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    © Mark Saville 2006  

Concrete Foundations

 
 
 

Movement is all around us in one form or another. The earth is is not a stationary object and the forces of nature are always at work. So what could be classed as the ultimate foundation? Concrete pinned with steel dowels to solid, flat bedrock 10 feet above any flood-plane. Problem is we don't always have that option presented to us. In 30 years I have never seen a foundation fail that followed these simple guidelines. Following these guidelines can save you thousands in costly concrete foundation repair.

 

 
 

1. An important first step although not a structural consideration. Make sure you or your builder have all the necessary permits and the ok from your local building authority to actually place a permanent structure in the location you have in mind. Make sure you are within the given lot coverage percentage, proper front-yard, side-yard and rear-yard allowances.

2. Build on good Solid Ground. A foundation is only as good as the ground it's sitting on. Always excavate out any organic matter, topsoil etc and get down to undisturbed soil. If your building site has any depth of organic type black soil then it must be dug out down to a solid layer. If you need to raise the elevation of your structure then you need to use good gravel as fill and compact it in 1 foot lifts to ensure a solid base.

If you are building in an area where there is an expansive clay soil problem make sure you or your builder read any soil report before deciding on a foundation type. Colorado is the State with the biggest problem but other States do have unstabe clay soil problems also. A pier and beam foundation is the best in areas of expansive soil.

Footings should always be below the frostline in colder climates and above the water table where possible otherwise an adequate mechanical drainage system must be installed. If in doubt about whether your ground will support your building project, have a soil test done and a review on what size of footing is required for the particular ground.

3. Always use an appropriate sized Footing. The type of soil base you have on your site will govern the size of footing you'll need to support the foundation walls. The softer the ground the wider the footing. In a case where you have bedrock and are using cast in place concrete you can pin the foundation walls right to the rock with 5/8" steel dowels.

Not all footings need to be of concrete. In the case of a pre-cast foundation, a layer of compacted crushed rock is used. Again, your soil conditions will govern the width and thickness of the base. A slab foundation must also have a footing poured right in and be sitting on a solid base of compacted gravel. Pier foundations must be sized for the firmness of the ground and the number of piers and their spacing depends on the load of your structure.

 

 
 

4. Always use steel reinforcing bar in any concrete pour. Concrete has tremendous compressive strength but will not tolerate shear or bending forces too well.

The strength of concrete is derived from a combination of it's own compressive strength and reinforcing bar working together. You might say it's the perfect match. Always have a minimum of two 5/8" bars spaced in the center of your footing along the vertical and split the width up in 3rds so you get an even amount of concrete between the bars and each side of the form. In any case always keep the rebar away from the form by a minimum of 2 inches.

 

concrete foundation
Foundations need to battle the elements, cold in some climates and poor ground conditions in other areas.

Concrete foundation repair can be very costly so a few hundred dollars worth of rebar and a day to install it is really cheap insurance.

For the ulitimate rebar system you should have 2 foot long dowels sticking out of the footing. Once you start your wall reinforcing, tie additional verticals for your foundation wall to the dowels and then at the very least on an 8 foot high foundation wall have a horizontal bar 1 foot up from the bottom, one in the center and 8 inches down from the top. 3 horizontal bars in total. This will keep the forces of the backfill in check and ensure your foundation can take any lateral pressure.

 

5. Place concrete in suitable conditions. Plan ahead. This is one time when a little planning will go a long way. Concrete does not like excessive heat or cold. On the cold side of the scale if the weather at night is going down to the freezing mark then the concrete must be heated. If it's going to be a hot one in the afternoon then water should be sprayed on the concrete after it sets up to ensure that it doesn't dry too quickly. This is what causes shrinkage cracks and although a shrinkage crack or two is not the end of the world, letting concrete dry as slowly as possible will minimize cracks.

On a large pour, have the concrete pumped instead of trying to place it by hand especially on a hot day. There's nothing worse than having to carry concrete in a wheelbarrow with 3 trucks sitting there waiting with the concrete on the verge of setting up. The few hundred dollars you spend on a pump truck will save the stress of that believe me. Adding too much water on the site is not a good thing which is what you'll need to do if the trucks are sitting too long. Concrete should be placed in a continuous pour with no more than 15 minutes between trucks on a hot day.

© Mark Saville 2006 Reprint with Permission only.
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