The Inside Outside Guys: Concrete basics
Welcome to Concrete 101; a primer for any homeowner intending to have a concrete patio, drive or sidewalk installed.
Concrete is a fascinating material whose use is documented back to 700BC where water storage cisterns made of the material were found beneath the desert floor. Both the Romans and Egyptians employed it in construction, most notably the dome in the Roman Pantheon that used un-reinforced concrete to span the roof of that beautiful edifice.
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Concrete is said to be the second most consumed material on earth – behind only water. It is made from readily attainable raw materials and has great characteristics for construction. It is mold-able to virtually any shape, has high compressive strength, is dimensionally stable and highly resistant to many chemicals and most weather.
Downsides to the material are complaints regarding the effect on Mother Earth in the production of the cement used in concrete and the lack of concrete’s ability to flex or bend.
In its base form concrete consists of cement, sand and gravel – mixed with just enough water to fully hydrate and integrate the mixture. More cement in design will translate to higher strength and higher cost. Today many additives exist through the benefit of science to strengthen concrete, slow or speed cure, give it greater strength, and even make it more “plastic” to reduce volume shrinkage and water usage. Fly Ash and other products are being used to minimize cement content and reduce environmental impact.
“Super” concrete has been developed to allow for taller buildings with smaller footprints, most notably the world’s tallest structure, the Burj Khalifa in Dubai which reaches over 2,700 feet into the air as a freestanding building!
A strength of concrete is that it does not compress when loaded. A weakness of the product is that it does not bend without breaking. To offset this we strategically place steel, which has a great ability to bend without breaking, in concrete pours. You will find embedded steel in roadways, drives, stadiums and high-rise buildings.
Steel placed in your concrete drive should be placed roughly center of the depth of pour, so about two inches into the nominal four inch thick slab. Not only can this help resist breaking, but it can also help resist “vertical displacement” or the tendency of slab pieces to sink or lift up.
Steel reinforcing should also be coated to protect it from chemicals and rusting.
Concrete is generally specified for residential placement by its ultimate compressive strength rated in PSI or pounds per square inch. In the Industry this is measured 28 days after placement where it is assumed the pour has reached around 90% of its “ultimate compressive strength, or UCS”. If you purchased and placed 3,500PSI concrete, we would assume it would test around 3,200PSI at 28 days.
A lot of mistakes can be made during placement that affect UCS. Using “short crews”, (too few people) or untrained crews on a job can encourage abuse. Short crews may add too much water to slow cure and make placement easier. Pouring with a short crew on very hot days may encourage them to begin finishing too early which can cause segregation of the materials and weaken the placement. With a lot of sun exposure the top may dry out while the center and bottom remain hydrated and slow curing.
Short crews may not trowel the curing pour to bring a good sealing paste to the top prior to putting on that “broom” finish. This makes the concrete weaker on top and more susceptible to salt degradation, staining, pitting and spalling. In colder weather short crews may add calcium chloride to the mix to speed up the hydration or cure. The Industry limits this to a maximum 2% by weight. Even then it may contribute to degrading embedded re-steel.
Untrained or short crews might not exercise care in the forming and initial placement such that the finished surface will not drain well or may have many depressions that catch and hold water. The surface of a residential placement for sidewalks, drives, patios, etc should be above existing grade and grass so that the concrete can drain to the surrounding soils in a controlled manner.
We noted the use of water to mix the ingredients of concrete. Design criteria for a batch of concrete will dictate the maximum amount of water to be used. Too little water and you may not fully integrate the materials. Too much water and you can radically diminish design strength and create a situation where you will have significant volume reduction when the concrete cures; also commonly called shrinkage.
This can be noted when a placement is made to the top of a form but five days later the surface of the concrete is well below the top of the form.
Newly placed concrete must be protected from freezing for a minimum 72 hours. Concrete cannot be placed on freezing ground or in standing water. A slow, cool cure is best.
Most all fresh concrete looks the same when new. The proof is in the specification, prep, placement and cure. A true professional will charge a little more up front, but they will “cost” a lot less in the long run when the concrete performs as it should.
For more information on this subject, read "That Sinking Feeling" online at detroitnews.com. And for more home improvement advice, listen to the Inside Outside Guys every Saturday and Sunday on News/Talk 760 WJR-AM from 10 a.m. to noon or contact us at insideoutsideguys.com with your questions.