Material effective building structures using high performance concrete

Transcription

1 Material effective building structures using high performance concrete Prof. Petr Hajek Faculty of Civil Engineering Czech Technical University in Prague, Czech Republic Ing. Magdalena Kynclova, Faculty of Civil Engineering, Czech Technical University in Prague, Czech Republic, Ing. Ctislav Fiala, Faculty of Civil Engineering, Czech Technical University in Prague, Czech Republic, Summary Concrete is the most used man-made material worldwide. Due to the amount of yearly production, concrete is responsible for approx. 7% of global CO 2 emissions. Therefore optimization of concrete structures can contribute to needed reduction of global environmental impacts. One possible way is utilizing of high performance and ultra high performance concrete in optimized structural shapes. Mechanical properties of these materials such as high compressive strength, durability, water tightness etc. create conditions for designing subtle structures that leads to saving up to 70% of material in comparison with ordinary concrete, and consequently to reduction of embodied CO 2 emissions. A combination of subtle precast RC frame structure with wooden based envelope and wooden internal structures represents one possible way to reduction of environmental impact associated to structural systems of buildings. The experimental investigation performed at CTU in Prague verified the possibilities of using HPC and UHPC in waffle floor structures and in timberconcrete composites. Utilizing of these materials enabled minimization of upper deck of waffle floor structure up to mm while keeping mechanical performance on required high level. Timberconcrete composite floor structures benefit from lower weight of HPC or UHPC deck while improving acoustic parameters and fire safety of the structure. The high quality of mechanical and environmental performance creates the potential for wider application of HPC and/or UHPC in building construction in the future. Keywords: high performance concrete, environmental profile, LCA 1. Introduction The production of concrete in the industrialized world annually amounts to tonne per capita. Concrete is used in construction of buildings, bridges, dams, roads, tunnels basically every contemporary construction contains concrete. Nowadays, even every timber house has some concrete elements (e.g. foundations). Concrete represents due to its mechanical properties, durability, and availability of resources and ability of variable design the mostly widespread structural material for construction of buildings. On the other hand cement production is associated with large energy consumption and CO 2 emissions. In consequence of a fact that world cement production has been 12 times increased in

2 the second half of the last century [1], the cement industry produces at present about 7% of global man-made CO 2 emissions. More over high amount of concrete use is associated with high transport needs and demands on production and demolition processes within the entire life cycle. This all has significant impact on the environment. One of the basic sustainability targets specified already in Agenda 21 for Sustainable Construction [2] is reduction of non-renewable raw material consumption. The reduction of concrete can be achieved by the development and application of new types of high performance concrete (HPC) silicate composites with improved mechanical properties in combination with shape optimization. A complex LCA analysis of two alternatives of RC floor structures (HPC105), one alternative of timber-concrete (HPC140) composite floor structure and reference RC full slab (C30/37) is presented and environmental impacts are compared and discussed. 2. LCA of concrete structures 2.1 Methodology The LCA methods and models should consider the whole life (from "cradle to grave") of a concrete product (element, structure, etc.) [3]. The typical life cycle of a concrete product should cover the following stages: raw material acquisition, production of concrete and structural components, design and construction, operation and maintenance, repair, renovation, demolition, recycling and waste disposal. The characteristic life cycle of a concrete structure with its typical material and energy flows and consequent environmental impacts is presented in Figure 1 [4]. Fig. 1 Life cycle of concrete structure material and energy flows and consequent environmental impacts 2.2 Regional specifics In general, the process of design and construction of concrete structures varies in different countries and regions. They are more or less determined by regional specifics due to different material bases (regionally available aggregates, type of steel, etc.), different climate conditions, different technology (based on the local labour cost, tradition in organization of work and different climate

3 conditions) and differences in cultural traditions. Some country/regional specifics are implemented in codes for structural design (e.g. National Application Documents - NAD in Eurocodes). Regional specifics should be considered when collecting embodied environmental data of different materials. The type of material sources, mining technology, transport means, transport distance, technology of production have a significant influence on final unit environmental embodied values. Relevant complex LCA of the product or entire structure should be based on local environmental data collected within the inventory phase of the LCA procedure. 2.3 Environmental profiles of cement Environmental profiles of cements were calculated for common cements available on the Czech market. The whole production cycle was processed in the inventory analysis of cement modules from the extraction of primary sources to the cement expedition. Aggregation of cement data for various types of cement are shown in following table (Tab 1.) Table 1 Aggregated balanced data of cement production The primary energy flow within the single process steps was monitored in the inventory analysis of individual cement modules. The preparation process of raw materials comprises energy necessary for extraction in quarries, crushing, grinding and transportation of components (energy plaster stone, fly ash). Clinker production process covers coal grinding, clinker firing and transportation of fuels. The cement production process covers slag and gypsum drying, cement milling and transportation of cement components. Operating energy of the cement plant is divided into heating and other operating overheads. Primary energy demands in absolute values are showed in Figure 2.

4 Fig. 2 Embodied primary energy consumption for cement production [MJ/t] It is apparent that the most energy intensive process from the perspective of technology is the formation of clinker. 3. Case study LCA of environmentally optimized HPC floor structures 3.1 Description of floor structures variants The analysis was performed for four various RC floor structures (three are from HPC), that were designed for four-storey residential building with ground plan 14.2 x 22.3 m. This analysis focuses primarily on floor structures and does not cover concrete beams and supporting structures. The analysis covers all significant life cycle stages: transport of the raw material to the concrete plant, concrete production, and transport to the building site, pumping of fresh concrete, formwork and demolition of structures. All assessed variants V1-V4 were designed for following conditions: theoretical span 4.4 m (simply supported), dead load (excluding self weight of the floor structure) g k = 4.0 kn/m 2 and live load q k = 2.0 kn/m 2. Variants V1, V2 and V4 were designed as one way slab, variant V3 as two way slab then. The variants considered in the study are shown in the Figure 3. Fig. 3 Schematic sections of floor structures alternatives V1 solid RC slab C30/37 thickness 200 mm, main reinforcement R10 ā 110 mm at the bottom surface, distributive reinforcement R8 ā 200 mm and reinforcing mesh W8/150/150 at the upper surface, ring beams reinforced by 4 R12 with stirrups R6 ā 200 mm. V2 prefab concrete panels HPC105 with fillers from recycled laminated drink cartons - thickness 200 mm, high performance fibre concrete with compressive strength of 105MPa, upper and bottom deck 30 mm without conventional reinforcement, reinforced only by fibres Fibrex A1 1% by volume, width of ribs 50 mm, ribs spacing 500 mm, main reinforcement 2 R16 ā 500 mm, filigree shear reinforcement R5 ā 250 mm, ring beams from C30/37 on external walls reinforced by 4 R12 with stirrups R6 ā 200 mm, ring beams on inner walls reinforced by 2 R12 with stirrups R6 ā 200 mm.

5 V3 waffle floor structure HPC105 thickness 160 mm, upper deck 30 mm, width of ribs in both directions mm, rib s spacing 600 mm, rib s reinforcement at the bottom surface R8 and R14 at upper surface in both directions, filigree shear reinforcement R5 ā 200 mm and R5 ā 180 mm, ring beams from HPC105 on external walls reinforced by 4 R12 with stirrups R6 ā 200 mm, ring beams on inner walls reinforced by 2 R12 with stirrups R6 ā 200 mm. V4 timber-concrete composite floor structure - thickness 190 mm, upper deck 30 mm from HPC140 reinforced by steel microfibers 13 mm long, timber beam 80/160, timber-concrete connection by gluing, ring beams from C30/37 on external walls reinforced by 4 R12 with stirrups R6 ā 200 mm, ring beams on inner walls were reinforced by 2 R12 with stirrups. The four alternatives were designed from three different concrete mixtures ordinary concrete C30/37, high performance fibre concrete HPC105 and HPC140. The HPC105 mixture was fibre concrete with 25 mm long steel fibres Fibrex A1. These fibres have tensile strength of only 350 MPa. The HPC140 mixture was designed as fine-grained with 13 mm long steel microfibres. The tensile strength of these fibres is 2400 MPa. The amount of steel fibres in both mixtures was 1% by volume. As suggested in designation, HPC105 has compressive strength of 105 MPa, HPC140 has 140 MPa then. 3.2 Input data for the analysis A set of environmental information data on concrete components and related processes has been collected and determined within the research performed at the CIDEAS centre of the Czech Technical University in Prague. These data are based on regionally available materials and on source data provided by companies producing and/or selling their products mainly on the Czech market. Energy and emission factors were taken from GEMIS [5]. Table 2 Balance of input data of construction life phase

6 In the following analysis the expected life span of concrete floor structures was considered for all variants equally 100 years. Two major repairs of 10% of concrete surface were considered for reference alternative V1 from ordinary concrete C30/37. The two floor alternatives from HPC105 (V2, V3) are planned to have a repair of 30% of balcony surfaces, one in a life span. No repair is considered in the case of the alternative V4 from HPC140, due to the significantly better surface quality and density of the concrete matrix. The location of the analysed building is in the town Kladno, Czech Republic. The concrete mix will be transported from a company 4 km away, concrete prefab panels from a precast concrete plant 23 km away and the demolition waste will be transported 26 km to the recycling plant. The balance of input data of construction phase is in table (Table 2). 3.3 Analysis results and discussion Three alternatives of floor structures from HPC V2, V3 and V4 were analyzed and compared with reference solid RC slab from standard concrete C30/37 V1. Graphs in Figures 4, 5 and 6 show aggregated environmental data achieved by detailed LCA analysis of all four variants of floor structures. Graph in the Figure 4 shows for all four alternatives detailed primary energy flows associated with particular material components, transport and construction processes. It is evident that the highest energy consumption is associated with cement production and steel use. The best results reaches alternative V4 composite timber- HPC floor structure, due to the use of timber beams with significantly lower primary energy demands. Top slab was made from very thin HPC140 slab precast elements. Variants V2 and V3 from HPC105 show lower primary energy consumption in comparison with reference solid slab (V1) due to more effective optimized hollow core and ribbed shape of floor cross section. Fig. 4 Aggregated data primary energy consumption in MJ Graph in the Figure 5 shows similar results for global warming potential (GWP). Again variant V4 timber-hpc shows the lowest GWP environmental impact. Both HPC105 variants V2 and V3 are again better than reference solid RC slab. The reason is same as stated for primary energy consumption more structurally efficient cross section shapes in the case V2 hollow core precast pannel and in the case V3 light ribbed structure. Graph in the Figure 6 shows relative comparison of selected aggregated LCA data GWP global warming potential, AP acidification potential, POCP photochemical ozone creation potential, raw material consumption, water use and primary energy consumption. 100% represents solid RC

7 slab from ordinary concrete (variant V1). All optimized alternatives have lower environmental impacts in all assessed environmental criteria. The best one is variant V4 timber-hpc composite ribbed structure. Fig. 5 Aggregated data global warming potential (GWP) in kg CO 2, equiv. Fig. 6 Aggregated data of assessed variants for whole life cycle (GWP global warming potential, AP acidification potential, POCP photochemical ozone creation potential), 100% is represented by V1 solid RC slab. 4. Conclusion The complex LCA and LCC of optimized HPC and UHPC 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 (climatic disasters or terrorist attacks). There is a big potential for the use of high performance silicate materials (application of HPC, UHPC) to form thin shell (ribbed, waffle, etc.) structures with reduction of the use of primary raw materials, and correspondent reduction of associated environmental impacts.

8 Increasing production of concrete is associated with increasing environmental impacts caused by high energy consumption 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. Case study presented in the paper showed, that wider implementation of these principles into construction practice is possible, applicable, feasible and sustainable. 5. Acknowledgements This outcome has been achieved with the financial support of the research project granted by Czech Grant Agency GACR P104/10/2153. All support is gratefully acknowledged. 6. References [1] Hájek, P Integrated environmental design and optimization of concrete slabs. In proc. Concrete in 3rd Millennium, Brisbane, [2] Agenda 21 on Sustainable Construction, CIB, Report Publication 237, Rotterdam [3] ISO 14040, ISO 14041, ISO 14042, ISO Environmental management Life cycle assessment, [4] fib bulletin 28. Environmental design, fib, [5] GEMIS (Global Emission Model for Integrated Systems) - version 4.6, database CZ, D 2010,