Buildings and structures manufactured with concrete can have a significant impact on the environment throughout their life cycles. How much of an impact, will depend on the sustainability of the concrete mix used in construction—and the makeup of the mix used will depend on the requirements or specifications put in place before a project begins.
In order to create truly sustainable buildings made from environmentally friendly concrete, design professionals, contractors and material suppliers must work together while still complying with the requirements put in place.
To understand how specifications for a concrete project requirement can impact sustainability, you first need to understand the two types of requirements most frequently imposed.
During a concrete construction project, specifications for the concrete are included in construction documents to establish project requirements with which the contractor and material suppliers must both comply.
Performance specifications are project specifications that adhere to industry standards. They outline the characteristics of the fresh and hardened concrete and will depend on the application of the concrete and the specifications needed to create the final structure.
Performance specifications should provide the necessary flexibility to the contractor and producer to provide concrete mixtures that meet the performance criteria. They should not restrict innovations.
By specifying the compressive strength a building or structure needs, a concrete producer can design a mixture to meet that criteria through experience and testing. The mixture proportions (how much water or aggregates should be used) are not specified. This leaves the manufacturer the ability to create a mixture that not only meets the requirement but does so economically and may also be sustainable and environmentally friendly. They have the freedom to innovate as long as the concrete meets the performance specification.
Prescriptive specifications impose constraints on concrete mixture proportions or means and methods of construction, rather than the result like the above example.
Examples of prescriptive criteria include:
Due to the limits of prescriptive specifications and the fact that many do not support sustainable efforts, they generally should not be placed on a project. The intent of these specifications is often to reduce the permeability of the concrete but establishing a minimum cement content takes away the ability of the concrete producer to optimize concrete formulation for strength, durability, sustainability, and more.
Often, these two requirements conflict and a compressive strength requirement in addition to a minimum cement content can inhibit the manufacturers ability to create an affordable concrete. This results in concrete that is more expensive (cement is the most expensive ingredient in concrete). It could also mean that the concrete might crack from high shrinkage or thermal effects. Cement also increases the carbon footprint of the concrete (since cement has a relatively high carbon footprint).
Most of these requirements do not support sustainability goals and often increase the cost of concrete.
Here are some examples of prescriptive requirements that put a damper on the ability to create innovative and sustainable concrete.
Specifications often restrict the type of cement used and from where it can be sourced. Unless there is a building code or additional reason for this restriction, it should be avoided.
These types of restrictions might:
These restrictions do not support environmental goals.
Some specifications might require the use of a specific aggregate source.
This could:
Often, the use of more than one type of SCM is prohibited. The only building code around SCMs is for exterior concretes that will be exposed to deicing chemicals.
Limits on the quantity of these materials include:
Many applications for concrete can accommodate the use of recycled aggregates or other materials without impacting the quality of the concrete. Crushed and returned concrete can be used as a portion of the aggregate for some applications to conserve virgin materials and minimize waste.
Use of recycled materials is industry standard and will:
These are just a few of the prescriptive requirements often placed on concrete that can inhibit the performance of concrete.
In the end, a building owner is less concerned with how much cement their concrete contains and more concerned with the environmental impact of the building and how long it will last.
Performance guidelines allow the concrete producer to create a mixture that not only meets performance criteria but also has a low environmental footprint, will last, and is innovative in its design.
Thank you to the NRMCA for providing the information in this blog. To learn more about the role concrete plays in a sustainable future, visit Build With Strength.