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I'm looking to make some weight plates for a home gym and so I am planning on using cement to make the plates. But I know that some water will be absorbed and not evaporated when making the cement so it will be hard for me to get the weight that I want.

How much heavier is cement after it has set? Do different conditions change this?

If I add more or less water will it affect this or will it only take a certain amount of water and then the rest will evaporate?

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@Mazura I imagine they won't last as long but unless you're dropping them they should last years. Weight plates are around $1 per pound, I can buy a hell of a lot of cement for $300. I checked on craigslist and a few others but there's nothing cheap near me. But I think it'd be more fun if I made it myself anyway – Aequitas Feb 1 at 10:36
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@Aequitas - Are you planning on using cement or concrete? You have stated cement, but the answers all seem to be about concrete. – AndyT Feb 1 at 17:06
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I don't mean to be negative but this seems like a lot of work for little gain. Yes, steel plates are pricey but if you are going to be working out at home I assume you'll be doing deadlifting which has a very high impact when you set the weight down. I can't image 1-2 inches of concrete holding together after hundreds of these. If you are not doing deadlifts setting weight back on the rack from squat/bench causes a lot of vibration. How are you planning on making a perfectly round concrete plate with a perfectly centered hole for the bar? You really don't want that thing to be out of balance. – Brad Feb 1 at 18:08
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You also never want to put yourself in the position of I don't want to drop these, they might break. - Drop it like it's hot, or welcome to a lifetime of back surgeries. – Mazura Feb 1 at 21:14
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@Mazura or where racking it vibrates a plate into breaking and the bar dangerously flips over itself. – Brad Feb 1 at 22:35

Several things affect the final weight of concrete.

Concrete has many different densities based on its composition. Standard "ready mix" concrete is often engineered for structural strength of 3000psi. Since concrete is not this strong when it is first poured, a standard was created for when to test concrete. This standard calls for the strength to be determined at 28 days old. At this point, concrete has achieved about 99% of its strength, and will continue to very slowly strengthen over the next few years. At 28 days old, I use a density of .085 pounds per cubic inch (146.9 pounds per cubic foot) when calculating structural load.

Concrete is porous. In humid weather, concrete will absorb moisture from the air while in dry weather it will release moisture. Even if you made the "perfect" weight today, it may decrease or increase over time. This is why, when you buy concrete weights, they are encased in plastic.

Regarding your question on adding more or less water: Contrary to popular belief, concrete doesn't "dry" to for it to cure. Concrete hardens as a chemical reaction. Adding more water than what is optimal to create the chemical reaction will only weaken the concrete without changing its final weight.

As a side note, concrete cracks easily. This is another reason why, when you buy concrete weights, they are encased in plastic. With that not being a reasonable option here, and based on your application, I would strengthen the concrete mesh or fiber. You can order "ready mix" with fiberglass fibers in it, or you can use a large hole mesh. Too small of holes will create a fault line. Aluminum works well because its density (.097) is close to concrete's density. In any case, this type of weight is not to be dropped from any height.

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You could make a pretty simple test to get almost exact data for your situation. My guess is that you are planning to use the pre-mixed stuff in the bags that you buy from the big box home store. Spend less than ten bucks and get a bag. Weigh it before opening the bag. Then mix up a batch according to the recipe on the bag and pour the mix into some random card board box to let it setup into a large brick. When it is hard and dried out weigh it again.

From this it will be easy to extrapolate target weight from the number of bags of pre-mix used.

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I was going to post the exact thing. All that you have to do is mix 40oz, use a small scale, and then see where it is in a week. Extrapolate for there. – DMoore Feb 1 at 5:06

According to my research, about 95% of the water used in the concrete mixing process will be in the final result, so you basically need to directly calculate how much you want the plates to weigh by adding the weight of the concrete and water together.

Sites suggest that you'll need about 10% more concrete than calculated just to be safe. Standard reinforced concrete is 150 pounds per cubic foot when dry, but there are "light" versions that may be under 100 pounds per cubic foot, or as heavy as 300 pounds per cubic foot.

If you want 10 pound plates using a standard concrete mix, the plates should be about 11 inches by 10 inches by 1 inch thick (note: that's a very off-the-back-of-a-napkin estimate, so experiment). In other words, if you take 8 pounds of concrete mix and add 2 pounds of water (a quart), you'll end up with approximately 10 pounds of final product (evaporation accounts for about 1.6 oz of weight for a 10 pound block).

You should be able to build a couple of small prototypes for just a couple dollars, as other answers suggested, but I thought I'd throw a starting point out there for you so you can have some idea of what to expect.

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Concrete is aggregate held together by cement, not pure cement. The weight variation in concrete comes (mainly) from varying the aggregate rather than varying the cement. – AndyT Feb 1 at 10:41
    
I believe we're saying the same thing. One pound cement, seven pounds of aggregate, and two pounds of water make about ten pounds of concrete, if I'm understating the resources I found. The aggregate makes up nearly seventy percent of the concrete. But, as I also said, "standard" mixes yield about 150 pounds per cubic foot. – phyrfox Feb 1 at 16:41

Adding different amounts of water will change the strength of the concrete. So you probably don't want to change that.

I don't know if there is a way to calculate the final weight, but the easiest thing to do is probably to make it heavier than needed, and sand or chip away at it until it is the exact weight you want.

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Almost all the water stays in the concrete. It does not dry out, but combines with the cement in a chemical reaction. So there should be relatively little difference between the weight when wet vs dry.

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if the water stays in the concrete why would that not affect the weight? if your first two sentences are true your conclusion should be the opposite – Aequitas Feb 1 at 4:44
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"The water not consumed in the hydration reaction will remain in the microstructure pore space." –UIUC @Aequitas - You have to weigh the water too. – Mazura Feb 1 at 4:45
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@Aequitas: I meant that the weight will not change as it dries. So if you weight out a 20 lb amount of wet concrete it will still be 20 lbs when it has cured a few weeks later. (Note that "cement" is the powdered binder and "concrete" is the mixed final product... not the same thing). – Henry Jackson Feb 1 at 15:23

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