Building My House

I have always wanted to build my own house. I am retired now, so I have the time. I found some land, designed a house that would fit the land and my needs and got started. I am doing all the work myself, so progress will be fairly slow. To read this blog from the beginning, start with the oldest archive and read posts from last to first.

Wednesday, November 19, 2008

52: Pour the Front Porch

It’s time to order the trusses. Well actually, the trusses are already up and installed, but let’s try to keep this blog in order. So, I had to pour the front porch concrete first. Why? Because at the corner of the porch sits a column, and on top of that column are two beams. Well, actually they are doubled 2x12’s, but they support two trusses that hold up the roof over the porch. I decided it would be much easier to pour and finish the concrete if the column wasn’t in the way.

Figuring how much concrete I would need wasn’t hard. It’s just length times width times depth. I figured about 20 cubic feet (a cubic yard is 27 cubic feet). I called around to a couple of redi-mix batch plants and they all wanted around $150 for a load this size. Realizing that the concrete truck can’t back up close enough to the porch to pour it directly, I would still have to use a wheelbarrow to get concrete to the porch.

Since I was going to have to use a wheelbarrow anyway, and because I already have a concrete mixer, and because I am pathologically frugal (okay, call it cheap…if you must) and because I like to do things myself, I decided to mix my own concrete. So it was off to buy concrete sand and pea gravel. A yard of each cost me $30 and two bags of Portland cement ran $16. Throw in $5 for gas (gas prices have come back down, now about $2 a gallon…enjoy it while it lasts, cause it ain’t gonna) and all my ingredients for the concrete came out about $50. Humm, one-third the cost of delivered concrete; now you’re talking my language. Or, to put it in terms that women shoppers understand, a 66% off retail sale. Hey guys, there’s bargains out there, but you have to be willing to do a little more than just drive to the store.

Okay, I said I bought two bags of Portland cement. But, it was quite an involved process figuring out how I needed two bags instead of three bags, or four, or even one. There is an old rule of thumb concrete mix out there that I’ve used before. It’s called the 3:2:1 rule; 3 parts gravel, 2 parts sand, 1 part cement. You can use either volume or weight for measuring the 3:2:1 parts, they both work (or are supposed to). To figure how much water you use the water-cement ratio, which should be about 50%, ie half as much water by weight as cement by weight. If you use that ratio you should have some pretty good concrete, but it is going to be very stiff (dry) and pretty hard to work (smooth). Conversely, if you put in enough water to make the concrete easy to work, you end up with weaker concrete. So, you go somewhere in the middle, balancing out workability versus strength. This concrete is for a porch, so it doesn’t have to be as strong as say concrete for a driveway that would have to support the weight of vehicles.

Okay, so you go buy a bag (or two) of Portland cement. It weighs 92.4 lbs (yes, that’s kinda heavy) and you want to know how much concrete that bag will make using the 3:2:1 rule. Well, let’s see. If I go by “weight” I’ll have 92 lbs of cement, twice that much weight in sand (184 lbs) and three times that much weight of gravel (276 lbs) and about 46 lbs of water. So far I have 598 lbs of ingredients, but does that make 598 lbs of concrete, and how many cubic feet of concrete is that (because in the end I still need 20 cubic feet). From my civil engineering days I remember that concrete weighs about 150 lbs per cubic foot, so 598 divided by 150 means this mix “should” yield me just under 4 cubic feet of concrete. At this rate I will need 5 bags of Portland to make 20 cubic feet.

Now this 3:2:1 rule is supposed to work both for weight of ingredients, AND for volume of ingredients. So, let’s do the same exercise with volume. Since the cement comes in a paper bag with rounded corners, it’s kinda hard to figure the volume of Portland we have in one bag. We could empty the cement out and then use the same bag over again for sand and rock (2 bags of sand, 3 bags of rock). But, the bag would never last long enough to do that. So, I dumped the cement into a box and then measured length, width and height of boxed cement to determine how many cubic feet were in one bag. So, you don’t have to do this for yourself, one 92.4 lb bag of Portland is 0.85 cubic feet. Okay, so 0.85 cubic feet plus twice that in sand (1.7 cubic feet) and three times that in rock (2.55 cubic feet) and 0.42 cubic feet of water adds up to 5.52 cubic feet of concrete out of one bag of Portland.

Okay, so by “weight”, one bag of Portland makes about 4 cubic feet, and by “volume” one bag makes 5.5 cubic feet, NOT the same number. This 3:2:1 method was supposed to work for both weight and volume. But, which one is right because they both can’t be. Here was my dilemma. I put the question up on a house building forum I am a member of, and one of the responses was, call a batch plant and ask them how much weight of ingredients they would use to make 20 cubic feet (3/4 of a yard) of concrete. I thought that was a prudent suggestion, so I called my favorite batch plant and they gave me the following: 1490 lbs of rock, 1009 lbs of sand, 285 lbs of Portland cement and 133 lbs of water. That reduces down to 0.6 yards of rock, 0.4 yards of sand, 3 bags of Portland and 16 gallons of water; and from that I should get 20 cubic feet of concrete.

Just adding up the volume of rock and sand you get 1 yard (I need ¾ yard, 20 cubic feet). This bothered me, thinking somehow they gave me the wrong numbers, or I misunderstood what they gave me. So, I called several other local batch plants; and it got even more interesting because they all gave me different numbers. While everyone quoted about the same numbers for rock and sand they were widely different in the amount of Portland cement I would need. I got numbers ranging from a low of 285 lbs, to a high of 500, with several numbers in between. Now, I’m really worried. Needing to make a decision I went with the 285 lb number because I have bought plenty of concrete from these guys and it’s always been good.

I already had the gravel and sand; so I bought a third bag of Portland cement and the girlfriend and I got started. I ran the mixer, and shoveled the wet concrete into the 5’ x 8’ work area and she worked it into a flat slab. It took about 3 hours to get all 11 or so mixer loads of concrete into place and then I took over with final smoothing of the slab. Thirty minutes later I was through with smoothing, so we misted it with water and covered it with visqueen. I left the visqueen on for a whole week while I worked on other things. After maybe four days I could see the slab edges were beginning to turn from the color of wet cement (dark gray) to the color of dried cement (light gray). After a week maybe 1/3 of the middle of the slab still looked wet, so I removed the visqueen and let it finish drying.

I’m happy with the slab. It didn’t turn out quite as smooth or uniform as it would have if I’d hired it done, or if I had been able to bring all the concrete in at once (in a truck). But it still looks pretty good. So, now it’s on to putting up roof trusses.

A note to future builders - If you try to mix your own, either limit yourself to a slab no bigger than this (5 x 8), or get a second concrete mixer going (with an extra person to run that mixer). You need to be able to keep a “wet edge” to ease the working (smoothing) of the slab. And to keep a wet edge you need to keep that concrete coming pretty continuously. Additionally, after three hours of concrete work, even though we both are in pretty good shape (work outs at the gym four days a week), we were still pretty tired. Any bigger slab and this project would have become real tough.


At 7:24 AM, Blogger Ed Abbey said...

Interesting post. I've mixed a lot of concrete and used the 3:2:1 rule but always by volume measured by whatever shovel I was using. I never heard that it applied to weight because in my experience, gravel and sand both can very dramatically based on size of the gravel and moisture content of the sand at the construction site. At batch plants where both of these can be controlled to a greater extent, I can see how they would then go to weight.

But in the end, I don't think it really matters either way for a porch slab. I'm sure your results will last longer than both of us.

At 9:39 AM, Blogger Tony said...

Actually I used pea gravel and the batch plants quoted that as 2500 lbs per yard. They quoted 2500 lbs per yard for dry sand and 2700 lbs if it was a "little" wet. In my calculations I used 2600.

In the end, I too used "volume" as you load a cement mixer with a shovel. I never bothered weighing anything.

At 7:00 PM, Blogger Tonerboy said...

WOW, it is really looking like a house. I spend several hours/wk "lurking" around building and woodworking forums trying to educate myself as much as possible.

While I haven't commited to building a house, I am feeling more and more each day like it may be the only way my wife and I will ever own a home again. Living on a fixed income is a little scary.


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