Rant: I always thought fly ash was just a cheap filler until I looked at the numbers

Was reading through some old ACI papers last week and found a study that showed mixes with 20-25% fly ash actually had BETTER long term strength after 90 days than straight portland cement. Everyone I work with says fly ash is just the cheap stuff you use when you're trying to save a buck. But the numbers from that 2019 DOT field test in Colorado are pretty clear. Has anyone else seen different results on actual job sites?

2 comments

2 Comments

baker.ben12d ago

Man, that bridge deck story hits home. I saw something similar on a parking garage ramp job where we used 30% fly ash and the owner was freaking out because the 7 day breaks were low. But after 6 months that slab was like a rock, way less map cracking than the sections done with straight portland. The slower cure thing is real though, we had to keep the thermal blankets on for an extra week in November.

johnthompson12d ago

Yeah, I've been there. The guys on my crew used to call it "budget dust" until we did a bridge deck pour with 25% fly ash. After a year, the core samples were denser and had less shrinkage cracking than the straight cement sections we did right next to it. The catch is you gotta let it cure longer, especially in cooler weather, because the initial strength gain is slower. That's probably why some job sites hate it when the schedule is tight.