Concrete Basics 101

Concrete has four main ingredients: Portland cement, large aggregate, small aggregate, and water. The large aggregate can be gravel or crushed stone. The small aggregate is almost always sand. Portland cement comes in five types, each formulated for specific characteristics. By choosing the right Portland cement type, the right size and type of aggregates, and adjusting the ratio of ingredients, you can create the best concrete for each job.

The job of aggregate is to increase the strength of the concrete while reducing the cost. The individual stones that make up the aggregate interlock with each other while the sand fills in the voids. The cement paste created from the cement and water is the glue that holds the rock and sand together. With less water comes more strength.

Portland Cement Types

• Type I General Purpose - This is the most widely used type, especially for residential projects.
• Type II - This generates less heat than type I, which is important in large volume jobs because it sets up more slowly to give crews more time to work during hot weather.
• Type III High Early- This sets and cures rapidly. It’s used for jobs that must bear weight shortly after being poured. It is also used during very cold weather to reduce the time in which to concrete is prone to freezing.
• Type IV - This produces very little heat as it cures and is used for huge-volume projects such as dams.
• Type V - This is very sulfate-resistant and is used in areas in which the soil or groundwater is very alkaline.

Admixtures For Concrete:

• Accelerators speed up the hydration (hardening) of the concrete.
• Retarders slow the hydration of concrete, and are used in large or difficult pours where partial setting before the pour is complete is undesirable.
• Air-entrainers add and distribute tiny air bubbles in the concrete, which will reduce damage during freeze-thaw cycles thereby increasing the concrete's durability. However, entrained air is a trade-off with strength, as each 1% of air may result in 5% decrease in compressive strength.
• Plasticizers (water-reducing admixtures) increase the workability of plastic or "fresh" concrete, allowing it be placed more easily, with less consolidating effort. Superplasticizers (high-range water-reducing admixtures) are a class of plasticizers which have fewer deleterious effects when used to significantly increase workability. Alternatively, plasticizers can be used to reduce the water content of a concrete (and have been called water reducers due to this application) while maintaining workability. This improves its strength and durability characteristics.
• Pigments can be used to change the color of concrete, for aesthetics.
• Corrosion inhibitors are used to minimize the corrosion of steel and steel bars in concrete.
• Bonding agents are used to create a bond between old and new concrete.
• Pumping aids improve pumpability, thicken the paste, and reduce dewatering – the tendency for the water to separate out of the paste.

 Accelerants reduce the curing time substantially so you can shorten the length of time needed to protect the concrete from freezing. Retarders are added to increase the setting time, allowing more time to finish the concrete especially in warm weather. Retarders also help to reduce the initial stress cracking that happens when concrete sets and cures too rapidly.

Water reducers let the concrete become more plastic and workable with less water. The benefit is a stronger concrete with less labor needed for placing and working it.

Air Entrainment admixtures allow for the formation of millions of microscopic air bubbles in the concrete mix. These bubbles give water in the concrete room to expand as it freezes during the freezes during the freeze/thaw cycles in cold climates. Air entrainment also acts as a plasticizer and allows the concrete to be more workable with less water.

Color pigments are used as a way to introduce color to concrete. The pigments must be insoluble in water, free from salts and acids, and colorfast in sunlight. Both liquid and powder pigments are available. It is also important to note that using white Portland cement instead of gray produces cleaner, brighter, more vivid colors.

Structural Reinforcement:

The cost of reinforcing concrete is minimal compared to the added value it gives. Concrete excels in the area of compressive strength yet leaves a lit to be desired in tensile strength. The addition of reinforcing can significantly offset this shortcoming. The most popular method of reinforcing is with welded wire. Other reinforcing methods include fiber mesh or wire mesh that is mixed into the concrete at the plant so the fibers are distributed through the batch.

Reinforced Concrete:

Three physical characteristics give reinforced concrete its special properties. First, the coefficient of thermal expansion of concrete is similar to that of steel, eliminating internal stresses due to differences in thermal expansion or contraction. Second, when the cement paste within the concrete hardens this conforms to the surface details of the steel, permitting any stress to be transmitted efficiently between the different materials. Usually steel bars are roughened or corrugated to further improve the bond or cohesion between the concrete and steel. Third, the alkaline chemical environment provided by calcium carbonate (lime) causes film to form on the surface of the steel, making it much more resistant to corrosion than it would be in neutral or acidic conditions.

The relative cross-sectional area of steel required for typical reinforced concrete is usually quite small and varies from 1% for most beams and slabs to 6% for some columns. Reinforcing bars are normally round in cross-section and vary in diameter. In the United States, rebar comes in two grades of carbon content, Grade 60 and Grade 40, which typically sell for the same price. Grade 60 has a higher carbon content and, therefore, a higher tensile strength, but its stiffness can make it difficult to bend and cut. Galvanized, epoxy-coated, and stainless steel rebar are also available for use in corrosive environments.

Fiber Reinforcement:

Polypropylene fibers are usually used in concrete to control plastic shrinkage cracking and drying shrinkage cracking. They also lower the permeability of concrete and thus reduce bleeding of water. Some types of fibers produce greater impact, abrasion and shatter resistance in concrete.

Generally fibers do not increase the flexural strength of concrete, so it can not replace moment resisting or structural steel reinforcement. Some fibers reduce the strength of concrete. The amount of fibers added to a concrete mix is measured as a percentage of the total volume of the composite. The volume of fibers typically ranges from 0.1 to 3%.

Concrete Strength:

The strength of concrete is typically measure in pounds per square inch (psi). Most residential project requires at least 2,500 psi. In the case of standard commercial and industrial applications the use of at least 3,000 psi concrete is required.

Curing Concrete:

The chemical reaction between water and cement is called hydration. In order to maintain proper hydration, and therefore proper curing, the rate of evaporation and the temperature of the water in the concrete must be controlled. If the water rapidly evaporates or freezes during the first seven days of curing, the concrete will be substandard. This is why weather plays a big role in concrete work.

Even thought the cement reaction with water is completed over period of time (normally about around 240 minutes which is referred to as final setting time), the hardening of concrete and gain of strength is over a period of time 95% to 98% strength is achieved in 3 weeks or about 28 days. During this period concrete needs to be maintained in ideal conditions by controlling temperature and humidity. In practice this is achieved by spraying or ponding water on the concrete surface there by protecting concrete mass from ill effects of ambient conditions.

Moisture curing is generally the best way to cure concrete. This can be accomplished in several ways such as ponding of water on the concrete shortly after the initial set of surface. In today’s fast passed world the use of plastic sheeting, burlap or straw is most prevalent with its ability to allow trades to work on the recently placed slab.

In lieu of water and plastic covering there are many types of liquid-membrane curing compounds available that one can easily spray on any type of concrete project. Once a curing compound is applied it will seal moisture in and the concrete should cure properly under all but freezing weather conditions.

It is important to us the correct compound for the application. Depending on its base, which can be acrylic, asphalt, rubber, wax, epoxy, or some other type, curing compounds have many side effects. For example some will discolor the concrete, some leave a film that must be removed for the proper adhesion for coatings, vinyl flooring or carpet. Worst of all some when left in place will chemically bind with lift truck tires leaving long lasting tire marks.

Service / Market Area:

For industrial and commercial flooring projects we work in Michigan cities such as Muskegon, Grand Haven, Norton Shores, Big Rapids, Cadillac, Reed City, Howell, Lansing, Jackson, Battle Creek, Kalamazoo, Hastings, Plainwell, Otsego, South Haven, Benton Harbor, St Joseph, Holland, Grand Haven, Grand Rapids, Wyoming, Kentwood, Grandville, Portage, Walker, Byron Center, Cascade Township, and Ada. For residential garage projects we generally only bid on work within 30 miles of Grand Rapids, MI.