IMPROVING SUSTAINABILITY OF CONCRETE CONSTRUCTION THE ROLE OF HIGH STRENGTH AND HIGH PERFORMANCE CONCRETE - PDF

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IMPROVING SUSTAINABILITY OF CONCRETE CONSTRUCTION THE ROLE OF HIGH STRENGTH AND HIGH PERFORMANCE CONCRETE Per Fidjesto, Elkem as Silicon Materials, Norway Rein Terje Thorstensen, Elkem as Silicon Materials,
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IMPROVING SUSTAINABILITY OF CONCRETE CONSTRUCTION THE ROLE OF HIGH STRENGTH AND HIGH PERFORMANCE CONCRETE Per Fidjesto, Elkem as Silicon Materials, Norway Rein Terje Thorstensen, Elkem as Silicon Materials, Norway 37th Conference on OUR WORLD IN CONCRETE & STRUCTURES: August 2012, Singapore Article Online Id: The online version of this article can be found at: This article is brought to you with the support of Singapore Concrete Institute All Rights reserved for CI Premier PTE LTD You are not Allowed to re distribute or re sale the article in any format without written approval of CI Premier PTE LTD Visit Our Website for more information 37 th Conference on Our World in Concrete and Structures August 2012, Singapore IMPROVING SUSTAINABILITY OF CONCRETE CONSTRUCTION THE ROLE OF HIGH STRENGTH AND HIGH PERFORMANCE CONCRETE Per Fidjestol and Rein Terje Thorstensen Elkem as Silicon Materials, Fiskaaveien, 4675 Kristiansand, Norway. PH , FAX , Keywords: Strength, durability, service life, CO2, sustainability, High strength concrete, SCM, LCA, high performance concrete Abstract. This study shows that by using mixture-optimized, high strength concrete, significant reductions in CO 2 contributions can be realized for concrete construction - in conventional buildings, as well as in major projects. More unorthodox design features can further reduce the carbon footprint. In the locality studied here, this improvement, in addition to reduced materials volumes, was accompanied by reduced cost of materials. Such results show that there are available methods for improving the environmental profile of concrete construction, use of high strength concrete is one such. This type of concrete also have excellent durability properties giving a long service life with minimal maintenance in several environments, and it has been found that heavy materials, such as concrete or stone, will contribute to reduced energy consumption throughout the life of the structure. INTRODUCTION Construction The concrete construction process contributes 5-10% of the CO 2 that is generated by human activity, most of it from the material. Therefore a reduction of CO 2 contributions from concrete construction will be a significant contribution to the popularly demanded reduction in GHG release. It has been advocated, e.g. by PK Mehta 1, that the concrete profession already has the means to make significant reductions in CO 2 without compromising quality rather with improved quality because the use of appropriately designed high strength concrete materials ensures improved sustainability and long-time serviceability of concrete construction. The three main steps in this approach for the construction process are: Reduce the volume of concrete required o By using concrete with higher strength, it is possible to reduce the cross section for building components. Use more durable concrete that will have a longer service life Reduce the binder content in the concrete mixture o Optimize mixture by Particle Packing. o Fill gaps in the concrete microstructure with better gradation of the aggregate so as to decrease the need for binder volume. Reduce clinker content of binder o Use additions, such as supplementary cementing materials (SCM) and fillers. As an example of a recent application: A reference case of high strength concrete reducing carbon imprint was documented for One Island East, a 308 meters tall tower on Hong Kong Island (Chan & al 2 ), concluding that the carbon footprint per unit floor area was reduced from 105 kg CO 2 /m 2 to 74 kg CO 2 /m 2 by replacing grade 45 concrete by grade 105 concrete. Durability There are plenty of examples and investigations of high performance concrete (HPC) for the use in demanding environments, such as the marine or in areas where ASR is a problem. The solution typically is HPC with SCM s for this type of exposure. Examples include the specification for marine concrete in Hong Kong and the requirement of the Norwegian Road Authority. Reams of pages have been written about the resistance of binary and ternary SCM blends to aggressive media, not least about silica fume and resistance to chlorides. The good durability of high strength concretes with SCM is therefore a valuable contribution to more sustainable concrete construction with the long service life and reduced maintenance. Operation Typically 10-20% of all energy is spent in the construction phase of a building, throughout the service life; the rest is spent on maintaining the climate inside the structure (heating/cooling/lighting) and on maintenance of the structure. As mentioned above, increased durability will improve the maintenance cost, maybe even close to eliminating it. This will also be the case for civil structures of various kinds. For buildings, where the climate is of importance, the ability of heavy materials, like concrete or stone, to accumulate heat and to release it will encompass a saving in heating and cooling of up to 10 or 20 %, something which has a huge impact on the accumulated energy consumption of a building 3. Recently, a lot of research and documentation has been performed on this issue, and the conclusion is that one can manufacture a structure with significantly less energy demand than traditional, lightweight structures 4. AN EXAMPLE CONSTRUCTION PROJECT Fig.1. Structure To investigate the structural approach, a study did: Produce the highest strength conventional concrete possible with (mainly) locally available materials Document properties of this concrete Design a 1000m 2 ground area, four floor building (fig. 1) using both this high strength concrete and traditional concrete. The building was traditionally designed as a column/slab(/beam) structure with shear walls in two corners. The shear walls are not included in the calculations of concrete volumes, but HSC would also reduce the volumes of concrete in the shear walls. Compare o environmental impact o cost o Service life in a coastal environment The building was designed according to Norwegian Standard (NS ). The buildings were compared in terms of CO 2, cost and service life in a chloride exposure. Four solutions were studied: Two concrete strengths (cylinder): Conventional structure: (B35) High-strength Concrete (HSC): 85 MPa, developed using particle packing model on local materials (B85) Different structural floor solutions were used for each concrete strength. Konvensjonelt flatdekke m.traditional beam/slab deck for conventional concrete in reference underliggende bjelkerbubbledeck, et nyutviklet prefabrikkert toveis hulldekke. Flat slab Traditional (for HSC) Eliminating much of the volume of concrete in the centre of the slabs. Here by using BubbleDeck 6, as a prefabricated two-way deck permanent form (fig. 2). 1 The objective is to reduce the volume of concrete in the center of the slab the slab being more than 90 % of the concrete in the structure. Other solutions, i e. various rib floors, the LiteDeck variant from South Africa; a solution with lightweight at the core, i.e. abeo,dk; all of these will also be relevant Fig. 2 BubbleDeck flat slab with area cleared for reinforcement at column-deck joint This project was organized as a Bachelor-thesis study at Agder University, and was reported there. 7 1 BubbleDeck contains plastic balls that will reduce concrete volume and decrease the weight of the deck. It is typically used as filigree elements with a 60 mm precast concrete layer serving as permanent formwork. Development of high-strength concrete, 100MPa. Per Fidjestol and Rein Terje Thorstensen The concrete was made using locally available aggregates, without special processing. Using particle packing with three different aggregate gradings, high strength cement, fly ash and Microsilica, a characteristic compressive strength of 98 was obtained (average about 105 MPa (cube)). The development process used EMMA 8, a particle packing software program. With the packing, EMMA optimized the mixture for a reasonably good flow. Table 0. Materials Cement: SCM s (Additions): Aggregates Three gradations from local supplier Reddal Sand AS. Admixtures Norcem Standard Anleggssement. Norwegian High strength cement (EN 197 Type , ASTM Type 1) Undensified Silica Fume: Elkem Microsilica 940U from Elkem as (satisfies EN13263 and ASTM C1240, Flyash: Material supplied by Norcem as corresponding to EN 450 and ASTM C618 class F Sand 0-6 mm Fine coarse 2-10 mm Coarse mm All concretes used a polycarboxylate-based superplasticiser Final mixture During mixture development, a total of 13 mixtures were tested, with cement (OPC) contents from 138 to 293 kg/m 3, fly ash up to 50% of total binder and silica fume from % of total. By comparison, the locally recommended mixture for the 45 MPa concrete today contains 433 kg OPC/m 3. Table 1. Mixture design (Mixture 11) kg/m 3 Norcem Anlegg cement type Silica Fume 940U 59 Fly Ash 44 Sand Coarse Coarse Water 130 HRWRA (weight of binder) 1,4% W/B Nominal 0.35 Real 0.36 Slump 150 mm The characteristic cube strength of this concrete (6 separate batches) was found to be 98 MPa at 28 days. Figure 3 shows the development of strength with time. 110.0 Strength development (mix 11) COmpressive strength (cube) Fig. 3 Strength development of selected mixture Fig. STRUCTURAL DESIGN, MATERIAL VOLUMES Fig. 4. Plan of building The structure is an office/light storage unit, measuring 24*42 m, i.e. covering an area of 1008 m 2 and with four floors (fig. 4). The reference structure is traditional column/beam/slab design with shear walls in two corners and with a stairwell (6*6.2 m). For the study, three slab systems were used: Traditional beam/slab solution or flat deck with Bubbledeck units in the center (and for comparison, a HDC with flat slab was included). Design was according to relevant Norwegian standards, primarily NS Two Microsoft Excel models were developed for the structural design: En for det konvensjonelle bygget, hvor det er mulig å velge mellom betongfastheter fra B30 til B95, og en for bygget med Bubbledeck, hvor man også kan velge mellom betongfastheter fra B30 til B95.One model for traditional buildings and one for buildings with Bubbledeck. In both models it is possible to choose different concrete qualities from B30 to B95. It is also possible to vary cross sections, span lengths and other variables. The model calculates percent utilization, and whether current requirements are met by the Norwegian design standard NS-3473. The selected design solutions gave concrete and reinforcement volumes as given below: Table 3. Concrete and steel volumes NC HSC HSC Conv. Conc HSC Conv. floor Conv. floor Flat Bubbledeck Bubbledeck slab Concrete m volume 54 Reinforcement tons Relative volumes Floors 89 % 92 % 96 % 97 % Beams 7 % 5 % 0 % 0 % Columns 4 % 2 % 4 % 3 % Column area The reduction in column size is sometimes referenced as a potential advantage of high strength concrete. As table 4 shows, the effect is marginal in the current situation. CO 2 -FOOTPRINT Table 4. Column areas Design Total column area, m 2 Std concrete and floor 13.5 HSC and std floor 6.4 Std concrete + Bubbledeck 10.1 HSC + Bubbledeck 5.2 Note that CO 2 is only estimated for the construction phase, and does not account for operation of the building. Furthermore, material production parameters are used in this study, while transport loads are only indicated. This works in this study, because local materials were used (apart from fly-ash from Denmark - where the CO 2 load due to transport would be 3-90 kg CO 2 /ton fly ash, depending on mode of transport 9 ) Aggregates CO 2 values for aggregates are calculated from a local concrete supplier s documentation for their concrete. Back-calculating from their total CO 2 number for concrete, it was estimated that 5.54 kg CO 2 per ton aggregate, considering the value for cement given by the manufacturer, will fit. Two comments: The secretiveness surrounding this information has been surprising to see. In Norway all electrical energy is renewable, so there is no CO 2 contribution from the energy used in aggregate production apart from transport. Cement The high strength CEM1 cement has a reported CO 2 contribution of 0.73 tons/ton cement. The low number is probably due to the extensive use of alternative fuels in the production. Reinforcement CO 2 numbers are contributed by Celsa Nordic, the steel supplier. The steel is manufactured from scrap processed in an electric arc furnace, and the number for this reinforcement is on the very low side becausethe electricity comes from renewable hydro-electric production. It is assumed that scrap steel essentially has no embodied CO 2. With this, the declared value for CO 2 from production is kg/ton steel including postprocessing of the reinforcement (cutting, bending etc). (If energy was coal based, the number for the EAF processing alone would be 790 kg CO 2 /ton reinforcement 10, which should then be added to the post-processing for a total on the order of 950 /ton steel) Bubble-deck elements CO 2 values were submitted by Bubbledeck Norway and are estimates based on the various components of the prefabricated bubble-deck elements. For the production of the plastic balls, the reported number is 0.8 kg CO 2 /m 2 floor. Energy consumption related to the actual production of the elements is considered to be negligible. Transport In the numbers for the concrete, estimated numbers for the delivery of raw materials to the concrete producer are included at 5 kg/m 3 concrete. CO 2 due to the delivery of concrete to site, delivery of reinforcement and delivery of bubble deck elements are not included, and would not make a difference in ranking the different design options. Table 5. Values used in CO 2 calculations Current concrete (B35) kg CO 2 /m 3 Concrete #B-11 (B85) kg CO 2 /m 3 Reinforcement Plastic spheres 194 kg CO 2 /ton steel 0.96 kg CO 2 /m 2 slab Energy for concrete production Since the energy is hydroelectric, the contribution from the actual production of concrete is only reported by the concrete producer to be 1.7 kg CO 2 /m 3 concrete. In a country where the electricity is coal-based, anything from 0.5. to 1 kg / kwh. CO 2 -summary Costs Table 6. CO 2 tons Conv. HSC HSC Conv. HSC concrete concrete Conv. floor Conv. floor Flat slab Bubblede ck Bubbled eck Concrete Reinforcement Spheres Total CO Table 7. Unit costs for total cost estimates (NOK). (1 NOK 0.18 USD) Conventional concrete 567 NOK/m 3 HSC 611 NOK /m 3 Labor/hour 300 NOK Bubbledeck elements 455 NOK/ m 2 The basis for costs calculated are shown in table 7. Table 8 shows accumulated construction costs (above ground) (1 USD 5.9 NOK?) Table 8. Total costs (NOK) Conv.Conc HSC HSC Conv.Concr HSC Conv.Floor Conv. Floor Flat slab Bubbledeck Bubbledeck Concrete Labor Reinforcement Elements total Notes to unit values: Reinforcement costs are from: and are not including transport and bending costs. The concrete volume includes only columns, beams and floor slabs (not stairwell and corner walls). Cost of concrete only includes raw materials. Contribution to the producer and transport is not included Labor costs are estimates from local contractor supplemented by a standard Norwegian reference estimation tool Bubbledeck element costs are estimates from the Norwegian distributor where production of precast elements, with concrete, steel and balls included. RESULTS, SHORT DISCUSSION The results are in table 9. The achieved results for CO 2, price, and lifetime are very promising. The lifetime was calculated using Life-365. The test result for traditional concrete B35, shows a lifetime of 52 years, while high strength concrete shows 206+ years, (Life 365 has a maximum analysis period of 200 years). There are a number of options for improving the analysis considered in this report, for example: The use of high strength concrete has an impact on the amount of reinforcement necessary to meet the design standard, and the use of high strength reinforcement can reduce the amount of steel necessary, as exemplified by the 20% reduction reported by Maingot 11. The impact of transport of raw materials and concrete on total CO 2 has to be included in a complete evaluation, and transport can conceivably be of decisive impact when long distances are involved. (As an example, fly ash might not be quite so attractive when it needs to be shipped km (6000 miles)). Another issue is the prevalent type of energy, and the corresponding CO 2 load. As for cost, it should be remembered that a number of factors are kept out of this model, the most important of which is Life Cycle Costs (LCC), where the service life of HPC is important. Also, contribution of high strength concrete to speed of construction is not considered in the costs. There is a marginal benefit in floor space of using high strength, in this case about 0.2% (up to 8m 2 ). Conv deck Table 9. Summary of results Building 1 Building 2 Building x Building 3 Building 4 Conv. HSC HSC Conv. HSC Conv. Flat slab Bubblede deck ck Bubble Bubble Bubbledeck Bubble Total CO ,3 [tonnes] CO 2 Saving Total NOK Service (years) DURABILITY cost life Chloride resistance (NTBUILD ) (Also AASHTO TP64) The chloride resistance of this high strength concrete was excellent: Transport coefficients determined using Nordtest NT BUILD 492: HSC (Mixture 11): Conventional concrete (C45): 0.48 *10-12 m 2 /sec 8,18 *10-12 m 2 /sec These values were then used to estimate service life in a chloride environment Life365 default). Table 10. Estimated service life in chloride environment (default environment) LIFE Conventional concrete HSC Service life (years) 52, using default exposure (the maximum the software would determine) Freeze-Thaw Many authors claim that frost resistance can be obtained without air-entrainment for high strength concrete, and much testing has shown that, say, more that 80 MPa will show good resistance to freeze thaw even in salt frost situations. However, it is difficult to find any supplier who will recommend doing this, obviously for responsibility reasons. We have, however, seen a number where the owner has specified no air entrainment in order to avoid strength penalty on the concrete. ASR Ternary mixes as developed here have a very high resistance to ASR. With a very low clinker content and abt. 20 % silica fume and 15% fly ash, the resistance to ASR, as calculated for instance by CSA A23.2, is very high.. From CSA A A Table 6 Combinations of SCMs When two or more SCMs are used together to control ASR, the minimum replacement levels given in this Table for the individual SCMs may be reduced provided that the sum of the parts of each SCM is greater than or equal to one. For example, when silica fume and slag are combined, the silica fume level may be reduced to one-third of the minimum silica fume level given in this Table provided that the slag level is at least two-thirds of the minimum slag level given in this Table. The effectiveness of other ternary blend combinations, using fly ashes, slag, silica fume, or natural pozzolans shall be demonstrated in accordance with CSA A A Other matters Ternary blends and low w/cm will also be effective towards other aggressive media, such as sulfate and even weak acids. It is, however, recommended to run a test on the appropriate mixture in order to verify, and possibly improve, the resistance. CONCLUSIONS The results show that the use of high strength concrete is highly relevant to ensuring the future of concrete construction industry. This example has shown that by using HSC/HPC in the chosen buildings, together with structural measures, it should possible to achieve more than 50% reduction in total construction CO 2 emissions. This is a step in the right direction towards a more sustainable concrete industry. It could be immediately used, and should be adapted in future building philosophy. Obviously, there are some reservations that must be made: CO 2 (and embodied energy) will vary
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