I tried a different mix for a driveway in Denver and it backfired hard

Everyone says to use a standard 3500 psi mix for residential driveways, but I wanted to try a 4000 psi mix with a little extra fly ash for a job last month. The idea was to get a stronger, more durable finish that would hold up better to freeze-thaw cycles. We poured it on a cool morning, around 45 degrees, and it set up way faster than expected, giving us maybe 20 minutes less working time. The finish ended up looking patchy and we had to grind down a couple of high spots the next day. Has anyone else had a mix surprise them like that on a cold pour?

3 comments

3 Comments

noran211mo ago

Was the fly ash a Class C or Class F type? That can really change how it sets, especially in the cold. The extra cement in a 4000 psi mix already heats up faster, and fly ash can sometimes make that reaction even quicker when the temps drop. You might have gotten a double hit on the set time. Next time, maybe ask your batch plant about a mid-range water reducer to buy back some of that workability without hurting strength.

noahhall1mo ago

Honestly have no clue what class it was, I just know it turned to stone before we could even finish screeding. Felt like we were racing against the mix itself. Next time I'm just gonna tell them to skip the fancy stuff and give me the dumb guy concrete. My back still hurts from trying to work that stuff.

dylan_rodriguez4d ago

I saw this one article a while back about how fly ash can actually slow down the initial set in cooler weather if it's Class F, but if they gave you Class C for that 4000 psi mix, it can make it kick off way faster because it's more reactive. Sounds like you got bit by the exact opposite of what you wanted. A lot of guys in Denver have been talking about that exact thing happening with the early morning pours when it's around 40-45 out, the concrete doesn't care about your plans. You probably would have been better off with a retarder added at the plant or just sticking with the 3500 psi and maybe a little air entrainment for the freeze-thaw stuff. That patchy finish and the grinding the next day, I bet that extra heat from the mix cooking off in the cold just dried everything out before you could get it smooth. Live and learn though, I've heard of plenty of people having to cut their losses on a driveway pour like that.

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