Reducing cement paste volume for production of SCC by adding fillers Professor Albert K.H. Kwan Department of Civil Engineering The University of Hong Kong Dr. Jaime S.K. Yeung Score Holdings Ltd.
Transcript
Slide 1
Reducing cement paste volume for production of SCC by adding
fillers Professor Albert K.H. Kwan Department of Civil Engineering
The University of Hong Kong Dr. Jaime S.K. Yeung Score Holdings
Ltd.
Slide 2
Introduction The complex shape of some of todays large scale
infrastructure demand the uses of very large and sophisticated
concrete moulds and exceedingly dense steel reinforcement, which
together render concreting a formidable task. The great
difficulties with the placing of concrete through closely spaced
reinforcing bars into every corner of the mould and with the
compaction of concrete placed inside confined space could lead to
unfilled corners, honeycombing, insufficient steel-concrete bond
and other defects. To improve the general quality of concrete
construction, the use of self-consolidating concrete (SCC) is
probably the best option.
Slide 3
Introduction SCC has excellent ability to deform and flow; fill
up confined spaces and far-reaching corners; pass through small
clearances between rebars; and achieve good consolidation without
compaction (or with facilitation of very little compaction in some
extremely difficult condition). Advantages of using SCC: enables a
highly automated concreting operation that allows reduction in the
number of concrete workers and improvement in site management.
Slide 4
Introduction Advantages of using SCC (continued): Without the
need of vibration, the concreting speed can be accelerated. Without
the need of vibration, the noise generated can be reduced by about
90% (8-10dB), leading to the possibility of extending the working
hours to the evening and even night time. The automated concreting
operation and longer working hours would together dramatically
speed up the construction. The necessity to cast the concrete
structure in stages can be eliminated and the provision of
construction joints can be avoided.
Slide 5
Introduction It is not at all easy to produce SCC. In order for
concrete to be classified as SCC, it should have the following
properties: (1) High workability; (2) High passing ability; and (3)
High segregation resistance. To achieve high passing ability and
high segregation resistance, the concrete needs to have relatively
high cohesiveness SCC must be designed to have high workability and
high cohesiveness, which are not easy to achieve concurrently.
Slide 6
Introduction The addition of more superplasticizer to increase
the workability would at the same time reduce the cohesiveness.
Concrete producers are forced to increase the paste volume so as to
achieve both high workability and high cohesiveness. In general a
paste volume of 30% to 38% is needed. quite large! What are the
problems with concrete with large paste volume composed only of
cementitious materials and water? (1) High material cost (2) Low
dimensional stability (3) High hydration heat generated (4) High
carbon footprint
Slide 7
Experimental Program Materials Cementitious Material: Ordinary
Portland cement (OPC) of strength class 52.5N + locally produced
pulverized fuel ash (PFA) Relative densities: OPC = 3.16; PFA =
2.52 Specific surface areas: OPC = 336 m 2 /kg; PFA = 369 m 2 /kg
Fine Aggregates: Crushed granite rocks Nominal maximum size: 5 mm
Relative density: 2.62
Slide 8
Experimental Program Materials Fine Aggregates (continued):
Fineness modulus: 3.26 Water absorption: 0.8% Coarse Aggregates:
Crushed granite rocks Nominal maximum size: 20 mm Relative density:
2.62 Fineness modulus: 6.46 Water absorption: 0.6%
Slide 9
Experimental Program Materials Superplasticiser (SP):
Polycarboxylate-based Relative density: 1.05 Solid content: 20%
Molecules have a comb-like structure consisting of a backbone chain
and a number of graft chains
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Experimental Program Materials Filler 1 Limestone Fines (LF)
Ground to have fineness similar to cement Volumetric mean particle
size: 8.4 m Filler 2 Ground Sand (GS) Ground to have maximum
particle size of 600 m Volumetric mean particle size: 302 m It was
expected that the LF would intermix with the cement paste to become
powder paste with a larger volume whilst the GS would intermix with
the mortar portion of the concrete to become part of the
mortar
Slide 11
Experimental Program Mix Proportions (Percentage of Concrete
Volume) Mix no. 1 LF content (%) GS content (%) Cement paste volume
(%) Fine aggregate content (%) Coarse aggregate content (%) W/CM
ratio A-0-0-0.400035 32.5 0.40 A-6-0-0.406029 A-8-0-0.408027
A-0-0-0.500035 32.5 0.50 A-6-0-0.506029 A-8-0-0.508027 Note: 1.
Mixes are identified by the convention: (Series) (LF content in %)
(GS content in %) W/C ratio 2. SP was added until the slump flow
reached at least 650 mm or the concrete mix was showing signs of
segregation.
Slide 12
Experimental Program Mix Proportions (Percentage of Concrete
Volume) Mix no. 1 LF content (%) GS content (%) Cement paste volume
(%) Fine aggregate content (%) Coarse aggregate content (%) W/CM
ratio B-0-8-0.400833 29.5 0.40 B-6-8-0.406827 B-8-8-0.408825
B-0-8-0.500833 29.5 0.50 B-6-8-0.506827 B-8-8-0.508825 Note: 1.
Mixes are identified by the convention: (Series) (LF content in %)
(GS content in %) W/C ratio 2. SP was added until the slump flow
reached at least 650 mm or the concrete mix was showing signs of
segregation.
Slide 13
Experimental Program Mixing, Testing and Casting Experimental
Procedures An electronic balance was used to weigh the materials
and a pan mixer was employed to produce each batch of concrete.
During production, the cementitious materials and water were first
added into the mixer. After a while of preliminary mixing, the
fillers, fine aggregate and coarse aggregate were added to the
mixer. SP was then added bit by bit and the mixing was continued
for about 10 minutes until the concrete mix appeared wet with paste
formed.
Slide 14
Experimental Program Mixing, Testing and Casting Experimental
Procedures (continued) Immediately after completion of the mixing
process, concrete samples were taken from the mixer for slump flow
test, L-box test and sieve segregation test, which were all
performed within 30 minutes to avoid significant workability loss
with time. After finishing these tests, the concrete samples were
put back into the mixer for remixing and then taken out of the
mixer for casting a total of nine 100 mm cubes. The cubes were cast
on a vibration table. At 24 hours after casting, the cubes were
demoulded and put into a lime-saturated water curing tank
controlled at a temperature of 27 2 C until the time of cube
compression test.
Slide 15
Experimental Program Mixing, Testing and Casting Slump Flow
Test The slump flow test for measuring the flowability, as
stipulated in the European Guidelines for SCC. It is very similar
to the slump test for conventional concrete stipulated in BS1881:
Part 102: 1983 and the same apparatus were employed. Unlike the
slump test, no tamping was applied when filling the concrete into
the slump cone.
Slide 16
Slump flow tests for all the concrete mixes A-0-0-0.4 A-6-0-0.4
A-8-0-0.4 A-0-0-0.5 A-6-0-0.5 A-8-0-0.5 B-0-8-0.4 B-6-8-0.4
B-8-8-0.4 B-0-8-0.5 B-6-8-0.5 B-8-8-0.5
Slide 17
Photos showing no segregation problem with all the concrete
mixes A-0-0-0.4 A-6-0-0.4 A-8-0-0.4 A-0-0-0.5 A-6-0-0.5 A-8-0-0.5
B-0-8-0.4 B-6-8-0.4 B-8-8-0.4 B-0-8-0.5 B-6-8-0.5 B-8-8-0.5
Slide 18
Experimental Program Mixing, Testing and Casting L-Box Test The
L-box test for measuring the passing ability, as stipulated in the
European Guidelines for SCC Apparatus: Note: All dimensions in mm
700 150 600 200 100 gate 2 12 smooth bars with gap = 59 mm for PL1
3 12 smooth bars with gap = 41 mm for PL2 H1H1 H 1 H2H2 L-box ratio
= H 1 /H 2
Experimental Program: Mixing, Testing and Casting Sieve
Segregation Test The sieve segregation test for measuring the
segregation resistance, as stipulated in the European Guidelines
for SCC Apparatus and the test: Base receiver 5 mm sieve Electronic
balance Sample container 500 mm 15 minutes
Discussions With the cement paste volume reduced from 35% to
25%, one immediate benefit is that the amount of cementitious
materials to be added can be decreased by as much as 29%. Reduction
of cementitious material by adding fillers Without the addition of
fillers, a cementitious materials content of about 450 kg/m 3 is
generally regarded as the minimum for the production of SCC. By
adding limestone fines into the paste to increase the paste volume,
the cementititous materials content has been reduced to 320 kg/m 3.
By adding also ground sand into the mortar to increase the mortar
volume, the cementitious materials content has been reduced to 300
kg/m 3.
Slide 24
Discussions Reduction of heat generation The substantial
decrease in cementitious materials content due to the reduction in
cement paste volume would significantly decrease the heat
generation of the concrete during curing. The addition of fillers
to reduce the cement paste volume to 25% can lower the adiabatic
temperature rise by the order of 10 to 16C at the W/CM ratios of
0.40 and 0.50. This will largely reduce the need of costly
temperature control for fresh concrete and the risk of thermal
crack formation, especially in thick section pours. SCC mixes with
cement paste volume reduced to 25% by the addition of fillers may
be regarded as low-heat SCC and Green Concrete.
Slide 25
Discussions Theoretically, reduction of the cement paste volume
would also decrease the shrinkage and creep, and increase the
Youngs modulus of the concrete. In other words, the reduction of
the cement paste volume down to 25% would significantly increase
the dimensional stability of the SCC. Hence, the problem with the
relatively low dimensional stability of SCC due to the large cement
paste volume could be overcome by adding suitable fine fillers to
reduce the cement paste volume.
Slide 26
Conclusions To study the feasibility of adding fillers to
reduce the cement paste volume in SCC, an experimental program, in
which two fillers, namely, limestone fines and ground sand, were
employed for the production of SCC, has been conducted. The
limestone fines, which has similar fineness as the cementitious
material, was added to replace an equal volume of cement paste
Whereas the ground sand, which has a mean particle size of 302 m,
was added to replace one quarter of its volume of cement paste and
three quarter of its volume of total aggregate.
Slide 27
Conclusions With up to 8% limestone fines and 8% ground sand
added, the cement paste volume could be reduced to 25% while still
satisfying the slump flow, passing ability and segregation
resistance requirements of SCC. With the cement paste volume so
reduced, the cementitious materials content could be decreased to
lower the material cost, carbon footprint and temperature rise at
early age. Moreover, the shrinkage and creep should be decreased
and the Youngs modulus should be increased.
Slide 28
Conclusions The various problems associated with the large
cement paste volume in SCC could be overcome by adding fillers to
reduce the cement paste volume. SCC with fillers added to reduce
the cement paste volume to only 25% should be regarded as second
generation SCC.