Steps to reduce overall carbon emissions associated with concrete pavements by about 50 percent

MIT – Concrete is one of the most extensively used materials worldwide — on average, more than two tons per year of the rock-like stuff is produced for every man, woman and child on Earth, making its use second only to water. And that vast amount of new concrete is responsible for somewhere between 5 and 10 percent of global greenhouse gas emissions, making it a significant target for improvements.

MIT in 2009 established a research group called the Concrete Sustainability Hub, with support from the cement industry. This month the CSH issued two major reports — one on concrete pavements (103 page pdf), the second on concrete buildings (115 page pdf) — that examine in detail those products’ life-cycle costs, in both money and greenhouse gas emissions.

Specific suggestions of actions that could improve a road’s life-cycle costs, emissions or both:

* Increase maintenance work on roadways to keep the surface smoother, thus improving the gas mileage of the cars and trucks that use it. For example, instead of scheduling road maintenance every 20 years, do it every 10 years.

* When pavement is replaced, pulverizing the old concrete and leaving it exposed for at least a year causes it to absorb carbon dioxide from the air, helping to cancel out part of the emissions released when the cement was produced.

* Even the color of a road can mitigate its overall effect on Earth’s climate: Lighter roads reflect more sunlight, while darker ones absorb it and get hotter. Just as white roofs can help to reduce warming of the climate, so can lighter pavements — which can be produced by adding lighter-colored aggregate (gravel or crushed rock) to the concrete mixture.

* Reassess the design criteria for road construction, to account for local and regional differences. Most specifications are now generic, which results in over-engineering many roads, making them stronger than they need to be. Simply reducing the paving thickness in places where this can be done without degrading performance could significantly reduce the amount of cement used, thus reducing both costs and emissions.

* Add more fly ash, a waste product scrubbed from the emissions of coal-fired powerplants, to the concrete mix. This material is already widely used, but increasing its use could displace more cement powder, which is a highly energy-intensive material to produce.


Note – China’s Broad Groups approach to factory mass produced buildings that are minimally assembled on site appears to address more of the issues of more radically reducing material usage and getting high efficiency buildings made.

The Can Be Built approach is 8-10 times more efficient with material, since it is replacing 72 tons of house and the extra buildings for offices and shops. It is 20 times more efficient if the higher occupancy levels are used.

Broad’s air conditioners are twice as energy efficient as conventional electric chillers of comparable size, and their CO2 emissions are four times lower.

Broad sustainable buildings are energy efficient – with CO2 savings on the order of 300 kg per square meter from materials, and automation of the production process. In 2010, Broad began building a factory for sustainable buildings which should be able to produce 10 million square meters of sustainable buildings.

The construction and maintenance of buildings is responsible for the majority of materials consumption in the United States. The operation of buildings is currently responsible for about 40% of national annual energy usage and about 70% of national electricity consumption.

Key findings of this report include the following:

* Total embodied GWP is approximately 27-69 lbs CO2e/ft2 (128-339 kg CO2e/m2) across residential and commercial buildings constructed in concrete, wood, and steel.

* In general, residential concrete buildings have higher embodied GWP than the wood alternative, while commercial concrete buildings are roughly equivalent to the steel alternative

* Annual operating GWP per square foot is approximately 8-18 lbs CO2e/ft2 (39-88 kg CO2e/m2) for residential and commercial buildings in Chicago and Phoenix.

* In general, the concrete structures have lower annual operating GWP than the alternate designs in wood or steel (ranging from 2%-10% in savings).

* Over a 60-year life cycle, the lower operating GWP outweighs the initially equal or higher embodied GWP for concrete buildings. This results in total life cycle GWP equal to or lower than alternate designs in steel or wood. The largest life cycle GWP reduction was 8%, for the single-family ICF house in Phoenix.

* Embodied GWP is equal to 2-8 years of annual operating GWP for a range of building types and materials.

* Over a 60-year lifetime, 88%-98% of CO2e emissions are due to the operating energy requirements for all buildings considered in this study.

* Increased substitution of fly ash or other SCMs, can reduce the embodied GWP of the concrete buildings considered here by 4% to 14%.

* While there are opportunities in the pre-use phase of the life cycle of concrete buildings, most carbon-reduction opportunities exist in the operating phase, including radiant cooling systems with chilled-water pipes embedded in concrete slabs.

* For residential buildings, life cycle cost analysis shows that reducing air infiltration in concrete houses and increasing the thermal resistance of concrete wall assemblies can be economically as well as environmentally attractive.

Improving the environmental performance of concrete buildings will require the attention of industry, government and the research community. A number of steps can be taken:

* Adopt life cycle design in the early design phase of new buildings, through LEED, state building codes and other means;

* Include in the life cycle design process both life cycle assessments based on key environmental metrics and life cycle cost assessments of the improved environmental performance;

* Include in cost assessments the downsizing of space conditioning equipment due to improvements to the building enclosure;

* Improve the formulation and application of concrete in buildings;

* Develop a public database of the simulated and measured performance of concrete buildings to more accurately assess the placement and amount of concrete and insulating materials in wall assemblies;

* Carry out field tests and document the performance of building space conditioning systems that enhance heat storage in thermal mass for a range of climates;

* Develop and promote low-carbon building design, complementing such current efforts as ASHRAE‘s Advanced Energy Design Guidelines to specify elimination of thermal bridges in building facades regardless of construction material and improved use of thermal mass.

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