Evaluation of Crack Propagation and Post-cracking Hinge-type Behavior in the Flexural Response of Steel Fiber Reinforced Concrete
International Journal of Concrete Structures and Materials
Evaluation of Crack Propagation and Post-cracking Hinge-type Behavior in the Flexural Response of Steel Fiber Reinforced Concrete
Sahith Gali 0
Kolluru V. L. Subramaniam 0
0 Department of Civil Engineering, Indian Institute of Technology Hyderabad , Hyderabad 502285 , India
An experimental evaluation of crack propagation and post-cracking behavior in steel fiber reinforced concrete (SFRC) beams, using full-field displacements obtained from the digital image correlation technique is presented. Surface displacements and strains during the fracture test of notched SFRC beams with volume fractions (Vf) of steel fibers equal to 0.5 and 0.75% are analyzed. An analysis procedure for determining the crack opening width over the depth of the beam during crack propagation in the flexure test is presented. The crack opening width is established as a function of the crack tip opening displacement and the residual flexural strength of SFRC beams. The softening in the post-peak load response is associated with the rapid surface crack propagation for small increases in crack tip opening displacement. The load recovery in the flexural response of SFRC is associated with a hinge-type behavior in the beam. For the stress gradient produced by flexure, the hinge is established before load recovery is initiated. The resistance provided by the fibers to the opening of the hinge produces the load recovery in the flexural response.
steel fibers; toughness; crack opening; post cracking response; softening; fracture; digital image correlation
The use of steel fibers as discrete structural reinforcement in
concrete has been explored and even recommended in some
codes of practice. Considerable research has been performed
on the fracture behavior of steel fiber reinforced concrete
(SFRC). Available information suggests that while there is a
nominal increase in flexural strength compared to plain
concrete there is a significant increase in toughness
(Gopalaratnam et al. 1991; Gopalaratnam and Gettu 1995; Armelin and
Banthia 1997; Barros and Figueiras 1999; di Prisco et al. 2009;
ACI 544.1R-96 2006). In SFRC, fibers enhance the
postcracking behavior by providing crack bridging stresses and
thus ensure stress transfer across the cracked sections
(Tadepalli et al. 2015; Sorensen et al. 2014). Several factors that
influence the crack formation and crack growth include, type
of fibers, volume of fibers, the distribution and orientation of
fibers at the crack location (Gettu et al. 2005; Laranjeira et al.
2012; Michels et al. 2013; Islam and Alam 2013; Abdallah
et al. 2016; Adjrad et al. 2016).
An understanding of the contribution of these fibers in
providing resistance to crack propagation and controlling
crack opening has been developed by combining fracture
mechanics-based approaches with local information provided
by the high resolution experimental techniques. Optical
techniques such as speckle pattern interferometry and Moire
interferometry have been used to investigate the fracture
process zone in concrete and fiber reinforced concrete (Shah
and Ouyang 1991). The information from these techniques has
been used primarily to evaluate the localization and
toughening mechanisms. The high resolution optical techniques
provide information within a limited field of view. The
information from the local measurement is then applied to a
numerical or analytical framework to relate the local material
behavior with the global response. Digital image correlation
(DIC) has emerged as a full-field optical technique, which
provides adequate resolution of the local strains close to the
crack tip while providing far-field information. DIC has been
applied successfully to obtain the cohesive interface fracture
behavior of the FRP-concrete bond (Ali-Ahmad et al. 2006;
Subramaniam et al. 2007; Carloni and Subramaniam
2010, 2013; Carloni et al. 2012). Limited number of
application of DIC technique for studying the crack growth in fiber
reinforced concrete have also been attempted (Robins et al.
2001; Subramaniam et al. 2015).
Several analytical formulations with different levels of
approximations have been proposed for predicting the load
response of concrete considering its post-cracking behavior.
In most representations, the behavior of the crack and the
fracture process zone is idealized within the geometric
relations imposed by geometry and loading (Hillerborg et al.
1976; ACI 544.8R 2016). A convenient idealization of the
facture behavior of concrete is the cracked hinge model,
where a hinge-type behavior is postulated for predicting the
response of concrete in flexure (Stang and Olesen 1998;
Olesen 2001). The non-linearity caused by the presence of
crack is assumed to be confined within a hinge region of
finite width and the stress bridging across the crack are
considered in the form of a stress-crack separation
relationship. The hinge is connected to elastic beams at each end
and the stress state in the elastic beam is the far-field stress
given by the elastic beam theory. While the idealization
provides adequate prediction of load response, direct
validation of the hinge-type behavior is not yet available.
The focus of this paper is to study the crack propagation using
a high resolution full-field optical technique, which provides
sufficient accuracy of the local strain localization behavior and
sufficient full-field view to allow the study of far-field response.
An experimental investigation into the flexural response of
notched SFRC beams is presented, where insights into the crack
propagation and post-cracking response in the composite are
obtained using DIC. Insights into crack propagation and the
post cracking response are obtained by combining information
from both the near-field and far-field measurements. The
surface displacements and strains are analyzed for evaluating the
crack growth in concrete in relation to the observed load
response in flexure. The measured crack opening at the tip of the
notch obtained from the DIC measurement is compared with the
crack tip opening displacement obtained using a surface
mounted gauge. The influence of fibers at the different points of
the flexural load response is evaluated in relation to the
measured crack length and profile.
2. Materials and Methods
Commercially available ordinary Portland cement
confirming to the requirements Grade 53 of Indian standard, IS
12269 with specific gravity of 3.1 and fineness modulus of
325 m2/kg was used for all concrete mixtures. Siliceous fly
ash conforming to the requirements of IS 3812-1 (2003) and
IS 1727 (1967) with specific gravity of 2.5 and fineness
modulus of 320 m2/kg was used as supplementary
cementitious material in concrete mixtures. Crushed sand with a
specific gravity of 2.67 and fineness modulus of 2.83 was
used as fine aggregate and crushed granite of specific gravity
of 2.63 was used as coarse aggregate. Two different coarse
aggregate fractions with maximum size in ranges of
10–4.75 mm and 20–10 mm were blended in equal
proportions for use in the concrete mix.
The cementitious content of the concrete mix was fixed at
340 and 140 kg/m3 fly ash was used. The
water-to-cementitious ratio equal to 0.48 and was kept constant in all mixes.
In the concrete mixture fine aggregates were taken as 45% of
the total aggregate volume fraction. One control mixture and
mixtures with different dosage of hooked end steel fibers
were prepared from one batch of ingredients. The control
mixture contained no fiber. Fiber reinforced concrete (FRC)
mixtures were prepared with fiber volume fractions (Vf)
equal to 0.50 and 0.75% and are labelled as SFRC50 and
150 mm Cubes and beams with dimensions 150 mm
(height) 9 150 mm (width) 9 500 mm (length) were cast
from each mixture. Concrete was prepared using a drum
mixer with a capacity of 0.25 m3. The ingredients were put
into the mixer in the decreasing order of sizes starting from
20 mm coarse aggregate to cement. Dry mixing of the
aggregates and cement was done for 2 min and then water
was added gradually in the rotating mixer and allowed to
mix for 5 min. During the mixing process, the walls and
bottom of mixer were scraped well to avoid sticking of
mortar. Super plasticizer (Gelenium) was used to increase
the workability of freshly prepared fiber reinforced concrete.
A consistent slump in the range 75–100 mm was maintained
for all mixtures. Finally, the fresh concrete was placed in
oiled moulds and compacted properly in three layers, each
layer being tamped 35 times using a tamping rod and
compacted using needle vibrator. After the initial setting of
concrete, the surface of the specimen was finished smooth
using a trowel. Immediately after casting, all specimens were
covered with wet burlap and plastic covers to minimize
moisture loss and stored room temperature, approximately
25 C. Specimens were demolded 24 h after casting and
stored under water up to 28 days of age.
The 28-day compressive strengths obtained from the
150 mm concrete cubes of the different mixtures evaluated
in this study are given in Table 1. An increase in the
compressive strength with increasing fiber volume in SFRC is
2.1 Three-Point-Flexure Test
The test procedure adopted was consistent with the
guidelines given in EN 14651:2005 and consisted of testing
prismatic specimens in a three-point bending configuration.
Specimens were rotated 90 along the longitudinal axis so
that casting surface was on the back side during the flexure
test and a notch of 25 mm depth was introduced at the
midspan using a circular saw as per the guidelines given in EN
14651:2005. The flexure test was conducted in crack mouth
opening displacement (CMOD) control. The loading fixture
conformed to the requirements of EN 14651:2005. The
corresponding deflection of the beam was measured using
the rectangular jig clamped to the specimen at mid-depth
directly over the supports. During the test, the crack tip
opening displacement (CTOD) was also measured using a
clip gauge mounted at the tip of the notch. A schematic
sketch of the test setup is shown in Fig. 1. The notched beam
was tested with a span equal to 450 mm. The rate of increase
of the CMOD during the flexure test was controlled in two
stages, at 0.05 mm/min for CMOD less than 0.1 mm and at
Table 1 Cube compressive strengths of concrete mixtures.
Fig. 1 Experimental setup for flexure test of notched specimens: a schematic sketch showing the placement of gauges;
b schematic representation of the DIC setup for obtaining images from the front face during flexure test.
0.1 mm/min for CMOD greater than 0.1 mm. All the tests
were ended when the CMOD reached a value of 4 mm. Six
specimens were tested for each concrete mixture.
2.2 Digital Image Correlation
For the use in image analysis, a sprayed-on speckle pattern
was created on the surface of the beam in a region close to
the notch. A uniform coat of white paint was applied of the
surface to provide a uniform background. The sprayed-on
speckle was created by spraying a black mist. During the
flexure test, digital images of the specimen were captured
using a high resolution camera (5 mega pixels). Uniform
light intensity was ensured across the surface of the cube
using normal white light. The camera was fitted with a
50 mm lens and was placed at a distance of 1 m from the
specimen surface. Calibration for the pixel size was
performed using a graduated rule placed in front of the
specimen. From this measurement, the physical calibration was
established and was in the range of 12–14 pixels per mm. A
reference image was captured in the undeformed state prior
to the initiation of loading program. Images of the specimen
were captured at regular intervals during the test.
DIC is a method based on digital image analysis, which
provides full-field, spatially continuous measurement of
displacements across the surface of the specimen. The
surface displacements are obtained from correlation of the
random pattern of speckles between images of the specimen
undergoing deformation and the specimen in the reference
(undeformed) configuration. The speckle pattern represents a
random pattern, which gives a unique distribution of pixel
gray level values. A two-dimensional displacement field was
obtained for all points on the surface from cross-correlating
the image of the deformed specimen with the image of the
specimen in the reference configuration. Correlations and
pattern identification is performed within small
neighborhoods called subsets (Sutton et al. 1983, 1988; Pan et al.
2009). In a given image, the pixel gray-levels in each subset
associated with the random sprayed-on pattern gives a
unique gray-level pattern, which differs from gray-level
distribution in another subset. In the analysis, positions
within the deformed image were mapped on to positions
within the reference subsets using second-order,
two-dimensional shape functions. Spatial domain cross-correlation
was performed to establish correspondence between
matching subsets in images of the reference and the
deformed states. Quintic B-spline interpolation of the gray
values was used to achieve sub-pixel accuracy. The cross
correlation analysis which maximizes the correlation
coefficient between gray levels in the subsets in the reference and
deformed images of the digital images was performed using
the VIC-2DTM software. Surface displacements and
displacement gradients at each loading stage were calculated at
each subset center, by evaluating the shape functions and
their partial derivatives at the center of the subset. For the
setup used in this study, the random error in the measured
displacement is in the range of 0.002 pixels. Strains were
computed from the gradients of the displacements.
The load-CMOD responses of the control and the SFRC
specimens are shown in Fig. 2a. The load-CMOD response
close to the peak is shown in Fig. 2b. There is an increase in
the peak load (identified as FL, the load corresponding to
limit of proportionality (LOP) as per EN 14651:2005) with
increasing Vf. There is softening in the load response
immediately following the FL. In FRC specimens there is an
increase in the load carrying capacity with increasing crack
opening after the softening load response. The load recovery
is initiated at a smaller value of crack opening displacement
and a higher load is achieved during the load recovery on
increasing the Vf. The increase in the residual load carrying
capacity with increasing CMOD indicates that the steel
fibers are effective in providing crack closing stresses with
increasing crack opening.
The FL and corresponding CTOD from the control and the
SFRC specimens are tabulated in Table 3. The load in the
post-peak at which load recovery is initiated, identified as
FcCriTtOD and the corresponding CTOD, identified as CTODcrit
are also tabulated in Table 2. There are systematic changes
in all values with an increase in the fiber volume content and
Fig. 2 Load-CMOD response from flexure tests a up to a CMOD = 3 mm; and b early response up to CMOD = 0.1 mm. The
average response of five beams is plotted with scatter bars indicating the range of variation at discrete values of CMOD.
Table 2 Load and CTOD measures from tests on notched beam.
the observed change in the mean values are outside the range
of scatter obtained from one group. The CTODcrit provides a
qualitative measure of the crack opening when the load
recovery is initiated. Increasing the fiber volume content
increases the resistance to crack opening, thereby resulting in
an earlier deviation from the descending portion of the load
response seen in control specimens. Thus, at larger volume
fractions, the fibers are effective at a smaller crack opening.
The available data indicates that load recovery is initiated
after a CTOD value of 250 lm and 175 lm for 0.5 and
0.75% fiber volume contents, respectively.
The results of the notched tests were analyzed using the
procedure given in of UNI 11039-2. The first crack flexural
strength (fIf), equivalent flexural strengths (feq(0-0.6),
feq(0.6-3)) were evaluated. In the current study the flexural
tests on notched beams were performed as per EN 14651
(2005), which recommends a three-point bending
configuration. The UNI 11039-2 standard formulae were developed
for four-point bending configuration. The first crack nominal
strength, fIf and the equivalent flexural strengths were
calculated using modified formulae, where a factor of 1.5 was
introduced as shown below.
Table 3 Strength values as per UNI 11039-2.
where b (150 mm), h (150 mm), l (450 mm) and a0 (25 mm)
are the width, the length, the height and the notch length,
respectively. U1 and U2 are given by the area under
loadCTOD (F(CTOD)) response as shown below
The equivalent flexural strengths for the control and the
SFRC beams are shown in Table 3. It can be seen that there
is an increasing trend in the first crack strength with
increasing fiber content. The values of feq(0-0.6) and feq(0.6-3)
show clear improvement on increasing the Vf from 0.5 to
3.1 Results of Digital Image Analysis
Contours of strain in the X-direction (exx) at distinct points
on the load response of an SFRC50 specimen are shown in
Fig. 3. A subset size of 35 pixels was used for the analysis.
The load points are identified on the load-CMOD response
of the specimen in Fig. 3a. The exx at progressively larger
Fig. 3 a Load CMOD response of SFRC specimen with Vf = 0.5%; Contours of exx at b Load Point 1 (CMOD = 0.014 mm);
c Load Point 2 (CMOD = 0.037 mm), d Load Point 5 (CMOD = 0.3 mm).
values of CMOD are shown in Figs. 3b–d. Strain
localization is evident in the pre-peak load response and with
increasing CMOD, leads to the formation of a single crack
emanating from the notch. The increase in the crack length
can clearly be identified with softening in the post peak load
response; as the crack propagates, there is a steady increase
in the crack opening and an associated decrease in the load.
The physical opening produced by the crack emanating from
the notch resulted in a loss of correlation within a region
corresponding to the subset size used for correlation centered
on the location of the crack.
The contours of displacement in the X-direction, Ux at the
load point with CMOD = 0.037 mm using different subset
sizes within an area of interest located over the notch (shown
marked in Fig. 3c) are shown in Figs. 4a–c. The closeness of
displacement contours directly over the notch indicates
localization. While using a smaller subset size results in the
local variations being accurately represented, the influence
of local heterogeneities in the material becomes more
prominent. Since concrete is a heterogeneous material, on
decreasing the subset size, the measured observations
contain significant contributions of local effects on the scale of
the inhomogeneity within the material. Increasing the subset
size has the effect of smearing out the sharp gradients in the
displacements. The subset size of 35 pixels was found to
provide sufficient resolution of the local behavior close to
the notch and provide adequate accuracy in the far-field
measurements. A conservative estimate of the resolution in
strain obtained from the digital correlation was 5 le (Bruck
et al. 1989; Schreier and Sutton 2002).
The Ux, and the exx along length of the member on a
horizontal line with fixed Y coordinate located at a distance
equal to 37 mm above the bottom face of the beam and
12 mm above the tip of the notch at different points in the
load response (in Fig. 3a) are shown in Fig. 5. The Ux away
from the notch, as X increases from -75 mm, exhibits a
steady increase with the X coordinate. The stress
concentration produced by the notch and subsequent crack
propagation associated with increasing CTOD produce sharp
gradients in Ux within a small region close to the notch
(values of X close to zero). In the far-field, away from the
region of high displacement gradients, Ux varies linearly
with a slope which is nominally constant. The magnitudes of
displacements in the far-field region are proportional to the
magnitude of applied stress. The variation of exx with X,
along the line shows a sharp increase in the magnitude of
strains to values significantly higher than the far field values
within the region of large displacement gradient close to the
notch, indicating localization of strain. There is an increase
in the magnitude of strain and there is also a sharper strain
gradient within the region of localization with increasing
The correlation coefficient at load points with CMOD values
equal to 0.3 mm, are shown in Fig. 6. The physical contour of
the crack can be identified by the region with a loss of
correlation. The physical separation in the material, produces a loss
in correlation, which occurs within subsets which overlap with
the opening. From the physical calibration, the width of the
region associated with the loss of correlation is approximately
0.4 mm. The correlations outside the region disturbed by the
crack were found to be within limits of acceptability.
The effect of the finite subset size is to smear out the sharp
strain gradients. The influence of local areas where
correlations are poor are also included in the local values of strain.
The available resolution from the DIC technique,
considering the finite subset size does not allow for determining
fracture parameters using local displacement and strain
measurements in regions with a physical crack. A procedure
Fig. 6 Contours of correlation coefficient at Load Point 5
(CMOD = 0.3 mm).
using asymptote matching of the far-field displacements was
developed to estimate the displacement discontinuity
associated with crack opening, which does not rely on local
measurements in regions overlapping with the crack as
shown in Fig. 7. The 3D contours of Ux obtained at Load
Point 5 (CMOD = 0.3 mm) are shown in Fig. 7a. The sharp
gradient in Ux is introduced by the crack emanating from the
notch. The asymptotic behavior in the displacement profile
with the X coordinate along a line of constant Y coordinate
is the far-field displacement associated with flexure. Slopes
of the lines on either side of the localization region were
determined by fitting straight line equations (linear equation
with constant slope) to the far-field displacements on either
side of the notch using least squares approximation. The
slopes of the two lines obtained from the linear fits to the
variation in the far-field displacement with X coordinate
were found to be nominally equal. The increment in the
displacement within the region of localization, which
represents the displacement discontinuity introduced by crack
was determined as the difference in the values of the
intercepts on the Y axis of the two lines fitted to the far-field
displacements. The displacement discontinuity along a line
of constant Y coordinate (at a given location above the
notch) determined using the procedure described above is
referred to as crack opening width (Shown in Fig. 7b).
Horizontal lines spaced at 80 pixels (approximately
6.0 mm spacing), located at different depths relative to the
notch, were used to obtain the crack opening width along the
depth of the beam. Crack opening widths were obtained at
different points on the load response. The area of interest
selected for correlations passes a few millimeters over the
notch. The exact location of the tip of the notch was
approximated using gray scale thresholding of pixel
intensities in the digital images of the specimen. From the gray
scale image of the notched specimen in the reference state,
the machine cut notch profile was identified using a pixel
intensity value equal to 10 on a grayscale intensity scale of
256. For lines with constant Y coordinate, the tip of the notch
was located as the position along Y where the threshold was
not exceeded on traversing along X coordinate.
The crack opening widths along the depth of a control
specimen at different values of CTOD are shown in Fig. 8.
The crack opening width exhibits a decreasing profile with
increasing height above the notch (increasing Y coordinate).
Initially, for small values of CTOD, the crack opening width
rapidly approaches a value of zero with increasing Y, away
from the notch. A crack opening width equal to zero
indicates that displacement profile does not show localized
increase in displacement; the asymptotic displacement
profiles intercept the Y axis at the same location, with no jump.
At this location, the displacement profile may be assumed to
be given by the far-field stress and the influence of the notch
or crack is insignificant. The available data suggests that
initially, the point of zero crack opening advances rapidly
along the depth of the beam for a small increment in the
CTOD. The point of zero crack opening progresses to a
depth in the range 110–120 mm very rapidly with increasing
CTOD between 0.035 and 0.082 mm. This occurs in the post
peak softening load response immediately following the
peak load. On increasing the CTOD further, while the crack
opening width indicates a wider opening along the depth, the
point of zero crack opening width remains relatively
stationary. The opening of the crack therefore exhibits a
hingetype behavior, as the crack opens about a fixed point along
the depth of the beam.
The observed crack opening width as a function of
depth with increasing CTOD can be interpreted in terms
of crack growth in the specimen. The rapid progression of
the location of the zero crack opening along the depth of
the beam is indicative of crack propagation. The initial
growth corresponds with the pre-peak and the immediate
post-peak part of the load response. In the initial growth
period, there is a very rapid increase in crack length along
the depth of the beam associated with a small increase in
CTOD. With increasing CTOD, the crack advances to a
depth of 110–120 mm, following which there is little or
no change in the location of the zero crack opening width.
There is however a continuous increase in the crack
opening width along the depth of the crack with
increasing CTOD. This experimental observation indicates
Fig. 7 a A 3-D representation of the displacement discontinuity introduced by the crack emanating from the notch; b Asymptote
matching procedure for determining the crack opening displacement.
the formation of a hinge along the depth of the beam.
After formation of the hinge there is a continuous opening
of the crack about a fixed location along the depth of the
beam. The subsequent load response is therefore governed
by the stress transfer across the crack. For control
specimen, the opening of the crack leads to a continuous
decrease in the load carrying capacity.
A validation of the crack profile in the beam determined
using DIC was attempted using the measured crack tip
opening displacement (CTOD) as a reference. The CTOD
was measured at the tip of the notch using the surface
mounted clip gauge, which was precisely attached at the tip
of the notch. The CTOD from the DIC measurement was
determined by extrapolating the crack opening width vs
depth line to zero depth (the tip of the notch) and is referred
to as CTODDIC. The extrapolation was performed by fitting a
line using a least square approximation to the points located
within 30 mm from the bottom of the area of interest used
for image correlation. This corresponds to data from five
equi-spaced lines located 6 mm apart, which were used for
generating the crack width profile. The CTODDIC values
were obtained for different images captured at different
values of CTOD. A comparison of the load-CTOD response
obtained from the physical measurement using the surface
mounted LVDT and that obtained from DIC measurements is
shown in Fig. 9. The response obtained using both CTOD
and CTODDIC show a good match, which provides
validation for the methodology for establishing crack profile width
using image analysis.
The crack opening widths as a function of depth for
different values of CTOD for an SFRC50 and an SFRC75
specimens are shown in Figs. 10 and 11 respectively. The
response is nominally similar to that observed in control
specimens. There is a rapid upward movement in the
location of the zero crack opening width in the softening
part of the load response immediately after the peak load.
A rapid advance to a depth 110–120 mm above the notch,
occurs up to crack opening displacements, which coincides
with the initiation of the load recovery. The load recovery
portion is associated with opening of the crack, with
insignificant crack advance. The crack therefore develops a
Fig. 9 Comparison of CTODDIC and CTOD measured using
surface mounted clip gauge.
hinge-type mechanism, where the equilibrium is maintained
by the stress transfer across the crack provided by the
fibers. For FRC specimen, the opening of the crack
associated with the hinge action leads to an increase in the load
carrying capacity. The results indicate that the crack
propagation in the matrix predominantly occurs in the
postpeak softening response immediately following the peak
load. The fibers provide resistance to opening of the hinge,
once it is established. The resistance to crack opening
which contributes to the opening of the hinge is due to
pullout of fibers from the matrix.
A plot of the crack extension in relation to the CTOD is
plotted in Fig. 10. The physical crack tip was located based
on the correlation values exceeding a threshold value
following a procedure established previously (Carloni and
Subramaniam 2010). The average value of correlation
obtained from a region away from the crack located in
regions X \ -20 mm and X [ 20 mm was used as the
threshold value, which was found to be 0.0015. The region
selected for obtaining the reference for the threshold value
was sufficiently removed from the crack to provide a
representative value for the correlation index within the
specimen. Using this criterion, the physical location of the crack
tip can be estimated to within the size of one subset. Depth
of crack thus calculated for different images is plotted with
against the corresponding CTOD in Fig. 12. The results
confirm that initially there is a very rapid extension of crack
for very small increase in crack opening. Subsequently, once
the hinge is established, there is very small increment in the
Fig. 10 Results from an SFRC50 specimen: a load-CTOD response of a typical specimen; b crack opening width as a function of
height above the notch for different values of CTOD (in mm).
Fig. 11 Results from an SFRC75 specimen: a load-CTOD response of a typical specimen; b crack opening width as a function of
height above the notch for different values of CTOD (in mm).
Fig. 12 Depth of crack extension with CTOD for control,
SFRC50 and SFRC75.
crack depth for a large increase in crack opening. The
influence of steel fibers at 0.5% Vf produce a deviation in the
response once the crack advances to a depth of 70 mm,
which occurs close to the peak flexural load. This relates to
the small increase in the peak load in the flexural test
response. At higher Vf equal to 0.75%, there is a significant
difference in the crack depth at all values of CTOD. There is
therefore a significant difference in the peak load and the
subsequent flexural test response.
4. Summary and Findings
The available information from the different test
techniques can now be combined to gain an insight into the crack
propagation and opening in steel fiber reinforced concrete in
flexure. Crack propagation in the FRC specimens is
primarily associated with the decreasing load in the post-peak
region immediately following the peak load. An asymptote
matching procedure which uses the far-field displacements
to estimate the displacement discontinuity produced by a
crack was developed. In this procedure the inaccuracies in
the local measurements at the crack location introduced by
the finite subset size and the lack of correlation due to the
physical opening of the crack are not considered. Subsequent
load recovery in flexural load response of SFRC beams is
due to hinge action associated with pullout of fibers. The
available information from DIC agrees with the standard test
measures of feq(0-0.6) obtained from UNI 11039. Feq(0-0.6)
provides a measure of energy in the early part of the post
peak, which is associated with hinge formation and initial
opening of the hinge. The feq(0.6-3) value is indicative of the
energy dissipated in pullout of fibers in a part of the load
response which is associated with opening of the hinge.
The findings of the experimental investigation are
1. In SFRC loaded in flexure, the surface crack propagates
rapidly through the depth of the specimen for small
crack openings. For Vf up to 0.75%, the resistance
provided by steel fibers during the crack propagation
stage is insufficient to prevent a softening in flexure.
The fibers do influence the peak load and the rate of
softening in post-peak produced by crack propagation.
2. Fiber volume content plays a significant role in improving the load recovery in the post-peak flexural
response, which contributes to increasing toughness.
The load recovery is associated with a hinge type
behavior. For the stress gradient produced by flexure,
the hinge is established at a crack tip opening
displacement before load recovery is initiated. After the
formation of a hinge, the resistance to the crack opening
is attributed to fiber pullout from the cementitious
matrix. The recovery is initiated with sufficient number
of fibers crossing the crack, which provide the required
resistance across the surface of the crack.
At fiber volume content equal to 0.75% there is a
significant decrease in the crack depth for a given crack
tip opening displacement. This produces significant
improvements in the peak load and the post peak load
resistance response in flexure.
The authors would like to acknowledge support of the
Center of Excellence in Sustainable Development at I.I.T.
Hyderabad, funded by the Ministry of Human Resource
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