Performance Specifications: The Key to Low-Carbon Concrete

Introduction

Concrete is unique among building materials. Design professionals and contractors have a greater influence on concrete’s product formulation than they do with other building products. Concrete can be made stronger, lighter, more flowable, stiffer, less permeable depending on performance needs using performance specifications. And most importantly, performance specifications provide the flexibility for contractors and concrete producers to meet traditional performance criteria for concrete while achieving lower carbon footprint.

Why is this Important?

According to the UN Environment’s Global Status Report 2017, the world is projected to add 2.5 trillion sq ft (230 billion sq meters) of buildings by 2060, doubling the current building stock. The report urges building industry to reduce operational carbon of building projects through disaster-resilience and net-zero energy consumption. It also urges the building industry to reduce the embodied carbon of building materials. The challenge for the concrete industry is to offer the benefits of concrete at a lower carbon footprint. Performance specifications are the mechanism that will help meet this challenge.

What is a Performance Specification?

A performance specification is a set of instructions that outlines the functional requirements for hardened concrete. The instructions should be clear, achievable, measurable and enforceable. For example, the traditional performance criteria for interior columns in a building might be compressive strength only. Traditional performance criteria for a parking garage might include strength, permeability, scaling, cracking and other criteria related to durability, usually identified by exposure class in the building code. More recently, embodied carbon limits are another performance criteria that have been placed on concrete.

Performance specifications should clearly specify the qualifications, submittals, test methods and the acceptance criteria that will be used to verify and enforce the requirements. For traditional performance criteria such as strength and durability, the building codes are very specific and well established. For carbon footprint, the concrete is qualified and accepted by requiring the carbon footprint of the concrete on the project be lower than the industry average carbon footprint by submitting Environmental Product Declarations (EPDs).

The specifications should provide flexibility to the contractor and producer to provide concrete mixtures that meet the performance criteria in the way they choose. The contractor and producer will also work together to develop a mix designs for the plastic concrete that meets additional requirement for placing and finishing such as flow and set time while ensuring that the performance requirements for the hardened concrete are not compromised.

What is the Concrete Industry Doing to Help?

To meet these challenges, the National Ready Mixed Concrete Association (NRMCA), USA, adopted the Architecture 2030 Challenge in 2012, which has goals to reduce operational carbon and embodied carbon from the built environment to net zero by 2050. NRMCA has outlined key strategies to incorporate carbon footprint as part of performance specifications in a document called The Top 10 Ways to Reduce Concrete’s Carbon Footprint.

All the strategies are important and should be implemented:

• Communicate carbon reduction goals
• Ensure good quality control and assurance
• Optimize concrete design
• Specify innovative cements
• Specify supplementary cementitious materials
• Specify admixtures
• Don’t limit ingredients
• Set targets for carbon footprint
• Sequester carbon dioxide in concrete
• Encourage innovation

This guidance document (Fig 1), available at www.nrmca.org/sustainability.com provides details of each strategy and recommended changes to specifications to help achieve lower carbon concrete.

Fig 1 NRMCA’s Guideline Document on Concrete’s Carbon Reduction

Tools and Resources Available from NRMCA

Visit www.nrmca.org/sustainability to download guidance on low-carbon concrete:

• Top 10 Ways to Reduce Concrete’s Carbon Footprint
• Guide to Specifications for Reducing Embodied Carbon
• Quantifying and Reducing the Carbon Footprint of a Concrete Building

Visit www.buildwithstrength.com/design-center for help with specification reviews and design guidance.

Visit www.concreteinnovations.com for education and award-winning concrete innovations.