workability of concretes produced with oarse · 3 sp: sikaplast 898 produced by sika; water: tap...
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WORKABILITY OF CONCRETES PRODUCED WITH COARSE
RECYCLED AGGREGATES
João Paulo Freitas Costa Lavado
Extended Abstract
Dissertation to obtain the Master Degree in Civil Engineering
Supervisors
Professor Doutor José Alexandre de Brito Aleixo Bogas
Professor Doutor Jorge Manuel Caliço Lopes de Brito
Jury
President: Professor Doutor Nuno Gonçalo Cordeiro Marques de Almeida
Supervisor: Professor Doutor José Alexandre de Brito Aleixo Bogas
Member: Professor Doutor Pedro Miguel Soares Raposeiro da Silva
June 2017
1
WORKABILITY OF CONCRETES PRODUCED WITH COARSE
RECYCLED AGGREGATES
João Paulo Freitas Costa Lavado, [email protected]
Supervisor: Prof. Dr. José Alexandre de Brito Aleixo Bogas
Co-Supervisor: Prof. Dr. Jorge Manuel Caliço Lopes de Brito
Abstract
The main purpose of this dissertation is to study the behaviour of concrete in the fresh state when coarse
recycled concrete aggregates are incorporated in the mix. Thus, three families of concrete with w/c ratios
of 0.55, 0.45 and 0.35 were prepared. The following tests were used: slump; flow table; inverted cone;
Vebe. The experimental results showed that concrete with coarse recycled concrete aggregates tend to
have a similar behaviour when 0.55 w/c ratio is used, mainly because in this type of concrete the paste
plays the main role in terms of workability. In lower w/c ratios, such as 0.35 and 0.45, the shape and
texture become more relevant leading to mixes with less fluidity and consistency. The mixes also
showed lower fluctuations in their fresh state for small variations in paste volume.
Keywords: Recycled coarse concrete aggregates; rheology; slump; spread; workability; loss of workability.
1. Introduction
1.1. Preliminary remarks
This study is intended to be a contribution to better understand the properties of concrete produced with
recycled aggregates (RA), leading to better solutions from an environmental and economic point of view.
The overall study has a major importance due to the high environmental impacts generated by the con-
struction sector: extraction of large quantities of raw materials, high energy consumption and significant
production of pollutants and waste.
CDW (construction and demolition wastes) result from: new construction, rehabilitation, reconstruction,
natural disasters, and building demolition. In Western Europe, the highest percentage of waste comes
from rehabilitation and demolition (80%). In Denmark, rehabilitation contributes with 20-25%, while
70-75% of waste comes from demolition (Mália 2010).
Having in mind the enormous cost with transportation, landfill disposal and treatment, recycled material
use is seen as an attractive alternative, also because it reduces the depletion of natural resources, limits
2
the high energetic/environmental impacts in traditional concrete production, and increases the life-cycle
of aggregates and concrete.
1.2. Scope and methodology of the investigation
The use of RA in concrete is still not consensual, and various studies are being made to better understand
its effects. In general replacing part or all natural aggregates (NA) with RA tends to produce concrete with
worst properties. This is true when replacing the coarse or the fine fraction, with major consequences on
the last ones (Brito 2005). The type of RA and concrete composition are two major factors that influence
concrete properties, and that is one of the reasons for some different conclusions in the literature.
The major’s differences between recycled concrete aggregates (RCA) and NA are:
higher absorption, affecting workability, w/c ratio, mechanical and durability properties of concrete;
lower compactness, with consequences on crushing strength and elasticity modulus of aggregates.
This happens because RCA have mortar attached to the original natural aggregates. The amount of
mortar attached to the aggregate has a direct relation with its size: smaller aggregates have more mortar
attached to them. Natural fines and cement are the constituents of this mortar with high porosity and
water absorption.
Analysing international and national experimental campaigns was the primary stage of this investigation.
The collected information constituted a repository that refers the most important properties of the aggre-
gates, the experimental test results, and the conclusions of each campaign. With high water absorption
of RAC it is necessary to have special attention when mixing all the components of concrete. It is very
important to compensate the water these aggregates will absorb so the effective w/c ratio remains the
same. Various campaigns did not have this in consideration, producing some inconsistent conclusions.
Afterwards, the experimental program was planned and executed. Recycled coarse concrete aggre-
gates (RCCA) and natural coarse aggregates (NCA) were characterized, but the results are not listed
in detail in this abstract. 20 concrete mixes where produced, varying the w/c ratio, 0.3, 0.4 and 0.5, paste
volume, and coarse aggregate replacement ratio. Super-plasticizer (SP) content was used in the mixes
with 0.3 and 0.4 w/c ratio to achieve the S3 class slump and still have maximum compactness according
to the Faury method.
Subsequently, the experimental results were discussed in detail. Correlations were established between
the properties of the recycled concrete coarse aggregates concrete (RCCAC) and of the reference con-
crete with NA (NAC).
2. Experimental program
2.1. Materials
RCCA: two types of RCA were used: type 1 being the remaining of a previous campaign that suf-
fered primary and secondary crushing, and type 2 with only primary crushing using a concrete jaw
crusher. The concrete was the same for both types of RCA and came from the precast industry.
NA: the NA used were gravel 1 and 2, “rice grain”, and coarse and fine sand;
Cement: ordinary CEM I 42.5 R Portland cement, produced in SECIL Outão;
3
SP: SikaPlast 898 produced by Sika;
Water: tap water was used for mixing and curing.
2.2. Mix design
Three concrete mixes were produced with water/cement ratio of 0.5, 0.4, and 0.3. The ones produced
with NA only were referred as references concrete, because they were the ones that served for com-
parison purposes. From these mixes RCCAC were produced replacing coarse NA with coarse RA. The
reference concrete mixes were designed to have a slump of 140 ± 10 mm. The proportions of the ma-
terials were determined on the basis of absolute volume of the constituents. The composition of the
NAC’s mixes is given in Table 1. The characteristics of the reference concrete are:
Slump class: S3;
Blinder: CEM I 42.5 R;
Aggregates’ maximum size: 22.5 mm.
Table 1 - Mixture proportions of natural aggregate concrete (NAC) mixes
2.3. Testing of aggregates
The particle size distribution was determined in accordance with EN 933-1 and EN 933-2; the particle
density and water absorption were measured following EN 1097-6; the water content was determined in
accordance with EN 1097-5; the water absorption over time, especially important when using RA, was
determined using a modified version of the test described in EN 1097-6, based on the work of Ferreira
(2007); the apparent bulk density and void percentage were measured following EN 1097-3; the aggre-
gates’ resistance to abrasion was measured by the Los Angeles test following LNEC E-237; the shape
index was measured following EN 933-4; and the flatness index was determined according to EN 933-3.
2.4. Testing of fresh concrete
Concrete was produced using a revolving drum concrete mixer. For each mix the slump in Abrams’ cone
following EN 12350-2 and the spread in the flow table test described in EN 12350-5 were tested. After-
wards concrete’s fresh density and air content were determined according to EN 12350-6 and
EN 12350-7, respectively.
At the starting point, i.e. when the mix is finished, the Vebe test defined in EN 12350-3 and the inverted
cone test, an empirical test widely used on site, described by Bogas (2011), were also performed.
NAC55 NAC45 NAC35
Natural aggregates
Type Volume (m3/m3) Volume (m3/m3) Volume (m3/m3)
Gravel 2 0,217 0,218 0,223
Gravel 1 0,105 0,108 0,112
“Rice grain” 0,079 0,078 0,082
Coarse sand 0,178 0,183 0,186
Fine sand 0,100 0,085 0,075
Total 0,679 0,672 0,678
Component Volume (m3/m3) Volume (m3/m3) Volume (m3/m3)
Cement 0,115 0,131 0,148
Water 0,193 0,180 0,158
SP (%) 0,0 0,2 0,5
4
Performing all the four test proved impossible when studying the loss of workability over time, because
a lot of water evaporated in the first moments, showing unreliable results.
2.5. Testing of hardened concrete
The 28-day compressive strength of concrete was determined in accordance with EN 12390-3.
Three cubes were filled for each mix when all the previous test were executed and three cubes were
also filled when analyzing the loss of workability over time in the first moments after mixing (around 5
minutes) to understand whether there is an impact in compressive strength when concrete is placed and
vibrated after the curing process starts.
3. Results and discussion
Concrete density were analyzed for NAC and for the mixes produced with the two types of CRA.
Contrary to what was expected, concrete density decreased from the 0.55 to the 0.45 w/c ratio and
increased, as expected, in the mixes with w/c of 0.35, for both NAC and RCCAC.
This can be justified by the fact that the mixes with different w/c ratios were also prepared with different
paste and aggregate volume, e.g. the mix with w/c of 0.55 is associated with a larger paste volume,
which corresponds to a decrease in the sand volume that has higher density. Thus, it was not possible
to directly correlate the w/c ratio with the density of concrete, since the proportion of the materials con-
stituting the mix, namely the aggregate-paste ratio, were modified.
When comparing the three types of concrete, it is possible to see a correlation: when replacing NCA
with RCCA-1 concrete density decreases 3% and when replaced with RCCA-2 it decreases 6%. This
differences are directly related to the mortar attached to the RCCA. Furthermore due to its higher po-
rosity, rougher texture and less rounded geometry, concrete tends to contain more air, affecting the
density of concrete in the fresh state.
When decreasing the w/c ratio and incorporating SP, the air content increased both in RCCAC and
NAC. However, this effect was more pronounced in RCCAC. It is important to highlight that the air con-
tent was less than 1.6% (16 l/m3) for all mixes produced in this study associated with a Dmax of 22.4 mm,
which is a low value, similar to what is common in standard concrete with Dmax of 25.4 mm (15 l/m3) in
ACI613, referred to by Coutinho (1988). In summary, it can be concluded that the incorporation of RCCA
will not significantly influence the air content.
Concrete with NAC was formulated in order to obtain a slump around 140 mm. For RCCAC the purpose
was to have the same composition, i.e. concrete with similar particle size distribution, same cement
content and equal amount of free water in the mix. Thus, a direct replacement of the coarse fraction was
made and the water was added to the mix to compensate the higher absorption of this type of aggre-
gates. Therefore it was possible to compare the influence of the physical characteristics of the aggre-
gates, such as shape, texture, density and specific surface.
The slump measured in Abrams’ cone is essentially influenced by the yield stress of the concrete. In
fact, the motion stops when the forces of gravity, exerted by the concrete's own weight, equal the yield
stress (Bogas 2011).
5
For concrete produced with NA or RCCA-1 similar slump values were obtained (Figure 1). Thus, the higher
porosity of the RCCA-1 and its more rugged texture had no significant influence on the yield stress. In mixes
produced with RCCA-2 slump was about 38% lower, evidencing an increase in the yield stress. This can
be justified by the interlocking forces that are generated between the particles, as these have more irregular
forms and rougher textures, which hinder the movement of concrete. Thus, despite workability being es-
sentially affected by the shape of the aggregates, it is possible to produce RAC with similar workability to
conventional concrete, if the crushing process produces aggregates with shapes similar to the NA.
Figure 1 - Fresh state tests for concretes with w/c ratio of 0.55
The inverted cone test forces concrete to pass through the lower aperture of the Abrams’ cone. This
test is more affected by the viscosity of concrete, which translates the flow rate of the mix. For more
viscous concrete, concrete has more difficulty in flowing through the opening, resulting in longer flowing
times. The shape of the funnel also evaluates the passing capacity, moving the particles closer to each
other, increasing the number of contacts between the aggregates and hindering their relative movement.
Figure 1 shows larger flow times for RCCAC-2 concrete and lower times for RCCAC-1 concrete when
compared to the time obtained for NAC. The geometric characteristics of RCCAC-2 led to a greater
entanglement between particles that increased the viscosity of concrete. The larger flow time in concrete
with RCCA results also from the fact that the force that mobilizes the test (self-weight) is lower in con-
crete with recycled aggregates. For the same reason, the similar lower value between NAC and concrete
with RCCA-1 may mean a more fluid behavior of the recycled aggregates concrete.
Despite the difference observed in the inverted cone test, the results of the flow table test indicate similar
viscosity in NAC and RCCAC-1. Both mixes had average diameters close to 45 cm. In RCCAC-2, the
smallest average diameter (40 cm) and higher time in the Vebe test confirms the higher viscosity of
these mixes compared to the other ones tested. Simultaneous analysis of the results of the inverted
cone and flow table suggests that RCCAC-1 mixes may be associated with greater flow capacity thru
narrow spaces.
It is also important to note that none of the mixes showed signs of segregation or exudation in all the
tests performed. Thus, it is possible to say that the worst characteristics of RCCAC-2 do not imply
14,1
3,09
45
1,91
13,8
2,03
45,25
1,68
8,75
4,13
40
2,59
0
5
10
15
20
25
30
35
40
45
50
slump inverted cone spread table Vebe test
Slu
mp
(cm
) /
Inve
rted
co
ne
tim
e (s
) /
spre
ad (
cm )
/ V
ebe
tim
e (s
)
NAC RCCAC-1 RCCAC-2
6
changes in the stability of the 0.55 w/c ratio mixes.
To achieve the same slump class, when analysing mixes with lower w/c ratio, SP was added. SP tends
to have an important effect on the yield stress and a smaller one on viscosity.
Figure 2 shows that SP was efficient when used in NAC and RAC produced with the same contents, in
terms of aggregate, cement and water. Only when 0.35 w/c ratio was prepared, associated with more
relevant SP content, some significant differences were detected.
Figure 2 - Slump, flow time, spread and Vebe time for concrete mixes with different w/c ratio
In order to make SP efficient when used in RAC, the pre-saturation of RCCA was performed.
In 0.55 and 0.45 w/c mixes the higher paste volume controls the viscosity’s decrease. This trend is
observed in NAC but also in RCA. However with recycled aggregates the flow time and the spread were
inconsistent: although the spread remained similar to that of NAC the flow time increased a lot. The
imbrication of the particles seems to be higher in RCCAC, which makes it more difficult to pass through
narrow areas and to flow.
In mixes with 0.35 w/c ratio, associated with lower water content and lower paste volume, there is al-
ready a significant increase in viscosity, translated in a longer flow and lower spread diameter.
NAC55 RC55 NAC45 RC45 NAC35 RC35
0
2
4
6
8
10
12
14
16
Slu
mp
(cm
)
Slump
NAC55 RC55 NAC45 RC45 NAC35 RC35
0
2
4
6
8
10
12
14
16
18
Flo
w t
ime
(s)
Inverted cone
NAC55 RC55 NAC45 RC45 NAC35 RC35
0
5
10
15
20
25
30
35
40
45
50
Ave
rage
sp
read
(cm
)
Flow table
NAC55 RC55 NAC45 RC45 NAC35 RC35
0,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
4,0
4,5
Veb
e ti
me
(s)
Vebe test
7
The Vebe test allows concluding that NAC and RCCAC with 0.55 w/c ratio have similar behaviour when
compacted. This was also noticed when the cubes for the compressive strength test were vibrated.
When lower w/c ratio mixes where produced the differences became more relevant. This may be related
with the fact that vibration is less efficient for more porous particles (Bogas 2011).
The loss of mortar from the aggregates during the mixing process increases the fines content. This
affects the size distribution and the surface area of the solid material of the mixes, making them more
susceptible to the variation of w/c ratio.
Mortsell et al. (1996) studied the behaviour of concrete in the fresh state, suggested the design and anal-
ysis of a type of curves, named "S-curves". Basically, these curves give an idea of the evolution of a given
property when the amount of paste in the mix varies, keeping the remaining parameters constant.
The S-curves were built experimentally taking into account the slump and flow table tests for various
mixes with different paste volumes (Figure 3). When replacing the CNA with RCA of similar character-
istics, the rheological behaviour of concrete tends to be similar, both in terms of slump and spread.
Figure 3 - S-Curves for NAC and RCCAC
On the other hand, concrete with RCCA-2 presents a different behaviour. These mixes produced with
aggregates subjected to primary crushing only tend to have less vertical S-curves. This is an indicator
that they are less sensitive to paste volume variations.
When analysing the loss of workability over time, it is also important to separate yield stress from viscosity.
The loss of slump in 0.55 w/c ratio mixes is small in the first 30 minutes (Figure 4). The loss becomes
more significant after 30 minutes, with a reduction of about 50% and 60%. The fact that a slightly higher
initial slump was obtained in RCCAC-1 conditions the analysis of the slump variation over time when
comparing the two mixes. Still, and considering that the slump class is the same, it can be considered
that the variation of 2 cm did not significantly influence the results. In the initial instants, the loss of
workability is mostly associated with slight losses of water by evaporation in the mix. At later ages, other
phenomena, such as the hydration reactions, justify the higher losses. Regardless of the phenomena
that led to the observed losses of workability, these occurred at the paste level, without influence of the
type of aggregate.
The introduction of SP led to higher slump losses shortly after the first 30 minutes. The SP tends to lose
8
its efficiency over time especially when it is progressively involved by the new hydration products.
In Figure 4 it is possible to observe that in absolute terms very similar values were obtained in the two
mixes with 0.45 w/c ratio.
Figure 4 - Loss of slump over time for NAC and RCCAC
In the mixes with 0.35 w/c ratio the evolution of the workability loss in RCCAC was not as expected.
RCCAC had lower losses compared to NAC in the first 30 minutes and higher losses after 90 minutes.
Taking into account the absolute values, it is possible that the higher initial value of the slump in RCCAC
conditioned the remaining test. The fact that it is more fluid in the first instants means that losses by
water evaporation do not cause such a significant impact on the loss of water.
The higher loss of workability observed in RCCAC may be related to the progressive loss of mortar from
RCCA during mixing, increasing the specific surface area of the solid material and consequently reduc-
ing the free water available in the mix. This assumes greater relevance in mixes that initially have a
higher content of fines.
The loss of spread over time was also determined. The results of this test for NAC and RCCAC and for
the three w/c ratios are analysed are presented in Figure 5.
Once again in 0.55 w/c ratio mixes the loss was only significant after 60 minutes. This is valid for NAC
and RCCAC.
The fact that in the first few minutes there were lower spread losses in RCCAC may be related to the
higher fines content from the attached mortar that tends to become loose from these aggregates when
they are inserted In the concrete mixer causing a lubrication effect and reducing the viscosity. For larger
volumes of material loss, the effect may be the contrary, since there is an increase in the surface area
of the solid material. In fact, as reported by Tattersall and Banfill (1983), there is an optimum content of
00:00 00:30 01:00 01:30
0%
20%
40%
60%
80%
100%
120%
Var
iati
on
of
the
slu
mp
in r
elat
ion
to
th
e in
itia
l in
stan
t
Instant (hours:minutes)
Loss of slump (NAC55)Loss of slump (RC55)Loss of slump (NAC45)Loss of slump (RC45)Loss of slump (NAC35)
12,1
14,413,7
14,7
12,1
14,9
11,4
14
8,2 8,1
6,4
8,7
6,4
7,4
5,5
4,23,4
3,94,5
5,5
3,6
2,3
1,1
00
2
4
6
8
10
12
14
16
NAC55 RC55 NAC45 RC45 NAC35 RC35
Slu
mp
(cm
)
0 minutos 30 minutos
60 minutos 90 minutos
9
fines after which the compactness and workability of the blends decrease. This would justify the fact that
mixes with lower w/c ratio are more sensitive to higher workability losses.
Figure 5 - Loss of spread over time for NAC and RCCAC
Analysing the compressive strength of the mixes (Table 2), it is possible to notice a decrease when
using RCCA. It is interesting to note that concrete produced with RCCA-2 that suffered only primary
crushing had similar compressive strength. This may be related to the fact that this analysis was per-
formed only in higher w/c ratio mixes, in which case the strength of the paste was conditioning to the
concrete strength. The fact that the compressive strength does not decrease substantially for this type
of aggregates can be further explained in the literature because the more angular and irregular the
aggregates are the better the bond between the paste and the aggregate is (Mehta and Monteiro 1994).
It is important to remember that the RCCA used in this experimental campaign were sourced from a
high-strength concrete.
Table 2 - Compressive strength of NAC and RCCAC
00:00 00:30 01:00 01:30
0%
20%
40%
60%
80%
100%
120%
Var
tiat
ion
of
spre
ad in
rel
atio
n t
o t
he
init
ial i
nst
ant
Instant (hours:minutes)
Loss of spread (NAC55)
Loss of spread (RC55)
Loss of spread (NAC45)
Loss of spread (RC45)
Loss of spread (NAC35)
Loss of spread (NC35)
42,75 43,5 43,25
46,5
38
47,25
43,2545,25
38
44,5
32,2533,5
36,25
41
35
38
29,25 29,5
36,7538
34,2536
25,5 25
0
5
10
15
20
25
30
35
40
45
50
NAC55 RC55 NAC45 RC45 NAC35 RC35
Spre
ad (
cm)
0 minutes 30 minutes
60 minutes 90 minutes
Concrete w/c fcm (MPa) Variation to NAC
NAC
0.35 78.52 0.00%
0.45 63.14 0.00%
0.55 52.29 0.00%
RCCAC-1
0.35 70.63 -10.05%
0.45 55.75 -11.71%
0.55 49.47 -5.39%
RCCAC-2
0.35 - -
0.45 - -
0.55 49.11 -6.10%
10
It was also possible to confirm that there is no significant loss in compressive strength when casting and
vibrating concrete after one hour and thirty minutes in both NAC and RCCAC. After this time concrete
has already started the setting process, making it more difficult to vibrate. In this case, the concrete
required a higher vibration energy that would make it difficult to operate on site.
4. Conclusions
This study makes it possible to demonstrate that tests that indirectly assess the rheology of concrete,
when affected by more than one parameter or factor, make their analysis complex, in particular with
regard to the establishment of comparisons between different types of concrete.
In general, it was possible to see that mixes with w/c of 0.35 and with SP tended to be more difficult to
work that the ones with w/c of 0.55, even though both mixes have the same slump. It means that other
parameters, such as the viscosity of the mixes, are also relevant when evaluating the workability.
In summary, it can be concluded from this study that workability is affected when replacing NA by RCCA
subjected to only one crushing phase. For RCCA with similar shape and texture as the NA and for mixes
with high w/c ratio, it seems that the paste plays the leading role in workability. For lower w/c ratio mixes
with SP, the loss of mortar attached to the aggregates, the higher water absorption and the irregular
shape and texture seem to make the behaviour of RCCAC different from that of NAC: making RCCAC
more fluid in the first ages but with higher losses afterwards.
5. References
BOGAS, J. (2011). - Caracterização de betões estruturais com agregados leves de argila expandida.
Tese de Doutoramento em Engenharia Civil. Volume I e II, Instituto Superior Técnico, Lisboa.
BRITO, J. (2005) - Agregados reciclados e a sua Influência nas propriedades dos betões. Lição de
síntese para provas de agregação, Instituto Superior Técnico, Lisboa.
HU, C., & DE LARRARD, F. (1996) - The rheology of fresh high performance concrete, in Cement and
Concrete Research, V. 26, N.º 2, pp. 283-294.
MÁLIA, M. (2010) - Indicadores de resíduos de construção e demolição. Dissertação de mestrado em
Engenharia Civil, Instituto Superior Técnico, Lisboa.
MEHTA, P. K., MONTEIRO, P. J. M. (1994) - Concreto: Estrutura, Propriedades, Materiais, São Paulo,
Pini.
MORTSELL, E., SMEPLASS, S., HAMMER, T.A., & MAGE, M. (1996) - FLOWCYL - How to determine
the flow properties of the matrix phase of high performance concrete. 4th International Symposium on
Utilization of High-strength / High-performance Concrete, Paris.
SANCHEZ, M. (2004) - Estudio sobre la utilización de árido reciclado para la fabricación de hormigón
estructural, Tese de Doctoramiento, Universidad Politécnica de Madrid, Madrid.
TATTERSALL, G. H., BANFILL, P. F. G. (1983) - The rheology of fresh concrete, Pitman.