A Case Study on the Eco-Balance of a Timber-Concrete Composite Structure in Comparison to other Construction Methods

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1 A Case Study on the Eco-Balance of a Timber-Concrete Composite Structure in Comparison to other Construction Methods Yannick Plüss, BSc in Civil Engineering HES-SO, research associate at College of Engineering and Architecture Fribourg, Univ. of Applied Sciences of Western Switzerland Daia Zwicky, Dr. sc. techn., MSc ETH, Professor at College of Engineering and Architecture Fribourg, Univ. of Applied Sciences of Western Switzerland ABSTRACT In today s building construction tenderings, ecological aspects are often taken into consideration. Therefore, it is of vital importance that all stakeholders architects, structural engineers, owners, constructors etc., have an idea on the eco-balance of different construction methods, in order to choose the one being the most appropriate for the tendered project, also in view of building labels. Nowadays, the most widely used construction material for mid-size buildings, such as multi-storey office buildings or schools, is reinforced concrete. Regrettably, concrete requires much energy for production and transportation, strongly draws upon non-renewable resources, and is rather challenging to recycle. Therefore, alternative construction methods are required and to be assessed with regard to their eco-balance. Construction with timber-concrete composite (TCC) elements allows significant reduction of concrete quantities while providing comparable serviceability performance. To provide first insight on the ecological performance of different construction methods, this paper shows case study results of a Life Cycle Assessment for a typical building floor slab. It compares several construction methods, i.e. a traditional reinforced concrete slab, TCC structures as well as a pure timber structure. Traditionally, composite action in TCC elements is provided by mechanical interface connectors (screws, dowels etc.), the latter being habitually designed with elasticity-based approaches. Recently, a new type of shear connection has been developed at the University of Applied Sciences Fribourg omitting the use of mechanical connectors. The new connector shows very stiff and extremely ductile behaviour, principally allowing the application of plasticity-based structural design methods. Based on this observation, the paper further analyses the impact of the applied structural design approach traditional elasticity-based or more sophisticated plasticity-based approaches on the eco-balance of the floor slab. It concludes by giving an outlook on the most important elements and concerned stakeholders for improving the eco-balance of typical mid-size buildings. Keywords: timber, concrete, composite structure, structural design approach, construction method, life cycle assessment

2 1. INTRODUCTION 1.1. Motivation Nowadays, the most widely used construction material for mid-size buildings, such as multistorey office buildings or schools, is reinforced concrete, regrettably being rather unsustainable. To reduce the volume of concrete used, composite structures are widely applied in building construction. Composite structures combine two or more materials such that they all can develop their full potential. Steel and concrete are mainly used for composite structures. However, timberconcrete composite (TCC) structures are used more and more, further improving the eco-balance. Traditional TCC structures combine timber joists with concrete slabs using mechanical connectors to transfer the interface shear between implied structural materials. Often, these connectors are metal screws. As for the TCC structures with the new corbel-type connectors, the resulting structural system is a concrete slab with timber ribs Plastic vs. elastic structural design of TCC elements Two different types of interface shear connections for TCC structures are considered in this study. Traditional mechanical steel connectors, elastically designed, and plastically designed TCC structures with corbel-type connectors. Former research [1] [2] has shown that corbel-type connectors provide sufficient plastic deformation capacity to allow structural design of TCC girders with limit analysis methods [3], thereby also allowing the optimisation of consumed material quantities Objectives The objectives of this study are: analyse the differences in eco-balance of different construction methods (concrete, pure timber, and a TCC structure) for floor slabs; and to explore if the connector type, associated with an elasticity- or plasticity-based structural design approach, has an impact on the eco-balance of TCC structures. A typical floor slab in the project of a school building in Switzerland is analysed as a case study. The type of the selected project represents the building size that is typically aimed at for the application of TCC structures. 2. CASE STUDY OBJECT Since private investors still hesitate to put considerable effort into ecological competitiveness of their projects, the authors decided to perform this case study on a typical public school building Delimitation The goal is to compare different construction methods for the slab structure. Therefore, the case study is limited to a 900 m 2 floor slab, Fig. 1(a). In order to guarantee comparability of the different structures, a composition of primary (load-bearing) and secondary (non-structural)

3 structure have been chosen for each slab structure which meets the same requirements with regard to serviceability, especially the fire resistance of the structure. Life-cycle analysis was performed including all aspects from cradle to gate of the production facility, transportation to the construction site and to the recycling facility, as well as disposal of the building materials. Fig. 1 (a) typical floor plan Fig. 1 (b) different floor types 2.2. Compared floor types Three main types of for floor slabs are compared, Fig. 1 (b): - Traditional concrete slab - Elastically designed TCC structure with mechanical connectors - Plastically designed TCC structure with corbel-type connectors In addition, a traditional pure timber structure as slab-on-beam construction is considered. This construction method, however, may not be completely comparable, as the requirements of acoustic insulation and vibration behaviour have not been explicitly verified Eco-balance indexes Multiple approaches and indexes exist in life-cycle analysis, Table 1. All are calculated on the basis of statistical data for the different construction materials used.

4 Table 1 - Eco-balance indexes Index Unit Remarks Primary energy PE [MJ] Only gives an indication on embedded energy, neglecting its renewability. Non-renewable primary energy NRE Emitted greenhouse gas EGG [MJ] Gives an indication on non-renewable embedded energy. This usually results in large differences for timber structures because of the renewable embedded energy [t CO2 eq. ] This index is the easiest to communicate; it is the most widely accepted and the best known in public. However, it neglects soil usage/consumption. UBP [-] This is the most comprehensive index, in addition to EGG; it also integrates soil usage/consumption. 3. ADMITTED CALCULATION HYPOTHESES FOR LIFE-CYCLE ANALYSIS 3.1. Service life The planned service life is estimated to 90 years for the load-bearing structure, and to 30 years for the replaceable secondary structure, i.e. considering two replacements over the structure s service life. The service life period is important since part of the total ecological impact occurs on a yearly basis Construction material Concrete type is admitted to be C30 [4] in all three structures. It is further admitted that the concrete is produced in a facility at 25 km distance and is delivered by trucks of less than 20 t. The bar reinforcement for the structural concrete is produced in a steel-plant at a distance of 100 km and delivered by trucks of more than 20 t. Glued laminated timber (glulam) elements are admitted. All timber products are produced at a distance of 50 km and delivered by trucks of less than 20 t. For the interface steel connectors, it is admitted that they have the same ecological impact properties as hot dip galvanised steel sheets. They are produced at 100 km distance and delivered on trucks of less than 20 t Fire protection cladding The structural dimensions are chosen such that a fire resistance of R60 is achieved. Where it is not possible to achieve this resistance by the structure alone, cladding is used to avoid that timber joists are exposed to fire. The cladding system is selected according Swiss recommendations [5] Heating energy demand The required heating energy is assumed to be at the upper limit of SIA 380/1 [6] for category IV Building, a mean temperature of 8.5 C and an inside temperature of 20 C. The difference to the

5 Minergie standard [7] is small and is not further considered. As the limit given in [6] is in relation to a net floor surface, it can be applied to the same surface as the other calculations. The heating energy is provided either by domestic fuel oil combustion, or by district heating Ecological impact of transportation Preliminary analysis on the impact of transportation from the production facility to the construction site showed an impact of less than 1% of the impact of the load-bearing structure. Furthermore, only small differences were observed for the different construction methods. Additionally, the distances between the construction site and the production facilities may vary considerably for other locations of the construction site. Therefore, it was decided to exclude the influence of the transportation from the further analysis. 4. CALCULATION METHOD 4.1. Data source All calculations are based on the public eco-impact database published by the Coordination of the Federal Services of Building and Real Estate (CFSC) [8]. The database provides values for the four indexes of Table 1. However, UBP is chosen to be the main index discussed here, mainly because of its comprehensiveness and because it is the only one that integrates soil consumption Procedure The different indexes are given in [8] normalised to a base quantity of the consumed building product, e.g. UBP per kg of concrete, heating energy or transportation method. Non-structural secondary structure (cladding, flooring etc.) are usually replaced during the service life of a building. It is assumed that all non-structural elements are replaced twice by similar products with the same ecological impact, i.e. considering three times the needed quantity of the good in question. Based on the normalised values given in [8], the UBP index can be calculated: Indicator N = K i M i Where K i is the specific amount of index N, in UBP, and M i is the quantity of material i Structural design Traditional construction methods The design of the traditional systems was performed using the Swiss standards for timber and concrete structures [9], [10], meeting all requirements of the Serviceability Limit State (SLS) and the Ultimate Limit State (ULS). n k=0

6 TCC structure The TCC structure with mechanical connectors is designed using an elastic approach. The connector stiffness was taken in account as proposed by the γ-method [11]. The TCC structure with the corbel-type connectors was designed based on a plastic approach, i.e. the total resistance of all connectors on a shear span are equal or superior to the totally applied interface shear (and are not arranged according to the elastic distribution of the interface shear) LCA indexes The different construction systems being structurally designed, the total quantities of all materials can be determined, Table 2. Table 2 - Material quantities of several construction methods for the case study mid-size building Concrete Slab TCC with screwtype TCC with corbel- Pure timber connectors type connectors Flat coating 91' ' ' '550 Planking 9'665 Concrete 579' ' '954 Reinforcement 20'000 9'000 9'000 Connectors 2'616 Timber joists 28'658 13'306 41'933 Rock wool 1'546 Ceiling cladding 15'706 15'706 15'706 [kg] [kg] [kg] [kg] 5. RESULTS AND DISCUSSION OF INFLUENCES ON ECO-BALANCE The results are compared under different aspects Load-bearing structure UBP [%] 140% 120% 100% Ecobalance of load bearing structure 80% 60% 40% 20% 0% Evacuation Fabrication Concrete TCC-P TCC-E Timber Fig. 2 - Eco-balance of load-bearing structure normalised to impact of concrete construction. Fig. 2 shows a relative comparison of the impact in UBPs of the load-bearing structure for the analysed systems (TCC-P: TCC structure plastically designed; TCC-E: TCC structure elastically designed). The concrete slab serves as reference. The ecological impact of a TCC structure with mechanical connectors has a higher ecological impact than the concrete structure. Plastic design considerably reduces the ecological impact of a TCC structure. The eco-balance of TCC structures may potentially be further improved if wood-based concrete compounds [12] instead of

7 traditional concrete are applied. The pure timber structure confirms its good reputation as ecological material; however, it cannot be directly compared to the other construction methods as the timber floor does not necessarily meet the same requirements with regard to vibrational behaviour and sound insulation Load-bearing (primary) and secondary structure Eco-balance of complete structure If the secondary structure is also considered, Fig. 3, the concrete slab gets a lower UBP score than 180% Secondary 160% the plastically designed TCC structure. The Load bearing 140% ecological impact of a pure timber structure is 120% slightly higher, too, and comparable to a 100% plastically designed TCC structures with a 80% generally better structural behaviour at SLS and ULS. UBP [%] 60% 40% 20% 0% Concrete TCC-P TCC-E Timber Fig. 3 - Eco-balance of primary and secondary structure normalised to impact of concrete construction. The reason for this more than proportional increase of ecological impact of TCC and timber structures is the high mass of secondary elements. Since the non-structural secondary structure is supposed to be replaced twice during service life of the building, it intervenes three times in the calculation of the UBP score. Also, the cladding considered to meet sound insulation and fire resistance requirements has a high impact. The eco-balance may be improved if cladding materials with lower ecological impact are used than those taken into account here. The differences in ecological impact of 20% to 60% should be considered with caution since architectural considerations are completely omitted from this case study. Visible concrete surfaces may be covered by additional cladding for aesthetical reasons; furthermore, the cladding of the TCC or timber structure may be replaced only once or twice during service life. All these options can easily shift the best eco-balance towards one or the other structure. Considering the EGG index for the load bearing structure gives slightly different results: the concrete slab and the plastically designed TCC structure perform at more or less the same level (both 68 t of CO 2eq per slab), whereas the elastically designed TCC structure has a considerably higher impact (130 t CO 2eq per slab); the traditional timber construction situates in between these two (113 t CO 2eq per slab).

8 5.3. Complete structure and heating energy Domestic fuel oil District heating UBP [x 10 6 ] UBP [x 10 6 ] Concrete TCC-P TCC-E Timber 0 Concrete TCC-P TCC-E Timber Load bearing structure Secondary structure Heating energy Fig. 4 - UBP including domestic fuel heating To get a sound view on eco-balances over the full period of service life, the heating consumed by the building must also be included. Since there are plenty of ways to heat buildings, two extremes are compared. On the one hand, if the demand of heating energy is provided by domestic fuel oil combustion or by district heating, on the other. Most practical cases will be situated between these two extremes. It can be observed, Fig. 4, that all efforts undertaken to reduce the ecological impact of loadbearing and secondary structure become superfluous if the building is heated by domestic fuel oil combustion. Meeting the requirements of reduced heating energy consumption, e.g. Minergie [7], will not considerably reduce this impact since the more severe requirement is not much lower (see section 3.4). 6. CONCLUSIONS The newly developed corbel-type connectors for TCC structures allow significantly reducing the ecological impact of the structure, compared to a traditional concrete slab. The use of timber instead of concrete implies a substantial reduction of embedded energy and therefore, a better ecological performance. Compared to traditional TCC structures, the corbel-type connector omits screw-type connectors which are energy-intense in production. Further, plastic design methods allow TCC structures that need less material, having an additionally lowered ecological impact.

9 Consequently, the applicable structural design method and the chosen connection means allow to improve the eco-balance of a TCC structure; in the presented case study, the ecological impact could be reduced by approx. 30% in comparison to a concrete structure, and by approx.45% in comparison to a traditionally designed TCC structure. Substitution of traditional concrete by potentially more ecological wood-based concrete compounds [12] may further improve the ecobalance of TCC structures. Surprisingly enough, a traditional concrete slab performs better in terms of UBP than TCC or timber structures if the secondary structure is taken into account in addition to the load-bearing primary structure. The secondary structure, needed for meeting fire resistance, acoustic and thermal insulation requirements, has a high impact on the eco-balance of a building; it may be further influenced by cladding for aesthetical reasons. Construction in pure timber principally confirms its good reputation as being ecological, even though the performance in vibrational behaviour and acoustic insulation may not be necessarily the same as for the other construction methods considered. But, this advantage in ecological performance is eliminated or even inverted, respectively, by the impact of the necessary secondary structure. However, the load-bearing structure has little impact on the global ecological performance of a building if heating energy is also considered. District heating instead of domestic fuel oil combustion implies a reduction of approx. 70% of the total impact. The choice of heating energy source is thus most important for having environment friendly buildings. The results of this case study on a mid-size public building show that the eco-balance is mainly influenced by the choices that owners, architects, building physicists, and HVAC engineers make with regard to heating energy supply and materials used for the secondary structure, necessary for satisfying acoustic and thermal insulation requirements. Structural engineers may also contribute to improving the ecological performance of a building by applying more sophisticated connections means and structural design methods, but their influence on the global eco-balance is rather limited. However, if all these constraints are taken into account, TCC structures have the potential to serve as environment friendly load-bearing structures for mid-size buildings.

10 7. REFERENCES 1. Plüss, Y. and Zwicky, D. Plastic Design of Timber Concrete Composite Girders. London : IABSE / IASS, Taller Lighter Longer. 2. Selçukoglu, E. and Zwicky, D. Towards the Plastic Design of Glulam Concrete Composite Structures. Bangkok : IABSE, Sustainable Infrastructure - Environment Friendly Safe and Resource Efficient. pp Nielsen, M. P. and Hoang, L. C. LimitAnalysis and Concrete Plasticity - 3rd edition. London : Taylor & Francis Group, Eurocode 2. Design of concrete structures - Part 1, General rules and rules for buildings. Brussels : European Comittee for Standardization, Lignum, SIA. Reccomendation 83, fire protection for buildings. Zurich : Lignum, SIA 380/1. Thermische Energie im Hochbau. Zurich : Swiss Society of Engineers and Architects (SIA), MINERGIE Head Office. Standards und Technik. [Online] MINERGIE, 13 January [Cited: 13 January 2014.] 8. CFSC: Coordination of the Federal Services of Building and Real Estate. Empfehlung, Ökobilanzdaten im Baubereich. 2009/1. Berne : CFSC, SIA 262. Concrete Structures. Zurich : Swiss Society of Engineers and Architects, SIA 265. Timber Structures. Zurich : Swiss Society of Engineers and Architects, Eurocode 5. Eurocode 5: Design of timber structures Part 1-1: General Common rules and rules for buildings. Brussels : European Comittee for Standardization, Macchi, N. and Zwicky, D. Wood-Based Concrete for Composite Building Construction with Timber. Oslo : Norvegian Concrete Association, Concrete Innovation Conference 2014.