How is concrete purchased
State and local governments purchase concrete for constructing buildings or transportation infrastructure such as roads, bridges, and sidewalks. For most buildings, governments are not purchasing the concrete directly. Instead, they hire an architect and structural engineer to design the building and a general contractor to construct the building. The general contractor then subcontracts out to a concrete producer to supply the mix design specified by the structural engineer. The labor to set and finish the concrete is not typically associated with the concrete supplier either.
Purchasing concrete for transportation infrastructure is much different than building construction. In these cases, governments directly hire a concrete producer to supply the concrete. Project mix designs are often driven by local or state construction specifications. Generally, specifications have become less prescriptive over time and allow for more flexibility in mix design.
Who needs to be involved
For building construction, the desire to obtain a low carbon concrete mix should be communicated as early as possible. It’s important that the architect and structural engineer be aware of the goal for low carbon concrete mixes during the design phase. The general contractor and the concrete producer should also be involved as early as possible. Ultimately, it’s up to the structural engineer to provide concrete mix design specifications to the general contractor that meet the project’s structural requirements. Specifications may include things like slump, air content, yield volume, water cement ratio, strength (at various time intervals), shrinkage, and set times. The general contractor controls the construction schedule and will hire a concrete producer that can deliver the desired mix designs. Early involvement of the concrete producer is also critical. Trial batches and past experience with high SCM mixes will help the producer inform the project schedule. The biggest challenge with low carbon concrete that has high SCM content are slow set times and low early strength. The contractor may need to leave the forms on longer and/or wait longer to reach the strength targets for that portion of the building.
Low early strength is the biggest issue that the general contractor needs to be aware of. Generally speaking, any concrete mix with SCM content above 30% may have low early strength. For example, a wall with 30% slag content may meet its 7 day strength target but a mix with 60% slag may take 14 days to meet the same early strength target. Overall, since the project schedule is affected by mix designs, the architect, engineer, general contractor, and the concrete producer should be involved as early as possible.
Where in procurement is intervention needed
In building construction, intervention is needed from the onset of hiring a design and structural engineering team. In fact, it’s prudent to ask the structural engineer before hiring them what their experience is with concrete mixes that have high SCM content. Are they comfortable specifying them? Have they done it before? What’s the highest SCM content they’ve specified in the past (anything over 40% SCM is fairly high in today’s market)?
The general contractor is a critical piece of the procurement process too. When project goals for low carbon concrete are communicated early with the general contractor, they can put specific requirements in their subcontract with the concrete producer that outlines their expectations. The exact nature of the language will vary depending on which purchasing strategy is selected. Cement maximums, SCM minimums, and EPDs are all purchasing strategies that can work their way into contract language.
Specific info needed to change purchasing behavior
Using SCM in concrete is not a new concept for concrete producers. So, at the most basic level, the organization purchasing concrete simply needs to ask for a low carbon concrete mix that has high SCM percentage. As highlighted in “Purchasing Strategies” section above, there are a variety of strategies to procure a low carbon mix. The first step is choosing the strategy that works for your organization.
The role of construction specifications
Most public agencies have construction specifications that dictate the requirements for public works projects. There is value in specifications to clearly communicate project requirements and have consistency of quality among public works projects. However, in some cases, construction specifications lag behind what is technically feasible and may limit an agency’s ability to procure low carbon concrete mixes. Short of changing the public specifications, most specifications will allow project by project variations if the mix designs are put through trial batches and proven to meet the strength and durability requirements of the project. Public agencies that are early adopters of low carbon concrete purchasing may have to pursue more trial batches and testing to demonstrate that the mix is appropriate.