Consider an example of a pervious concrete project using a #89 crushed limestone aggregate with a specific gravity of 2.70 and a dry rodded unit weight of 100 lbs/CF. The mix is designed and produced with 2,700 lbs of stone, 600 pounds of cement and 21 gallons of water (175 lbs). This mix has a cement/aggregate ratio of .22 with 22% paste by volume (water/cement ratio of .29). Admixtures are used to control hydration and improve workability and so the mix is easily discharged from the truck and easily compacted due to its "fluid" nature.
Now here comes the challenge....what would happen if we threw in 5 lbs/CF of sand (into the stone at the quary) without the producer's knowledge? In other words, what if the gradation changed in this way during the course of the project?
Look at this week's Poll and try to pick the right answer or answers.
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If I may, I believe you have made an error in your calculations. 600pds of cement equals 11.234% volume (of a yard)and 21 gal of water equals 10.0748% for a total of 21.39% volume paste, not 17% as you state.
ReplyDeleteBased on the aggregate SG of 2.70 at 2,700 pds, the void content of the agg is 40.7%. Subtract the paste (21.39) for a final void content of 19.31%. (Actually probably slightly less with the admix's).
Adding 135 pds (per/yd) of sand (at an assumed SG of 2.70) adds an additional 2.966% of volume bring the void content down to 16.344%.
Given my understanding that as the void content approaches 15% there is a rapid reduction in the interconnectedness of the voids, I would conclude that this mix with the addition of the sand would likely not perform as designed. Since however this is not one of the choices my next best response is that the the mix will appear dry.
Thanks David!
ReplyDeleteActually, the total past volume is 5.74 CF, or 22% (assuming the cement SG is 3.1 and paste air is .0) leaving 19% voids (yes, probably less with admixtures, but we don’t need to be that detailed).
I’ve changed the post with the corrections and I’ll let the Challenge run for extra time.
Dale