HIGH PERFORMANCE CONCRETE AS A SUSTAINABLE MATERIAL

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1 HIGH PERFORMANCE CONCRETE AS A SUSTAINABLE MATERIAL Magdalena Kynclova 1) Ctislav Fiala 2) and Petr Hajek 3) Department of Building Structures, Faculty of Civil Engineering, Czech Technical University in Prague, Czech Republic. magdalena.kynclova@fsv.cvut.cz 1), ctislav.fiala@fsv.cvut.cz 2), petr.hajek@fsv.cvut.cz 3), Fax : Biography: Magdalena Kynclova is a doctoral student at the Department of Building Structures at CTU in Prague and researcher at the Centre for Integrated Design of Advanced Structures (CIDEAS). Her specialization is in high performance concrete, ultra high performance concrete and alkali-activated concrete. She has already cooperated on 3 research projects focused on sustainable construction of buildings and on optimization of structures from fibre concrete and HPC. In 2010 she spent 8 months at Kassel University in Germany, working under supervision of prof. M. Schmidt on the research of alkali-activated concrete and UHPC. Ctislav Fiala is a lecturer at the Department of Building Structures at CTU in Prague and researcher at the CIDEAS centre. His professional focus is on optimization and integrated design of concrete structures and consequent LCA analysis. He has recently finished a doctoral thesis on Integrated Design of Floor Structures Using of High Performance Concrete. He has already cooperated on 5 research projects dealing with sustainable development and optimization of concrete structures. In 2009 he spent 3 months at Kassel University in Germany, working under supervision of prof. M. Schmidt on the research focused to environmental impacts of UHPC. Petr Hajek is a Professor and Head of the Department of Building Structures at CTU in Prague and deputy head of CIDEAS centre. His professional focus is on sustainable and environmental friendly building design, optimization of load bearing structures and the use of recycled materials in building constructions. In 1989, he spent six months at US universities where he co-operated with prof. D. M. Frangopol on optimization research. Since 1991, P. Hájek has co-operated with prof. K.S. Virdi from the City University of London (UK) in the area of floor slabs optimization. P. Hájek co-ordinated around 10 research projects and published more than 140 publications. P. Hájek is a member of the international Commission fib C3 - Environmental Aspects in Design and Construction. Since 2002, he has been the head of the fib Task Group C3.7 - Life-Cycle Assessment 1

2 and Optimisation of Concrete Structures and board member of international organization iisbe International Initiative for Sustainable Built Environment. ABSTRACT Concrete is after water the second mostly used material and the most widely used construction material in the world. The production of cement creates more than 7% of worldwide man-made CO 2 emissions. Therefore optimization of concrete structures can lead to the significant environmental savings. Experimental investigation and case studies performed by authors in the frame of long term research, focused on environmental optimization of building structures, support the expectation that it will be possible to reach factor 3 or even more through utilization of high performance concrete (HPC) while keeping structural reliability on the needed high level. Developed structural concepts have been proved not only by theoretical and experimental results, but also by practical application in construction of several buildings. Paper presents three case studies ribbed / waffle floor structure with minimized thickness of upper deck to 30 mm, light precast RC balcony element and light precast RC frame for passive house. Keywords: High Performance Concrete, Structural Efficiency, Sustainable Structure 1. INTRODUCTION Concrete is due to its mechanical parameters, durability, availability of the original materials and possibility of variable design undoubtedly the most wide spread structural material. Production of concrete in the industrialised world annually amounts to tonne per capita. World cement production was 12 times increased during the second half of the last century (Hajek, 2003). The extraction of raw materials for construction, manufacturing of structural elements, construction processes and waste landfill at the end of life cycle are associated with corresponding environmental impacts. From the above mentioned state of affairs follows the undisputed need for significant reduction of consumption of primary non-renewable materials one of the basic principles of Sustainable Construction. Development of construction materials, structures and construction technologies should be thus based on the struggle for the reduction of primary nonrenewable material and energy resources, while keeping performance quality, safety and durability of the structure on the required high level. New composite high performance concrete reaches significantly better properties from the aspect of mechanical resistance, durability and resistance against extra loads. These high performance materials could be used for construction of stronger, more durable and at the same time slender shell structures, enabling design with significantly reduced use of materials. This 2

3 leads to reduction of environmental impacts associated with the use of primary natural sources and with depositing and recycling of the structure at the end of its life cycle. It has been already shown that for many structures it is possible to reduce amount of used concrete by 30 60% - when materially and shape optimized solution is applied (Hájek & Fiala 2007). The complex LCA and LCC of optimized HPC structures would show not only environmental benefits, but also the cost efficiency - in spite of the fact that HPC is more expensive and has higher values of unit embodied parameters. Moreover, high performance material properties (higher ductility, fire safety, water tightness, frost resistance, etc.) make structures more durable and more resistant against climatic effects and also safer in case of exceptional loads (like climatic disasters or terrorist attacks). This all creates the potential for wider application of HPC in building construction in the future. Three examples of applications of optimized HPC structures for the use in building construction, followed by evaluation of environmental profiles and showing advantages of proposed solutions are presented in chapters 3, 4 and ENVIRONMENTAL IMPACT OF CONCRETE The specific amount of harmful environmental impacts embodied in 1 kg of concrete is relatively small in comparison with other building materials. However, due to the amount of produced concrete the final negative environmental impact of concrete structures is very high (just cement industry produces more than 7% of global man-made CO 2 emissions). There is a big potential for the employment of high performance silicate materials (HPC, UHPC) to form ultra thin shell (ribbed, waffle, etc.) structures with reduced use of primary raw materials, and correspondent reduction of associated environmental impacts. The complex LCA and LCC of optimized HPC structures could show not only environmental benefits, but also the cost efficiency - in spite of the fact that HPC is more expensive and has higher values of unit embodied parameters in comparison to ordinary concrete. The quality of results of LCA analysis is determined by the quality of input data describing environmental impacts of concrete components, production processes and aggregated data for concrete and concrete structures. Only complex aggregated LCA results covering all life cycle of the structure can show real environmental benefits. Moreover, high performance material properties (higher ductility, fire safety, water tightness, frost resistance, etc.) make structures more durable and more resistant against climatic effects and also safer in case of exceptional loads (like climatic disasters or terrorist attacks). This all creates the potential for wider application of HPC in construction in the future. A set of environmental impact data of concrete components and related processes has been collected and determined within the research performed at the CIDEAS centre (Centre for Integrated Design of Advanced Structures) of the Czech Technical University in Prague. These data 3

4 are based on regionally available materials and on source data provided by companies producing and/or selling their products mainly on the Czech market. A LCA inventory analysis of material and energy flows (inputs and outputs) are balanced and quantified in data modules, i.e. consumption of raw materials, products and by-products, auxiliary materials, energy, water and transport, emissions, by-products and waste from manufacturing processes. Energy data and emission factors used in the assessment are from GEMIS (Global Emission Model for Integrated Systems) Version 4.6 (GEMIS, 2010). Table 1 shows aggregated environmental impact data for two types of HPC and ordinary concrete C30/37. It is evident, that both types of HPC have higher unit environmental impacts in comparison to ordinary concrete C30/37. This is due to higher content of cement and due to the use of steel micro fibres in the concrete mix. However, following case studies show environmental benefits of utilization of HPC, when considered entire structure and its whole life. Table 1 Aggregated data per m 3 of two HPC alternatives and comparison with C30/37 concrete 3. LIGHT PRECAST BALCONY ELEMENT 3.1 Element description The shape solution of new fibre concrete railing comes from the shape of standard railing ground plan. The shape is demonstrated on following pictures, the length is 3410 mm and the height is 1050 mm. The original railing balcony element was designed from ordinary reinforced concrete having constant thickness of railing slab 80 mm and conventional reinforcement. The aim of optimization was to eliminate the amount of the conventional reinforcement (2 reinforcing meshes) in the slab and to minimize the railing thickness in order to achieve maximal savings in structural 4

5 materials, concrete and steel. The new design of railing slab has the thickness of 40 mm and therefore it cannot be effectively reinforced be the conventional reinforcement. Hence, it is designed from the fibre concrete. The railing shape and the shape of stiffening rib along the prefabricate element perimeter come out from the cross-section optimization. The stiffening rib has the thickness of 120 mm, the height of 60 mm and under the angle of 45 runs to the thickness of 40 mm (Fig.1). Fig. 1 Cross-section of the railing Five different mixtures containing polypropylene fibres with different admixtures (CSF microsilica Chryso, MK metakaolin Metaver I) were designed and used for experimental verification of cross-sections, see Table 2. Table 2 Properties of concrete for the experimental balcony railing sections Railing section - series A B C D E Compressive strength [MPa] 75.9 ± ± ± ± ± 2.0 Tensile bending [MPa] 11.1 ± ± ± ± ± 0.5 Density [kg/m 3 ] 2136 ± ± ± ± ± 30 Admixture CSF 8% MK 4% MK 4% MK 5% CSF 8% Fibres Chryso 22 mm 3,2 kg/m 3 2,5 kg/m 3 5 kg/m 3 5 kg/m 3 5 kg/m 3 5

6 Fig. 2 Sections of balcony railings Experimental verification of mechanical properties of a thin railing slab between the reinforcing ribs was performed on 1.0 m wide testing specimens. The shape of the test sample is apparent on the Fig. 2. The theoretical span of specimen for four-point bending test was 1.0 m. Concrete properties of various balcony railing sections were tested in laboratories ŽPSV Uherský Ostroh and they are listed in the Table. 3. Railing section - series A B C D E M exp [knm] M cal. [knm] F max. [kn] f ctm,fl [MPa] Table 3 Summary of static values for each experimental railing cross-section A standard four-point bending test was performed in order to verify mechanical properties of cross sections fibre concrete balcony railings. The best in terms of deflections and strains appeared in the case of mixture B, which contained 4% metakaolin Metaver I and 2.5 kg/m 3 Chryso fibre length of 22 mm. Measured deflections for all 5-mixtures ranged between approximately 0.5 to 1.0 mm, which represents the deflection of approximately L/2000 to L/1000. Deflection limit L/250 is 4.0 mm (for the span L = 1.0 m). 6

7 2.2 Environmental evaluation Embodied energy, embodied emissions CO 2,equiv, embodied emissions SO 2,equiv were compared in the environmental evaluation of prefab balcony railing alternatives. Environmental impacts and self weight of individual variants of railing are shown in following graph (Fig. 3). Fig. 3 Relative comparison of environmental data It is obvious from the relative comparison that fibre concrete railings variants reduce the weight of utilized structural materials (concrete and steel) by approx. 35%. The second variant with polypropylene fibres (series A) is the most favourable solution from the environmental point of view; in contrary to standard solution it has lower environmental impact by approx. 25%. Environmental and economical benefits are related to (i) decrease of raw material consumption (savings in concrete and its components aggregates, cements etc.), (ii) decrease in transportation demands and material handling (lower amount of concrete), (iii) savings in supporting structures and (iv) the longer durability of prefabricated elements. 7

8 4. WAFFLE FLOOR STRUCTURE FROM HPC 4.1 Description of the structure Representative segments of waffle floor structure with minimized thickness of upper deck (30 mm) were made from two different mixtures: HPC105 with 25 mm long steel fibres Fibrex A1(tensile strength of 350 MPa) and HPC140 with 13 mm long steel fibres (tensile strength of 2400 MPa) from Stratec GmbH. Deck of this thickness cannot be reinforced by conventional reinforcement therefore utilization of fibres was tested. Both mixtures contained 1% per volume of fibers. The first mixture HPC105 had the maximal grain size of 8 mm whereas the HPC140 was designed from fine aggregate with maximal grain size of 0.6 mm. The compressive strength of HPC105 was 105 MPa, respectively 140 MPa for HPC140. Test samples had the following dimensions: upper deck 30 mm, ribs 50-70/170 mm, the size of the section 1.2 x 1.25 m. Ribs were reinforced by steel rods 10 mm in diameter and contained no conventional shear or torsion reinforcement. Samples were tested on combination of flexure and torsion. Fig. 4 Bottom view on the test specimen of the waffle floor structure Mechanical tests of specimens showed very good structural performance and verified the concept of light waffle slab with very thin top slab and slim ribs without conventional shear and torsion reinforcement from fibre reinforced HPC (Hájek, Kynčlová, Fiala, 2009). 4.2 Environmental evaluation Four alternatives of floor structures have been compared: i) full RC slab from ordinary concrete C30/37, ii) waffle floor structure from ordinary concrete C30/37, iii) waffle floor structure from HPC105 and iv) waffle floor structure from HPC140. All structures were designed for the same performance dead load 4 kn/m 2, live load 1,5 kn/m 2, span 5 x 5 m, same thickness of 200 8

9 mm. The waffle floor from ordinary concrete had 60 mm thick upper deck and the width of ribs was 80 mm. While waffle slab from HPC105 and HPC140 had dimensions: upper deck 30 mm, ribs 50/170 mm. The data source used in the analysis was Passivehaus-Bauteilkatalog (Waltjen 2008) and (Schießl&Stengelt 2007). The graph in the Fig. 5 shows evident environmental advantages of all waffle structures. The reduction of concrete consumption in optimized shape of waffle FRC floor structure can reach up to 50 to 70 % in comparison with full RC slab. Moreover this results in lower load from self weight and consequently lower load on supporting structures (columns, walls, foundations). Fig. 5 Comparison of environmental parameters of RC floor slabs. Reference level 100% is represented by a full RC slab 5. LIGHT PRECAST RC FRAME FOR PASSIVE HOUSE 5.1 Description of the construction system A combination of light subtle RC frame structure with external walls and internal partitions from timber elements represents effective structural solution from economical as well as environmental point of view. Significant savings in concrete and steel consumption follow from subtle sections of precast RC members. Consequently savings in transport and manipulation costs are evident. This all makes this structural concept more environmental efficient. This approach has been utilized in the construction of passive family house on the suburb of Prague, Czech Republic. Load bearing structure of the 1 st floor is made from precast RC light 9

10 frame. The section of precast columns is 150 x 250 mm. It was made from common concrete C35-45XC1, reinforcement was 4 x R12. Edge columns were composed from two column elements creating L shape section. Columns supporting beams have thickness 150 mm. Floor slabs are RC composite with precast filligran lower part and cast in site upper part total thickness 210 mm (Fig. 6) Fig. 6 Light precast RC frame structure Load bearing structure of the 2 nd floor and roof structure is timber structure (Fig. 7). Entire load bearing structure is covered with timber external wall containing 400 mm of thermal insulation from mineral wool. The total expected energy consumption will be less than 20 kwh/m 2 per year. Fig. 7 Timber frame structure on the top of RC precast part; fixing of timber external wall 10

11 4.2 Environmental evaluation Three alternatives of structure applied for construction of the same family house have been assessed in the study. Reference alternative is common structural solution from ceramic brick blocks Porotherm 44 P+D (thickness 440 mm) and RC floor structures with ceramic hollow fillers MIAKO (VAR. 1). Second alternative VAR. 2 has also load bearing structure from ceramic bricks (Porotherm 24 P+D) and the same type of ceramic floor slab. The external walls are insulated with 300 mm of PPS (polystyrene). VAR. 3 is light RC frame structure with timber external walls and timber internal partitions This alternative was applied in the construction of family house in Prague - Modrany (Fig. 6 and 7). Fig. 8 Comparison of environmental parameters of three alternatives of load bearing structure for passive family house The analysis has been made just for one storey (1 st storey) in which the combination of light RC frame and timber external walls and partitions was used. In the Fig. 8 are presented results - comparison of environmental profiles of described three alternatives. It is evident that alternative with light RC precast frame and timber external wall show better results in embodied energy (10% savings in relation to reference alternative) and embodied CO 2 emissions (32% less). The thickness of external wall is also the lowest this can represent important economical advantage, especially in urban regulated areas with high density of houses. The environmental advantages of light RC frame structure are evident although ordinary concrete C35/45 was used. There was not need to use higher strengths concrete for this small 11

12 building. In the case of application of light frame for structure of multi-storey building the use of HPC would bring significantly higher level of environmental savings due to reduced amount of needed raw materials. 5. CONCLUSIONS Increasing production of concrete is associated with increasing environmental impacts caused by high energy consumption (mainly by cement production) and high non-renewable material use. It has been already shown that utilization of optimized light subtle concrete structures can result in reduction of concrete consumption up to 50 70%. This could be achieved e.g. by the use of high performance concrete with significantly better mechanical properties and higher durability in combination with shape optimization. Application of this approach can lead to environmental savings and represents important contribution to sustainable building. Three case studies presented in the paper showed, that wider implementation of these principles into construction practice is possible, applicable, feasible and sustainable. ACKNOWLEDGMENTS This outcome has been achieved with the financial support of the Grant Agency of the Czech Republic Grant Project No. P104/10/2153 and with the financial support of the Ministry of Education, Youth and Sports of the Czech Republic, project CIDEAS No. 1M0579. All support is gratefully acknowledged. REFERENCES 1. GEMIS Global Emission Model for Integrated Systems - version 4.6, database CZ, D 2010, 2. Hájek, P Integrated environmental design and optimization of concrete slabs. In proc. Concrete in 3rd Millennium, Brisbane, Hájek, P. & Fiala, C Environmental design and assessment of alternatives of RC floor structures, Sustainable building 2007, Torino, Italy, p Hájek, P., Kynčlová, M., Fiala, C Large scale tests and environmental evaluation of the waffle floor slabs from fibre concrete, Fibre Concrete 2009, Praha, CTU: p Schießl,P.&Stengelt, T Der kumulierte Energieaufwand ausgewählter Baustoffe für die ökologische Bewetung von Betonbauteilen, Wissenschatl. Kurzbericht Nr Waltejn, T : Passivhaus-Bauteilkatalog 2008 Ökologisch bewertetekonstruktionen, Springer-Verlag, Wien 12