DEVELOPMENT IN HIGH PERFORMANCE CONCRETE TECHNOLOGY

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1 DEVELOPMENT IN HIGH PERFORMANCE CONCRETE TECHNOLOGY Marijan Skazlic 1, Dubravka Bjegovic 2, Mladen Jambresic 3 1 Faculty of Civil Engineering, University of Zagreb, Kaciceva 26, Zagreb, Croatia, skazle@grad.hr 2 Faculty of Civil Engineering, University of Zagreb, Kaciceva 26, Zagreb, Croatia, dubravka@grad.hr 3 Faculty of Civil Engineering, University of Zagreb, Kaciceva 26, Zagreb, Croatia, jmladen@grad.hr Received ; accepted Abstract. Development of new materials based on cement binder, and improvement of properties of existing materials have made great advances over the last few decades. All this can be achieved only if development of concrete microstructure, concrete technology and concrete performance are closely related. The modern, innovative types of concrete are expected to have outstanding durability properties, should be environmentally friendly, costeffective, and readily used in construction. High performance concrete (HPC) has been a promising material for many decades. Development of new high performance concrete types has a great success within the last few years. One type of such innovative concrete types are ultra high performance concrete types (UHPC), the development of and researches on which started late in the 20th century. UHPC exhibit a very high compressive (even above 200 MPa) and flexural strengths and possess outstanding properties such as high durability etc. Beside that, high performance concretes with the strength range between 100 and 200 MPa are already integrating because of their lower price related to UHPC. This paper deals with the development of new high performance concrete types. The experimental results of testing mechanical and durability properties of different high performance concrete types (ordinary HPC, ultra high performance fibre reinforced concrete (UHPFRC), high performance fibre reinforced self compacting concrete (HPFRSCC), and high performance sprayed concrete (HPSC)) are presented. Advantage and disadvantage of these concrete types are critically commented with the aim to find the optimum practical application of different high performance concrete types. Keywords: concrete, durability, high performance concrete, high strength, mechanical properties, HPFRSCC, HPSC, ultra high strength, UHPC, UHPFRC 1. Introduction Increasing demands for construction of new, modern buildings and repair of deteriorated constructions in the last few decades led to invention, development and application of new concrete types high performance concretes (HPC). HPC is material based on cement binder with higher strength and at least one improved property than of normal strength concrete (NSC) [1]. Its high performance properties can be achieved only by improving and correlating concrete microstructure, concrete technology and performance in a proper way [2]. Development of HPC technology started in the middle of the 20 th century with the invention of high strength concretes (HSC). According to existing Croatian standards NSC are concrete types of compressive strength classes up to C50/60. HSC correspond to concrete compressive strength classes from C55/67 to C100/115 [3]. If HSC has other improved properties beside compressive strength, than it is called HPC. Concretes with compressive strength higher of compressive strength class C100/115 are called ultra high strength concretes (UHSC). The development of UHSC started late in the 20 th century by modifying some of the existing rules for designing concrete composition and selecting materials in HSC types. Beside ultra-high compressive strength (even above 200 MPa), UHSC also have considerably improved tensile strength, stiffness and durability compared to other concrete types. Therefore, they are also called ultra high performance concretes (UHPC) [4].

In this paper experimental results of testing mechanical and durability properties of different high performance concrete types (ordinary HPC, ultra high performance fibre reinforced concrete

2 In this paper experimental results of testing mechanical and durability properties of different high performance concrete types (ordinary HPC, ultra high performance fibre reinforced concrete (UHPFRC), high performance fibre reinforced self compacting concrete (HPFRSCC), and high performance sprayed concrete (HPSC)) are presented. Advantage and disadvantage of these concrete types are critically commented with the aim to present the optimum practical application of different high performance concrete types. adhesion of fibres and cement matrix, and the quality of the cement matrix itself [6]. 2. HPC and UHPC mix design The mix design and production of HPC is more complicated than design of NSC. By increasing of compressive strength the concrete properties are in correlation not only with water/cement (w/c) ratio, as it is in design of NSC, but also with several other parameters (especially porosity of concrete) [5]. To achieve higher strengths or to improve other properties, special attention should be given to the selection and compatibility of components (cement, superplasticizer, aggregate, mineral additives, fibres, etc.) and technology (production, placement, curing, transport, and quality control). In comparison to NSC, HPC are much more homogenous and less porous. Strength and other properties of HPC grow with the higher number of contacts among particles, reduced porosity and defects within the structure. Considering that HPC contain high quantities of binders, the size of maximum aggregate grain size should be also reduced. Reduction of porosity is achieved by using a low water/binder (w/b) ratio, adding superplasticizer (providing sufficient workability in fresh state), and replacing a portion of cement with pozzolanic additives. Less water in the composition of HPC than in NSC reduces the space between cement grains and mineral additives in the fresh state. In this way capillary porosity is also reduced and so is the space to be filled with the products of hydration [4, 5]. A reduction in water/binder ratio and the use of mineral additives have a positive effect on an improvement in the interface between cement matrix and aggregates as the weakest link in the concrete structure. The most efficient admixture to cement is silica fume. Because of its very small grains (about 10 times smaller than a cement grain) and large specific area, silica fume has a positive effect on an increase in density of the area surrounding cement particles and, because of higher reactivity, on accelerated hydration. Furthermore, silica fume reacts with free lime - the poorest component of cement thereby making CSH gel [2]. Concrete brittleness grows with the increase of its compressive strength. Therefore HSC/HPC are much more brittle than those exhibiting less strength. By using fibres in this weak, brittle matrix, ductility of the cement matrix can be improved (Figure 1). The degree of improvement resulting from fibre reinforcement varies depending on the amount and kind of fibres admixed, Fig 1. Improving ductility of HPC by using hybrid steel fibres Fig 2. Multiple cracking of UHPFRC due to higher ductility Besides taking into account the facts mentioned above, to obtain UHSC/UHPC the concrete designer should adhere to the basic principles as follows: o increase of homogeneity by completely or partially eliminating coarse aggregate, o increase of density of the placed concrete by optimizing grain size distribution in such a way to achieve maximum packing of particles, o improvement to the structure of placed concrete by heat treatment, o water quantity in concrete is maximally reduced and, because of this, its quantity is insufficient for cement hydration; this principle results in the reduced quantity of free water, which can result in the formation of micro cracks because of desiccation; non-hydrated cement acts as reactive micro aggregate of high modulus of elasticity that can hydrate subsequently, o improvement to ductility by adding higher quantity of fibres. By adhering to the first four principles high compressive strengths can be achieved, while by adding fibres tensile strength and ductility are improved. This has an effect on the ductility of the cross-section and structural elements, thereby ensuring deformation and redistribution of cutting forces and prevention of brittle failure of the structure or the test element [6].

3 High performance fibre reinforced self compacting concrete (HPFRSCC) has to have, beside properties of normal self compacting concrete (filling and passing ability, resistance to segregation, prevention of blowholes making on the surface), higher mechanical (e.g. ductility) and durability properties. This can be obtained by modifying composition of ordinary HPC by using: o higher volumes of superplasticizers, cement, and silica fume, o higher quality cement, o lower aggregate/binder (a/b) ratio, o steel fibres, o higher quality aggregate (e.g. diabase), and o larger content of fine aggregate. HPSC can be obtained using the basic principles of high performance fibre reinforced shotcrete technology laid down at the end of the 20 th century. Specifically, HPSC is obtained by modifying shotcrete composition by using: o smaller maximum aggregate size (up to 4 mm), o higher quality aggregate (e.g. quartz sand or igneous sand), o shorter fibres and larger volumes of steel fibres o (even up to 2 vol.%), and larger volumes of cement, silica fume and superplasticizers. The use of these components greatly reduces rebound during application, and makes it possible for HPSC to be applied in a thick layer without falling off [6]. 3. Experimental work In the laboratory of the Department of Materials at the Faculty of Civil Engineering, University of Zagreb, experimental researches have been conducted into above mentioned new types of concrete. These researches have been carried out by investigating, in parallel, properties and technology of concrete. The aim of the researches was to get new types of concrete that exhibit improved mechanical and durability properties in relation to other concrete types. At the same time, they also needed to fulfil the condition that the obtained concrete types can be prepared at the existing concrete production plants. The preceding researches yielded the components for these concretes which were used in the subject researches [2, 7]. In order that the production price of the new concretes is as favourable as possible the components available in the Croatian market were used. Table 1 shows the compositions of the mixtures of HPC, UHPFRC, HPFRSCC, and HPSC tested. The design of the compositions of mentioned mixtures, selection of their components and production were governed by the principles described in previous chapter. Two types of cement were used, CEM II/B-S 42,5 for HPC and CEM I 52,5 for other three concrete types. Mineralogical compositions of cements used in experimental work, calculated by Bogue formula according to the results of chemical analysis, are shown in Table 2. Table 1. Compositions of the mixtures tested Mix components UHP (kg/m 3 HPC ) FRC HPFRSCC HPSC Water/binder (w/b) ratio Max. aggregate size (mm) Aggregate/binder (a/b) ratio Amount of silica fume (% m cem) Superplasticizer amount (% m cem+sil) CEM I 42, CEM I 52, Silica fume Fine aggregate (dolomite) Coarse aggregate (dolomite) Fine aggregate (diabase) Coarse aggregate (diabase) Quartz sand Water Superplasticizer /0.15, flat /0.2, curved 13/0.15, flat /0.55, curved /0.5, curved /0.8, curved /1.0, curved Silica fume used, was in dry state and packed in bags. It had following characteristics: o specific surface cm 2 /g, and o content of total SiO 2 93,02 %. Table 2. Mineralogical composition of cements used Cement C 3S Mineral content (%) C 2S C 3A C 4AF CEM II/B-S 42,5 37,7 12,2 11,1 13,4 CEM I 52,5 70,7 3,5 8,2 9,1 All mixtures had plastic consistency in fresh state. In hardened state, they were tested for the following properties (according to existing Croatian standards): o Compressive strength (HRN EN ) (Figure 3) o Flexural strength (HRN EN or HRN EN 196-1) o Static modulus of elasticity (HRN.U.M1.025) o Toughness (ASTM C1609) o Gas permeability (EN 993-4) o Capillary water absorption (HRN.U.M8.300) o Diffusion of chloride ions (ASTM C 1202)

Compressive strength testing of HPSC Due to size effect in high performance concretes, samples taken, for mechanical properties tests, were of different sizes depending on each concrete type.

4 The results of testing mechanical and durability properties are shown in Table 3. Statistical analysis of the results obtained by the single-parameter model of analysis of variance showed that they are repeatable for all four concrete types. Fig 3. Compressive strength testing of HPSC Due to size effect in high performance concretes, samples taken, for mechanical properties tests, were of different sizes depending on each concrete type. For HPC and UHPFRC compression strength testing cube samples of 15x15x15 cm dimension were used. Cubes of 4x4x4 cm dimension were used for compression strength testing of HPFRSCC. Samples for compression testing of HPSC were cylinders of Φ5/10 cm dimension, cored from test panels (Figure 1). Test panels had plan dimensions of 60x60 cm and thickness of 20 cm. Dimensions of test panels as well as spraying of HPSC were according to EFNARC European specifications for sprayed concrete (Figure 4). Dimensions of samples (prisms) for flexural strength testing were 10x10x cm (for HPC and HPFRC) and 4x4x16 cm (for HPFRSCC and HPSC) (Figure 5). Table 3. Test results Property HPC UHPFRC HPFRSCC HPSC Compressive strength (MPa) Flexural strength (MPa) Static modulus of elasticity (GPa) Coefficient of gas permeability (m 2 ) Coefficient of capillary water absorption (kg/m 2 h 1/2 ) Diffusion of chlorine ions (Coulomb) By comparison of the test results of mechanical properties given in Table 3, it can be seen that concrete types UHPFRC and HPFRSCC have much better mechanical properties than the ordinary HPC and HPSC. Except increased values of compressive strengths in comparison with the other three concrete types, UHPFRC has also high flexural strength and toughness owing to high quantities of steel fibres contained in its composition. With the increase in concrete homogeneity, their stiffness grows too. As for the values of static and dynamic modulus of elasticity, Table 3 shows that UHPFRC and HPFRSCC have also higher values of modulus of elasticity compared to HPC and HPSC. Figure 6 shows that UHPFRC has much higher toughness than other three concrete types, while HPSC and HPFRSCC have similar toughness curves. Furthermore, it can be seen that ordinary HPC performs like brittle material because it has no fibres in its composition Fig 4. Placing of HPSC test panels force (kn) UHPFRC 10 HPC HPFRSCC HPSC displacement (mm) Fig 6. Diagrams showing toughness of concrete types tested Fig 5. UHPFRC samples taking for flexural strength tests Results of testing durability properties show that the UHPFRC exhibits the best durability properties, and then

5 follow HPFRSCC and HPC in this order (Table 3). HPSC was not tested on durability properties. Such results are primarily due to significantly reduced capillary and total porosity of UHPFRC compared to the other two concrete types. 4. Possible structural applications of tested high performance concretes Presently, ordinary HPC is mainly used in precast or in situ constructions of load bearing elements of viaducts and bridges, as well as for high-rise building columns. HPC is also used in construction of concrete pavements (e.g. highways, airports) and for repair of damaged concrete structures. The use of UHPFRC for structures allows the designer to reduce the sizes of structural elements. Modulus of elasticity of these concrete types does not raise proportionally to the strength values. High flexural strength of UHPFRC, in certain applications, makes possible to eliminate ordinary reinforcement and to only pre-stress cross-sections of structural elements. As a result, structural engineers have gained freedom in structural shaping of load-bearing elements. For this reason, in structural application of high-strength and ultra-high strength concretes, slim cross-sections are used [4]. On the basis of the above facts, it can be concluded that UHPFRC can be used for high-building construction, for bridge construction, and for the construction of slim and lightweight structures such as thin-walled roof structures for roofs having wide spans or for girders of low self-weight intended for building additional storey or rising. Considering all properties of UHPFRC, this concrete type could also be used for precast segments of secondary tunnel linings, impact protection panels against missiles and explosion, containers intended for storage of radioactive and other non-degradable waste, waste water pipes, vaults in which valuable things are kept safely, etc. Due to the fact that HPFRSCC contains fine particles which make the structure of a material especially fine, and with the possibility for admixing colour pigments and replacing steel fibres with polymer ones, fabrication of more complicated architectural elements is possible. Furthermore, HPFRSCC is more cost-effective than UHPFRC, and its compressive strength and other mechanical properties are sufficient for the majority of similar structural applications. HPSC can be successfully applied as a shotcrete in placing of primary tunnel lining. In this manner the thickness of the placed HPSC would be reduced thus ensuring flexibility of the primary tunnel support. Rapid growth in strengths would allow it to immediately take over a large portion of load; additionally, because of a large binder quantity, rebound, i.e. the loss of material during installation would be reduced. HPSC can also be applied in rock masses of poorer quality with reasonable certainty that in time such concrete will not lose its durability properties, which could result in the collapse of a tunnel [8]. By using fibres in this concrete type, it gains high ductility and energy absorption along with small self-weight. Therefore, this kind of concrete can also be used for strengthening and repairs of existing structures. 5. Conclusion Described high performance concretes exhibited much better mechanical and durability properties than other concrete types. Outstanding characteristics of these materials enable their structural application in various fields of civil engineering. Excluding ordinary HPC which is in use for a longer time, other three tested concrete types (UHPFRC, HPFRSCC and HPSC) have been applied for the last five to ten years. However, the first structures built of these concrete types were test structures aimed primary to monitoring material and structure behaviour with time. So it can be generally said that the real structural application is yet to take hold. The properties and production technology of these concretes should further be improved with the aim to use them in the structures in which the properties of material will be used optimally. Experimental work performed at the Faculty of Civil Engineering in Zagreb was one valuable step to increase future applications of these materials. References 1. Nawy, E. Fundamentals of high-performance concrete. 2 nd Edition, New York: John Wiley & Sons Inc, Skazlic, M. High performance hybrid fibre reinforced concrete. Master s Thesis, Zagreb: Faculty of Civil Engineering, University of Zagreb, (in Croatian). 3. Radic, J. Concrete structures handbook, Zagreb: Faculty of Civil Engineering, University of Zagreb, 2006., p (in Croatian). 4. Skazlic, M., Bjegovic, D. Perspectives of designing with new concrete types. Zagreb: Annual 2005 of the Croatian Academy of Engineering, 2005., p Aitcin, P. C. High-performance concrete, London: E&FN SPON, Skazlic, M. Precast fibre reinforced segments of secondary tunnel lining. Dissertation, Zagreb: Faculty of Civil Engineering, University of Zagreb, (in Croatian). 7. Skazlic, M., Bjegovic, D. High strength concretes, Gradjevinar 56 (2004) 2, p (in Croatian) 8. Skazlic, M., Skazlic, Z., Majer, J. Application of high performance fibre reinforced shotcrete for tunnel primary support, Proceedings of the 10th International Conference - Shotcrete for Underground Support, Whistler, p

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