Concrete yet to perform the mix design of concrete

Concrete is a composite material made out of coarse
aggregates bonded together with fluid cement that solidifies over the time. In
concrete, Ordinary Portland cement (OPC) is widely used as the main binder in
the concrete. Globally,
an estimated 46 million tonnes of concrete and mortar were consumed in 2013. Production
of OPC is currently topping 2.6 billion tons per year. Manufacture of 1 kg of OPC emits nearly
equivalent amount of CO2 in to the atmosphere Russell et al., 1999.
Therefore OPC production is one of the main contributor to the global warming. Therefore, it
will be much worth, if another material is found instead of cement with
equivalent properties or if cement is partly replaced by some other
supplementary cementitious materials (SCMs). Search of such materials can lead
to a sustainable development incorporating a minimum impact on the
environment.  Presently OPC is partly
replaced with SCMs obtained by industrial by products such as Rice husk ash
(RHA), Fly ash, Silica Fume, Ground Furnace Slag, High Reactive Meta kaolin
etc. SCMs enhance the concrete properties mainly in two ways.
These materials increase the generation of more Calcium Silicate Hydrate
through the pozzolanic reaction and they also contribute in providing denser
concrete due to better packing of particles.

Rice
husk contains high amount of organic materials. However, it has less percentage
of inorganic material which is rich in silica. It was made sure through X-Ray
fluorescence (XRF) and X-ray Diffraction (XRD) analyses of RHA (discussed
later). Therefore, it can be used as supplementary cementitious materials in
concrete production due to its high pozzolanic property. Although
there are many investigations have been documented in this regards, a proper
guidelines are not developed yet to perform the mix design of concrete consist
of RHA. Therefore it was planned to extend the k-value concept stated in EN
206-1: 2013 for the concrete consists of Rice Husk Ash through systematic
statistical approach to use it as a guide for the mix design.

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Traditionally,
the application of concrete recycled coarse aggregate (RCA) is used as
landfill. Nowadays, the applications of recycled aggregate in the concrete
production is getting popular around the World due to the higher demand of
natural aggregate. However, the use of recycled coarse aggregates, which have
different characteristics and properties compared to natural coarse aggregates,
could modify the concrete properties. However, number of issues associated with
RCA are not solved yet.

There
are many investigations have been documented to examine the behaviour of
concrete with either RHA or RCA. However, only a few studies have investigated
the combined use of both RHA and RCA. Therefore, it was planned to investigate
the effect of RHA together with RCA on engineering properties of high strength
concrete to ensure the sustainable development of concrete industry. Sixteen
mix propositions are made by changing percentage of RHA and RCA to investigate engineering
properties of concrete consists of both RCA and RHA. Thereby, some guidelines
were made to use it as a tool for the mix design of concrete consists of RHA
and RCA.

2. Literature Review

Rice
milling industry produces a lot of rice husk during milling of paddy which
originates from the fields. Many ways are being practiced for utilising them by
making commercial use of RHA. RHA can also be used as supplementary
cementitious material in the concrete production industry due to its high
pozzolanic property Kishore et.al 2011. Numerous investigations have been
reported in this regards. Various levels of RHA replacement to cement was
examined on compressive strength characteristics of concrete in previous
studies Makarata et al., 2013;  Deepa et
al., 2008; Suresh et al., 2006; Sata et al.,2006.

The
results of the studies made by Makarata et al., 2013 indicated that the
concrete mixed with 20% of ground RHA had a compressive strength at 7 days
higher than that of concrete without RHA. Sata et al., 2006 also reported that the higher fineness of
RHA could be used as a pozzolanic material in making high strength concrete.
The maximum compressive strength of 80 MPa was obtained and reported in their
study Sata et al., 2006. Furthermore, Deepa et al., 2008 reported that the
incorporation of RHA in concretes resulted in improved compressive strength and
flexural strength. It can be concluded from the results of these studies that
the RHA has the high pozzolanic property and could be used as a good
supplementary cementing material in concrete production. However, some other
investigations showed the results in the other way around Rupali et al.,
2017.  Moreover, the results made by
Khassaf et al., 2004 showed that the 10% replacement of cement by RHA gives
peak compressive strength. They also revealed that the increment of rice husk
ash percentage above 10% showed the decline in compressive strength. Tensile
strength of the concrete containing RHA was almost similar as the compressive
strength variation. Marand et al., 2006 found that the addition of RHA in
concrete becomes cohesive and more plastic and thus permits easier placing and
finishing of concrete. In contrast, Rupali et al., 2017 and Avinash et al.,
2004 revealed that the workability was decreased with the increment of RHA. Further
investigations are necessary to solve the uncertainties associates with these
issues.

RCA
is obtained mainly by crushing and processing of old concrete structural
elements. RCA may contain bricks, tiles, metals and other miscellaneous
materials such as glass, wood, paper, plastic and other debris along with
crushed concrete.
It has been established in literature that increase in RCA
amount at the same w/c ratio leads to decrease in compressive strength,
generally up to 10% lower than that of natural aggregate concrete Elhakam et
al., 2012; Fonseca et al., 2011; Rahal et al., 2007. Kou et al., 2008 reported
that the compressive strength of RCA added concrete was 20% less than compressive
strength of conventional Concrete. However some other researchers observed that
the compressive strength of concrete remains unaffected, or increases slightly
for replacement of Natural aggregate by RCA up to 25% Etxeberria et al.,2007;
Rao et al., 2011; Kwan et al.,2012. Moreover, research conducted by Padmini
et al.,2009 showed that the rate of strength gain in RCA made with 20%, 50% and
100% RA was more after 28 days as compared to that of conventional concrete
Padmini et al.,2009.

The
split tensile strength of RCA has been observed to be dependent on a variety of
factors such as RCA replacement, water-binder ratio, mixing methods, type of
cement, curing age and RA quality Bairagi et al.,1993. The results of the
study made by Bairagi et al.,1993 showed that the split tensile strength of RCA
were 6%, 10% and 40% less than that of conventional concrete when RAC was made
with 25%, 50% and 100% RA replacement, respectively. Many studies
showed that the tensile strength of concrete for replacement ratio of RCA up to
30% is same or even exceeds the tensile strength of natural aggregate concrete
Padmini et al., 2009.

However, only few investigations have been
reported about partial replacement of untreated RHA together with RCA on the
engineering properties of HSC. It is worth to find out the influencing factors
and its effects on concrete containing both RHA and RCA to determine the
sustainable mix design of designated concrete. Therefore this research study
examines the engineering properties of concrete containing both RCA and RHA for
the sustainable development of modern concrete industry.

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