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ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 268-274 © IAEME
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STUDIES ON COMPRESSION AND FLEXURAL STRENGTH
CHARACTERISTICS OF TRIPLE BLENDED HIGH STRENGTH
RECYCLED AGGREGATE CONCRETE
M.V.S.S. Sastri1
, Dr. K. Jagannadha Rao2
, Dr. V. Bhiksma3
1
(Assoc.Professor, Department of Civil Engineering, Vasavi College of Engineering, Ibrahimbagh,
Hyderabad- 500031 (AP), India)
2
(Professor, Department of Civil Engg. Chaitanya Bharathi Institute of Technology,
Hyderabad-500075 (AP) India)
3
(Professor, Department of Civil Engg, O.U.College of Engg (A). Osmania University,
Hyderabad-500007 (AP) India)
ABSTRACT
The suitability of recycled coarse aggregate (RCA) in the production of a high-strength
concrete using triple blended industrial by-products is tested in laboratory. The by-products used are
fly ash and condensed silica fume as binders at different percentages and recycled aggregates as
partial replacement to natural aggregates. The concrete mixtures containing both supplementary
cementitious materials and recycled aggregates had shown high compressive strength (>70 MPa),
high flexural strength and split tensile strength compared to control concrete.
Keywords: Triple Blending, High Strength Concrete, Recycled Aggregate, Sustainability.
1.0 INTRODUCTION
In order to reduce resource depletion from the construction sector, an effort to use recycled
and secondary materials in concrete production has been introduced decades ago. The use of
secondary materials in concrete is still largely limited to low-strength concrete products such as base
courses for roads and 80% of the fly ash ends up in low value applications [1]. However, some
industrial by-products show excellent properties as construction materials, which means that they
could be used in concrete production not only for resource preservation but also to improve the final
product but exhibits different properties compared to conventional materials. In order to safely use
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2. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),
ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 268-274 © IAEME
269
them in concrete production they should undergo thorough quality control testing and their properties
must be taken into account in the concrete mixture design.The changes in material properties or in
production techniques generally take place for strengths more than 40 MPa. Earlier studies on
conventional strength concrete [6, 7, 8] reported that there is no significant variation in the strength
and other mechanical properties of recycle aggregate concrete compared to the natural aggregate
concrete.
1.1 Recycled aggregate
In the present scenario of construction, building demolition waste (BDW) concrete handling
and management is the new primary challenge faced by the countries all over the world. The
problem has to be tackled in an indigenous manner, it is desirable to completely recycle the waste in
order to protect natural resources and reduce environmental pollution. Recycled concrete aggregates
contain not only the original aggregates, but also a little hydrated cement paste. This paste reduces
the specific gravity and increases the porosity compared to similar virgin aggregates. Higher porosity
of recycled aggregates leads to a higher absorption [9, 10, 11, 12]. Quality requirements of recycled
aggregates produced from the poorest quality concrete have to be same as that of conventional
aggregates.
1.2 Recycled Aggregate Triple Blended Concrete Mixes
In the present experimental investigation triple blending has been carried out by mixing fly
ash and condensed silica fume in various proportions as replacements to ordinary Portland cement.
Three percentages of fly ash (20, 30 and 40) and four percentages of CSF (0, 5, 10 and 15) were used
as replacement to cement for triple blending. Recycled aggregate also varied at 0, 25 and 50% by
weight. In all 28 concrete mixes were cast and tested. The objective of the present investigation is to
find out the strength parameters, in specific, the compressive, flexural and split tensile strength of
recycled aggregate triple blended high strength concrete and compare the same with that of ordinary
concrete. In turn, the project is aimed towards experimentally proving the usage of recycled
aggregate in structural usage over ordinary concrete and thus fostering its usage for not only greater
strength and durability but also in view of the economic and environmental considerations involved.
2.0 EXPERIMENTAL INVESTIGATION
2.1 Cement
The Ordinary Portland Cement (OPC) of UltraTech 53 grade confirming to Indian standard
IS 12269-1987 was used.
2.2 Fine aggregate
Fine aggregate used for this entire study of investigation for concrete was river sand
confirming to zone-1 of IS: 383-1987.
2.3 Coarse aggregate
Crushed hard granite chips of maximum size 20 mm were used in concrete mixes.
2.4 Water
Potable water available in the college was used for casting and curing.
2.5 Condensed Silica Fume
The CSF was obtained from M/s V.B. Ferro Alloys Pvt. Ltd., Hyderabad.
3. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),
ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 268-274 © IAEME
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2.6 Fly Ash
The material was procured from Ramagundam Thermal Power Plant (A.P).
2.7 Super Plasticizer (SP) of M/s Fosroc Industries Ltd Conplast SP 430 was used.
2.8 Recycled aggregate
The building demolition waste was collected from a school building at the time of road
widening and the age of the building is about 20 years. The concrete debris was broken into pieces of
approximately 80 mm size with the help of hammer & drilling machine. The foreign matters were
sorted out from the pieces. Further, those pieces were hand crushed in the lab and mechanically
sieved through sieve of 4.75 mm to remove the finer particles. The recycled coarse aggregates were
washed and dried and collected for use in concrete mix.
Table-1: Summary of Physical properties of Coarse Aggregate
Water
absorption%
Impact
strength %
Los Angeles
abrasion
value %
Aggregate
crushing
value %
Voids
%
Specific
gravity
Fineness
Modulus
NA 1.25 26 27 28 42 2.66 6.98
RA 2.71 31 31 47 51 2.50 6.925
2.9 Reference Concrete Mix
Design of M-80 grade concrete mix was carried out by using Design of Experiments method.
Quantity of cement is 650 kg/m3
with a water cement ratio of 0.28. The details of mix proportions
are given in table-2.
Table-2: Summary of Mix proportions
Mix w/c ratio Water (litre) Cement(kg) NA(kg) FA(kg) Mix
M80 0.28 182 650 1316.25 406.25
1:0.625:2.025 with
0.28 w/c and 1.5% SP
3.0 CASTING AND CURING OF SPECIMENS
Casting of Specimens was done by batching of materials, preparation of moulds and placing
of concrete in the moulds. Vibrator was used after every 1/3 filling of material into the mould and the
top surface was properly leveled at the end. They were allowed to dry for 24 hrs and proper
identification marks were written and kept into the curing tank for various ages of testing.
4.0 TESTS CONDUCTED ON HARDENED CONCRETE
4.1 Compressive strength
Three specimens of size 100 mm x 100 mm x 100 mm were used for compression testing for
each batch of mix.
4.2 Split Tensile strength Test
Split tensile test was conducted on cylinders of size 100 mm diameter and 200 mm height.
4.3 Flexural strength
The prisms of size 100x100x500mm were tested to evaluate the flexural strength of the
concrete by two point loading. All the above tests are conducted as per IS specifications.
4. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),
ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 268-274 © IAEME
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5.0 TEST RESULTS AND DISCUSSIONS
The test results on hardened concrete are reported in tables 3 and 4 and figures 1 to 6.
5.1 Workability of Recycled Triple Blended, High Strength Concrete Mix
When various percentages of condensed silica fume along with recycled aggregate was added
the workability was becoming very low. Hence, superplasticizer was added up to a maximum
percentage of 1.5% to maintain workability. A higher dosage of superplasticizer is required for high
strength concrete mixes particularly when recycled aggregate and mineral admixtures like condensed
silica fume is used. But the increment of fly ash has shown improvement in workability.
Fig.1: Compressive strength of 25% RA at
various ages in MPa
Fig.2: Compressive strength of 50% RA at
various ages in MPa
Fig.3: Flexural strength of 25% RA in MPa Fig.4: Flexural strength of 50% RA in MPa
Fig.5: Split tensile strength of 25% RA in MPa Fig.6: Split tensile strength of 50% RA in MPa
Mix: first numerical in the parenthesis indicates %fly ash; second is %condensed silica fume and
third is % recycled aggregate.
5. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),
ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 268-274 © IAEME
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5.2 Influence of the Mineral Admixtures on the Compressive Strength
The variation of compressive strength at 7, 28, 56 and 90 days with recycled aggregate triple
blended concrete along with percentage increment over control mixes is shown in table 3 and in fig 1
and 2. It is observed that condensed silica fume contributes towards increase in the compressive
strength of triple blended, high strength concrete mix. The compressive strength of the concrete is
showing increasing trend when fly ash is added along with condensed silica fume. Fly ash is
pozzolanic in nature and is reacting slowly as it needs longer curing periods hence even beyond 28
days the strength of concrete is improving particularly when percentage is more. Fly ash content of
20 percent and 5 percent condensed silica fume was found to be optimum for all the ages without
recycled aggregate. Highest compressive strength was obtained at 5% condensed silica fume with
20% fly ash. This value is 94 MPa. The compressive strength of the reference mix without any
mineral admixtures was obtained as 90.2 MPa at 90 days. There is an increase of nearly 4% in
compressive strength over the reference mix. It is observed from the tables that as the Fly ash
percentage increases, the compressive strength is gradually decreasing. This happened in the case of
all other combinations. The strength of the triple blended recycled aggregate concrete mixes with
20% fly ash along with various percentages of condensed silica fume and recycled aggregates
considered are above the design strength i.e. 80 MPa, but the observed strengths are lower than the
control concrete. When the cement is replaced by fly ash at 30% along with various percentages of
condensed silica fume and recycled aggregates considered the strength of recycled aggregate triple
blended concrete mixes are in the range of 10 to 29% less than the control concrete. When the
percentage replacement of fly ash at 40% and condensed silica fume at its maximum the strength of
50% recycled aggregate concrete mix is of 50% of control mix which is significant as the total
amount of cement used is 357.5 kg/m3
only. M80 grade concrete with 25% recycled aggregate could
meet the design strength requirement with 20% and 5% replacement of cement with fly ash and
condensed silica fume respectively.
5.3 Influence of the Mineral Admixtures on the Flexural Strength
Referring to table 4 and figure 3 and 4, it can be seen that silica fume contributes towards
increase in the flexural strength up to 10% but after that the strength is decreasing drastically and it
shows that optimum level of condensed silica fume has reached along with fly ash. Highest flexural
strength of 9.2 MPa was obtained at 5% CSF with 20% fly ash which is equivalent to control
concrete strength.
Table 3: Average compressive strength at all ages in MPa for typical combinations and
corresponding increase /decrease over control concrete
MIX
CF compressive strength (MPa)
7 day % 28 day % 56 day % 90 day %
(0,0,0) 0.86 54.5 0% 85.0 0% 88.6 0% 90.2 0%
(20,5,0) 0.88 58.2 7% 85.7 1% 88.5 0% 94.0 4%
(20,10,0) 0.84 51.0 -6% 80.3 -6% 85.6 -3% 90.2 0%
(20,15,0) 0.816 45.8 -16% 73.5 -14% 82.3 -7% 84.5 -6%
(30,5,0) 0.89 46.5 -15% 57.4 -32% 78.3 -12% 89.2 -1%
(30,10,0) 0.84 43.5 -20% 68.1 -20% 73.3 -17% 80.9 -10%
(30,15,0) 0.88 33.8 -38% 64.3 -24% 66.5 -25% 67.9 -25%
(40,5,0) 0.91 35.6 -35% 66.0 -22% 70.3 -21% 72.5 -20%
(40,10,0) 0.89 38.6 -29% 67.2 -21% 71.3 -20% 73.5 -19%
(40,15,0) 0.88 28.2 -48% 42.6 -50% 48.4 -45% 54.5 -40%
6. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),
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It can be seen from figure 3 and 4 that as the fly ash, condensed silica fume and recycled
aggregate percentage increases, the flexural strength is gradually decreasing. As discussed earlier
the optimum percentage of mineral admixture is obtained as 20% fly ash with 5% CSF without
recycled aggregate. The flexural strength of concrete mix with 20% fly ash, 10% condensed silica
fume along with recycled aggregate by 50% replacement is less by 9 percent to control mix, hence
can be neglected.
5.4 Influence of the Mineral Admixtures on the Split Tensile Strength
Referring to table 5 and figures 5 and 6 it is observed that condense silica fume contributes
towards increase in the split tensile strength without recycled aggregate. Highest split tensile strength
of 4.5 MPa was obtained at 10% CSF with 20% fly ash which is 5 percent more than control
concrete. From the figures it is observed that as the fly ash, condensed silica fume and recycled
aggregate percentage increases, the split tensile strength is gradually decreasing and it follows the
same trend of flexural strength. As discussed earlier the optimum percentage of mineral admixture is
obtained as 20% fly ash with 5% CSF without recycled aggregate.
Table 4: Average 28 day flexural, split tensile strengths in MPa for all the typical combinations
and the percentage increase/ decrease over control concrete
MIX Split Tensile Strength (MPa) Flexural Strength (MPa)
(0,0,0) 4.3 0% 9.2 0%
(20,5,0) 4.4 3% 9.2 0%
(20,10,0) 4.5 5% 8.4 -9%
(20,15,0) 4.4 3% 7.5 -19%
(30,5,0) 3.5 -17% 7.3 -21%
(30,10,0) 3.4 -21% 7.0 -24%
(30,15,0) 3.4 -21% 6.8 -26%
(40,5,0) 3.1 -27% 6.3 -31%
The split tensile strength of recycled aggregate triple blended concrete mix having 20% fly
ash, 10% condensed silica fume and recycled aggregate with 50% replacement is less by 4 percent
compared to control concrete, which is negligible and further these strengths can be improved by
adding fibres.
5.5 Optimum Mix for Triple Blended High Strength Recycled Aggregate Concrete
The strength of triple blended concrete with fly ash percentage of 20%, condensed silica
fume percentage of 5% and 25% recycled aggregate considered the strength is on par with control
concrete. On the overall, strength loss with the higher percentages of fly ash is compensated by silica
fume. With the fly ash percentage of 20% and with the increase of silica fume percentage up to an
optimum of 10% along with 50% recycled aggregate the strength reduction is negligible. Thus an
optimum high strength concrete mix possessing optimum strength properties can be obtained
resorting to triple blended recycled aggregate concrete.
6.0 CONCLUSIONS
Based on the present experimental investigation the following main conclusions are drawn.
1. Higher dosages of superplasticizer are required for high strength concrete mixes particularly
when mineral admixtures and recycled aggregates were employed to maintain workability.
7. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),
ISSN 0976 – 6316(Online) Volume 5, Issue 3, March (2014), pp. 268-274 © IAEME
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2. Twenty percent fly ash generates marginal increase in strength beyond which decreases with
higher percentages of fly ash.
3. The use of 50% of RCA as partial replacement of natural aggregate reduced the strength of
triple blended high strength concrete marginally. But it would help in consuming the
construction and demolition waste to some extent apart from consuming the industrial wastes,
thereby achieving the sustainability.
4. It is recommended to use 20 percent fly ash and 10 percent silica fume as partial replacement
of cement and 50 percent recycled aggregates as replacement of natural aggregate for the
optimum strength properties.
5. It is the right time to seriously think of reusing demolished concrete for the production of
recycled concrete in our country. Recycling would not only conserve the resources but would
also promote safe and economic use of such concrete which is the need of the hour for a
country like India.
ACKNOWLEDGEMENTS
The authors express their sincere regard and gratitude to the management of ACE
Engineering College, Hyderabad, for the facilities provided for the experimentation work in
connection with the present paper. Our special thanks to Professor & Head Dr. P.J.Rao, for his
constant encouragement and help.
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