Hydro-mechanical properties of fresh cement pastes

containing polycarboxylate superplasticizer

A. Perrot1, D. Rangeard2, V. Picandet1, Y. Mélinge2

1 Laboratoire d’Ingénierie des MATériaux de Bretagne, (LIMATB)Université de Bretagne Sud – Université Européenne de BretagneCentre de Recherche de Saint-Maudé, BP 92116, 56321 Lorient, Cedex, Francearnaud.perrot@univ-ubs.frvincent.picandet@univ-ubs.fr

2 Laboratoire de Génie Civil Génie Mécanique (GCGM)Institut National des Sciences Appliquées – Université Européenne de Bretagne20, avenue des buttes de CoësmesCS 14315, 35043 Rennes, Cedex, Francedamien.rangeard@insa-rennes.frYannick.mélinge@insa-rennes.fr

Abstract A novel oedometric cell has been used to study the permeability and compressibility

of freshly made cement-based pastes. This cell allows rapid tests providing an accurate

estimation of compressibility, permeability and thus consolidation coefficient with no cement

hydration effect. The influence of the test-induced flow on the results obtained is discussed in

order to find an optimized procedure. The effects of common mix-design parameters such as

W/C ratio and polycarboxylate amount, is discussed. Especially, it is shown that

polycarboxylate high range water reducing admixture reduces the cement paste permeability

and does not influence the compressibility of the cement grains network. The effect of W/C

and admixture amount on paste bleeding is also discussed.

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Keywords: permeability, consolidation, bleeding, superplasticizer

I Introduction

Fresh cement pastes and generally fresh cement-based materials are suspensions of grains in a

liquid phase. However, fresh cement pastes are generally regarded as homogeneous

thixotropic yield stress materials [1-5]. This assumption is commonly used to predict the

mechanical behaviour and stability of these fresh pastes during casting or pre-casting.

However, both a liquid phase and a granular skeleton are involved and could be separately

considered. For instance, water bleeding depends on the interaction forces between grains on

the pore structure and on the liquid viscosity [6]. On a larger scale, sedimentation of

aggregates in cement paste depends on both aggregates and cement paste properties [7].

Moreover, some time-dependent issues of fresh cement-based materials are directly linked to

the interactions between cement grains (and aggregates) and the liquid phase. The hydraulic

conductivity (commonly known as permeability) of the matrix made of solid particles

monitors the liquid phase migration during the first hours of curing. Firstly, it controls the

increase of the pore water pressure while the material is sheared [8]. During setting,

permeability also should act on the rate of capillary pressure build-up, and consequently on

the plastic shrinkage [9-11]. Then, permeability is a key parameter to predict the drying

kinetic after casting. [12]. As a consequence, the efficiency of the mortar used for masonry is

also linked to its permeability, as it depends on the drying and liquid desaturation kinetics

[13-16]. Finally, we can mention that the permeability of freshly cast underwater concrete is

also involved in strength loss in a hardened state [17].

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Considering cement-based materials as bi-phased materials including a large amount of

micrometric solid particles, soils mechanic assumptions can be used to provide a theoretical

framework based on hydro-mechanical theories. Before the setting time (i.e. during the

dormant period), cement-based materials are considered as frictional granular [9, 18]. For

example, the consolidation theory has been successfully used for the development of

extrudability or pumping criteria for cement-based materials [19, 20]. It has also been applied

to predict the amount of bleeding water and the settlement of concrete in formwork after

casting [21-26]. In this case, the permeability of the granular skeleton and its compressibility

coefficient () (which describes the mechanical behaviour of the granular skeleton), are

required to predict the settlement, and make it possible to compute the consolidation

coefficient Cv which describes the whole consolidation process [27]. It should be noted that

bleeding may induce a mechanical anisotropy created by the vertical water flow in a concrete

formwork [28]. In this case, a mechanical heterogeneity of the concrete's compressive

strength is also observed. It was also demonstrated that the consolidation theory can be used

to model the process of vacuum-dewatered concrete [29] and the development of pressure

acting on formwork [30-33].

It appears that the determination of hydro-mechanical parameters such as permeability,

compressibility and consolidation coefficients can be very useful to understand and predict

the above-listed phenomena occurring in fresh concrete. The determination of the

permeability of fresh cement pastes have already been the subject of previous studies [14, 34-

37] using imbibition tests, traditional filtration tests, drying tests, water retention tests or

controlled oedometer.

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In this study, we have used the controlled oedometer/permeameter device presented in

Picandet et al. [35] in order to accurately determine the permeability, the compressibility and

the consolidation coefficients.

First, the permeability of the fresh cement pastes is studied. The liquid phase flow induced to

perform permeation tests may affect the sample properties; its effect on the measured

permeability is studied, using sequential measurement periods at various hydration times (age

0 is defined by the end of mixing).

The evolution of the sample undergoing a constant flow since the the beginning of the test is

used to compared our results. The influence of mix-design parameters, such as W/C ratio or

the use of polycarboxylate-type admixture, is then studied. The analysis is extended to the

relationship between bleeding and permeability.

Secondly, the mechanical property of the granular skeleton inside cement pastes is studied

from the measurement of the compressibility coefficient. The effects of the mix-designed

parameter such as W/C and admixture content are presented.

Finally, the granular skeleton compressibility and the consolidation coefficient of cement

paste are investigated.

II Materials and methods

II-1 Materials

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A CEM I type cement of 3.15 specific density is used in this study. The Particle Size

Distribution (PSD) was measured in ethanol using a laser particle size analyzer (Figure 1).

The specific surface of this cement, measured using a Blaine apparatus, is 3390 cm²/g. The

Water to Cement mass ratio (W/C), and admixture to cement ratio, were chosen to cover the

rheology range of cement pastes used in industrial practice. For permeability measurements,

the targeted yield stress is 100 Pa. This initial yield stress is expected to provide sufficient

cohesion to undergo the vertical flow of the liquid without loss of structure in the sample. For

compressibility measurements, no specific yield stress is targeted. So, high W/C ratio pastes

were prepared in order to reveal a possible change in mechanical behaviour.

Figure 1. Cement particle size distribution.

The High Range Water Reducing Agent (HRWRA) used in this study is a commercial

polycarboxylate type polymer. It consists in a liquid solution with 20 % of dry material,

designed to be added to the mix with a mass ratio of agent to cement (A/C) ranging from, 0.3

to 3%. In this study, the A/C ratio ranges from 1 to 3 %. The HRWRA is added to the mixing

water before water/cement contact. This water is then mixed with cement in a planetary

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Hobart mixer. The mixing phase consists in two steps: 2 minutes at 140 rpm and 3 minutes at

280 rpm. It is important to note that the materials (HRWRA and cement) and mixing protocol

are the same as those already used in a previous study on the relationship between yield stress

and bleeding [6].

II-2 Method and theorical background

In this study, the same device is used for both permeability and compressibility measurements

of the pastes. The apparatus consists in an oedometer device placed under a compression press

as presented in Picandet et al. [35].

A 50 mm diameter and 20 mm height oedometer ring is used for the study. The oedometer

ring immersion provides constant water head conditions at the top face of the material. Porous

stones are placed upside and downside the sample. Two thin paper filters are placed between

the porous stones and the sample in order to avoid migration of the solid through the drainage

system. A perforated piston placed on the top face of the specimen induces a one-way

drained condition.

The vertical stress applied to the sample is recorded during the entire test. The sample height

variation is measured by a displacement transducer with an accuracy of 0.01 mm. A pressure

transducer measures the pore water pressure on the bottom face.

A schematic view of the device is given in figure 2.

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Figure 2. Measurement device

The experimental protocol consists in two main stages: a loading stage, corresponding to a

conventional one-dimensional consolidation test, and a permeability measurement stage. The

test configuration used during the permeability measurement stage is the usual rigid wall

permeability cell. The permeability measurement is performed after a loading phase, when

the specimen height is kept constant and the excess pore water pressure due to consolidation

is completely dissipated as indicated in Picandet et al. [35]..

The permeability tests are conducted using the constant head method with bottom up water

flow. A constant hydraulic head is applied by means of a Mariotte siphon. This test is

conducted with the device described in figure 2, maintaining valve A open.

The permeability value is calculated from Darcy’s law:

H.S.th.Vk i

(1)

With V/t the rate of inlet water volume, hi the height of the sample during the permeability

measurement stage, and H the applied fluid pressure (m), which corresponds to the

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difference between the fluid pressure at the top and at the bottom of the sample. During the

test, we use H = 1 m and hi=0.02 m. Then the hydraulic gradient H/hi is set to 50.

Different stages of permeability measurement can be performed successively and quickly for

decreasing values of W/C ratio on the same sample (four W/C ratios during one hour of tests).

The mechanical behaviour of the grains assembly is studied during the loading phase, using

the one-dimensional soil consolidation. This is based on the following major assumption

provided by Terzaghi [27]: the total stress applied is the sum of both the stress applied on

the granular skeleton (called effective stress ’), and the pressure applied on the liquid phase

(called pore pressure u). This can be written as follow:

u' (2)

In soils mechanics, the soil porosity is commonly expressed as the void ratio, e (i.e.: void to

solid volume ratio). It should be noted that cement pastes are considered as saturated media.

Thus, the entrapped air is neglected, as already considered in previous studies dealing with the

consolidation of concrete [35]. The void volume is then assumed to be equal to the liquid

volume, and the void ratio e can be written as a function of the W/C ratio and the specific unit

weights of cement particles c (kN/m-3) and of water at 20°C w (kN/m-3). The weight to

volume ratios of the pastes are checked before each permeability stage.

w

c

γγ

CW

e (3)

To analyze data, the common Terzaghi equations [27] are used. They provide the following

differential equation allowing computation of the void ratio e according to the time:

2

2

1 ze

²eC

te v

(4)

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The mechanical behaviour and stiffness of the granular skeleton is studied from the

compressibility coefficient which linearly links the void ratio variation e to (ln ’)

according to [27]:

)'(lne (5)

It is well-known that the Terzaghi equation does not describe completely the behavior of

saturated soils, as the permeability and compressibility coefficient are both dependent on e.

However, we consider that during one compression stage, for a limited range of strain, the

variation of e remains limited and e is assumed equal to a constant value. This makes it

possible to use eq. (3) to eq. (5) for our data analysis.

A constant ram advance rate of 0.5 mm.min-1 is applied to the specimen while the vertical

stress and the excess pore pressure are recorded simultaneously (valve A is closed in Figure

2). The effective stress is thus calculated from total stress and excess pore pressure using the

Terzaghi relationship.

The compressibility curve is obtained by recording the evolution of the vertical effective

stress as a function of the void ratio. The effective stress is calculated from total stress and

excess pore pressure using eq. (2).

Assuming that the dry weight of the sample is constant (and according to volume

conservation), the height variation h of the sample induces a variation of the void ratio, ∆e.

Such variations can be computed by the following equation (6):

00

1 ehhe (6)

where h0 represents the initial sample height, and e0 the initial void ratio. Consequently, can

be computed from eq. (6).

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II.3 Discussion on test duration

The test method requires an upward flow of water through the sample that could affect the

sample nature and homogeneity. The finest grains can migrate through the sample due to the

pressure-driven flow, and hydration kinetics may be influenced.

A comparison of the results of tests performed on same batch pastes with various durations

(from 30 to 150), at different ages (0, 40 and 80 minutes), shows the effect of this intrusive

flow on the measured permeability as shown on figure 3.

Figure 3. Comparison of the measured permeability between samples undergoing continuous

measurement (i.e; a continuous pressure-driven flow over 150 minutes) and samples

undergoing sequential measurements (i.e, a pressure driven flow over a time lapse of 40

minutes) after 0, 40 and 80 minutes of hydration time.

Figure 3 clearly shows that the measured permeability is affected over time. During the first

hour, the permeability of the two tested samples is the same. Beyond one hour of hydration

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time, the measurement carried out on the virgin sample shows a permeability 15% smaller

than the sample continuously tested (with a pressure driven flow percolating the sample

during one hour). The difference is higher at two hours: the permeability is 25% smaller for

the virgin sample.

These results can be easily explained by the effect of the induced water flow which may

prevent the formation of hydrates in the cross-connected pores. We may also assume that the

flow leads to a migration of soluble minerals as well as the smallest grains. However, this last

assumption is not in agreement with the previous study of Picandet et al. [35] concluding that

there is no significant effect of the hydraulic gradient on the measured permeability, and

therefore, negligible solid migration inside samples.

The test procedure has noticeable effects on the pore structure and the water flow paths. It

seems that upward flows tend to keep the capillary path opened, and to slow down the rate of

the diminution in permeability commonly observed. Tests should have a limited duration and

should be carried out on a virgin sample in order to give a representative permeability of the

sample at a given hydration time.

From theses observations, the authors suggest limiting the test duration to one hour. This

duration is short enough to avoid the ageing effect tending to reduce the water flow, and long

enough to measure the initial permeability of the material with a good level of accuracy, while

using lower induced pressure gradients.

This observation on the effect of an upward flow on the hydration of the cement matrix may

have practical interest: during the first hour in a formwork, the concrete is submitted to a

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vertical stress which induces vertical consolidation of the paste [22, 23, 26, 36]. This effect is

magnified with the formwork height, or with the use of highly bleeding concrete that induces

concrete heterogeneity and anisotropic mechanical behaviour [28]. To go one step further , it

may be interesting to quantify the effect of this upward flow on the heterogeneity of hardened

concrete and the variation of its mechanical properties.

III – Permeability

III.1 Influence of W/C

As shown by previous studies [26, 35, 36], the evolution of the permeability of cement pastes

according to void ratio, e, is well described by the Kozeny-Carman or Taylor equations.

The Taylor model [38], based on the empirical equation used in the study of fine soil grains,

requires two fitting parameters and predicts a linear relationship between the void ratio e, and

log (k):

010 ek

10 klogCek log (8)

Where 1/Ck is the slope and ke=0 is the intercept with the vertical axis. The most famous model

to predict permeability as a function of the void ratio is probably the Kozeny-Carman [39]

equations (cf. eq. 7). Solving the Poiseuille problem, the authors predict the hydraulic

conductivity k as a function of the void ratio for a network of capillary tubes. The specific

area S (m²/kg) and a material constant C are the constant parameters relative to the porous

material as shown by equation (9):

e1e

)²/( S²1

ρ μg Ck

3

wsww

(9)

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C is shown equal to 0.2 for spherical particles. However, this value of the coefficient C is

inappropriate for cement-based materials as it does not take into account micro-scaled

interactions between micrometer grains and water phase [40]. For cement-based materials, the

flow is probably influenced by the flocculation state. As a consequence, we choose to adjust

the specific area as a fitting parameter.

As shown by Picandet et al. [35], the determination of fitting coefficients are required for

those two models for an accurate prediction of permeability. In figure 4, the evolution of k is

plotted for mixes with various A/C mass contents of HRWRA ranging from 0 to 3%. As

expected, the permeability increases with the void ratio and best fitting parameters of Taylor

and Kozeny-Carman modelling for all A/C ratios are summed up in table 1.

Figure 4. Evolution of the permeability, k (m.s-1), versus the void ratio e of cement paste with

various contents of HRWRA. Comparison with model provided by Taylor (lines) and Kozeny-

Carman (dotted lines).

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  Kozeny-Carman TaylorA/C (%) C S (m²/kg) R² Ck k e=0 R²

0 0.0103 188 0.92 0.61 3.6x10-9 0.991 0.0081 256 0.64 0.51 8.51x10-10 0.772 0.0057 340 0.86 0.42 2.05x10-10 0.983 0.00143 335 0.85 0.42 5.73x10-11 0.91

Table 1: Kozeny-Carman and Taylor models, best fit parameters for the various HRWRA

dosages.

III.2 Admixture influence and discussion of its effect on bleeding

The addition of a polycarboxylate-based admixture reduces the permeability of the cement-

based mixes as shown on figure 4. The admixture dosage influences the permeability value in

the range of tested A/C (mass ratio of admixture to cement).

The diminution of permeability induced by the admixture can be explained by its

deflocculating properties. Polycarboxylate induces a strong steric hindrance which allows the

separation of the cement grains [4, 6, 41, 42]. Then, for the tested mixes, the average distance

between grains at the contact points can be multiplied by four when the HRWRA is used at

the saturation dosage (i.e. when A/C=3%, see [6]). The admixture changes an assembly of

densely flocculated grains into a better dispersed grains assembly [41, 43] which presents the

smallest number of accessible capillaries for the water flow. Without any admixture, dense

aggregates of flocculated cement grains are formed, creating a wider percolated network

throughout the sample [44, 45]. A recent analysis of the effect of polycaboxylate HRWRA on

the dispersion of grains by cryo-microscopy analysis [46] brings new insight to the grains

dispersion status with and without HRWRA, and shows that without HRWRA, the finest

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particles are packed together, leaving the largest porous networks inside the cement paste. On

the contrary, when polycarboxylate is added, the finest particles are better dispersed, which

makes the pores smaller. These observations are in agreement with the fitted values of

specific area obtained from the Kozeny-Carman model in table 1. For well-dispersed

suspensions (with 2 and 3% of A/C mass ratio), the fitted specific area is close to the Blaine

measured one. On the contrary, for non-dispersed suspensions, we obtained a smaller value.

In this second case, the specific area to be taken into account is linked to the flocculation

state.

Figure 5: Taylor model of permeability according to the A/C ratio for W/C=0.25 ; W/C=0.3

and W/C=0.35.

Figure 5 shows that the permeability diminution is about a decade with the addition of 3 % of

admixture (9 times, 12 times and 16 times less than A/C=0% for respectively W/C=0.35;

W/C=0.3 and W/C=0.25). This implies a better stability of concrete with a high amount of

superplasticizer, such as self-compacting concrete. However, a recent description of the

relationship between yield stress and bleeding of paste highlights that cement paste stability

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enhancement is not only influenced by permeability, but also by the network of forces

between particles [6]. On the other hand, the widespread idea that HRWRA automatically

increases bleeding is also false.

Figure 6 illustrates how HRWRA influences the bleeding of cement pastes. The possibility of

bleeding is directly driven by the interaction between particles, and then by the distance

separating the particles. In fact, bleeding may occur if the gravity forces acting on the cement

particles are sufficient to overcome the attractive Van der Waals forces within the particles

network [6]. The addition of polycarboxylate increases the separation and reduces the

colloidal forces. Brownian motion cannot stabilize the suspension and prevent the settling of

cement particles, since the force associated to gravity is several orders of magnitude higher

than the force associated to Brownian motion [6]. Thus, a cement paste is able to sustain an

homogeneous suspension if the following condition is fulfilled;

 A0a*/12H2 > Δρgπd503/6 (10)

Where a* is the curvature radius of the cement particles “contact” points (300 nm see [4]), H

is the surface-to-surface separation distance at “contact” points, A0 is the non-retarded

Hamaker constant (1.6x10-20), is the density difference, and d50 is the average diameter (10

m) [6]. The separation distance H at the contact point depends on the HRWRA content, and

can be computed from [47], using the surface coverage ratio θ, which governs the statistical

distribution of interactions. The surface coverage values were both indirectly computed from

the Yodel model [43] with yield stress measurements data, and measured using TOC (Total

Organic Carbon) analysis for the tested cement paste and HRWRA [6]. Then, using eq. (10)

and the computed separation distance H at a given HRWRA dosage, a critical W/C ratio

relative to bleeding can be predicted [6].

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At such a critical W/C ratio (displayed by a solid line on figure 6), colloidal forces do not

become sufficient to handle gravity, and bleeding may appear. In this configuration, the paste

bleeds until a static equilibrium is reached. This equilibrium can be reached if sufficient water

has gone out (then, particles can get closer and the colloidal forces increase), or if the

hydration has induced enough stiffness to the paste. For instance, a paste without HRWRA at

W/C=0.32 displays no bleeding. A dosage of 2% of HRWRA with the same W/C ratio

overcomes the critical dosage, and the paste exhibits bleeding (while the paste permeability

has decreased). A supplementary addition of HRWRA (for example providing an A/C ratio of

3%) reduces the permeability of the paste and the bleeding rate. This is consistent with the

observations given in Josserand et al. [22].

It must be noted that within the bleeding area drawn on figure 6, the permeability value of a

given dosage provides information on the initial bleeding rate only. The amount of bleeding

cannot be predicted since it depends on many other factors such as hydration time, the amount

of water needed to go to the “no bleeding” area, etc...

For a stiffer paste (below W/C=0.26), the addition of HRWRA reduces the permeability with

no risk of reaching the critical dosage. This is of great value for applications such as

extrusion, where the homogeneity of the paste is required along the whole process [19].

Otherwise, fluid pastes, (yield stress of 25 Pa) exhibit bleeding at all HRWRA dosages.

Figure 7 shows the evolution of the permeability of pastes exhibiting the same yield stress. At

saturation dosage of HRWRA, the permeability is divided by two decades. Thus, it appears

that the bleeding rate of pastes with admixture will be far smaller than pastes without

admixture. However, to avoid any bleeding phenomenon, viscosity agents can be used and

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added to the paste. For common pastes (around 100 Pa yield stress), an A/C mass ratio of

HRWRA equal to1.5% is enough to avoid any risk of bleeding.

Figure 6: Map of permeability values for mixes with A/C ranging from 0 to 3% and W/C

ranging from 0.2 to 0.45. The line shows the frontier between a stable paste and a paste that

is expected to bleed.

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Figure 7: Evolution of permeability at iso-rheology (same yield stress) according to the A/C

mass ratio. The filled area represents an A/C ratio displaying no bleeding.

IV- Compressibility and consolidation coefficient

IV-1 Fresh cement paste with admixture

The bleeding phenomenon is not only governed by the fresh-paste permeability that induces

the initial bleeding rate. Actually, the amount of bleeding is also governed by the granular

skeleton's mechanical behaviour. Eq. (5) highlights that the total volume of bleeding is

directly linked to the stress acting on the cement particles' network. Therefore, the description

of the compressibility of the granular assembly, commonly modelled by a constant parameter

, is used to model the amount of bleeding and to predict the magnitude of concrete

settlement in high formwork [22, 23, 36]. As a consequence, the effect of polycarboxylate

addition on particle network compressibility deserves further investigation.

A one-dimensional compression test is performed by applying a constant strain rate to the

sample. The effective stress acting on the granular skeleton is deduced from the total vertical

stress applied and the recorded excess pore water pressure. Then, the void ratio is computed

from the settlement of the tested sample (see eq. 6).

The void ratio versus log(’) during the test performed on high W/C cement paste (from 0.5

to 0.55), with and without admixture, are plotted in figure 8. All tested pastes display the

same behaviour with three distinct linear slopes:

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- If the void ratio is over 1.2, (i.e, if the solid volume fraction of 0.46 as =(1+e)-1), the

maximum packing solid volume fraction m is 0.59 as shown in [6]. The random loose

packing is close to 0.8m for cement-based mixes [4, 8, 48-50], then below this solid

volume fraction (i.e. over the corresponding void ratio equal to 1.2), the compression

behaviour is monitored by hydrodynamical contribution, and equilibrium is achieved

by colloidal interaction. In this part, the granular skeleton is the most compressible

and the load sensor is not enough sensitive to compute any value.

- If the void ratio is below 1.2 (i.e. the volume fraction is beyond the random loose

packing) a rigid percolation network exists and the compression behaviour is then due

to inter granular contacts and interactions. In this case, the average compressibility is

 = 0.06 and the granular skeleton is stiffer than for e > 1.2. In this configuration, the

compression behaviour of the granular skeleton does not depend on the admixture

dosage but mainly on the particles properties.

- If the void ratio is still below 1.2 but the applied stress decreases after excess pore

water pressure has vanished, the granular skeleton is over-consolidated at a stress level

relative to the applied load at which it has already been consolidated. A swelling

coefficient equal to 0.0050 is measured. This corresponds to a quasi-elastic response

and consequently traduces a reversible behaviour.

The compressibility behaviour of cement paste is not trivial and depends on both solid volume

fraction and stress path history.

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Figure 8: Compressibility curve obtained for cement pastes with different admixture dosage.

IV-2 Method to predict the consolidation coefficient

In the Terzaghi consolidation theory, the coupled effect of permeability and granular skeleton

compressibility can be integrated in only one modelling consolidation coefficient Cv [27]. In

this case, the whole consolidation process may be predicted using eq. (4). This relationship

makes it possible to quantify the bleeding according to the resting time.

Thus the consolidation coefficient Cv (m.s-2) can be expressed in function of the already

determined permeability and compressibility, as follow (eq. 11):

wv .a)e(kCv

1

(11)

With

'eav

(12)

Combining the equation (5), (8) and (11), the Cv coefficient can be expressed using the

equation 12:

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440

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442

443

444

445

446

447

448

449

450

451

452

(13)

Where Nis the void ratio corresponding to the intercept of the compressibility curve (in the

frictional behaviour area, i.e, intercept of the fitted straight line drawn from the lower part of

the graph in figure 8), with the vertical axis of the effective stress ’ equal to 1 kPa..

The evolution of the consolidation coefficient Cv versus the void ratio deduced from eq. (13)

is plotted in figure 9 for the cement paste. Thus, the Cv values deduced from the experimental

data provided by the loading phase using a standard method (Taylor’s method as used in

ASTM standard [51]) are also presented in figure 9 for the two tested materials.

A continuous evolution of the Cv factor versus the void ratio is obtained with the proposed

method. The standard method only gives an approximate value of Cv for an average void

ratio. The values obtained using the two methods are in agreement.

Both performed phases are here required for the determination of Cv: the permeability

measurement phase (under a range of void ratio), and a compressibility test which

corresponds to the loading phase (under constant strain rate).

453

454

455

456

457

458

459

460

461

462

463

464

465

466

467

468

469

Figure 9: Evolution of the Cv coefficient versus void ratio for the tested cement paste and

comparison between the results obtained with the proposed method and with the ASTM

standard [51]

It is important to note that the compressibility of the cement paste decreases with the

hydration time. In further studies, the evolution of compressibility (and of the associated

coefficient of consolidation) could be investigated.

V – Conclusions

Original concepts and results about the hydromechanical behaviour of fresh cement paste

containing admixtures have been developed in this study. Using a specific oedometer, the

permeability and compressibility of fresh mixes is measured. The main advances are listed

below:

- The permeability test produces a water flow percolating through the sample which can

induce a measurement artefact, since it tends to reduce the hydration inside the

capillary path of the fresh cement paste. As a consequence, such measurement leads to

a slight underestimation of the rate of the decrease in permeability according to the

hydration time of cement paste at rest. As expected, results confirm that short

consecutive measurements sequences limit the intrusive water flow inside the material

compared to the continuous percolation method.

- The addition of polycarboxylate allows a better dispersion of the cement grains,

inducing a smaller porous network (i.e. the average pore radius is lower and the specific

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471

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473

474

475

476

477

478

479

480

481

482

483

484

485

486

487

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489

490

491

492

493

494

area is higher) that increases the flow resistance inside the material. At saturation content,

the permeability is decreased by about a decade and the bleeding rate is decreased by the

same order of magnitude.

- The decrease of permeability due to polycarboxylate addition does not automatically

lead to a decrease in bleeding. Polycarboxylate addition induces an increase of the average

contact point distance and then a decrease of the interparticle force network. Therefore, if

the effect of gravity overcomes the vanishing interparticle force network, the paste will

bleed in spite of the addition of polycarboxylate [6].

- The compressibility of the cement grains skeleton mostly depends on the solid volume

fraction (i.e. on the void ratio). Beyond a critical volume fraction equal to 0.8m (where m

is the solid fraction of packing volume fraction), the material behaviour is frictional, while

below this value, the material behaves like a colloidal suspension.

In further investigations, it should be interesting to extend this study to concrete mixes. Also,

the evolution of compressibility and consolidation of cement pastes until setting time should

be studied to apprehend the whole casting period.

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