Study on high performance roller compacted concrete

Transcription

1 Materials and Structures/Matériaux et Constructions, Vol. 33, July 2000, pp TECHNICAL REPORTS Study on high performance roller compacted concrete A. C. Bettencourt Ribeiro 1 and I. R. de Almeida 2 (1) National Laboratory of Civil Engineering, LNEC, Lisbon, Portugal (2) Building Construction Materials - UFF, Niterói, Brazil Paper received: September 23, 1999; Paper accepted: December 21, 1999 A B S T R A C T Results are presented of a study about mixture design, production and characterisation of a concrete conjugating the typical features of roller compacted concrete with the mixture features of high performance concrete. The compressive strength, the tensile strength and the abrasion resistance of the material produced were determined together with the modulus of elasticity. It has been verified that the material had a very high compressive strength, an abrasion resistance higher than the very coarse aggregate, which formed its composition, a modulus of elasticity comparatively proportional to the compressive strength and a tensile strength not very developed. It has also been assumed that, due mainly to the energy spent in the vibration of that concrete, the binding material used was three times more effective than when used in normal concrete. R É S U M É On présente les résultats d une étude sur le mélange, la production et la caractérisation d un béton associant les caractéristiques typiques d un béton compacté au rouleau avec les caractéristiques de dosage d un béton à haute performance. La résistance à la compression, la résistance à la traction et la résistance à l abrasion du matériau produit ont été déterminées simultanément avec le module d élasticité. On a vérifié que le matériau avait une résistance à la compression très haute, une résistance à l abrasion plus grande que celle de l agrégat lui-même, entrant dans sa composition, et, finalement, avait un module d élasticité relativement proportionnel à la résistance à la compression, pour une résistance à la traction peu élevée. On a aussi constaté qu à cause de l énergie utilisée pour la vibration de ce béton, le matériau agglomérant utilisé a été trois fois plus efficace que s il avait été utilisé dans un béton courant. 1. INTRODUCTION Roller compacted concrete (RCC) is a material used in the construction of dams and pavements. It is a dry concrete, consolidated by a very powerful external vibration, using the vibrating cylinders usually employed in the compaction of soils. This material differs from conventional concrete (CC) not only as regards the placement method but also as regards its consistency and mixture. In fact, the consistency should be designed to allow the circulation of the heavy compacting cylinders on the concrete surface. Its mixture design is usually characterised by the addition of a low cement dosage. The limitation in the cement dosage is due, in the case of dams, to the need of preventing a high release of hydration heat by the binder. In the case of pavements, the RCC is seldom used as course layer and therefore, a high mechanical performance is not required from it. Considering that the first dams constructed with RCC did not present a performance similar to the conventional concrete dams, due mainly to the scarce knowledge available on this material, there was an initial trend to consider that this type of concrete could not have a behaviour similar to CC [1]. Nevertheless, more recent studies [2-4] have proved that the RCC may present similar and even better performance than conventional concrete, due to the high compaction energy used in its consolidation, provided that the mixture is dully formulated and good quality component materials are used. Thus, the study on the behaviour of RCC made with unusual mixtures, may extend the field of application of that material, as for instance, in slabs on ground, top pavement layers, dam elements subject to significant erosion, etc. The usual features of a RCC are, to some extent, opposed to those of high performance concrete (HPC), Editorial Note LNEC (National Laboratory of Civil Engineering) is a RILEM Titular Member /00 RILEM 398

2 Bettencourt Ribeiro, de Almeida since the latter tends to present mixtures with a high binder dosage. Furthermore, the HPC, in some applications, requires a fairly fluid consistency, almost self-levelling and self-compacting, so as to require a low or even none vibration energy. Since its prevailing use occurs in slenderer structural parts, as for instance the columns of high rise buildings, the problem related with the dissipation of the hydration heat is not usually worrying [5]. Despite that relative antagonism in their features, it has been decided that the possibility of making a high performance roller compacted concrete should be investigated. Thus, in this work, an analysis is made to the resistance and deformability of a RCC made with a mixture similar to those used in HPC, i.e., with a high cement dosage, about 10% of silica fume, a superplasticizer admixture and aggregates with a high mechanical resistance. 2. MATERIALS AND MIXTURES The composition of that concrete, designated as HPRCC, included a type I, class 52.5 Portland cement, a densified silica fume, basalt coarse aggregates, siliceous river sand, as well as a superplasticizer. Tables 1 to 4 show the characteristics of the component materials. Table 5 shows the concrete mixture used. The grain size curve of the solid particles of the concrete followed the theoretical Faury curve [6, 7], parameters A and B being 18 and 1, respectively. This refers to concrete with crushed coarse aggregate, rounded sand and dry workability. The water dosage has been established in order to provide the HPRCC with a consistency of 13 ± 3 Vebe seconds. The consistency has been measured in accordance with specification ASTM C [8], method A, with a compaction surcharge of 22.7 kgf. Fig. 1 shows the appearance of the concrete immediately after leaving the concrete mixer and Fig. 2 shows the moulding of a 150 mm test cube, with a surcharge of 9.1 kgf. 3. TESTS With the composition shown in Table 5, the concrete has been produced using the same equipment and following the same manufacturing procedures as those usually used in a technologic control laboratory. Table 6 shows the tests performed. The test samples for the execution of those tests have been prepared in accordance with specification ASTM C [9]. Table 6 also indicates the test standard or specification, the number and size of the specimens prepared, as well as the test ages. 4. RESULTS Table 7 shows the results of the compressive strength tests. Table 8 shows the results obtained from the other determinations performed on the HPRCC. Table 1 Mechanical behaviour of cement I 52.5 Characteristics Values Compressive 2 days 30.7 strength 7 days 48.9 (MPa) 28 days 59.2 Flexural 2 days 5.2 strength 7 days 7.6 (MPa) 28 days 8.5 Specific weight (kg/m 3 ) Soundness (mm) 1 Initial setting time (h:min) 2:30 Table 2 Chemical and physical analysis of cement and condensed silica fume Tests Cement I 52.5 Condensed silica fume Loss on ignition (%) Insoluble residue (%) SiO 2 (%) Fe 2 O 3 (%) Al 2 O 3 (%) CaO (%) MgO (%) Na 2 O (%) K 2 O (%) SO 3 (%) Cl - (%) free lime, CaO (%) Fineness: BET (m 2 /g) Blaine (m 2 /g) Average grain size (mm) * * particles agglomeration Table 3 Physical characteristics of aggregates Characteristics C.A. 2 C.A. 1 C.A. 0 Sand Specific weigh (kg/m 3 ) Absorption (%) Maximum size (mm) Abrasion - Los Angeles (%) 16 - Compressive strength Saturated/dry state (MPa) 298/293 - Fineness modulus Modulus of elasticity (GPa) Porosity (%) Abrasion resistance at 200m (mm)

3 Materials and Structures/Matériaux et Constructions, Vol. 33, July 2000 Table 4 Characteristics of the admixture (information provided by the manufacturer) Type modified polycarboxylic ether Relative density 1.1 ph 6.6 Table 5 Concrete mixture Component Dosage Coarse aggregate kg/m 3 Coarse aggregate kg/m 3 Coarse aggregate kg/m 3 Sand kg/m 3 Cement kg/m 3 Condensed silica fume 49.4 kg/m 3 Superlasticizer l/m 3 Water l/m 3 W/(C+CSF) Table 6 Tests on HPRCC Test Specification Test specimens (mm) Number of Testing ages specimens (days) Compressive strength pr EN-ISO 4012/1 cubes , 90 (1994) [10] cylinders 150 X , 7, 28, 90 Tensile splitting strength pr EN-ISO 4108 cylinders 150 X (1994) [11] Abrasion resistance LNEC E 396 plates 6 X 6 X 2, method A [12] Modulus of elasticity LNEC E 397 [13] cylinders 150 X Table 7 Simple compressive strength of the concrete (MPa) Test specimens (mm) Age (days) Cylinders (150 x 300) Cubes (150) Fig. 1 Aspect of the concrete immediately after leaving the concrete mixer. are about 4 times and half higher than those obtained in normal concrete at 28 and 90 days. At 3 days, the compressive strength of the HPRCC, in values estimated for cubes, is already higher than 100 MPa. At 90 days, the resistance in test cubes is higher than 170 MPa, which is a fairly high value considering the cement and silica fume dosages used. By comparing the effectiveness of a kilogram of binder for the compressive strength at 28 days of the HPRCC with a normal concrete C20/25, made with 300 kg/m 3 of binder and with a resistance of 33 MPa, the values obtained were MPa/kg in the first case, and 0.11 in the second case, i.e., an effectiveness about three times higher in high performance concrete. The compressive strength values measured in that concrete are significantly higher than the reference values of high performance concrete made in Portugal. In fact, the compressive strength at 28 days of a HPC, for binder dosages of about 500 kg/m 3, ranges, in general, from 100 to 110 MPa [5]. The high strength obtained in Table 8 Other characteristics of the concrete Characteristics Testing age Value Tensile splitting strength 28 days 8.0 MPa Abrasion 28 days < 0.1 mm Modulus of elasticity 28 days 48.6 GPa 5. ANALYSES OF RESULTS Fig. 1 shows the evolution of the compressive strength with the age of the HPRCC and of a C20/25 normal concrete. The results obtained in the HPRCC Fig. 2 Moulding of a 150 mm test cube using a vibrating table and a surcharge of 9.1 kgf (ASTM C ). 400

4 Bettencourt Ribeiro, de Almeida of Eurocode 2 [14] (for the estimation of the modulus of elasticity of the concrete), E = fc, where: E - modulus of elasticity, in GPa, f c - compressive strength (cylinders, in MPa). The value, 48.5 GPa, is almost equal to the result obtained in the test performed at 28 days, which has been 48.6 GPa. 6. FINAL CONSIDERATIONS Fig. 3 Simple compressive strength of HPRCC, on 150 mm test cubes and on 150 x 300 mm test cylinders, as well as of a normal concrete C20/25, on test cubes. the HPRCC, even as refers to the HPC, was considered to be due to the following factors: the use of a highly resistant basalt rock, which increases significantly the energy necessary for the failure of aggregate particles; the use of a highly powerful superplasticizer, which makes possible to decrease significantly the dosage of mixing water; the high compaction energy used in the production of RCC, which makes possible to consolidate a comparatively dry concrete; the dry consistency of RCC, which makes possible to reduce considerably the dosage of sand used in the mixture. All these factors contribute to obtain a concrete mass formed mainly by a highly resistant paste matrix (W/C+CSF = 0.20) and by rock particles also highly resistant. Besides, the reduced sand dosage leads to the existence of a lower number of paste-aggregate interfaces and, therefore, the failure surfaces cross less areas of those zones potentially weaker, increasing thus the energy required to produce the failure of the test sample. The abrasion resistance of HPRCC was also very high. The wear occurred was so small that it has not been possible to measure it with the test methodology used. It is interesting to observe that the high resistance of the paste produced an abrasion resistance of the concrete even higher than the abrasion resistance of the very aggregate. The tensile splitting strength was comparatively limited, in view of the values obtained in the compressive strength. In fact, the tensile splitting strength at 28 days was only 5.1% of the compressive strength measured on test cubes. That type of result was expected, considering that the relation between the splitting tensile strength and the compressive strength decreases usually with the increasing resistance of concrete. The modulus of elasticity of the HPRCC presented a value that can be considered as normal for the compressive strength shown. For verification purposes, it has been calculated the value resulting from the expression In this work, studies have been done in laboratory, about proportioning, production and determination of some properties of a roller compacted high performance concrete. The results have proved that very high compressive strengths can be obtained with a HPRCC. Compressive strength values higher than 150 MPa at 90 days can be easily obtained, when a very resistant coarse aggregate is used, mainly because it is possible to reduce significantly the dosage of sand and water in that type of concrete, and also because a high compaction energy is used. The abrasion resistance measured in that HPRCC has also been considered high, which means that it can be used in various situations, as for instance, on floors and pavements subject to high wear. The modulus of elasticity and the splitting tensile strength presented values compatible with those of the compressive strength. The possibility of obtaining HPC with the manufacturing and placement procedures used in the RCC works, makes it possible, undoubtedly, to extend the range of practical applications of that material that has been mainly used on elements where the high compressive strength and abrasion resistance of the concrete are not aspects considered as relevant. Since the binder dosage in HPRCC is comparatively high, for massive structures, further studies are required about properties that control cracking tendency, namely: autogenous shrinkage, early creep/relaxation and heat of hydration. ACKNOWLEDGEMENTS The authors wish to thank to CAPES/MEC and CNPq/SCT (Brazil) for their financial support, as well as to LNEC (Portugal) for the technical assistance provided during the execution of this research work. REFERENCES [1] Moler, W. A. and Moore, J. F., Design of seepage control systems for RCC Dams, in Roller Compacted Concrete II, Proceedings of an International Conference, San Diego, California, Feb. 29 March 2, 1988 (American Society of Civil Engineers, New York, 1988) [2] Andriolo, F. R., Contributions to the knowledge and development of Roller Compacted Concrete (Barber-Greene, Brazil, 1989) (Only available in Portuguese). 401

5 Materials and Structures/Matériaux et Constructions, Vol. 33, July 2000 [3] Ribeiro, A. B., Characterisation and mixture proportioning of Roller Compacted Concrete, Report 57/97 NAB, Portuguese National Laboratory of Civil Engineering, 1997 (Only available in Portuguese). [4] Oliveira, P. J., Salles, F. M. and Andriolo, F. R., Studies of various types of RCC mix designs laboratory test results, in Roller Compacted Concrete Dams, Proceedings of an International Symposium, Santander, Spain, 2-4 Oct (Spanish Institute of Cement and its Applications, Madrid, 1995) [5] Almeida, I. R., High Performance Concrete. Characteristics and Composition, Doctoral thesis, Instituto Superior Técnico da Universidade Técnica de Lisboa, Portugal, 1990 (Portuguese National Laboratory of Civil Engineering, Lisbon, 1990) (Only available in Portuguese). [6] Faury, J., Le Béton, third edition, (Dunod, Paris, 1958). [7] Dreux, J., Connaissance du Béton (Société de Diffusion des Techniques du Bâtiment et des Travaux Publics, Paris, 1964). [8] American Society for Testing and Materials, ASTM C , Standard test method for determining consistency and density of Roller-Compacted Concrete using a vibrating table Annual Book of ASTM Standards, vol. 4.02, (ASTM, United States of America, 1997) [9] American Society for Testing and Materials, ASTM C , Standard practice for making Roller-Compacted Concrete in cylinder moulds using a vibrating table Annual Book of ASTM Standards, vol. 4.02, (ASTM, United States of America, 1997) [10] European Committee for Standardization CEN/TC 104, pr EN-ISO 4012/1 1994, Testing concrete Determination of compressive strength of test specimens, Draft: Dec (European Committee for Standardization, Brussels, 1994). [11] European Committee for Standardization CEN/TC 104, pr EN-ISO , Testing concrete Determination of tensile splitting strength of test specimens, Draft: July 1994 (European Committee for Standardization, Brussels, 1994). [12] National Laboratory of Civil Engineering, LNEC E Concrete. Determination of abrasion resistance (Portuguese National Laboratory of Civil Engineering, Lisbon, 1993) (Only available in Portuguese). [13] National Laboratory of Civil Engineering, LNEC E , Concrete. Determination of elastic modulus in compression (Portuguese National Laboratory of Civil Engineering, Lisbon, 1993) (Only available in Portuguese). [14] European Committee for Standardization, European Prestandard ENV , Eurocode 2, Design of Concrete Structures. Part 1 General Rules and rules for buildings, Dec. 1991, (European Committee for Standardization, Brussels, 1991). 402