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Peachtree City, GA

Pouring Concrete in Atlanta, GA

Why would I start off an article like this by limiting my opinions here to such a relatively small region like the Atlanta metro area? Why not say 'the southeastern U.S.' or not mention a location to begin with? After all, concrete is concrete right?

 

Wrong. The concrete industry is a difficult one to research on the internet, and its always been that way, because mix designs and installation practices vary substantially from region to region, state to state, and even city to city, here in Georgia. There are many reasons for this.

Soil type is a factor. There are two basic classes of soil that I will talk about for the purposes of this article...sandy soil and clay soil. Atlanta-metro is mostly clay deposits, while two hours south of Atlanta the soil quickly turns to sand all the way to the ocean. Clay soil, or clay 'subgrade' is the better of the two; Clay is more stable, generally easier to compact (unless its too wet), and is less susceptible to freeze/thaw cycle problems that wreak havoc on concrete installed north of the Mason/Dixon line. Its important to know what soil type your dealing with, because if sand is your subgrade, you would be wise to include some type of steel reinforcement in your concrete design. Concrete contractors will often remove 4" of sandy subgrade and replace it with gravel, lessening the climates effect on the concrete above it. With clay as a subgrade, steel reinforcement isn't generally a requirement (meaning your local building inspector probably won't require its use) for concrete driveways, patios, and sidewalks, although concrete foundations and slabs will always need some type of reinforcement.

Temperature variations can cause problems. This is especially true the farther north you go. Your local frost line depth is a good example of this. In Atlanta, our frost line is 12", meaning that nothing deeper than 12" below the surface of the ground should ever freeze. I've heard that the frost line in Pennsylvania is 42"...that's almost four times deeper than what I'm used to in Peachtree City, GA. Frigid temperatures can cause frost heaving which can literally lift up and drop a concrete slab as the ground freezes and thaws, cracking and tearing the concrete apart. Concrete cannot survive these conditions long term without some type of reinforcement. Our temperate climate in Atlanta protects us from frost heaving or freeze/thaw cycle issues.

Concrete mix designs can vary from region to region, plant to plant, and company to company. In Atlanta, 3000 PSI is the basic strength specification....Farther north you would start with 4000 PSI. Below 3000 PSI is never recommended. The higher the PSI number, the more cement and the stronger and more durable the mix will be. Other variables include the size and type of aggregate, coarseness and moisture content of the sand, water ratio, and type/quantity of any additives such as calcium chloride.

I can't speak for other areas, but here in Atlanta the basic mix design is 3000 PSI at 4" thick. Here the aggregate is mostly granite. Reinforcement, including wire mesh, fiber mesh, rebar is good for the mix design if you can spend the extra money, but its not required by most municipalities. To be sure, you can always contact your local building department. A gravel subgrade isn't necessary for patios, driveways, and sidewalks as a general rule.