Absorption determination method i Possibilities and limitations

An important piece of the puzzle in evaluating effect of IC is to assess absorption capacity of SAP and thus to verify whether the material carries all the fluid added for IC reasons. The task is not new. To date, however, mainly because of complex sorptivity behaviour of SAP as well as limited number of methods to test SAP absorption which underwent critical review, with only few in concrete [Ass 13][Ass 14], it remains chink in armour for successful internal curing optimization.

Many of methods known mainly from industrial experiences as being summarized in Appendix E cannot be implemented for the goals of this study. This relates to the fact that either no load is or can be applied during measurement or provision of the required and often changeable conditions during absorption is not feasible. Furthermore, testing multiple samples differentiated by size and where the load would be distributed uniformly or the opposite (e.g. as on exposure to shear) also appears to be the apparent difficulty of the methods proposed.

Concluding, knowing absorption capacity of IC agent after it has been already introduced into concrete would be an advantage of solution found. Only in such conditions all factors controlling sorption behaviour of SAP (i.e. exposure to one or more modes of shear/compression as well as varying composition and ionic strength of pore solution) would be in play.

With this in mind, some non-destructive methods and stereology-based approaches could become of interest. Summary of various methods including Neutron Tomography, Nuclear Magnetic Resonance, Computer Tomography, calorimetry tests and evaluation of cross- section of hardened samples has been recently made in [TC 225-SAP]. All other disadvantages of particular approaches aside, their utilization though mostly scientifically correct would not be handy if numerous tests were to be performed. This becomes understood when acknowledging that with amount and type of SAP being held constant, absorption changes with variation of concrete composition, see Section 4.3.3 for example. Another argument against some method application is yielded in case when both shape and size of pores ascribed to air and accommodated SAP particles are similar (the present case!), therefore where demand of production with vacuum appears as necessity to run the estimation e.g. the case of stereology-based approaches. Still, some of the methods could be applied for other as important purpose, viz. tracing behaviour of IC agent in concrete and thus for verifying its applicability for IC, see Section 5.3-5.4 and Appendix H.

As alternative, it has been suggested [TC 196-ICC] that empirical tests on fresh concrete could be a useful tool for the purpose of absorption capacity estimation. Both Mönnig [Moe 09] and Assmann [Ass 13] including group work [Ass 14] carried out comparison of slump flow as measured at different ages for cement-based mixtures without and with IC. A certain difficulty of the method proposed yet considered as necessary to avoid formation of clumps was using additional mixing time within the intervals between the measurements. If applied especially to viscous UHPC, it could mean possibility of introducing extra amounts of technological air31, impacting workability in similar manner to clumps, i.e. rather negatively that positively. Furthermore, when mixing (and thus shearing of particles) is continued, some water could be potentially lost from SAP (due to solvent release, solvent detachment e.g. [Ver 03]) leading to increase of effective w/c. This adding to specific behaviour of SAP, which

31 _{It is recalled that SAP-enriched mixtures did generally contain higher content of air compared to control}

mixes, see Section 4.1.2. Using extended mixing in the intervals between measurements would therefore increase the concern.

might be releasing water prematurely and independently of load, a resultant increase of slump flow could be expected. That is all to say, there is no guarantee that test for absorption capacity is finished within reasonable frame of time. E.g. fluidity loss was followed for UHPC without and with IC in study of Huang and Wang [Hua 12]. In contrast to control mix, lack of stable trend in slump flow changes until the age of 120 minutes (i.e. the end of test) was reported for one of tested mixtures with SAP.

It was thus concluded that in cases like present only the maximum absorption capacity and not equilibrium swelling of SAP could be assessed from the slump flow measurement. In stating so, it is also assumed that any changes occurred subsequently to time of obtaining the characteristic value is to be interpreted as premature effect of IC, having impact e.g. on capillary porosity that forms.

ii. Solution, establishments and comments

Approach based on test of consistency was applied in own study. Consistency of mixtures containing the IC agent was adjusted by using extra water during mix production but only in amount strictly needed for SAP to attain its maximum absorption capacity. Experimentally, this meant that slump flow comparable to that of F-R was aimed at for IC-incorporating mixtures. The choice to follow particular approach (with only one measurement of consistency per composition) was made having knowledge of both: specific sorption characteristic of SAP under investigation and earliest possible times of the spread record. Both of them and the comparison of their results revealed to be very favourable for the goal of finding maximum absorption capacity of SAP under investigation.

Firstly, the tea-bag test results (Figure 4.26) have been analysed. The investigation for which cement paste filtrate had been used yielded information about shortest swelling time until attaining the maximum absorption for particular SAP. This is in contrast to tea-bag tests with distilled water which, as can be followed based on Section 2.5.1, should lead to revealing the longest swelling time. With some further adjustment in the interpretation of results (e.g. arbitrary shifting of absorption peak to nearest maximum value since having similar value to that of peak)32,33, important observation was made: It was revealed that despite some changes

32 _{It should be pointed out that maximum of absorption when the concrete is already being mixed is likely to be}

acquired later than in free conditions due to exposure to shear loading, otherwise leading to at least partial water release from polymer at any stage of its absorption [Zan 02]. This implies that further limitation of left ‘window of absorption’ borderline but being ascribed to swelling in pore solution should result purely from behaviour of SAP itself. Please see later discussion in the Section.

in absorption depending on solvent type, maximum absorption for SAP studied falls into very limited time-frame, which herein is proposed to be referred to as ‘window of absorption’. This ‘window of absorption’ should be expected to differ from one IC agent to another.

Secondly, realistic times of the spread records were assessed. This time was comprised of mixing times (‘wet mixing’; changing with mixture type, see Section 3.2.6), extra time taken to start the slump flow measurement and one needed for fresh concrete to flow until stop. After new information was obtained, comparison of both pictures regarding time was made.

0 10 20 30 40 50 60 70 80 0 10 20 30 40 50 60 Time [min] F lu id a b s o rb e d [ g /g S A P ] distilled water cement paste filtrate 0 4 8 12 16 20 Time [min]

Figure 4.27 shows results of regarded comparison. A clear overlap was found. This meant that estimation of maximum absorption capacity in particular conditions could be performed in first measurement after mixing. This protocol was applied in Section 4.3.3. It is belief of the author that in future ‘window of absorption’ could be a very useful method for making the choice about slump flow record time or otherwise to decide whether method itself is fit for assessing the absorption in case of rival SAP material chosen.

The remaining question is the role of two simplifications made in the approach with regards to the tea-bag result viz. lack of externally applied load and other pore solution composition. For the moment, the following assumptions can be made with respect to issues not accounted for in measurement:

33 _{Absorption rate of SAP in artificial pore solution (i.e. similar to that met in concrete) has been showed to be}

higher than in cement paste filtrate [Pou 13], the latter being used for own measurements. This again limits the ‘window of absorption’ and makes the estimation more precise.

Figure 4.26: Determination of ‘window of absorption’

from tea-bag test results.

Figure 4.27: Comparison between slump flow testing

times and time range (‘window of absorption’) in which studied SAP attained maximum absorption.

Window of absorption

the earliest measured slump flow (e.g. for F-R-mm, small cone slump flow

at the latests measured slump flow (e.g. for Ff-R, with custom-built set-up)

- the role of the load

In concrete, this term should be associated with impacts related to mixing as well as the outcomes of gravity, and/or perhaps compaction, if applied [Lee 10]. Such processes bring about added stresses on polymer phase, this including shearing and compressive stresses. Should they be present, there will be shift of the (Gibbs) free energy minimum in SAP towards a new position, see Figure 4.28. It is the type of load as well as state of swelling [Buc 05] which decides about/determines the course of this change and the point in time when the new equilibrium is attained.

Purely theoretically, effect of exposure to shear and compression on volume can be expected as negative i.e. the size of SAP should be smaller compared to case without load. Applied to own case, however, some details regarding extent of exposure and the exposure time clearly differ the individual impacts.

In case of shear, the outcome depends on loading rate applied and theoretically can result in more phenomena [Zan 02][Ver 03]. Ones being relevant in respect to absorption include reversible slow release of solvent (being hindered by simultaneous molecule attraction) and permanent release of liquid once absorbed for the low and high strains applied, respectively. However, these were also reported to hold true after SAP has already swollen and carried appreciable amounts of liquid (in trend: the higher the swelling the higher potential solvent release) [Zan 02][Ver 03]. This is different for the case of mixed UHPC with IC. Considering that: 1) absorption takes place in pore solution, meaning relatively small amount of fluid absorbed by the IC agent, 2) uptake proceeds at appreciable rate for SAP under investigation while 3) passage of time until slump flow record is sufficient to cover any loss of fluid since ‘wet mixing’, low likelihood of effect or otherwise its negligible impact on maximum absorption capacity as well as time required for it could be hypothesized.

Figure 4.28: Schematic of a shift of the minimum of

the free energy (F) caused by imposed deformation, and the resultant change in volume (V) [Ura 12].

Stretched V'' V0 V' V F Compressed No load

In comparison, the effect of compressive load on swelling of ionic gel (polyelectrolyte) could be addressed using chemical potentials (= pressures) instead of energy acc. to Eq. 4.2:

(

)

(

)

gel π π π π π π π

π = + − = + + − (4.2)

where πmix is the osmotic pressure difference due to entropy and enthalpy of the mixing of the

polymer network and water (i.e. osmotic contribution reflecting polymer-solvent interactions), πel is the osmotic pressure difference due to rubber elasticity of the polymer network (i.e.

elastic contribution associated with chain deformations in the network and determined by the chain conformation properties) and πion is the osmotic pressure difference due to counterions

of the polymer (i.e. contribution from ion-solvent mixing and electrostatic effects).

Based on equation, it is evident that exposure to compressive load makes the static pressure inside gel to increase in contrast to osmotic pressure (of counterions [Ver 05][Gan 10]), undergoing the opposite change. This is accompanied by decrease in entropy and, as stated in [Gan 10], reduced repulsion between monomers caused by van der Waals interactions. The result is reduced volume and diminished sorption capability, see Figure 4.29.

Last information implies that when compression is applied at least some difference in the rates of swelling compared to behaviour without load could plausibly result as well. However, whether this turns into delay of absorption capacity approach time, this is known to be decided by more factors. The rule of thumb is it is network porosity which governs the main absorption mechanism [Gan 10], and consequently decides about the absorption rate as well [Zoh 08][Omi 12]. In fact, any limitation of transport pathways would be disadvantageous from perspective of the latter34. On the other hand, substantial difference in sorption behaviour presumably emerges also from mode of swelling allowed that can be either uniaxial

34 _{It should be pointed out that the longer the SAP absorption in concrete lasts, the lower likelihood that capacity}

attains the maximum which could be attributed to competition for water molecules (diminishing potential of absorbing fluid free of any interactions) and/or their entrapment. This follows the hypothesis of Jensen and Hansen [Jen 02] claiming restricted absorption of large SAP particles. It is another argument why according to present author the load effect is insignificant when SAP absorption determination time is concerned.

Figure 4.29: Predicted effect of external load on degree of

swelling. Based on [Dub 94][San 04].

Sw e lli n g d e g re e Pressure applied Free swelling

or three-dimensional [Dub 94]. Among other reasons (e.g. particle size distribution and density of functional groups being another pieces of puzzle), these could be used to explain why both lack of effect [Bud 97] and, presenting the worst case scenario, pronounced delay [Sal 10] of equilibrium swelling acquire time have been reported.

In respect to last aspect, it should be recalled that tested SAP were characterized with low functional group density, leading to much lower absorption in distilled water compared to SAP of [Sal 10]. For such SAP, osmotic compression might be as important, if not more, as external load. Therefore, the delay that in [Sal 10] related to attaining small percentage of total achievable absorption capacity (less or much less than 25 %) was considered as being too little to change the picture observed in swelling measurements. Since in mixing of UHPC with IC load is also exerted from mixing start35, impact of compressive load is highly unlikely to have retardation effect.

- the role of the ionic concentration

It is recalled that tested SAP is a polyelectrolyte. This means that, among other contributions, the swelling pressure constitutes of additional ionic component (see Eq. 4.2) that strongly depends on the ionic composition of solution. The rule of thumb is swelling pressure will rapidly decrease in response to increase in the concentration of ions, e.g. [Dub 94].

In fresh concrete the pore solution produced from gradual dissolution of reactive components, regardless whether modified by chemical admixture of not, undergoes continues changes. Still, at any stage of development, its ionic concentration can be assumed to be of lower magnitude compared to that of the cement filtrate as used in tea-bag tests. This should be attributed to the low w/c of UHPC (bringing about lower concentration of e.g. Ca2+ per g of cement [Hos 09]) and presence of PCE as well as silica fume (‘capturing’ Ca2+ ions present in pore solution, see [Cha 81][Pla 09] and [Oga 80][Pla 09], respectively). It directly translates into lower artificial, isotropic compressive load [Hor 88][Dub 94], also known as osmotic compression. On one hand, the consequence is that peak of absorption appears somewhat later than that recorded in cement paste filtrate and narrower ‘window of absorption’. On the other

35 _{It should be pointed out that swelling under load of equal magnitude may [Dub 90] but needn’t be [Hor 88]}

affected depending whether three- or one-dimensional mode of network swelling is allowed. The decisive factors for observation are, among others, size of polymeric sample [Dub 89], and, assumed by present author, the state of swelling when loading is applied as well as type of change considered (e.g. volume and not absorption time!). This would set another new challenge in accounting for the effect of load in concrete. For simplicity, this effect is not discussed further.

hand, because chelating of ions still present takes place, thus restricting absorption, importance of shearing and compression in selfsame respect becomes diminished. Again, this would be apparent advantage for the IC water estimation proposed.

4.3.3 Investigations on consistency of fresh concrete