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7/31/2019 Constructon Material -Idv

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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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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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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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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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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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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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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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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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