Strength and Workability of Recycled Aggregate Concrete

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1

INTRODUCTION


The amount of concrete debris collected from d
e-
mo
l
ished structures is huge. Without proper trea
t-
ments, it can cause secondary p
ollution. The applic
a-
tion of building rubble collected from damaged and
demolished structures become an important issue in
every country. After crushing and screening, this m
a-
terial could serve as recycled aggregate in concrete.
Thus, recycling and reusing

this concrete debris can
not only reduce the waste but also transform them
i
n
to aggregate resources.

In Taiwan, lots of researches and dissertations r
e-
garding recycled concrete have been reported recen
t-
ly. The possible effects of recycled aggregate upon
c
oncrete properties such as workability, strength and
durability have been discussed in several papers [1
,
2
]. In most of the literature the main concerns were
the variations in recycled aggregate properties
caused by the native waste paste and their effect
s
upon the concrete properties [
3
-
6
]. Due to lower
compressive strength and worse workability of RAC,
it will be limited in the application of construction.
A series of experiments using recycled aggregate of
three original strength levels from tested conc
rete
cylinders in the laboratory was conducted.

Moreover,
p
ozzolans such as fly ash and slag
could increase compressive strength and workability
when it applies on concrete

[
7
]. In this research fly
ash and slag were used to replace part of cement for
impr
oving the properties of recycled concrete. The
RACs were produced
with

same water
/
binder ratio
and different replacing percentage of fly ash and slag
to cement. The engineering properties of RAC were
investigated to determine the optimum replacing
amount a
nd rate of fly ash and slag.

Most buildings in Asia were constructed of rei
n-
forced concrete accompanied with brick and tile m
a-
terials. Thus, building rubble collected from da
m-
aged structures includes bricks and tile as well as
waste concrete. The effects o
f brick and tile particles
on the properties of recycled concrete are less well
known in the literature. In light of the difficulty of
splitting the concrete waste and the brick rubbles,
this research also investigated recycled aggregates
made from buildin
g rubble, containing waste co
n-
crete, bricks and tiles. A series of tests were carried
out to investigate the effects of recycled aggregates
containing various brick contents on the properties
of the recycled concrete.

2

EXPERIMENTAL PROGRAM


The present stud
y consists of three parts.

The D
max

of
recycled aggregate

and natural aggregate is 25 mm.

The first part is to produce the recycled coarse a
g-
gregate by the crushing of tested concrete cylinders
Strength and Workability of Recycled

Aggregate Concrete

H
.
S
.

Peng
,

H
.
J
.

Chen

&
T
.

Yen

Dep
t.

of Civil Engineering, National Chung
-
Hsing University, Taichung
,

Taiwan











ABSTRACT:
Recycled aggregate concrete (RAC) has lower compressive strength and wor
se workability than
normal concrete. This research intends to investigate the effect of the strength of recycled coarse aggregate on
the strength and workability of RAC. The recycled coarse aggregate was produced by the crushing of tested
concrete cylinder
s in the laboratory, in which three strength levels of concrete cylinders were selected. Mor
e-
over, pozzolans such as fly ash and slag were applied on RAC to replace parts of cement. Test results show
that it could increase the compressive strength and the
workability of RAC to reach the same level with nat
u-
ral aggregate concrete for using 10%~30% of fly ash or 30%~50% of slag replacement. On the other hand, the
recycled aggregate from construction waste contains not only waste concrete but large amount of b
ricks. In
light of the difficulty of splitting the waste concrete and the bricks, this research also intends to
use the bricks

as
recycled
aggregate. Test results show that brick contents in recycled aggregate have little influence on the
compressive stren
gth and workability of RAC. Therefore, this article recommends the application of bricks i
n-
to recycled aggregate.

in the laboratory, in which three strength levels of
concrete
cylinders were selected. In order to evaluate
the effect on the properties of concrete

made from
different strength levels of wasted concrete

used to
produce

recycled aggregate
, concrete was made of
natural aggregate
as

a control group
.

The mix pr
o-
portions

are shown in
Table

1
,
where
0.5 represents
the water
/
cement ratio, N means natural aggregate,
and R2
0

indicate that the original strength of
old
concrete

is
at

the 20 MPa level (20

±
2.5

MPa)
.




Table 1
.

Mix Proportions (
Part 1
).




















The
second one is also to manufacture the rec
y-
cled coarse aggregates by crushing concrete cyli
n-
ders

as the original
strength
of

old concrete

is at the
range of

20 MPa to 35 MPa
. In the study, different
percentage of fly ash and slag are used to replace
parts o
f the portland cement. The mix proportions
are shown in
Table

2. These mixtures had same w
a-
ter
/binder

ratios of 0.60.

Accordingly, the
y were
l
a-
beled NC, R0%, RF30% and RS30%, w
here

NC i
n-
dicate the natural aggregate concrete, R means
recycled aggregate conc
rete, and F30%, S30% repr
e-
sents 30% of fly ash or slag replacement.



Table 2
.


Mix Proportions (
Part 2
).
















The last part is to evaluate the properties of co
n-
crete

made with various recycled aggregate compos
i-
tions, five groups of recycled con
crete made with
washed recycled aggregate, with the same w
a-
ter
/
cement ratio of 0.5 were adapted for testing. In
these five groups, the bricks and tiles content in the
recycled aggregate were the only variables. The
weight proportions of brick and tile part
icles in the
recycled aggregates
were

0%, 17%, 33%, 50%, and
67%. They were numbered BT0%, BT17%, BT33%,
BT50% and BT67% respectively.

Additionally, co
n-
crete made of natural aggregate with the same w
a-
ter
/
cement ratio served
as

the control batch
.

The mix
pr
oportions are shown in
Table
s 3.



Table 3
.


Mix Proportions (
Part 3
).














The compressive strength and the slump test of
recycled concrete and normal concrete were tested
for comparison
.

The test method
s

followed ASTM
specifications. Specimens w
ere cured in a curing
room and then taken out
to air dry
one day before

the
tests
.

Specimens used in

the

test
s

for compressive
strength were
ψ
100×200 mm.

3

RESULTS AND DISCUSSI
ONS

3.1

Effect of recycled aggregate strength on
r
ecycle
c
oncrete

T
he compressive stre
ngth
of
recycled concrete and
normal

concrete with different aggregate strength
levels are shown in
Figure

1.

The strength of rec
y-
cled aggregate in the mix will govern the percentage
reduction in the recycled concrete strength. As the
strength of recycled
aggregate is increased, the pe
r-
centage reduction in recycled concrete strength d
e-
creases. A
t
the water
/
cement ratio of 0.5, the co
m-
pressive strength of recycled concrete with highest
aggregate strength can reach 90
%
of its correspon
d-
ing
normal

concrete.

Th
e strength of recycled co
n-
crete with lowest aggregate strength can reach only
about 85
%
. For higher water
/
cement ratios of 0.6
and 0.7, the compressive strength of recycled co
n-
crete can reach at least 89
%
that of
normal

concrete,
respectively.

The main rea
son is that the strength of the paste
greatly

increase
s
at low water
/
cement ratios.
A
ccor
d-
ing to the composite material theory, the recycled
aggregate
becomes
a weak material

and

its bearing
capacity is
small
er

thus lead
ing

to

a decrease in

co
n-
crete

streng
th. This effect will dominate the strength
of recycled concrete.

The test results of fresh concrete are shown in
Figure

2.

R
egard
ing

the
slump, under the same mix
proportion, the natural aggregate concrete has a
Item
Water
Cement
Fly Ash
Slag
Natural
Sand
Natural
Coarse
Aggregate
Recycled
Coarse
Aggregate
NC
345
0
0
847
839
0
R0%
345
0
0
844
RF10%
310.5
34.5
0
834
RF20%
276
69
0
823
RF30%
241.5
103.5
0
813
RS30%
241.5
0
103.5
836
RS40%
207
0
138
834
RS50%
172.5
0
172.5
831
207
810
0
Unit : kg/m
3
Waste
Concrete
Brick and
Tile
NC
1123
0
0
BT0%
0
969
0
BT17%
0
808
161
BT33%
0
646
323
BT50%
0
485
485
BT67%
0
323
646
Unit : kg/m
3
Item
Water
Cement
Natural
Sand
Recycled Coarse
Natural
Coarse
Aggregate
190
380
637
Item
Water
Cement
Natural
Sand
Natural
Coarse
Aggregate
Recycled
Coarse
Aggregate
0.5N
938
0
0.5R20
0
865
0.5R25
0
851
0.5R30
0
861
0.6N
938
0
0.6R20
0
865
0.6R25
0
851
0.6R30
0
861
0.7N
938
0
0.7R20
0
865
0.7R25
0
851
0.7R30
0
861
Unit : kg/m
3
208
208
208
416
734
347
791
297
833
higher slump.

If
the
natural aggregate is re
placed
with the

recycled aggregate, the slump may decrease.
This is
due to

the
flatter
particle shape

and
rough

surface

of recycled aggregate than that of natural a
g-
gregate.

The change of recycled aggregate strength
will not make the situation different.






















Figure

1
.

Effect of original strength on strength
.























Figure

2
.

Effect of original strength on slump
.



3.2

Effect of fly ash replacement on
r
ecycle
c
oncrete

Using suitable
amount of
fly ash to replace part of
cement
can decrease the water demand of the mi
x-
ture. This character has a beneficial effect in the
strength of concrete. Because the pozolanic reaction
is much slower than the initial cementitious rea
c-
tions, the strength of concrete made with fly ash is
lower tha
n that of concrete made without the cement
replacement by fly ash at early ages. But the ultimate
strength is nearly the same or greater.

The test results are shown in
Figure

3. The co
m-
pressive strength of RAC were obviously increased
when the replacement
of fly ash is 10%, it may su
r-
pass the normal strength requirement at 28 days. Up
to 20% replacement, it can also meet the normal
strength requirement at 56 days and surpass that at
91 days. It shows that the optimum replacing amount
and rate of fly ash for

RAC is about 10% ~20%, as
well as normal concrete.

The addition of fly ash improves the workability
because of the fineness of the material and its sphe
r-
ical particle shape.
Figure

4 shows that the slump
values of RAC are greater than its corresponding
no
rmal concrete when the replacement of fly ash is
more than 20%. So, the defects of inferior workabi
l-
ity and of RAC could be greatly improved.




















Figure

3
.

Effect of fly ash contents on strength
.





















Figure

4
.

Effect of f
ly ash contents on slump.



3.3

Effect of slag replacement on
r
ecycle
c
oncrete

Slag
is a siliceous material that
possess little c
e-
mentitious property
,
and

can react with the calcium
hydroxide of concrete to form cementitious co
m-
pounds. The test results are sho
wn in
Figure

5. The
compressive strength of RAC were obviously i
n-
creased when the replacement of slag is 30%, it may
surpass the normal strength requirement at 28 days.
Up to 40% replacement, it can also surpass the no
r-

0%
10%
20%
30%
F
l
y

a
s
h

c
o
n
t
e
n
t
s
20
25
30
35
40
C
o
m
p
r
e
s
s
i
v
e

s
t
r
e
n
g
t
h

(
M
P
a
)
A
g
e
s
91 days
56 days
28 days
N
C

(
9
1

d
a
y
s
)
N
C

(
5
6

d
a
y
s
)
N
C

(
2
8

d
a
y
s
)
0%
10%
20%
30%
F
l
y

a
s
h

c
o
n
t
e
n
t
s
0
2.5
5
7.5
10
12.5
S
l
u
m
p

(
c
m
)
T
i
m
e
45 mins
30 mins
0 mins
N
C

(
0

m
i
n
s
)
N
C

(
3
0

m
i
n
s
)
N
C

(
4
5

m
i
n
s
)
20
25
30
¡ ]MPa¡ ^
O
r
i
g
i
n
a
l

s
t
r
e
n
g
t
h

o
f

o
l
d

c
o
n
c
r
e
t
e
10
20
30
40
50
C
o
m
p
r
e
s
s
i
v
e

s
t
r
e
n
g
t
h

(
M
P
a
)
85%
86%
90%
89%
93%
95%
89%
91%
99%
W/C=0.5
W/C=0.6
W/C=0.7
0
.
5
N

(
1
0
0
%
)
0
.
6
N

(
1
0
0
%
)
0
.
7
N

(
1
0
0
%
)
20
25
30
¡ ]MPa¡ ^
O
r
i
g
i
n
a
l

s
t
r
e
n
g
t
h

o
f

o
l
d

c
o
n
c
r
e
t
e
0
5
10
15
20
S
l
u
m
p

(
c
m
)
92%
83%
75%
86%
83%
83%
84%
84%
90%
W/C=0.5
W/C=0.6
W/C=0.7
0
.
5
N

(
1
0
0
%
)
0
.
7
N

(
1
0
0
%
)
0
.
6
N

(
1
0
0
%
)
mal strength requirement at 56 days.
It shows that
the optimum replacing amount and rate of
slag

for
RAC is about 30% ~40%.

In g
eneral, concrete made with slag replacement
has well workability than that made without it.
Fi
g-
ure

6 shows that it could
enhanc
e the slump value of
RAC to the n
orm
al

concrete level for using
the
a
p-
propriat
ely

slag replacement.






















Figure

5
.

Effect of slag contents on strength
.























Figure

6
.

Effect of slag contents on slump
.



3.4

Effect of
b
rick and
t
itle
c
ontent on
r
ecycle
c
oncrete

The compressive strength
of

the 5 groups of recycled
concrete made with various recycled coarse aggr
e-
gate compositions is shown in
Figure

7.

The test r
e-
sults show that the compressive strength of concrete
made with recycled coarse aggregate containing
bric
k and tile particles is about 75

~ 85
%
that of
normal

concrete. The greater the brick and tile co
n-
tent, the lower the compressive strength of the rec
y-
cled concrete. However, within a 10% variation of
the compressive strength, it means that recycled a
g-
grega
te
may
contains brick and tile particles up to a
certain amount. Because

these materials posses

b
asic
strength, their impact upon the compressive strength
of recycled concrete is relatively limited.

I
t

can be seen from
Figure

8 that
the
slump of r
e-
cycled c
oncrete is about 71

~

83
%
that of
normal

concrete.

There

is

no
major

difference
among

the
workability of recycled concrete mixes containing
various amounts of brick and tile. From the results, it
can be concluded that brick and tile particles will
have som
e impact upon
the
workability of recycled
concrete. Using brick and tile particles in recycled
aggregate is acceptable
in

recycled concrete produ
c-
tion when the brick and tile content in recycled a
g-
gregate
i
s lower than 67
%
.






















Figure

7
.

Effect of brick contents on strength
.























Figure

8
.

Effect of brick contents on slump
.



4

CONCLUSIONS

From

the result
s

and discussion
,

the conclusions

are

drawn

as follows:



0%
30%
40%
50%
S
l
a
g

c
o
n
t
e
n
t
s
20
25
30
35
40
C
o
m
p
r
e
s
s
i
v
e

s
t
r
e
n
g
t
h

(
M
P
a
)
A
g
e
s
91 days
56 days
28 days
N
C

(
9
1

d
a
y
s
)
N
C

(
5
6

d
a
y
s
)
N
C

(
2
8

d
a
y
s
)

0%
30%
40%
50%
S
l
a
g

c
o
n
t
e
n
t
s
0
2
4
6
8
10
S
l
u
m
p

(
c
m
)
T
i
m
e
45 mins
30 mins
0 mins
N
C

(
0

m
i
m
s
)
N
C

(
3
0

m
i
n
s
)
N
C

(
4
5

m
i
n
s
)

67%
50%
33%
17%
0%
B
r
i
c
k

a
n
d

t
i
l
e

c
o
n
t
e
n
t
s
0
10
20
30
40
50
C
o
m
p
r
e
s
s
i
v
e

s
t
r
e
n
g
t
h

(
M
P
a
)
75%
80%
80%
77%
85%
N
C

(
1
0
0
%
)

67%
50%
33%
17%
0%
B
r
i
c
k

a
n
d

t
i
l
e

c
o
n
t
e
n
t
s
0
3
6
9
12
15
S
l
u
m
p

(
c
m
)
71%
71%
71%
83%
75%
N
C

(
1
0
0
%
)
1.

As the original strength of recycled aggregate is
increased, t
he percentage reduction in recycled
concrete strength decreases. For higher w
a-
ter
/
cement ratios, the compressive strength of r
e-
cycled concrete can reach the same level

that of
normal

concrete.


2.

When
the
water
/binder

ratio is
0.6
, t
he
compre
s-
sive strength

o
f RAC can reach the same or above
of its corresponding normal concrete

as the
r
e-
placement
of

fly ash
is at the range of

10% ~20%

or
the
replacement
of
slag
is at the range of
30%
~40%
.


3.

It could increase the slump value of RAC to reach
the same level with
natural aggregate concrete for
using
below
30% of fly ash or
below
50% of slag
replacement.


4.

Building rubble could be transformed into useful
recycled aggregate through proper processing.
The properties of tested recycled concrete were
generally worse than

those of normal concrete.
When the
b
rick and tile
content

is lower than
67%
, t
he
effect

on
the
propert
ies

of recycled co
n-
crete
is relatively limited.

5

REFERENCES

[1]

Buck
,

A
.
D. 1977
.

Recycled Concrete as a Source of A
g-
gregate
.

ACI Journal Proceedings

74(5):
21
2
-
219.

[2]

Arnold
,

C.J. 1988
.
Recycling Concrete Pavements
.

Co
n-
crete Construction
.

[3]

Tavakoli
,

M.

&

Soroushian
,

P. 1996
.
Strength of Rec
y-
cled Aggregate Concrete Made Using Field
-
D
emolished
Concrete as Aggregate
.

ACI Materials

Journal

93
(
2)
:
182
-
190
.

[4]

Hansen
,
T.C.

& Narud
,

H. 1983
.
Strength of Recycled
Concrete Made from Crushed Concrete Coarse Aggr
e-
gate
.
Concrete International

5(1)
.

[5]

Barragi
,
N.K., Vidyadhare
,

H.S. & Ravande
,

K. 1990
.
Mix
D
esign
P
rocedure for

R
ecycled
A
ggregate
C
oncrete
.

Construction & Building Mat
erial

4
(
4
).

[6]

Chesner
,

W
.
H. 1999
.
Selected State Engineering and E
n-
vironmental Specifications, Policies and Regulations for
the Beneficial Use of By
-
Product Materials in Constru
c-
tion Applications
.

Technical Conference on the Beneficial
Use of By
-
Product Mate
rials in Construction Applic
a-
tions
.
Albany, New York.

[7]

Peng, H.S.,
Ch
e
n
, H.J.

&

Yen
,

T
.

2002.
Effects of Pozz
o-
lan
s
on the

Strength and Workability of Recycled

Aggr
e-
gate Concrete
.

The 6th National Conference on Structural
Engineering

Paper No. C07
.

Pingtung, Taiwan
.

(in Ch
i-
nese)