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Construction Materials
Concrete Technology laboratory
Mix design & Proportioning
Specified characteristic strength-Grade 60
Target mean strength- 73.1N/mm2 with standard
deviation of 8N/mm2 for less than 40 results
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Concrete Mix Design-Trial Mix
British Mix Design Method (DOE) is adopted.
A wrong method for C60 concrete which is classified as high strength concrete.
DOE is suitable for targeted strength smaller or equal to 40N/mm2 without any
admixture.
Over-designed ( 67.2N/mm2@7-day >51.2N/mm2)
The graph on the left shows the strength development of
concrete contains 335kg/m3 cement with Type I is the Ordinary
Portland cement. By assuming that percentage of gained
strength V.S. time do not change with the cement content for
the small mass of concrete (i.e. a 100mm cube). We can find
that the compressive strength at 7-days is about 70% of that in
28-days. (i.e.73.1x0.7=51.2)
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Concrete Mix Design-Trial Mix
Too much cement content (661.6kg/m3 >
500kg/m3 given by the teacher) may result in
thermal cracks after one day to two/three weeks.
The early thermal cracks are due to the C3S and
C3A in the cement which have a rapid rate of
hydration and medium to very high heat of
hydration. Data ( Concrete Technology, p. 168)shows that cracks will occur if the peak
temperature of the concrete and the surrounding
temperature is larger than 20C for flint gravel and
40C for limestone aggregate. It is, however, the
occurrence of cracks also affected by the
fineness, relative humidity, ambient temperature,size of the structure, curing methods, heat
capacity & coefficient of thermal expansion of the
constituent materials.
The graph shows the development of heat of
hydration of different Portland cements cured at
21C with w/c 0.4. Type I is the Ordinary Portland
cement. It will expect that the value will be higher
for w/c 0.32. The fineness, relative humidity,
ambient temperature, size of the structure, curing
methods, heat capacityaffects the dispersion ofheat and result the increase in temperature.
Thermal expansion affects the strain due to
temperature changes.
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Concrete Mix Design- Modified Absolute volume approach is adopted.
Mainly based on past experience (Tips from the lecture notes and the laboratorytechnician).
Cement content is between 400-500kg/m3
w/c ratiois between 0.35 to 0.25. It can be as low as 0.20.
Cement content of 450kg/m3 with w/c 0.32 produces about 73N/mm2 compressivestrength.
Aggregate / cement ratio is between 3.5 4.5 Aggregate should be no more than 10% of flaky/elongated particles.
Coarse / fine aggregate ratio is between 1.5 1.8. It can be high as 2.0.
Large size aggregate should be avoided in order to prevent micro-cracking,shrinkage, creep.
Super-plasticizer should be fully compatible with cement.
PFA is preferable.
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Concrete Mix Design- Modified
The above tips can be explained in the following:
Limited w/c ratio
Abrams Law (1919)
Shows the compressive strength is a functionof w/c but not the amount of cement used. Thecompressive strength increases with thedecrease in w/c ratio. The w/c ratio shouldhave a maximum value for the production of
high strength concrete (> 60N/mm2). Theminimum w/c ratio is given is due to theinsufficiency of compaction as the graphshown on the right hand side.
c/w
2
1c
K
Kf
Com
pressiveStrength
Water/Cement Ratio
Fully Compacted
ConcreteInsufficiently
Compacted Concrete
Vibration
Hand compaction
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Concrete Mix Design- Modified
Limited cement content & aggregate/cement ratio
Experiments shows that retrogression occurs when a mix of very low water-cementratio and high cement content (>530kg/m3) is used with large size of aggregate. This
combines with the effect of heat of hydration (in slide 3), shrinkage limit the maximum
amount of cement content.
The control of the strength and cost also limit the cement content &
aggregate/cement ratio. Since the range of w/c ratio is limited, the effect of water
absorption ability increases when there is a higher aggregate/cement ratio. The effectwill lower if the ratio is lower. A balance should be reached with considerable
workability ( i.e less super-plasticizer is used). From the experience the value should
be 3.5 4.5. The limited aggregate/cement ratio also limtis the range of cement
content.
3
3
3
3
3
/ 3.5- 4.5
500 /
1750 2250 /
388 642 /
Rejected those > 500kg/m
Cement content 400 500 /
Aggregate cement
Maximum cement content kg m
Aggregate kg m
Cement content kg m
kg m
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Concrete Mix Design- Modified
Aggregates- limited in 10% of flaky/elongated particles
The mass of flaky & elongated particle are expressed as flakiness index & elongation
index. The British standard has put a limit on flakiness index and elongation index in
both the design of normal or high strength concrete due to the fact that the higher the
index , the lower the workability. The specified value is , of course higher than 10%.
We want to reduce the reduction of the workability by the flaky/elongated particles by
setting the value at lower than 10%. It would be better if the value is zero, but it isimpossible for normal operation, especially for crushed rock, due to the mechanical
disintegration from the parent rock. Aggregate with thicknessless than 0.6 times the
mean sieve size is
classified as flaky.
Aggregate with length more than 1.8
times the mean sieve size id
classified as elongated.
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Concrete Mix Design- ModifiedAggregates- maximum size of aggregate
Experiments shows that the strength decreases when the increase in the size ofaggregate in the high strength concrete. The increase in size of aggregate reduce the
bonding area in the transition zone and hence a larger stress is result when load is
applied. Discontinuities is also resulted.
Diagrammaticrepresentation of the
composition of
hardened cementpaste
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Concrete Mix Design- Modified
Super-plasticizer (S.P.)
The design of super-plasticizer is based on trial and error. There is a point where
an increase in super-plasticizer but no decrease in w/c ratio. It is the optimum
dosage, but we have not done such a test in the laboratory.
The use of super-plasticizer is used for the following three purposes:1. Increase a higher workability by maintain the same strength of concrete.
2. Increase the strength of concrete by reducing the w/c ratio and maintain the same
slump.
3. Reduce the amount of cement used and maintain the same workability and slump.
During the mix of concrete, we do the trial and error during the mix of concrete.
(1)It is found that the effect is much significant if we add the super-plasticizer in the
water then in the concrete than directly add it on the concrete. (2) The increase of
slump is not quite significant in the later stage of addition. These are because:
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Concrete Mix Design- Modified
1. The super-plasticizer works by disperse the cement grains and release the trapped
water. The S.P. will spread more evenly over the cement grains if it is added withwater. And adding the dosage seperately is similar to re-dosing in Fig. 2.
2. There is moisture loss during the mixing. Since the trial
and error approach is adopted during the mixing, more
time is taken for mix and the slump test. The addition of S.P.
Further worsen the phenomenon by accelerate the speed of
loss of slump, Fig 3.
Fig.3Fig.2
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Concrete Mix Design- Modified
Pulverized Fly ash (PFA)
The PFA used in the mix is Class F PFA. It is mainly composed of SiO2.It does nothave cementitious properties itself. The properties are result the reaction with
Ca(OH)2.
PFA can act as micro-filler. It reduces the size of capillary pores and increase the are
of bonding. It reduce the present of micro-cracking which is a great concern in the
high strength concrete.
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Concrete Mix Design- Modified
The PFA produced in Hong Kong does not have water-reducing property. During themixing, we even found that it may lower the workability as the fine / total aggregateratio is 0.36 which is within the acceptable range. (Given by the laboratory
technician). It is due to the increase in wetted surface area.
The reaction of PFA is slow because its glass material, which prevent it from reaction,
breaks when PH value is at least 13 and it takes a considerable time for the gain of
PH .
The presence of PFA can help cement gel to block the pores and hence a reduction
in permeability. The increase in permeability can help to educe sulfate attack.
PFA react with Ca(OH)2 which is one of the causes of sulfate attack.
PFA can also minimize the allkali-silica reaction.
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Concrete Mix Design- Modified
The design of the amount of PFA is based on the following chart ( for rough estimation
from the result of trial mix).
The chart shows the influence of
percentage of direct replacemet of
PFA on the relative strength
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CompactionVibrating table is used for the compaction of the fresh concrete. It provides an uniform
vibration and relatively reliable means of compaction.Significance of Compaction
Compaction removes the entrapped
air and make the constituents closer
together. The compaction has direct
influence on the strength of the
concrete. It is, however,
over-compaction will result in
segregation which the separate the
mixture into a layer of cement paste
and a layer of aggregate. The
non-uniformity will result in a lowerstrength.
It is expected that the same mix content will have a lower strength in the site than
that makes in the laboratory as internal vibrators are commonly used with larger in
size. More air is entrapped.
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Bleeding
A thin layer of water with some grey flocs
floating on the surface is observed (Bleeding)It is, however, the bleeding is not quite
obvious and serious.
Serious bleeding may be fatal to the strength
of concrete due to the following reasons:
1. The top layer of concrete may
be too wet, a weak layer can be resulted.
2. The bleeding water may be trapped on the
underside of large aggregate and result in
poor bonding. The water flow to the upper
part of the concrete in one direction and
result in high permeability in the direction and lower the resistance of the concrete.
3. If the speed of evaporation of the bleeding water is faster than the bleeding rate, plasticshrinkage cracks may be resulted.
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Results
Mix proportion
The estimated density of fresh
concrete: 2437.5 kg/m3
The cost for 1m3 concrete:$285.19
Three cubes are tested each time. The
Average Compressive strength v.s.
time is plotted below.3
3
3
3
3
10
3
20
3
450 /
112.5 /
185 /
614 /
360 /
716 /
1.249 /
fin
cor
cor
Cement kg m
PFA kg m
Water kg m
Agg kg m
Agg kg m
Agg kg m
Superplasticiser kg m
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Results
Average slump test result = 20mm
which is smaller than the required
range.
Two cylinders are used in the splitting
test. The average splitting test result =
132.2kN
Calculated tensile strength of concrete
= 4.688N/mm2
Cylinder compressive test
=56.55N/mm2
Static modulus of
Elasticity=33300N/mm2
Failure mode
There are three phrase in the high
strength concrete : Mortar , Aggregate,
Mortar-Aggregate inter-phase . From
the observed failure mode of the
concrete. It is found that the concrete
fail in interface at day 2 and the failuremode becomes matrix failure at
7,14,28 days.
Interface failure Matrix failure
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Discussion on the results
Although the splitting test give a more uniform results than other tension test, the
experimental tensile strength is not the actual tensile strength. It is about 5-12%higher than the direct tensile strength. The direct tensile strength is about 4.185-
4.465N/mm2.
The early rapid increase in the compressive strength is caused by the reaction of C3S
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Discussion on the results Relation between Tensile & compressive strength
If the compressive strength is determined by cubic testBritish Stabdard method ft= 0.12(fc)
0.7
is used.
If the compressive strength is determined by cylinder test
ACI method ft= 0.3(fc)2/3
is used.
The calculation shows that the ACI method of 4.42N/mm2 is much closer to thetested tensile strength of 4.688N/mm2 while that is the British Standard is 2.52N/mm2.
Relation between cubic compressive strength & cylindrical compressive strength
cylindrical compressive strength=0.67 cubic compressive strength
Calculation shows that 0.67 cubic strength = 52N/mm2 while that in the measuredcylinder test is of 56.55N/mm2
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