Volumetric Measurement in Water Bath- Inappropriate Method to Meaure Autogenous Strain of Cement...

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PCA R&D Serial No. 2925

Volumetric Measurement in Water Bath an Inappropriate Method to MeasureAutogenous Strain of Cement Paste

by Pietro Lura and Ole Mejlhede JensenBYG DTU - Department Civil Engineering

Technical University of DenmarkBrovej

DTU - Building 118DK-2800 Kgs. LyngbyTel: +45 45 25 17 58Fax: +45 45 88 32 82

Portland Cement Association 2005All rights reserved


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Volumetric measurement in water bath- an inappropriate method to measureautogenous strain of cement paste

Technical report

Pietro Lura and Ole Mejlhede Jensen

Technical University of Denmark, Department of Civil Engineering, February 2005


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AbstractVolumetric measurements of autogenous strain are frequently performed by placing the fresh cement

paste in an elastic rubber membrane immersed in water. The change in volume of the cement paste is

measured by the amount of water displaced by the immersed sample, for example by measuring its

weight change underwater. Volumetric and linear measurements of autogenous strain should in

principle give identical results. However, the measuring results from the volumetric method aretypically 3-5 times the results from the linear technique, depending on the type of cement paste and

the experimental conditions.

Some sources of error in the volumetric method have been identified by different authors and

eliminated or compensated for. In this study, absorption of water from the buoyancy bath through

the rubber membrane is identified as the principal artifact of the volumetric method and cause of

most of the discrepancies between volumetric and linear measurements. Water absorption is driven

by a lowering of the water activity in the cement paste due to self desiccation and to dissolved salts

in the pore fluid. Very high water absorption in the membrane-enclosed cement paste samples was

registered from the moment of casting. The influence of this error on the volumetric measurements

was of the same order of magnitude as the autogenous strain itself.

By performing the measurements in a paraffin oil bath instead of a water bath, water absorption

through the elastic membrane was eliminated. Volumetric measurements performed in paraffin oil

were almost identical to linear measurements performed on the same cement pastes. This shows that

the principal artifact of the volumetric method was individuated and eliminated. Nevertheless, the

presence of many other artifacts makes the volumetric method very difficult to use in practical

applications.


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IntroductionThe title of this report makes explicit reference to a notable paper by Sidney Diamond [1], where the

author thoroughly discussed the applicability of the mercury intrusion porosimetry technique for

cement based materials. In a similar way, the present paper aims at discussing the volumetric

technique for autogenous strain measurements in the form it has taken in the recent years:

measurements of the underwater weight of an elastic membrane filled with cement paste or mortar.An alternative to this method is to monitor the water level in a capillary tube, which measures the

water displaced by the membrane filled with cementitious material.

Literature review

Measurements of autogenous volumetric strain in the early literature were performed with different

principles. In 1928, Neville and Jones [2] described an apparatus for measurement of volumetric

strain of cement paste during sealed hardening at constant temperature: the cement paste was cast in

a deformable petrolatum mould and topped with paraffin oil, whose change of level in a capillary

tube indicated the volume change of the cement paste. In a similar way, Del Campo [3] poured fresh

cement paste on an elastic membrane and used mercury as a displacement liquid. Slate and Matheus

[4] inserted fresh cement paste or concrete in a container filled with a mineral oil and monitored thechange of oil level in a vertical capillary tube.

The fundamental elements of the volumetric method as it is used today were already present in the

work of Yates in 1941 [5]: here a rubber membrane containing cement paste is suspended in a water-

filled container and the level of water in a capillary tube is measured up to 1 year. A similar method

was used for concrete by Wuerpel [6] and for de-aired cement paste by LHermite and Grieu [7].

Also Edmeades et al. [8] used a similar method for cement paste, but readings were taken on a

horizontal mercury capillary tube. De Haas et al. [9] measured instead the weight of a beaker that

exchanged water with the bath when the volume of the specimen changed. Yamazaky et al. [10]

measured the underwater weight of the sample enclosed in a rubber membrane.

In 1978, Setter and Roy [11] presented volumetric measurements on cement pastes based on a

method similar to de Haas et al. [9]. A very poignant discussion of their paper was written by Baron

and Buil [12], based on Buils doctoral thesis work [13], which was perhaps the first international

debate on measuring techniques of autogenous strain. Baron and Buil [12] compared a volumetric

measurement, by means of the previously described apparatus by Del Campo [3], and a linear one,

showing that the strain values in the volumetric technique were one order of magnitude greater.

Baron and Buil [12] pointed out two principal artefacts in the volumetric technique: 1) In the first

hours after setting, the volumetric method may in reality measure the chemical shrinkage because of

reabsorption of a layer of bleeding water. Elimination of the bleeding water with a syringe [11] may

not be complete; 2) The pressure exerted by the measuring liquid and by the elastic membrane

causes contraction of the paste, especially around setting when it is weakest. The method used in[11] will most likely also be influenced by absorption of water through the membrane. This artefact

was not mentioned by Baron and Buil [12], possibly because the apparatus by Del Campo [3] did not

suffer from this shortcoming.


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4

In the early 1980ies Ziegeldorf and co-workers [14, 15] cast fresh cement paste into 0.4-mm thick

polyethylene bags, which were immersed in a container filled with de-aired water, whose level was

monitored. They observed that the measured volumetric strain critically depended on bleeding and

varied greatly with specimen size and shape.

The volumetric technique for autogenous strain measurements received renewed attention from themid 1990ies in a series of papers by Justnes, Sellevold and co-workers [16-22], who used

measurements of underwater weight of rubber membranes (latex condoms) filled with cement paste.

The known artefact due to the reabsorbtion of bleeding water was addressed by rotating the samples.

Most of the following papers by different authors on the volumetric technique make explicit

reference to this particular version of the volumetric method and use it in a similar form [23-35]. An

exception is the apparatus devised by Gagn et al. [36], where the sample is contained in a rubber

membrane sustained by a perforated metallic sheet and immersed in a water bath; the volumetric

autogenous strain is registered by level changes on a graduated capillary tube.

A debate about the appropriateness of the volumetric and of the linear technique started again in the

1990ies [37, 18, 20], but comparison of data on the same materials and systematic analysis ofartefacts of both techniques was still missing. Both a critical approach and a deep investigation of

the measuring techniques are present in Barcelo et al. [38]. Barcelo et al. [38] performed volumetric

measurements of autogenous strain on rotated samples, similar to the procedure by Justnes et al.

[17], but monitoring continuously the level of the displacement liquid. Comparison of volumetric

and linear measurements on the same paste revealed that, even after 1 d, the volumetric autogenous

strain rate was about 3 times the linear. Differences before and around setting could be explained by

a number of reasons, including temperature differences, pressure of the membrane in the volumetric

method, anisotropy of autogenous strain due to settlement, and friction in the linear method. The

discrepancies after a few days were instead much more difficult to explain. Barcelo et al. [38]

hypothesized that autogenous strain might not be isotropic even after setting; a second hypothesis

was that underpressure in the rubber membrane, which was isolated from external pressure, mightincrease the shrinkage in comparison with the linear measurements. The hypothesis of anisotropy of

autogenous strains after setting was later proven wrong: Charron et al. [39] found that the linear

autogenous strain of 2 mortars on 50-mm prisms was identical in 3 different directions.

Nevertheless, the linear autogenous shrinkage rate was 1/10 of the volumetric one after setting. As

reasons for the discrepancies, they suggested the internal underpressure in the volumetric specimen

or the suction of the elastic membrane into the air voids at the surface of the sample.

Osmosis through the membrane

One fundamental reason for the inconsistency between volumetric and linear autogenous strain

measurements after setting is transport of water through the rubber membrane, occurring when the

buoyancy liquid used is water [40]. Penetrated water may partially fill the internal voids produced bychemical shrinkage, causing an increase in the submerged weight that is finally interpreted as

volumetric shrinkage. On the other hand, water penetration reduces the shrinkage of the sample or in

extreme induces swelling. It is important to emphasize that the change in immersed weight due to

water absorption overrides the buoyancy change due to shrinkage of the paste.


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5

Rubber membranes used in the volumetric method are not waterproof. For example, condoms filled

with water and exposed to air at low relative humidity lost about 0.5% of their water content per day

[41, 42]. According to some authors [41, 43], the influence of the permeability of the membrane on

the volumetric measurements would be insignificant until self-desiccation occurs in the cement

paste, a few days after casting. Self-desiccation would then cause a relative-humidity gradient across

the membrane and drive water from the water bath into the sample.

However, significant water transport through the condom does not start some days after casting, but

instead occurs right from the beginning. A difference in water activity across the membrane exists

from the very start of the measurements due to dissolved salts in the pore fluid [40, 44]. This

difference in water activity constitutes a driving force for osmosis of water through the membrane

into the paste. To show this effect, Marciniak [42] measured the water absorption of condoms filled

with synthetic pore solution, immersed in a distilled water bath. The weight of the condoms filled

with pore solution increased by 0.1 % per day, while condoms filled with distilled water did not

show any appreciable weight change. Beltzung and Wittmann [27] observed a 0.02-0.05% mass

increase of cement paste samples immersed in water (w/c 0.29) in the first 3 d and attributed this

phenomenon to water diffusion through the latex membrane due to osmosis. Douglas and Hover [45,46] also measured a substantial weight change of condoms filled with cement paste and mortars

immersed in a water bath. They excluded osmosis as a driving mechanism [46], after observing no

relevant mass change of elastic membranes filled with a lime-saturated solution immersed in a water

bath. However, the composition of the pore fluid in a cement paste is very different from a saturated

lime solution [47]. Barcelo [48] tested different latex membranes and observed that all allowed water

transfer; as driving force of the water absorption, he suggested the reduced air pressure in the

capillary system of the cement paste hydrating in the elastic membrane.

Aim of study

This study aims at investigating the phenomenon of water absorption into the samples in the

volumetric method. In particular, its driving mechanism(s) needs to be ascertained and its influenceon the measured strain quantified. The final aim is to compensate for the absorption, if possible, or

instead to modify the volumetric measurement as to exclude this artifact. To verify the results of the

volumetric measurements, linear autogenous strain was measured on the same cement pastes with a

technique based on corrugated moulds [37].

In this research, widely different cement pastes were used, which had previously been investigated

with the linear technique [49]. The choice of the pastes was done to cover a range of different

deformational behaviors: 2 high performance cement pastes with w/c ratio 0.30 and 20% silica fume

addition, one with significant self-desiccation and high initial shrinkage, the other with initial

expansion due to the addition of superabsorbent polymers, SAP; finally, a plain cement paste of w/c

0.35, which showed initial expansion followed by moderate shrinkage.


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MaterialsSynthetic pore solution

Synthetic pore solutions were produced based on the compositions reported by Page and Vennesland

[47]. The composition of the synthetic pore fluid was: 0.4 mol/l Na+, 0.4 mol/l K

+, 0.002 mol/l Ca

2+,

0.724 mol/l OH-and 0.04 mol/l SO4

2-. This was obtained by mixing 16.00 g of NaOH, 6.97 g K2SO4,

17.95 g KOH and 0.15 g Ca(OH)2 in 1 l of demineralized water. Another solution with half the ionicstrength was obtained by dilution with 1 l of demineralized water.

Cement pastes

The following cement paste compositions were investigated:

A: w/c=0.30, 20% silica fume addition;

B: w/c=0.30, 20% silica fume addition, 0.6% superabsorbent polymer, SAP, by weight of cement;

C: w/c=0.35, 0% silica fume.

The cement used is a low-alkali Danish white Portland cement produced by Aalborg Portland.

Blaine fineness is 420 m2/kg and the Bogue-calculated phase composition (in wt.%) is: C3S: 66.1,

C2S: 21.2, C3A: 4.3, C4AF: 1.1, CS :: 3.5, free CaO: 1.96, Na2O eq.: 0.17.

The silica fume in pastes A and B was added as a dry powder at a rate of 20 wt.% of cement. The

specific surface of the silica fume is 17.5 m2/g (BET method). The chemical composition is (in

wt.%): SiO2: 94.1, Fe2O3: 1.00, Al2O3: 0.13, MgO: 0.71, SO3: 0.43, and Na2O eq.: 1.09.

In mixtures A and B, a naphtalene-based dry powder superplasticizer was added at a rate of 1.0 wt.%

of cement+silica fume.

The SAP used in mixture B are suspension-polymerized covalently cross-linked acrylamide/acrylic

acid copolymers [49]. The spherical particles have diameters about 100-150 m in the dry state. The

size of the swollen SAP particles in the cement pastes and mortars is about three times larger due topore fluid absorption. The SAP were added at a rate of 0.6 wt.% of cement. In mixture B, extra

mixing water was added in the amount sufficient to saturate the SAP particles. The amount adsorbed

in the SAP corresponds to an entrained w/c of 0.075 [49].

The cement pastes were mixed in a 5-l epicyclic mixer [50]. Cement and all other admixtures,

included the SAP in mix B, were put into the bowl. Mixing was at low speed for 1 minute, while

gradually adding about 3/4 of the demineralised water. Mixing continued at high speed for 1 minute,

after which it was stopped and the paste was scraped off from the blade and the walls of the bowl for

1 minute. Mixing was resumed for 1 minute at low speed while the rest of the water was added; a

last minute at high speed followed. The water was added in two steps to assure the homogeneity of

the mix and the dispersion of the silica fume. Total mixing time from first water addition was 5minutes. The temperature of the ingredients was approximately 20C at mixing.

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MethodsAutogenous relative humidity measurements

Autogenous relative humidity (RH) of cement paste was measured by Rotronic Hygroscope DT

(Rotronic, Basserdorf, Switzerland) stations equipped with WA-14TH and WA-40TH measuring

cells, which were built into a thermostatically controlled box (0.1C). Before and after every

experiment, the equipment was calibrated with saturated salt solutions in the range 75100% RH.Autogenous RH change was measured simultaneously on two identical samples. A thorough

description of the Rotronic Hygroscope and the sample preparation is published elsewhere [51].

Autogenous relative humidity was measured in this research for two principal reasons: 1) As shown

in previous research, the measured RH drop is related to the linear autogenous strain measured [52]

and under certain conditions the measured strains can be calculated based on the internal RH

development [53]; 2) The autogenous RH is identical to the water activity of the pore fluid in the

cement paste [53], which is a driving force of the water transport across the elastic membrane, as

will be illustrated in the following.

Volumetric measurements of autogenous strain

Measurements of volumetric autogenous strain were performed by monitoring the weight of cementpaste samples contained in elastic membranes and immersed in a buoyancy liquid, either distilled

water or paraffin oil. The experiments were done in a climate room at 200.2C.About 100 to 150 g of fresh cement paste were poured into a membrane, either a latex or

polyurethane condom. The latex condoms were of type Plan by RFSU, with thickness 0.06 mm and

without gliding crme or reservoir. The polyurethane condoms were of type Avanti by Durex, with

thickness 0.04 mm, gliding crme and reservoir. The gliding crme was removed from the external

surface of the condom with a paper towel before filling the condom with cement paste.

The filled membrane was tightly closed with a knot; attention was paid to avoid entrapment of air

bubbles. The excess part was then cut off and a 0.12-mm mono-filament Silicon-PTFE string

(fishing line) was tied to the sample by means of a plastic strap. The string, about 400 mm long, was

tied to a stainless steel hook at the other end. In the case of measurements in water, the knot of therubber membrane was sealed with a drop of silicone before immersion in the liquid. Typical samples

are shown in Figure 1.

Figure 1. Polyurethane membrane with reservoir (left)

and latex membrane without reservoir (right) filled

with cement paste. The picture shows also the plastic

strap around the knot, to which a thin string was tied,

ending with a steel hook that was used to hang the

sample under the balance.

During the measurements, the samples hung from a hook beneath the balance plate. The balance was

a Sartorious CP 225 D, with sensitivity 0.01 mg for weights below 80 g and 0.1 mg for weights until

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220 g. All measurements were performed in the 220 g measuring range. The 0.1 mg accuracy in this

range was fully sufficient for the measurements; in addition the stability of the readings from the

balance was improved. The 0.1 mg weighing accuracy results in a nominal accuracy on the

measured strain of 0.4-0.8 m/m, considering typical sample sizes, different buoyancy liquids, andisotropic deformations. The actual uncertainty on the measured strain is much higher, being

dominated by a number of factors, included temperature oscillations and drift of the balance. Thebalance was placed on a 100-kg concrete weighing table to minimize vibrations. The table was

provided with a 110-mm circular hole through which the fishing line passed. A view of the test setup

is shown in Figure 2.

In the case of measurement in water, the sample was submerged in a water bath, consisting of a

container of dimensions 0.50.80.4 m, holding about 100 l of distilled water. Water circulation in

the bath, which would have helped keeping the sample temperature constant, was avoided to

improve the stability of the weight measurements. Most of the surface of the bath was covered with a

lid to reduce evaporation; this was done to avoid changes in the water level and thermal gradients. In

the case of measurements in paraffin oil, a smaller cylindrical container ( 130 mm, height 200 mm)

was filled with paraffin oil; the container was immersed in the larger bathtub filled with water tohelp keeping the sample temperature constant. Temperature measurements by means of

thermocouples showed a maximum temperature increase of 0.5 C in the samples measured in thewater bath; the maximum temperature increase of the samples measured in the paraffin oil bath was

1.5 C. In both cases, the temperature peak occurred at 8 h from casting and the temperatureequilibrated at 24 h.

The immersed weight of the sample was measured and recorded automatically at regular intervals by

controlling software. Measurements were generally recorded every 10 minutes from about

30 minutes after casting up to about 2 weeks. Upon immersion of the sample in the buoyancy liquid,

air bubbles on the upper outer part of the membrane were gently removed. The first measurement

immediately after immersion was discarded because of the influence of the oscillations of the sampleon the measurements, though oscillations were quickly damped in the viscous paraffin oil. Another

source of error in the very beginning were drops of buoyancy liquid attached to the string, but these

fell into the bath in a few seconds.

Linear measurements of autogenous strain

Linear autogenous strain of cement pastes was measured by a special measuring technique, where

the cement paste is encapsulated in thin, corrugated polyethylene moulds with length/diameter

approximately 300:30 mm [37]. This ensures insignificant restraint of the hardening cement paste

and permits measurements to start at an early age. The cement paste was cast under vibration into

the moulds; the specimens were then placed in a dilatometer equipped with automatic data-logging

and electronic linear displacement transducers. The dilatometer with samples was immersed into atemperature-controlled glycol bath at 200.1C. Two samples were tested simultaneously in thedilatometer, with a measuring accuracy of 5 m/m. A separate sample with an embeddedthermocouple registered the temperature evolution. Measurements were performed every 15 minutes

and started 30 minutes after casting. A detailed description of the dilatometric technique can be

found in the literature [37].


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Absorption measurements

To test absorption of buoyancy liquids through the different membranes used, a series of mass

measurements on the balance plate was performed on different samples. Membranes filled with

either synthetic pore solutions or cement pastes were immersed in the buoyancy liquids and their

mass was recorded at regular intervals. Before every measurement, the samples were removed from

the bath and the surface of the membranes was gently but accurately wiped with a paper towel toremove the excess water or paraffin oil.

Figure 2. Setup for testing volumetric autogenous strain of

cement paste. A Sartorious CP 225 D balance is placed on a

massive weighing table. Underneath the balance there is a

hole in the table through which the sample is suspended by

means of a hook and a string. The sample is immersed in a

buoyancy bath, either water or paraffin oil.

Results and discussionWater transport through membranes filled with different solutions

To quantify the influence of osmosis on the volumetric measurements, rubber membranes filled with

either synthetic pore solution, 50% diluted, synthetic pore solution, or demineralized water, were

immersed in a demineralized water bath and their mass was monitored. Both latex and polyurethane

membranes were tested. The membranes contained amount of solutions varying from 70 to 140 g.

Figures 3 and 4 show the results of the experiments. Water diffusion through the condoms was

proportional to the solute concentration. In particular, the mass of condoms filled with synthetic pore

solution increased by 0.26% per day, while the mass of the condoms filled with 50% diluted

synthetic pore solution increased at about half the rate. The mass of membranes filled with

demineralized water remained almost unchanged. No substantial differences in absorption rate were

observed between latex and polyurethane membranes. However, after a few days in the water bath,the polyurethane membranes started leaking and lost weight. Examination of the polyurethane

membranes revealed extensive degradation.

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The scatter between identical membranes filled with similar quantities of synthetic pore solution was

quite substantial, as shown by the standard deviation bars in Figures 3 and 4. The main reason is

believed to be variability in thickness between the membranes and possibly presence ofm-leaksand pinholes in individual membranes; this fact is confirmed by the literature about properties of

condoms [54].

0

3

6

9

12

15

0 7 14 21 28

Time [days]

Masschange[%]

Synthetic pore solution 50% diluted

POLYURETHANE

Synthetic pore solution

POLYURETHANE


LATEX

Figure 3. Mass change of membranes containing different solutions immersed in a demineralized water bath

at 20.0C. The average and the standard deviation of 3 samples is shown in the case of the latex membranes

filled with synthetic pore solution and of 2 samples in the case of the polyurethane membranes with synthetic

pore solution. For the polyurethane membranes with diluted (50%) synthetic pore solution, a single

measurement is shown.

-1

0

1

2

3

4

5

6

7

0 500 1000 1500 2000

Concentration of solution [mmol/l]

Masschange[%]


50% diluted

Demineralized water


Figure 4. Mass change of condoms containing synthetic pore solution, diluted (50%) synthetic pore solution,

and demineralized water, immersed in a demineralized water bath at 20.0C for 21 days. Each measurement

shows the average and the standard deviation based on 3 samples.

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Osmotic transport is driven by differences of water activity across the membrane. The activity of

water in an ideal solution can be calculated as:

WW xa = (1)wherexW[mol/mol] is the mole fraction of water in the solution. For the pore solution used, the

calculated water activity is 0.973, supposing all species are fully dissociated in the solution. The

osmotic pressure that can be established at thermodynamic equilibrium across a semi-permeablemembrane between a pure liquid and a solution can be calculated by van't Hoffs formula:

icRTPosmotic = (2)where c [mol/m

3] is the concentration of the solution, i [-] is the ionization factor of the solute,

R = 8.314 J/(molK) is the ideal gas constant, and T[K] is the absolute temperature, in this study293.15 K. For the synthetic pore solution used in this research, Posmotic,PS = 3.8 MPa, considering all

ions as dissociated. In the membranes filled with synthetic pore solution, a pressure equal to the

osmotic pressure would build up at equilibrium. This equivalent pressure can be used as a measure

of the driving force that causes ingress of water into the membranes. As seen from eq.(2), the

osmotic pressure is proportional to the solute concentration. The mass gain of the samples in Figure

4 depended linearly on the concentration of the pore solution, an indication that this process is

indeed driven by osmosis. Moreover, the linear dependence also on time, as seen in Figure 3,

confirms that the driving force remains almost constant through the experiment, as it is expected

since almost no dilution of the pore fluid occurred during the test.

Figures 1 and 2 are consistent with results obtained by Marciniak [42], who measured mass change

of latex condoms filled with a synthetic pore solution with composition very similar to the one used

in this research. However, Douglas [46] observed no relevant difference in the mass change of latex

condoms filled with a lime-saturated solution immersed in a water bath compared to pure water. As

observed before, the composition of the pore fluid in a cement paste is very different from a

saturated lime solution [47, 44], especially because of its alkali ions content. In particular, due to the

very low solubility of the calcium hydroxide in water, 0.022 mol/l at 20 C [55], the water activity ofa saturated solution of calcium hydroxide calculated with eq.(1) is close to unity, or 0.999. Theosmotic pressure that can be developed across the membrane has an upper bound at 0.16 MPa,

considering the calcium hydroxide as fully dissociated in water. This value is much lower than the

value calculated with eq.(2) for the synthetic pore solution used in this research.

The conclusion of the experiments on elastic membranes filled with synthetic pore solution and

immersed in a water bath is that osmosis through the membrane is a driving force able to account for

the mass gain of the samples. By extension, it is a plausible driving force also for absorption of

water into elastic membranes filled with cement paste [27, 46]. Moreover, it is a mechanism which is

active from the very start of the volumetric measurement, because a high ionic concentration in the

pore fluid is established rapidly when the cement is brought into contact with water.

Relative humidity measurements on cement pastes

Figure 5 shows the autogenous RH in the investigated hardening cement pastes. In paste A, with w/c

0.30 and 20% silica fume addition, a significant autogenous lowering of the RH is observed. After

12 days of sealed hardening, the RH is approximately 85% due to the hydration reactions. Water

entrainment based on SAP addition is seen to be efficacious in counteracting autogenous RH

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change: the autogenous RH change is almost fully counteracted in paste B, with 0.6% SAP addition.

The slight autogenous RH change to about 98% RH of paste B can be accounted for by dissolved

salts in the pore fluid [52]. Paste C shows a RH drop after 2 days until it stabilizes after about 10

days at 91% RH.

The initial internal RH, 98-99% in the 3 mixes, can be attributed to the dissolved salts in the pore

fluid of the cement pastes [53]. The equilibrium RH of a solution is described by Raoults law:wS xRH = (3)

wherexw [mol/mol] is the molar fraction of water in the solution. Sincexw corresponds also to the

activity of the solution, see eq.(1), the activity value calculated for the synthetic pore fluid in the

previous paragraph would correspond to a relative humidity of 97% RH. This value is in quite good

agreement with the initial RH in the three pastes, especially if considered that the composition of the

pore solution was not measured on the pastes, but instead derived from the literature [47].

85

90

95

100

0 1 2 3 4 5 6 7 8 9 10 11 12 13 1

Time (days)

Relativehumidity(%)

Paste B: w/c 0.30 + 20% SF + 0.6% SAP

Paste A: w/c 0.30 + 20% SF

Paste C: w/c 0.35

4

Figure 5. Autogenous RH of cement pastes A, B and C. Silica fume, SF, and superabsorbent polymer, SAP,

additions are given by weight of cement, where 0.6% corresponds to an entrained w/c of 0.075. Basic w/c is

0.3 for pastes A and B and 0.35 for paste C. Temperature: 20.0C.

More generally, since water vapor is close to an ideal gas, the measured internal RH of a cement

paste is equal to the water activity in its pore fluid. Water activity is affected not only by the

presence of dissolved salts, but also by the presence of air-water menisci in the pores of the cement

paste. The menisci are a consequence of chemical shrinkage and induce a tensile stress in the pore

fluid [53]. The influence on the RH from the menisci is described by Kelvins equation. The RH of a

cement paste can be described as a product of two terms, one due to dissolved salts in the pore fluid,the other due to the presence of the menisci [53]:

==

RT

MexpxRHRHRH

cap

wKS

(4)

where cap [Pa] is the stress in the pore fluid,M=0.01802 kg/mol is the molar weight of water, and

=1000 kg/m3

is the density of water. The derivation of eq.(4) is presented in reference [44].

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Results of the absorption measurement are shown in Figure 6. Pastes A and B are expected to have

similar permeability throughout the hydration process, since the presence of SAP should have only

limited impact on the transport property of the paste [49]. Therefore it is possible to analyze the

water uptake of the two pastes only in terms of the acting driving forces. Long-term water absorption

in cement paste B with SAP is lower than in paste A, whereas the absorption in the first hours aftermixing is quite similar. According to the analysis presented in the previous paragraph, osmosis

dominates the water uptake of the samples in the first hours. Osmotic forces depend on the

concentration of the pore solution, which is expected to be quite similar in pastes A and B because

the same cement and admixtures have been used. At a later age, self desiccation occurs in paste B

and becomes the dominant driving force for water uptake. This explains the higher water absorption

of paste B at later ages. It is noticed that water transport into the sample due to osmosis in the first

hours occurs at a very high rate compared to later ages. In fact, high porosity and low tortuosity of

the cement paste pore system at early age both promote transport of water into the cement paste.

Moreover, at early age the transport of water into the sample is entirely liquid based, instead of the

slower, combined liquid-gas transport that occurs when pores in the cement paste have been partially

emptied by self-desiccation. Whereas at early ages the permeability of the elastic membrane is thelimiting factor of the transport process, at later ages the reduced permeability of the cement pastes

becomes dominant.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 7 14 21 28

Time (days)

Massch

ange(%)

Paste A: w/c 0.3 + 20%SF

Paste B: w/c 0.3 + 20%SF + 0.6%SAP

Paste C: w/c 0.35

Figure 6. Mass change of latex membranes filled with cement pastes immersed in a water bath at 20C. The

average and the standard deviation of 5 samples are shown for pastes A and B, while 3 samples were

measured for paste C.

Paste C shows a higher absorption in the first hours after casting, but is close to paste A in the

following days, confirming that self-desiccation does indeed influence water uptake. Comparison of

paste C with the other two pastes is complicated, because the water uptake is influenced also by the

transport properties of the paste. In particular, paste C, with higher w/c and no silica fume, should

have a much higher permeability than pastes A and B at any time.

14


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A simple calculation confirms that the water uptake in the first day might be caused primarily by

osmosis. The absorption rate of latex membranes filled with synthetic pore fluid is about 0.4% by

weight per day (Figure 3). The cement pastes studied have initial water to solids volume ratio close

to 0.5, which corresponds to the volume fraction of pore fluid in the samples. It is therefore possible

to estimate the absorption in the first day as 0.4%0.5=0.2% by weight of cement paste. This valueis indeed very close to the measured values, around 0.15% by weight of cement paste in the first day

of hydration (Figure 6).

When water is absorbed from the buoyancy bath into the samples during a volumetric measurement

of autogenous strain, the measurements are affected in two ways: 1) the mass of the sample is

increased since the water is absorbed into internal voids; this is measured as an apparent shrinkage

after setting; 2) the water content of the sample is increased, which potentially increases the w/c

before setting but especially reduces self-desiccation and self-desiccation shrinkage of the hardening

cement paste.

-5000

-4000

-3000

-2000

-1000

0

0 7 14 21 28

Time (days)

Erroronautogenousstrain(m/m)

Paste A: w/c 0.3 + 20%SF

Paste C: w/c 0.35

Paste B: w/c 0.3 + 20%SF + 0.6%SAP

Figure 7. Error on the measured autogenous strain due to water uptake of cement paste in latex membranes

immersed in water bath at 20C. Linear strain was calculated assuming isotropic behavior; shrinkage is

plotted as negative. The average and the standard deviation of 5 samples are shown for pastes A and B, while

3 samples were measured for paste C.

In Figure 7, the estimated error on volumetric autogenous strain measurements that would result

from the weight change of the immersed samples is shown. The error was calculated based on theassumption that all the water absorbed into the sample filled voids produced by chemical shrinkage.

This is a rough assumption, especially before setting, when no voids are present in the pastes and the

absorbed water might instead contribute to raise the w/c ratio of the pastes. The error has been

recalculated as linear shrinkage, assuming isotropic deformation, in order to facilitate comparisons

with linear measurements in the following sections of this report; that means that the calculated error

on the volumetric strain has been divided by 3. Absorption of water from the bath produces a

15


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16

fictitious shrinkage after 3 weeks of 2000 m/m for paste B, 4000 m/m for paste A and more than5000 m/m for paste C. Already at 1 day after casting, the measuring error is substantial, varyingfrom 500 to 1500 m/m for the 3 pastes.

Besides the artifacts resulting from the measuring technique, absorption of water from the bath

affects the curing conditions of the sample, which will not be autogenous any longer. To performmeasurements of autogenous strain, the cementitious system must be: 1) sealed, 2) kept at constant

temperature, and 3) not subjected to external forces [52]. Therefore, in the case of water penetrating

the membrane the measured strain is not autogenous strain. According to Barcelo [48] the effect of

water absorbed into the sample can be described as an increase of the average water/cement ratio of

the cement paste; with this assumption, he showed that the effect on self desiccation was negligible

in the first week of hydration. However, it is misleading to assimilate the absorbed water to an

increase of w/c ratio: the situation is more similar to a cement paste with a given w/c ratio cured with

an external supply of water. This external supply of water will have a much greater influence on the

autogenous shrinkage of the paste than if it was supplied initially in the form of a higher w/c ratio

[49].

Some authors [33, 35, 41, 43] stated that absorption through the elastic membrane is a slow process

and its impact on the volumetric measurements can be neglected in short-term measurements;

therefore, use of the volumetric technique would be justified for measurements in the first day after

casting. However, no data on water uptake of volumetric samples was published by these authors to

confirm their hypothesis. On the contrary, Figure 7 shows that the impact of water absorption from

the buoyancy bath on the volumetric measurements is great enough to compromise the whole

measurement from the time of casting, since an error on the measured strain of 500-1500 m/m in1 d is of the same order of magnitude as the true strain itself.

Having proved that the water uptake of the samples is a significant artifact of the volumetric

technique and capable of disrupting its measurement results from the beginning, it is possible totackle the problem in two different ways: compensate for the absorption or eliminate the absorption.

Barcelo [48, 56] compensated for this artifact by measuring the weight increase of immersed

samples over a long period, about one month, and using a linear regression to fit a correction to the

measured strain. Once this correction was applied, a part of the discrepancy between linear and

volumetric measurements was eliminated, according to Barcelo [48]. Besides ignoring the non

autogenous conditions of the sample, this type of correction suffers from a number of other

drawbacks:

1) According to the measurements in Figure 6, absorption of water into the samples proceeds at afaster rate in the beginning, because of osmosis and high permeability of the paste (Figure 6). A

linear correction factor does not take this into account;2) Absorption shows a very high variability between samples, as shown both in Figure 6 and in

reference [46]. The variability in absorption is comparable to the measured strain and questions

the use of an average correction factor. To be accurate, one should measure absorption and

volumetric strain at the same time on the same sample, which is very problematic and would

preclude continuous measurements of volumetric autogenous strain;


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17

3) It is uncertain how absorbed water influences the volumetric measurements before and aroundsetting; this is the moment when we are mostly interested in the volumetric measurements, since

linear measurements cannot be carried out at this point. In fact, if there are no capillary pores to

be filled before setting, the absorbed water might have little influence on the buoyancy of the

sample; the weight of the sample would increase, but this would be compensated by an increase

of the volume. Since setting time is difficult to determine and it is a gradual transition, it isuncertain when the correction factor should be applied to the measured strain;

4) More generally, it appears unsound to apply a correction factor that is of the same order ofmagnitude as the measurement results.

Another procedure to correct the volumetric measurements was proposed by Douglas [45, 46]. He

observed that not only did the samples absorb water from the bath during the volumetric test, but

that the latex membranes themselves absorbed water. Finally, by stripping the membrane after the

measurement and weighing both membrane and sample, he found that about 70% of the water was

absorbed into the cement paste and most of the rest remained as a surface layer, while only a very

minor amount was absorbed by the membrane itself. On the basis of these observations, Douglas

[45, 46] proposed to correct the volumetric measurements with multiple and nonlinear correctionfactors, taking into account all this complex behavior. The same considerations advanced in the

previous paragraph apply also to this case, with the further remark that the number of corrections

needed, their order of magnitude, and their uncertainty makes the method extremely difficult to

apply.

The other possible solution of this artifact is to eliminate the absorption of water through the elastic

membrane. The most obvious way is to choose an impermeable membrane. With this aim, Barcelo

[57] measured a number of latex membranes but found that they all were permeable. Douglas [46]

tested a number of different latex, polyurethane and neoprene membranes and all were permeable to

water. Also in the present research, latex and polyurethane membranes were found to have similar

permeability (Figure 4). The lack of tightness of the membranes is due both to their intrinsicpermeability and to the presence of defects such as pinholes. Pinholes would be especially

significant to increase water uptake at early ages, while their importance at later ages would be

lowered by the reduced permeability of the cement paste. Making the membrane thicker, while

improving the tightness, would also increases its stiffness and restrain the cement paste, especially

around setting time [12]. It is not excluded that a satisfactory balance between stiffness and tightness

of the elastic membrane can be achieved; however, this is not the case with the type of membranes

used in the volumetric measurements in the last 15 years, i.e. latex condoms.

A more radical approach to eliminate the water uptake of the samples in the volumetric test is to

substitute the water with a different buoyancy liquid. As documented in the introduction to this

report, volumetric measurements with displacement liquids different than water, i.e. oil or mercury,are not a novelty in the literature [2-4]. However, water has been the displacement liquid of choice in

the last 25 years [14-36]. Douglas [46] performed one single measurement using latex balloons

cured in 100% RH air between the measurements and another measurement using polyurethane

condoms cured in vegetable oil. The reason for not using latex membranes in oil is that latex is

highly unstable in oils: it swells up and rapidly degrades in a matter of a few hours.


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18

Douglas [46] observed first a mass decrease of polyurethane condoms filled with mortar in vegetable

oil in the first day, and then a small increase until 7 d. The total variation was about 0.1%, while the

water uptake of latex condoms filled with mortar in a water bath was about 0.5% in the same period.

The mass decrease of the polyurethane condom filled with mortar was attributed to partial

disintegration of the condom as it reacted with the vegetable oil, which afterwards allowed some oil

to penetrate into the sample, explaining the later weight increase.

In this research, measurements of mass change of polyurethane condoms filled with different cement

pastes and immersed in paraffin oil were performed. The mass gain in the first week of immersion

was about 0.005%, which is about 1/20 to 1/35 the mass change observed in the same period on

latex condoms filled with cement paste and immersed in a water bath. This mass change would lead

to an error on the measured autogenous strain of less than 50 m/m in one week. A few samplesshowed a mass gain much higher, up to 0.03% in 4 days; however, in this case it was easy to verify

the presence of leaks in the membranes by observing darker areas on the surface of the cement paste.

It was also observed that when the measured volumetric strain curve showed abrupt variations, the

samples showed a mass gain and stains on the cement paste due to paraffin oil were also present; in

this case, the results were discarded.

Volumetric measurements of autogenous strain on cement pastes

On the same pastes tested for weight change underwater, the autogenous strain was measured with

three different methods:

1) Measurements of immersed weight of cement paste samples cast in latex condoms and immersed

in a water bath;

2) Measurements of immersed weight of cement paste samples cast in polyurethane condoms and

immersed in a paraffin oil bath;

3) Measurement of length changes of samples cast in corrugated moulds in the dilatometer [37].

Changing the buoyancy liquid from water to paraffin oil eliminates the influence on the volumetricmeasurements of water absorption from the bath into the sample. Linear measurements were

performed in parallel for comparison with the volumetric ones. In all cases, the mass of the samples

before and after testing was measured. Mass change of the samples was generally negligible in the

case of measurements of type 2) and 3), whereas the mass gain of samples tested in a water bath

confirmed the findings in the previous section (Figure 6).

In Figure 8-10, measurements of autogenous strain with the three techniques are presented. In

addition, another curve is calculated adding to the measurements in paraffin oil the error due to

water absorption (Figure 7); this curve shows the effect on the volumetric measurements of the

change of buoyancy due to water absorption.

In Figure 8, measurements are shown for paste A, with w/c 0.3 and 20% silica fume addition.

Volumetric measurements have been converted into linear under the assumption of isotropic

behavior, i.e. dividing the volumetric strains by a factor 3. The measurements have been zeroed at

6 h, corresponding to the setting time. Linear measurements and volumetric measurements in

paraffin oil run parallel for several days, showing an initial expansion followed by shrinkage. The

volumetric measurements in water deviate significantly from the other two from about 12 h, with a


20/26

shrinkage rate about 5 times greater; they also show a higher experimental scatter than the other

types of measurements. If the error due to absorption calculated for samples stored in a water bath

(Figure 7) is added to the volumetric measurement in paraffin oil, a shrinkage rate close to the one

measured in water is obtained.

-3000

-2500

-2000

-1500

-1000

-500

0

500

1000

0 1 2 3 4 5 6 7

Time (days)

A

utogenousstrain(m/m)

Volumetric - Water

Linear

Volumetric - Paraffin oil


+ Absorption

Figure 8. Measurements of autogenous strain with different techniques at 20C on cement paste with w/c

ratio 0.3 and 20% silica fume addition (paste A). Conversion from volumetric to linear strain assumed

isotropic behavior; the curves have been zeroed at setting, 6 h after casting; shrinkage is plotted as negative.

In Figure 9, measurements are shown for paste B, with 0.6% superabsorbent polymers by weight of

cement. Also in this case, results of volumetric measurements have been converted into linear strainassuming isotropic behavior. The measurements have been zeroed at 6 h, corresponding to the

setting time. After an initial shrinkage, the pastes started to expand in all types of measurements.

Linear measurements and volumetric measurements in paraffin oil follow a similar pattern after

setting, though with a slightly greater initial expansion in the linear measurements. Measurements in

water, however, significantly deviate from the other two from about 12 h. They show shrinkage,

whereas according to the other measurements the paste continued to expand for a couple of days.

Moreover, duplicate measurements in water show a considerable scatter in the expansion phase,

probably due to different absorption of the single specimens (compare with the scatter in Figure 6).

The subsequent shrinkage is instead quite similar in the two samples. When the error due to

absorption for samples stored in a water bath (Figure 7) is added to the measurements in paraffin oil,

a shrinkage rate similar to the measurements in water is obtained.

19


21/26

-3000

-2500

-2000

-1500

-1000

-500

0

500

1000

0 1 2 3 4 5 6 7

Time (days)

Autogenousstrain

(m/m)

Volumetric - Water

Volumetric - Paraffin oilLinear


+ Absorption

Figure 9. Measurements of autogenous strain with different techniques at 20C on cement paste with w/cratio 0.3, 20% silica fume addition and 0.6% superabsorbent polymers (SAP) by weight of cement (paste B).

Conversion from volumetric to linear strain assumed isotropic behavior; the curves have been zeroed at

setting, 6 hours after casting; shrinkage is plotted as negative.

-3000

-2500

-2000

-1500

-1000

-500

0

500

1000

0 1 2 3 4 5 6 7

Time (days)

Autogenousstrain(m/m)

Linear


Volumetric - Water


+ Absorption

Figure 10. Measurements of autogenous strain with different techniques at 20C on cement paste with w/c

ratio 0.35 (paste C). Conversion from volumetric to linear strain assumed isotropic behavior; the curves havebeen zeroed at setting, 3.25 h after casting; shrinkage is plotted as negative.

20


22/26

21

Figure 10 shows measurements of autogenous strain on paste C, with w/c 0.35. Also in this case,

volumetric measurements have been converted into linear strain assuming isotropic behavior. The

measured strain has been zeroed at setting, corresponding to 3.25 h; notice that in this case setting

occurred earlier than in the other two cement pastes because no superplasticizers were used. Linear

measurements and volumetric measurements in paraffin oil show some differences in the first day

after casting, while the measured shrinkage rate in the following days is almost identical. Themeasurements in water deviate from the other two from about 12 h; after a first short expansion,

shrinkage is measured, whereas according to the other measurements the paste continued to expand

for a couple of days. Moreover, the measurements in water show a considerable scatter, probably

due to different absorption of the single specimens (Figure 6). Also in this case an additional curve is

shows the measurement in paraffin oil plus the error due to absorption in samples stored in water

(Figure 7); the calculated curve follows closely the volumetric measurements in water, included the

first short expansion and the following shrinkage rate.

Measurements in Figures 8-10 show a number of common features. Linear measurements performed

with the dilatometer [37] and volumetric measurements in paraffin oil fundamentally agree after

some differences in the first few hours. Moreover, their results are in qualitative agreement with RHmeasurements: for example, paste B shows almost no lowering of the internal RH in the first week

(Figure 5) and this agrees with the negligible shrinkage measured both by the linear technique and

by the volumetric technique in paraffin oil (Figure 9). The same considerations apply for the start of

self-desiccation of paste A and C, which corresponds to the onset of shrinkage according to the

linear technique and the volumetric technique in paraffin oil. Moreover, both linear measurements

and volumetric measurements in paraffin oil are quite reproducible and show little scatter throughout

the measuring period. This good agreement, obtained for 3 cement pastes with very different

autogenous strain behavior, confirms the validity of both techniques.

On the contrary, the volumetric measurements in water fundamentally disagree with the other two

types of strain measurements and with the autogenous RH measurements. The measured shrinkage issystematically higher than for the other two techniques. This agrees with previous data in the

literature about the volumetric technique in water bath [38, 39, 48]. As explained in the previous

paragraphs, absorption of water through the elastic membrane into the samples is a principal cause

of this discrepancy [38, 46]. In Figures 8-10, the estimated error due to water absorption (Figure 6)

was added to the volumetric measurements in paraffin oil; this calculated curve followed closely the

volumetric measurements in water bath. In particular, a very good agreement is obtained with the

shrinkage rate after a couple of days: this shows that almost all the shrinkage measured with the

volumetric method in water is an experimental artifact due to water absorption into the sample.

Moreover, volumetric measurements in water show a poor reproducibility in the first hours or days,

while the strain rate at later ages is more similar between the samples. Similarly, mass gain of the

samples stored in water showed a high scatter especially in the first period, which may be due todifferent permeability of the elastic membranes. The high initial scatter in the volumetric

measurements in water is another consequence of the water absorption through the membrane, which

varies from sample to sample especially in the first hours. Considering the high variation in the

results of the volumetric measurements in water bath, the strategy of correcting those measurements

using a coefficient calculated from the average measured absorption [38, 46] does not appear

practicable neither convenient. In fact, volumetric measurements in paraffin oil show a better


23/26

22

reproducibility, they do not need calculation of any correction factor, and finally they do not alter the

curing condition of the sample.

Some brief additional comments about the differences between the linear technique and the

volumetric technique in paraffin oil in the first hours after setting are listed in the following

paragraph. In the expansion phase measured for pastes B and C (Figures 9 and 10), the minimum ofthe strain is shifted at later ages in the volumetric measurements and the expansion is lower; after the

first expansion phase, however, the measured strain is very similar. Causes of these differences are

possibly a result of the well-known artifacts of both techniques:

- Reabsorption of bleed water may lead to a measured additional shrinkage in the volumetric method

that may partially override the initial expansion of the paste. On the other hand, it may lead to an

increased expansion in the linear technique. Bleeding might have occurred in paste C (Figure 10),

but it is unlikely for paste B, with low w/c, silica fume and SAP (Figure 9);

- Absorption of air bubbles and suction of the membrane into the small irregularities on the surface

of the volumetric sample would be a relevant mechanism for both pastes B and C. Observation of

volumetric samples after the test showed that indeed this had occurred. This artifact would result in

apparent extra shrinkage that overrides the initial expansion;- In the volumetric method, the pressure of the elastic membrane [13] acts on the weak interface

layer between membrane and pastes, whose deformation should be much greater than the ones of the

bulk.

All these mechanisms should be especially relevant in the period around and immediately after

setting, which corresponds to the period when expansion is measured. Their relevance in the later

period would be minor.

ConclusionsIn this report, absorption of water from the buoyancy bath through the elastic membrane is identified

as the principal artifact of the volumetric method and cause of most of the discrepancies between

volumetric and linear measurements. Water absorption is driven by a lowered water activity in thecement paste, due to dissolved salts and to self-desiccation. The importance of osmosis through the

elastic membrane was shown by measuring mass change of membranes filled with synthetic pore

solution and immersed in water. Water is absorbed into the cement paste sample at a high rate from

the moment of immersion and the error it produces on the measured strain is of the same order of

magnitude as the autogenous strain itself. All measurement results obtained with the volumetric

technique using latex condoms to encapsulate the sample and a water bath [16-35] should therefore

be considered with suspect, due to absorption of water into the sample. Notice that none of these

papers reported the mass of the samples before and after the test. Moreover, all papers that make use

of these results should be questioned as well. The strategy of limiting the measurements to the first

day of hydration [33, 35, 41, 43] does not solve the problem, because absorption in that period is a

significant source of error. In all volumetric measurements, the mass of the sample before and aftermeasurement should be measured to verify absorption of the buoyancy liquid into the sample or

possibly dissolution of the membrane; if a mass change is observed, the whole measurement should

be disregarded.


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23

An artifact resulting from water absorption through the membrane is very difficult to compensate

[38, 46], especially because of high scatter in the absorption of different samples. Even if correction

factors are applied, discrepancies between linear and volumetric measurements results do not appear

to be fully eliminated [38]. In this paper, absorption of water through the membrane was avoided by

paraffin oil as buoyancy liquid. Samples weighed in a paraffin oil bath did not show any appreciable

mass change in the testing period of one week. It was shown that adding the measuring error due tothe absorption to the measurement results in paraffin oil, a shrinkage rate much higher and similar to

the one measured in water was obtained. This confirmed that, in volumetric measurements in water,

the measurement results are dominated by the effect of water absorption through the membrane.

The volumetric autogenous strain of three cement paste measured in a paraffin oil bath was in good

agreement with measurements performed with a linear technique, the dilatometer [37]. Some minor

differences between the two measuring techniques in a couple of hours around setting can be object

of further research.

Acknowledgements

This study is part of a 3-year project dedicated to examine measuring methods for autogenous strain.The funding by the Danish Technical Scientific Research Council (STVF) is gratefully

acknowledged.

Portland Cement Association was not a sponsor of this research for SN2925. The contents of thisreport reflect the views of the authors, who are responsible for the facts and accuracy of the datapresented. The contents do not necessarily reflect the views of the Portland Cement Association.

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