Water soluble polymers such as cellulosic quintessences or amylum quintessences are frequently included in the mix-design of Self Compacting Concretes ( SCCs ) in order to better their stableness and hardiness. The stableness, including opposition to liquid-solid separation and deposit, may be attributed to the addition of the viscousness of the liquid stage due to the inspissating consequence of the polymer. The later is so referred to as a Viscosity-Modifying Admixture ( VMA ) . In the present survey, we consider the influence of VMAs on the rheological belongingss of the stuff at cement graduated table degree. In peculiar, the alteration in the thixotropic belongingss of the cement paste due to the inclusion of VMA is investigated. It is found that add-on of VMA significantly enhances rebuild-up dynamicss at remainder following shearing at high shear-rate. The influence of VMA on the steady province rheological belongingss is besides considered. As already reported in the literature, the output emphasis is found to monotonically increase with VMA content, while the consistence presents a minimal bespeaking the being of an optimal value of the VMA for which the workability of the cement paste is maximal.
Keywords: cement paste ; self-compacting concretes ; thixotropy ; viscosity-modifying alloy.
( * ) Writer for correspondence: chaouche @ lmt.ens-cachan.fr
1. Introduction
Viscosity-modifying alloies ( VMAs ) are frequently used in mix-design of extremely unstable cementitious stuffs, including self-compacting concretes ( SCCs ) , pumpable concretes, etc. , to avoid solid-liquid separation and to better the hardiness of the preparation. That is the primary aim when utilizing these alloies. However, one can anticipate some influence on the rheological behavior since add-on of VMA will alter the rheology of the aqueous stage and the interactions between the solid atoms. This job has been investigated in the literature, in peculiar by sing the combination of VMAs and Superplastizers ( SPs ) [ 1-7 ] . Phan et Al. [ 7 ] showed that the influence of VMA on the rheological belongingss of cement pastes is really little compared to that of SPs. This has been explained by the fact that VMAs act chiefly on the liquid stage, while SPs modify the farinaceous constellation ( deflocculation ) , which really dominates the rheological behavior in such a extremely concentrated suspension.
The reported consequences refering the influence of VMAs on the rheological belongingss of cementitious stuffs ( the surveies concerned in general cement pastes ) converge to the decision that add-on of this alloy decreases the fluidness ( including output emphasis and viscousness ) of the stuff ( see for case [ 5 ] and mentions in this ) . The consequences presented here show that the influence of VMA on the rheological parametric quantities is really more complex, due the fact that this alloy may play an equivocal function: on one manus it may increase the liquid stage viscousness, which would take to an addition of the paste ‘s viscousness, and on the other manus, it may lubricate the contacts between the solid atoms, which would take to a lessening of the viscousness.
A figure of writers pointed out that self-compacting concretes ( SCCs ) show extremely thixotropic belongingss [ 8-10 ] , without nevertheless describing clear experimental consequences. Thixotropy is frequently a important belongings for edifice stuffs. Thixotropy may uncover advantageous, and sometime necessary, for the application procedure of the stuff. This is the instance for case for pigments or rendering howitzers. On the other manus, thixotropy may be unwanted for case when covering with the procedure of consecutive concreting. The influence of mix-design of such stuff on their thixotropic behavior is so a important issue. In the present survey this rheological belongings is quantitatively investigated. We consider both microstructure breakage at a high shear rate and reconstruct up at remainder.
2. EXPERMENTAL Procedure
2.1. Materials
The cement pastes are composed of tap H2O, Portland cement ( CEM I 52,5 PM ES CP2 from Teil in France ) , fillers ( Ground limestone with a similar farinaceous size distribution than that of the cement ) , a Polycarboxylate-type Superplastizer ( Glenium 27 from Degussa ) and a polysaccharide-type viscosity-modifying alloy ( Foxcrete from AVEBE ) . The function of the SP is to scatter the cement particles that are prone to aggregation due to colloidal interactions. In the instance of the SP used here the scattering is due to both steric and electrostatic effects of the adsorbed polymer. It is shown here that the cement pastes ( even without VMA ) are shear cutting, bespeaking that we do non hold complete scattering of the collection for the SP dose rate used here.
The composing of the cement paste is reported in Table 1. The composing reported in this table corresponds to the mention paste, which is used in pattern to proportion self-levelling concretes. Here we consider the alteration in the rheological belongingss when changing the concentration by weight of the VMA. Then, 4 other pastes are prepared by increasing or diminishing the VMA dose rate. The 5 cement pastes considered are so: REF ( cite paste ) , REF-50 ( obtained by spliting the VMA dose rate by 2 ) , REF-100 ( without VMA ) , REF+50 ( obtained by increasing the mention dose rate by 50 % ) and REF+100 ( obtained by increasing the mention dose rate by 100 % ) .
Table 1: Composition of the cement paste [ 6-7 ]
cement ( g )
filler ( g )
H2O ( g )
SuperPlastizer ( SP ) ( g )
VMA ( g )
1000
330
300
7 g
2 g
The mixing process consists of the same set of stairss for all the pastes considered in order to better the duplicability of the trials. A laboratory paddle-pan sociable is used. The entire continuance of commixture is rather high ( 11.5 min ) to see homogenous suspensions. The mixing process is described in Table 2.
Table 2: Mix process
Measure
Cement+filler
Water+SP +VMA
Addition
Blending at low velocity ( 95 RPM )
Blending at high velocity ( 165 RPM )
Duration of commixture ( min )
5
0.5
4
2
2.2 Rheologic measurings
2.2.1 Apparatus
The rheological measurings were performed utilizing a stress-controlled shear rheometer ( AR2000 from TA Instruments ) equipped with the vane geometry ( Fig. 1 ) . In such geometry, the tested stuff is non subjected to a unvarying shear rate. This status is normally required in rheological experiments in order to mensurate existent stuff belongingss, and to acquire a simple relationship between the measured torque/rotational speed and the shear-stress/shear-rate. Vane geometry is however recognized to be appropriate for farinaceous suspensions such as howitzers [ 11-12 ] since slippage can be avoided and the stuff is sheared in volume.
60mm
34mm 34mm
30mm
45mm
Figure 1: Rheologic measuring system: a ) vane, B ) inner cylinder.
The spread ( distance between the fringe of the Vane and the outer cylinder ) is 5.5 millimeter, which is more than an order of magnitude larger that the maximal size of the cement or filler atoms ( about 0.1 millimeter ) .
The shear-rate and the shear-stress are inferred from the torsion and the rotational speed of the vane by graduating with a Newtonian fluid.
The temperature was regulated at 25 & A ; deg ; C ( to within 0.1 & A ; deg ; C ) thanks to a circulating H2O system. In order to forestall vaporization of the paste ‘s H2O the measuring system was sealed.
2.2.2 Measurement process
All the rheological measurings were undertaken during the initiation period during which the hydration rate of the cement is really low and may hold non-significant influence on rheology. In order to look into that this was really the instance, two successive and same rheological measurings with the same sample were performed, bespeaking that there was no irreversible transmutation ( hydration ) of the stuff up to 2 hours. The fact that the initiation period is so long is due to the presence of the organic stages, including the SP and the VMA.
Before get downing measurings, the samples are pre-sheared at 200 s-1 and so keep at remainder during 1 min.
The steady province rheograms ( shear-stress versus shear-rate ) are determined by subjecting the stuff to rhythms of increase-decrease of shear-rate. An approximative steady province is obtained within 4-5 cringles. The flow curves reported here correspond to the descendent subdivision of the last cringle. For each paste, the rheological parametric quantities reported here correspond to average values over at least 5 trials undertaken utilizing a newly prepared sample.
The transeunt behavior is considered individually in inside informations here. Figure 2 represents the flow way used to look into the interruption down at high shear rates and reconstruct up at remainder. In order to mime rebuilding at remainder ( to suit the precise definition of thixotropy ) , the stuff is subjected to a really low shear-rate ( 0.01 s-1 ) during this period.
200 s-1 during 4min
Shear rate
0.01 s-1 during 45 min
Time
Figure 2: Flow way used to see transient belongingss ( thixotropy ) , including breakdown at a high shear rate and reconstruct up at remainder ( really low shear rate ) .
3. RESULTS AND DISCUSSION
3.1 Thixotropic behavior
Figure 3 represents the temporal development of the emphasis when the paste is subjected to a comparatively high shear-rate ( 200 s-1 ) , matching the dislocation of the microstructure. As it can be seen in Figure 3, the breakdown dynamicss can be reasonably good fitted by the amount of two exponentials in the instance of pastes incorporating VMA. The correlativity coefficient of the tantrum is in all instances really near to 1 ( 0.999… ) . For pastes without VMA a simple exponential decay turned out to be adequate to account for the breakdown dynamicss.
Figure 3: Development of the emphasis versus clip for different VMA dose rates at a comparatively high shear rate ( 200 s-1 ) , stand foring the microstructure interrupt down. The uninterrupted lines correspond to the best tantrum with the amount of two exponentials. ( ) REF-100, ( ) REF-50, ( ) REF, ( ) REF+50, ( + ) REF+100.
The breakdown dynamicss of the pastes devoid of VMA can be understood in footings of a competition between shear-induced breakage of cement and filler sums and Brownian induced collection of the colloidal portion of the atoms. In the presence of VMA, the polymer is capable, on one manus, to alignment and disentanglement under flow and, on the other manus, to relaxation towards maximal informations due Brownian gesture. Due to the really high molecular weight of the VMA polymer ( order of 1000000s ) , its mean relaxation clip would be rather high. One can so presume that the dislocation kineticss is governed by two chief characteristic times. Then it is straightforward to demo, utilizing a basic thixotropy theoretical account, that the temporal development of the emphasis can be modelled by the amount of two exponentials in understanding with the experimental consequences reported in Fig. 3.
A big figure of more or less sophisticated theoretical accounts for thixotropy have been reported in the literature [ 10-16 ] . To cover with thixotropy the simplest theoretical account has to incorporate at least a parametric quantity cubic decimeter that characterises the grade of interconnectedness of the stuff s microstructure at a given clip and shear rate. The precise physical significance of cubic decimeter depends upon the existent microstructure germinating under flow and that has important effects on the mensural rheological belongings ( output emphasis, evident viscousness, etc. ) . In general it is assumed that cubic decimeter = 0 for a to the full broken down microstructure and l=1 when the microstructure is to the full built up. The development of the parametric quantity cubic decimeter is governed by the competition between the microstructure rebuild up, that takes topographic point with a characteristic clip T, and the microstructure dislocation whose dynamicss can be assumed to be relative to the shear-rate.
Our experimental consequences ( Fig. 3 ) , as discussed above, strongly suggest that the rebuild up dynamicss would be governed by at least two different characteristic times tg and tp, matching severally to the granular and the polymer ( VMA ) norm relaxation times. The fact that we have merely one characteristic clip to account for breakdown dynamicss of the paste without VMA supports this vision. We can so compose down the undermentioned kinetic equation for the structural parametric quantity cubic decimeter:
( 1 )
This corresponds to the simplest theoretical account for thixotropy as proposed many old ages ago by Moore [ 17 ] . In peculiar, this simple theoretical account does non take into history eventual shear-induced collection. More sophisticated theoretical account can be used [ 18-20 ] , nevertheless Equation ( 1 ) is adequate for a qualitative reading of our experimental consequences.
For a given shear-rate, the emphasis s can be assumed to scale to a first estimate with the parametric quantity l. That is: s=al=a ( lg+lp ) , where a is a changeless and the indexes g and P refer severally to the farinaceous and polymer parts to the grade of microstructure interconnectedness. Solving Equation 1 leads so to the transeunt behavior of the emphasis:
s=sg+sp = ( 2 )
where the superiors 0 and ? refer severally to the initial and steady province values of the emphasis.
tg ( s )
tp ( s )
T R ( s )
REF-100
3,77
–
2382,10
REF-50
1,30
11,64
1062,20
Referee
0,77
7,50
760,28
REF+50
2,20
17,95
642,29
REF+100
1,95
12,43
555,09
Table 3: Characteristic times for dislocation and that of rebuild up for different dose rates of VMA.
The best tantrum of the experimental emphasis relaxation leads to the values of the relaxation times for the different pastes considered. They are reported in Table 3. One can detect that the polymer relaxation times are an order of magnitude higher than the farinaceous 1s. This can be expected since the molecular weight of the polymer is really high. The development of the relaxation times for microstructure breakage is non humdrum. One can detect a minimal value for the mention paste. The ground of such development of microstructure dislocation dynamicss as a map of VMA content is non clear. This point deserves more probe.
Figure 5 represents a typical temporal development of the emphasis ( stand foring the microstructure reconstruct up ) when the paste is hold at remainder ( really low shear rate ) following shearing at a high shear-rate ( 200 s-1 ) . The illustration in Figure 4 corresponds to the mention paste. The consequences for the other pastes are similar.
Figure 4: Typical temporal development of the emphasis at a really little shear rate ( 0.01 s-1 ) stand foring the microstructure rebuild up after interrupt down at a high shear rate ( 200 s-1 ) . The uninterrupted line corresponds to the best tantrum with a stretched exponential.
In contrast with breakage the rebuild up dynamicss can non be described by the amount of two exponentials. In this instance a stretched exponential is more suited to suit the emphasis growing curves. A stretched exponential is kindred to treat affecting a whole distribution of relaxation times. One can understand physically such a behavior by the fact that since the paste is subjected to a really low shear rate a full distribution of relaxation times may be mobilized, including those matching to the grains and the polymer ironss. During the interruption down procedure merely relaxation times on the order ( or smaller ) of the characteristic clip of flow ( opposite of applied shear-rate ) may be mobilized. Stretched exponential response of thixtropic fluids has already been reported in the literature [ 10 ] .
Once once more a simple theoretical account of thixotropy, including an infinite set of independent relaxation times, can be used to account for a stretched exponential behavior of the construction rebuild up.
The best tantrum of the experimental curves with stretched exponentials leads the characteristic times ( tr ) of the rebuild up procedure, that is:
( 3 )
where and are severally the equilibrium and the initial emphasiss.
The development of rebuild up characteristic clip for different dose rates of VMA is reported in Table 3. Our experimental consequences clearly show that the VMA speeds up the rebuild up belongingss of the paste. The physical reading of this consequence is non straightforward and needs farther probe. In practise VMAs are sometimes referred to as thixotropic agents, our experimental consequences confirm that in a more quantitative manner.
3.2 Steady province behavior
Figure 5 represents the steady province flow curves of the different pastes considered. The influence of VMA dose rate is rather little. This has been attributed ( see Ref. [ 7 ] ) to the fact that the rheological behavior of such a extremely concentrated farinaceous suspension is dominated by the farinaceous stage, whereas VMA has influence chiefly on the liquid stage. The consequence of VMA dose rate depends qualitatively upon the shear rate interval considered, in understanding with the consequences reported by other writers [ 5 ] . This will be discussed in more inside informations below when sing the rheological parametric quantities.
Figure 6 represents the development of the plastic viscousness, which is defined as the derived function of the shear-stress with regard to the shear-rate, in map of shear-rate for different dose rates. It is to be noted that we use here fictile viscousness alternatively of evident viscousness ( shear-stress divided by shear-rate ) because it is the incline of the flow curve that determines the sensitiveness of the emphasis to the fluctuation of shear-rate. Since we deal with output emphasis fluids the two types of viscousness are different.
Figure 5: Flow curves for different VMA dose rates. ( ) REF-100, ( ) REF-50, ( ) Referee
( ) REF+50, ( ) REF+100
Figure 6 shows that the cement pastes incorporating VMA are shear-thickening ( fictile viscousness additions with shear-rate ) throughout the whole shear-rate interval considered. On the other manus the inserted zoom-in graph in Figure 6 shows that the rheological behavior of the cement paste without VMA is non humdrum. This paste is shear-thinning at low shear-rates and shear-thickening at high shear-rates.
Figure 6: Development of the fictile viscousness for different VMA contents: ( ) REF-100, ( ) REF-50, ( ) REF, ( ) REF+50, ( ) REF+100. The inserted graph rapid climbs in the behavior of REF-100 at low shear rates.
Shear-thinning behavior is kindred to flocculated suspensions and entangled polymer solutions or thaws. Shear-thinning is by and large attributed to shear-induced deflocculation in the former instance and to polymer-chains unsnarling and alliance in the later. In our instance, the shear cutting may be attributed to both phenomena.
Shear-thickening is by and large attributed to repulsive interactions between both colloidal and non-colloidal atoms in the instance of suspensions [ 21-22 ] and shear-induced constructions in polymer solutions [ 23-24 ] . Both two phenomena may lend to shear inspissating in our instance since aqueous solutions of polyose are known to exhibit shear inspissating at sufficiently high shear-rates [ 25 ] .
The steady province rheological parametric quantities can be determined by the best tantrum with a Herschel-Bulkley ( H-B ) theoretical account:
( 4 )
where ? is the shear-stress, the shear-rate, ?y the dynamic output emphasis, k the consistence and n the fluidness index.
In the instance where the behavior is non humdrum ( paste without VMA ) the shear cutting and shear inspissating zones of the flow curve are fitted individually.
The development of the dynamic output emphasis as a map VMA dose rate is reported in Table 4. The cement paste output emphasis monotonically increases with VMA content in understanding with old surveies reported in the literature [ 2, 4, 5 ] . Such behavior is expected and may be attributed in peculiar to entanglement and entwining of the VMA polymer ironss at low shear rates.
In contrast to the output emphasis, the development of the consistence ( in the shear-thickening
zone ) when increasing VMA dose rate is non-monotonous ( see Table 4 ) . The consistence foremost decreases when adding VMA and so additions. Such behavior has already been reported in the literature in the instance another type of a cementitious stuff [ 26 ] and attributed in peculiar to air-entraining effects of the polymer alloy. One can besides raise the fact that the polymer may really play an equivocal function. On one manus, it may increase the liquid stage viscousness taking to an addition of the paste viscousness, and on the other manus, it may lubricate the contacts between the solid atoms taking to a lessening of the paste viscousness. The competition between the two effects would take to the behavior observed here. A similar treatment has already been reported in the instance of silicon oxide suspensions and cement pastes whose composing is near to that of ours [ 27 ] .
Paste
Output emphasis
Consistency
Fluidity index
REF-100
0.09
0.062
1.72
REF-50
0.15
0.012
2.15
Referee
0.35
0.013
2.16
REF+50
0.39
0.014
2.13
REF+100
0.72
0.029
1.98
Table 4: steady province rheological parametric quantities of pastes.
Similarly to the consistence, the behavior of the fluidness index ( in the shear inspissating zone ) is non humdrum, showing a maximal value around the mention paste
( see Table 4 ) . This may besides be attributed a dual function that may be played by the VMA: on one manus polysaccharide aqueous solutions are known to exhibit shear-thickening at high shear-rates [ 25 ] and, on the other manus, its lubricating consequence would diminish the part of the farinaceous contacts to shear-thickening.
4. Decision
The rheological behavior of cement pastes involved in mix-design of extremely unstable concretes was investigated by experimentation. In peculiar the influence of VMAs on the rheological belongingss was considered. Both transient ( thixotropy ) and steady province behaviors were investigated. Without VMA the pastes exhibited complex steady-state flow-curves, including a shear-thinning subdivision at low shear-rate and a shear-thickening 1 at high shear-rates. It was found that the shear-thinning subdivision was absent for pastes with VMA. The development of the steady province rheological parametric quantities was found to establish to be complex: the dynamic output emphasis monotonically increases with VMA content, while the consistence and the fluidness index present an extremum value around the mention dose of VMA ( the 1 used to mix-design self-levelling concretes ) .
The influence of the VMA on thixotropic behavior was investigated by sing breakdown dynamicss under high rates and reconstruct up at remainder or really low shear-rate. It was found that the dislocation dynamicss was governed by two chief characteristic times ( the relaxation curves could be fitted with the amount of two exponentials ) which differ by an order of magnitude. This was attributed to the two different components of paste, viz. the VMA polymer and the farinaceous stage, whose relaxation kineticss would take topographic point at rather different timescales.
On the other manus, the rebuild up dynamicss was found to follow a stretched exponential-like procedure. This was attributed to the fact that, at remainder or really low shear-rate, a big set of relaxation times, including those matching to the polymer and the grains, may be mobilized.
