A New Method of Controlling Water in SelfCompacting Concrete Production
Hai-Thong Ngo 1, Abdelhak Kaci 1, El-Hadj Kadri 1, Tien-Tung Ngo 1, Alain Trudel
2
, Sylvie Lecrux 2
1
Laboratory of Mechanics and Materials of Civil Engineering, University of CergyPontoise, France
2
Centre Technique National, Cemex France, France
Abstract: Online water control is one of the most important processes to improve
self-compacting-concrete (SCC) production regularity. This control is generally
implemented during the mixing process based on the stabilisation of mixing power
measurement. However, as for concrete with admixtures, typical SCC mixture takes
a certain amount of time to recognise the stabilisation of mixing power which is
sometimes difficult to achieve due to the effects of admixtures on concrete
workability. In addition, for very fluid concrete like SCC, when the concrete water
content is high, the variation of the power measurement becomes very small; thus,
the water measurement precision becomes insufficient. In order to improve the
control process based on the mixing power measurement and optimise the mixing
time, this research proposes a criterion to determine the stabilisation of power curves
by using power fluctuation over time. The stabilisation time estimated by this
criterion is tested for measuring the concrete water content through an experimental
program consisting of 14 SCC truck mixer productions of different compositions
produced at an industrial batching plant. The results show that the new processing
method allows obtaining a ±3.6 L/m3 precision of water content measurement.
Besides, the reduction in mixing time can be made by considering the stabilisation
time of power curves as an appropriate mixing time for each concrete batch
production.
Keywords: Mixing power; Stabilisation time, Concrete plant, Water control, Selfcompacting concrete production
265
K.H. Khayat, SCC 2016 - 8th International RILEM
Symposium on Self-Compacting Concrete,
ISBN: 978-2-35158-156-8 © RILEM 2016
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Hai-Thong Ngo, Abdelhak Kaci, El-Hadj Kadri et al.
Introduction
With the growing popularity of self-compacting concrete (SCC), the requirement for
adequate quality control in ready-mixed concrete plants has increased considerably.
Compared with conventional concretes, SCC mixtures contain a greater dosage of
admixture and a higher powder content which makes the concrete more difficult to
mix and more sensitive to all kinds of disturbance. Field experience also shows that
the industrial production of these new mix designs does not tolerate any
approximation due to the sensitivity of SCC to compositional deviations, especially
the variation of water dosage. The control of the water content is consequently
crucial in SCC production.
In the ready-mixed concrete plants, the application of online water measurements in
quality control has become more and more common for an improvement of SCC
production regularity [1-3]. These measurements assist the operator in the regulation
of the batch composition by adjusting the water content for forthcoming batches of
a same truck based on the information collected from the current batch [4]. The most
current measurement method used to make water correction is based on the
stabilisation of mixing power measurement. Some of the researches regarding the
correlation between the concrete water content and the stabilisation power level have
been found in the literature [4, 5]. Nevertheless, for very fluid concrete like SCC, an
increase in water content of the mix does not affect the power level significantly.
This leads to an insufficient accuracy of water measurement. Moreover, it takes
several seconds to notice that the signal no longer changes. This waiting period
results in a loss of time and thus productivity.
Concrete mixtures influence the mixing time needed for complete dispersion of all
the concrete components. As shown in [6], mixtures with higher viscosity tend to
require longer mixing time and vice versa. In practice, batches of a given concrete
design contain different water contents. Therefore, by monitoring their mixing
power stabilisation, we realise that the mixing time varies from one batch to another.
Considering these observations, the present study focuses on the use of the
stabilisation time, which is probably linked with the deviation of water content in
the batch, for improving the process of online water control. In order to evaluate the
performance of this method in water content measurement, an experimental test
campaign is performed in a full-scale concrete plant.
Experimental Program
The experimental campaign is conducted in a mixing plant equipped with a 2 m³
concrete pan-mixer. The power consumption vs. time evolution is recorded every
0.5 s. The power consumption is measured in percentage of the nominal mixing
power of the mixer motor (37 kW). The order of constituent loading and the
duration of the loading period set in this concrete plant are given as follows:
A New Method of Controlling Water in SCC Production
267
- Coarse aggregates and sand (natural moisture) are first introduced into the mixer.
- The loading of fines (cement and limestone filler) occurs after about 10 s.
- Concrete admixtures pre-mixed with water are introduced after 20 s of dry mixing
of coarse aggregates and fine elements.
Liquid components are sprayed onto the upper part of the mixture in 9 to 11 s
approximately, depending on the additional water amount. For each batch produced,
the mixing time is set long enough to avoid incomplete mixing. As reported by
RILEM [7], the mixing time was defined as the time between the loading of all
constituents and the beginning of concrete discharge.
In this study, 14 truck mixers have been filled with a total of 52 batches produced
from 4 different SCC mix designs with several batch sizes. The analysis of all these
batches is consistent. We present herein only 20 batches of two mix designs with a
volume equals to 2 m³. These two concrete formulas are given in Table I. For each
mix design, the water content was varied from one batch to the next. While SCC1,
characterised by a lower workability, is used for horizontal applications, SCC2 with
higher workability is employed for vertical applications.
Table I. Concrete compositions.
Component (kg/m³)
Cement
Limestone filler
Sand 1 (0/4)
Sand 2 (0/4)
Gravel 1 (4/14)
Gravel 2 (4/20)
Superplasticizer
Viscosity agent
Total water
Number of batches
SCC1
295
85
300
552
886
4.37
187
8
SCC2
290
90
298
602
900
5.7
0.51
185
12
Total water in the mixture includes the moisture of aggregates, added water and
water content in additives as well. The natural humidity of sand is measured online
for each batch through the use of a microwave moisture probe. This moisture value
is also checked and adjusted by drying before the fabrication. Coarse aggregate
moisture is considered constant during the day of production. Both controls
performed through the gravimetric method before and after production show close
values. From typical fluctuations of moisture measurements, the measurement
precision of water content in the batch concrete has been estimated at around
1.4 L/m3 (standard deviation), which is very high within an industrial environment.
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Hai-Thong Ngo, Abdelhak Kaci, El-Hadj Kadri et al.
Concrete fluidity is characterised by means of a test: slump-flow measurement at the
batching plant at the end of each mixing. The slump-flow values vary between 62
and 75 cm for both two mix designs.
Stabilisation Criterion of Power Curves
The definition of a stabilisation criterion of power curves is likely linked to the
determination of stabilisation time Ts which is the time needed to reach the
stabilisation power level. Regarding concrete industrial production, this
stabilisation time of the power consumption curve is generally considered as the
shortest necessary mixing time after which the evolution of fresh concrete
properties is negligible and the concrete can be discharged.
The determination of Ts for each mix design can be made by using certain
mathematical models suggested in the literature to fit the experimental power curves
recorded during mixing. Many authors agreed that the power–time curves can be
fitted by some exponential functions [6, 8]. These mathematical expressions mostly
suit for the regular concrete and some SCC mix designs with high water to powder
ratio (W/P). In these mixtures, the experimental mixing power evolution with
respect to time shows that the power consumption increases with the constituents
loading and then decreases during mixing until reaching a relative signal
stabilisation. However, the power consumption curve of lower W/P ratio SCC
cannot be treated with these models. Indeed, a second rise of power before the
power final stabilisation for batch with low water dosage makes these models
inappropriate. The highlight of this second power peak of high viscosity concrete,
which marks the increase in material cohesion during mixing, was already
mentioned in some works [1, 9, 10].
For this reason, one alternative method is to use the fluctuation in time of the power
consumption, called "power fluctuation" associated with the filtered power
consumption vs. time (Figure 1). This fluctuation is assessed herein by calculating
the standard deviation of twenty consecutive power measurements (10 s after a
given mixing time). The stabilisation time is then determined by the time needed
for the power standard deviation to reach a defined limiting value. More
specifically, by observing and analysing all experimental power curves, as soon as
the power standard deviation decreases below the threshold value α set at 0.15% in
absolute value, during at least 10 s, the stabilisation time of power curves is reached.
The "end-point" of the stabilisation time in the power to time chart is called the
"stabilisation point" after which there is no longer strong fluctuation in mixing
power. The power measured at this point is called the "stabilisation power" Ps. The
chosen value of α allows getting a minimum value of 55 s for Ts among the
considered mixtures. Indeed, this time is the minimum mixing time recommended
by the French standard for SCC mixtures [11].
Mixing
power
mixing
power
Power
standard-deviation
power
standard-deviation
Liquid loading
70
269
0.8
0.7
60
Stabilisation time
0.6
50
0.5
Stabilisation
point
40
0.4
30
0.3
20
0.2
α = 0.15%
10
Power standard deviation (%)
Percent of nominal mixing power (%)
A New Method of Controlling Water in SCC Production
0.1
0
0.0
0
20
40
60
80
100
120
140
160
180
200
Time (s)
Figure 1. Power consumption and its standard deviation during the mixing of a
SCC2 batch with 171 L/m3 are used to determine the stabilisation time.
Results and Discussion
Stabilisation time versus total water content
As already mentioned, the shortest necessary mixing time (i.e. stabilisation time Ts)
depends on concrete composition, especially concrete water content [6, 8]. The
correlation between stabilisation time Ts and total water content is confirmed in the
Figure 2 for the two tested concrete mix designs. It can be noticed that the
stabilisation time decreases with an increase in concrete water content.
Furthermore, it could be observed that the plots fall into two groups depending on
the use of viscosity agents in the mix. With the same water dosage, the presence of
the viscosity agent leads to a higher stabilisation time. Consequently, mixtures with
higher viscosity by using viscosity agents tend to require a longer mixing time. On
the contrary, adding more water in the mix could reduce the friction between the
particles and make them easier to be mixed. The diminution of stabilisation time is
hence obvious.
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Hai-Thong Ngo, Abdelhak Kaci, El-Hadj Kadri et al.
160
SCC1 (without viscosity agent)
Stabilisation time (s)
140
SCC2 (with viscosity agent)
120
R² = 0.91
100
80
60
R² = 0.94
40
167
177
187
197
207
Total water content (L/m³)
Figure 2. Relationship between water content and stabilisation time for the two
SCC mix designs (with and without viscosity agent).
Calculated water content L/m3)
205
195
185
175
165
165
175
185
195
205
3
Measured water content (L/m )
Figure 3. Measured versus Calculated water content for two SCC mix designs.
A New Method of Controlling Water in SCC Production
271
The linear correlation between the stabilisation time and the water content observed
in each mix design demonstrates that the stabilisation time of power curves could
be a good indicator of concrete water content. The correspondence between the
measured total water content and that calculated with the stabilisation time is given
in
Figure. For SCC mixtures tested here, water measurement precision based on the
stabilisation time is better than ±3.6 L/m³ (standard deviation of 2.2 L/m3).
Stabilisation line
The experimental mixing power evolutions of seven SCC2 batches are shown in
Figure which also represents the corresponding stabilisation points. It appears that
the stabilisation points are located on a master curve called "stabilisation line" in
the descending order of the water content. After the intersection of this line and the
power curves, the variation of power consumption becomes negligible (power
standard-deviation smaller than 0.15%). Thus, further mixing is not necessary when
the stabilisation time is reached.
Percent of nominal mixing power (%)
70
End of liquid loading
Total water
content (L/m³)
60
SCC2
(with viscosity agent)
171
50
175
177
Stabilisation line
40
186
189
30
192
20
200
10
Stabilisation point
0
0
50
100
150
200
Time (s)
Figure 4. Power consumption vs. time for some SCC2 batches at different total
water dosages.
If the idea of setting an appropriate mixing time for each concrete batch is applied
to the SCC production in the ready-mixed concrete plants, it would be possible to
significantly reduce the mixing time and the energy consumption of the plant mixer.
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Hai-Thong Ngo, Abdelhak Kaci, El-Hadj Kadri et al.
When using the stabilisation time of power curves as the sufficient mixing time, at
least 10 s can be reduced from the mixing time for each concrete batch production
(see Figure 4).
30
Stabilisaation power (%)
25
Stabilisation line
20
15
SCC1 (without viscosity agent)
10
SCC2 (with viscosity agent)
5
40
60
80
100
120
140
160
Stabilisation time (s)
Figure 5. The location of stabilisation points for the twenty batches of two SCC
mix designs (with and without viscosity agent).
For the pan-mixer investigated here, the stabilisation line of the twenty tested SCC
batches can be fitted by a linear evolution of stabilisation power in stabilisation time
(see Figure 5). For a given industrial mixing condition including mixer type and
loading sequences, there exists a unique stabilisation line for each mix design (with
a given batch volume). If several concrete mixtures have a similarity in the
composition, it is possible to have one stabilisation line for all these mixtures (e.g.,
the two mix designs tested here). It should be also noted that the calibration of the
stabilisation line is essentially influenced by modifying mechanical parts of the
mixer or the loading sequences.
Conclusions
The proposed criterion of power consumption stabilisation in this paper allows
determining the stabilisation time of power curves which can be used as a relative
indication of SCC water content. With this new processing method, water
measurement precision is estimated at ±3.6 L/m3 under real manufacturing
conditions. In addition, the use of the stabilisation time as an indicator informing
A New Method of Controlling Water in SCC Production
273
the end of the mixing time will allow avoiding unnecessary mixing time and
increase the productivity in concrete plants.
In the mixing power vs. mixing time chart, the stabilisation points corresponding to
the mixing end-points of all SCC batches tested here are positioned on the
stabilisation line in the decreasing order of the water dosage. For further
applications, this line should be determined before being implemented in the
concrete production process in order to optimise the mixing time and improve the
online water control of SCC production. Thanks to industrial data processing, the
intersection of the stabilisation line with the power consumption curve of the current
batch may allow indentifying the actual water content and then making water
adjustment for the next batch with the purpose of achieving the correct total water
content of the mixed concrete.
Acknowledgements
The authors would like to thank Cemex Company for technical and financial
support of the research work.
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