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Design with PT

Design with post-tensioned concrete compared to reinforced concrete can offer up to 37% savings of CO2 emission

PT instead of reinforced concrete

The emission of greenhouse gases, specifically CO2, is the most common way of measuring the environmental impact relating to the energy use for an activity. However, the constant and rapid changes within the construction industry and the nature of the projects add to the complexity. Greenhouse gases are emitted during the production, transportation and assembling of materials but one also has to take into account the construction method, operation, maintenance and eventual demolition of the structures. The energy consumed during construction of an office building represents only between 5% and 15% of the total energy used during a 100 year design life.

As an example, consider a typical office building. Various features of post-tensioning - VSL’s core business - help reduce CO2 emissions of projects when compared with the same building constructed in ordinary reinforced concrete or composite steel concrete. Optimised post-tensioned structural cross-sections allow thinner floors and slabs giving more slender structures that result in immediate reductions in concrete volume of the order of up to 25%. More slender floor construction also results in savings in building heights of 5% or more, which brings with it savings in curtain wall costs, mechanical and electrical installation costs and in general fitout costs. There can also be reductions in the visual impact of a building. In addition, the use of high-strength prestressing steel, rather than only reinforcing steel, can lead to a reduction of the total tonnage of steel to be installed by up to 65%. Slender floors also reduce the foundation requirements. Maintenance needs are often reduced and design life extended. And buildings with post-tensioned floors generally provide more usable space and offer increased flexibility in use. Overall, post-tensioned structures have a significantly reduced environmental impact when compared with more traditional construction methods.

CO2 footprint with materials for PT

CO2 emissions for concrete are mainly due to cement production (Portland clinker) as the CO2 emissions for other constituents are marginal:

Global average0.85t CO2 / tonne cement
Low0.65t CO2/ tonne cement
High0.92t CO2 / tonne cement

However, some allowance should be made for concrete mixing. This adds about 15% to the CO2 total and so a value of 1t CO2 / tonne cement is suggested for consideration in any assessment. Concrete reabsorbs CO2 during its life due to carbonation, with major absorption at the time of demolition when the concrete is crushed. CO2 emissions can also be attributed to the reinforcing and prestressing steel:

Global average0.59t CO2 / tonne steel
Low0.15t CO2 / tonne steel
High1.08t CO2 / tonne steel

These values apply to the manufacture of bars and rods in an electric arc furnace. Production methods of rod for reinforcing and prestressing steel have similarities and so in a first approximation, the CO2 emissions for both steels are taken as equal. Structural steel sections produced in a blast furnace have a global average of 1.97t CO2 / tonne steel.

Post-tensioning to reduce greenhouse gases

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VSL Mexico participated in the construction of a commercial and hotel building in Cancun. The structure’s 32,500m² slabs and foundations were all post-tensioned: VSL’s technical alternative allowed for a reduction in the height of the steel beams from 800mm to 400mm and reduced the overall height of the building by 4m

In a typical office building, floors with VSL post-tensioning systems require less material, using it more efficiently. The savings in concrete and steel are of the order of 25% and 55% respectively, resulting in an overall reduction in the carbon footprint of between 25% and 30% for post-tensioned floors. In office buildings, floors and foundations represent about 50% of the cost of the structure, accounting for roughly 50% of the materials and up to 75% in the case of car parks. As a consequence, post-tensioned floors achieve a significant reduction of about 15% in the CO2 emissions for the total building construction. In long span floors with increased floor thickness, it is possible to introduce void formers and thus reduce the concrete volume and CO2 emissions by an additional 20%. Hence, VSL post-tensioned floors can reduce the emission of greenhouse gases for typical office buildings by 15% to 25%.

Post-tensioning offers significant reductions

Post-tensioning offers significant reductions

Generally the use of VSL Post-tensioning delivers the maximum cost-benefit for a project and has as well a beneficial impact on its sustainability and CO2 emissions during construction. Compared with conventional reinforced concrete slabs, the use of post-tensioning results in more durable structures with reduced concrete volumes, lowering the CO2 emissions by up to 37%.

Materials and quantitiesRC(kg CO2/m²)PC(kg CO2/m²)
Energy541340
Materials7,5674,760
Freight2,1621,360
Site staff transportation216136
Plant10868
Site waste216136
Total10,8116,800

The overall reduction of CO2 emission can achieve up to 37%!

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