Lexington, KY - Concrete is everywhere. This might not seem like a very sexy topic, but wait until you’re about to invest a sizable chunk of change in a new concrete driveway. Then, if you want to understand what you’re paying for and get a good job done, the topic could suddenly turn pretty darn interesting.
Concrete is composed primarily of aggregate, cement and water. The aggregate is generally coarse gravel or crushed rocks along with a fine aggregate, like sand. Cement is a binder, a substance that sets and hardens independently, and holds the other materials together.
Cement manufacturing is a complicated topic. There are many different types, each with specific ingredients and manufacturing method. While homeowners don’t need to know about all of that, here’s something worthy of note: despite plenty of research, the process by which cement sets, when mixed with water –– called hydration –– is not completely understood, even though this material has been used since Roman times.
There are safety issues with this product. Bags of cement have warnings on them because cement is highly alkaline and it releases heat as it is setting. As a result, wet cement is strongly caustic and can cause skin burns if not promptly washed off with water. Dry cement powder can cause severe eye or respiratory irritation.
And now, let us consider your driveway. Even though a good pavement contractor will be knowledgeable and careful about your job, it never hurts to be an educated consumer.
The first step in your project, after the design work, is site preparation. Your existing driveway can easily be removed and carted away. If there is a gravel base, at least part of this can be reused. It will need to be compacted with a vibratory plate compactor.
Next, the side forms will be placed and securely nailed to stakes that should be no more than four feet apart. The forms will likely allow for a slab thickness of four inches, which is considered sufficient for a residential driveway. Isolation joints need to be installed wherever your new driveway will meet existing pavement. These joints are typically made by inserting pre-molded joint material along the edge of the existing pavement.
The concrete mixture is very important to the success of your project. There are many aspects to consider but here are three basics: cement content, water content and air content.
Cement comprises 10-15 percent of the concrete mix, by volume. The recommended minimum cement content is 564 pounds per cubic yard of concrete, often referred to as a “six bag mix,” since one bag of cement weights about 94 pounds.
Concrete is mixed with more water than is needed for the hydration reactions, in order to give sufficient workability. Too much water reduces the concrete strength, while too little will make the concrete unworkable.
Air content is extremely important. Air-entrained concrete contains billions of microscopic air cells per cubic foot. These air pockets relieve internal pressure by providing tiny chambers for water to expand into when it freezes, essential in places like Kentucky where we experience winter freeze-thaw cycles. The amount of entrained air is usually between 4-7 percent of the volume of the concrete.
Back to your driveway. The concrete truck has just arrived. This is where it gets exciting. It’s fascinating to watch a driveway installation from pour to sealing.
Your contractor will spray or brush the forms with form oil. The concrete is chuted directly onto the driveway or into wheelbarrows and wheeled to its final location. A straight piece of lumber or a magnesium straight edge may be used to manually smooth the surface –– called “striking off” the concrete –– or this can be accomplished by mechanical means. Immediately after strike off, a magnesium bull float is worked back and forth across the surface to remove irregularities. If the concrete is overworked, the surface may be weakened –– a maximum of two passes on any part of the surface area is recommended.
Any surface bleed water must evaporate before proceeding with final finishing. Properly specified concrete should allow finishing to begin almost immediately. A concrete broom may be used for the final finish, when the surface is stiff enough to retain the grooves left by the bristles. A broom finish is often recommended for driveways because the rougher surface improves traction. There are also many decorative finishes, including exposed aggregate, colored concrete and stamped concrete, designed to resemble brick or stone.
Now your driveway needs to cure properly, meaning that an adequate moisture content must be maintained for proper hydration to occur. Without proper hydration, concrete will be less durable and more likely to crack. This can be accomplished by continuously sprinkling with water, covering the concrete with plastic sheets or applying a special cure and seal compound. Continuous sprinkling is rather onerous and plastic sheeting can leave dark streaks wherever a wrinkle touches the concrete, so curing compounds seem to be the best way to go. A single coat may be adequate, but where possible a second coat, applied at right angles to the first, is desirable for even coverage.
Cracks in concrete cannot be prevented entirely, but they can be controlled and minimized by properly designed control joints. These control joints can be hand-tooled or saw cut and should be no less than one inch deep, and no more than 10 feet apart (for a four inch thick slab). Driveways that are 12 feet wide, or more, also require a control joint down the center.
Do not use any de-icing chemical on your new driveway for the first year. Use sand as needed for traction. Later on, if you wish to use a de-icing product, avoid the use of magnesium chloride since it is known to be more physically damaging than sodium or calcium chloride.
Pressure washing and re-sealing your driveway every two to three years is recommended.