|STEEL vs. CONCRETE|
|GREENING EAST CAMPUS|
|Concrete Versus Steel
This section evaluates the potential impacts and tradeoffs at stake in deciding to use steel or reinforced concrete to construct the superstructure of the East Campus Project building. Modern construction techniques can incorporate each type of superstructure into a building design without significant differences in the experience afforded to building users and maintenance personnel, or in the performance of other systems within the building. Aesthetics are also not impacted by the choice of structural material as the proposed building fašade can be achieved with either steel or concrete as the superstructure. Although concrete may be used as thermal mass as part of a passive heating and cooling strategy, such an option will not be considered herein because concrete could be strategically used in conjunction with either a steel or concrete superstructure if thermal mass is desired. Therefore, because impacts of the structural material during the usage phase of the building are minimal, they are not considered in this case study. The relevant impacts of concrete and steel to be compared are production of the materials, construction of the building, and demolition and disposal of the building at the end of its life.
Estimated environmental and worker safety impacts of concrete and steel superstructures for the East Campus Project building, assessed using Athena EIE (an environmental LCA tool) and published statistical information, are summarized in Table 1. The results indicate that a steel superstructure is preferable over a concrete structure for seven of the eleven impact categories evaluated. No significant difference was found in two of the impact categories (transportation of materials and fatal occupational injuries). The only impact category in which a concrete superstructure is preferable is Water Pollution Emissions. This is due to the large amount of water used in steel-making processes for cooling, quenching, and pollution control and the greater abundance of toxic chemicals used in steel making. More detailed information on each impact category is included in the following Detailed Results section.
The poor performance of concrete relative to steel in most impact categories is primarily caused by the large amount of material that goes into concrete structural elements. According to the Athena EIE model constructed to evaluate several of the impact categories, over six times as much material (by mass) is required to produce the concrete superstructure than is required for the steel superstructure. The modeled concrete superstructure requires approximately 21,000 cubic meters (m3) of concrete (46,000 standard tons assuming a density of 2,000 kg per m3) and about 1,500 tons of iron and steel for wire and rebar. A steel superstructure for the East Campus Project building would require approximately 3,000 m3 of concrete (7,000 standard tons) and about 1,000 standard tons of iron and steel.
It should be noted that if further investigations into the engineering requirements of the East Campus Project building show that the building would require more or less materials than the above figures for either type of superstructure, the given results can be adjusted proportionately. Because the modeled impacts occur primarily in the manufacturing phase, their magnitude is proportional to the amount of material produced. Therefore, if the amount of steel or concrete that will actually be required is found to be less than or more than that indicated by the Athena EIE, the Athena EIE results can simply be multiplied by the ratio of the two totals to obtain an approximation of the new result.
This analysis of concrete versus steel for the superstructure of the East Campus Project building is a classic example of how reducing the amount of material used for a product or service reduces the environmental impact of that product or service. Concrete would require an estimated siz times more material (by mass) than steel, and its estimated environmental impacts are consequently much higher. The results of the Athena EIE model, summarized in the graph below, show that steel produces far less environmental impact than concrete in every category with the exception of water emissions.
The comparison for safety of production workers shows that concrete and steel do not differ significantly for fatal injuries. However, non0fatal injuries and illness are estimated to be 12 perent higher for steel vs. concrete.
Methodology - Athena EIE model
Five of the impact categories (Energy Consumption, Natural Resource Use, Solid Waste Production, Air Pollution Emissions, and Water Pollution Emissions) were evaluated using the Athena Environmental Impact Evaluator (EIE) (version 2.0) software developed by the Athena Sustainable Materials Institute in Merrickville, Ontario, Canada. Athena EIE is an LCA program that allows for the modeling of environmental impacts of different building types, designs, and materials. The program calculates the environmental impact of a building design input by the user. A user may choose from an array of building components, such as beam and column systems, floor and roof systems, etc. Calculations are based on databases stored within the program that contain information on the resources consumed and the pollution produced by the production of various building materials from resource extraction through construction and use, to final disposal of the building materials.
Based on conceptual drawings of the East Campus Building provided by the MIT Facilities Department, the building was modeled as an eight-story 450,000 square foot (450 feet by 100 feet) building. The concrete superstructure scenario was modeled using a reinforced concrete gerber beam and column system and pre-cast concrete double-T floors and roof with a concrete and steel mesh topping on the floors. The steel superstructure scenario was modeled using a wide flange steel beam and hollow structural steel column system with open web steel joist and steel decking floor and roof with concrete topping added to the floors. No walls, envelope, foundation, or any other component (including the parking garage proposed to be built beneath the building) were added to the model as it is assumed that no significant change to their design will result from the choice between a concrete or a steel superstructure for the building.
Background - Production Process Overview
Concrete is produced by the mixing of hydraulic cement and aggregate materials such as sand, gravel, or crushed stone. Aggregate materials are mined from sand and gravel pits or open quarries or are obtained as waste from stone mining operations. Hydraulic cement generally consists of aluminum and silica from clay or shale, and a calcareous material such as limestone or chalk, all of which are also mined. Portland cement, the most common type produced in the US, also contains iron. Hydraulic cement is manufactured by combining these materials, heating and fusing them in a rotary kiln, cooling the product and reducing it to a fine powder. The cement and aggregate are combined, with each other at a concrete batching plant. The concrete mixture is then either combined with water and cast into prefabricated shapes, or transported to the building site to be cast in place , .
Structural steel is produced primarily from iron ore, coke, and scrap steel. Iron ore and coal are mined domestically in areas primarily located in the Great Lakes Region (iron ore) and in the Appalachian and Rocky Mountain Ranges (coal). The coal is heated in the absence of oxygen to produce coke. Iron ore is melted down in a blast furnace with coke and limestone to produce pig iron and slag. Pig iron is then processed with coke and scrap steel in a basic oxygen furnace to produce carbon steel. Various finishing and shaping processes can be used to alter the properties of the steel. Steel is also produced through a recycling process in which scrap steel is melted down in an electric arc furnace.
Summary of Concrete vs. Steel Assessment
for the Superstructure of the East Campus Project