Wednesday, December 26, 2012


A lot of folks think that concrete and cement are the same thing.   It's not and it's important to know the difference.  

We poured concrete for our footers.  Concrete is a mix of sand, gravel, cement and water - often with other additives.   Concrete contains  cement.

Cement doesn't contain concrete.

Now you know.

There are a lot of different recipes for concrete.  The Romans had some interesting ones.  From the wiki:

 "'Concrete additives have been used since Roman and Egyptian times, when it was discovered that adding volcanic ash to the mix allowed it to set under water. Similarly, the Romans knew that adding horse hair made concrete less liable to crack while it hardened and adding blood made it more frost-resistant. '

I kind of don't want to know how they figured out the blood thing.


When the truck comes to pour your concrete, they have a mix of the dry stuff in one tank and water in another tank.   They mix it after they get to you.    This way they can change the mix according to what the different parts of the pour need. 

We needed thick stuff for the steps in our footer.  We needed thinner stuff for the tops to move it from one place to another. They measure concrete thickness by 'slump'.   The wiki explains it in detail.

Basically, to test thickness, the concrete is packed into a cone.  The cone is turned upside down and taken off the concrete - sand castle style.    Then they measure how much the concrete slumps.   Thick stuff won't slump much.  Thinner stuff with more water in it slumps a lot.  In the US, they measure by inches.  

The lower the number, the thicker the concrete.  Thicker concrete is stronger, but it's hard to finish smoothly.   Thicker concrete is also hard to move along.   If you need to move it along, you need thinner concrete.

We used 4" slump in our steps and 6-7" slump for the rest.

Here are a few things we learned:

1.  For changes in elevation/level in your pour use 4" slump.

2.  For concrete you need to move along the form to get to the other side, use 6-7" slump.  

3.  If you build a plywood chute to move concrete in, get it wet before you start.   Dry plywood really holds the concrete in place.   Once the chute is wet, the concrete moves nicely along.

4.  Garden rakes are great tools for working with concrete.   Narrow rakes are better.   For the foundation we might need to use hoes because the foundation will only be 8" wide.

5.  2x4s are your friends.   We used them to smooth and level and to put our feet on when we were poised over the wet concrete.

6.  Wear gloves and clothes you don't mind getting really really dirty.  A lot of it will wash up, but some might not. 

7.  A good attitude and good communication skills make the job a lot easier.  Seriously - this is the most important thing you can bring to any construction job. 

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