A Comparative Analysis of Mix Proportioning for Prescriptive and Performance-Based Specifications for Sustainability
With the recent advancements in concrete technology, design and production of “green concrete” is increasing in demand. Although, the production of structures that are entirely sustainable is still challenging, industry has shown progress by motivating alternative solutions to conventional concrete such as using recycled aggregates, binary and ternary mixes with high levels of supplementary cementitious materials, and alternative binders with different chemistry with lower carbon footprints than portland cement. However, mix proportioning guidelines for performance-based specifications have fallen behind these advancements. ACI 211, the mostcommonly used mix proportioning guideline for prescriptive-based specifications, is primarily focused on structural concrete, and has not been updated after the recent developments in concrete technology. ACI 211 is conservative in recommending the cementitious materials content which negatively impacts sustainability, despite the well known relation between cement production and carbon dioxide emissions.
This paper addresses the effect of mix proportioning on sustainability by comparing the corresponding carbon footprint of cementitious materials required when proportioned based on ACI 211 mix proportioning guidelines and performance requirements for concrete pavements. The performance criteria for concrete pavements were selected, and the quantity of materials was calculated following the ACI 211 mix proportioning guidelines to achieve the desired workability and strength. Sixty different binary and ternary mixes were proportioned based on performance requirements which were verified by testing fresh and hardened properties. The comparison between the calculated cementitious materials content recommended by ACI 211 with the required cementitious materials content of performance-based mixes to achieve the same given performance criteria, as well as their associated carbon footprints are presented and discussed.