CHAPTER 6 SUSTAINABLITY OF HIGH VOLUME SLAG CONCRETE

Transcription

1 135 CHAPTER 6 SUSTAINABLITY OF HIGH VOLUME SLAG CONCRETE 6.1 GENERAL The construction industry is a rapidly growing one and large amount of men, materials and money are involved with it. The infrastructure boom has gained enormous fortunes for the civil engineering community but with a huge environmental cost. The environmental impact cannot be neglected due to the global warming and climate change Carbon Emissions Carbon footprint of a material is the quantity of CO 2 released in kg or tonnes during the life cycle of the material that is from cradle to site. It is also referred as embodied carbon. The production of construction materials is believed to be responsible for 10 % to 20% of total global CO 2 emissions 69. Carbon emission is a major sustainability issue and it is the responsibility of civil engineering fraternity to reduce carbon emissions and thereby reduce the harm done to our planet. Concrete is the most widely used construction material and cement is its primary ingredient. The carbon footprint of Ordinary Portland Cement is very high since its manufacture is highly energy intensive and calcination of lime stone emits huge volumes of CO 2. The cement industry alone is responsible for 5% percent of the global carbon emissions 70.

2 136 Table 6.1 Carbon Emissions during Production of 1 Tonne OPC Source CO 2 Emitted(kg) Comment Calcination (breakdown of limestone) 500 Unavoidable Fuel 350 Use waste fuel Electricity 80 - The carbon emission due to cement production can be reduced by replacing cement with pozzolonic materials like fly ash, Ground Granulated Blast Furnace Slag (GGBS), silica fume etc which are by-products of high temperature processes. The increased use of GGBS in the manufacture of concrete has a direct impact on reducing the CO 2 emissions. The reduction in cement consumption lowers the fuel consumption and calcination of limestone. The resulting concrete has lesser embodied energy and can be termed as Sustainable Concrete. Sustainable concrete is defined as one which incorporates waste material for its production and does not cause environmental destruction. Sustainable concrete has high performance and durability. The green materials like fly ash, GGBS are sustainable and have low embodied energy. OPC is the source of environmental burdens since it consumes 2% of global primary energy and is responsible for 5% of total global carbon dioxide emissions. Therefore, high volume replacement of cement with GGBS is an effort to attain sustainability in concrete industry. 6.2 EMBODIED ENERGY The construction industry is a fast growing and highly energy consuming one. The energy required to extract, process and transport a building material to site is known as embodied energy of the material.

3 137 energy is the total energy required to convert a raw material into product. As a thumb rule the embodied energy is an indicator of the impact of the material on the environment. Higher the value of embodied energy greater will be environmental impact. Energy use is a major environmental issue, most of the energy generated is by burning of fossil fuels (oil, gas and coal) which are non renewable sources. Burning of fossil fuels releases carbon dioxide a greenhouse gas which causes global warming. energy can be classified in to two categories, namely initial embodied energy and recurring embodied energy. Figure 6.1 presents the block diagram of embodied energy classification Initial Energy Initial embodied energy is divided into two components, namely indirect energy and direct energy. Indirect energy is the non renewable energy required to extract the raw materials, process and manufacture them. Direct energy is the energy required to transport the products to the construction site and construct the building Recurring Energy Once the building becomes operational energy is consumed to maintain, repair or replace components during the life of the building. This energy is termed as recurring embodied energy. Sometimes energy has to be spent even to demolish the building. Figure 6.1 provides the details about embodied energy calculation.

4 138 Primary Energy Extraction of Raw Transport of Raw Processing/ Manufacture Transport of Product to Site Building of Structure Maintenance Demolition Recurring Direct Indirect Energy Figure 6.1 Block Diagram of Energy Calculation Life Cycle Assessment (LCA) A life cycle assessment is the study and estimation of the environmental impacts of a given material caused or necessitated by its existence. Life cycle assessment is defined by two boundary conditions, namely cradle-to-site and cradle-to-grave. Cradle-to-site boundary condition determines the energy required and quantity of carbon emitted from extraction of the raw material until it reaches the building site. Cradle-to-grave includes the energy required and quantity of carbon emitted from extraction until demolition.

5 EMBODIED ENERGY MEASUREMENT energy is measured in terms of kilowatt hour/ unit weight of the material. One kilo watt hour is 3.6 Mega Joules or MJ. British thermal unit (BTU or Btu) is used to measure energy in power, heating and air conditioning industries. energy of material is denoted as kilowatt hours per tonne (kwh/t) or kilojoules per kilogram (kj/kg) or gigajoules per square metre (GJ/m 2 ) and it depends on the material that is being measured. The embodied energy calculation can vary with the inclusion of energy spent on transportation. s like timber, aggregates etc require significant amount of energy for transportation. Therefore, their embodied energy will be less if locally available materials are used. 6.4 EMBODIED ENERGY OF BUILDING MATERIALS Building materials that are naturally available and need less processing are classified as low embodied energy materials namely aggregates. s that require medium amount of processing are termed as moderately embodied energy like cement, bricks, plaster boards and materials obtained as a result of intensive industrial processes like steel, glass are termed as high embodied energy materials. This classification is dependent on the source as transporting these materials over large distances can increase the embodied energy drastically. s like steel, cement consume more energy during their manufacture whereas bulky materials like timber; aggregates etc require more energy for transport from source to site. Therefore, usage of locally available materials reduces the embodied energy in a building. The cradle to site boundary condition was employed to determine the embodied energy of the building materials in the current study. The values adopted in Table 6.2 are

6 140 from the database provided by Auroville Earth Institute 3. In the present study, Ordinary Portland Cement (53Grade) was used. The specimens were cast using M 40 grade of concrete, designed as per the IS 10262:2009. Locally available river sand and crushed stone aggregates were used. The superplasticizer used was Glenium B1-233 which is a commercial high range water reducing agent with low alkali content. Table 6.2 Energy per Unit Weight of s 3 Sl. No Energy(MJ/kg) 1. Cement Fine Aggregate Coarse Aggregate Water 0 5. GGBS Fly ash 0 7. Steel (virgin) The embodied energy of cement manufactured by dry process is 4.2 MJ/kg, energy used for transportation has also been included and hence the embodied energy value of cement was taken as 4.6 MJ/kg. Only the energy required for transportation is considered in the case of natural sand and it is about MJ/kg. In the case of coarse aggregate embodied energy considered is about 0.22 MJ/kg. The embodied energy of coarse aggregate is higher than fine aggregate, since energy is used to crush the aggregate to the required size.

7 141 In order to obtain GGBS, the granulated slag has to be ground, so the embodied energy value of GGBS is taken as 0.33 MJ/kg. The embodied energy of water is taken as zero. The embodied energy of concrete is much less than that of steel but large quantity of concrete is used hence the environmental effects are magnified. Three mixes were used in casting the specimens. Beam-column specimens were cast with 0% GGBS mix OPCC, 40% GGBS mix GGBS 40 and 50% GGBS mix GGBS 50 as a partial replacement to cement. The mix ratios are provided in Table 6.3. Table 6.3 OPC and GGBS Concrete Mix Ratios Mix (Grade -M40) OPCC GGBS 40 GGBS 50 Replacement % Water-cement ratio Cement (kg/m 3 ) GGBS(kg/ m 3 ) Fine Aggregate (kg/ m 3 ) Coarse Aggregate (kg/ m 3 ) Water (kg/ m 3 ) Superplasticizer (%) Figure 6.2 to gives the details of the quantity and percentage of material constituents in kg for 1m 3 of OPCC.

8 kg 417 kg 1224 kg 678 kg Cement Fine Aggregate Coarse Aggregate Water Figure 6.2 Quantity of Constituents in kg for 1m 3 of OPCC Figure 6.3 to Figure 6.5 give of the details of the percentage of material constituents in 1m 3 of OPCC, GGBS 40 and GGBS 50. Water 7% GGBS 0% Cement 17% Coarse Aggregate 49% Fine Aggregate 27% Figure 6.3 Percentage of Constituents in 1m 3 of OPCC

9 143 It is observed from Figure 6.4 that inclusion of 40% GGBS as a replacement to cement in the GGBS 40 mix constitutes nearly 7% of the total volume. It is observed from Figure 6.5 that addition of 50% GGBS as a replacement to cement in the GGBS 50 mix constitutes nearly 8% of the total volume. Water 7% GGBS 7% Cement 10% Fine Aggregate 27% Coarse Aggregate 49% Figure 6.4 Percentage of Constituents in GGBS 40 Water 7% GGBS 8% Cement 9% Fine Aggregate 27% Coarse Aggregate 49% Figure 6.5 Percentage of Constituents in GGBS 50

10 144 Figure 6.6 to Figure 6.8 give the percentage share of the embodied energy of materials in OPCC, GGBS 40 and GGBS 50. Fine Aggregate 1% Coarse Aggregate 12% Water 0% Cement 87% Figure 6.6 Percentage Share of Energy of s in OPCC Coarse Aggregate 19% Water 0% GGBS 4% Fine Aggregate 1% Cement 76% Figure 6.7 Percentage Share of Energy of s in GGBS 40

11 145 Fine Aggregate 1% Water 0% Coarse Aggregate 21% GGBS 5% Cement 73% Figure 6.8 Percentage Share of Energy of s in GGBS 50 It is observed from Figure 6.7 and Figure 6.8 that the percentage share of GGBS in the total embodied energy of GGBS 40 is 4% and it is about 5% in GGBS 50. Each beam-column specimen requires m 3 of concrete and m 3 of steel. The quantities of materials required to cast the beam-column specimen have been tabulated in Table 6.4.

12 146 Table 6.4 Quantity of s Required for the Beam-Column Specimens Sl. No for Reinforced Concrete Beam- Columns Specimens Quantity Required in kg for M 40 Grade of Concrete OPCC GGBS 40 GGBS Cement Fine Aggregate Coarse Aggregate Water GGBS Glenium Steel (virgin) Table 6.5 gives the details of the total embodied energy of the OPCC beam-column specimen. Table 6.5 Energy in OPCC Beam-Column Specimen Sl. No Energy of the (MJ/kg) Quantity Required (kg) Energy in the Specimen (MJ) 1. Cement Fine Aggregate Coarse Aggregate Water GGBS Glenium Steel (virgin) Total Energy Table 6.6 and Table 6.7 give the details of the embodied energy of the GGBS 40 and GGBS 50 beam-column specimen. The quantities of the constituent materials have been tabulated and the total embodied energy has been calculated.

13 147 Table 6.6 Energy in GGBS 40 Beam-Column Specimen Sl. No Energy of the (MJ/kg) Quantity Required (kg) Energy in the Specimen (MJ) 1. Cement Fine Aggregate Coarse Aggregate Water GGBS Glenium Steel (virgin) Total Energy 688 Table 6.7 Energy in GGBS 50 Beam-Column Specimen Sl. No Energy of the (MJ/kg) Quantity Required (kg) Energy in the Specimen (MJ) 1. Cement Fine Aggregate Coarse Aggregate Water GGBS Glenium Steel (virgin) Total Energy

14 148 Figure 6.9 shows that there is considerable reduction in the embodied energy of the specimens when there is partial replacement of cement with GGBS Energy (MJ) Energy 0 OPCC GGBS 40 GGBS 50 Concrete Mix Figure 6.9 Comparison of Energy of Beam-Column Specimens The beam-column specimen cast with OPCC has an embodied energy of MJ while the embodied energy of GGBS 40 beam-column specimen is 688 MJ. The embodied energy is MJ for beam-column specimen cast with GGBS 50. The embodied energy reduces by 6.4% when 40% GGBS replaces cement and there is 7.9 % reduction in embodied energy when 50% cement is replaced with GGBS. Hence it is evident that the material choice in the buildings will have an important effect on the embodied energy.

15 CARBON EMISSIONS REDUCTION IN HIGH VOLUME SLAG CONCRETE The CO 2 released into the atmosphere due to the extraction, processing, manufacturing and transport of the material from cradle to site is termed as carbon footprint of the material. It is also termed as embodied carbon (eco 2 ). It is the sum of the amount of greenhouse gas (GHG) emissions throughout the production and supply chain of a material or product. It is usually measured in kilograms or tonnes of CO 2 emitted. Table 6.8 provides the embodied carbon in OPCC beam-column specimen; the carbon emission values adopted are from the database provided by Auroville Earth Institute 3. Table 6.8 Carbon in OPCC Specimen Sl. No Carbon Emission (eco 2 ) kg CO 2 /kg Quantity Required in (kg) Carbon (eco 2 ) 1. Cement Fine Aggregate Coarse Aggregate Water GGBS Glenium Steel (virgin) Total Energy kg Table 6.9 and Table 6.10 provide the embodied carbon in beamcolumn specimens with GGBS 40 and GGBS 50 respectively.

16 150 Table 6.9 Carbon in GGBS 40 Specimen Sl. No Carbon Emission of the (eco 2 ) Quantity Required Carbon of the Specimen (eco 2 ) kg CO 2 /kg kg kg 1. Cement Fine Aggregate Coarse Aggregate Water GGBS Glenium Steel (virgin) Total Energy Table 6.10 Carbon in GGBS 50 Specimens Sl. No Carbon Emission of the (eco 2 ) Quantity Required Carbon of the Specimen (eco 2 ) kg CO 2 /kg kg kg 1. Cement Fine Aggregate Coarse Aggregate Water GGBS Glenium Steel (virgin) Total Energy 66.72

17 151 Figure 6.10 shows that there is considerable reduction in the carbon emissions when there is partial replacement of cement with GGBS. Carbon eco2 (kg) Carbon 0 OPCC GGBS 40 GGBS 50 Concrete Mix Figure 6.10 Comparison of Carbon of Beam-Column Specimens It is observed that in a single beam-column specimen the carbon emission is reduced by 11.2% when cement is replaced with 40% GGBS and 14 % in reduction Carbon emission is observed when cement is replaced with 50% GGBS. The above study reveals that the embodied energy and carbon emissions are directly related to the materials used in a building. It is evident that there is about 6.4% decrease in the embodied energy when cement is replaced by 40% GGBS in M 40 concrete and 7.8% reduction when cement is replaced by 50% GGBS. If a small scaled down sample has such significant reduction in embodied energy, then commendable reduction in embodied energy can be achieved if GGBS is used as a high volume replacement to cement in the entire building. Hence, it is evident that reduction in embodied energy will also reduce carbon emissions. This would be a way out to the

18 152 three pronged problem of energy crisis, global warming and climate change which is a huge challenge before humankind at present. 6.6 SUMMARY The sustainability concepts of GGBS concrete was discussed in this chapter. The embodied energy and carbon emissions of beam-columns with OPC and GGBS were estimated and compared. The findings indicate that inclusion of GGBS in concrete as a replacement to cement can cut down embodied energy and carbon emissions significantly.