Evolution and sustainability of in-situ concrete flat slabs in office buildings more

XXXVII IAHS World Congress on Housing October 26 – 29, 2010, Santander, Spain Evolution and sustainability of in-situ concrete flat slabs in office buildings Liébana, O.1; G. Pulido1, MD; Gómez Hermoso J. 2 1 University of San Pablo CEU (Madrid), 2University Politécnica de Madrid 1 e-mail: oliebana@ceu.es, Key words: in-situ concrete, sustainability, flat slab, minimum reinforcing ratios, CO2 emission Abstract In the current office building construction practice, flat plate is the most common floor structural system due to its economic and practical advantages. Traditionally, high material costs have had a great impact on total construction cost, so voided options as ribbed or waffle slab have been used extensively. However, recently there is a tendency to design solid slabs options, especially due to rising labor costs, simpler and quicker construction, which reduces execution timeframe and increases construction safety. The use of post-tensioned (PT) floors in building structures has been growing in recent years. This type of construction allows thinner slabs and thus, it creates lighter structures, produces a large reduction in rebar tonnage, with the subsequent advantages in transportation, storage or labor. Also, these slabs have other advantages as reduced cracking and deflections, reduced floor to floor height or quick construction. In most countries these design mechanical and economical features have enabled the system to compete economically with traditional in-situ concrete floor slabs; however, this is not the case in some other countries like Spain. Introduction of sustainability criteria and specialization in construction can allow for this system to be introduced in the market, which can also mean lower costs, improved performance and focusing on sustainability in construction. Different solutions have been studied for an actual project, in terms of material quantities, minimum structural thickness and it has also been checked the impact of costs and environmental criteria based on CO2 emission. With these data, we could assess that the current status of low production of flat slabs in Spain it is not related to real economic reasons; it is actually related to a combination of reasons as lack of technical knowledge of designers or builders, inconsistencies or deficiencies in current national codes in each country. The construction industry is following an inertial period that resists changing known systems, apparently satisfactory, and that especially shows an unsustainable view of the construction. Post-tensioned concrete slabs will not always be the most suitable and sustainable option, but it should be evaluated while considering other more familiar techniques of construction with updated considerations. Liébana, O,; G. Pulido, MD.; Gómez Hermoso, J 1 Introduction Flat plate is very popular because it offers advantages such as low cost to ease of construction in office buildings. But post-tensioned (PT) flat plate slab system is more efficient, since the PT flat plate provides improved crack and deflection control, and allows relatively large slab span to depth ratios, on order of 35 to 45, an then with low floor-to-floor height. Waffle slab is a very common floor in Spanish office buildings. However, recently there is a tendency to design solid slabs options because materials quantities have not been a biggest problem in recent years. Why do PT floors in building structures compete economically with traditional in-situ concrete floor slabs in other countries? Low specialization in construction can be an answer. Introduction of sustainability criteria in construction may be a necessary evolution. 2 2.1 Sustainability LEED rating The United States Green Building Council (USGBC) [1] developed a rating system for sustainability in construction: Leadership in Energy and Environmental Design (LEED). The Spain Green Building Council is starting very slowly; few buildings have been certified in the first ten years. As far as buildings structures are concerned, LEED rating system is deficient and we need to change it to properly reflect the significant contributions of PT concrete structural efficiency to real sustainability. For example, LEED does not reward the immediate savings in materials quantities, the associated transportation, storage, labor, or reduced waste. It does not reward the tremendous long-term energy savings achieved by reducing building heating and cooling systems or the shorter distance the elevator travels, and so on, over the life of the structure [2]. PT concrete can reduce vertical construction elements; the less material used the less building weight (dead load). The reduced weight can allow for smaller foundations and walls and this can minimize excavation requirements. With a reduced building height due to bonded post-tensioned concrete, potential savings in construction like reduced floor-to-floor height, reduced freight, CO2, pollution, labor or manufacturing can be obtained. As for the lifetime of the building, the use of PT slabs reduces the total volume and therefore contributes to reduce significantly the energy consumption necessary to heat their space conditioning. 2.2 Sustainability index in EHE-08 Environmental sustainability index from the EHE-08 Spanish code [3] quantifies the contribution of structures to sustainable development in design or construction phases, through the application of indicators. Mainly it promotes the study of the life cycle of buildings, the recycling of concrete waste, the reduction and mitigation of impacts by extending shelf life. Also it promotes the use of recycled materials and reduces the impacts from the construction of the structure. However, it does not reward suitably more sustainable solutions aiming at the optimization of materials as it only assesses the correct use of the solution. XXXVII IAHS, October 26-28, 2010, Santander (Cantabria). Spain 2.3 CO2 emissions Carbon dioxide is produced in cement making as a result of the production of a process ingredient called 'Clinker'. Clinker is made when limestone is heated so as to produce lime, but substantial amounts of carbon dioxide are also formed during this reaction. The final amount of carbon dioxide produced varies depending on the type of cement being made. Globally, this source of carbon dioxide is estimated to amount to 100 million tonnes of carbon emission to the atmosphere each year. Future trading of carbon credits and the likely increasing financial cost of greenhouse gas emissions to individual companies should help promote substantial cuts in carbon dioxide emissions from industry through improved efficiency. The materials used in concrete represent twice as much the world’s production for all of the remaining construction materials. Cement manufacturing process liberates approximately 5% of the total emissions delivered in the atmosphere annually. These numbers highlight the intense use of natural resources and the gas emission which significantly contributes to global warming. At present, part of the cement is being replaced by mineral additions, such as fly ash, slag, and other by-products, in order to decrease the environmental impact and increase concrete durability. Concrete, the most consumed construction material worldwide, is the ideal repository to shelter industrial residues as blast-furnace, slag and fly ash. These cementitious materials have technical, economical, and social advantages. Energy consumption decreases heavily with the content increase of mineral additions because the cement, alone, contains more than 80% of the total embedded concrete’s energy [4]. 3 3.1 Study Floor building, loads and materials The floor studied is part of an office building in Madrid; in plan, the building is shaped like a square with mean length of 50x50 meters. This module of the building has three levels, two of which are located underground. The first level corresponds to parking areas; the next one has areas for loading and unloading of goods, and a great storage. The third level corresponds to the lobby of the office building. The slabs are supported on a mesh of columns with a distance of 8x8 m. Three structural solutions have been designed; flat plate, waffle slab and post-tensioned flat slab, also the depth of the slabs and the width of the ribs have been changed. Finally, a part of the rebar has been replaced by structural steel fibers. We have used the same materials for each solution of slab. In PT slab options, the prestressing is bonded and flat ducts have been used in order to maximize the eccentricity of the prestressing and minimize the reinforcement. The tendons are formed by 4 or 3 strands of 0.62”, grouped together in a single flat duct. The width of the duct is 70mm and its depth, only 20mm. Table 1: Design information Materials HA-35/B/20/IIa HP-35/B/15/IIa HAF-35/A/B/IIa B-500-S Y-1860-S7 Floor System Solid Flat Plate (SFP) Waffle Slab (WS) Bonded PT flat plate (BPTP) Design Loads (kN/m2) Floor Live Loads Dead loads Storage 15 2 Lobby 5 3 Parking 4 0.5 Liébana, O,; G. Pulido, MD.; Gómez Hermoso, J 3.2 Bonded tendons Bonded PT systems have important advantages: higher flexural capacity, good flexural crack distribution, good corrosion protection, flexibility for later holes and easier demolition. Although we have chosen this option for sustainability, there are disadvantages such as an additional operation for grouting and more laborious installation. But the use of bonded tendons contributes to reduce the overall cost more than the use of unbonded system. For unbonded tendons the post-tensioning price may be less, but the overall cost of reinforcing materials is greater and therefore the amount of steel is lower and more sustainable in the case of bonded tendons. 3.3 The minimum thickness 3.3.1 Un-tensioned slabs In choosing the slab thickness, the designer must consider deflection control, shear resistance, fire resistance (especially for voided solutions) and corrosion protection for the reinforcement. The selection is often based on personal experience or on recommended maximum span to depth ratios. EHE-08 and EC-2 show basic ratios span/affective depth for reinforce slabs (in practice for voided solutions), but the selection of thickness is influenced mainly by imposed loads. Many codes or guidelines give typical imposed load capacities for a variety of flat slabs, usually the ratios for solid flat slab have values that are similar to what is proposed for ACI-318-08 [6]; in this option it is always necessary to check deflections. When the imposed load is very strong, with flat plate, unless drop panels are used, the load-carrying capacity of such thin two-way slab will typically be governed by the punching shear resistance around the columns. 3.3.2 Post-tensioned slabs The depth of a flat plate is usually controlled by deflection requirements or by the punching shear capacity around the column. PT improves control of deflections and enhances shear capacity. Although post-tensioned floors are usually thinner and lighter than reinforced floors, vibration is not normally a problem and it is typically catered for by controlling the span/depth ratio. Concrete Society [7] recommends ratios for PT slabs depending on the type of floor and the imposed loads. Knowing the span and imposed loading requirements a suitable span/depth ratio for the section type being considered can be used, 25 for storage level, 32 for lobby area and 40 for parking level (Table 2). In the figures below thicknesses are compared according to different codes and guidelines, for reinforced slab (figure 3) and for PT slab (figure 4). Post-Tensioning Institute [8] recommends higher ratios than those usually used. Table 2: Thickness used (mm) Slab Solid Flat Slab (SFS) Waffle Slab (WS) PT Solid Flat Slab (PTS) Storage 450 420 400+120/200 400+100/200 400+100/160 320 Public 350 320 250+100/160 100 Parking 250 220 250+100/160 200+100/160 200 XXXVII IAHS, October 26-28, 2010, Santander (Cantabria). Spain Figures 1-2: Minimum depth according to different codes and guidelines 3.4 The minimum bonded reinforcement for crack control 3.4.1 Un-tensioned slabs and steel fiber For non-prestressed two-way slab, a minimum amount of bonded reinforcement is required for shrinkage and temperature; reinforcement shall be provided in accordance with EHE-08. This quantity is very similar to other codes. Steel fiber reinforcement offers constructors an efficient and costeffective alternative for mesh or rebar. Working with fiber reinforcement not only means using less steel, but it also takes less man hours to put in place. The annexed 14 of EHE-08 allows the substitution of steel reinforcements, it is also possible with prestressing. 3.4.2 Post-tensioned flat slabs EHE-08 does not show minimum bonded reinforcement for PT slabs, then we use the amounts specified in the ACI318-08 code. All flat slabs should have minimum bonded reinforcement in negative moment areas at column position to distribute cracking: As≥0.00075 Acf (1) Bonded reinforcement shall not be required in positive moment areas if the extreme fiber stress in tension does not exceed 0.17√f ’c, in accordance with ACI-318. According to EC2, in prestressed members no minimum reinforcement is required in sections where, under the characteristic combination of loads and the characteristic value of prestress, the concrete stress is below σct,p. For members with only unbonded tendons, the requirements for reinforced concrete elements apply. But EHE-08 does not allow that bonded PT tendons replace rebar, and then minimum reinforcement should be determined: Amin, g ≥ 1.8 h (2) Also, in a flat slab, minimum area of reinforcement steel with the tensile zone will be greater than: Amin, g ≥ 40 h · fcd/fyd (3) If designer uses these requirements, in our example, for the storage floor, the solution will be more expensive than in other countries. This decision means more than 5.5 kg/m2 of rebar, very common in Spain now. According to the recommendation of HP9.96 from ATEP [9], in the underside of the slab Liébana, O,; G. Pulido, MD.; Gómez Hermoso, J should have minimum grid reinforcement to distribute cracking and increase the ductility, but it is a recommendation for unbonded tendons: Amin = 0.0023b·h (3) 3.5 Serviceability requirements. Durability and crack control According EHE-08, a limiting calculated crack width, wmax, taking into account the proposed function and exposed class, should be established. With EHE-08 wmax must be less 0,3mm, and for IIa exposure class, in addition, decompression should be checked under the quasi-permanent combination of loads (60% of live loads). This should be revised as it requires a great quantity of prestressing, which makes it an uneconomic procedure. In practice, the concrete stress is limited to concrete tensile strength under checked under the quasi-permanent combination of loads. We have chosen the control of cracking without direct calculation according EC2, where the crack widths are not likely to be excessive, if for cracks caused mainly by loading, either the provisions of maximum size of bar or the provisions of maximum bar spacing are complied with certain stresses. Steel stress should be calculated on the basis of a cracked section under the relevant combination of actions. For PT concrete, the provisions may be used with the stress in this reinforcement calculated with the effect of prestressing forces included. 3.6 Measuring emissions from the quantities of materials In this paper we have introduced a quantitative criterion of sustainability widely recognized based on CO2 emission. Carbon dioxide emissions have been calculated for each type of slab from its components according to data bank “BEDEC 2010” from the “Institut de Tecnologia de la Construcció de Catalunya, ITEC [10]. This source offers environmental data (waste, energy costs and CO2 emissions) and other construction information. These values have been obtained from data provided by ICAEN (Instituto Catalán de Energía) and other data obtained from research teams from the UPC. They have consulted several databases, as Ecoinvent, although some data have been compared and completed after consultation and information provided by some construction companies. Energy cost includes the extraction and transformation of its components (extraction, transport from the source to the factory and the manufacturing process). The transformation of material in a specific item or transport to work is not included. Figure 3: % CO2 by components in 320mm PT flat slab XXXVII IAHS, October 26-28, 2010, Santander (Cantabria). Spain 4 Results Figure 4: Ton of CO2 depending on use and type of slab Figure 5: Ton of CO2 – solid flat plates Figure 6: % of CO2 of flat slabs Liébana, O,; G. Pulido, MD.; Gómez Hermoso, J 5 Conclusions At present, the selection criteria for in-situ concrete flat slabs in office buildings are not governed by actual technical criteria; on the contrary, they are clearly unsustainable and are based on the tendency to design known solutions, with no evolution of specialized building techniques. An approach which takes into account project specialization and execution of in-situ concrete flat slabs should be adopted in order to develop a sustainable evolution in current building construction practice. EHE-08 is an innovative code with respect to special concrete issues or durability purposes. However, in the case of building PT slabs, it is clearly separated from other standards. All code sections specifically related to bonded PT slabs should be revised urgently. For the case study example, in a typical range of spans, the PT solution is always the most sustainable option, particularly for high loading. The difference is not so important in voided options with moderate loading. For the case of high loading and longer spans, voided PT solutions may be very competitive. Small changes in thickness, in width of the joist or in the topping slab do not significantly improve CO2 emission data. Replacement of rebar for steel fibers does not involve a decrease in emissions; in the case of thicker slabs it may even aggravate the situation. It is interesting to simplify the construction of voided slabs with moderate loads as mesh, punching and shear reinforcement would not be required and consequently the execution of construction would improve. References [1] USGBC. U. S. Green Building Council. Green Building Design and Construction Reference Guide, 2009 Edition. Raiting System. www.usgbc.org. 2009. [2] POST-TENSIONING INSTITUTE. PTI. PT-Gray is the new Green. PTI Journal. 41. Sustainability and Post-Tensioned Concrete. August 2009. V.7. nº1. 2009. [3] EHE-08. Instrucción de Hormigón Estructural. Comisión Permanente del Hormigón. 1º edición revisada. Ministerio de Fomento. Madrid. 2008 [4] Isaia, G.C. and Gastaldini, A.L, G. Concrete sustainability with very high amount of fly ash and slag. Ibracon Structures and materials journal, Vol. 2/3 (September 2009), pp. 244-253. [5] EC-2. Eurocode 2: Design of concrete structures- Part 1:General rules and rules for buildings. CEN European Committee for Standardization. Brussels. 2002 [6] Building Code Requirements for Structural Concrete (ACI 318-08) and Commentary. An ACI Standard. Reported by ACI Committee 318. Structural Building Code. American Concrete Institute. USA.2008 [7] CONCRETE SOCIETY. CS. Post-tensioned concrete floors. Design Handbook. Concrete Society Technical Report nº43. 2ªed. UK. 2005 [8] POST-TENSIONING INSTITUTE. PTI. Post-Tensioning Manual. 6th Edition. 2006. [9] ATEP. Recomendaciones para el proyecto y construcción de losas postesadas con tendones no adherentes HP9.96. Manuales de la ATEP. 1996 [10] Institut de Tecnologia de la Construcció de Catalunya. ITEC. Banco estructurado de elementos constructivos BEDEC. http://www.itec.es/noumetabase2.e/Presentacio.aspx?page=bancbedec