Pull-Out Behaviour of Hooked End Steel Fibres Embedded in Ultra-high Performance Mortar with Various W/B Ratios
International Journal of Concrete Structures and Materials
Pull-Out Behaviour of Hooked End Steel Fibres Embedded in Ultra-high Performance Mortar with Various W/B Ratios
Sadoon Abdallah 0
Mizi Fan 0
Xiangming Zhou 0
0 Civil Engineering, College of Engineering, Design and Physical Sciences, Brunel University London , Uxbridge, Middlesex UB8 3PH , UK
This paper presents the fibre-matrix interfacial properties of hooked end steel fibres embedded in ultra-high performance mortars with various water/binder (W/B) ratios. The principle objective was to improve bond behaviour in terms of bond strength by reducing the (W/B) ratio to a minimum. Results show that a decrease in W/B ratio has a significant effect on the bondslip behaviour of both types of 3D fibres, especially when the W/B ratio was reduced from 0.25 to 0.15. Furthermore, the optimization in maximizing pullout load and total pullout work is found to be more prominent for the 3D fibres with a larger diameter than for fibres with a smaller diameter. On the contrary, increasing the embedded length of the 3D fibres did not result in an improvement on the maximum pullout load, but increase in the total pullout work.
pullout behaviour; bond mechanisms; water/binder ratio; hook geometry; embedment length and fibre-matrix interface
Nowadays one of the main challenges that the concrete
industry facing; is how to improve the tensile strength and
durability of mortar and concrete (Petrone et al. 2016;
Alberti et al. 2014). The addition of randomised short fibres
into mortar has shown to be capable of controlling crack
progression as well as resisting tensile stresses (Adjrad et al
2016; Hwang et al. 2016; El-Mal et al. 2015; Sorensen et al.
2014; Islam and Alam 2013a, b; Romualdi et al. 1968). The
efficiency of fibres in transferring applied stresses is greatly
dependent on the fibre-matrix interface properties of fibre
reinforced concrete (FRC) (Abdallah et al. 2016; Dinh et al.
2016; Lu et al. 2016; Tadepalli et al. 2015). Therefore, the
bond-slip characteristics play a crucial role in controlling the
tensile behaviour of FRC (Li and Liu 2016; Srikar et al.
2016; Bentur et al. 1995).
To improve the bonding strength of fibre and matrix
interfaces, a number of modifications may be adopted. The
densification of interfacial transition zone (ITZ) (Chan and
Li 1997) by (a) increasing the fineness of materials particles
(Wille and Naaman 2013), (b) reducing water/binder ratio
(W/B) (Beglarigale and Yazıcı 2015; Beygi et al. 2013;
Markovic 2006), (c) using pozzolanic materials such silica
fume and quartz powder (Shaikh et al. 2016; Abbas et al.
2016; Chan and Chu 2004), (d) optimising surface properties
through chemical treatment to increase adhesion or enhance
surface roughness (Wille and Naaman 2013) and use of
deformed fibres, such as corrugated fibre and hooked end
fibre is a very efficient in improving bond properties and
mechanical properties due to mechanical anchorage (Wu
et al. 2016; Wille and Naaman 2012). One of the most
recommended methods to investigate the bond
characteristics is through single fibre pullout test (Laranjeira de Oliveira
2010). The test configuration is able to simulate the realistic
case of cracking bridging by fibres in a FRC element (joo
Kim, 2009). These test results could provide a
comprehensive understanding on bond-slip characteristics and enable
further improvement to fibre-matrix interfacial properties
(Wille and Naaman 2010).
Ultra-high performance concrete (UHPC) mainly consists
of very fine materials (less than 0.5 mm) and is designated to
generate compressive strength exceeding 150 MPa (Yoo and
Yoon, 2016; Shi et al. 2015; Wang et al. 2015; Wille et al.
2014). Research concerned with incorporating various types
of steel fibres in UHPC has raised significantly interests due
to its ability to achieve ductile behaviour under tension (Lim
and Hong 2016; Abdallah et al. 2016; Kang et al. 2016;
Wille et al. 2014; Lee et al. 2010). Yet, for the same
ingredients of UHPC a significant difference in compressive
strength can be found due to different W/B ratio (Torregrosa
2014). The great effect of W/B ratio on concrete
performance may arise as a result of controlling the capillary pores
which lead to increased concrete strength (Torregrosa 2014).
Although numerous researches has been conducted to
investigate the pullout behaviour of steel fibres embedded in
UHPC matrix, research in the optimization of bond
characteristics of UHPC is limited (Wille and Naaman 2012).
Optimising the fibre-matrix bond characteristics in terms of
bond strength is the key parameter that enhances the tensile
response of UHP-FRC composites (Abdallah et al. 2016;
Cunha 2010). To further achieve the effect of fibre-matrix
properties on pullout behaviour, the quality of ultra-high
performance matrix could be improved by lowering W/B
The objectives of this research are first to understand the
mechanisms of interfacial bonding and bond-slip
characteristics of UHP-FRC composites through a comprehensive
determination of the influence of various parameters, such as
fibre diameter and embedded length. The research is then
aimed at further reducing W/B ratio to a minimum (i.e.\0.2)
and investigating its effect on pull-out behaviour with taking
into account the workability and rheological properties of the
2. Experimental Programme
The ultra-high performance fibre reinforced mortar matrix
(UHP-FRM) with different W/B ratio (W/B = 0.15, 0.20
and 0.25) considered here is produced by the following
ingredients: Portland cement CEM III 52.5 N confirming to
BS EN 197-1; densified silica fume; fine sand (150–600)
micrometres (lm); ground quartz with average particle size
(10 lm); superplasticizer (TamCem23SSR), accelerator (203
accelerator and frostproofer) and water. The mix proportion
adopted in this study is summarized in Table 1. Two types of
commercially available and commonly used 3Dramix
hooked-end steel fibres (3DH) were used to reinforce the
UHPM (Table 2). The geometrical properties of each fibre
type are depicted in Fig. 1 and detailed in Table 2. For each
type of fibre, the fibres were electronically scanned and
measurements of the end-hook geometry were acquired
using the electron microscope (SUPRA 35 VP) (Fig. 2).
2.2 Sample Preparation
The pull-out test specimens prepared were cylinders with a
diameter of 100 mm and height of 50 mm. In each test
specimen, a single steel fibre was carefully placed through a hole
which was made in the bottom of moulds (Fig. 3). Three
different embedded lengths LE (10, 15 and 30 mm) were
investigated in this study. For compressive strength test three
cubes of (100 9 100 9 100 mm) were prepared for each
mixture differentiated in W/B ratio. During mortar fabrication,
the components were firstly dry mixed for approximately
1 min followed by the addition of water and superplasticizer to
the dry mixture, which were then mixed for 11 min. After
casting and vibration, the specimens were covered with a thin
polyethylene film and left for 24 h at room temperature. Then
specimens were removed from their moulds and cured for a
further 28 days in a conditioning chamber (20 ± 2 C,
96 ± 4% RH). The free end of the steel fibre for the pullout
specimens was covered with an anti-rust coating to prevent the
corrosion during the curing to occur. For all series, the test was
carried out at an age of 30 ± 2 days and the average of three
specimens was adopted.
2.3 Test Setup
The pull-out tests were performed using a specially
designed grip system, as illustrated in Fig. 4, which was
attached to an Instron 5584 universal testing machine. The
grips were designed such that the forces applied to the fibre
provided a true reflection of the real situation experienced by
fibres bridging a crack. The body of the gripping system was
machined in a lathe using mild steel and had a tapered end to
allow the insertion of four M4 grub screws (Fig. 4). These
were then tightened around the steel fibre to an equal torque
to ensure an even distribution of gripping pressure and to
minimise deformation or breakage of the fibre ends. Two
linear variable differential transformer (LVDT) transducers
were used to measure the distance travelled by the steel fibre
relative to the concrete face during testing (i.e. the pull-out
distance). They were held in place using aluminium sleeves
on either side of the main grip body (Fig. 4). The LVDTs
had ball bearings at the tips to allow for accurate readings on
the face of the samples. The sample was secured to the
Instron base using clamps with riser blocks and M16 studs.
The specimen was positioned on a brass round disc to
remove any discrepancies in the sample base and allow for
distortion. In all pull-out tests, a displacement rate of 10 lm/
s was adopted.
Table 1 Constituents and proportions of ultra-high performance mortars (kg/m3).
Cement type III 52.5 N
Table 2 General properties of hooked end steel fibres.
2.4 Theoretical Consideration of rmax, sav
In order to assess and compare the pullout behaviour of
the two types of hooked end steel fibres embedded in
different ultra-high performance mortars (UHPMs), the
following parameters are considered based on the experimental
results (Wille and Naaman 2012):
Maximum fibre tensile stress, rmax that can be obtained
by dividing the maximum pullout load, Pmax over
nominal cross-sectional area of the fibre, Af .
Average bond strength, sav, can be defined as the
maximum pullout load based on the initial embedment length
surface area (Wille and Naaman 2013).
where sav is the average bond strength, Pmax is the maximum
pullout load, df is the fibre diameter, and LE is the
embedment length of steel fibre.
Equivalent bond strength, seq can be defined as the
average bond strength based on the total pullout work during the
entire fibre pullout (joo Kim 2009).
Fig. 1 Geometrical properties of hooked end fibres.
Fig. 2 SEM image shows measuring the hook dimensions.
where seq is the equivalent bond strength, Wp is the total
pullout work, df is the fibre diameter, and LE is the
embedment length of steel fibre.
Fig. 3 Single fiber pullout moulds and specimens. a Moulds, b specimens.
3. Results and Discussion
3.1 Fresh and Hardened Properties of UHPMs
To evaluate the workability and rheological properties
of fresh mortars, the slump-flow test according to EN
12350-8:2010 (2010) were performed. It can be seen from
Table 3 that all UHPMs mixtures had excellent rheological
and self-compacting properties (Fig. 5). However, the
reducing of W/B ratio leads to a decrease in slump-flow
diameter (SFD), while time to reach 500 mm spread (T500)
is increased. This is in agreement with other results
reported by (Deeb et al. 2012). The average compressive
strength was remarkably enhanced for all UHPMs by
decreasing W/B ratio (Table 3). This indicates that an
excessive water in the matrix may result in adverse effect
on the formulation of microstructure and hence the property
of the concrete.
3.2 Effect of Water/Binder Ratio on Pullout Behaviour
The average pullout load-slip curves of the two types of 3D
hooked end steel fibres embedded in ultra-high performance
mortar matrix with three different water/binder ratios (W/
B = 0.15, 0.20, 0.25) are presented in Figs. 6 and 7. The
Fig. 4 Pull-out test setup.
maximum pullout load and the total pullout work (the area
under the pullout curve) of both types of hooked end fibres
increase as the W/B ratio decreases (Table 4). It can also be
seen from the curves that decreasing W/B ratio from 0.25 to
0.15 remarkably enhances the maximum pullout load and
pullout work. However, for 3DH1 fibre with LE (15 mm) and
Fig. 5 Slump flow test of UHPM1.
Table 3 Properties of fresh and hardened UHPMs.
Slump flow test
* Average of three specimens.
Fig. 6 Average pullout load-slip curves of 3DH1 fibres. a Embedded length (10 mm), b embedded length (16 mm).
3DH2 fibre with LE (30 mm) a slight difference in pullout
behaviour is observed when W/B ratio decreased from 0.25 to
0.20. The high compressive strength associated with 0.15 W/
B ratio (fc = 172 MPa) and the close compressive strengths
for 0.20 W/B ratio (fc = 152 MPa) and 0.25 W/B ratio
(fc = 149 MPa) can interpret the better pullout behaviour of
the specimens with 0.15 W/B ratio and the similar behaviour
of the specimens with 0.20 W/B and 0.25 W/B ratios.
On the other hand, the pullout response of both types of
fibres exhibit somewhat different slip behaviour before the
second drop of the pullout load. As can be seen in Figs. 6
and 7, the slip capacity of both fibres noticeably increases
when W/B ratio decreases. It has been found that the decease
of W/B ratio not only increases the maximum pullout load
but also effectively enhances the Dpeak (Table 4). This effect
may be attributed to the significant improvement in the
fibrematrix interfacial properties in term of bond strength.
Furthermore, a significant difference in the total pullout work
can be observed due to different slip capacities.
For the 3DH2 fibre with embedded length of 10 mm, the
maximum pullout load is increased by 38.88%, while the
corresponding increase for 3DH1 fibre is only 16.75%, when
W/B ratio decreased from 0.25 to 0.15. In contrast, the
decrease of W/B ratio from 0.25 to 0.20 the maximum
pullout load of the 3DH2 fibre with embedded length of
30 mm is only increased by 1.41%, while for the 3DH1 fibre
with embedded length of 15 mm it is decreased by 4.41%.
This indicates that the decrease W/B ratio from 0.25 to 0.20
does not offer any improvement in maximum pullout load.
On the other hand, the improvement in total pullout work
due to decrease in W/B ratio is relatively more significant
than that in maximum pullout load for both types of fibres
with LE (10 mm). The total pullout work is increased by
52.11 and 25.9% for 3DH2 and 3DH1 fibres, respectively
when W/B decreased from 0.25 to 0.15 (Table 4). These
results are directly related to significant improvement in
bond strength which increases the consumed energy during
the pullout process. Beglarigale et al. (Beglarigale and Yazıcı
2015) have also reported the similar results. It has been
found that increasing the W/B ratio of the reactive powder
concrete (RPC) mixture from 0.2 to 0.3, 0.4, 0.5, and 0.6,
leads to decrease the maximum pull-out load values by 0–8,
Fig. 7 Average pullout load-slip curves of 3DH2 fibres. a embedded length (10 mm), b embedded length (30 mm).
17–20, 37–47, and 43–53%, while, the corresponding
decrease in the total pullout work is 6–16, 4–20, 37–46, and
According to Fig. 8, it can be seen that the average and
equivalent bond strength are remarkably increase due to the
decrease in W/B ratio. However, the great effect of
decreasing W/B ratio is found to be optimal at the W/B ratio
of 0.15, which has the highest values for both the average
and equivalent bond strength. The significant enhancement
in bond strength due to a decrease in W/B ratio from 0.25 to
0.15 may help to explain the noticeably high values of
pullout load and total pullout work. For the 3DH2 fibre with
embedded length of 10 mm, the average and equivalent
bond strength are increased by 38.84 and 52.28%,
respectively, whereas for the 3DH1 fibre it only increased by 25.83
and 16.82%, respectively, when W/B ratio decreased from
0.25 to 0.15.
Figure 9 shows the images of fibres after pullout tests with
various W/B ratios. It can be seen that the end hook of both
types of fibres was somehow straightened, with those pulled
out from the matrix with the W/B ratio of 0.15 being more
straight compared with those in matrix with other W/B
ratios. The reason for this behaviour may be explained by
the enhancement of the fibre-matrix interfacial properties.
This was also confirmed by the remarkable improvement in
pullout behaviour, equivalent bond strength and average
bond strength (Table 4). Although the fibres embedded in
the matrix with 0.20 and 0.25 W/B ratios were completely
deformed, the full straightening of their end hook did not
occur. Nevertheless, the straightening of the end hook of
both types of fibres embedded in matrixes with 0.20 and
0.25 W/B ratios are similar. This reinforces the conclusions
that the decreasing W/B ratio from 0.25 to 0.20 may not
improve the interfacial bond characteristics as in case of
The tensile stress induced in fibre or the maximum fibre
stress is then interpreted and summarized in Table 4 and
Figs. 6 and 7. Although the values of induced stress in both
types of fibres are comparable in matrixes with 0.20 and
0.25 W/B ratios, a significant improvement in the maximum
Table 4 The experimental parameters of pullout test.
fibre stress was achieved for all fibres in matrix with 0.15 W/
B ratio. The maximum fibre stress of the 3DH2 fibre with
embedded length of 10 mm is increased by 25.41%, while
for the 3DH1 fibre only 5.90% when W/B ratio decreases
from 0.25 to 0.20. However, a further decrease in W/B ratio
to 0.15 leads to remarkable increase in the maximum tensile
stress about 38.58, and 16.68% for the 3DH2 and 3DH1
fibres, respectively. This represents an utilisation of about 64
and 67% of extra tensile capacity of these fibres,
3.3 Effect of Fibre Embedment Length on Pullout Behaviour
In order to evaluate the influence of embedment length of
hooked end steel fibres on pullout behaviour, two different
embedded lengths (LE) for each type of fibres have been
considered in this study. For the 3DH1 fibre, the embedment
length investigated is 10 and 15 mm, while that for the
3DH2 fibre is 10 and 30 mm.
Overall, both types of fibres showed extremely similar
pullout behaviour but difference in maximum pullout load
and pullout work (Figs. 6, 7). It is apparent that the increase
of embedment length has no great effect on the maximum
pullout load but it relatively increases the total pullout work.
This can be explained by the slightly higher maximum
pullout load was observed for both types of fibres in 0.20 W/
B ratio series with an embedded length of 10 mm than those
of 15 and 30 mm (Table 4). This is also in accordance with
the results of other researchers (Markovic 2006; Van Gysel
On the other hand, since the measured lengths (L1 ? L2)
of the end hook of the 3DH1 and 3DH2 fibres were
approximately 4.80 and 5 mm (Table 2), respectively. It is
believed that an embedment length of 10 mm which is
roughly twice of the length of the end hook is efficient to
achieve full mobilization and straightened end hook. This
indicates that the pullout behaviour is drastically governed
by the hook component and increasing in embedment length
does not have significant contribution to the maximum
pullout load. On the basis of this, it can be concluded that if
the fibre is fully deformed and straightened, it seems that the
fibres with a shorter embedded length (10 mm) can be used
to obtain the same efficiency as fibres with a larger
embedded length (15 or 30 mm). This was also confirmed
from the results of average and equivalent bond strength in
Table 3. Although the bond strength is drastically enhanced
Fig. 8 Effect of W/B ratio on the bond strength of a 3DH1 fibres, b 3DH2 fibres.
by W/B ratio, the increase in fibre embedment length
remarkably decreases both the average and equivalent
strength. In addition, there is nearly no significant increase in
the maximum pullout load relative to the increase in
embedded length which leads to decrease the bond strength.
It seems that the maximum pullout load significantly
influenced by the plastic deformation of the fibre hook and
increase the embedded length only enhances the frictional
Table 4 also summarizes the key parameters of pullout
behaviour of all series of tests performed in this study. It can
be observed that the maximum fibre stress of the 3DH1
fibres is somewhat higher than that generated by the 3DH2
fibres. On the other hand, although reducing W/B ratio
particularly from 0.25 to 0.15 considerably enhances the
fibres stress-slip behaviour, the increase in embedded length
has no remarkable effect on the maximum fibre stress. It is
noteworthy from Table 4 that for the matrix with 0.20 W/B
ratio the increase in embedded length of both types of fibres
did not improve fibre stress and the values of the maximum
fibre stress is found to be very similar.
In comparing the pullout behaviour of 3DH1 fibre with an
embedded length of 10 mm (Fig. 6a) with that of 15 mm
(Fig. 6b) embedded length, no clear difference is observed,
particularly, for the matrix of W/B ratio of 0.20 or 0.25.
Similarly the 3DH2 fibre showed that the increase in the
embedded length from 10 to 30 mm slightly enhancing the
maximum fibre stress (Fig. 7). This leads to the conclusion
that the increase in fibre embedment length after specific
limit which is 10 mm in this study does not contribute to
maximum fibre stress but only improve the total pullout
work. Although many studies reported that increase the
embedded length of straight fibres can develop higher tensile
stresses during pullout, it appears not to be the case for
hooked-end steel fibres (Markovic 2006). Since the
embedded length of 10 mm seems to be enough for
achieving the full deformation and straightening of the hook,
an increase of embedded length is no longer play an
important role for maximum fibre stress. This behaviour was
also confirmed from the results (Table 4) of the fibre
efficiency ratio (rmax/fy), which represents the maximum tensile
stress induced during pullout, rmax over the fibre tensile
strength, fy. A slight difference is also observed between the
values of (rmax/fy) ratio when the embedded length
increases. Although the increase in embedded length slightly
increases the maximum tensile stress induced by fibres, both
types of fibres embedded in the matrix with 0.20 W/B
showed the same value of (rmax/fy) ratio even with
embedded length increases from 10 to 15 and 30 mm. These results
strongly proved that the end hook of fibres with embedded
Fig. 9 Image shows the comparison between 3DH1 and
3DH2 fibres after pullout.
length of 10 mm can be fully deformed and straightened,
and any increase in the embedded length does not affect
much the pullout behaviour.
3.4 Microscopic Observations (SEM—Scanning Electron Microscopy)
Figure 10a shows SEM images of the steel fibre-matrix
interface of the UHPM1 mixture (W/B = 0.15). It can be
seen from this figure that the particle dispersion and packing
density at fibre-matrix interface is well-developed in the
UHPM1 matrix. This is mainly due to the low W/B and
pozzolanic reactions between silica fume and calcium
hydroxide (CH), which consumes most of the CH crystals
and transforms them to C–S–H (Shi et al. 2015; Chan and
Chu 2004). The densification of microstructures in the ITZ
due to congestion of the hydration products significantly
enhances the bond properties between fibre and matrix. (Lee
and Jacobsen 2011) found that mortar with 0.3 W/B had
higher debonding loads and fracture energies than that of the
0.5 W/B. It has been reported that the incorporation of silica
fume can effectively improve the interfacial bond by
reducing the porosity, refining the pores, and increasing the
density and content of the C–S–H (Zeng et al. 2012;
AbuLebdeh et al. 2011). Also, the lower the porosity, the higher
is the particle packing density in the ITZ and bulk the matrix.
Thus, a higher content of the cement hydration products
such as C–S–H which are important to enhance the
microstructure and microhardness of the ITZ, resulting in
improves the transmission of stress between the fibre and
matrix (Wu et al. 2016).
Figure 10b shows the SEM images of the fibre-matrix
interface of the UHPM2 mixture (W/B = 0.20). It can be
observed from this figure that some pores are formed in the
ITZ. A according to (Lee and Jacobsen 2011), although the
incorporation of 10% silica fume has a positive effect on the
fracture and compressive energies, the improvement in
debonding loads was not observed. They revealed that if
silica fume particle is not dispersed properly in the mortar,
an increased amount of C–S–H through the pozzolanic
reaction cannot be achieved, regardless of W/B. With further
increase in W/B from 0.20 to 0.25, a numerous small and
large pores were observed which have formed along the ITZ
(Fig. 10c). Basically, higher water content is responsible for
forming the pores, ultimately leads to decrease the bond
strength of the UHPM3 mixture significantly. (Wu et al.
2016) has also observed a large porous zone located within
50 mm from the fibre edge for UHPM with 0.18 W/B. This
weak zone could significantly reduce the contact surface area
between fibre and matrix. These facts may help to explain
the relatively lower pull out load and total pull-out work of
the matrix with 0.25 W/B ratio compared with 0.15 W/B.
3.5 Mechanical Anchorage Contribution
of the End Hook to Pullout Behaviour
To get better understanding of the contribution provided
by the end hook to pullout behaviour, a quantitative account
for hook mechanisms has been adopted. This follows that
the proposed procedure is mainly dependent on the
measured hook lengths which are approximately 4.78 and 5 mm
for the 3DH1 and 3DH2 fibres respectively (Table 2). As can
be seen from Fig. 11, the end hook contribution is nearly
being finished at 4.78 mm for the 3DH1 fibre and 5 mm for
the 3DH2 fibre, which corresponds to decay of pullout load
due to complete deformation and straightening of the end
hook. Consecutively, the friction resistance contribution
initiates and continues until fibres completely pullout. On the
other hand, while test results revealed that the debonding
process finishes at fibre slips up to less than 0.1 mm; its
contribution to total pullout work is found to be lower than
1% for all fibres series. Therefore, the contribution due to
debonding process can be neglected. This procedure
provides basic information about the effects of the
parameters such as W/B ratio, diameter, embedded length and
tensile strength of fibres on pullout behaviour.
The percentage contribution of the end hook and frictional
resistance on the total pullout work is summarized in
Table 5. From the experimental results of the both types of
hooked fibres (Table 5), it can be observed that as the
embedded length of fibre increases, the percentage of hook
contribution in terms of total pullout work dramatically
decreases. Although the percentage of hook contribution
with embedded length of 10 mm is significantly higher than
that of the frictional resistance, the increase in embedded
length especially of the 3DH2 fibres drastically increase the
contribution percentage of frictional resistance. The increase
of embedded length of the 3DH1 fibres from 10 to 15 mm
leads to slight decrease in the hook contribution, while for
the 3DH2 fibres the increase in embedded length from 10 to
30 mm results in a sharp decrease in hook contribution up to
half of that in case of 10 mm embedded length. This can be
attributed to large surface area of fibre in contact with
surrounding matrix which increases the frictional resistance to
pullout. Based on experimental results, it appears that the
embedded length has a greater effect on total pullout work
than maximum pullout load.
Table 5 The end hook and frictional resistance contribution to the total pullout work.
End hook contribution %
Fig. 12 Pullout process of a hooked end steel fibre.
3.6 Difference in the Pullout Behaviour of Two
Hooked End Fibres
The observed pullout load-slip curves of hooked end
steel fibres embedded in UHPM is generally characterised
by a steady increase up to peak load as a result of the
combination of two mechanisms which are: detachment of
the fibre-matrix bond and mechanical anchorage of the end
hook. Once the fibre-matrix bond is fully detached, two
plastic hinges of the fibre hook undergo cold work causing
deformation and bending of the end hook (Alwan et al.
1999), in Fig. 12, the two plastic hinges are identified as 1
and 2. As a result of deformation and slippage of the first
plastic hinge a sharp decrease in pullout load takes place.
Nevertheless, initial increase in pullout load can be
observed due to the progressive deformation of the second
plastic hinge in conjunction with straightening of the end
hook. The last stage of the pullout will occur under sliding
friction until complete pullout of fibre from the mortar
The comparison of the pullout behaviour between the two
hooked end fibres shows that the maximum pullout load of
the 3DH2 fibres is approximately more than two times that
of the 3DH1 fibres for all W/B ratios. Moreover, the total
pullout work of the 3DH2 fibres embedded up to half fibre
length (LE = 30 mm) is roughly five times that the 3DH1
fibres (LE = 15 mm). It is believed that the reason for the
enhanced pullout work is due to the increased embedded
length which leads to large surface area of fibre in contact
with surrounding matrix. Note that the embedded length of
the 3DH2 fibre (LE = 30 mm) is two times that of the 3DH1
fibre (LE = 15 mm), to allow fair comparison an
embedment length of 10 mm for both types of fibres is considered
here. The pullout work of the 3DH2 fibre is also
approximately three times greater than that of the 3DH1 fibre. This
may be attributed to the fibre diameter that increases the
bending stiffness of fibre hook, because more energy is
required during the pullout process.
On the other hand, although the decrease in W/B ratio
has positive effect for both types of fibres, this effect is
more pronounced for the 3DH2 (diameter of 0.9 mm) than
that of the 3DH1 (diameter of 0.55 mm) fibre. The
reduction of W/B ratio from 0.25 to 0.15 leads to increases in
maximum pullout load of the 3DH2 is 38.88%, which is
approximately more than two times that achieved by the
3DH1 fibres (16.75%). These results suggest that the
fibre with larger diameter is considerably influenced by
enhancing fibre-matrix interfacial properties than that with
For the fibre stress-slip, the induced stress in the 3DH1
fibres which have smaller fibre diameter (df = 0.55), is higher
than that of the 3DH2 fibres with (df = 0.90) (Fig. 12). This
may be due to the larger cross-sectional area of the 3DH2
fibre which is approximately 2.6 times greater than that of
the 3DH1 fibre. The maximum tensile stress induced in the
3DH1 fibres with the embedded length of 10 mm is higher
by 21.68, 22.11 and 44.52% than those in the 3DH2 fibres
for 0.15, 0.20 and 0.25 W/B ratio, respectively. Despite
the fact that the tensile strength of the 3DH1 fibres
(fy = 1345 MPa) is higher than that of the 3DH2 fibres
(fy = 1160 MPa), a slight difference in the values of (rmax/fy)
ratio were observed. This indicates that the 3DH2 and 3DH1
fibres have somewhat similar efficiency in the utilization of
tensile strength capacity.
The effect of W/B ratio of ultra-high performance mortar
on pullout behaviour of two types of hooked end steel fibres
has been investigated. Based on experimental results, the
following conclusions can be drawn:
1. The maximum pullout load of the 3DH2 fibres was more than two times that of the 3DH1 fibres. In
addition, in case of the same embedded length the total
pullout work of the 3DH2 fibres was about three times
that of the 3DH1 fibres.
For the same fibre geometry, an increase in embedded
length had no appreciable effect on the maximum
pullout load but resulted in a slight improvement in the
total pullout work due to larger surface area of fibre in
contact with surrounding matrix. The little effect of fibre
embedded length on bond properties is due to very
limited difference in length and significant mechanical
anchorage associated with hooked end.
The decrease in W/B ratio from 0.2 to 0.15 had a
significant effect on the overall pullout behaviour.
However, no remarkable contribution could be observed
when W/B ratio decreased from 0.25 to 0.20. The
hooked end fibres with larger diameter would be a better
choice with lower W/B ratio.
For the same embedded length, the equivalent bond
strength of the 3DH2 fibres was approximately two
times greater than that of the 3DH1 fibres for all series.
Though the tensile strength of the 3DH1 fibres was
higher by 16% than that of the 3DH2 fibres, both fibres
showed similar efficiency of utilising its tensile stress
capacity. The mechanical contribution of the 3DH2
fibres would be highly effective if the fibre tensile
strength can be increased.
The first author gratefully acknowledges the financial
support of the Ministry of Higher Education and Scientific
Research of Iraqi Government for this Ph.D. project.
This article is distributed under the terms of the
Creative Commons Attribution 4.0 International License
permits unrestricted use, distribution, and reproduction in
any medium, provided you give appropriate credit to the
original author(s) and the source, provide a link to the
Creative Commons license, and indicate if changes were
Abbas , S. , Nehdi , M. , & Saleem , M. ( 2016 ). Ultra-high performance concrete: Mechanical performance, durability, sustainability and implementation challenges . International Journal of Concrete Structures and Materials , 10 ( 3 ), 271 - 295 .
Abdallah , S. , Fan , M. , & Rees , D. W. A. ( 2016a ). Analysis and modelling of mechanical anchorage of 4D/5D hooked end steel fibres . Materials and Design , 112 , 539 - 552 .
Abdallah , S. , Fan , M. , & Zhou , X. ( 2016b ). Effect of hookedend steel fibres geometry on pull-out behaviour of ultrahigh performance concrete . World Academy of Science, Engineering and Technology, International Journal of Civil , Environmental, Structural, Construction and Architectural Engineering , 10 ( 12 ), 1530 - 1535 .
Abdallah , S. , Fan , M. , Zhou , X. , & Geyt , S. ( 2016c ). Anchorage effects of various steel fibre architectures for concrete reinforcement . International Journal of Concrete Structures and Materials , 10 , 325 - 335 .
Abu-Lebdeh , T. , Hamoush , S. , Heard , W. , & Zornig , B. ( 2011 ). Effect of matrix strength on pullout behavior of steel fiber reinforced very-high strength concrete composites . Construction and Building Materials , 25 ( 1 ), 39 - 46 .
Adjrad , A. , Bouafia , Y. , Kachi , M. , & Ghazi , F. ( 2016 ). Prediction of the rupture of circular sections of reinforced concrete and fiber reinforced concrete . International Journal of Concrete Structures and Materials , 1 , 373 - 381 .
Alberti , M. G. , Enfedaque , A. , Ga´lvez, J. C., Ca´novas , M. F. , & Osorio , I. R. ( 2014 ). Polyolefin fiber-reinforced concrete enhanced with steel-hooked fibers in low proportions . Materials and Design , 60 , 57 - 65 .
Alwan , J. M. , Naaman , A. E. , & Guerrero , P. ( 1999 ). Effect of mechanical clamping on the pull-out response of hooked steel fibers embedded in cementitious matrices . Concrete Science and Engineering , 1 ( 1 ), 15 - 25 .
Beglarigale , A. , & Yazıcı , H. ( 2015 ). Pull-out behavior of steel fiber embedded in flowable RPC and ordinary mortar . Construction and Building Materials , 75 , 255 - 265 .
Bentur , A. , Wu , S. , Banthia , N. , Baggott , R. , Hansen , W. , Katz , A. , Leung , C. , Li , V. , Mobasher , B. , & Naaman , A. ( 1995 ). Fiber-matrix interfaces . In A. Naaman & Reinhardt H. (Eds.), Chapter 5 van pre-proceedings 2nd international workshop high performance fiber reinforced cement composites ( HPFRCC-95) , 11 - 14 juni (pp. 139 - 182 ). Ann Arbor, MI, USA.
Beygi , M. H. A. , Kazemi , M. T. , Nikbin , I. M. , & Amiri , J. V. ( 2013 ). The effect of water to cement ratio on fracture parameters and brittleness of self-compacting concrete . Materials and Design , 50 , 267 - 276 .
Chan , Y. , & Chu , S. ( 2004 ). Effect of silica fume on steel fiber bond characteristics in reactive powder concrete . Cement and Concrete Research , 34 ( 7 ), 1167 - 1172 .
Chan , Y. , & Li , V. C. ( 1997 ). Effects of transition zone densification on fiber/cement paste bond strength improvement . Advanced Cement Based Materials , 5 ( 1 ), 8 - 17 .
Cunha , V. M. ( 2010 ). Steel fibre reinforced self-compacting concrete (from micromechanics to composite behavior) .
Deeb , R. , Ghanbari , A. , & Karihaloo , B. L. ( 2012 ). Development of self-compacting high and ultra high performance concretes with and without steel fibres . Cement & Concrete Composites , 34 ( 2 ), 185 - 190 .
Dinh , N. , Choi , K. , & Kim , H. ( 2016 ). Mechanical properties and modeling of amorphous metallic fiber-reinforced concrete in compression . International Journal of Concrete Structures and Materials , 10 , 221 - 236 .
El-Mal , H. A. , Sherbini , A. , & Sallam , H. ( 2015 ). Mode II fracture toughness of hybrid FRCs . International Journal of Concrete Structures and Materials , 9 ( 4 ), 475 - 486 .
EN , B. 12350 - 8 . ( 2010 ). Testing fresh concrete. Self-compacting concrete. Slump-flow test .
Hwang , J. , Lee , D. H. , Ju , H. , Kim , K. S. , Kang, T. H. , & Pan , Z. ( 2016 ). Shear deformation of steel fiber-reinforced prestressed concrete beams . International Journal of Concrete Structures and Materials , 10 ( 3 ), 53 - 63 .
Islam , M. S. , & Alam , S. ( 2013 ). Principal component and multiple regression analysis for steel fiber reinforced concrete (SFRC) beams . International Journal of Concrete Structures and Materials , 7 ( 4 ), 303 - 317 .
Joo Kim , D. ( 2009 ). Strain rate effect on high performance fiber reinforced cementitious composites using slip hardening high strength deformed steel fibers . ProQuest.
Kang , S. , Lee , K. , Choi , J. , Lee , Y. , Felekog˘ lu, B. , & Lee , B. Y. ( 2016 ). Control of tensile behavior of ultra-high performance concrete through artificial flaws and fiber hybridization . International Journal of Concrete Structures and Materials , 10 ( 3 ), 33 - 41 .
Laranjeira de Oliveira , F. ( 2010 ). Design-oriented constitutive model for steel fiber reinforced concrete . Unpublished doctoral dissertation . Universitat Polite`cnica de Catalunya , Spain.
Lee , S. F. , & Jacobsen , S. ( 2011 ). Study of interfacial microstructure, fracture energy, compressive energy and debonding load of steel fiber-reinforced mortar . Materials and Structures , 44 ( 8 ), 1451 - 1465 .
Lee , Y. , Kang , S. , & Kim , J. ( 2010 ). Pullout behavior of inclined steel fiber in an ultra-high strength cementitious matrix . Construction and Building Materials , 24 ( 10 ), 2030 - 2041 .
Li , H. , & Liu , G. ( 2016 ). Tensile properties of hybrid fiberreinforced reactive powder concrete after exposure to elevated temperatures . International Journal of Concrete Structures and Materials , 10 , 29 - 37 .
Lim , W. , & Hong , S. ( 2016 ). Shear tests for ultra-high performance fiber reinforced concrete (UHPFRC) beams with shear reinforcement . International Journal of Concrete Structures and Materials , 10 , 177 - 188 .
Lu , L. , Tadepalli , P. , Mo , Y. , & Hsu , T. ( 2016 ). Simulation of prestressed steel fiber concrete beams subjected to shear . International Journal of Concrete Structures and Materials , 10 ( 3 ), 297 - 306 .
Markovic , I. ( 2006 ). High-performance hybrid-fibre concrete: Development and utilisation . Delft: IOS Press.
Petrone , F. , Shan , L. , & Kunnath , S. K. ( 2016 ). Modeling of RC frame buildings for progressive collapse analysis . International Journal of Concrete Structures and Materials , 10 ( 1 ), 1 - 13 .
Romualdi , J. P. , Ramey , M. , & Sanday , S. C. ( 1968 ). Prevention and control of cracking by use of short random fibers . Special Publication , 20 , 179 - 204 .
Shaikh , F. U. A., Shafaei , Y. , & Sarker , P. K. ( 2016 ). Effect of nano and micro-silica on bond behaviour of steel and polypropylene fibres in high volume fly ash mortar . Construction and Building Materials , 115 , 690 - 698 .
Shi , C. , Wu , Z. , Xiao , J. , Wang , D. , Huang , Z. , & Fang , Z. ( 2015 ). A review on ultra high performance concrete: Part I. Raw materials and mixture design . Construction and Building Materials , 101 , 741 - 751 .
Sorensen , C. , Berge , E. , & Nikolaisen , E. B. ( 2014 ). Investigation of fiber distribution in concrete batches discharged from ready-mix truck . International Journal of Concrete Structures and Materials , 8 ( 4 ), 279 - 287 .
Srikar , G. , Anand , G. , & Prakash , S. S. ( 2016 ). A study on residual compression behavior of structural fiber reinforced concrete exposed to moderate temperature using digital image correlation . International Journal of Concrete Structures and Materials , 10 , 75 - 85 .
Tadepalli , P. R. , Dhonde , H. B. , Mo , Y. , & Hsu , T. T. ( 2015 ). Shear strength of prestressed steel fiber concrete I-beams . International Journal of Concrete Structures and Materials , 9 ( 3 ), 267 - 281 .
Torregrosa , C. E. E. ( 2014 ). Dosage optimization and bolted connections for UHPFRC ties .
Van Gysel , A. ( 2000 ). Studie van het uittrekgedrag van staalvezels ingebed in een cementgebonden matrix met toepassing op staalvezelbeton onderworpen aan buiging . Published.
Wang , D. , Shi , C. , Wu , Z. , Xiao , J. , Huang , Z. , & Fang , Z. ( 2015 ). A review on ultra high performance concrete: Part II. Hydration, microstructure and properties . Construction and Building Materials , 96 , 368 - 377 .
Wille , K. , El-Tawil , S. , & Naaman , A. E. ( 2014 ). Properties of strain hardening ultra high performance fiber reinforced concrete (UHP-FRC) under direct tensile loading . Cement & Concrete Composites , 48 , 53 - 66 .
Wille , K. , & Naaman , A. ( 2010 ). Bond stress-slip behavior of steel fibers embedded in ultra high performance concrete . Proceedings of 18th European conference on fracture and damage of advanced fiber-reinforced cement-based materials (99-111).
Wille , K. , & Naaman , A. E. ( 2012 ). Pullout behavior of highstrength steel fibers embedded in ultra-high-performance concrete . ACI Materials Journal , 109 ( 4 ), 479 - 588 .
Wille , K. , & Naaman , A. E. ( 2013 ). Effect of ultra-high-performance concrete on pullout behavior of high-strength brass-coated straight steel fibers . ACI Materials Journal , 110 ( 4 ), 451 - 462 .
Wu , Z. , Shi , C. , He , W. , & Wu , L. ( 2016a ). Effects of steel fiber content and shape on mechanical properties of ultra high performance concrete . Construction and Building Materials , 103 , 8 - 14 .
Wu , Z. , Shi , C. , & Khayat , K. H. ( 2016b ). Influence of silica fume content on microstructure development and bond to steel fiber in ultra-high strength cement-based materials (UHSC) . Cement & Concrete Composites , 71 , 97 - 109 .
Yoo , D. , & Yoon , Y. ( 2016 ). A review on structural behavior, design, and application of ultra-high-performance fiber-reinforced concrete . International Journal of Concrete Structures and Materials , 10 , 125 - 142 .
Zeng , Q. , Li , K. , Fen-chong , T., & Dangla , P. ( 2012 ). Pore structure characterization of cement pastes blended with high-volume fly-ash . Cement and Concrete Research , 42 ( 1 ), 194 - 204 .