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Concrete mixing procedure for home repairs, small projects, or design-build deck projects:
Here we describe in detail how to buy & mix concrete by hand for small home repair jobs such as filling a hole in a basement floor slab. We explain how to figure how much concrete you'll need, how many bags of concrete to buy, what other tools are needed, and how to do the whole job.
This article series tells how to mix your own concrete or how & when to decide to order a concrete delivery when building a deck, porch, or exterior stairs, pouring a floor slab, building a concrete foundation or other structure as well as how to handle concrete for small do-it-yourself jobs. This article series describes construction details for decks and porches.
This article describes how to do your own very small DIY concrete job such as filling a hole or building a stair landing. The steps in the slightly larger concrete jobs shown at page top and just above are detailed in a separate article, CONCRETE DELIVERY & MIXING.
I need to fill in the hole left in our basement floor slab when a large old furnace was replaced by a new one with a smaller footprint. The new furnace was set on blocks by the heating company, but that has left a square hole in our basement floor that exposes dirt and moisture.
We measured the length, width, and depth of the floor opening: it is 3.8 cubic feet. How much concrete do I need to fill this hole and how should I mix and install the cement. - Anon. in Duluth, MN, by private conversation with DF, 2016/03/07
Reply: Suggestions for a complete but small DIY Concrete Job
Here are my suggestions for buying concrete and mixing it up to fill a 3.8 cu. ft. hole in a basement around a new furnace or boiler that was set up on blocks. If you want to go ahead and buy the concrete ahead of time that'd be fine. Doing the math I don't want to haul 13 bags in Joanne's car's trunk.
Attitude DIY Concrete Work
Don't be nervous. Millions of people stupider than us (and we're pretty concrete-stupid) have bought and used pre-mixed concrete for use around their homes. Most of them are still alive and very few were injured. You'll have fun. But bottom line, if you absolutely hate the idea of mixing and placing your own concrete, even for an easy small job like this one, then it's best to hire a mason or a handyman. If you hire the latter, be sure she reads the instructions on the bags of concrete mix and that all cleanup is completed at the end of the job.
Buy These DIY Concrete Job Supplies
Concrete enough to fill the hole - you'll need to convert concrete bags (by weight) to cubic feet to calculate the number needed (I've done this below for a 3.8 cu. ft. hole)
You want Quikcrete concrete mix (or equivalent) - these are ready to mix, just add water. Sold in various sizes. A 60 lb bag is around $3.00
For a 3.8 cu. ft. hole in the floor you need this many bags of concrete mix (depending on bag size by weight)
A 80 lb (36.3 kg) bag yields approximately 0.60 cu ft (17 L) - you'd need 6 bags (really 6.3) - and 6 pints of water per bag
A 60 lb (27.2 kg) bag yields approximately 0.45 cu ft (12.7 L) - you'd need 8 bags (really 8.5) - and 4 pints of water per bag
A 40 lb (18.1 kg) bag yields approximately 0.30 cu ft (8.5 L) - you'd need 13 bags (really 12.7) - and 3 pints of water per bag
as I said, when you mix and pour concrete into a hole it spreads out and sort-of "disappears" at a surprising rate.
A mixing bucket (5 gallon plastic) If you were going to do a lot of this I'd consider using a metal wheelbarrow or a mortar pan to mix concrete. If you already have a metal wheelbarrow that you can get into the basement and if it's not leaky, use that to do the mixing.
A small shovel, or a garden hoe is handy for mixing the concrete/water or you can do it by hand with a garden trowel or with a triangular mason's trowel (as I do) but those 2 options are nastier. Pros who mix by hand use a larger hoe designed for mixing concrete (it has 2 holes in it but you'll never use it for anything else so don't buy one of these)
Optional: a flat, rectangular concrete trowel. Else you'll need a makeshift tool: a flat-bladed shovel or even a length of board can work for smoothing the concrete surface at the end of the job.
Recommended: Heavy rubber gloves - concrete, if you get a lot on your hands, is caustic and can leave your hands uncomfortable.
Watch out: do not get concrete in your eyes - it's caustic; and avoid breathing concrete dust (hence the mask)A
A few disposable dust masks (N-90 rated is best)
Have These On Hand for Your Small Concrete Job
Water - For each 80 lb (36.3 kg) bag of mix, add approximately 6 pt (2.8 L) of clean water. Other water quantities are given above
Garden hose for delivering water and later for outside use for wash-up
Cleanup rags and mops
Details of How to Mix Concrete by Hand
Drag bit bit, eventually all of the dry mix into the water or wet mix, don't leave any dry material as it won't make nice concrete and may leave voids. You will see the dry concrete when you paw through the mix with your hoe - it will appear as dry light gray powder.
DO NOT add excessive water - add a little water at a time and mix - too much water makes a weak mix.
If you've mixed all the concrete and water and see standing puddles of water you've added too much water. If you do add too much water you can add more concrete mix to stiffen it up unless you're on your last bag.
You can mix concrete by hand to prepare for placing it in a several of ways:
In a 5 gallon plastic bucket by hand - requires some arm strength unless you mix just 1/2 bucket at a time
On the floor or outside on the ground: concrete is poured on the floor (wear a DUST MASK) and arranged into a ring, water is poured into the middle of the ring and you use a hoe to drag concrete into the water to make a mess of concrete right on the floor - one bag at a time. This is then squeezed over into the hole and leveled out. When finished you'll have a gray patch on the floor which is OK if it's concrete but otherwise will need to be cleaned up with water and rags. (This is how concrete is mixed in many countries)
Right in the hole that's to be filled-in: this may work if the hole is convenient, has a fairly hard bottom, and lets you push each mixed bag to the far end of the hole and under the boiler or furnace.
Place the Concrete Where You Want It
If you're not mixing the concrete right in the hole where it's to be used (something that works ok in a large shallow hole but not in a deck pier) you'll need to drag or pour your concrete into the opening where it belongs. If you are making a landing at the end of a run of stairs you will have first built concrete forms using horizontal boards to mark the edges of your slab and if necessary you'll have done any needed excavation.
Watch out: while you don't normally need control joints in small slabs such as a stair landing or a repair around a heating furnace on a basement slab, if you are building a multiple-section sidewalk or an entire concrete slab or floor, see CONTROL JOINT CRACKS in CONCRETE.
Smooth the Concrete Surface
For concrete filling in a hole in a slab or at a landing, you might want use a small trowel or you can use a flat board to smooth the concrete top to make it pretty. Aesthetically it'd be nice to bring the concrete to the top level with the rest of the floor but in terms of avoiding a moisture entry path and rodents, if it's slightly lower that's only a cosmetic defect as long as it's not going to be a trip hazard.
Toweling the concrete surface will usually leave a smooth top layer without a lot of rocks (or any) showing.
Avoid over-toweling - that raises water to the concrete surface and can cause the top thickness of concrete to be soft and chalky after it's cured.
If your concrete work is to serve as a walking surface such as at a deck stair landing, when the concrete has set but has not fully hardened, use a coarse stiff-bristle push broom to roughen the surface. Drag the broom gently across the slab surface to leave brush lines in its surface. These form a more slip-resistant surface.
Clean Up After Placing the Concrete
When you like the filled-in hole in the floor or your new stair landing or walkway section:
Wipe off any concrete that splashed up onto the steel jacket of your boiler or furnace and scrape off blobs that splashed onto the concrete blocks on which the boiler/furnace sits
(Optional) carve your initials in the new concrete in a corner along with the date
Wash off tools outside using a garden hose; if you see a bit of concrete on the grass, water it down until it disappears. Pick up any stones and throw them at the dog who's been pooping near your door. Or try snagging a walleye.
Concrete DIY Project References
Quikcrete concrete mix # 1101, retrieved 2016/03/08, original source: https://www.quikrete.com/PDFs/DATA_SHEET-Concrete%20Mix%201101.pdf
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