How much concrete do I need ?

[John Timmins]

I'm thinking of having a pad poured in front of my workshop. It will be for light duty use like working on a 14 foot aluminum boat with a 10 HP motor or rolling my sandblaster outside to use it.

I'm not interested in footers or rebar. Do I need to have that fiber stuff added to the mix ?????

The slab in question is 9'10 inches wide by 20' long. I suppose it is going to be 4 inches thick. How many yards is this ?

A permit will have to be pulled to do this job in my city - $30 and no problem. What I have issue with is the local cement company has a 5 yard minimum.

Another small outfit called 3 Yard concrete has a 3 yard minimum for $375 plus tax....but they will deliver less if I want.

What I DON"T WANT is some ******* cement mixer driver who wants to rinse out and clean out his truck and dump all that slop in my yard. So I want to order as close as possible to the amount I need.

Just in case I'll have a form prepared to have a place to put access and make a pad for the trash can.

This posting obviously shows my no experience with concrete.

 

[Stuart in MN]

9 foot 10 inches = 118"
20 feet = 240"
volume = 118 x 240 x 4 = 113280 cubic inches
1 cubic inch = .00002143 cubic yards
113280 x .00002143 = 2.43 cubic yards

 

[holdover]

or you can do it this way round up to 10' X20' = 200sqft, 4" thick which is 1/3 of a foot, divide the 200 sq ft by 1/3 this is how many cubic feet of cement you need, there are 27 cu ft to a yd, divide 27 into the cu.ft. for you amount. If you want the slab 6" thick divide the sqft by 1/2 =100 and divide that by 27

 

[mtwaterguy]

or you can do it this way round up to 10' X20' = 200sqft, 4" thick which is 1/3 of a foot, divide the 200 sq ft by 1/3 this is how many cubic feet of cement you need, there are 27 cu ft to a yd, divide 27 into the cu.ft. for you amount. If you want the slab 6" thick divide the sqft by 1/2 =100 and divide that by 27

Since he's looking for an exact amount, rounding up probably will not work for him.

 

[TommyK]

9 foot 10 inches is 9.83 feet.

4 inches is .33 feet.

9.83 x 20 x .33/27=2.4 CY.

Get the three yards. They don't dump the leftovers in your yard. They will want to wash the chute though.

 

[geologist]

If you figure in that, on average, more often than not, you're not going to have a perfect 4 inches in depth (I went with 5 inches) then you'll end up using 3.03 yards of concrete. That .03 yards equates to 0.81 cubic feet. That is to say that 3 yards of concrete will give you enough to pour 95% of your pad 5" thick.

 

[ddawg16]

But no rebar? In the sandy soil of Florida? First crack will show up in less than a month.....within a year you will be tripping over cracks......

 

[rburke65]

He has already stated that he is NOT interested in rebar. A yard of concrete will cover an area of 81 square feet, or a 8'x10 area at 4" thick.......approx.

 

[Nighttrain]

At least go with wire mesh, as Dawg stated you will be tripping over the cracks.

 

[bofg33]

Here is the formula I have always used... convert everything to inches and then mulitply all three measurements then divide by 45656
118 x 240 x 4=113280 divide by 45656=2.48 yrds.
But i agree with the above posts get 3yrds.

 

[pro machine Engineering]

always better to have to much and have an ******* truck driver dump it in your yard. Than to have not enough to finish your pour. Which makes you the ******* when you start cussing yourself and the driver for the mistake

 

[Diverbill45]

Here's a simple formula that I have used for many years. I got it from a concrete co. that I used to order concrete from, when I had my own company. It's quite simple and the only number you have to remember is 80, since most slabs are 4" thick. For each 2" added to the thickness of the slab you just subtract 20.

1 cu. yard of concrete 4" thick covers 80 sq. feet, 6" thick covers 60 sq. feet, 8" thick covers 40 sq. feet.

So all you need to do is multiply the width by the length of the slab to get the total sq. feet, then divide the sq. feet by whatever the thickness of the slab will be and that gives you the total cubic yardage of concrete needed.

Example: I just poured a slab in front of my shop about 2 months ago that was 12' long and 24' wide. It was going to be 6" thick. I also added 1/2" rebar on 2' centers, which adds to the strength of the concrete.

12' x 24' = 288 sq. feet. Divide 288 sq. feet by 60 (slab being 6" thick) = 4.8 cubic yards of concrete. Seeing that this was almost 1/4 yard short of being 5 even yards, I ordered 5 yards of concrete.

Lesson learned quickly from many years ago; always order (by at least 1/4 yard) more concrete than needed. The worst thing that can happen is being short on concrete on a nice HOT sunny day. If it happens, you'll never let it happen again, AND IT DOES HAPPEN EVERY SO OFTEN.

ANOTHER WORD OF WARNING: (Yesteryear compared to today.) Before the computer age all concrete mixed at batch plants was mixed by hand. Same as free poured drinks at a bar, compared to metered poured drinks. Using this as an example: With free poured drinks you usually got more booze in your drink. Now days most all batch plants mix concrete with computers, so the chance of getting a little more concrete than needed, is kinda slim. You're more likely to get the exact yardage you ordered and if you have a couple of low spots in the area graded for the slab (and this can happen unless you're grading with a laser level, which will help in eliminating low spots) and you order the exact yardage you need, you'll come up short, compared to mixing by hand where there's a better chance of getting a little more than what you ordered.

What I did for the extra amount of concrete I ordered for the slab I poured, was make a form for a little slab in front of the man door going into my shop and had the extra concrete (2 tenths of a yard) dumped in that form. There was still a little bit of concrete left on the truck and we had it dumped in a low spot, in front of the big slab on the ground, spread it out and covered it up with gravel.

In your case 2.4 yards are needed, so I'd order 2.75 yards, to be on the safe side and the extra could go in the pad for the garbage cans.

Hope this helps.

 

[jclem40c]

length x width x thickness in inches x .0031 = total yards needed. When you get the answer round it off to the high side and order it. Better to have a little to much than be short. tell him to take the leftover back with him. Learned this building bridges for the county highway dept for 20 years.

John

 

[camarotoolman]

I live down the beach, put some wire or bars in it. If you live in Ormond beach you can afrord a few more bucks.

 

[Zeke]

Since the figure is coming out around 2.5 yds. and people say buy 3 since short load costs just about give it to you, thicken the edges a little. Make a little trench around the perimeter at the forms 6" deep and 6" wide. This is good especially at the front where you will drive up on the slab.

 

[NUTTSGT]

always better to have to much and have an ******* truck driver dump it in your yard. Than to have not enough to finish your pour. Which makes you the ******* when you start cussing yourself and the driver for the mistake

Since the figure is coming out around 2.5 yds. and people say buy 3 since short load costs just about give it to you, thicken the edges a little. Make a little trench around the perimeter at the forms 6" deep and 6" wide. This is good especially at the front where you will drive up on the slab.

Two very good posts.

 

[John Timmins]

Thanks for all the responces; all are helpful. The reason I didn't think rebar was needed is because swales were added last year to my street. Some driveways were dug out to put a culvert under them. Others like mine have a slight "V" shaped dip in the end of the driveway. The contractor dug a ditch from the road edge to the edge of the right of way for each driveway and renewed the driveway ends with new concrete.

All of the concrete used was fiber added stuff and no rebar was added. I assumed that if the city engineer didn't require rebar in the driveways, I wouldn't need it. What do I know?

I think the best thing to do is hire a local guy with a Bobcat and a general contractor's license, and just let him do the entire job from pulling the permit to hauling the forms away. I'll ask him what he thinks about the wire mesh.

I am not concerned about the cost of some rebar or mesh. The lot is on a salt water canal. Digging a hole gets you in salt water about 2-3 feet down depending on the tide. I'm surprised that my tomato plants even grow in that soil ! Corrosion is always something I think about....

The suggestion to fatten up the end of the slab is a good one. Again, thanks for all the replys !

John

 

[The Maxx]

Call the company you intend to order from and ask these questions. If they are customer oriented they can give you guidance on the amount and mix.

You can then ask them if the "******* driver" will need to place his full load on site, or if they can return the unused portion to be recycled. No one around here makes you take every bit, they all dump it out and crush it into base rock.

No matter who you call they will need to clean out the chutes and wash them on site. When the end of the pour is near, don't dump any more concrete in the chutes then you need. If it's in the chute it will have to go somewhere, most drivers will not shovel it back up into the truck. And with proper planning they shouldn't have to.

 

[Sureshot]

Don't they have mix on the fly units there that give you exactly what you need?