I'm going to be pouring a 3 x 5 slab 8" thick. 2" below ground and 6" above. It will need to support about 500lbs.
I'll be using quikrete for this. Do you think I'll need rebar for this small slab?
I am curious why so thick? You are holding up a bit more than 30 pounds/SF. Hand batching that much sack goods will have you worn out. 15-20 bags of 80 #. Normal sack goods makes about 2500 PSF concrete if you don't add excess water (most people do).
______________________________ Keep the whole world singing . . . . DanG (remove the sevens) snipped-for-privacy@7cox.net
I need to lift surface high enough so that it won't flood during heavy rains. putting it 6" above ground level will insure that. The most I've seen in that area is 2" of water during an extremely heavy downpour bfore the water had a chance to drain off.
Just want to be safe.
I will rent a mixer for this job for sure!
I'd use welded wire grid, an inch and a half from the bottom and top.
Either that, or place about half the concrete, and then dump rocks in to bulk it up, and then another layer of concrete. (Always assuming I could resist the temptation to cast a storage vault under the damn thing.)
I'm w/ Dan--why in the world use an 8" pour for such a small load? What is the footprint of the object(s) to be mounted on the slab? If the purpose of the thickness is to get the height above surrounding grade, I'd recommend strongly considering accomplishing that by grade adjustment and a resulting thinner slab. A standard nominal 4" slab would easily support the load unless it is a very small footprint (and in that case it would make a lot more sense to make local mounting piers thicker rather than the whole slab).
As for the bar, I wouldn't worry about bar, but I'd probably use 6" square mesh just for doing it (and because I've got a roll remnant around :) ).
HTH, and better or more specific answers would undoubtedly be forthcoming w/ some more info on what the objective actually is...
BTW, iirc (and I think I do) the 80-lb Quikrete sack is 2/3 cu-ft so your proposal would be a roughly 3x5x(2/3) cu-ft * (3/2) bags/cu-ft -->
15 bags. That's a lot of hand mixing considering the time element since you presumably want a continuous pour.
Before you mix all that concrete, look at some alternatives.
Can there be a platform on blocks? 500 pounds is not much so building a wood platform with a 2 x 4 frame and decking boards on top it will be sufficient. I'd use four concrete blocks to support it all. Once the concrete is in place, you have a very permanent slab. With wood and block, you can move, modify, eliminate, add height, as needed.
The concrete slab will weigh about 1600 pounds. Do you really want to handle that when something a couple hundred will do the job just as good?
No, you shouldn't need rebar, but as someone else suggested wire mesh instead. The rebar is there for bending stress, it won't stop it from cracking though.
Besides you didn't specify which gauge rebar you wanted to use, there's only about 50 different types (well actually that's an exaggeration). Anyway if it were me, I'd go with the other suggestions of pillar and post.
OK teach me something about slab building as I am a rookie.
The object is a 500bs box that is 2' x 4'. It is fairly evenly distributed over the box so load is not a problem.
Like I said in a previous post I'd like the surface of the slab to be 5" -
6" above the current ground level. This is in south texas so frost line is not a major problem.how would you recommend constructing this slab?
Mook, Would it be fine if the grass formed a little hill with this slab on top? Never pour concrete on grass. Remove sod as required. Use gravel, fill sand, broken cement blocks, old bricks, rocks, or select fill (compactable sand/gravel). Set a 2x4 edge form at the height and slope that you want to end up with. Pack the bottom edge with dirt or whatever so the concrete can't run out. Pour and finish. Strip forms and add back top soil and sod.
If you want it high above the grass with nicely finished 6-8" face. Set an edge form (2x8?) around the intended slab. You still need to have removed the sod. Raise as much of the center with rubble or select fill as possible, leaving the top and sides at least 2" thick. Think about the finished concrete looking a bit like an upside down cake pan.
______________________________ Keep the whole world singing . . . . DanG (remove the sevens) snipped-for-privacy@7cox.net
whats it going to be used for? a generator?
I would bui;ld a block foundation and rise it to a foot so flooding will never matter..
if its for emergenmcy equiptement like a generator
Eek!
Consider a few 4x4s, a decking of 2x6s, four (maybe six) concrete blocks for support. You'll need a saw, a tape measure, and some nails. Total time of construction for one person: 2 hours.
It'll take two hours just to MIX twenty 80-pound bags of concrete.
Also, consider what you'll have to do when the wife says: "It would look better over there."
...
I'd do one of several things, assuming you don't want a wooden deck type solution (which, in So TX I can certainly see) which as someone else mentioned would certainly not be difficult to make to support the load easily.
I'll outline basic choices/alternatives to consider--what, specifically works for you depends on factors I don't have any way of knowing--where you are, what sort of tools you have or have access to, etc., etc., ...
First choice would be to simply make a small "hill" to place the slab on top of...will need some fill material from somewhere which can be a pain if you're in a large metro area and don't have any hauling capabilities. If you're lucky you've got enough of a lot you can scrounge enough soil from one place to make the area you need large enough high enough above the surrounding area. Assuming it's a yard, ideally you would want it graded enough to plant back to grass and mow along w/ the rest of the yard.
Next would be variations of the "wall and slab" method...rather than a slab this thick all the way through, go ahead and form the full height around the outer edge but fill in the center portion except for an area around 4-6" wide (say) w/ fill material so that the bulk of the pour is only 3-4" thick. Again, need something for some fill--can be old brick, rock, clean dirt (not mixed w/ sod, sticks, etc.) or whatever else you can find. Need it to be good enough that it can be packed solidly, so settling isn't an issue later.
More sophisticated versions of the above include laying an outer wall of block or brick, etc., and essentially proceeding as above from there. Really all the point is is that there should be far less labor-intensive and expensive ways to get the end result w/o pouring such a huge block of solid concrete.
Second alternative would be to
You guessed it. Its a Generac 15kW unit and it will be a perminant addition to the house and won't move.
I thought about a wood solution but I don't like the idea of hot generator on wood. Wood rot and termites are also issues to deal with.
Tell me about building a block foundation?
Pour a standard 4" slab. Get 4 precast concrete deck anchor blocks. Drill 4 holes in alignment with the anchor holes in your genny's bottom/frame. Use threaded anchors to mount about 1 foot lengths of 1/2" or 5/8" threaded rod. Fit the four blocks over the threaded rods and mortar the blocks to the slab and fill the center hole in the blocks around the threaded rods with morter. Set your genny on top of the blocks with the threaded rods through the mounting holes in the bottom of the genny and tighten into place with some washers and nuts. This will provide an air space under the genny to keep it dry reducing corrosion, help it cool with in operation and high enough to prevent minor flooding from touching it. I did a similar installation in my garage for a diesel generator.
What about motor mounts or similar things? I'm thinking rubber mounts or polyurethane mounts - last for decades even under harsh conditions - like the kind in your engine compartment.
Say split the difference, 4" concrete slab with diesel engine mounts - should put it well over 10" total height.
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