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1
PROPORTIONING OF CONCRETE
MIXES FOR PRECAST ELEMENTS
Dr. R. Nagendra
Senior Director
One-Day Seminar on Precast Technologies,
ICI-KBC
STEDRANT Technclinic Pvt. Ltd.
A Centre for all Civil Engg. Solutions
NABL Accredited Laboratory
Bangalore-Hyderabad-VijayawadaBy
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TYPICAL PRECAST CONCRETE CONSTRUCTION SITE
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What is Precast Concrete ?
Precast concrete means a concrete member that
is cast and cured at a location other than its final
designated location.
Advantages of Precast Concrete
➢Saves Construction Time
➢Quality Assurance
➢Usage of Prestressed Concrete
➢Cost effective
➢Durability
➢Aesthetics
➢Safe working Platform
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Disadvantages of Precast Concrete:
➢High Initial Investment
➢Transportation Issue
➢Handling Difficulties
➢Modification
➢Sensitive Connection Works
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Precast concrete ensures fast construction
times, high profitability and excellent quality.
Still, requirements for beautiful, modern
buildings are not compromised.
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IMPORTANCE OF CONCRETE TECHNOLOGY IN
PRECAST CONSTRUCTION
➢Visual (surface) quality comes primarily from the concrete
mix being used
➢ Structural (Load bearing capacity) quality comes primarily
from the mix being used
➢Cost of concrete primarily decides the cost of precast
construction
➢Use of industrial byproducts from the environmental effect
point of view.
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CONCRETE INGREDIENTS:
➢CEMENT
➢MINERAL ADMIXTURES – POZZOLANIC AND
HYDRAULIC – FLYASH, GGBS
➢ULTRA FINE MATERIALS- MICROSILICA, NANOSILICA &
ULTRAFINE GGBS (ALCOFINE)
➢ FINE AGGREGATE- CRSUHED STONE SAND FROM A VSI
CRUSHER
➢COARSE AGGREGATE- FROM A VSI CRUSHER
➢WATER
➢CHEMICAL ADMIXTURES
➢ FIBRES
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Different factors affect the consistency andcompactability of the concrete mix
➢ Water increases flowability and reduces strength.➢ Binders increase the need for water.➢ Round aggregate shapes improve compactability and
reduce tensile strength.➢ An optimal grading curve improves compactability.➢ Plasticizers reduce water demand.➢ An increase in temperature – or a delay in casting –
reduces compactability.
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In a concrete mix-
➢Maximize the aggregate volume
➢Minimize the volumes of cement and water
Remember that aggregate is a natural material
and the variation in quality and properties is
larger in comparison to cement and other
components.
Emphasis in QC in a factory lab should be on
aggregates
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10
JAW CRUSHER AGGREGARES- FLAKY ELONGATED
VSI CRUSHER AGGREGARES
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MIX PROPORTIONING OF PRECAST CONCRETE IS SIMILAR TO
SITE MIXED CONCRETE OR READY-MIXED CONCRETE
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The main properties of Precast Concrete are:
➢Cost effective mix design (Factory like QC)
➢Strength according to the design demand
➢Fast and controlled hardening and importance of
early strength
➢Proper workability according to the casting
method
➢Good durability according to the design
requirements
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WAYS OF ACHIEVING DESIRED EARLY
STRENGTH
1. USE OF RAPID HARDENING CEMENT/ OPC 53 GRADE
CEMENT/ HIGH EARLY STRENGTH CEMENT
2. USE OF TEMPERATURE CURING/STEAM CURING
3. USE OF ACCELERATING ADMIXTURES –PCE BASED
4. PROPER COMBINATIONS OF CEMENT + GGBS/FLYASH +
MICROSILICA/ALCOFINE
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Steam curing is curing in water vapor at atmospheric
or higher pressures. When cured at atmospheric
pressure, the enclosure temperatures are usually
between 40 and 70°C . Steam curing is used where
early strength gain is needed and where heat is
required for hydration, such as in cold weather.
In prestressing and precast plants, steam at
atmospheric pressure provides high early strengths,
enabling rapid demolding and reuse of forms.
Excessive rates of heating and cooling may result in
large, harmful volumetric changes and should be
avoided.
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MIX DESIGN TRIALS FOR PRECAST
CONCRETE
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SL. NO. Cement usedMix
Designation
Aggregate
Max. size
(mm)
Cement
Content
(kg/cu.m)
UGGBFS
(kg/cu.m)
Free
Water
Cement
ratio
(max)
Suggested Mix
Proportions
C:CSS:CA
1 53 Grade OPC-
High 1 day
strength
M40 20 380 - 0.30 1: 2.39: 3.28
2 M40 20 380 - 0.30 1: 2.39: 3.28
3 53 Grade OPC-
Low 1 day
strength
M40 20 410 - 0.30 1: 2.16: 2.96
4 M40 20 410 - 0.30 1: 2.16: 2.96
5Normal 53 Grade
OPC
M 35 20 380 - 0.41 1 : 1.91 : 3.00
6 M 40 20 380 - 0.38 1 :1.94 : 3.05
7
OPC 53 S with 7
day strength not
less than 37.5 MPa
M40 20 375 - 0.30 1: 2.33: 3.46
8 M40 20 380 - 0.31 1: 2.27: 3.38
9 M40 20 380 - 0.31 1: 2.38: 3.27
10Normal 53 Grade
OPC
M40 20 400 30 0.3 1 : 1.78 : 2.69
11 M40 20 440 - 0.3 1 : 1.73 : 2.61
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SL. NO.
Dosage of
Admixture by
weight of cement
(%)
Slump Obtained
(mm)Compressive Strength (N/mm2)
Initial 1 Hour 12 hour 14 hour 24 hour 7 day 28 day
1 1.0 Collapse 110 17.3 19.8 28.9 65.5 74.7
2 1.0 Collapse 120 17.7 20.7 29.5 66.9 75.2
3 0.9 Collapse 105 13.8 15.8 24.0 63.7 78.5
4 0.9 Collapse 110 13.5 15.5 23.6 63.0 77.6
5 0.2 35 - - - - 29.8 44.8
6 0.15 40 - - - - 37.8 50.6
7 0.75 Collapse 100 15.3 19.1 29.9 66.5 75.0
8 0.70 Collapse 120 15.3 18.4 29.2 66.0 74.8
9 0.70 Collapse 80 15.2 18.8 28.8 65.8 74.3
10 0.5 Collapse 120 - 23.0 - 80.3 87.1
11 0.5 Collapse 110 - 22.1 - 77.2 82.8
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INFERENCES:
1. WHILE ACHIEVING HIGH EARLY STRENGTH OF
CONCRETE ONE-DAY STRENGH OF CEMENT CAN BE A
CRITERIA. RANGE OF 1-DAY STRENGTH IS 12 MPa TO 18
MPa.
2. ONE DAY STRENGTH OF CEMENT CAN BE AS HIGH AS 23
MPa
3. IT IS OBSERVED THAT CEMENTS HAVING HIGHER ONE
DAY STRENGTH OF CEMENT HAVE PERFORMED BETTER
IN TERMS OF HOURLY STRENGTHS STARTING FROM 12
HrTO 24 Hr.
4. SLEEPER GRADE CEEMNTS CAN BE USED TO GET HIGH
EARLY STRENGTH OF CONCRETE
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Optimization of Curing Regimes for Precast Prestressed Members with Early-Strength
Concrete
EXPERIMENTAL WORK By
Songhee Lee, Ngocchien Nguyen, Thi Suong Le and
Chadon Lee
Chung-Ang University Seoul
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FINDINGS OFTHE EXPERIMENTALWORK
Early-strength-concrete (ESC) made of Type I cement with a high Blaine
value of 500 m2/kg reaches approximately 60 % of its compressive strength
in 1 day at ambient temperature. Based on the 210 compressive test results,
a generalized rate constant material model was presented to predict the
development of compressive strengths of ESC at different equivalent ages (9,
12, 18, 24, 36, 100 and 168 h) and maximum temperatures (20, 30, 40, 50
and 60 C) for design compressive strengths of 30, 40 and 50 MPa.
The developed material model was used to find optimum curing regimes for
precast prestressed members with ESC. The results indicated that depending
on design compressive strength, conservatively 25–40 % savings could be
realized for a total curing duration of 18 h with the maximum temperature
of 60 C, compared with those observed in a typical curing regime for
concrete withType I cement.
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IN CONCLUSION….
Concrete is the most commonly used building material
in the world, and precast concrete components and
structures provide a creative way to extend the use of
concrete.
Challenges on the industry- climate change, energy
saving and environmental protection, engineers need a
new vision for the potential of precast structures as
well as for the sustainable use of concrete in general.
In thinking about sustainable construction, we need
to find innovative ways to reduce the impact of
construction activity on our environment.
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44
For your kind attention