It’s hard to escape the promotion of high-volume fly ash (HVFA) concrete in recent years. Perhaps you wonder, “Why bother?” If replacing portland cement with fly ash still results in gray concrete that cracks, is it really worth your time? Let’s take a different tack: Using HVFA is just plain smart. It is good engineering to adopt a proven material that brings benefit to the finished product and makes better use of our natural resources.
Portland cement in typical mixes accounts for 85 percent of the embodied energy and 95 percent of the “carbon footprint” of the mix. Therefore, it makes sense to replace cement with a material that would otherwise be sent to the landfill. It also makes sense to expend a little more effort with your client, owner, and contractor to get good strength, good quality, workable, smart concrete. “But I approve mix designs with 20 percent fly ash content all the time,” you protest. While that is common and well proven, an HVFA mix, as defined by Malhotra and Mehta in High-Performance, High Volume Fly Ash Concrete, must contain at least 50 percent fly ash by weight of total cementitious materials.
If you aren’t already familiar with the use of fly ash, take some time to learn about it and its benefits to concrete. Review ASCE’s recently released Sustainability Guidelines for the Structural Engineer, among other resources available (www.acaa-usa.org or www.tcaug.org).
The Mitchell Physics Buildings at Texas A&M University in College Station, Texas are a good case study for the practical aspects of HVFA concrete. Sustainability goals informed the decision and the design team was well educated on the benefits and use of HVFA. As well, Vaughn Construction, the project construction manager-at-risk, had extensive experience with HVFA. Since they were engaged in the project during the design phase, they were aware of the HVFA requirements and took steps to prepare the local concrete suppliers to address the projects requirements. Both the primary and backup suppliers prepared test batches that met the project specifications and production elements (piers, walls) were used to verify the mix designs. If using production elements to prove a mix design scares you, prepare your general contractor to perhaps use some site preparation pours (such as temporary building pads, mockup foundations, or non-structural site pours) to test HVFA mixes and finishing techniques. Maybe your suppliers have provided HVFA mixes in the past? Consider offering some flexibility in the age of acceptable test data if the supplier’s material sources haven’t changed, and allow a single test batch to confirm current performance.
Also, Vaughn was wary of getting “fear factor” cost markups from concrete suppliers because of their lack of experience with HVFA concrete. Proactive steps to educate suppliers and subs can mitigate this tendency.
Don’t be over-zealous with HVFA in concrete placements that have finished surfaces. Fly ash can act as a retarder and though this may be a desirable quality in certain applications (slurry piers, mat foundations), deck pours that take 30-plus hours to set up are a contractor’s worst nightmare. On the Mitchell project, we specified 50 percent replacement in drilled piers, 40 percent in basement walls and columns, and only 25 percent in slabs-on-grade and elevated decks. Be wary of cold weather placement and be prepared to work with your contractor to reduce fly ash content when floor finishing durations become unreasonable. Also, consider using a 56-day standard rather than the usual 28-day; HVFA concrete gains relatively more strength after 28 days. For elements that see sustained loads late in construction (lower level columns, foundations), higher replacement percentages are more feasible with a 56-day standard.
What’s the bottom line? The Mitchell Physics buildings used approximately 4,400 cubic yards of concrete in the foundations and basement walls alone. A typical concrete mix likely would have replaced approximately 20 percent of the cement with fly ash; however, by using 40 and 50 percent replacement percentages, the project reduced the need for approximately 300,000 pounds of cement. Put another way, this saved roughly the same amount of CO2 emissions as those that would be emitted during a 250,000-mile drive. All this was accomplished without a cost premium or an increase in schedule. With a little education, you can design a smart structure that makes intelligent use of readily available materials with an established performance record.
Jamison Smith, P.E., LEED AP, is a senior associate and project manager with the Houston office of Walter P Moore and was the firm’s structural project manager for the Mitchell Physics Buildings project. He can be reached at firstname.lastname@example.org. Brad Wendler is a project manager with the Bryan/College Station office of Vaughn Construction and served as the project manager for the Mitchell Physics Buildings project. He can be reached at email@example.com.