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Long term behavior of concrete in nuclear waste repositories
Nuclear Engineering and Design 138 (1992) 157-164 North-Holland
157
Long term behavior of concrete in nuclear waste repositories F. Jacobs a n d F.H. W i t t m a n n
Institute for Building Materials, ETH-Ziirich, Switzerland Received 20 July 1992
It is necessary to protect the environment from contamination by radioactive waste repositories. Predictions of the long term behavior of the materials used in a repository are therefore necessary. Investigations on 28 years old concrete and 91 days old concrete are presented with respect to permeability and porosity. It was found tfiat the continuing hydration does not strongly affect the properties of the concrete. With regard to permeability it is shown that the degree of the water saturation of the concrete is the main influence on the gas permeability.
1. Introduction Radioactive waste repositories should provide sealing of the waste for long time periods. Therefore it is necessary to make predictions of the long term durability and behavior of the repository. Concrete will be present as a liner or as a backfill material in the repository. The ongoing hydration of the concrete and the chemical interactions between inflowing groundwater or air remaining in the repository and the concrete will influence the integrity and properties of the concrete. A main task of the backfill material is to ensure during the service life of the construction that the gas, which is generated in the repository, can be released from the repository. Two methods are available to investigate the alteration behavior; accelerated aging tests and the study of several decades old Portland Cement concrete. Here the second method was chosen. In this investigation, permeability and pore size distribution were studied on normal concrete up to 28 years old. These measurements were compared with concrete of the same composition and an age of 91 days.
2. Samples The composition of the 28 years old concrete cubes varied in the type of cement, fineness and w / c ratio
Correspondence to: Mr. Frank Jacobs, Institute for Building Materials, Swiss Federal Institute of Technology, ZiirichHSnggerberg, CH-8093 Ziirich, Switzerland.
(table 1). For each composition, two to four specimens were investigated. These samples (cubes with an edge length of 10 cm) were stored at approximately 60% r.h. and 23°C. New specimens were made with similar cements and the same composition in the same way as 28 years before. Previous results (Griif [1], Jacobs [2]) showed, that a large scatter in the permeability coefficients can be obtained by using several moulds for one batch. For each batch only one specimen (30 × 40 x 50 cm) was made. They were kept wet until the 7th day and then stored at 60% r.h. and 23°C.
3. Experiments 3.1. Specimen preparation and handling Cores with 96 mm diameter and 100 mm in length were drilled from the old cubes. The upper and lower part was removed to leave a 60 mm long core. The depth of carbonation was determined on the upper and lower part (table 2). It can be seen that only the Blast Furnace Slag (BFS) cores with a w / c ratio of 0.8 had already undergone a carbonation process and hence a change in the fabric. At least 63 days elapsed prior to the first measurements were started on these specimens. Cores (diameter of 150 mm) were drilled from the remade concrete at the age of 28 days perpendicular to the direction of concreting and subsequently cut into slices of 60 mm length and stored again until the 91st day at 60% r.h..
0 0 2 9 - 5 4 9 3 / 9 2 / $ 0 5 . 0 0 © 1992 - Elsevier Science P u b l i s h e r s B.V. All rights reserved
158
F Jacobs, FH. Wittmann / Long term behavior of concrete
Table 1 Composition of the 28 years old concrete and the remade concrete
the change in the water content due the storage condition and the permeability test.
W/C ratio
3.2. Experimental setup and performance
Type of cement, fineness [cruZ/g]
0.45
Cement content [kg/m 3]
Aggregate content [kg/m 3]
420
1775 80 wt%
300
1902 75 wt%
240
1922 81 Wt%
Portland Cement (PC): 3900-4900 a, 4500 b 0.60
0.80
Blast Furnace Slag cement (BFS): 3600-4300 a, 3400 b
a 28 years old concrete, b remade concrete, 91 days old.
At least 63 days before testing the specimens were prepared by cutting and drilling, but it was expected that the moisture content in the discs would not be in equilibrium with 60% r.h.. In this paper these specimens are called moist. The specimens for the determination of the pore size distribution by mercury intrusion porosimetry (MIP) were drilled from the remaining parts of the concrete cubes and cut into appropriate pieces (20 mm diameter, 20 mm length) for the MIP sample holder. The specimens were evacuated and d-dried prior to MIP measurements. On all samples permeability for gas and water were determined. After determination of the gas permeability, the specimens were stored for at least 24 hours in tap water. On these wet specimens water permeability was determined over a period of one week. Afterwards gas permeability was determined on the water saturated specimens. Several samples were subsequently dried at 105°C to constant weight and gas permeability was measured again. Before and after each permeability test the specimen weight was measured to estimate
Table 2 Depth of carbonation from specimens stored for 28 years at approximately 60% r.h. and 23°C Depths of carbonation [mrn] w/c ratio Type of cement
0.45
0.6
0.8
PC BFS
7_+3 6_+3
10_+3 13_+4
15_+5 25_+5
3. 2.1. Gas permeability Gas permeability was measured for uniaxial flow through the upper and lower face of the concrete cylinders. The specimens were placed in specimen cells, using a polyurethane seal and a tube around the specimens. By pressurising the tube at 10 bar, the seal is pressed gas tight against the sample (method developed by [3]). Gas permeability was determined by using hydrogen gas, as an inert gas, due to the small molecule size and the fact that it will be produced in a repository. Atkinson et al. [4] showed that smaller molecules give greater permeability coefficients and that it is not possible to use one gas as an analogue for another. A computer controlled device, developed at our Institute, was used to measure the permeability and to control the inlet pressure of the sample. The permeability for the investigated specimens was measured at different pressure steps to determine a threshold pressure. The following excess pressures (above atmospheric) were chosen as pressure steps: 0.05, 0.2, 0.5, 1, 2, 4, 6, 8, 7, 5, 3, 1, 0.5, 0,05 bar. Three pressure transducers were used to measure and control the exact inlet pressures. In principle, at each pressure step flow was allowed to continue until a steady state flow occured. Four mass flow meters allowed the measurement of flow rates between 1 x 10 -3 cm3/s and 3 × 10 z cm3/s. At each pressure step a certain minimum duration of the measurement was required, to allow the gas to flow through the specimen. This depends strongly on the pore size distribution, the water content and the height of the specimen. In this investigation, the minimum time was calculated by multiplying the inlet pressure in mbar by a factor of 0.5 s/mbar. This means that at an inlet pressure of 1000 mbar the waiting time was 500 seconds. Additionally a maximum time for each pressure step was defined after which the pressure was changed. To simplify matters, a constant value of 100 minutes was chosen. If no flow was observed during this time, it was assumed that the pressure was below the threshold pressure and the specimens were gas tight. The flow was measured in groups, consisting of 50 measurements, lasting in total for about 1 minute. After the fiftieth measurement the mean value and the relative error was calculated, If the relative error was less than 0.1%, a constant flow was assumed, and the pressure was incremented.
F. Jacob& F.H. Wittmann / Long term behavior of concrete 3.2.2. Water permeability The same specimen cells as for gas permeability tests were used. An ordinary tap water served as the measurement fluid. The pressure was achieved by applying a gas pressure of 7.5 bar at the top of a water column, which forced the water to flow under this pressure into the specimen. The outflow from the specimen was collected in calibrated burettes and was monitored for up to one week.
159
0.003
% 0.002
f
0.001
0
, MIk
0.01
0.1
1
,~., 10
diameter ~m]
3.2.3. Pore size distribution Pore size distribution was determined by mercury intrusion porosimetry (MIP). An Autopore 9220, manufactured by Micromeritics, was used. Applied pressures between 30 mbar and 4000 bar are available, corresponding to 0.3 mm and 3 nm pore diameter. Before starting the mercury intrusion, the specimens were evacuated until a pressure of 50 /~m Hg was achieved. This pressure was maintained for 5 minutes. Intrusion was determined at 151 pressure points using an equilibration time of 20 seconds at each pressure step. The results were corrected by data obtained from blank runs, which allowed for the volume change of the Hg and the sample holder due to pressure and temperature.
h
Fig. 1. Pore size distribution of concrete made from PC cement after 91 days storage at 60% r.h.. The solid line denotes a w/c ratio of 0.45, the dashed line 0.6 and the dotted line 0.8.
0.003
" •0.002
II
g -
.~ o.0o~
I
t/'~
I ! |
3.2.4. Degree o f water saturation The degree of the water saturation was calculated by the difference in weight between specimens dried at 105°C and directly after the water permeability tests.
4. Results
0.01
0.1
1
tO
diameter [ ~ m ]
Fig. 2. Pore size distribution of concrete made from PC cement after 28 years storage at 60% r.h.. The solid line denotes a w/c ratio of 0.45, the dashed line 0.6 and the dotted line 0.8.
4.1. Porosity The pore size distributions of the mixes made from Portland Cement are shown in figs. 1 and 2 and for concrete made from Blast Furnace Slag Cement (BFS) in figs. 3 and 4. The MIP results show for both PC and BFS a reduction of the pore volume up to about one third with increasing age. BFS mixes show a lower pore volume than the PC mixes (figs. 1-4 and table 3).
0.003
0.002 '5
•~- 0.001
x
_
4.2. Permeability 10 1 diometer [,am] Fig. 3. Pore size distribution of concrete made from BFS cement after 91 days storage at 60% r.h., The solid line denotes a w/c ratio of 0.45, the dashed line 0.6 and the dotted line 0.8. 0.01
The pressure at which gas starts to flow the threshold pressure - is shown in table 4 for concretes with different degrees of water saturation, ages and w / c ratios. The first pressure at which gas permeability i s measured is 0.05 bar. If gas starts to flow at this
0.1
160
F. Jacobs, F.H. Wittmann / Long term behavior of concrete TSO
O. O0.~,
125 mix with BFS w/c ratio 0.8 / ~ -4 100
z~
0.002
~
c6
~
m
i
x
with BFS w/c ratio 0.6
75
~ 50
o.oo~
25 - -
o.oq
oi
q
1o
:
i
i
t
J
i
i
F
l
2
5
4
5
6
7
8
lime [d]
diameter [,um]
Fig. 4. Pore size distribution of concrete made from BFS cement after 28 years storage at 60% r.h.. The solid line denotes a w / c ratio of 0.45, the dashed line 0.6 and the dotted line 0.8.
pressure, this pressure is also called threshold pressure (but the threshold pressure may be at lower pressures). Dried specimens show a threshold pressure < 0.05 bar, independent of age and w / c ratio. A trend can be seen, in that with increasing moisture content and decreasing w / c ratio the threshold pressure increases. Moist specimens show an increase in the threshold pressure with increasing age. W a t e r saturated specimens show equal threshold pressures. Samples made of
Table 3 Porosity [%] of concrete mixes (made from PC and BFS) determined by MIP Type of cement w / c ratio
91 days
PC 28 years
91 days
BFS 28 years
0.45 0.60 0.80
8.9 12.0 13.9
7.0 8.2 9.7
7.1 11.0 12.4
6.4 8.0 9.6
Fig. 5. The amount of water flowing through different types of concrete is shown as a function of the elapsed time.
BFS cement show higher threshold pressures than do PC samples. In the case of moist and wet specimens two factors influence the gas permeability coefficient: (1) the degree of water saturation changes with increasing pressure and measured time and (2) by increasing the gas pressure the mean free path length of the gas molecules decreases and hence the flow rate through the specimen decreases (Iriya et al. [5], Klinkenberg [6]). In the case of moist and wet specimens the permeability coefficient measured at the highest pressure (lowest saturation) is taken as a representative value. For dried specimens a reduction in the permeability coefficient (determined at 8 bar) to about the half or one quarter of the initial value (determined at 50 mbar) can be seen. In this case the permeability coefficient is calculated for infinite pressure (Klinkenberg [6]). Water permeability tests show a decrease in permeability with time (fig. 5) by 10 to 20%. Representative permeability coefficients values were taken to be those measured in the last 24 hours.
Table 4 Threshold pressure of the 28 years and 91 days old mixes containing various w / c ratios, degrees of water saturation and types of cement. PC denotes concrete made of Portland cement and BFS concrete made of Blast Furnace Slag cement. If at the first pressure step (0.05 bar) gas flow could be observed, the threshold pressure is given as 0.05 bar. Each value of the threshold pressures represents 3 to 6 measurements. The scatter of the threshold pressures is 20%. Threshold pressure [bar]
0.45 PC
BFS
0.60 PC
BFS
0.80 PC
BFS
Dry (105°C)
28 years
0.05
0.05
0.05
0.05
0.05
0.05
Moist ( > 60% r.h)
91 days 28 years
0.1 2.0
0.2 2.0
0.05 1.0
0.1 2.0
0.05 1.0
0.05 1.0
Water saturated
91 days 28 years
8.0 8.0
8.0 8.0
5.0 4.0
6.0 6.0
6.0 4.0
6.0 5.0
161
17. Jacobs, F.H. Wittmann / Long term behavior o f concrete 1[-15
1E-16
gas ~rmeobility, 60 % r . h .
IE-16 "e 1E-17
~
gos permeobility, woter soturoted
RPC
IE-18
"~,~ I E- 20
IE-19
, 20
~ 40
, 60
, ", 80
, 100
IE-21
a"~ 0.4
i ~~ ~~ , 0.5
water permeability ,
0.6
' 0.7
I error bar ' 0.8
w/c r0ti0
degree ot woter saturation [%]
Fig. 6. The gas permeability as a function of the water content for 28 years old concrete. The solid line denotes a w/c ratio of 0.45, the broken line one of 0.60. BFS, OPC or RPC denotes concrete made from Blast Furnace Slag Cement, Ordinary Portland Cement or fine ground Portland Cement.
Fig. 8. Gas ( ) and water ( - - - - - - ) permeability of concrete made from BFS and cured for 91 days and 28 years as a function of the w/c ratio and the storage condition. zx: 28 years old specimens, x : 91 days old specimens.
In fig. 6, the gas permeability is given as a function of the water content of the specimens. With increasing water content in the specimens gas permeability decreases. It can be seen that the moist specimens still have a degree of water saturation between 20 and 40%. Additionally, it can be seen that the influence of the fineness of the cement on the gas permeability is not significant. Therefore, in the following diagrams the concrete types are only distinguished by the w / c ratio and the type of cement. In figs. 7 and 8 the permeability coefficient for PC and BFS samples is plotted against the w / c ratio for different degrees of water saturation and age. Each point in the diagram represents between three and six measurements. The values were calculated from the mean value of the logarithms of the permeability coefficients. 28 years old concrete shows lower permeability
coefficients for gas and water than the 91 days old mixes only for low w / c ratios and high degrees of water saturation. Water permeability is always lower than gas permeability. Concrete made of BFS cement shows slightly lower water and gas permeability coefficients than PC specimens for the 91 days old mixes with low w / c ratios and high degrees of water saturation. No influence of carbonation could be recognized for the different depths of carbonation for BFS samples with a w / c ratio of 0.8.
IE-15 gas permeobility, 60 % r.h.
IE-16
gos permeobility,watersoturoted
IE-17 1E-18 &
IE-19
~-~
IE-20
woterpermeobility I error bar
~r1 t ~ IE-21 0.4
0.5
0.6
0.7
0.8
w/c ratio ) and water (------) permeability of Fig. 7. Gas ( concrete made from PC and cured for 91 days and 28 years as a function of the w/c ratio and the storage condition. A : 28 years old specimens, × : 91 days old specimens.
5. Discussion 5.1. I n f l u e n c e o f age
The measured permeability coefficients and pore size distributions are consistent with the data of [1,4,8]. The influence of the w / c ratio agrees with the data of Grail [1]. Permeability decreases with age and decreasing w / c ratio. The influence of age is due to the continuation of the hydration of the cement and the resulting alteration of the pore structure (figs. 1-4, table 3). Feldman and Cheng-yi [7] showed by MIP measurements the decrease in pore volume and a shift of the threshold diameter to smaller diameter for samples with an age between one and 180 days of hydration. The rate of decrease and shift decreases with time. The consequent change in porosity and threshold diameter is the reason that the threshold pressures are equal or higher and the permeabilities for wet specimens are equal or lower with increasing age. Wet storage conditions emphasise this alteration of the
162
F. Jacobs, F.11. Wittmann / Long term behat;ior of concrete
fabric by a swelling of the hardened cement paste. The lower the w / c ratio (higher cement content) the higher the possible swelling of the matrix. This effect is shown for gas and water permeability in fig. 7 for PC samples. For BFS samples (fig. 8) it can only be observed for water permeability tests. With respect to the scatter of the measurements the difference in the pore structure is too small to be identified with such a low number of gas permeability measurements for dried storage conditions. Investigations [3,5,8,10] report a decrease in gas permeability for moist and dry curing conditions of one order of magnitude during the first 28 days and up to 91 days a further decrease of 1/10. Storage under water increases the decrease in permeability to three orders of magnitude for specimens with an age of less than one week to those measured after one year. This investigation shows that permeability decreases by less than one order of magnitude from 3 months to nearly 30 years for water saturated specimens. For dry storage conditions the decrease is even less. Hence the rate of decrease in permeability decreases strongly with time. According to these data the next decrease of one order of magnitude will be expected after more than several hundreds of years. In this estimation chemical reactions are not taken into consideration. 5.2. Influence o f moisture content
As can be seen in fig. 6 the gas permeability depends on the moisture content of the specimens. The higher the moisture content the lower the remaining space in the specimen available for the gas flow. Additionally, a possible swelling of the matrix by moist storage and chemical/physical reactions between the tap water and the hardened cement paste could reduce the remaining air-filled pore volume. The threshold pressure decreases with decreasing moisture content. The measured threshold pressures for water saturated specimens between 6 and 8 bar (table 4) correspond according to the Washburn equation to capillary pressures of pores with radii of approx. 0.2/xm. These pore radii r T are emptied by the gas pressure at the threshold pressure. They correspond to the increase of.the pore size maxima in figs. 1 to 4. For moist storage of the specimens the threshold pressure decreases to less than 2 bar. This could probabely indicate that due to the swelling the pores with radii r T became smaller and hence a lower threshold pressure was determined. The influence of the water saturation of the specimens on gas permeability is small when the main pores
are free of water. The main gas pathways, corresponding approximately to the maxima in the pore size distribution, dominate the gas flow (see 5.3). Hence a change in the saturation of smaller pores does not influence the permeability strongly. Specimens saturated up to 20 to 30% give only a small reduction in gas permeability (fig. 6). Gas permeability drops nearly two orders of magnitude for specimens cured first at 60% r.h. and being saturated afterwards with water. This agrees with data of Atkinson et al. [4], who measured a difference in gas permeability of nearly three orders of magnitude for dry and at 100% r.h. cured specimens. By monitoring the weight of the specimens before and after each measurement it was found, that during the water permeability experiments, beside the uptake of water and expelling of gas, a compression of air remaining in the specimens takes place. This gives an additional increase in the degree of water saturation during the water permeability tests. The weight of the specimens is the highest immediately after the water permeability test and slowly decreases afterwards due to the expansion of the included air bubbles and the expulsion of water from the specimen. Nevertheless, the weight of the specimens is still the same or higher after the water permeability test. Due to these phenomena, the gas permeability measured under "water saturated conditions" corresponds to only 80 to 90% of full water saturation. A reduction in the water permeability with increasing duration of experiment is observed (fig. 5). It is uncertain if and at which value this reduction stabilizes. A blocking of the main pathways leads to a decrease in permeability. Several explanations can be given for this blocking: A possible swelling of the matrix or changes in the fabric due to the water flow could lead to a reduction in the pore volume. During water storage an expansion of the specimens can be observed [2,11]. The change in volume is approx. 2%. Chemical interactions between e.g. sulphate and carbonate of the tap water and the hardened cement paste may take place. It can be noted that even after more than two years of water storage and several weeks of water permeability testing air still remains in the specimens (Jacobs [2]). A decrease in the saturation during water flow by the release of dissolved air, and a blocking of capillaries is plausible, too. A1Manaseer et al. [12] have discussed this possibility. Hearn [13] showed that the decrease in permeability depends mainly on the curing of the specimens and is explained as self-sealing of microcracks due to drying shrinkage.
163
F. Jacobs, F.H. Wittmann / Long term behavior of concrete
Comparing gas and water permeabilities (figs. 7,8) it is obvious that water permeability is always lower than gas permeability. The higher the gas permeability the smaller is the difference between both values. This result is confirmed by different authors [2,9,14]. So far it is not clear why this discrepancy exists. Some possibilities are discussed in a previous paragraph.
14
12 "-'-'"'I0
K 6
0
5.3. Equivalent pore radii
0.01
A comparison of MIP with permeability measurements after drying to constant weight at 105°C is only valid for the same conditions of the fabric. D-drying reduces the water content to approximately the same value as heating to constant weight at 105°C (Taylor [151).
Plotting the cumulative pore volume (obtained from mercury intrusion porosimetry) versus the pore radius, a median pore radius (pore radius at 50% of total intrusion volume) can be found for each concrete. Making the extrapolation of the Hagen-Poiseuille law for single capillaries to porous media, the permeability coefficient k is introduced by k = r 2 p / 8 . From the permeability values and the porosity an equivalent pore radius was calculated (fig. 9). The equivalent pore radius varies more strongly with changing permeability than the median pore value. In the calculation of the median pore value a few large air voids can have a significant influence on the median value. A correlation between the radius values and the gas permeabil-
IE-14
&
IE-15 1E-16
& 1E-17
2
IE-18 ,.
0.00
0.10
0.20
0.30 0.40 rodius [~m]
0.50
0,60
Fig. 9. Median pore radii ( * ) and equivalent pore radii (o) as a function of gas permeability. The median pore radii corresponds to the pore radii at 50% of the maximum intrusion volume. The equivalent pore radii is calculated by following formula: r = V/~ * 8 ) / p . The scatter in the pore radii for one permeability coefficient is due to the different pore size distributions and porosities from different samples. The line denotes a correlation which was calculated by the equation r = ~[(k * 8 ) / p , by assuming a porosity of 8%.
",,, cumulotive volume[o/o] "-,
vo e 0.1
. . . . . I
10
diometer ~um]
Fig. 10. Incremental pore volume in %o and cumulative pore volume in % as function of the pore size distribution for normal concrete after 91 days water storage; A: diameter corresponding to the threshold pressure; B: equivalent pore diameter.
ity is observed. This correlation coincides approximately with the line in fig. 9 which denotes the correlation between k and r for a given porosity of 8%. For low permeabilities, an equivalent pore radius of 0.05 p.m and for high permeabilities one of 0.2 to 0.3/~m is found. Comparing these results with the MIP curves (figs. 1 to 4) the pore maxima for w / c ratio 0.45 is around 0.05 izm and for w / c ratio 0,8 around 0.2 tzm. The increase to the pore size maxima coincides with the pore size calculated from the threshold pressure. Figure 10 summarizes the characteristics from the pore size distribution for the permeability on the pore size distribution of 91 days old normal concrete (w/c ratio 0.8). The pore size equivalent to the threshold pressure is at the increase to the pore maxima, the equivalent pore size coincides with the pore size maxima or is shifted to bigger pore size. One half of the porosity is created by pores bigger than the equivalent pore size. Smaller pores (gel pores) created by the continuation of the hydration of the cement have a negligible influence on permeability.
6. Conclusions The aging of highly hydrated specimens (91 days) leads to a small change in the pore structure and the permeability. The pore structure and permeability are not strongly correlated with the composition of the concrete (type of cement) and not strongly influenced by the w / c ratio. The water saturation of the specimens generally has a significant influence on permeability. The investi-
164
F. Jacobs, F.H. Wittmann / Long term behauior of concrete
gated normal concrete samples showed a strong decrease in permeability by increasing the water saturation from 40 to 80%. The threshold pressure agrees with the capillary pressure of pores with a radius of approximately at the increase of the pore size maxima whereas the gas permeability agrees with the maxima in the pores size distribution (fig. 10). An attempt to minimize the water content should be a priority in order to increase the gas flow and to decrease the quantity of expelling water. Artifical air voids, capillary barriers or hydrophobing agents can reduce the water uptake of concrete, but the hydrostatic pressure must be taken into consideration.
Acknowledgement We gratefully acknowledge financial support by N A G R A and additionally the kindness of Dipl. Ing. Miiller, T U Munich for leaving us 28 years old concrete samples. W e have to thank Dipl. Phys. Mayer from our institute for his help in the experimental setup and fruitful discussions.
References [1] H. Gr~if, (/ber die Porosit~it und die Durchl~issigkeit von Zementstein, M6rtel und Beton und ihren Einfluss auf Gebrauchseigenschaften von Beton, Ph.D. Thesis University of Essen (1988), 200 p. [2] F. Jacobs, Untersuchung zementgebundener Materialien in Hinblick auf Porosit~it und Permeabilit~it sowie begleitender Eigenschaften, unpubl, report, Institute for Building Materials, ETH Zurich (1991), 72 p. [3] H. Gr~if, H. Grube, Verfahren zur Priifung der Durchl~issigkeit von M6rtel und Beton gegeniiber Gasen
und Wasser, Beton 5 (1986) pp. 184-187, 6 (1986) pp. 222-226. [4] A. Atkinson, P.A. Claisse, A.W. Harris, A.K. Nickerson, Transport of gas through concrete, UKAEA Harwell Report, AERE-G 4977 (1988), 30 p. [5} K. Iriya, F. Jacobs, B. Knecht, F.H. Wittmann, Cementitious backfill materials for a L/ILW repository - Investigations of gas transport properties, Nucl. Engrg. Des. 129, 1 (1991) pp. 49-56. [6] L.J. Klinkenberg, The permeability of porous media to liquids and Bases, Drilling Prod. Pract. Am. Petrol. Inst. (1941) 200-213. [7] R.F. Feldman, H. Cheng-yi, Properties of portland cement-silica fume pastes, I. Porosity and surface properties, Cem. Concr. Res. 15 (1985) pp. 765-774. [8] The cembureau co-operative programme on permeability measurements for concrete, in: Pore Structure and Construction Materials, Vol. I, ed. J,C. Maso (Chapman and Hall, London, 1987) pp. 41-48. [9] R.K. Dhir, P.C. Hewlett, Y.N. Chan, Near surface characteristics of concrete: intrinsic permeability, Mag. Concr. Res. 41 (1989) pp. 87-97. [10] J.G. Cabrera C.J. Lynsdale, A new gas permeameter for measuring the permeability of mortar and concrete, Mag. Concr. Res. 40 (1988) pp. 177-182. [11] A. Atkinson, A. Haxby and J.A. Hearne, The chemistry and expansion of limestone-Portland cement mortars exposed to sulphate containing solutions, UKAEA Hatwell Report, Nirex Safty studies NSS/R127 (1988), 23 p. [12] A.A. AI-Manaseer, M. Onofrei, M.N. Gray, B.S. Shenton, The effect of silica fume and water/cement ratio on the hydraulic conductivity of cement-based grout, MRS Fall Meeting (1990). [13] N. Hearn, a recording permeameter for measuring timesensitive permeability of concrete, in: Ceramic Transactions 16, ed. S. Mindess (1990) pp. 463-475. [14] P.B. Bamforth, The relationship between permeability coefficients for concrete obtained using liquid and gas, Mag. Concr. Res. 39 (1987) pp. 3-11. [15] H.F. Taylor, Cement Chemistry (Academic Press Ltd., London, 1990), 475 p.
157
Long term behavior of concrete in nuclear waste repositories F. Jacobs a n d F.H. W i t t m a n n
Institute for Building Materials, ETH-Ziirich, Switzerland Received 20 July 1992
It is necessary to protect the environment from contamination by radioactive waste repositories. Predictions of the long term behavior of the materials used in a repository are therefore necessary. Investigations on 28 years old concrete and 91 days old concrete are presented with respect to permeability and porosity. It was found tfiat the continuing hydration does not strongly affect the properties of the concrete. With regard to permeability it is shown that the degree of the water saturation of the concrete is the main influence on the gas permeability.
1. Introduction Radioactive waste repositories should provide sealing of the waste for long time periods. Therefore it is necessary to make predictions of the long term durability and behavior of the repository. Concrete will be present as a liner or as a backfill material in the repository. The ongoing hydration of the concrete and the chemical interactions between inflowing groundwater or air remaining in the repository and the concrete will influence the integrity and properties of the concrete. A main task of the backfill material is to ensure during the service life of the construction that the gas, which is generated in the repository, can be released from the repository. Two methods are available to investigate the alteration behavior; accelerated aging tests and the study of several decades old Portland Cement concrete. Here the second method was chosen. In this investigation, permeability and pore size distribution were studied on normal concrete up to 28 years old. These measurements were compared with concrete of the same composition and an age of 91 days.
2. Samples The composition of the 28 years old concrete cubes varied in the type of cement, fineness and w / c ratio
Correspondence to: Mr. Frank Jacobs, Institute for Building Materials, Swiss Federal Institute of Technology, ZiirichHSnggerberg, CH-8093 Ziirich, Switzerland.
(table 1). For each composition, two to four specimens were investigated. These samples (cubes with an edge length of 10 cm) were stored at approximately 60% r.h. and 23°C. New specimens were made with similar cements and the same composition in the same way as 28 years before. Previous results (Griif [1], Jacobs [2]) showed, that a large scatter in the permeability coefficients can be obtained by using several moulds for one batch. For each batch only one specimen (30 × 40 x 50 cm) was made. They were kept wet until the 7th day and then stored at 60% r.h. and 23°C.
3. Experiments 3.1. Specimen preparation and handling Cores with 96 mm diameter and 100 mm in length were drilled from the old cubes. The upper and lower part was removed to leave a 60 mm long core. The depth of carbonation was determined on the upper and lower part (table 2). It can be seen that only the Blast Furnace Slag (BFS) cores with a w / c ratio of 0.8 had already undergone a carbonation process and hence a change in the fabric. At least 63 days elapsed prior to the first measurements were started on these specimens. Cores (diameter of 150 mm) were drilled from the remade concrete at the age of 28 days perpendicular to the direction of concreting and subsequently cut into slices of 60 mm length and stored again until the 91st day at 60% r.h..
0 0 2 9 - 5 4 9 3 / 9 2 / $ 0 5 . 0 0 © 1992 - Elsevier Science P u b l i s h e r s B.V. All rights reserved
158
F Jacobs, FH. Wittmann / Long term behavior of concrete
Table 1 Composition of the 28 years old concrete and the remade concrete
the change in the water content due the storage condition and the permeability test.
W/C ratio
3.2. Experimental setup and performance
Type of cement, fineness [cruZ/g]
0.45
Cement content [kg/m 3]
Aggregate content [kg/m 3]
420
1775 80 wt%
300
1902 75 wt%
240
1922 81 Wt%
Portland Cement (PC): 3900-4900 a, 4500 b 0.60
0.80
Blast Furnace Slag cement (BFS): 3600-4300 a, 3400 b
a 28 years old concrete, b remade concrete, 91 days old.
At least 63 days before testing the specimens were prepared by cutting and drilling, but it was expected that the moisture content in the discs would not be in equilibrium with 60% r.h.. In this paper these specimens are called moist. The specimens for the determination of the pore size distribution by mercury intrusion porosimetry (MIP) were drilled from the remaining parts of the concrete cubes and cut into appropriate pieces (20 mm diameter, 20 mm length) for the MIP sample holder. The specimens were evacuated and d-dried prior to MIP measurements. On all samples permeability for gas and water were determined. After determination of the gas permeability, the specimens were stored for at least 24 hours in tap water. On these wet specimens water permeability was determined over a period of one week. Afterwards gas permeability was determined on the water saturated specimens. Several samples were subsequently dried at 105°C to constant weight and gas permeability was measured again. Before and after each permeability test the specimen weight was measured to estimate
Table 2 Depth of carbonation from specimens stored for 28 years at approximately 60% r.h. and 23°C Depths of carbonation [mrn] w/c ratio Type of cement
0.45
0.6
0.8
PC BFS
7_+3 6_+3
10_+3 13_+4
15_+5 25_+5
3. 2.1. Gas permeability Gas permeability was measured for uniaxial flow through the upper and lower face of the concrete cylinders. The specimens were placed in specimen cells, using a polyurethane seal and a tube around the specimens. By pressurising the tube at 10 bar, the seal is pressed gas tight against the sample (method developed by [3]). Gas permeability was determined by using hydrogen gas, as an inert gas, due to the small molecule size and the fact that it will be produced in a repository. Atkinson et al. [4] showed that smaller molecules give greater permeability coefficients and that it is not possible to use one gas as an analogue for another. A computer controlled device, developed at our Institute, was used to measure the permeability and to control the inlet pressure of the sample. The permeability for the investigated specimens was measured at different pressure steps to determine a threshold pressure. The following excess pressures (above atmospheric) were chosen as pressure steps: 0.05, 0.2, 0.5, 1, 2, 4, 6, 8, 7, 5, 3, 1, 0.5, 0,05 bar. Three pressure transducers were used to measure and control the exact inlet pressures. In principle, at each pressure step flow was allowed to continue until a steady state flow occured. Four mass flow meters allowed the measurement of flow rates between 1 x 10 -3 cm3/s and 3 × 10 z cm3/s. At each pressure step a certain minimum duration of the measurement was required, to allow the gas to flow through the specimen. This depends strongly on the pore size distribution, the water content and the height of the specimen. In this investigation, the minimum time was calculated by multiplying the inlet pressure in mbar by a factor of 0.5 s/mbar. This means that at an inlet pressure of 1000 mbar the waiting time was 500 seconds. Additionally a maximum time for each pressure step was defined after which the pressure was changed. To simplify matters, a constant value of 100 minutes was chosen. If no flow was observed during this time, it was assumed that the pressure was below the threshold pressure and the specimens were gas tight. The flow was measured in groups, consisting of 50 measurements, lasting in total for about 1 minute. After the fiftieth measurement the mean value and the relative error was calculated, If the relative error was less than 0.1%, a constant flow was assumed, and the pressure was incremented.
F. Jacob& F.H. Wittmann / Long term behavior of concrete 3.2.2. Water permeability The same specimen cells as for gas permeability tests were used. An ordinary tap water served as the measurement fluid. The pressure was achieved by applying a gas pressure of 7.5 bar at the top of a water column, which forced the water to flow under this pressure into the specimen. The outflow from the specimen was collected in calibrated burettes and was monitored for up to one week.
159
0.003
% 0.002
f
0.001
0
, MIk
0.01
0.1
1
,~., 10
diameter ~m]
3.2.3. Pore size distribution Pore size distribution was determined by mercury intrusion porosimetry (MIP). An Autopore 9220, manufactured by Micromeritics, was used. Applied pressures between 30 mbar and 4000 bar are available, corresponding to 0.3 mm and 3 nm pore diameter. Before starting the mercury intrusion, the specimens were evacuated until a pressure of 50 /~m Hg was achieved. This pressure was maintained for 5 minutes. Intrusion was determined at 151 pressure points using an equilibration time of 20 seconds at each pressure step. The results were corrected by data obtained from blank runs, which allowed for the volume change of the Hg and the sample holder due to pressure and temperature.
h
Fig. 1. Pore size distribution of concrete made from PC cement after 91 days storage at 60% r.h.. The solid line denotes a w/c ratio of 0.45, the dashed line 0.6 and the dotted line 0.8.
0.003
" •0.002
II
g -
.~ o.0o~
I
t/'~
I ! |
3.2.4. Degree o f water saturation The degree of the water saturation was calculated by the difference in weight between specimens dried at 105°C and directly after the water permeability tests.
4. Results
0.01
0.1
1
tO
diameter [ ~ m ]
Fig. 2. Pore size distribution of concrete made from PC cement after 28 years storage at 60% r.h.. The solid line denotes a w/c ratio of 0.45, the dashed line 0.6 and the dotted line 0.8.
4.1. Porosity The pore size distributions of the mixes made from Portland Cement are shown in figs. 1 and 2 and for concrete made from Blast Furnace Slag Cement (BFS) in figs. 3 and 4. The MIP results show for both PC and BFS a reduction of the pore volume up to about one third with increasing age. BFS mixes show a lower pore volume than the PC mixes (figs. 1-4 and table 3).
0.003
0.002 '5
•~- 0.001
x
_
4.2. Permeability 10 1 diometer [,am] Fig. 3. Pore size distribution of concrete made from BFS cement after 91 days storage at 60% r.h., The solid line denotes a w/c ratio of 0.45, the dashed line 0.6 and the dotted line 0.8. 0.01
The pressure at which gas starts to flow the threshold pressure - is shown in table 4 for concretes with different degrees of water saturation, ages and w / c ratios. The first pressure at which gas permeability i s measured is 0.05 bar. If gas starts to flow at this
0.1
160
F. Jacobs, F.H. Wittmann / Long term behavior of concrete TSO
O. O0.~,
125 mix with BFS w/c ratio 0.8 / ~ -4 100
z~
0.002
~
c6
~
m
i
x
with BFS w/c ratio 0.6
75
~ 50
o.oo~
25 - -
o.oq
oi
q
1o
:
i
i
t
J
i
i
F
l
2
5
4
5
6
7
8
lime [d]
diameter [,um]
Fig. 4. Pore size distribution of concrete made from BFS cement after 28 years storage at 60% r.h.. The solid line denotes a w / c ratio of 0.45, the dashed line 0.6 and the dotted line 0.8.
pressure, this pressure is also called threshold pressure (but the threshold pressure may be at lower pressures). Dried specimens show a threshold pressure < 0.05 bar, independent of age and w / c ratio. A trend can be seen, in that with increasing moisture content and decreasing w / c ratio the threshold pressure increases. Moist specimens show an increase in the threshold pressure with increasing age. W a t e r saturated specimens show equal threshold pressures. Samples made of
Table 3 Porosity [%] of concrete mixes (made from PC and BFS) determined by MIP Type of cement w / c ratio
91 days
PC 28 years
91 days
BFS 28 years
0.45 0.60 0.80
8.9 12.0 13.9
7.0 8.2 9.7
7.1 11.0 12.4
6.4 8.0 9.6
Fig. 5. The amount of water flowing through different types of concrete is shown as a function of the elapsed time.
BFS cement show higher threshold pressures than do PC samples. In the case of moist and wet specimens two factors influence the gas permeability coefficient: (1) the degree of water saturation changes with increasing pressure and measured time and (2) by increasing the gas pressure the mean free path length of the gas molecules decreases and hence the flow rate through the specimen decreases (Iriya et al. [5], Klinkenberg [6]). In the case of moist and wet specimens the permeability coefficient measured at the highest pressure (lowest saturation) is taken as a representative value. For dried specimens a reduction in the permeability coefficient (determined at 8 bar) to about the half or one quarter of the initial value (determined at 50 mbar) can be seen. In this case the permeability coefficient is calculated for infinite pressure (Klinkenberg [6]). Water permeability tests show a decrease in permeability with time (fig. 5) by 10 to 20%. Representative permeability coefficients values were taken to be those measured in the last 24 hours.
Table 4 Threshold pressure of the 28 years and 91 days old mixes containing various w / c ratios, degrees of water saturation and types of cement. PC denotes concrete made of Portland cement and BFS concrete made of Blast Furnace Slag cement. If at the first pressure step (0.05 bar) gas flow could be observed, the threshold pressure is given as 0.05 bar. Each value of the threshold pressures represents 3 to 6 measurements. The scatter of the threshold pressures is 20%. Threshold pressure [bar]
0.45 PC
BFS
0.60 PC
BFS
0.80 PC
BFS
Dry (105°C)
28 years
0.05
0.05
0.05
0.05
0.05
0.05
Moist ( > 60% r.h)
91 days 28 years
0.1 2.0
0.2 2.0
0.05 1.0
0.1 2.0
0.05 1.0
0.05 1.0
Water saturated
91 days 28 years
8.0 8.0
8.0 8.0
5.0 4.0
6.0 6.0
6.0 4.0
6.0 5.0
161
17. Jacobs, F.H. Wittmann / Long term behavior o f concrete 1[-15
1E-16
gas ~rmeobility, 60 % r . h .
IE-16 "e 1E-17
~
gos permeobility, woter soturoted
RPC
IE-18
"~,~ I E- 20
IE-19
, 20
~ 40
, 60
, ", 80
, 100
IE-21
a"~ 0.4
i ~~ ~~ , 0.5
water permeability ,
0.6
' 0.7
I error bar ' 0.8
w/c r0ti0
degree ot woter saturation [%]
Fig. 6. The gas permeability as a function of the water content for 28 years old concrete. The solid line denotes a w/c ratio of 0.45, the broken line one of 0.60. BFS, OPC or RPC denotes concrete made from Blast Furnace Slag Cement, Ordinary Portland Cement or fine ground Portland Cement.
Fig. 8. Gas ( ) and water ( - - - - - - ) permeability of concrete made from BFS and cured for 91 days and 28 years as a function of the w/c ratio and the storage condition. zx: 28 years old specimens, x : 91 days old specimens.
In fig. 6, the gas permeability is given as a function of the water content of the specimens. With increasing water content in the specimens gas permeability decreases. It can be seen that the moist specimens still have a degree of water saturation between 20 and 40%. Additionally, it can be seen that the influence of the fineness of the cement on the gas permeability is not significant. Therefore, in the following diagrams the concrete types are only distinguished by the w / c ratio and the type of cement. In figs. 7 and 8 the permeability coefficient for PC and BFS samples is plotted against the w / c ratio for different degrees of water saturation and age. Each point in the diagram represents between three and six measurements. The values were calculated from the mean value of the logarithms of the permeability coefficients. 28 years old concrete shows lower permeability
coefficients for gas and water than the 91 days old mixes only for low w / c ratios and high degrees of water saturation. Water permeability is always lower than gas permeability. Concrete made of BFS cement shows slightly lower water and gas permeability coefficients than PC specimens for the 91 days old mixes with low w / c ratios and high degrees of water saturation. No influence of carbonation could be recognized for the different depths of carbonation for BFS samples with a w / c ratio of 0.8.
IE-15 gas permeobility, 60 % r.h.
IE-16
gos permeobility,watersoturoted
IE-17 1E-18 &
IE-19
~-~
IE-20
woterpermeobility I error bar
~r1 t ~ IE-21 0.4
0.5
0.6
0.7
0.8
w/c ratio ) and water (------) permeability of Fig. 7. Gas ( concrete made from PC and cured for 91 days and 28 years as a function of the w/c ratio and the storage condition. A : 28 years old specimens, × : 91 days old specimens.
5. Discussion 5.1. I n f l u e n c e o f age
The measured permeability coefficients and pore size distributions are consistent with the data of [1,4,8]. The influence of the w / c ratio agrees with the data of Grail [1]. Permeability decreases with age and decreasing w / c ratio. The influence of age is due to the continuation of the hydration of the cement and the resulting alteration of the pore structure (figs. 1-4, table 3). Feldman and Cheng-yi [7] showed by MIP measurements the decrease in pore volume and a shift of the threshold diameter to smaller diameter for samples with an age between one and 180 days of hydration. The rate of decrease and shift decreases with time. The consequent change in porosity and threshold diameter is the reason that the threshold pressures are equal or higher and the permeabilities for wet specimens are equal or lower with increasing age. Wet storage conditions emphasise this alteration of the
162
F. Jacobs, F.11. Wittmann / Long term behat;ior of concrete
fabric by a swelling of the hardened cement paste. The lower the w / c ratio (higher cement content) the higher the possible swelling of the matrix. This effect is shown for gas and water permeability in fig. 7 for PC samples. For BFS samples (fig. 8) it can only be observed for water permeability tests. With respect to the scatter of the measurements the difference in the pore structure is too small to be identified with such a low number of gas permeability measurements for dried storage conditions. Investigations [3,5,8,10] report a decrease in gas permeability for moist and dry curing conditions of one order of magnitude during the first 28 days and up to 91 days a further decrease of 1/10. Storage under water increases the decrease in permeability to three orders of magnitude for specimens with an age of less than one week to those measured after one year. This investigation shows that permeability decreases by less than one order of magnitude from 3 months to nearly 30 years for water saturated specimens. For dry storage conditions the decrease is even less. Hence the rate of decrease in permeability decreases strongly with time. According to these data the next decrease of one order of magnitude will be expected after more than several hundreds of years. In this estimation chemical reactions are not taken into consideration. 5.2. Influence o f moisture content
As can be seen in fig. 6 the gas permeability depends on the moisture content of the specimens. The higher the moisture content the lower the remaining space in the specimen available for the gas flow. Additionally, a possible swelling of the matrix by moist storage and chemical/physical reactions between the tap water and the hardened cement paste could reduce the remaining air-filled pore volume. The threshold pressure decreases with decreasing moisture content. The measured threshold pressures for water saturated specimens between 6 and 8 bar (table 4) correspond according to the Washburn equation to capillary pressures of pores with radii of approx. 0.2/xm. These pore radii r T are emptied by the gas pressure at the threshold pressure. They correspond to the increase of.the pore size maxima in figs. 1 to 4. For moist storage of the specimens the threshold pressure decreases to less than 2 bar. This could probabely indicate that due to the swelling the pores with radii r T became smaller and hence a lower threshold pressure was determined. The influence of the water saturation of the specimens on gas permeability is small when the main pores
are free of water. The main gas pathways, corresponding approximately to the maxima in the pore size distribution, dominate the gas flow (see 5.3). Hence a change in the saturation of smaller pores does not influence the permeability strongly. Specimens saturated up to 20 to 30% give only a small reduction in gas permeability (fig. 6). Gas permeability drops nearly two orders of magnitude for specimens cured first at 60% r.h. and being saturated afterwards with water. This agrees with data of Atkinson et al. [4], who measured a difference in gas permeability of nearly three orders of magnitude for dry and at 100% r.h. cured specimens. By monitoring the weight of the specimens before and after each measurement it was found, that during the water permeability experiments, beside the uptake of water and expelling of gas, a compression of air remaining in the specimens takes place. This gives an additional increase in the degree of water saturation during the water permeability tests. The weight of the specimens is the highest immediately after the water permeability test and slowly decreases afterwards due to the expansion of the included air bubbles and the expulsion of water from the specimen. Nevertheless, the weight of the specimens is still the same or higher after the water permeability test. Due to these phenomena, the gas permeability measured under "water saturated conditions" corresponds to only 80 to 90% of full water saturation. A reduction in the water permeability with increasing duration of experiment is observed (fig. 5). It is uncertain if and at which value this reduction stabilizes. A blocking of the main pathways leads to a decrease in permeability. Several explanations can be given for this blocking: A possible swelling of the matrix or changes in the fabric due to the water flow could lead to a reduction in the pore volume. During water storage an expansion of the specimens can be observed [2,11]. The change in volume is approx. 2%. Chemical interactions between e.g. sulphate and carbonate of the tap water and the hardened cement paste may take place. It can be noted that even after more than two years of water storage and several weeks of water permeability testing air still remains in the specimens (Jacobs [2]). A decrease in the saturation during water flow by the release of dissolved air, and a blocking of capillaries is plausible, too. A1Manaseer et al. [12] have discussed this possibility. Hearn [13] showed that the decrease in permeability depends mainly on the curing of the specimens and is explained as self-sealing of microcracks due to drying shrinkage.
163
F. Jacobs, F.H. Wittmann / Long term behavior of concrete
Comparing gas and water permeabilities (figs. 7,8) it is obvious that water permeability is always lower than gas permeability. The higher the gas permeability the smaller is the difference between both values. This result is confirmed by different authors [2,9,14]. So far it is not clear why this discrepancy exists. Some possibilities are discussed in a previous paragraph.
14
12 "-'-'"'I0
K 6
0
5.3. Equivalent pore radii
0.01
A comparison of MIP with permeability measurements after drying to constant weight at 105°C is only valid for the same conditions of the fabric. D-drying reduces the water content to approximately the same value as heating to constant weight at 105°C (Taylor [151).
Plotting the cumulative pore volume (obtained from mercury intrusion porosimetry) versus the pore radius, a median pore radius (pore radius at 50% of total intrusion volume) can be found for each concrete. Making the extrapolation of the Hagen-Poiseuille law for single capillaries to porous media, the permeability coefficient k is introduced by k = r 2 p / 8 . From the permeability values and the porosity an equivalent pore radius was calculated (fig. 9). The equivalent pore radius varies more strongly with changing permeability than the median pore value. In the calculation of the median pore value a few large air voids can have a significant influence on the median value. A correlation between the radius values and the gas permeabil-
IE-14
&
IE-15 1E-16
& 1E-17
2
IE-18 ,.
0.00
0.10
0.20
0.30 0.40 rodius [~m]
0.50
0,60
Fig. 9. Median pore radii ( * ) and equivalent pore radii (o) as a function of gas permeability. The median pore radii corresponds to the pore radii at 50% of the maximum intrusion volume. The equivalent pore radii is calculated by following formula: r = V/~ * 8 ) / p . The scatter in the pore radii for one permeability coefficient is due to the different pore size distributions and porosities from different samples. The line denotes a correlation which was calculated by the equation r = ~[(k * 8 ) / p , by assuming a porosity of 8%.
",,, cumulotive volume[o/o] "-,
vo e 0.1
. . . . . I
10
diometer ~um]
Fig. 10. Incremental pore volume in %o and cumulative pore volume in % as function of the pore size distribution for normal concrete after 91 days water storage; A: diameter corresponding to the threshold pressure; B: equivalent pore diameter.
ity is observed. This correlation coincides approximately with the line in fig. 9 which denotes the correlation between k and r for a given porosity of 8%. For low permeabilities, an equivalent pore radius of 0.05 p.m and for high permeabilities one of 0.2 to 0.3/~m is found. Comparing these results with the MIP curves (figs. 1 to 4) the pore maxima for w / c ratio 0.45 is around 0.05 izm and for w / c ratio 0,8 around 0.2 tzm. The increase to the pore size maxima coincides with the pore size calculated from the threshold pressure. Figure 10 summarizes the characteristics from the pore size distribution for the permeability on the pore size distribution of 91 days old normal concrete (w/c ratio 0.8). The pore size equivalent to the threshold pressure is at the increase to the pore maxima, the equivalent pore size coincides with the pore size maxima or is shifted to bigger pore size. One half of the porosity is created by pores bigger than the equivalent pore size. Smaller pores (gel pores) created by the continuation of the hydration of the cement have a negligible influence on permeability.
6. Conclusions The aging of highly hydrated specimens (91 days) leads to a small change in the pore structure and the permeability. The pore structure and permeability are not strongly correlated with the composition of the concrete (type of cement) and not strongly influenced by the w / c ratio. The water saturation of the specimens generally has a significant influence on permeability. The investi-
164
F. Jacobs, F.H. Wittmann / Long term behauior of concrete
gated normal concrete samples showed a strong decrease in permeability by increasing the water saturation from 40 to 80%. The threshold pressure agrees with the capillary pressure of pores with a radius of approximately at the increase of the pore size maxima whereas the gas permeability agrees with the maxima in the pores size distribution (fig. 10). An attempt to minimize the water content should be a priority in order to increase the gas flow and to decrease the quantity of expelling water. Artifical air voids, capillary barriers or hydrophobing agents can reduce the water uptake of concrete, but the hydrostatic pressure must be taken into consideration.
Acknowledgement We gratefully acknowledge financial support by N A G R A and additionally the kindness of Dipl. Ing. Miiller, T U Munich for leaving us 28 years old concrete samples. W e have to thank Dipl. Phys. Mayer from our institute for his help in the experimental setup and fruitful discussions.
References [1] H. Gr~if, (/ber die Porosit~it und die Durchl~issigkeit von Zementstein, M6rtel und Beton und ihren Einfluss auf Gebrauchseigenschaften von Beton, Ph.D. Thesis University of Essen (1988), 200 p. [2] F. Jacobs, Untersuchung zementgebundener Materialien in Hinblick auf Porosit~it und Permeabilit~it sowie begleitender Eigenschaften, unpubl, report, Institute for Building Materials, ETH Zurich (1991), 72 p. [3] H. Gr~if, H. Grube, Verfahren zur Priifung der Durchl~issigkeit von M6rtel und Beton gegeniiber Gasen
und Wasser, Beton 5 (1986) pp. 184-187, 6 (1986) pp. 222-226. [4] A. Atkinson, P.A. Claisse, A.W. Harris, A.K. Nickerson, Transport of gas through concrete, UKAEA Harwell Report, AERE-G 4977 (1988), 30 p. [5} K. Iriya, F. Jacobs, B. Knecht, F.H. Wittmann, Cementitious backfill materials for a L/ILW repository - Investigations of gas transport properties, Nucl. Engrg. Des. 129, 1 (1991) pp. 49-56. [6] L.J. Klinkenberg, The permeability of porous media to liquids and Bases, Drilling Prod. Pract. Am. Petrol. Inst. (1941) 200-213. [7] R.F. Feldman, H. Cheng-yi, Properties of portland cement-silica fume pastes, I. Porosity and surface properties, Cem. Concr. Res. 15 (1985) pp. 765-774. [8] The cembureau co-operative programme on permeability measurements for concrete, in: Pore Structure and Construction Materials, Vol. I, ed. J,C. Maso (Chapman and Hall, London, 1987) pp. 41-48. [9] R.K. Dhir, P.C. Hewlett, Y.N. Chan, Near surface characteristics of concrete: intrinsic permeability, Mag. Concr. Res. 41 (1989) pp. 87-97. [10] J.G. Cabrera C.J. Lynsdale, A new gas permeameter for measuring the permeability of mortar and concrete, Mag. Concr. Res. 40 (1988) pp. 177-182. [11] A. Atkinson, A. Haxby and J.A. Hearne, The chemistry and expansion of limestone-Portland cement mortars exposed to sulphate containing solutions, UKAEA Hatwell Report, Nirex Safty studies NSS/R127 (1988), 23 p. [12] A.A. AI-Manaseer, M. Onofrei, M.N. Gray, B.S. Shenton, The effect of silica fume and water/cement ratio on the hydraulic conductivity of cement-based grout, MRS Fall Meeting (1990). [13] N. Hearn, a recording permeameter for measuring timesensitive permeability of concrete, in: Ceramic Transactions 16, ed. S. Mindess (1990) pp. 463-475. [14] P.B. Bamforth, The relationship between permeability coefficients for concrete obtained using liquid and gas, Mag. Concr. Res. 39 (1987) pp. 3-11. [15] H.F. Taylor, Cement Chemistry (Academic Press Ltd., London, 1990), 475 p.