Precise Permeability Measurement for High Strength and Ultra Low PermeabilityConcrete under Controlled Temperature+1

Masaji Kato1, Yoshitaka Nara2,+2 and Kazutoshi Shibuya3

1Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan2Graduate School of Engineering, Kyoto University, Kyoto 615-8540, Japan3Taiheiyo Consultant Co. Ltd., Tokyo 103-0004, Japan

High strength and ultra low permeability concrete (HSULPC) is being considered as a material used to package transuranic (TRU) wastefor disposal in geological repositories. Therefore, information on the permeability of HSULPC is essential. Permeability tests need to be highlyaccurate to determine the hydraulic conductivity of HSULPC because of its ultralow permeability. In our study, we measured the permeability ofHSULPC samples using the transient pulse method. The temperature of the concrete was finely controlled and held constant. The hydraulicconductivities were determined from the measurements to be around 10¹13 to 10¹12m/s for confining pressures between 2 and 10MPa. The porepressure was a constant 1MPa. The results further showed that the permeability of HSULPC had a hysteretic dependence on the effectiveconfining pressure. We found that the hydraulic conductivity of HSULPC is comparable to or less than that of intact Toki granite obtained fromGifu Prefecture in central Japan. It was also considered that the hydraulic conductivity of HSULPC stabilized at around 10¹13m/s after beingburied and stressed. The high density and impermeability of HSULPC would enable it to effectively confine 14C radionuclides found in TRUwaste. [doi:10.2320/matertrans.Z-M2020846]

(Received July 3, 2020; Accepted August 5, 2020; Published October 25, 2020)

Keywords: high strength and ultra low permeability concrete, permeability, transient pulse method, radioactive waste disposal

1. Introduction

An important aspect of geological disposal of radioactivewaste is the confining ability of the disposal system. Forengineered or natural rock mass barriers, this ability mainlyconsists of retardation of migration and adsorption ofradionuclides by clay (specifically bentonite).1,2) Conse-quently, the material properties of clay and rock areessential. However, transuranic (TRU) waste often containsradionuclides, which are difficult for engineered or naturalbarriers to absorb such as 129I and 14C. To ensure that theconfining ability of the disposal system works well, analternative disposal technique has been suggested for 129Iand 14C, which are the main radionuclides in the safetyevaluation of radioactive waste disposal.3) Long-termconfinement of radioactive waste has been proposed as analternative technique for 14C.4)

Figure 1 schematically illustrates one option for the long-term confinement of radionuclides using a cementitiousmaterial. This concept proposes using High Strength andUltra Low Permeability Concrete (HSULPC) to retardground water migration. In this concept, radioactive waste(TRU waste) is stored in a canister inside a metal box.HSULPC is then used to cover the metal box containingthe canister with TRU waste. Due to the high confiningability of HSULPC, groundwater cannot penetrate the wasteover the long term (i.e., 60,000 years), which correspondsto ten-times of the 14C half-life.4) In this alternative concept,the permeability and time-dependent properties of crackpropagation and closure should be investigated.

Nara et al.5,6) studied the influence of the surroundingenvironment on the crack velocity by measuring subcritical

crack growth in HSULPC. The crack velocity increasedsignificantly in water.5) Additionally, neither the temperaturenor the humidity had a negligible influence on the crackvelocity in HSULPC in air.6) Fukuda et al.7­9) examinedcrack closure in HSULPC kept in artificial sea water. Crackclosures occurred significantly at the end of the sampleaccording to X-ray CT observations.7) Moreover, crackclosure was easier for a smaller crack aperture.8) Scanningelectron photomicrographs revealed that the precipitation ofcalcium carbonates strongly affected the crack closure ofHSULPC in water.9)

Various studies have investigated crack propagation andclosure in HSULPC. The permeability properties ofHSULPC have also been studied,10,11) but permeabilitymeasurements are difficult using conventional methods.12)

Large scattering occurs because HSULPC is dense and hasan extremely low permeability.13,14) Therefore, HSULPC

Fig. 1 Schematic illustration of alternative concept using HSULPC toradioactive waste packaging.5)

+1This Paper was Originally Published in Japanese in J. Soc. Mater. Sci.,Japan 69 (2020) 263­268. Acknowledgement is added.

+2Corresponding author, E-mail: nara.yoshitaka.2n@kyoto-u.ac.jp

Materials Transactions, Vol. 61, No. 11 (2020) pp. 2134 to 2138©2020 The Society of Materials Science, Japan

requires precise permeability measurements via a methodapplicable to low-permeability materials.

This study investigates the applicability of permeabilitymeasurements for HSULPC using the transient pulsemethod.15) In particular, we conduct permeability measure-ments with the transient pulse method by minimizing thechange in the temperature of the surrounding environmentand evaluate the permeability precisely. Considering theinfluence of the external load applied on radioactive wasteafter backfilling, the influence of the pressure on thepermeability of HSULPC is also investigated. In addition,the permeability of HSULPC is compared to that of granite,which is a typical low-permeability rock material, tounderstand the low-permeability property of HSULPC.

2. Sample Material

The sample material was HSULPC, which was preparedaccording to the guidelines published by the ConcreteCommittee of Japan Society of Civil Engineers.16) Table 1summarizes the composition. HSULPC used in this studywas made by placing concrete in a mold, steam curing ata temperature of 363K for 2 days, and subsequently holdingfor 2 days in air at a temperature of 293K. The relativehumidity during steam curing was 99%.

For the measurements, we used the HSULPC sampleafter holding under ambient air conditions. It had P-wavevelocities in the three orthogonal directions of 4.98, 5.06, and5.08 km/s. The uniaxial compressive strength and the tensilestrength determined by the diametral compression test were203MPa and 10.9MPa, respectively.5)

Figure 2 shows the cylindrical specimen used in thepermeability measurements. Its diameter and length were50mm and 25mm, respectively. Prior to the permeabilitymeasurements, the specimen was saturated by placing indistilled water under vacuum conditions for several days.

3. Experimental Method

3.1 Permeability test systemFigure 3 shows the permeability test system. It consisted of

a temperature-controlled chamber with triple thermal insu-lation walls and an air conditioner set in the outermost placeof the temperature-controlled chamber.17,18) The pressurevessel, including the specimen, was set in the innermostchamber, which lacked heat and light sources. Figure 4 showsa photo of the inside of the temperature-controlled chamber.The temperature change around the pressure vessel wasaround 0.1°C during the permeability measurements.

3.2 Experimental procedureAlthough the permeability test system could support

different testing methods for the measurements, this studyadopted the transient pulse method because HSULPC is a

Table 1 Composition of HSULPC.5)

Fig. 2 Photo of cylindrical specimen of HSULPC for permeability test.The diameter and height of the specimen are 50 and 25mm, respectively.

Fig. 3 Schematics of permeability test system. (IC: Triple insulatedchamber, AC: Air conditioner, RT1-3: Resistance thermometer 1 to 3,BR: Barometer, PT: Pressure transducer, DPT: Differential pressuretransducer, PV: Pressure vessel, ER: Extra reservoir, UL: Upstream line,DL: Downstream line, SV: Separation valve, PV: Pressure pulse valve,EP: Evacuating port, AP: Air discharge port, WP: Water supply port, SC:Specimen, EC: End caps, HT: Heat shrinkable tube, DP: Double plungerpump, SP: Syringe pump, CU: Controlling unit for syringe pumps, LG:Data logger, PC: Laptop computer)

Precise Permeability Measurement for High Strength and Ultra Low Permeability Concrete under Controlled Temperature 2135

low-permeability material. A previous study demonstratedthat the transient pulse method is suitable for measurementsof low-permeability materials.15)

The experimental procedure of the transient pulse methodin this study involved the following steps:(1) The specimen saturated by distilled water was set in

the pressure vessel between two end pieces, whichprovided the flow of the pore fluid, and covered itwith a jacket. Here, the jacket was a heat shrink tube.This jacket prevented the flow of the pressurizingmedium into the specimen when a confining pressurewas applied. Then the specimen placed between theend pieces was set in the specimen closure connectedwith the line of the pore fluid.

(2) All the lines of the water flow and the extra reservoirwere filled with distilled water. A vacuum pumpremoved the air, and a syringe pump applied waterto the lines of water flow while maintaining a constantpressure.

(3) During a measurement, the temperature around thepressure vessel in the innermost chamber was constant.Additionally, we checked for leaks by monitoring theamount of water flow from the syringe pump.

(4) After confirming that the system was not leaking, theconfining and pore pressures were set. The temperaturefluctuation was less than 0.1°C. The confining pressurewas set in the range of 2­10MPa. The pore pressurewas 1MPa. After setting the confining pressure andpore pressure to the predetermined values for eachmeasurement, we waited for several hours until thepressure values settled and remained constant. In thisstudy, the confining pressure was initially increasedfrom 2MPa to 10MPa. Then it was decreased from10MPa to 2MPa to investigate the hysteresis of theresults of the permeability measurements.

(5) The separation valve was closed to divide the upstreamand downstream lines of water flow. Then the

fluctuation in the pore pressure difference of theupstream and downstream was monitored to detectwater leakage. If leaks were detected, we returned tostep (3). After closing the valve, several hours werenecessary for the water head distribution in thespecimen to become uniform.

(6) If there was not an observable fluctuation in theupstream and downstream pore pressure difference, apore pressure pulse (25­100 kPa) was applied to theupstream side of the specimen. The pore pressuredifference decreased with elapsed time after applyingthe pore pressure pulse. The measurement was finishedonce the pore pressure difference approached the valueof the difference before applying the pore pressurepulse. For confining pressures of 2, 3, and 4MPa, eachmeasurement using the transient pulse method wasrepeated twice. For other confining pressures, eachmeasurement was conducted once.

(7) As suggested in Refs. 15), 19), the nonlinear leastsquare method by applying the solution of the transientpulse method was used to evaluate the hydraulicconductivity. The temporal change of the pore pressuredifference was obtained in the measurement accordingto

�hðtÞH

¼ exp �KAt

l

1

Suþ 1

Sd

� �� �ð1Þ

where t [s] is the time after applying the pore pressurepulse, ¦h(t) [m] is the difference of the upstreamand downstream water heads, H [m] is the water headpulse (the initial difference between the upstream anddownstream water heads), l [m] is the specimen length,A [m2] is the cross-sectional area of the specimen, K[m/s] is the hydraulic conductivity, Su [m2] is thecompressional storage of the upstream reservoir, and Sd[m2] is the compressional storage of the downstreamreservoir. Su and Sd were determined by the methodproposed by Zhang et al.20) In this study, both Su and Sdwere 8.5 © 10¹10m2.

4. Results

For the transient pulse permeability test, the hydraulicconductivity was evaluated using the temporal change ofthe pore pressure difference between the upstream side andthe downstream side of the specimen. The data from thedifferential pressure indicated a change in the temperature ofthe surrounding environment. In addition, the difference ofthe thermal expansion or contraction in the reservoirs andwater flow lines between the upstream and downstream alsoinfluenced the pore pressure difference data. Consequently,the data of the temporal change indicated disturbances in thepore pressure difference. Figure 5 shows an example of thetemporal change of the pore pressure difference, where theconfining pressure, pore pressure, and pore pressure pulse are2MPa, 1MPa, and 28 kPa, respectively.

Figure 6 shows the temperature changes in the outermostand the innermost places in the temperature-controlledchamber during a measurement. For the temperature in theoutermost place (measured with the resistance thermometer

Fig. 4 Photo of triple insulated chamber.

M. Kato, Y. Nara and K. Shibuya2136

“RT1”), the influence of the air conditioner appeared as aperiodic change in the temperature. On the other hand, thechange of the temperature in the innermost place (measuredwith the resistance thermometer “RT3”) was around 0.1°C,indicating highly accurate permeability measurements andevaluation of the hydraulic conductivity in this study.17)

Figure 7 shows the hydraulic conductivity of HSULPCevaluated under various effective confining pressures. Thehydraulic conductivity of Toki granite is shown for

comparison. HSULPC has a hydraulic conductivity valueon the order of 10¹13­10¹12m/s. The hydraulic conductivityof HSULPC decreased as the effective confining pressureincreased during the loading confining pressure procedure.The hydraulic conductivity measured for HSULPC showeda hysteresis because the procedure to unload the confiningpressure was lower than that of the loading confining pressure.

5. Discussion

Various studies have investigated crack propagation andclosure using HSULPC.5­9) Since the crack propagation inHSULPC accelerates in water, the formation of water flowpathways by the crack propagation must be avoided if theradioactive waste repository is submerged by ground water.In contrast, since crack sealing by mineral precipitationshas been observed in aqueous conditions, HSULPC is asuitable material to confine radioactive waste. Therefore, thepermeability measurement of HSULPC is meaningful.

In general, a permeability test using a low permeabilitymaterial does not provide accurate measurement results dueto the long measurement time and significant influence ofthe temperature change in the surrounding environment. Thetransient pulse method typically has a shorter measurementtime than other methods, minimizing the impact of thetemperature change. Using the transient pulse method toconduct permeability measurements under conditions wherethe temperature change is small, the hydraulic conductivitycan be evaluated with a high accuracy. The change of thetemperature at the innermost place in the temperature-controlled chamber is around 0.1°C (Fig. 6). By conductingthe permeability test with the transient pulse method witha small temperature change, the hydraulic conductivity ofHSULPC is measured successfully.

Figure 7 shows the hydraulic conductivities of granite aswell as those of HSULPC. The hydraulic conductivities ofgranite are those previously reported for an intact sample ofToki granite by Nara et al.21) The hydraulic conductivity forHSULPC is similar or less than that for intact Toki granite,indicating that HSULPC is a dense and low permeabilitymaterial, which can effectively confine radionuclides.

The existence of cracks and pores remarkably affects thepermeability22) because they provide a pathway for waterflow. Introducing an open crack in a dense material increasesthe hydraulic conductivity significantly,23­25) whereas closinga crack by pressure or mineral precipitation decreases thehydraulic conductivity.26­30) Although the hydraulic con-ductivity of HSULPC is quite low, it is important toinvestigate the water flow pathway in HSULPC.

We conducted microscopic observations of a thin sectionof HSULPC. HSULPC contains a few microcracks andisolated pores. Figure 8 shows a photomicrograph with amicrocrack in the central part. The aperture and aspect ratioof this microcrack are 1 µm and 10¹3, respectively.

Generally, a higher pressure is necessary to close cracksand pores mechanically as the aspect ratio becomes higher.31)

However, materials containing numerous pores and a highaspect ratio do not exhibit a detectable change in thepermeability even as the applied pressure is increased.32,33)

In this study, HSULPC has a decreased hydraulic con-

0

10

20

30

0 5000 10000 15000 20000

Diff

eren

tial p

ress

ure

(kPa

)

Time (s)

Confining pressure: 2 MPaPore pressure: 1 MPa

Fig. 5 Decline curve of differential pressure during transient pulsepermeability test.

Fig. 6 Temperature variations in air-conditioned room during permeabilitytest corresponding to Fig. 3.

0 2 4 6 8 1010-14

10-13

10-12

10-11

10-10

10-9

Effective confining pressure (MPa)

Hyd

raul

ic c

ondu

ctiv

ity (m

/s) : HSULPC

: Toki granite

Loading

Unloading

Fig. 7 Hydraulic conductivities of HSULPC comparing to those of intactToki granite.

Precise Permeability Measurement for High Strength and Ultra Low Permeability Concrete under Controlled Temperature 2137

ductivity due to the closure of the microcrack with a lowaspect ratio (Fig. 8).

HSULPC shows a similar hysteresis as that for rockmaterials.34) After placing a TRU waste package usingHSULPC, backfilling an underground repository of radio-active waste was conducted using buffer and backfilledmaterials. In this case, the following factors should beconsidered: the pore pressure increases due to the increasedgroundwater level, the overburden pressure due to back-filling, the swelling pressure generated from buffer materialsand the backfill materials. To assess these factors an externalpressure was applied on the TRU waste package usingHSULPC place in the underground repository for radioactivewaste. The hydraulic conductivity of HSULPC is on the orderof 10¹13m/s due to the effect of the external pressure.

6. Conclusion

The transient pulse method using HSULPC realizes ahighly precise permeability test when the confining pressureis 2­10MPa and the pore pressure is 1MPa. The hydraulicconductivity of HSULPC is on the order of 10¹13­10¹12m/s.This value is similar or lower than that of intact granite. Thehydraulic conductivity measured in HSULPC shows hys-teresis. The hydraulic conductivity decreases as the confiningpressure increases (Fig. 7), although it is a low value eventhough the confining pressure decreases.

HSULPC is suitable for the long-term confinement of 14Cdue to its density and low permeability. After placing a TRUwaste package using HSULPC in the repository, backfillingof the underground repository of radioactive waste wasconducted using buffer and backfilled materials. Assumingthat the pore pressure increases with the groundwater level,the overburden pressure is due to backfilling, the swellingpressure is generated from buffer materials, and the backfillmaterials are known, applying an external pressure on theTRU waste package using HSULPC results in the hydraulicconductivity of HSULPC around the order of 10¹13m/s.

Acknowledgment

We appreciate the support from Supporting Program forInteraction-based Initiative Team Studies (SPIRITS) ofKyoto University for English proofreading.

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M. Kato, Y. Nara and K. Shibuya2138