Concrete

Intro: Concrete

PopularMechanics.com
For more on concrete, check out the original story.

Concrete is an age-old pillar of civilization, on a par with beer, steel and the plow. Master concrete (we're talking about simple backyard cement and water here, not engineer-grade flexible concrete.) and you can build a backyard empire--starting with a place to fire up your grill.

For small jobs, you can buy bags of cement that already contain sand and gravel--you just add water. On larger jobs, it's more economical to buy cement and aggregate, and combine the ingredients in a motorized mixer. These rent for about $50 a day and can be towed home. Cement comes in 94-pound bags; about six will make a cubic yard. The sand-and-gravel aggregate should include a range of sizes, from grains to 3/4-in. stones. If the aggregate in your area is too fine, add 3/8- to 3/4-in. gravel.

As for time commitment, you can easily pour a small slab in an afternoon. Here are the steps to get the job done.

STEP 1: Tools of the Trade

Float: A wooden or magnesium surfacing tool (wood is slightly coarser) used to smooth a screeded, or leveled, surface.

Edger: To round the corners of a pad, run the edger back and forth while pressing down.

Trowel: A steel surfacing tool for an extra-fine finish. It's optional for outdoor work.

Groover: To make contraction and expansion joints, which help control cracking, use the groover to section off the concrete.

STEP 2: Create Form

Build the form out of 2 x 4s, nailed at the corners with 16d nails. We chose duplex nails, because their double heads make them easy to remove. Long drywall screws can also make form stripping easier. Then use a shovel to slice the sod into manageable squares.

STEP 3: Excavate Soil

Rough out the rest of the form bed, but be careful to not dig too deeply. Backfilled soil settles, so over-excavation will require compacted sand, which adds work and expense to the project.

Set the form in place and shave away soil from high spots and check with a level. A slight slope of about 1/8 in. per foot helps shed water.

STEP 4: Stakes

When you have the bed roughed out, set the form in place. Drive 1-in. stakes around the perimeter to just below the top of the form. This prevents the concrete from pushing the form apart.

STEP 5: Screw in Stakes

Fasten the stakes with 2-in. drywall screws, leveling the form in both directions as you go. After you're done, recheck the form bed for high spots. A slight slope of about 1/8 in. per foot helps shed water.

STEP 6: Mixing Ingredients

Pour about 2 gal. of water into the mixer drum and then add the aggregate. If you have a sand-and-gravel combination, throw in one shovel of cement for every five shovels of aggregate.

STEP 7: Coarse Aggregate

If you have a separate pile of coarse aggregate, use 2 1/2 shovels of sand, 2 1/2 shovels of coarse gravel and one shovel of cement. We find it works best when you keep the shovel topped off so the quantities are uniform.

When the drum is about three-quarters full, gradually add more water--the exact amount will vary, depending on the moisture content of the aggregate.

When the mixture reaches a gray/green color with hardly any of the rock color showing through, stop the mixer and pull out a small handful. Form it into a 3-in. ball and toss it from one hand to the other. If it crumbles, it's too dry. If it splatters, it's too wet. Cautiously add a little water or more aggregate and cement to correct the consistency, then run the mixer for 3 more minutes.

Concrete from a relatively dry mix will always be stronger than that from a wet mix, so it's best to err on the dry side. After touching the concrete, immediately give your hands a thorough wash.

STEP 8: Prepare to Pour

After you're satisfied with the consistency, load the mix into a wheelbarrow and deliver it to the form.

Wash your hands immediately after handling wet concrete. It contains alkaline compounds like calcium oxide that can irritate skin, can cause up to third-degree burns.

STEP 9: Fill Form

Dump the mix into the form, being careful to avoid skin contact with the wet concrete.

STEP 10: Spread With a Screed

Spread the concrete around with a shovel and press it against the sides. Then mix another batch. When you have about half the form filled, use a 2 x 4 screed board to level the surface, but leave it slightly high. Fill the other half of the form in the same way. A larger slab requires rebar or steel mesh to help control cracking, but a small slab can do without that kind of reinforcement.

Then, steadily saw the screed board side to side while moving it along the form. If you find a low spot, fill it and saw through that area again.

STEP 11: Smooth With Float

Finishing is all about timing--and surface water is the timer. Each pass with a tool brings up water and cement, but overworking the surface can weaken it. When the water from the previous pass is no longer visible, it's okay to move to the next operation.

After screeding, wait a few minutes before using the float. When the surface appears dull, hold the float on the concrete with extra pressure on the trailing edge. Run it along the pad edges and across the surface in wide arcs.

STEP 12: Edge the Slab

We recommend you round the edges of the slab, as that prevents chipping. Use an edging tool, running it back and forth along the form with downward pressure until the edge is neatly rounded.

STEP 13: Trowel Finish

A rougher finish may be good for outdoor slabs, as they provide more traction. But using a trowel will give you a smoother surface, if that's what you're looking for. Again, wait for the surface water to disappear. Trowel the edges, then sweep the surface in wide arcs. If you can't avoid sweep lines, wait 15 minutes and trowel again.

Concrete cures best between 50 F and 70 F. It achieves most of its strength--60 to 75 percent--during the first week, and about 95 percent over the first three weeks. To make sure the concrete dries correctly, keep the job damp for five days by covering it evenly with a plastic vapor barrier or by using a sprinkler. Alternatively, you can also apply a curing-and-sealing compound.

Comments

[Stopher c]

Personally I would use reinforced steel ! Even though it's not a large slab I think it's important especially if it will be holding a substantial amount of weight!

[FCSjamie]

Small slabs can be done by the home owner. A concrete mixer is cheap to hire for a day and even if you stuff it up its easy to resurface concrete. Its important to seal concrete when your happy with the finish. concrete sealing is necessary if its going to be a barbecue or car area to prevent permanent oil staining.

[darr64]

I'm building a 4X8 slab for a 4X6 brick Bar B Q grill. Do I need to lay down some rebarb, how high should it be? and how much concrete should I buy

[BodhidharmaCharma]

Awesome! Although I'm awesome with a soldering iron, construction still evades me. Thanks for putting this up, you've convinced me to hire someone :p

[StepsoftheSun]

Any suggestions on how to do this without a mixer or wheelbarrow? Can you just slap a tarp in a shallow hole and mix on top of that? I'll be building a ferrocement water tank this summer and wanted to hear your ideas.

[Popular Mechanics]

Concrete and mortar can be mixed on a sheet of plywood or by
using a heavy-duty plastic sheet. The crete sheet is one company's answer to replacing a mixer and wheelbarrow.

[captain Jack]

that sheet is very clever!

[smokehill]

There is a clever little device sold in various mail-order catalogs for small batches of concrete or mortar mix. Basically, it's a five-gallon drywall bucket (more or less), with fins molded into the interior, and a screw-on lid. Just throw the right ingredient mix inside, close the lid, and roll it around on the ground. I think my Great Danes & Boxers might think this is great fun as a dog toy .... Since you can get screw-on lids for drywall-type buckets, it wouldn't take much creativity to bolt or pop-rivet your own "fins" on the inside of a bucket as a cheap home-brewed alternative. Pieces from another drywall bucker, with shelf brackets, should be all you'd need ... Anyone feel like doing an instructable on this? I'm way behind in winter projects already. If one had a lot of mortar to mix, you could always motorize this with a belt, or rig up an adapter to the drive wheel of a car or truck, so you could jack it up, engage the tranny, and rotate the drum. Sort of like those wonderful old log-splitters that bolted to your rear wheel.

[PKM]

How about getting one of those bucket moxers, then tell your kids they can have a time trial race pushing the barrel'o'cement around the garden, and whoever does it the fastest gets an extra slice of pie :) $pie for children < $cement mixer hire... no, wait, child labour laws. Ok, scratch that idea.

[Full Frontal Graphic]

You can get a fairly sturdy tub (mud tub) at H*** D**** or most places that cel concrete supplies for just a few bucks, that will work fine to mix in cement in, and I imagine it would work all the better if you put it in a close-fitting shallow hole.

If you are mixing a lot of cement, get a wheelbarrow at least. It will help you lug those 90 pound sacks a lot easier.

[smokehill]

I'm sure this is old news to most people, but for things like fence posts or deck supports you don't even need to mix the Sakrete to get a solid "hold." You just dig the hole, spray it lightly with water, dump in the Sakrete and fill the hole with water. Next day it's a solid chunk. I didn't believe this until I saw it, but have never had a failure, with ten decks and thousands of fence posts. My tool shed and tractor shed are all supported the same way, too. Generally, if I have a long piece of pipe or prybar handy, I'll bang it into the mix a few times, just to mix it a bit, but I can't see that it makes much difference. The theory behind this seems to be that the concrete mix will absorb whatever water it needs and the rest will leach out into the soil. One school of thought says that this also spreads the adhesion out to the adjacent soil as well, instead of just having a slug of concrete in a hole. Since there is a very old, primitive technique that involves just mixing cement into loose soil, then wetting it, to create a very hard surface, this makes some sense to me. Almost every professional fence-builder does it this way, I've found, so I feel less like a "cheater" now when I do it. One fencing pro sticks a cylinder of welded wire into the hole before he dumps the Sakrete, but just when he's doing a two-bag pour, like for a big farm gate post, or a chain link corner post.

[servant74]

For a fence post it doesn't matter. For concrete that needs to be tested for strength or look pretty, it matters. Actually concrete keeps getting stronger over its entire life. After a day it is normally about 30% of 28day strength. 7 days gives 90%, but even after 28 days it keeps gaining strength almost forever, but the rate slows down..

[Culturedropout]

I'm just impressed that he managed to do the whole thing without getting a _single_ spot of dirt or concrete on his clothes. I always end up covered in the stuff when I'm doing something like this... :-/

[ampeyro]

magnesium?, why, I've worked in construction some summers and I never hear about it

[Derin]

my grandpa who used to work at constructions said a 1-1 ratio will be very strong

[Derin]

Also,if you're too cheap to rent a mixer (which is not available where i live) you can make a pile with the correct ratio,and shovel it around while someone else adds water with a hose.They use this method at commercial building too.

[Drackar]

While I was working a fairly varied job, including concrete, I had something pointed out to me. The pre-mixed bags of concrete is to week. If you add a bit more cement, it will last longer, and be more durable...as is, the stock mix will shatter in just a few years.

[smokehill]

I've never had a real problem with Sakrete right out of the bag, though most of my litttle pours aren't stressed hard. I slopped together a sloping ramp (no rebar, no real soil tamping) 16 years ago and it's doing great. Probably doesn't do a bit of harm to add more cement, but does anyone know how much more cement to add? I do know that if you raise the cement proportion in brick mortar, you get a really mean joint that is far tougher than the regular mix. I've heard it called "flamingo" mortar by some old-time masons, but it seems to be a generic term rather than some brand; none of them knew the origin of the term. My brick house was made, I'm told, with that mortar, and after 50 years there isn't a single settlement crack, and I haven't had to point up one single joint. It is extremely difficult to drill through that mortar, though. When running some water supply lines I burned out two bit\s and had to go buy a 50-dollar industrial bit to finish the second hole. I've forgotten how much more cement they told me was added to make this hard-boiled mortar, but it didn't seem like much.

[Drackar]

One of my old jobs was for this old ...quasi hippie guy. Didn't like spending a penny more than he needed to, and took every deal he could. Kept bringing in pallets of bagged pre-mixed concrete. The foreman on the job actually had a fair bit of experience, and dealt with it.... Anyway, he used a three to one ratio, for anything that wasn't obsessively structural.

[Full Frontal Graphic]

3 to 1 is normal (3 sand 1 cement) mortar, 1 to 1 will make some hard core mortar.