Mix Challenge 2

Here are the poll results of the Mix Design Challenge:

16% said Mix will perform as designed...
33% said Yield will increase...
33% said Yield will decrease...
16% said Mix will appear wet...
50% said Mix will appear dry...

As you can see, the majority thinks that the mix will not perform as designed and they are correct, but as for what will happen to the mix seems to be a guess.  50% of those polled think the mix will be dry and, as for the yield, half think it will increase and half think it will decrease. 

The correct answers are: The mix will appear wet and the yield will decrease.

I'll explain why.....

In our example, the gradation changed without the producer's knowledge.  So what started as yard of pervious concrete with 2,700 lbs of aggregate became .95 yards of pervious concrete with 2,565 lbs of #89 limestone aggregate and 135 lbs of sand (this explains why the yield will decrease).  The mix is batched with 600 pounds of cement which translates to 600 lbs per .95CY or 632 lbs per CY.  It has 21 gallons of water (175 lbs) per .95CY or 22 gallons per CY.  The water/cement ratio stays the same but the sand will increase the paste volume from .22% to .26%.  The voids will decrease to 15% and, seeming to seal easily, "jelled" spots will appear and make the the mix look as if it is to wet, and indeed, if any water is added to the truck, it may seal completely (these "jelled" spots are a good indicator that there are unplanned fines in the mix).

Now here's the next challenge.  How do you fix it?

Look at this week's Poll and try to pick the right answer or answers.

Mix Challenge

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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