CEMENT and CONCRETE RESEARCH. Vol. 22, pp. 319-324, 1992. Printed in the USA. 0008-8846/92 $5.00 + .00. Copyright © 1992 Pergamon Press Ltd.

VERY HIGH PERFORMANCE MICRO-CONCRETES FOR THE CONFINEMENT OF INDUSTRIAL WASTE

- G. Cochet * - B. Cariou **

• Docteur Ing6nieur- Lafarge Copp6e Recherche BP 15 - 38291 La Verpilli~re Cedex - France

• * Ing~nieur de Recherche - Lafarge Copp~e Recherche BP 8 - 07220 Viviers sur Rh6ne - France

A b s t r a c t Industrial waste is very often immobilized in metallic casks which are then installed

in concrete confinement enclosures. High performance concrete types can provide an elegant solution for the storage of waste, with the possibility of avoiding double-wall confinement.

This study analyses the behavioural evolutions of very high performance micro- concretes with a fibre content, with particular attention to the nature of the aggre- gates and the nature, the dimension and the quantity of the fibres. The characteristics obtained, which are generally of a veryhigh order, result from the very low porosity and permeability of the matrix.

In a similar manner we deal with the problem of sealing the enclosure, by using a special mortar which provides excellent sealing for the container through perfect binding between the walls of the enclosure and the plug.

1. INTRODUCTION

The confinement of industrial waste has always involved serious problems with respect to the durability of the containers given the requirement to safeguard the environment. One of the solutions developed for storing waste, in particular radio- active waste, consists in confining it in an envelope, generally metallic, which itself is placed within a concrete enclosure.

The object of this study concerns the development of micro- concretes with suffi- cient performance to enable a new concept in the immobilization or storage of waste, without referring to the double-wall principle,to be achieved. The compositions envisaged must present performance parameters (mechanical,

physical, chemical) such that they constitute an efficient and durable barrier between the waste and the outside world.

On the basis of this principle, and based on formulations for very high performance micro-concretes with a fibre content, derived from published research works [1 - 4 ] , we have investigated the influence of different parameters such as: - nature of the aggregates, - nature, dimension and quantity of fibres, - consistency of the composition; in such a manner as to account for the behavioural and characteristic modifications of the materials obtained.

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320 G. Cochet and B. Cariou Vol. 22, Nos. 2/3

One of the recognized weak points in the concrete enclosures used to immobilize waste is the sealing efficiency of the "plug", whether it is cast in situ or added and grouted. In both cases, shrinkage of the plug or seal can lead to the development of cracks between the plug and the walls of the enclosure.

To avoid this nuisance, we have developed a special mortar with a compensated shrinkage, the characteristics of which are defined in relation with those of the very high performance matrix used for the enclosure. Its properties are characterized byhigh strength, very low dimensional variations and the absence of any shrinkage; these commend it for sealing.

2. VERY HIGH PERFORMANCE MICRO-CONCRETES

On the basis of conventional formulations containing a cement matrix with an appropriate composition we have investigated the effect of certain parameters on the characteristics of the products.

The cement matrix is constituted from: - a Portland cement in sufficient quantity,

mineral additions (fillers and/or silica fume) to complete the granular distribution of the composition and to react with the Portlandite released in the course of binder hydration,

- organic additives to limit the quantity of water required for mixing and hence improving the characteristics of the products.

2.1 .Influence of the nature of the aggregates Table 1 illustrates the mechanical characteristics of the two micro-concretes used

on the basis of the same composition and implemented at the same consistency level: 13 cm slump on an Abrams cone. The variable factor resides in the nature of the aggregates : siliceous or corundum type.

Table 1 Influence of the nature of the aggregates - slump 13 cm - samples dia. 11, h 22 cm

aggregates mixing water % compressive strength(MPa) splitting (MPa 1 day 28 days 28 days

siliceous up to 8 mm 5 39 113 7,5 siliceous up to 1 mm 4,4 45 131 8,3 corundum 1-8 mm

Analysing this table, it becomes evident that: the micro-concretes envisaged are very high performance products and are

characterized by a low water requirement, a relatively fluid consistency and high mechanical strength.

in comparison with siliceous aggregates, corundum aggregates require a lower quantity of mixing water which translates into higher mechanical strength.

2.2. Influence of the nature, the quantity and the dimension of the fibres To complete this study we have used the two compositions (table 1) adjusting.

the quantity of fibres in such a manner that for a previously defined quantity of m~xmg water, the composition should offer a fluid consistency rate corresponding with a 13 cm slump.

To obtain high performance products the quantities of mixing water have been

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empirically restricted to 5.1% (for corundum), 6.6 % and 7.8% (for siliceous aggre- gates) to retain porosity and strength characteristics which are compatible with very high performance.

Four types of fibres have been used: - 2 types of steel fibres, of different dimensions - amorphous cast-iron fibres - alkali-resistant glass fibres

Table 2. Influence of the nature, the quantity and the dimension of the fibres - slump 13 cm- samples o 11, h 22 cm - %fibres, in weight, are ( )-

siliceous up to 8 mm

steel O 0,2- 1 17 mm steel O 0,5- 1 30 mm amorphous cast iron 30xl,5x0,03 mm alkali-resistant glass

(4) (5) (10) (12)

(2)

siliceous up to 1 mm corundum 1 - 8 mm

(3,4)

(0,6)

water % 6,6 7,8 6,6 7,8 6,6 6,6 5,1

splitting(MPa) 10,7 11 15,8 15,9 8,1 7 14,4 compressive s~ength (MPa) 1 day 37 31 38 33 29 26 48 28days 115 95 120 97 101 90 147

An analysis of the results in table 2 shows that in comparison with the specimen product with no fibres made with the same consistency: - the integration of amorphous cast-iron fibres and glass fibres results in a loss in the compressive strength, without any modification in resistance levels through split- ting.

the introduction of steel fibres, with 6.6% of mixing water, does not modify the compressive strength, but leads to a significant improvement in resistance to splitting, an improvement which is more noticable for long fibres.

Increasing the amount of fibres leads to an increase in the quantity of mixing water (in as much as an attempt is made to work with a constant consistency factor) and, at a later point in time, to a decrease in compressive strength. Resistance to splitting is however retained. Note: The percentage of large dimension steel fibres is clearly higher, in weight, than that of small fibres. The number of long fibres present in the composition however remains lower.

Incorporating 3.4% of short steel fibres in the corundum based composition (the equivalent of 4% of these same fibres in a siliceous matrix, in view of the differences in aggregate density) translates into an appreciable improvement in compressive strength and a very marked improvement in splitting resistance.

The complementary characteristics for the steel fibre and amorphous cast-iron fibre products indicated in table 3, shows evidence that these two products provide low

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and very close dimensional variations, a low rate of porosity and a very good rate of permeability to air.

Table 3 Complementary characteristics at 28 days - Siliceous aggregates up to 8 mm slump 13 cm - prisms 7x7 x28cm

Fibres shrinkage expansion porosity to air perme- static modulus (% in weight) 20°(2-50% RH wn/m water (%) ability (m2) of elasticity (MPa)

steel ¢ 0,2- 1 17 mm - 19 (4%) - 350 + 20 5,9 < 10 49000 amorphous cast iron - 19 (2%) 30x 1,5x0~03mm - 440 + 30 7,5 < 10 46800

2.3. Influence of the introduction of fibres on the workability of the composi- tion

The introduction of further quantities of fibres in the composition, when the quantity of mixing water is kept at the same level, translates into a notable decrease in work ability. In the same way, the mechanical strength of products increase in a significant manner when steel fibres are in contact with a siliceous matrix, but the increase in strength is not very significant in a corundum matrix(table 4).

Table 4 - Influence of the fibre % (in weight)- Mixing water-- 7.8% - samples o 11 x 22 cm

siliceous up to 8 mm siliceous up to lmm corundum 1- 8 mm

steel ¢ 0,2- 1 17 mm 5 7 3,4 5,2 steel ¢~ 0,5- 1 30 mm 12 13,5

slump (cm) 13 8 13 10 13 10 compressive strength 1 day 31 40 33 38 48 48 (MPa) 28 days 95 123 97 117 147 150

splitting (MPa) 28days 11~0 13,2 15~9 17~4 14,4 14~7

2.4.Conclusions These investigations have shown evidence that under certain conditions it was

possible to introduce relatively significant quantities of fibres into very high perfor- mance micro- concrete compositions. The presence of these fibres does not change the porosity and permeability of the cement matrix in any significant way, it enables the retention of mechanical strength. Levels of splitting are appreciably improved, which translates into clearly lower (compressive strength/splitting resistance) levels than those conventionally obtained with non- fibre concretes. In view of the fact that the fibre structure develops a 3D internal matrix, these materials offer further properties which are of particular interest with respect to shock or impact.

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3. MORTAR FOR THE PLUG

Plugging the concrete enclosures with a hydraulic binder-based material comes up against problems in the field of both sealing and bonding between the wall of the enclosure and the plug.

In a concrete or mortar, shrinkage can come from three sources: hydraulic shrinkage resulting from binder hydration, hygrometric shrinkage, resulting from dessication of the material,

- thermal shrinkage.

While the last two types of shrinkage are a direct consequence of the environment and cannot be controlled in practical terms, it is possible to control hydraulic shrin- kage by modifying the cement composition. Using an expansive binder in the plug (in which case the rate of expansion must be perfectly mastered and controlled), deve- lops stress at the enclosure wall. As long as these stress levels remain, the material will not be able to contract, and plug cracking or separation cannot occur.

On the basis of this principle, we have developed a formulated mortar the charac- teristics of which are presented under table 5.

Table 5 - Plug mortar - characteristics

water slump mechanical strength (11 x 22) expansion air perme- (%) (cm) compression (MPa) splitting (MPa) 28 days-lam/m ability (m2)

1 day 28 days 28 days ?x7x28 28 days

-19 plug mortar 11 6 44 75 5,3 + 250 10

The product is characterized by high levels of mechanical strength, low dimensional variations and a very high level of permeability to air.

To characterize the sealing effect of the enclosure-plug assembly, there is no purpose in measuring the free shrinkage of a mortar of this type. In our opinion, it appeared more appropriate to characterize the link directly, using permeability measurements on the wall/plug interface. Figure 1 simulates this type of test using hollow cylindrical specimens (outer dia. 80,

inner dia. 25 mm)constituted of very high performance fibre content micro- concrete, filled internally with the expansive mortar.

The specimens are stored before testing in different atmospheres, but are always tested after a further cure at 20°C - 65% RH, for 15 days.

The results, in terms of air permeability on 30 mm dia. cylinders (integrating the complete linking structure), are close to 5.10"17m 2" This confirms the excellent behaviour of these products and the very high quality of the bond.

324 G. Cochet and B. Cariou Vol. 22. Nos. 2/3

MORTAR PLUG (0 25 mm)

TEST SPECIMEN (0 30 mm)

MICRO-CONCRETE

( 0 80 mm )

Figure 1 - Bonding diagram - specimen characterizing enclosure and plug

4. CONCLUSIONS

The containment of waste is generally done using double-envelope systems. The work described shows that the characteristics of very high performance fibre micro- concretes are sufficient to enable the consideration of single-envelope systems. In fact the introduction, within an appropriate cementitious matrix, of selected aggre- gates and metallic fibres leads to very high performance products which are characterized by high mechanical strength, especially in terms of splitting, by small dimensional variations, and by very low porosity and permeability rates.

This set of characteristics, in combination with the particular structure of the pro- duct where the 3D fibre network plays the role of an internal structure, leads to the assumption of good behavioural characteristics for the material in terms of durability, both in terms of chemical aggression and in terms of shock or impact.

Further more, the plugging of these enclosures with a special expansive mortar (the properties of which are defined and are compatible with those of the envelope matrix), gives the monolith very interesting characteristics. In particular, the excellent sealing performance at the level of the enclosure-plug interface will contribute to enhance the containment of the waste, thus avoiding contaminating the surrounding environment.

5. REFERENCES

1 G. Bernier - Conference Composites Ciment-Fibres, ENS Cachan - France 22-25 Nov. 1988 - Tome 3.

2 Amer. Conc. Inst. Committee 363, Report R 363 - R 84, 1984 3 P.C. AYtcin, P. Laplante, C. Bedard - American Concrete Institute, SP 87, 1988,

p. 51-70 4 P.C. AYtcin - Conference pr~sent~e au S~minaire de rATILH, Paris, 7 Nov.

1988