Abstract
Bendable
concrete also known as Engineered Cementitious Composites abbreviated as ECC is
class of ultra-ductile fiber reinforced cementitious composites, characterized
by high ductility and tight crack width control. This material is capable to exhibit
considerably enhanced flexibility. An ECC has a strain capacity of more than 3%
and thus acts more like a ductile metal rather than like a brittle glass. A
bendable concrete is reinforced with micromechanically designed polymer fibres.
In this paper literature survey of fresh and mechanical properties of different
ECC mixtures are evaluated by incorporating supplementary cementitious
material, i.e., fly ash and different aggregate type considering various
parameters, i.e., types of fibers, compressive strength, flexural strength and
deflection.
INTRODUCTION
Conventional
concretes are almost unbendable and have a strain capacity of only 0.1% making
them highly brittle and rigid. This lack of bendability is a major cause of
failure under strain and has been a pushing factor in the development of an
elegant material namely, bendable concrete also known as Engineered
Cementitious Composites abbreviated as ECC. This material is capable to exhibit
considerably enhanced flexibility. A bendable concrete is reinforced with
micromechanically designed polymer fibres. ECC is made from the same basic
ingredients as conventional concrete but with the addition of High-Range Water
Reducing (HRWR) agent is required to impart good workability.
However,
coarse aggregates are not used in ECCs (hence it is a mortar rather than
concrete). The powder content of ECC is relatively high. Cementitious
materials, such as fly ash, silica fume, blast furnace slag, silica fume, etc.,
may be used in addition to cement to increase the paste content. Additionally,
ECC uses low amounts, typically 2% by volume, of short, discontinuous fibres.
ECC incorporates super fine silica sand and tiny Polyvinyl Alcohol-fibres
covered with a very thin (nanometer thick), slick coating. This surface coating
allows the fibre to begin slipping when they are over loaded so they are not
fracturing.

It
prevents the fibre from rupturing which would lead to large cracking. Thus an
ECC deforms much more than a normal concrete but without fracturing. Figure 1
represents the behavior of ECC under flexural loading and it can be seen that
the beam can deform well without direct failure. The different ingredients of
ECC work together to share the applied load. ECC has proved to be 50 times more
flexible than traditional concrete, and 40 times lighter, which could even
influence design choices in skyscrapers. Additionally, the excellent energy
absorbing properties of ECC make it especially suitable for critical elements
in seismic zones.
LITERATURE
REVIEW
Qian et
al. (2010) carried out experimental study to investigate the self-healing
behavior of ECC with focus on the influence of curing condition and precracking
time. Four-point bending tests was used to pre crack ECC beams at different
age, followed by different curing conditions, including air curing, 3% CO2
concentration curing, cyclic wet/dry (dry under 3% CO2 concentration) curing
and water curing. For all curing conditions, deflection capacity after
self-healing can recover or even exceed that from virgin samples with almost
all precracking ages. After self-healing, flexural stiffness was also retained
significantly compared with that from virgin samples, even though the level of
retaining decreases with the increase of precracking time. The flexural strength
increases for samples pre-cracked at the age of 14 days and 28 days, presumably
due to continuous hydration of cementitious materials afterwards.
Furthermore,
it was promising to utilize nanoclay as distributed internal water reservoirs
to promote selfhealing behavior within ECC without relying on external water
supply. Victor C Li et al. (2012) carried out experimental study to improve the
fibre distribution by adjusting the mixing sequence. With the standard mixing
sequence, fibres are added after all solid and liquid materials are mixed. The
undesirable plastic viscosity before the fibre addition may cause poor fibre
distribution and results in poor hardened properties. With the adjusted mixing
sequence, the mix of solid materials with the liquid material is divided into
two steps and the addition of fibres is between the two steps. In this paper,
the influence of different water mixing sequences was investigated by comparing
the experimental results of the uniaxial tensile test and the fibre distribution
analysis. The result was concluded that compared with the standard mixing
sequence, the adjusted mixing sequence increases the tensile strain capacity
and ultimate tensile strength of ECC and improves the fibre distribution.
Yu Zhu et
al. (2012) carried out an experimental study to develop a kind of green ECC
with high tensile ductility and strong enough matrix strength, especially at
early age. A series of investigations was carried out to evaluate mechanical
properties and drying shrinkage of ECC with 70% combination mineral admixtures
of FA and ground granulated blast furnace slag (SL). Four ECC mixtures with
constant W/B of 0.25 are prepared with combined inclusion of FA and SL as
constant cement replacement level of 70%. The laboratory measurements are
carried out, including direct tensile test, fourpoint bending test, and
compressive strength and drying shrinkage. The experimental results show that
ECC with combination mineral admixtures can achieve strain hardening behavior,
tensile capacity of ECC can be more than 2.5% at 90 days. Meanwhile, compared
to ECC only with fly ash, slag and fly ash can effectively increase compressive
strength of ECC, especially at early age. Incorporating SL into matrix can
slightly increase drying shrinkage of ECC.
However,
among four ECC mixtures, ECC with 30% SL and 40% FA presents the lowest drying
shrinkage at later ages. Jun Zhang et al. (2013) carried out an experimental
study on the potential applications of the fibre reinforced engineered
cementitious composite with characteristic of low drying shrinkage (LSECC) in
concrete pavements for the purpose of eliminating joints that are normally used
to accommodate temperature and shrinkage deformation. It was found that a
composite slab containing both plain concrete and LSECC, with steel bars at the
LSECC/concrete interface, and designed construction procedures, it is possible
to localize the tensile cracks into the LSECC strip instead of cracking in
adjacent concrete slab.
The
crucial problem that interfacial failure in composite slab was prevented by
using reinforcing bars across the interfaces. Due to the strain-hardening and
high strain capacity of the LSECC, the overall strain capacity and the
integrity of the composite slab can be significantly improved. The temperature
and shrinkage deformations can be accommodated by adequate selection on the
length ratio of LSECC strip and concrete slab. Mustafa Sahmaran et al. (2013)
carried out experimental work for 36 different ECC mixtures to evaluate the
combined effects of the following factors on workability and rheological
properties: water-binder (w/b), sand-binder (s/b), superplasticizer-binder (SP/
b) ratios and maximum aggregate size (Dmax).
A
mini-slump cone, a Marsh cone and a rotational viscometer was used to evaluate
the workability and rheological properties of ECC mixtures. Compressive
strength and four point bending tests was used for mechanical characteristics
of ECC mixtures at 28 days. The effects of studied parameters (w/b, s/b, SP/b
and Dmax) was characterized and analyzed using regression models, which can
identify the primary factors and their interactions on the measured properties.
Statistically significant regression models was developed for all tested
parameters as function of w/b, s/b, SP/b and Dmax.
To find
out the best possible ECC mixture under the range of parameters investigated
for the desired workability and mechanical characteristics, a multi-objective
optimization problem was defined and solved based on the developed regression
models. Experimental results indicate that w/b, s/b and SP/b parameters affect
the rheological and workability properties. On the other hand, for the range of
studied aggregate sizes, Dmax is found to be statistically insignificant on the
rheological and workability properties of ECC, also in addition to that the
mid-span beam deflection capacities, which reflect material ductility, of ECC
mixtures varied noticeably with the change of s/b and Dmax design parameters.
Both of these two parameters negatively affect the deflection capacity of the
ECC mixtures. The other parameters have almost no effect on the mid-span beam
deflection capacities of ECC mixtures.
Yu Zhu et
al. (2014) carried out experimental study to investigate the mechanical
properties of ECC produced by high volume mineral admixtures which are fly ash,
slag and silica fume. Emphasis of this study is placed on building the
correlation between compressive strength and the parameters obtained in
load–deflection curves of 12 different ECC mixtures in binary and ternary
system of binder materials with different mineral admixtures (FA, SL and silica
fume) and to build the correlation between compressive strength and durability
of ECC. The water-binder materials ratio (W/B) is kept at 0.25 for various ECC
mixtures. The replacement levels of different mineral admixtures in all ECCs in
binary systems of binder materials are 50%, 60%, 70% and 80%, respectively (FA
+ cement and SL + cement). In ternary system (FA + SL + cement and FA + SF +
cement), the total replacement of mineral admixtures is 70%, the ratios of
FA/SL and FA/SF are different in ECC mixture proportions.
The
toughness behavior and compressive strength of 12 different ECC mixtures are
firstly measured by fourpoint bending test and compressive strength test,
respectively. The results indicate that the compressive strength has an inverse
relationship with deflection, toughness index and fracture energy,
respectively; but the compressive strength have an direct proportional relation
with flexural strength, first cracking load, and peaking load, respectively.
Additionally, in the binary system of binder materials, the ductility of ECC
can be obviously improved by introducing high volume fly ash and slag replacing
the cement, respectively. However, the compressive strength of ECC with fly ash
and slag can reduce 40% and 14%, respectively. For the ternary system of binder
materials with replacement 70% of cement, the combination of fly ash and slag
can keep not only the excellent ductility of ECC, but also enough stronger
matrix strength.
Meanwhile,
the combination of fly ash and silica fume only increase the compressive
strength, but weaken the toughness of ECC. Tahir Kemal Erdem (2014) carried out
experimental work to study size effect on the residual properties of ECC was
investigated on the specimens exposed to high temperatures up to 800°C.
Cylindrical specimens having different sizes were produced with a standard ECC
mixture. Changes in pore structure, residual compressive strength and
stress–strain curves due to high temperatures were determined after air
cooling. Standard ECC mixture (M45) with a fly ash-cement ratio (FA/C) of 1.2
by mass was used in this investigation which was prepared in a standard mortar
mixer at water to cementitious material ratio of 0.27.
Experimental
results indicate that despite the increase of specimen size, no explosive
spalling occurred in any of the specimens during the high temperature exposure.
Increasing the specimen size and exposure temperature decreased the compressive
strength and stiffness. Percent reduction in compressive strength and stiffness
due to high temperature was similar for all specimen sizes. Bensaid Boulekbache
et al. (2012) carried out experimental study to examining the influence of the
paste yield stress and compressive strength on the behavior of Fibre-
Reinforced Concrete (FRC) versus direct shear. The parameters studied are the
steel fibre contents, the aspect ratio of fibres and the concrete strength.
Prismatic specimens of dimensions 10 * 10 * 35 cm made of concrete of various
yield stress reinforced with steel fibres hooked at the ends with three fibre
volume fractions (i.e., 0%, 0.5% and 1%) and two aspects ratio (65 and 80) were
tested to direct shear.
Three
types of concretes with various compressive strength and yield stress were
tested, an Ordinary Concrete (OC), a Self-Compacting Concrete (SCC) and a High
Strength Concrete (HSC). The concrete strengths investigated include 30 MPa for
OC, 60 MPa for SCC and 80 MPa for HSC. The results show that the shear strength
and ductility are affected and have been improved very significantly by the
fibre contents, fibre aspect ratio and concrete strength. As the compressive
strength and the volume fraction of fibres increase, the shear strength
increases. The ductility was much higher for ordinary and self-compacting.
Soutsos et al. (2012) carried out experimental study on commercially available
steel and synthetic fibres.
Flexural
stress – deflection relationships have been used to determine: flexural strength,
flexural toughness, equivalent flexural strength, and equivalent flexural
strength ratio. The flexural toughness of concrete was found to increase
considerably when steel and synthetic fibres were used. However, equal dosages
of different fibres did not result in specimens with the same flexural
toughness. Pajak and Ponikiewski (2013) carried out experimental study to
investigate the flexural behavior of self-compacting concrete reinforced with
straight and hooked end steel fibres at levels of 0.5%, 1.0% and 1.5% and
compare it to Normally Vibrated Concrete (NVC). The laboratory tests were
determined according to RILEM TC 162-TD
The
flexural behavior of SCC appeared to be comparable to NCV, where the increase
of fibres volume ratio cause the increase in pre peak and postpeak parameters
of SCC. Nevertheless, the type of steel fibres influences much this dependency.
However, the SCC achieves the maximum crack mouth displacement for lower
deflections than NVC. Albert et al. (2014) carried out experimental study on
Polyolefin fibre-reinforced concrete enhanced with steel-hooked fibres in low
proportions. Four types of conventional fibrereinforced concrete with steel and
polyolefin fibres were produced on the basis of the same self-compacting
concrete also manufactured as reference.
These
concrete mixtures were manufactured separately with the same fibre contents
being subsequently used for two more hybrid mixtures. Fracture properties, in
addition to fresh and mechanical properties, were assessed. The result revealed
that it is possible to produce a hybrid fibre reinforced selfcompacting
concrete with a combination of hooked steel fibres and macro polyolefin fibres,
preserving the high performance fresh properties within the most common
selfcompacting requirements. It should also be noted that the addition of
Fibres did not noticeably change the compressive strength, indirect tensile
strength or modulus of elasticity of the reference SCC for any of the amounts,
types or combination of fibres used.
CONCLUSION
•
Compressive strength is directly related to the Flexural strength and inversely
related to deflection but if the compressive strength is kept in limited
ranges, the desirable value of related parameters can be obtained.
•
Compressive strength decreases with the increase in the cementitious material
i.e. fly ash, silica fume, LP etc.
•
Incorporation SL into matrix can effectively increase compressive strength at
all ages, especially at early age
• The
water to cementitious material (w/c) ratio 0.27 gives the best result.
• Compared
with the standard mixing sequence, by adjusting mixing sequence increases the
tensile strain capacity and ultimate tensile strength of ECC and improves the
fibre distribution.
•
Increasing the specimen size and exposure temperature decreased the compressive
strength and stiffness.
• In
Hybrid fibres mixture the compressive strength decreases with decreasing
flexural strength.
• The
ductility in direct shear depends on the fibre orientation and is significantly
improved when the fibres are perpendicular to the shear plane.
• The
Polycarboxylate based superplasticizer mortar mixes give more workability and
higher compressive strength at all ages compare with sulphonated melamine
formaldehyde based SP
No comments:
Post a Comment