Concrete

Reinforced concrete presents a great combination of strength, durability, and fire resistance, and can be green, too.

1 Flyash concrete
2 Recycled rubble
3 Prefabricated form boards

Because foundations carry the entire weight of the building and suffer wetness and microbial action from the ground, they are almost invariably made of reinforced concrete. Larger buildings also use concrete for floors, walls, and structural frames when loads and fire risks are high. The construction industry is working to lessen the environmental impacts of conventional concrete; every building project has the opportunity to support the use of better kinds of concrete and foundations.

Goal: Make concrete with sustainable materials

Concrete made with flyash

Reinforced concrete is a blend of water, cement, sand, and gravel (or "aggregate") poured or sprayed into formwork around steel reinforcing bars, where it cures and hardens. Cement production involves mining of limestone and a very energy–intensive burning process that produces a substantial percentage (8% or more) of the world's CO2 greenhouse gas emissions. Acquiring sand and gravel involves extraction and transportation of virgin materials. And concrete's wood formwork is often thrown away after a single use. Building sustainably with concrete means using alternatives to these conventional materials and techniques that perform as well but have lower impacts and lower costs. Reinforcing steel typically has a high percentage (60% or more) of recycled content; it is an important element of sustainable building as well.

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Use flyash in concrete

What is this?

Flyash is a byproduct of coal–burning power plants that has binding properties similar to cement, and can be substituted for a large portion of the cement usually used to make concrete.

Why do it?

Substituting flyash, a waste material, for manufactured cement saves natural resources used in cement production and often saves money as well. Flyash also makes concrete stronger, more waterproof, and more durable, although it can slow curing time.

How to do this?

Many California concrete companies already provide some flyash in their standard concrete mix. Ask for the largest amount of flyash they are willing to provide and include this in written specifications. Structural engineers can specify even higher percentages of flyash if they are familiar with current research on its use.

Who does this?

Concrete companies and subcontractors, architects, structural engineers, specification writers.

Making Better Concrete

The County of San Mateo Department of Public Works has a policy relating to the use of Fly Ash. You can download this policy and the related engineering documents at Programs and Policies.

Technical information on fly ash from Build It Green

Everything you want to know and more can be found in Making Better Concrete by Bruce King available at Green Building Press


area one

Use recycled aggregate in non–structural concrete

What is this?

Crushed waste concrete from building and sidewalk demolition is often suitable for use as concrete aggregate — the gravel–sized particles that are held together by the cement and sand matrix of concrete in place of gravel or crushed rock from quarries.

Why do it?

Substituting a common waste material for virgin materials reduces the environmental impacts of materials mining, processing and transportation. It also saves landfill space and the fees charged to your project for dumping the rubble your project may generate in demolition.

How to do this?

Recycled aggregate is not appropriate for structural concrete, but can be used for sidewalks, low site walls, and topping slabs. In decorative applications, recycled materials can include native rocks, broken ceramic tiles, or glass. Also, consider simply using stacked stone or concrete rubble for site retaining walls — these could then include 100% recycled material.

Who does this?

Concrete companies and subcontractors, landscape contractors.


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Use prefabricated forms or save and reuse wood form boards

What is this?

Wood form boards hold concrete while it cures from a semiliquid to a hard finished material. By taking some care when removing wood form boards, they can be reused many times. Alternately, prefabricated forms, typically made of aluminum, can be substituted for wood forms.

Why do it?

Saving wood — especially the large–dimension boards often used for framework, reduces forest loss and especially harvesting of old–growth trees. Reusing forms also saves material costs, and prefabricated forms can be faster to install as well.

How to do this?

Plan out wood forms so that the boards can be removed in condition for reuse. Prefabricated or aluminum forms can be rented or purchased. Write specifications to prevent the construction of wasteful single–use formwork.

Who does this?

Concrete companies and subcontractors.