

ST10 1.85 0.925 1.3875 1.85 3.7 5.55 7.4 9.25

ST20 2.3 1.15 1.725 2.3 4.6 6.9 9.2 11.5

ST30 2.6 1.3 1.95 2.6 5.2 7.8 10.4 13

Figure (4): Monotonic behavior for various mix designs

In the presented hysteresis curves for each mix

design, the corresponding monotonic curves are also

placed in the diagram to show that the monotonic curves

always cover hysteresis loops, and because of

cumulative damage increment with the increase of

loading amplitude and cycles, the gap between the two

curves becomes more obvious.

Hysteresis curves indicate that cyclic behavior of the

0

10000

20000

30000

40000

50000

60000

70000

0 10 20 30 40 50

For

ce (

N)

Mid Displacement (mm)

CONT

ST10

ST20

ST30


- 489 -

specimens is linear before cracking and after that, in non-

linear zone, the behavior of the specimens begins to

change. Until the yielding point, stiffness and strength

degradation caused by cyclic loading are negligibles, but

after cracking, slope of the curves (secant stiffness)

decreases with the increase of displacement. Results

achieved based on Figure 5 may be summarized as follows:

In the specimen using reference concrete design,

cracking of the concrete's cover was severe, while in

specimens containing fiber, this phenomenon was rarely

observed. In fibrous specimens, fiber causes integration

in cement matrix and results in less cracking in concrete

cover.

Even though more than two cracks were observed in

all beam specimens, with increasing the amplitude of

displacement, two major cracks propagate on top and

bottom of the beams in a distance equal to the width of

loading plates representing two plastic hinges.

It is assumed that by using fiber in concrete mixture,

instead of the occurrence of a few wide cracks, the

number of cracks increases while their width

diminishes. The width of cracks observed in FRC beam

specimens was less than in the specimen without fiber.

This difference in crack width is more obvious in

loading cycles with higher amplitude than the yielding

point.

In similar and consecutive loading cycles, stiffness

and strength degradations were observed. With

increasing loading cycles, the cumulative damage

effects become more obvious and the behavioral

parameters of the specimens confront more degradation.

Figure (5): Hysteresis curves for different mix

designs: (a) CONT; (b) ST10; (c) ST20; (d) ST30

Envelope Curve

To study the effects of adding fiber on the bearing

capacity of the specimens, the maximum force values

obtained from both positive and negative bending cycles of

each specimen are considered. The resulting curve from

connecting the maximum points is presented as the envelope

curve and is demonstrated in Figure 6 for all 4 mix designs.

-40000

-30000

-20000

-10000

0

10000

20000

30000

40000

-10 -5 0 5 10

For

ce (

N)


Cyclic

Mono 1

Mono 2

(a)

-50000

-40000

-30000

-20000

-10000

0

10000

20000

30000

40000

50000

-15 -10 -5 0 5 10 15

For

ce (

N)

Mid displacement (mm)

Cyclic

Mono 1

Mono 2

(b)

-60000

-40000

-20000

0

20000

40000

60000

-15 -10 -5 0 5 10 15

For

ce (

N)


Cyclic

Mono 1

Mono 2

(c)

-60000

-40000

-20000

0

20000

40000

60000

-15 -10 -5 0 5 10 15 20

Fro

ce (

N)


Cyclic

Mono 1

Mono 2

(d)


- 490 -

Figure (6): Envelope curves

The difference observed in the force-displacement

behavior between positive and negative bending

positions is because of two main reasons. In the stage of

pouring concrete into the framework, the fibers tend to

sediment within the concrete's mixture. Therefore, the

distribution of fibers differs from top to bottom (more in

the bottom) and accordingly the bending capacity of the

section in positive bending position would be more.

Another reason for this difference is the crack

occurrence. In the elastic zone, the force values for

similar displacements in both positive and negative

bending positions are approximately equal. But, in the

inelastic behavior zone, with the specimen cracking in

positive bending position, a drop in strength parameters

and force capacity would result; therefore, with

changing the bending position to negative, for the same

displacement amplitude, less force would be achieved.

It is essential to note that because of the difference in

the yielding point of each mix design, the displacement

amplitude of loading for each of them varies. However,

to compare the results, a same displacement can be used

and as demonstrated in Figure 6, with the increase of

fiber dosage in concrete, the force capacity of the

specimen increases.

Dissipated Energy

The energy entering into structures has two forms:

dissipated energy and recoverable energy. Total

absorbed energy of the structure is equal to the sum of

dissipated energy and recoverable energy. Recoverable

energy includes the element capacity to retain its

primary conditions (Elmenshawi and Brown, 2010 a).

Dissipated energy in a cycle is represented by the area

enclosed by the corresponding load-deflection curve of

the specimen (Figure 7) and is representative of the

structural element capacity to extinguish the earthquake

effects through inelastic behavior of reinforcement steel

bars, which causes excessive fractions and perdurable

deformations (Elmenshawi and Brown, 2010a;

Gençoğlu and Eren, 2002; Maekawa and Dhakal, 2001;

Qiu et al., 2002; Xue et al., 2008). Energy of oscillatory

systems could be dissipated via different mechanisms

and mostly there would be more than one mechanism at

the same time. In vibration of a reinforced concrete

structure, these mechanisms comprise: cracking,

reinforcement yielding and friction between concrete

and steel bars during the slippage. In reinforced fibrous

concrete, friction between fiber and matrix, fiber

yielding, inelastic behavior, fraction and pulling out

fiber from matrix are additional mechanisms of

dissipating energy (Chopra, 2007; Laurent and Ahmed,

2002). Non-linear static methods for design use

parameters dependent on energy dissipation capacity for

assessing non-linear seismic response of structures and

for describing strength and stiffness deterioration in

reinforced concrete elements which are subjected to

cyclic loading (Eom and Park, 2010; Rodrigues et al.,

2012). More dissipated energy represents better

performance of the element (Elmenshawi and Brown,

2010 b).

Because of the different input excitations for each

mix design, to normalize the dissipated energy values,

yielding force and displacement parameters were

adopted from the corresponding monotonic loading

curves. The results achieved from the normalized

dissipated energy in the first cycle of every step of cyclic

loading are presented in Figure 8. As demonstrated, with

the increase of loading amplitude, the dissipated energy

values increase as well.

Also, adding fiber increases the dissipated energy.

As far as in the 6th step of loading, the percentage of

-50000

-40000

-30000

-20000

-10000

0

10000

20000

30000

40000

50000

-15 -10 -5 0 5 10 15

For

ce (

N)


CONT

ST10

ST20

ST30


- 491 -

dissipated energy increment in comparison to the

reference concrete for 0.1%, 0.2% and 0.3% of fiber

steel percentage is 26%, 28% and 42%, respectively.

The effect of adding fiber is more obvious in higher

amplitudes, where the specimen has cracked and the

bond effect of fiber becomes more significant.

Figure (7): Dissipated energy loop with definitions

Figure (8): Normalized dissipated energy

0

0.5

1

1.5

2

2.5

3

3.5

1 2 3 4 5 6 7

Nor

mal

ized

Dis

sipa

ted

ener

gy

Loading step

CONT

ST10

ST20

ST30


- 492 -

CONCLUSIONS

An experimental study is accomplished on 12

reinforced self-compacting concrete beams containing

different fractions of steel fiber. Monotonic and cyclic

loadings were used to assess the mechanical behavior of

specimens in each mix design. Effects of the amount of

steel fiber on various parameters are investigated and the

results are summarized as follows:

1. By adding more fiber in concrete mix design,

yielding point in monotonic behavior would improve

and the specimen would undergo more force and

displacement elastically.

2. In cyclic loading, by increasing steel fiber in

concrete, force capacity of the members would

increase in an identical displacement, as it is obvious

from the envelope curves.

3. By increasing steel fiber in concrete, more dissipated

energy can be observed from the hysteresis curves.

In elastic amplitudes, the amounts of dissipated

energy for different mix designs are close to each

other and negligible. However, for amplitudes more

than the yield point, differences would appear.

REFERENCES

ATC-24. (1992). "Guidelines for cyclic seismic testing of

components of steel structures". ATC-24, Redwood

City, CA.

Balaguru, P., and Shah, S. (1992). "Fiber-reinforced cement

composites". McGraw-Hill, Inc., New York, USA.

Chopra, A.K. (2007). "Dynamics of structures: theory and

applications to earthquake engineering". Prentice-Hall.

Elmenshawi, A., and Brown, T. (2010 a). "Hysteretic energy

and damping capacity of flexural elements constructed

with different concrete strengths". Engineering

Structures, 32 (1), 297-305.

Elmenshawi, A., and Brown, T. (2010b). "Seismic behavior

of 150 MPa (22 ksi) concrete flexural elements". ACI

Structural Journal, 107 (3), 311.

Eom, T.-S., and Park, H.-G. (2010). "Evaluation of energy

dissipation of slender reinforced concrete members and

its applications". Engineering Structures, 32 (9), 2884-

2893.

Fernandes, C., Melo, J., Varum, H., and Costa, A. (2013).

"Cyclic behavior of sub-standard reinforced concrete

beam-column joints with plain bars". ACI Structural

Journal, 110 (1), 137.

Gençoğlu, M., and Eren, I. (2002). "An experimental study

on the effect of steel fiber-reinforced concrete on the

behavior of the exterior beam-column joints subjected to

reversal cyclic loading". Turkish Journal of Engineering

and Environmental Sciences, 26 (6), 493-502.

Laurent, D., and Ahmed, L. (2002). "Behavior of high-

strength fiber-reinforced concrete beams under cyclic

loading". Structural Journal, 99 (3).

Maekawa, K., and Dhakal, R. (2001). "Post-peak cyclic

response analysis and energy dissipation capacity of RC

columns".

Masud, M., and Chorzepa, M. G. (2015). "Impact resilience

of multi-scale fibre-reinforced composites". Magazine

of Concrete Research, 68 (8), 379-390.

Okamura, H. (1997). "Self-compacting high-performance

concrete".

Okamura, H., and Ozawa, K. (1994). "Self-compactable

high-performance concrete in Japan". International

Workshop on High-Performance Concrete (Edited by

Paul Zia).

Ozawa, K., Maekawa, K., and Okamura, H. (1996). "Self-

compacting high-performance concrete". Collected

Papers (University of Tokyo: Department of Civil

Engineering), 34, 135-149.


- 493 -

Qiu, F., Li, W., Pan, P., and Qian, J. (2002). "Experimental

tests on reinforced concrete columns under biaxial

quasi-static loading". Engineering Structures, 24 (4),

419-428.

Rodrigues, H., Varum, H., Arêde, A., and Costa, A. (2012).

"A comparative analysis of energy dissipation and

equivalent viscous damping of RC columns subjected to

uniaxial and biaxial loading". Engineering Structures,

35, 149-164.

Shaheen, H., Rakib, T., Hashem, Y., Shaaban, I., and

Abdelrahman, A. (2003). "Behaviour of RC columns

retrofitted by fiber-reinforced polymers under cyclic

loads". Proceedings of the Sixth International

Symposium on FRP Reinforcement for Concrete

Structures (FRPRCS‐6), 10.

Song, P., Hwang, S., and Sheu, B. (2004). "Statistical

evaluation for impact resistance of steel-fibre-reinforced

concretes". Magazine of Concrete Research, 56 (8), 437-

442.

Tavakoli, H. R., Jalali, P., and Mahmoudi, S. (2018). "A

comparative study of the effect of adding steel fiber on

the mechanical behavior of RSCC beams under

displacement and force control conditions".

Construction and Building Materials, 166, 315-323.

Tavakoli, H. R., Mahmoudi, S., Goltabar, A. R., and Jalali,

P. (2017). "Experimental evaluation of the effects of

reverse cyclic loading rate on the mechanical behavior

of reinforced SCC beams". Construction and Building

Materials, 131, 254-266.

Valipour, H., Vessali, N., and Foster, S. (2015). "Fibre-

reinforced concrete beam assemblages subject to

column loss". Magazine of Concrete Research, 68 (6),

305-317.

Wang, J., Morikawa, H., and Kawaguchi, T. (2015).

"Experimental and analytical investigations of bonding

ultrahigh-strength fibre-concrete panels on reinforced

concrete structures with low concrete strength".

Magazine of Concrete Research, 67 (24), 1329-1339.

Xue, W., Li, L., Cheng, B., and Li, J. (2008). "The reversed

cyclic load tests of normal and pre-stressed concrete

beams". Engineering Structures, 30 (4), 1014-1023.

Yun, H.-D., Kim, S.-W., Jeon, E., Park, W.-S., and Lee, Y.-

T. (2008). "Effects of fibre-reinforced cement

composites' ductility on the seismic performance of

short coupling beams". Magazine of Concrete Research,

60 (3), 223-233.

Documents

Experimental Evaluation of the Effects of Steel Fiber