How is concrete made?

­When you get right down to it, concrete is a mixture of two components: aggregates and paste. In primitive brick construction, straw served as the aggregate and mud served as the paste. Once the mud had hardened, the embedded straw strengthened the structure of the brick.

In modern concrete, a variety of materials help make up aggregates, including sand and various sizes of gravel and crushed stone. Aggregates are divided into two categories based on particle size: fine aggregate, which is 0.2 inches (5 millimeters) or smaller, and coarse aggregate as large as 1.5 inches (38.1 millimeters). Concrete makers have even found ways to reuse such items as oyster shells and even waste ash from coal-fired power plants.

Cement serves as the paste in modern concrete. Dry cement mixes contain a host of different materials that typically include limestone, clay, gypsum, and various other chemicals and minerals. After the materials have been crushed, mixed in appropriate proportions, burned and ground, the resulting mixture is ready to be combined with water. When this occurs, a chemical process called hydration takes place, and the resulting paste dries into a solid stonelike mass. Portland cement is the most common variety of cement and is typically used in concrete production.

Supplementary cementitious materials (SCM) such as pozzolans (such as power-plant ash) and slag (a byproduct of iron smelting) are sometimes added to the cement mixture as well. Pozzolans are materials that exhibit cementlike properties when combined with lime.

When it comes to ensuring proper proportions, making concrete is a lot like baking. Mess up the contents of the mixing bowl, and you're likely to have a disaster of a cake on your hands. Likewise, cement, water and aggregate measurements have a huge influence on concrete's strength, permeability and durability. The amount of water used can also influence how well concrete bonds with reinforcement and how much shrinking and cracking take place during drying.

Additional ingredients, called admixtures, further complicate the process. An admixture's effect depends greatly on the other proportions in the concrete but, if added during mixing, they can serve as a kind of power-up for the concrete. Some admixtures affect color, hydration time or shrinkage rate, while others provide such benefits as minimized corrosion for steel beams inside the concrete block.

Once the best ingredient proportions have been determined for a given job, it's time to mix it up and pour a few slabs.

­After all the required dry materials have been assembled, they typically go into one of two destinations: a bag or a mixer. Ready-mix concretes are premixed bags of dry concrete ingredients that only require water and mixing. These are ideal for small projects, and manufacturers tend to offer mixtures that are ideal for various specialized home projects. Users can generally add water in a mixing ­pail and apply the concrete with a shovel.

If you're looking to pour a lot of concrete though, premixed bags get expensive really fast -- to say nothing of the packaging waste. For larger projects, some construction companies mix concrete on-site. Otherwise, mixing takes place at the concrete plant, and the batch is subsequently transferred to the job site in a rotating drum mixer. These devices keep the concrete from setting by constantly tumbling the mixture around and are usually either truck or trailer mounted. However, this hot potato act can't go on indefinitely. Most batches of concrete need to be discharged 90 minutes or 300 revolutions after the addition of water. After that point, the batch experiences slump loss, which refers to decreased workability. If construction crews know in advance that time will be an issue, certain admixtures can make it possible for concrete to travel longer in drum mixers.

Once at the job site, it's time to pour the concrete into prearranged forms. These wooden molds are carefully measured to meet the necessary slab specifications. In the case of reinforced concrete, the form will contain steel rods or mesh. Then the concrete can be compressed and smoothed over as needed. Temperature and moisture are both important factors in proper concrete drying. As the concrete dries, it inevitably shrinks. To prevent this process from taking place unevenly and potentially warping the finished slab, it's important to keep the surface of the concrete damp to slow the shrinkage uniformly.

Extremely low or high temperatures can also pose a problem. Extreme heat will cause rapid curing, while chilly temperatures can draw the process out and produce weaker concrete. As such, summer construction crews typically work during the cooler portion of the day and use warming additives during cold weather. Calcium chloride, for instance, will speed up the chemical curing process, which produces heat. A good sealant will help protect the concrete once the slab has fully dried.

Just as you can avoid baking mishaps by simply buying a prebaked pie, you can also purchase various sizes and shapes of precast slabs directly from the concrete plant. Sorry, mobsters -- they're typically not available in boot size.

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