Characterization of Recycled Aggregate Concrete

8/13/2019 Characterization of Recycled Aggregate Concrete

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International Journal of Advanced Engineering Technology E-ISSN 0976-3945

IJAET/Vol.II/ Issue IV/October-December, 2011/321-330

Research Article

CHARACTERIZATION OF RECYCLED AGGREGATE CONCRETED. N. Parekh

1and Dr. C. D. Modhera

2

Address for Correspondence1Research Scholar; Applied Mechanics Department; SVNIT; Surat

2

Professor; AMD; SVNIT; SuratABSTRACT

Use of recycled aggregate in concrete can be useful for environmental protection and economical terms. Recycled aggregates are

the materials for the future. The application of recycled aggregate has been started in many construction projects in manyEuropean, American and Asian countries. Many countries are giving many infrastructural laws relaxation for increase the use of

recycled aggregate. Codal guidelines of recycled aggregates concrete in various countries were stated here with their effects, onconcreting work. Paper reports the basic properties of recycled fine aggregate and recycled coarse aggregate. It also comparesthese properties with natural aggregates. Basic changes in all aggregate properties were determined and their effects on concreting

work were discussed at length. Similarly the properties of recycled aggregate concrete were also determined and explained here.Basic concrete properties like compressive strength, flexural strength, workability etc are explained here for different combinations

of recycled aggregate with natural aggregate. Use of recycled aggregate has been found useful for pavement construction. Reasons,of use of recycled aggregate concrete in pavement construction, with technical proofs are explained here in detail. Individualperformance of recycled fine aggregate in concrete, use of silica fumes in recycled aggregate concrete, use of fly ash in recycled

aggregate concrete etc are shown with experimental reasons.

In general, present status of recycled aggregate in India with their future need and its successful utilization were discussed here.KEY WORDS: Recycled aggregate concrete, silica fume, fly ash.

1.1 PRELIMINARY REMARKS

1.1.1 Introduction

Concrete is the premier construction material across the

world and the most widely used in all types of civil

engineering works, including infrastructure, low and

high-rise buildings, defense installations, environment

protection and local/domestic developments. Concrete

is a manufactured product, essentially consisting of

cement, aggregates, water and admixture(s). Among

these, aggregates, i.e. inert granular materials such as

sand, crushed stone or gravel form the major part.

Traditionally aggregates have been readily available at

economic prices and of qualities to suit all purposes.

However, in recent years the wisdom of our continued

wholesale extraction and use of aggregates from

natural resources has been questioned at an

international level. This is mainly because of the

depletion of quality primary aggregates and greater

awareness of environmental protection. In light of this,

the availability of natural resources to future

generations has also been realized.

1.1.2 Reasons for Use of Recycled Aggregate

It is now widely accepted that there is a significant

potential for reclaiming and recycling demolished

debris for use in value added applications to maximize

economic and environmental benefits. As a direct

result of this, recycling industries in many part of the

world, including South Africa, at present converts low-

value waste into secondary construction materials such

as a variety of aggregate grades, road materials and

aggregate fines (dust)[7]

. Often these materials are used

in as road construction, backfill for retaining walls,

low-grade concrete production, drainage and brickwork

and block work for low-cost housing.1.2 RECYCLED AGGREGATES AND THEIR

PROPPERTIES

As described earlier, recycled aggregate is a crushed

and processed concrete product. For obvious reasons,

this aggregate is bound to be different from natural

aggregate in many respects. The most notable aspect of

the recycled aggregate is the component of attached

cement mortar with the original aggregate. This

component of attached cement mortar has significant

bearing on most of the properties of recycled

aggregate.

All of the above parameters or the properties of

recycled aggregates are discussed, one by one in detail,

in following pages.

1.2.1 Grading, Particle Shape and Texture

It is generally assumed that natural rock, when fed to a

crusher, will break according to a straight line

distribution, where 15% of the crusher product may be

of a size above the crusher setting in a closed position.

If hardened concrete were to break according to a

straight line distribution, no recycled aggregate should

be generated between 20 and 30mm, 34 % between 10

and 20mm and 17% of crusher fines should be

generated below 5mm. Test results of particle size

distribution reported by Hansen and Narud[13]

are in

reasonably good agreement with the above said

predictions. Similar results have been obtained by

Fergus[9]

. Usually, particle size distribution of crusher

outputs approximate Fuller Curves. Thus, it may be

concluded that the crushing characteristics of hardened

concrete are similar to those of natural rock and are not

significantly affected by the grade of original concrete.

Studies conducted in Japan and reported by B. C. S.

J.[5]

indicate that, irrespective of quality or grade of

source concrete, approximately 20% by weight of fine

recycled aggregate below 5mm is produced when old

concrete is crushed in jaw crusher with an opening of

33mm. Further, with jaw openings of 60mm, 80mm,and 120mm corresponding percentages of fine recycled

aggregate produced are found to be 14.1%, 10.6% and

7.0% respectively. Ravindrarajah and Tam[26]

found

that quantities of fine material below 5mm to be


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23.1%, 25.7% and 26.5% by weight for 37MPa,

30MPa and 22MPa concretes respectively, when the

opening of the jaw crusher is 20mm.

Regarding particle shape and texture, Ravindrarajah

and Tam[26]

reported that, the particle shape of recycled

aggregate is angular and surface texture is more rough

and porous. The surface texture of granite aggregate,

on the other hand is of a rough and crystalline nature.Visual observations on the recycled aggregate reveal

that, not all surfaces of the granite particles are coated

with old materials, whilst some of the aggregate

particles are entirely made up of mortar. While most of

the researcher confirm these findings, Malhotra[20]

has

made observations contrary to the above indicating a

more rounded shape and smooth texture for recycled

aggregate.

1.2.2 Attached Mortar and Cement Paste

When old concrete is crushed, a certain amount of

mortar from the original concrete remains adhered to

virgin aggregate particles. Different size fractions ofrecycled aggregate have varying amount of attached

mortar to it. This component of attached cement mortar

cause significant difference in the properties of

recycled aggregate. Researchers, therefore, found it

necessary to know the amount of adhered cement

mortar.

Hansen and Narud[13]

, on the basis of an investigation

carried out at the Technical University of Denmark (M.

Sc. Thesis), reported that the volume percentage of

mortar attached to natural gravel particle is between

25% and 35% for 16 32mm size fraction, 40% for 8

16mm size fractions and 60% for 48mm size fraction

of recycled aggregate. Hasaba et al.[11]

in his report

indicated a mean of 35.5% of old mortar attached to

natural gravel particle in 25 5mm recycled coarse

aggregate produced by the crushing of original

concrete having a compressive strength of 24MPa.

Corresponding figures are 36.7% mortar content for

41MPa concrete and 38.4% for 51MPa concrete.

The results of a Japanese investigation reported by B.

C. S. J.[5]

indicate that, approximately 20% of cement

paste is attached to 20 30mm size aggregate, while 0

0.3mm filler fraction of recycled fine aggregate

contains 45 65% of old cement paste. Further,

Ravindrarajah and Tam[26]

observed that, in general,

the recycled aggregate contain on average of above

50% by volume, of mortar from original aggregate.

Significant values for the coefficient of variation

observed by Ravindrarajah and Tam indicate the

randomness of the distribution of mortar content in

recycled aggregate.

1.2.3 Specific Gravity and Water Absorption

Specific gravity and water absorption are two vital

properties, which are directly affected due to highly

porous nature of attached old cement mortar toaggregate particles. Most common observations are

significant decrease in the specific gravity (ssd) value,

accompanied by sharp increase in the water absorption

capacity.


in their investigation, found 4.6%

to 6.5% decrease in the specific gravity (ssd) value,

when it is compared with specific gravity (ssd) of

natural gravel. Correspondingly, water absorption

values are 2.3 to 4.6 times the value of absorption

capacity for natural gravel, which is highly significant.

Another notable observation by Hansen and Narud is

that, the values of specific gravity and water absorptionare not significantly affected by grade of original

concrete. Hasaba et. Al[11]

reported about 10%

decrease in specific gravity (ssd) accompanied by

about 6 times increase in water absorption capacity for

recycled aggregate. His report further indicates that, the

above properties are independent of the quality of

original concrete.

Japanese investigation reported by B.C.S.J.[5]

reveal

that dry specific gravity of recycled aggregates varied

between 2.12 and 2.43, corresponding to ssd specific

gravity between 2.29 to 2.51 for recycled aggregates

from a wide range of original concretes.Investigations carried out by Ravindrarajah and Tam

[26]

indicated similar trend. Results of tests conducted on

recycled aggregate obtained from high grade, medium

grade and low grade original concrete are, more or less,

similar thus indicating no significant influence of the

quality of original concrete on the specific gravity and

water absorption property. However, a notable

decrease in specific gravity (ssd) by amount 8.6%

accompanied by a steep rise in water absorption by

about 15 to 18 times is reported. Further, Ravindrarajah

and Tam, observed that highly porous nature of

recycled aggregate particles lead to significant increase

in the value of apparent specific gravity over the

specific gravity (ssd).

M. C. Limbachiya et al[18]

have also found the same

observation that RCA had 3 to 10% lower density and

3 to 5 times higher water absorption than NA in the

saturated surface dry state, reflecting the porosity of

cement paste surrounding the RCA.

1.2.4 Sulphate Soundness

Durability of recycled aggregate is studied in terms of

sulphate soundness. There are very limited results

available on this aspect of recycled aggregate. B. C. S.

J.[5]

reported sodium sulphate soundness loss

percentage after five cycles, ranging from 18.4% to

58.9% for recycled aggregate obtained from 15 original

concrete of different compressive strengths. Contrary

to this observation, Fergus[9]

found magnesium

sulphate soundness loss ranging from 0.9 to 2%.

Strand[27]

observed sulphate soundness loss of 3% for

recycled aggregate compared with 5% for

corresponding virgin aggregate. However, latest

investigation by Shigetoshi and Kobayashi et al[14]

reveal higher percentage lost mass ranging from 22.7%

to 31.5%. Thus, there is a wide difference inobservations on this property.

1.2.5 Mechanical Properties

In general, recycled aggregate are found to be weaker

than corresponding virgin aggregate against


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mechanical action such as aggregate impact value,

aggregate crushing value, ten percent fines value and

Los Angeles aggregate abrasion value. Such a

behavior is expected because of weak mortar

component and mortar aggregate bond in the recycled

aggregate.


observed LA aggregate abrasion

percentages ranging from 22.4% for 16 32mmrecycled aggregate produced from a high strength

original concrete, to 41.4% fro 4 8mm recycled

aggregate from a low strength original concrete.

Corresponding BS aggregate crushing values are

reported to be ranging from 20.4% to 29.6%.

Hasaba et al[11]

report BS aggregate crushing values

ranging from 23.0 for 25.5 mm recycled aggregate

produced from an original high strength concrete to

24.6% for 25-5mm recycled aggregate produced from

an original low strength concrete. Corresponding BS

ten percent fines values are found to be 13.3 tonnes and

11.3 tonnes.B. C. S. J.

[5] found LA aggregate abrasion percentage

ranging from 25.1% to 35.1% for recycled aggregates

from 15 different concretes of widely different

strengths.

Yoshikane[31]

found LA aggregate abrasion percentage

ranging from 20.1% for 13-5mm recycled aggregate

produced from an original high strength (40MPa)

concrete to 28.7% for a 13-5mm recycled aggregate

produced from an original low strength (16MPa)

concrete.


in their investigations

reported aggregate impact value ranging from 26% for

recycled aggregate from a high grade concrete to 31%

for recycled aggregate from a low grade concrete.

Corresponding aggregate crushing values are 28.7%

and 33.5% and that of LA aggregate abrasion values

are 37.2% to 40.8%. Thus, a higher quality original

concrete seems to produce recycled aggregate having

marginally higher resistance to these mechanical

actions than lower quality original concrete.

The results reported above indicate that the recycled

aggregate obtained from poorest quality concrete may

pass ASTM, BS and IS requirements on mechanical

properties of coarse aggregate.

1.2.6 Contaminants

One of the problems associated with the recycled

aggregate is the possibility of contaminants in original

demolition debris passing in to new concrete. These

contaminants may be in the forms of clay balls,

bitumen joint seals, expansion joint fillers, gypsum,

refractory bricks, chlorides, organic materials,

chemical admixtures, tramp steel and other metals,

glass, lightweight bricks and concrete, weathered or

fire damaged particles, particles susceptible to frost or

alkali reactions, industrial chemical sands, reactivesubstances and high alumina cement concrete. The

presence of some of these contaminants in the recycled

aggregate concrete may prove detrimental to the

performance of concrete in one way or the other

B.C.S.J.[5]

report the results of a study of the effect of

various contaminants on the strength properties of

recycled aggregate concrete. The results of the B.C.S.J.

study reveal that the impurities in the form of tiles and

window glass have little influence on the compressive

strength of recycled aggregate concrete. Recycled

aggregate concrete with 3% by weight of gypsum

plaster reduces strength by 15% when concrete is drycured and by up to 50% when it is wet cured. Addition

of 30 volume percent of asphalt to recycled aggregate

reduced concrete compressive strength by

approximately 30%. Fergus[9]

also obtained similar

results.

Chlorides in concrete can give rise to severe

reinforcement corrosion. Original concrete can be

contaminated by chlorides in several ways. When

chloride affected original concrete is crushed for

manufacture of recycled aggregate, the processed

recycled aggregate may carry chloride ions. Presence

of such chloride ions in recycled aggregate concretemay promote corrosion of reinforcement and thus

affect the durability of structure. However, if chloride

ions in concrete prior to service exposure are within

prescribed limits specified by ACI committee or any

other standards, then the durability of concrete is not

affected. Hansen and Hedegaard[12]

conducted some

studies on this aspect. They further investigated the

properties of recycled aggregate concrete as affected by

chemical admixtures in original concretes. They

observed that, as long as plasticizing, air-entraining and

retarding admixtures are used in quantities not

exceeding the manufacturers prescribed dosage, the

presence of such admixtures in recycled aggregate

concrete and on compressive strength of hardened

concrete.

1.3 RECYCLED AGGREGATE CONCRETE &

ITS PROPERTIES:

It is important that various constituents of concrete are

proportioned appropriately to obtain the desired

properties of fresh and hardened concrete most

economically. While it is possible to employ the same

conventional mix design methods for proportioning

recycled aggregate concrete, the behavior of recycled

aggregate concrete mix in its fresh state does not

remain same as that of the conventional concrete of

identical mix proportions. The reasons for this can be

attributed to change in the properties of recycled

aggregate.

1.3.1Mix Design

All available reports indicate that existing conventional

mix design methods can be employed for proportioning

various constituents of recycled aggregates concrete.

However, due to higher water absorption

characteristics of recycled aggregate, a change in water

demand and thus cement demand is noticed by severalinvestigators. Ravindrarajah and Tam

[26] however

indicated a requirement of 8% more free water for

recycled aggregate concrete for similar purpose.

Further it was observed that 5% to 8% extra cement is


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required to maintain the water cement ratio at the

same level and to obtain the same strength as that of

corresponding conventional concrete.

According to B.C.S.J.[5]

, the optimum ratio of fine to

coarse aggregate remains approximately the same for

recycled aggregate concrete as for conventional

concrete. Due to possible variation in quality of

recycled aggregate, appropriate standard deviation isdesired to be employed while designing a recycled

aggregate concrete mix. B. C. S. J. and CUR found the

coefficients of variation for compressive strength of

recycled aggregate concrete in the laboratory not to be

much different from that of conventional concrete,

when one and the same recycled aggregate was used

throughout production. Hansen and Narud[13]

later

confirmed these findings. However, when recycled

aggregate concretes are produced from original

concrete of different qualities, the coefficient of

variation for compressive strength is much larger than

when the same recycled aggregate is used in allbatches.

Although, the researcher have suggested some

modifications while designing recycled aggregate

concretes by conventional methods, these modification

appear to be more general in nature and purely

approximate, requiring more trial for finalization of

mix proportions. Thus, no precise guidelines based on

rational basis, are available for proportioning recycled

aggregate concrete mixes.

1.3.2 Workability

Workability of concrete which determines the mobility

and placeability of concrete mix is measured in terms

of slump, compaction factor and/or vee bee time.

Available reports indicate that, the workability of

recycled aggregate concrete closely matches the

workability of conventional concrete. However, other

researcher have also made it clear that, workability of

recycled aggregate concrete only if the additional water

demand of 5 to 8% required by recycled aggregate

concrete is appropriately met with. Poon et al.[22]

showed that the slump of recycled aggregate concrete

was dependent on the moisture state of the recycled

aggregate. When oven dry recycled aggregate was

used, a high initial slump was observed due to the high

amount of water that was used to compensate for the

high water absorption of the recycled aggregate[16]

.

Another notable observation by Hansen and Narud[13]

is about the rapid loss of workability of recycled

aggregate concrete with elapsed time. The workability

(slump and compacting factor) and stability (bleeding

and segregation) of recycled aggregate concrete mixes

were determined. In addition, in order to assess the

effect of recycled aggregate on the retention of the

workability and maintenance of entrained air content

with time, further tests were carried out up to 150 and60 minutes respectively. In general, results showed a

reduction in slump value with increasing recycled

aggregate content in the mix, but this remained

essentially within the specified tolerances of 25mm,

of BS 5328.[18]

However, the stability of the mixes

containing greater than 50% recycled aggregate content

was reduced. In the main such mixes were found to be

harsh, less cohesive and exhibited increased bleeding

when compared to the corresponding NA concrete.[18]

1.3.3 Wet density and Air Content

Many researchers found that the natural air content of

fresh recycled aggregate concretes were higher andvaried more than air contents of fresh conventional

concrete mixes. Wet density of fresh recycled

aggregate concrete varied from 2020 to 2210 kg/m3,

which is between 85% to 95% of conventional concrete

mixes.


conducted systematic

investigation, the results of which reveal that natural air

contents of recycled aggregate concretes may be up to

0.6% higher than natural air contents of fresh

conventional concrete mixes. Correspondingly, wet

density for recycled aggregate concretes varied from

2200 to 2250 kg.m3

, which is more than 95% ofconventional concrete mixes.

In general, for fresh recycled aggregate concrete, 5% to

15% reduction in wet density accompanied by slight

increase in air contents is more commonly observed.

However, the extent of increase in air content and

reduction in wet density depends on the mix design and

efficiency of compaction. The reports further indicate

that, it is possible to produce recycled aggregate

concrete with no significant increase in air content and

less than 5% lower density, compared with

corresponding conventional concrete.

1.3.4 Compressive Strength and Rate of Strength

Development:

On the basis of experiments B. C. S. J.[5]

in Japan

derived the conclusions on the compressive strength

results, which showed compressive strength of recycled

aggregate concrete to be between 14% and 32% lower

than that of conventional concrete. Apparent

correlation obtained between compressive strengths of

conventional and recycled aggregate concrete reveal

that, recycled aggregate concrete consistently had 10%

lower compressive strength than control concrete made

with natural aggregate. Later Ravindrajah and Tam[26]

found recycled aggregate concrete to have between 8%

and 24% lower compressive strength than

corresponding concretes made with conventional

aggregates.


conducted a series of

experiments for find out the effect of grade of original

concrete on the compressive strength of recycled

aggregate concrete. They obtained three types of

recycled aggregates namely H, M, and L by crushing

the laboratory made conventional concretes of high

strength, medium strength and low strength

respectively. Recycled aggregate concrete of H, M, andL having same mix proportions as the three original

concretes, but with all nine possible combinations of

aggregates were prepared and tested for compressive

strength.


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Table 1 Results of Compressive strengths for the fly ash replaced specimen for W/B ratio 0.45

Table 2 Results of Compressive strengths for fly ash replaced specimen for W/B ratio 0.55

The test results of these experimental reveal that, the

compressive strength of recycled aggregate concrete

depends on the strength of original concrete, and that it

is largely controlled by a combination of the water

cement ratio of the original concrete and water

cement ratio of the recycled aggregate concrete, when

other factors are essentially identical. If the water

cement ratio of the original concrete is the same as or

lower than that of the recycled aggregate concrete, then

the strength of recycled aggregate concrete can also be

as good as or higher than the strength of the original

concrete.

M. C. Limbachiya et al[18]

has shown the results with

help of graph (Fig. 2) that up to 30% coarse recycled

coarse aggregate has no effect on concrete strength, but

thereafter a gradual reduction with increasing recycled

aggregate content occurs.

Fig 2 Test results of compressive strength of cube[18]

Kou Shi Cong et al[16]

had replaced cement with fly ash

for different W/B ratio of 0.45 and 0.55. Results of theboth are shown in the table 1 and 2 respectively.

Dhir et al.[8]

showed that the compressive strength of

concrete prepared with 100% coarse and 50% fine

recycled aggregates was between 20 and 30% lower

than that of the corresponding natural aggregate

concrete. However, the reduction in strength can be

minimized if the mixing procedure is modified.[16]

Some other researcher also found the strength

development with age to be similar for conventional

and recycled aggregate concrete made with recycled

coarse aggregate and/or natural sand.

1.3.5 Modulus of Elasticity

Due to large amount of old mortar with comparatively

low modulus of elasticity which is attached to original

aggregate particles in recycled aggregates, the modulus

of elasticity of recycled aggregate concrete is always

lower than that of corresponding conventional

concretes. Static modulus of elasticity of recycled

aggregate concrete is generally reported to be on lower

side.

Japanese investigations reported by B. C. S. J. indicate

10% to 30% lower modulus of elasticity for recycled

aggregate concretes.Hansen

[13] had conducted a series of experiments to

determine the effect of grade of original concrete on

modulus of elasticity. They obtained three types of

aggregate namely, H, M, and L by crushing laboratory

made conventional concrete of high strength, medium

strength and lower strength respectively. Recycled

aggregate concretes of high strength, medium strength

and low strength, having same mix proportions as the

three original concretes, but with all nine possible

combinations of aggregates were prepared and tested

for modulus of elasticity. The test results of these set of

experiments reveal that, both dynamic and staticmodulus of elasticity are from 14% to 28% lower for

recycled aggregate concrete than for corresponding

conventional concretes. However it is evident that

difference in modulus of elasticity would have been


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much larger if the high strength, concrete had been

made with softer aggregate than the natural aggregate

which was actually used in the experiment. In one

particular case Hansen and Boegh found the modulus

of elasticity of recycled aggregate concrete which was

made with recycled aggregate that consisted of low

quality crushed mortar to be 45% lower than the

modulus of elasticity of corresponding conventional

concretes.


, in their investigation

confirmed the above findings. They also found the

relationship between compressive strength and

modulus of elasticity which is different from that for

conventional concrete proposed by various authorities

such as CEB, FIP, and BS code.

In his investigation, Topcu[29]

obtained the complete

stressstrain curve of recycled aggregate concrete with

the RCA replacement percentages of 0%, 30%, 50%,

70% and 100%, and he found that with the increase of

recycled coarse aggregate amount, the values of the

elastic modulus decrease by about 30%.

Fig 3 Modulus of Elasticity for recycled aggregate

concrete

Xiao et al[30]

has noted that the elastic modulus of

recycled aggregate concrete is lower than that of the

normal concrete. It decreases as the recycled aggregate

content increases. For a recycled aggregate

replacement percentage equals 100%, the elastic

modulus is reduced by 45% (Fig 3).

1.3.6 Tensile and Flexural Strength

According to B. C. S. J.[5]

significant difference in

indirect tensile strength (split tensile strength) of

conventional concrete and corresponding recycled

aggregate concrete. Further they[5]

, found that the

flexural strength of recycled aggregate concrete is

somewhere between 1/5 and 1/8 of its compressive

strength, similar to what is the case for conventional

concrete. However, no experimental data are presented.


at first found no significant

difference in flexural strength of conventional concrete

and recycled aggregate concrete. However, later they

only has reported that, both tensile and flexural

strength of recycled aggregate concrete is consistently

10% lower than corresponding conventional concrete.However Kou Shi Cong et al

[16] has determined the

tensile splitting strength of concrete with partial fly ash

replacement. He showed that tensile splitting strength

of the concrete mixture decreased as the recycled

aggregate content increased. At the same recycled

aggregate replacement level, the use of fly ash as a

partial replacement of cement reduced the tensile

splitting strength of the concrete. Results of those

experiments are given in the table 3 and 4 for W/B

ratio of 0.45 and 0.55 respectively.

Table 3 Results of Tensile Splitting Strength for fly ash replaced specimen for W/B ratio 0.45[16]

Table 4 Results of Tensile Splitting Strength for fly ash replaced specimen for W/B ratio 0.55[16]


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Table 5 Mix proportioning details of condensed silica fume concrete[15]

Table 6 Strength properties of high performance concrete[15]

Kalaiarasu and Subramanian[15]

has experimented

silica fumes with recycled aggregate concrete by using

mix proportions as given in table 5, and reported the

tensile splitting strength test results shown in table 6.

Detail conclusion is given below in the same word.

Cement replacement level of 15% with silica fume in

M60 grade of HPC (40% artificial sand and 60%

natural sand) is found to be the optimum level to obtain

higher values of compressive strength, split tensile

strength and elastic modules. Concrete mixes

containing silica fume showed higher values of acidresistance and impermeability of chloride ions

[15].

1.3.7 Creep and Drying Shrinkage

It is a time dependent phenomenon, which is quite

significant for any type of concrete. Concrete creeps

considerably under sustained loads for a long time and

thus deserves careful attention. Accordingly, some of

the researchers have investigated this aspect carefully.


found creep of recycled

aggregate concrete to be 30% to 60% higher than creep

of corresponding conventional concretes. Further, the

investigators observed that specific creep of recycled

aggregate concrete was greater than that ofconventional concrete. These differences appeared to

have developed over a period of 250 300 days after

loading. However later the rate of increase in creep

strain for both concrete gradually became smaller with

further increase in time. Creep strain increased

considerably with an increase in the water cement ratio,

but the difference in creep between recycled aggregate

concrete and ordinary concrete remained almost

constant at any water cement ratio and at any sustained

load level. Thus it is not expected that creep of

recycled aggregate concrete shall give rise to any

problem provided its magnitude is taken into account.

Kou Shi Kong et al[16]

have shown that the creep of

concrete increased with an increasing recycled

aggregate content. The use of fly ash as a partial

replacement of cement was able to reduce the creep of

concrete as a result of the greater long term strength

development due to the pozzolanic reaction of fly ash.[16]

Shrinkage is a term generally used to describe various

aspects of volume change in concrete. Volume change

is one of the most detrimental properties of concrete,

which affects long term strength durability. Although,

it is difficult to get concrete without shrinkage, it is

equally important to control the shrinkage in the body

of the concrete.Hasaba et al

[11] conducted tests to evaluate drying

shrinkage and they found drying shrinkage of recycled

aggregate concrete to be 40% to 50% higher than

drying shrinkage of corresponding conventional

concretes.


conducted experiments on

low, medium and high strength recycled aggregates

concretes, each having made using recycled aggregate

obtained form similar conventional concretes of low,

medium and high strengths. The test results indicated

as low as 14% to as high as 95% higher drying

shrinkage for recycled aggregate concrete of varioustypes. Further, the test results conclude that, the use of

recycled aggregate from lower grade original concrete

seems to be beneficial in reducing drying shrinkage.

Limbachiya M. C. et. al.[18]

found that the ultimate

shrinkage and creep strains were found to increase with

recycled aggregate content in the mix. This is due to

the increased proportions of cement content in such

concrete mixes, as the w/c ratio of this mix was

reduced by increasing cement content to achieve 28-

day strength equivalent to corresponding natural

aggregate concrete. Previously it has been reported that

the presence of attached mortar in the recycled

aggregate is also a contributory factor for higher

shrinkage and creep strains in concrete with high

proportions of recycled aggregate[19]

.


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Koi Shi Cong et. al.[16]

had shown that use of fly ash as

a substitute for cement improved the resistance to

chloride ion penetration and decreased the drying

shrinkage and creep of the recycled aggregate concrete

and drying shrinkage and creep can increased with

increase of the recycled aggregate content. They

further explained the results that one of the practical

ways to utilize a high percentage of recycled aggregatein structural concrete is by incorporating 2535% of fly

ash as some of the drawbacks induced by the use of

recycled aggregates in concrete could be minimized.

They also further derived that the decrease in the W/B

ratio can also led to a reduction in the drying shrinkage.[16]

Comparisons for the different properties with different

percentage of recycled aggregates are shown in table

7.[18]

Table 7 Comparison of engineering and durability

performance of NA and RA concrete

1.3.8 Permeability and Water Absorption

The rate of most kinds of concrete deterioration

depends on concrete permeability. This is because

water absorption is indirectly related to permeability of

hardened concrete and penetration of water into

concrete is required for most deterioration mechanism

to be effective.

B. C. S. J.[5] conducted water permeability tests on

concretes which were made with water cement ratio

of 0.5 0.7 and with slump values around 21 cms. The

results show that the water permeability of recycled

aggregate concrete is 2 5 times that of corresponding

conventional concretes and that the scatter of result is

larger.

Abou-Zeid et al.[1]

reported that recycled aggregate

concrete exhibited higher water permeability and lower

resistance to chloride ion penetration compared to

conventional concrete.

1.3.9 Frost Resistance

The frost resistance of conventional and recycled

aggregate concrete which were produced with a variety

of water cement ratios, was compared and observed

that, the freeze thaw resistance of recycled aggregate

concrete is higher than that of corresponding

conventional concrete.

While American, French and Dutch results on frost

resistance are encouraging, Japanese results are less

convulsive. More research is, therefore desired to be

carried out to establish the fact.

Limbachiya M. C. et. al.[18]

reported that the durability

factors obtained following procedures described inASTM C666

[4] showed that the concrete produced

using up to 100% coarse RCA had durability factors in

excess of 95%, indicating little or no deterioration

under freeze/thaw attack. The results also showed

minor effect of RCA content on abrasion resistance.

Typically, the differences between concrete made with

NA and 100% coarse RCA at design strength 35 and

45 N/mm2were 0.09 and 0.05mm respectively.

Salem et al.[24]

showed that recycled aggregate concrete

had a lower resistance to freezing and thawing

compared to natural concrete.

1.3.10 Some Other DetailsKalaiarasu M. and Suramanian K.

[15]had reported that

even a partial replacement of cement with silica fume

in concrete mixes would lead to considerable saving in

consumption of cement and natural sand. Therefore

they have concluded that replacement of cement with

15% of silica fume would render concrete (with 40%

artificial sand) more strong and durable.

Lee S. T. et. al.[17]

had reported that high replacement

levels of recycled fines, especially those with high

water absorption, resulted in poor resistance against

both sodium and magnesium sulfate attack.

This study further confirms that the absorption

characteristics and replacement level of recycled fines

used have a decisive influence on the durability,

especially sulphate resistance, of hardened cement

matrix.[3]

1.4 CONCLUDING REMARKS

Some of the concluding remarks based on various

experiments are given below.

The potential compressive strength of concretecontaining recycled concrete as aggregate is

controlled largely by the compressive strength

of the concrete to be recycled, provided the fine

aggregate is crushed rock or natural sand of

suitable quality.

A substantial reduction in potential compressivestrength may result when the conventional fine

aggregate is replaced in whole or in part by fine

aggregate derived from the recycled concrete.

Further material smaller than 2 mm in recycled

concrete should be screened and wasted.

Use of recycled concrete decreases workabilityof fresh concrete at given water content,

increases water requirements for given

consistency, increases drying shrinkage at givenwater content, and reduces modulus of elasticity

at given water-cement ratio. The effects are

greatest when the recycled concrete is used as

both coarse and fine aggregate.


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Freezing and thawing resistance of the newconcrete relates to many factors, including the

properties of the recycled concrete in terms of

compressive strength, parameters of the air-

void system, and frost resistance of the

aggregate included in the recycled concrete as

well as the parameters of the air-void system

and other qualities of the cementitious matrix ofthe new concrete.

Chemical admixtures, air-entrainingadmixtures, and mineral admixtures included in

the recycled concrete will not modify

significantly the properties of the fresh or

hardened, new concrete. High concentrations of

water soluble chloride ion in the recycled

concrete may contribute to accelerated

corrosion of steel embedments in the new

concrete.

Prospective sources of recycled concrete maybe unsound or have been rendered unsound inservice, such as presence of physically unsound

or chemically reactive aggregate, deterioration

by aggressive chemical attack or leaching,

damage by fire or service at high temperature,

and so on.

Significance of contaminants in the recycledconcrete should be analysed in relation to the

anticipated service, such as presence of

noxious, toxic, or radioactive substances;

presence of bituminous materials that may

impair air entrainment; appreciable

concentrations of organic materials that my

produce excessive air entrainment; inclusion of

metallic embedments that may cause rust

staining or blistering of surfaces; and excessive

fragments of glass, including bottle glass, that

are expected to produce harmful effects of

alkali-silica reaction.

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European Commission.

3. ASTM (2005) Standard DescriptiveNomenclature for Constituents of ConcreteAggregates, C 29405: 910

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Characterization of Recycled Aggregate Concrete

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