Timber UHPC composite floor structures environmental study

Transcription

1 Timber UHPC composite floor structures environmental study Petr Hajek 1, Magdalena Kynclova 1, Ctislav Fiala 1 1: Faculty of Civil Engineering, CTU in Prague, Czech Republic The experimental investigation performed at CTU in Prague verified the possibilities of using UHPC in timber-concrete composites. Timber-concrete composite floor structures benefit from lower weight of UHPC deck while improving acoustic parameters and fire safety of the structure. One of the key problems is the connection system between timber beam and UHPC deck. Wet process is not suitable due to the effect on wood and slim UHPC deck does not provide space for mechanical joint. The experimental results have showed that this can be solved by gluing. A complex LCA analysis of four alternatives of RC floor structures is presented and environmental impacts are compared and discussed in the paper. The results show that the high quality of mechanical and environmental performance creates the potential for wider application of UHPC in building construction in the future. Keywords: timber-uhpc composites, glued connections, environmental study 1 Introduction Optimization of concrete structures can contribute to needed reduction of global environmental impacts. One possible way is utilizing of 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. The composite structures based on high performance silicates and wood represent the beneficial alternative to the timber floor structures. The timber structures have problems to achieve sufficient stiffness; the lack of mass causes troubles with acoustics, inflammability of wood limits the use from the perspective of fire safety. These disadvantages can be reduced by utilizing timber-silicate composites. Timber-concrete composite floor structures benefit from lower weight of UHPC deck in comparison to deck from OPC while improving acoustic parameters and fire safety of the structure. The combination of thin silicate slab and timber beam represents the effective crosssection from the perspective of bending stress. One of the key problems is the connection system between timber beam and UHPC deck. Wet process is not suitable due to the effect on wood and slim UHPC deck does not provide space for mechanical joint. The experimental results have showed that this can be solved by gluing. A complex LCA analysis of various alternatives of RC floor structures is presented and environmental impacts are compared and discussed in the paper. Relevant complex LCA is based on local environmental data collected within the inventory phase of the LCA procedure. The results show that the high quality of mechanical and environmental performance creates the potential for wider application of UHPC in building construction in the future. 2 Experimental verification of timber-concrete glued connections The primary research focuses on developing UHPC mixture from materials available in Czech Republic. Broad spectrum of cements, superplasticizers, microsilicas and sands from various producers were tested. Only Stratec fibres were brought from Germany. The mixture used in

2 these experiments has a compressive strength of 160 MPa tested on cubes a=100 mm. The optimalization of UHPC mixture is still in progress. The first step in the research of the timber-concrete composite floor structures was to verify the performance of glued connection. The shear test was proposed. Both sides of concrete prism 100/100/400 were glued to two timber prisms 80/160/320. The arrangement of the test is apparent from figure 1. Fig. 1 Shear test arrangement of timber - concrete composite In the first set of experiments there were tested two types of concrete (OPC ordinary portland cement concrete and UHPC ultra high performance concrete) and three various glues (Sikadur 30, Sikadur 330, SikaFloor 156). Timber prisms were from glued laminated wood. Eventhough, there were quite high variances in results, trend was obvious. Difference between OPC and UHPC timber composites was in the type of failure the rapture in timber- OPC composites was mainly in concrete while in UHPC-timber composite the rapture was in the timber (figure 2). The best results were achieved with SikaFloor 156 despite having the worst workability. This type of glue is too liquid for that type of application. Therefore, for the second set of experiments a specific filler (3% and 5% by mass) into the SikaFloor 156 was introduced. The glue with 5% of filler proved well both from the point of workability and shear strength. Fig. 2 Shear failure of timber - OPC composite and timber-uhpc composite Large scale experiments are now under preparation. Slender UHPC deck 1200x2400 mm with thickness of only 30 mm is being glued to two timber beams. The results of the experiment will be presented at the conference.

3 3 LCA of four various concrete floor structures Timber UHPC composite floor structures environmental study Description of floor structures variants The analysis was performed for four various RC floor structures, that were designed for fourstorey 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. 34 Schematic sections of floor structures alternatives V1 full 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. 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 UHPC160 reinforced by steel microfibers 13 mm long, timber beam 80/160, timberconcrete 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 UHPC160. The HPC105 mixture was fibre concrete with 25 mm long steel fibres Fibrex A1. These fibres have tensile strength of only 350 MPa. The UHPC160 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

4 compressive strength of 105 MPa, UHPC160 has 160 MPa then. 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 [1]. 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 [2]. In the following analysis the expected life span of concrete floor structures was considered for all alternatives 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 UHPC160, 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. Analysis results and discussion Three alternatives of floor structures from HPC V2, V3 and UHPC V4 were analyzed and compared with reference solid RC slab from standard concrete C30/37 V1. Graphs in Figures 5, 6 and 7 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- UHPC floor structure, due to the use of timber beams with significantly lower primary energy demands. Top slab was made from very thin UHPC160 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-uhpc shows the lowest GWP environmental impact. Both HPC105

5 Timber UHPC composite floor structures environmental study alternatives 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 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-uhpc 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 Conclusions New types of high performance silicate composites enable design of ultra thin decks with thickness less than 30 mm. Some applications use even mm thin UHPC decks. That thickness disables to use ordinary mechanical kinds of connection. Glued connection represents an effective option. Presented experimental results of timber-concrete composite shear test proved the potential of timber-concrete glued connection. Therefore, there is a chance to design the timber-concrete

6 composite with HPC or UHPC slender deck with thickness of only 30 mm or less. Environmental efficiency of such composite structure was shown in the LCA case study. The research of timber-concrete composite floor structures is continuing with the large scale bending tests of composite beams with HPC slender slab. 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. References [1] Hájek, P., Fiala, C., Kynčlová, M.: Life cycle assessments of concrete structures a step towards environmental savings, Structural Concrete, Journal of the fib, Vol 12/1, March 2011, ISSN , pp , [2] GEMIS (Global Emission Model for Integrated Systems) - version 4.6, database CZ, D 2010,