INFLUENCE OF ATTACHED MORTAR CONTENT ON THE
PROPERTIES OF RECYCLED CONCRETE AGGREGATE
Marta Sánchez de Juan(1), Pilar Alaejos Gutiérrez(2)
(1) Laboratorio Central de Estructuras y Materiales (CEDEX). Spain.
(2) Laboratorio Central de Estructuras y Materiales (CEDEX). Head of the Material Science
Area. Spain.
Abstract ID Number: …..346 ……..
Author contacts
Authors
Marta Sánchez de Juan
E-Mail
Fax
Postal address
msanchez@cedex.es 34 91 3357422
Laboratorio Central de
Estructuras y Materiales
C/Alfonso XII, 3 y 5
28014 Madrid
Pilar Alaejos Gutiérrez
palaejos@cedex.es
34 91 3357422
Laboratorio Central de
Estructuras y Materiales
C/Alfonso XII, 3 y 5
28014 Madrid
Contact person for the paper: Marta Sánchez de Juan
Presenter of the paper during the Conference: Marta Sánchez de Juan
1
Total number of pages of the paper (this one excluded):
9
Page 0
INFLUENCE OF ATTACHED MORTAR CONTENT ON
PROPERTIES OF RECYCLED CONCRETE AGGREGATE
THE
Marta Sánchez de Juan(1), Pilar Alaejos Gutiérrez(2)
(1)
Laboratorio de Estructuras y Materiales (CEDEX). Spain.
(2)
Laboratorio de Estructuras y Materiales (CEDEX). Head of the concrete technology
department. Spain.
Abstract
Recycled concrete aggregates mainly differ from natural aggregates in that they are
composed by two materials of different nature: natural aggregate and cement mortar attached
to it. This last material is the origin of the different and worse properties of recycled
aggregates.
The objective of this study is to relate the attached mortar content to recycled aggregate
properties, to establish the mortar content from which aggregate properties essentially get
worse. This aspect can be useful for recycling plants as they will be able to adjust the
production processes to improve aggregate quality, for example, by means of several crushing
processes.
The results show that the main properties unfavourably affected by the attached mortar
content are density, absorption and Los Angeles abrasion. However, the attached mortar
content is not the only determining factor on recycled aggregate properties, also mortar and
natural aggregate quality influence them.
Keywords: coarse recycled concrete aggregate, attached mortar, recycled aggregate quality.
1.
INTRODUCTION
Recycled concrete aggregates mainly differ from natural aggregates in that they are
composed by two materials of different nature: natural aggregate and cement mortar attached
to it. This last material is the cause of the different and worse properties of recycled
aggregates: lower density, higher absorption, sulphate soundness and sulphate content.
These properties have a negative influence on recycled concrete quality, mainly affecting
to properties related with deformation (elasticity, shrinkage, creep), also durability, and to a
lesser extent to strength.
One aspect that influences on old mortar content is the number of crushing processes in
the production plants. Increasing the number of crushing processes, attached mortar content
can be diminished, and therefore, aggregate quality can be improved.
However, this fact causes an increase of production costs, so that it will be necessary to
find a balance between number of stage-crushing processes and aggregate quality required
according to each application.
Page 1
This study is included into the experimental works carried out by The Central Laboratory
of Structures and Materials (CEDEX), aimed to establish the requirements that recycled
concrete aggregates should fulfil, in order to guarantee their safe utilization for structural
concrete.
2.
EXPERIMENTAL OUTLINE
In this study, a periodical control along one year of fifteen samples coming from a
stationary recycling plant in Madrid (TEC REC Tecnología y Reciclado) has been done.
The plant produces recycled aggregates for bulk fills, bases and subbases for road
construction. There is a visual inspection of the incoming waste materials, that allows to
classify them according their nature and to establish separated stockpiles (concrete, masonry
and asphalt).
The processing consists of a double crushing with impact crushers and vibrating screens.
It also has systems to remove metallic elements by a magnetic conveyor belt and a manual
separation of big impurities, such as plastics, papers, glass and so on.
The batched samples were graded at the Laboratory selecting 4/16 mm fraction prior
testing, and were divided into two new fractions, 4/8 mm and 8/16 mm, in order to establish
aggregate size influence on aggregate quality. Properties of 4/16 mm fraction, have been
obtained as an average, according to each sample grading.
In addition to mortar content, other properties required to natural aggregate for structural
concrete, according to Spanish Specification EHE(1), and that are related to mortar content,
have been studied. These properties are: density, absorption, Los Angeles abrasion and
sulphate content.
All the tests have been carried out for the fourteen samples, except chemical tests, that
have been done only for several samples.
Furthermore, in each sampling at the recycling plant, four cores were extracted from
several concrete blocks stocked, and compressive strength tests were made. This will allow to
asses if mortar content and original concrete quality are related.
3.
ATTACHED MORTAR CONTENT
As we mentioned before, recycled concrete aggregate is composed by natural aggregate
and cement mortar attached to it. Photograph 1 shows the appearance of these components,
that can be found separately or together.
Mortar
Aggregate+mortar
Aggregate
Figure 1: Detail of recycled concrete aggregate
Page 2
3.1
Test method
As there is not any standardized method to quantify attached mortar content in recycled
aggregate, we have valued those found in bibliography, which are described below:
-Treatment with a solution of hydrochloric acid(2): This method consists on the dissolution
of cement paste attached to stone particles when recycled aggregate is immersed in a solution
of hydrochloric acid. The problem is that it can not be used with limestone, because acid also
attacks this kind of natural aggregate.
-Production of a new concrete(3): This method consists on the production of a new
concrete made with recycled aggregate and new coloured cement, so you can determine, in
several slices, the mortar surface by the different colour from both natural aggregate an new
mortar.
-Thermal treatment(4): This method has been developed by the Universidad Politécnica of
Barcelona, and consists on a treatment including a soaking and drying of the aggregate.
The thermal treatment has been chosen in this study, as it can be used for all kind of
aggregates (including limestone), it is easier to do and its results are very accurate. Next, a
more detailed explanation of this method is included.
70
60
50
40
30
M-XIV
M-XII
M-XIII
M-X
M-XI
M-IX
M-VII
M-VIII
M-V
M-VI
M-III
M-IV
M-I
20
M-II
One factor that mainly affects
the mortar content is the aggregate
size. Attached mortar content is
generally higher in the fine
fraction than in the coarse one.
The fine fraction 4/8 mm had a
mortar content between 33% and
55%, while in coarse fraction, it
ranged from 23% to 44%. Graphic
1 shows these results.
Attached mortar content (%)
3.2
A recycled aggregate sample (mi) is prepared, having previously removed all kind of
impurities as bricks, asphalt, plastics, wood, etc. (Photo 2). The sample is immersed in
water for two hours, so that attached mortar saturates but not the natural aggregates
(Photo 3).
Next, the sample is dried in a muffle, at a temperature of 500ºC for two hours (Photo
4), to achieve the evaporation of the water included into the mortar pores. This causes
stress into the mortar so that it can be easily removed. From the muffle, the sample is
immersed into water (Photo 5).
After these operations, some mortar usually remains attached (Photo 6), so it is
necessary to remove it by means of a rubber hammer or scratching the surface (Photos
7 and 8).
When all the mortar has been removed, the sample is screened by 4 mm sieve,
obtaining the coarse aggregate weight (mf).
Mortar content is calculated as follows:
m − mf
% attached mortar = i
⋅ 100
mi
Results
Samples
4/8 mm fraction
Total (4/16 mm)
8/16 mm fraction
Graphic 1: Attached mortar content.
Page 3
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
4.
RELATIONSHIP BETWEEN
AGGREGATE PROPERTIES
MORTAR
CONTENT
AND
OTHER
As we said before, attached mortar unfavourably affects the properties of recycled
aggregate, as mortar is a porous material, and has lower density and higher absorption.
Other properties affected are Los Angeles abrasion and sulphate content. Table 1
summarizes the test methods and the Spanish specifications for each property.
Table 1: Test methods
PROPERTY
Attached mortar (%)
Apparent specific gravity
(kg/dm3)
Water saturated surface dry
density (kg/dm3)
Water absorption (%)
Los Angeles abrasion (%)
Total sulfur content as SO3= (%)
Rc of tested cores (N/mm2)
Ref. 4
Spanish Specification
(EHE)
-
UNE 83134:98
≥2
UNE 83134:98
-
UNE 83134:98
UNE-EN 1097-2:99
UNE-EN 1744-1:99
UNE 83.304:84
≤5
≤40
≤1
-
Test method
Page 4
4.1
Density
Bulk specific density
(kg/m3)
2500
D= -8,8175x + 2546,4
R = 0,73
2400
2300
2200
2100
2000
20
30
40
50
Attached mortar (%)
4/8 mm fraction
Lineal (all results )
60
8/16 mm fraction
Graphic 2: Relationship between mortar
content and bulk specific density
4.2
Saturated surface-dry density
(kg/m3)
Bulk specific density of recycled aggregate ranged from 2.010 kg/m3 to 2.300 kg/m3 in
fraction 4/8 mm, and from 2.100 kg/m3 to 2.400 kg/m3 in fraction 8/16 mm. On the other
hand, the ranges for saturated surface-dry density were 2.240-2.420 kg/m3 and 2.330-2.470
kg/m3, respectively.
Graphics 2 and 3 show the relationship between mortar content and density. The
correlations show that the higher amount of attached mortar, the lower the density is.
If we consider the intersection points of the these correlations with axis of coordinates
(x=0), we can obtain the average value of bulk specific density and saturated surface-dry
density of natural aggregate, set on 2.546 kg/m3 and 2.579 kg/m3 respectively. In a same way,
the points when all recycled aggregate is mortar (x=100%), give the average values of bulk
specific density and saturated surface-dry density of a normal mortar, set on 1.600 kg/m3 and
2.000 kg/m3. These results are coherent with the real experimental values usually obtained.
DSS = -5,6283x + 2579,3
R = 0,72
2500
2400
2300
2200
20
30
40
50
Attached mortar (%)
4/8 mm fraction
Lineal (all results)
60
8/16 mm fraction
Graphic 3: Relationship between mortar
content and saturated surface-dry density
Absorption
Water absorption of recycled aggregates was found to be in the range of 5,1% to 11,5% in
4/8 mm fraction, and in the rage of 4,2% to 8,8% in 8/16 mm fraction.
Graphic 4 gives the relation between the amount of attached mortar and water absorption
of recycled aggregate. It shows that when mortar content increases, water absorption
increases too, specially in the finer fraction 4/8 mm.
The dispersion of results, as in the case of density, points out that water absorption of
recycled aggregate is affected by others factors than original mortar amount, as original
mortar quality.
Page 5
4.3
y = 0,177x + 0,36
R = 0,71
12
Absorption (%)
To
satisfy
Spanish
specification regarding water
absorption, fixed in 5%, mortar
content should be very low, less
than 26%. This value is hardly
reached, at least by the more
coarse fractions.
According this correlation, the
average water absorption of
natural aggregate (x=0%) would
be 0,36% (lower than the usual
one), while mortar absorption
would be placed in 18%.
10
8
6
4
2
20
30
40
Attached mortar (%)
4/8 mm
EHE limit
50
8/16 mm
Lineal (Todos)
Graphic 4: Relationship between mortar content and
absorption
Los Angeles abrasión
Los Angeles abrasion
coeficient (%)
Los Angeles abrasion loss percentage of recycled aggregate ranged from 35% to 42%. In
Los Angeles abrasion test, all the attached mortar is removed, besides the abrasion suffered
by natural aggregate. For this reason, both properties are expected to be related.
Graphic 5 shows that when
attached mortar content is high,
50
Los Angeles abrasion is high too.
Although correlation is not
y = 0,19x + 31,6
45
R = 0,60
good enough, it obtains an average
loss for natural aggregate (x=0) of
40
32%, and for a 100% attached
mortar of 50%, that can be
35
considered coherent values.
30
Conforming
to
Spanish
44 50
20
30
40
specification, Los Angeles abrasion
Attached mortar (%)
loss should be lower than 40%. All
samples with an attached mortar
Experimental
content lower than 44% fulfil this
limit.
Graphic 5: Relationship between mortar content
and Los Angeles abrasion
Additionally, if those samples with an amount of mortar higher than 44% are rejected,
according to Graphic 2, the rest of samples have a density higher than 2.160 kg/m3 and an
absorption lower than 8% (Graphic 4).
4.4
Sulphate content
Recycled aggregates can have a higher sulphate content due, among other causes, to
sulphate compounds from cement of attached mortar.
According to Spanish Instruction for Cement (RC-03)(5), sulphate content in cements has
to be lower than 4%. For an usual concrete dosage, this maximum sulphate content, referred
to concrete, would be set on 0,5%, and referred to mortar, on 1%. To estimate concrete and
mortar weight, combined water has been considered the 23% of the cement weight.
Page 6
Usual dosage:
1300 kg.
600 kg.
275 kg.
180 l.
coarse aggregate
sand
cement
water
Sulphate content (%)
% SO3= (in
y = 0,088x 0,473
R = 0,60
1
0,8
0,6
0,4
0,2
0
20
30
40
50
60
Mortar content (%)
4/8 mm
8/16 mm
Experimental results have ranged between
0,35-0,75%, so we can consider that all
sulphate content is provided by cement paste.
Graphic 7 confirms this relation, and shows
that those samples with a higher mortar content
have higher sulphate content, and that for 0%
and 100% of mortar content, SO3= would be set
on 0% and 0,8% respectively. Furthermore, the
mortar content of this dosage is:
%mortar content=
P o tencial (A ll samples)
Graphic 7: Relationship between mortar
content and sulphur content
4.5
0.04 * 275
= 1,2%
600 + 275 + ( 0.23 * 275 )
0.04 * 275
concrete)=
= 0 ,5%
1300 + 600 + 275 + ( 0.23 * 275 )
% SO3= (in mortar)=
600 + 275 + ( 0 ,23 ⋅ 275 )
= 42%
1300 + 600 + 275 + ( 0 ,23 ⋅ 275 )
The sulphate content that corresponds to this
value, according Graphic 7 would be 0,52%,
close to the estimated value of 0,5% of % SO3=
in concrete.
Alkalis content
Although experimental tests have not been finished, a theoretical calculation can be made
as follows.
Maximum alkalis content in recycled aggregate, provided by cement, can also be
estimated with a similar analysis as before (when we suppose that natural aggregates do not
provide alkalis). Alkalis content in Spanish cements is usually lower than 1%, so that referred
to concrete, the maximum alkalis content will be 0,12% and referred to mortar will be 0,29%.
1300 kg.
600 kg.
275 kg.
180 l.
Coarse aggregate
sand
cement
water
% 0,658K2O+Na2O (in concrete)=
% 0,658K2O+Na2O (in mortar)=
0.01 * 275
= 0 ,12 %
1300 + 600 + 275 + ( 0.23 * 275 )
0.01 * 275
= 0 ,29%
600 + 275 + ( 0 ,23 ⋅ 275 )
Therefore, to use a coarse recycled aggregate with a maximum alkalis content of 0,12%,
in a new concrete with the same dosage, would be equivalent to use a cement with an
additional alkalis content of ( 0 ,12 ⋅1300 ) = 0 ,56% .
275
5.
RELATIONSHIP BETWEEN MORTAR CONTENT AND COMPRESSIVE
STRENGHT OF ORIGINAL CONCRETE
The influence of original concrete quality on attached mortar content has been evaluated
for the same crushing process.
In Graphic 9, mortar content of recycled aggregate and compressive strength of each four
cores are compared. If concretes with a compressive strength below 25 N/mm2 are rejected,
we select samples that have an attached mortar percentage lower than 44%. However, with
this strength criterion, some samples with low mortar contents will also be rejected. Anyway,
these three samples with reduced mortar percentages, gave a high absorption and low density.
In any case, strength criterion rejects samples with high mortar content or low mortar quality.
Page 7
Compressive strength of original
concrete (N/mm2)
55
50
45
40
35
30
25
M- IX, X, XI
20
15
M-VI,
VIII, XII
10
26
28
30
32
34
36
38
40
42
44
46
48
Mortar content (%)
M-II
M-III
M-IV
M-V
M-VI
M-X
M-XI
M-XII
M-XIII
M-XIV
M-VII
M-VIII
M-IX
Graphic 9: Relationship between attached mortar content and compressive
strength of original concrete
6.
-
-
-
-
-
CONCLUSIONS
Quality of recycled concrete aggregate is lower than natural aggregate quality, due to
the mortar which remains attached to natural aggregate.
Usual mortar content is about 23-44% for 8/16 mm fraction and 33-55% for 4/8 mm
fraction. Generally, amount of mortar attached to fine fraction is higher than to coarse
fraction.
The main properties unfavourable affected by mortar content are absorption, density
and Los Angeles abrasion.
To limit mortar content to 44% is considered appropriate, as the coarse fraction of
recycled concrete aggregate can fulfil Spanish specification of Los Angeles abrasion
loss percentage (40%). With this criterion, recycled aggregate are expected to have
absorption lower than 8% and density higher than 2.160 kg/m3.
Other properties, as sulphate content or alkalis content are also related to mortar
content, so aggregates with a high mortar content seem to have higher sulphate and
alkalis content.
For the same processing, the methods to reduce mortar content of recycled aggregates
are:
o To limit size fraction of recycled aggregate, as fine fraction has poor quality
due to it higher mortar content. For this reason, it is not recommended to use
fine recycled concrete aggregate for most applications, specially for production
of new concrete.
o To limit original concrete quality; though to a lesser extent, original concrete
quality also have influence on recycled aggregate quality, and generally, the
lower the strength of the original concrete, the lower will be the quality of the
recycled aggregate.
A solution to control recycled aggregate quality for production of structural concrete
could be to control original concrete, rejecting those concretes with compressive
Page 8
strength lower than 25 N/mm2. With this criterion, samples of coarse recycled
aggregate with a high mortar content (more than 44%) or samples with a low mortar
quality should be rejected.
ACKNOWLEDGEMENTS
This experimental work has been supported by the General Direction of Quality and
Environmental Evaluation of the Ministry of Environment.
Recycled aggregate samples have been collected thanks to the collaboration of the
company TEC REC Tecnología y Reciclado S.R.L.
Thanks to General Directions of CEDEX for initiatives to promote postgraduate training
and development of doctoral thesis. Thanks to College of Civil Engineer for all the support at
all stages during the preparation of this experimental study.
REFERENCES
[1] EHE. Instrucción de Hormigón Estructural. Ministerio de Fomento. 2000.
[2] Yagishita, F., Sano, M., Yamada, M.: "Behaviour of reinforced concrete beams
containing recycled coarse aggregate”. Demolition and Reuse of Concrete. Edited by
Erik K. Lauritzen. Rilem. p. 331-342. 1994.
[3] Ravindrarajah, R.S., Tam, C.T.: “Properties of concrete made with crushed concrete as
coarse aggregate”. Magazine of Concrete Research: Vol 37, Nº130. March 1985.
pp.29-38.
[4] Barra, M.: “Estudio de la durabilidad del hormigón de árido reciclado en su aplicación
como hormigón estructural”. Tesis doctoral. Universidad Politécnica de Cataluña.
1996.
[5] RC-03. Instrucción para la Recepción de Cementos. Ministerio de Fomento. 2003.
Page 9