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.
Prescriptive Vs Performance
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.
What are Performance Specifications
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.
Example of a Concrete Performance Specification: Strength
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.
What are Prescriptive Specifications for Concrete?
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:
- Limits on the composition of the concrete mixture such as minimum cement content.
- Limits on the quantity and characteristics of supplementary cementitious materials (SCM).
- Maximum water to cement ratio.
- Grading of aggregates.
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.
A Common Problem:
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.
Prescriptive Concrete Requirements
and Their Impact on Sustainability
Here are some examples of prescriptive requirements that put a damper on the ability to create innovative and sustainable concrete.
Restrictions on Types and Sources of Cement
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:
- Force the use of materials unfamiliar to the producer.
- Cause incompatibility with other materials and/ or require material to be transported a longer distance.
- Limit the use of innovative products.
- Increase the cost of concrete.
These restrictions do not support environmental goals.
Restrictions on Types and Sources of Concrete Aggregates
Some specifications might require the use of a specific aggregate source.
- Prevent mixtures from being optimized for performance.
- Increase the cost of the aggregate due to transportation.
- Adversely impact performance.
- Limit sustainable development of the product.
Content of Supplementary Cementitious Materials (SCM)
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:
- Preventing optimization for strength and durability.
- Increases cost.
- Not supporting sustainable development.
- Impacting performance capabilities.
- Impacting the long-term life of the structure.
Recycled Aggregates and Materials Not Permitted
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:
- Reduce cost.
- Conserve natural resources.
- Reduce landfill space.
These are just a few of the prescriptive requirements often placed on concrete that can inhibit the performance of concrete.
Performance Over Prescriptive
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.