Dry mix or wet concrete for ballast box

[s219]

So I'm about to fill my semi-custom ballast box, and am debating whether to fill with dry mix, or go through the routine of mixing the concrete with water.

Dry concrete mix is about 160-165 pounds per cubic foot (assumes minimal air), whereas wet mix ends up curing at 145 pounds per cubic foot. For my size box, dry mix will result in about 90-95 pounds more weight overall.

I know over time, the dry mix will setup hard, so in the end the results will be more or less the same -- a solid block of concrete in the box.

Anybody have a reason to avoid the dry mix? Only disadvantage I can think of is dust, so I'd have to cover the box. Or could fill primarily with dry mix and then cap off the top with a wet mix layer.

 

[RedNeckRacin]

If the mix is entirely contained I don't see a big difference between one way or the other. The only issue I could really see is if you need the strength for the box to keep it from deforming if you back into something, or if you puncture a hole in the box and dry mix starts pouring out. The box should be heavy enough that 90lbs won't make a big difference.

 

[CurlyDave]

...Anybody have a reason to avoid the dry mix? Only disadvantage I can think of is dust, so I'd have to cover the box. Or could fill primarily with dry mix and then cap off the top with a wet mix layer.

I suspect that there is a significant volume expansion between dry mix and cured concrete. You are adding water, but the density is decreasing. The only way that can happen is if volume increases. Since the concrete will eventually harden if put in dry and exposed to air, I think there is probably a volume increase there also.

This can generate a lot of pressure in a container. More than enough to damage the box.

I would take the time to mix first, that way you know the box will be the same size and shape at the end as when you made it.

 

[TemporaryKubotaOwnerMark]

I think that's backwards. Dry concrete mix should be LESS dense than cured. The cement takes water in as part of its chemical structure (concrete does not dry, it cures) and the resulting material has BOTH components in the same space.

The rule of thumb is to consider that a 94 lb. bag of cement will make one cubic foot when it is freshly packed.

Density Of Concrete - How Much Does Concrete Weigh?



Where are you getting that 165 lbs. number?

 

[EddieWalker]

Why use concrete mix? how important is every pound? Will you ever need to lessen the weight?

One of the best ballasts that I've seen was lengths of half inch rebar. He just kept adding them until he got the weight he wanted.

I've thought about five gallon buckets with water on my disk for more weight. I like the ability to remove them, add water for more weight, or just use the buckets for other things.

Considering that you already have the box, why not use sand?

 

[s219]

I suspect that there is a significant volume expansion between dry mix and cured concrete. You are adding water, but the density is decreasing. The only way that can happen is if volume increases. Since the concrete will eventually harden if put in dry and exposed to air, I think there is probably a volume increase there also.

This can generate a lot of pressure in a container. More than enough to damage the box.

I would take the time to mix first, that way you know the box will be the same size and shape at the end as when you made it.

See, now that is a good point and makes me glad I asked for opinions!! I have no idea what happens to dry mix when it hardens by itself, since I think most of us only do that for post holes and don't ever judge the volume to see if/how it changes. I have gotten a few rock-hard bags of concrete mix at the bottom of a pallet of bags, and they do indeed seem to be straining at the seams. Not sure if it's because they were at the bottom getting squeezed, or because hardening caused some increase in volume, but it's got me wondering....

 

[s219]

I think that's backwards. Dry concrete mix should be LESS dense than cured. The cement takes water in as part of its chemical structure (concrete does not dry, it cures) and the resulting material has BOTH components in the same space.

Density Of Concrete - How Much Does Concrete Weigh?

Where are you getting that 165 lbs. number?

I pulled it out of an engineering handbook and also did a couple different calculations to verify. Seems counterintuitive at first, but does make sense when I run the math.

One way to think about it is that cured concrete at 145 #/cu.ft is a mixture of lower-density water (62.4 #/cu.ft) and the bag mix. For the final product to end up at 145 #/cu.ft means that the bag mix has to be higher density than 145 #/cu/ft -- because the water, being lighter, mass-averages the mix's density down.

If you look at typical sack mix, making 1 "cured" cubic foot requires 1.375 gals of water and ~ 133-134 # of mix. That is ~ 133# of mix and 11.5# of water. Run the numbers for mass/density/etc and it should work out to the dry mix being about 165#/cu.ft density. That number assumes no air in the mix. From what I read, most dry mix has about 4% air by volume. So it's very well compacted, but still has room to "soak" up some water between particles beyond what goes into the mixing/curing. That will change the math a little bit, but I was too lazy to factor in for 4% air....

 

[s219]

Why use concrete mix? how important is every pound? Will you ever need to lessen the weight?

One of the best ballasts that I've seen was lengths of half inch rebar. He just kept adding them until he got the weight he wanted.

I've thought about five gallon buckets with water on my disk for more weight. I like the ability to remove them, add water for more weight, or just use the buckets for other things.

Considering that you already have the box, why not use sand?

Well, I was trying to maximize the weight I can get into the box volume, and I happen to have some sack mix leftover from another project. It's in my garage with a dehumidifier, but it will probably still harden up before I can use it for another purpose. So it's kind of got "ballast box" written all over it.

I have also considered adding scrap steel or lead, but the cost makes that less reasonable.

 

[9973720wb19]

i just filled mine with red bricks

 

[boomer1025]

I'd paint the inside and fill it with sand.