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.

Comments

[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

[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.

[silver912targa]

And if you keep it moist as long as possible the cement gets much harder. The longer it takes to dry the harder the cement gets.

[Full Frontal Graphic]

yeah, but EVENLY moist so like use a blanket or gunny sacks to hold moisture on it, or the like, and if there are puddles or spots where puddles form, keep the 'other' part really wet too, or sweep the puddles away, etc. moist in one place and not-so-moist in others stresses the concrete as it sets, and not necessarily in a good way or a bad way, but in an unpredictable-ish way.

[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.

[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]

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.

[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!

[Full Frontal Graphic]

The pictures don't show it but I totally recommend safety goggles and respirator or good well-fitted facemask. Cement in the eyes or lungs is hard core. The dust from unmixed cement goes everywhere in the slightest breeze. You can skip the respirator once the cement is wet and not powdery, but you still should use some safety glasses to keep spalshes and plops off of your eyeballs. Even a little cement in your eyes sucks for days.

[cpotoso]

Don't you need some re-bar and/or re-mesh to make it stronger?

[Popular Mechanics]

Here are a few more comments on rebar and welded wire mesh: Steel reinforcement does two things: 1. Its tensile strength improves the bending characteristics and strength of monolithic concrete structures. 2. Welded wire mesh holds concrete together so that when its inevitable cracking occurs, the mesh holds the pieces in position. However, since steel rusts and expands, this raises problem for concrete. You only use steel where you have to. A slab of this size does not require rebar or welded wire mesh.

[Pkranger88]

Rebar and mesh are simply support to give the concrete extra support capabilities i.e. holding the weight of a car or the foundation of a building for example. If it is only going to hold a grill, and you pour it around 4 inches thick, holding a grill will be a piece of cake.

[Pkranger88]

This is a great instructable for starting a concrete project. I have some concerns to voice though. First off, if the pad is going to support any substantial weight, a car etc, add reinforcement. When excavating you ground it is usually a good idea to lay down a layer of sand, just enough to create a flat surface. Some people say that this allows to much moisture to leave the concrete to fast, so dampen then sand first. Good Luck.

[royalestel]

Wow, It's pretty cool that PM is posting here. I'm sure at some point I'll refer to this. Thanks much.

[ongissim]

Sweet! I just learned how to do this at the Waukasha Expo Center for a career fair in WI.

[mdmoose29]

holy cow... ur from waukesha wisconsin? i used to live there.. what a small world instructables is.