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What is a Concrete Block?

By Eric Tallberg
Updated May 17, 2024
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As the name implies, a concrete block is a building or construction block manufactured of concrete. Concrete is an amalgamation of Portland cement, aggregate and water. Often the aggregate used in the manufacture of concrete block is fly ash, or bottom ash. Both fly ash and bottom ash is the residue, or cinders, resulting from burning coal. Hence, some concrete blocks are known as cinder blocks.

Concrete blocks can be made in nearly any size, but some of the most common are 6 X 8 X 16 inches (15.24 X 20.32 X 40.64 cm) and 8 X 8 X 16 inches (20.32 X 20.32 X 40.64 cm). These blocks weigh about 40 to 45 pounds (18.14 kg to 20.41 kg). Blocks may be solid or contain large holes in the middle. There are also cement blocks, known as concrete bricks, manufactured to the specifications of the common, red brick.

A lighter weight concrete block, manufactured of aerated concrete is also available for construction. This type of block is known aerated concrete (AAC) block, or as an autoclave cellular concrete (ACC) block. This type of concrete block is manufactured from a mixture containing a binder with smaller portions of cement than in the more common block, and an aluminum powder aggregate, along with water for mixing. Though just as strong as regular concrete blocks, AAC blocks are much lighter. The AAC block is, however, much more expensive than the common concrete block, thus is not used extensively.

Most common concrete block is cored, having either two or three holes on the center of the block. This is done for a number of reasons. First, this lessens the weight of the block. Secondly, rebar, or long steel bars, can be inserted through the cores to reinforce a block load-bearing wall. Third, the cores in the block may be filled with sand or gravel in a further reinforcement procedure.

Other sizes and shapes of concrete blocks are also available for different purposes. There are notched blocks for window and door sills, and angled blocks for corners. There are also wedge-shaped blocks on a straight base known as capping units. Additionally, there are header and bull-nosed blocks. Specialty blocks may be obtained suitable to the unique requirements of any sort of masonry structure.

Concrete block, because of its rather bland appearance, is used more frequently in utilitarian structures than for residential construction. Blocks can, however, be painted or coated with a thin, tinted cement veneer to make them more attractive. Blocks are also coated to ensure proper weatherizing, and to provide further insulation for the interior of the structure.

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Discussion Comments
By anon996036 — On Jun 29, 2016

I love the different kinds of cinder blocks you've posted! Maybe you would know where I could find some as great as the ones on the picture!

I went to Home Depot and Lowe's, but they only have the 2 core cinder blocks.

By poppyseed — On Aug 10, 2011

My parent’s actually live in a solid concrete block home. They paint it just like you would a wooden structure that doesn’t have vinyl siding or such.

It’s really incredibly sturdy, and I just love it.

My love comes in no small part that they live out in the country in the middle of a big old field where you can see the storms coming at you from afar.

That’s pretty scary to behold, especially during hurricane and tornado season. However, a block house can weather just about anything at all.

Unless a tornado lands smack dab on top of it, you are about as safe as you can get inside that house during the worst weather.

I will say though that it is very difficult to heat and to cool. That would be the downside I suppose.

By geekish — On Aug 09, 2011

@speechie - Concrete block and poured concrete are typically your most common foundations used.

There are pros and cons to both. One I have read about is that poured concrete is a solid face so it may resist water intrusion.

Another thing that can make a difference is if the builder pours cement based mortar or poured concrete into the holes in concrete brick then I have read a builder's blog that feels this is makes the concrete block much like the poured concrete.

By Speechie — On Aug 09, 2011

@cafe41 - Thanks for sharing, living here in the eastern part of North Carolina you definitely do not think about using concrete block for your house, but I bet many coastal areas could learn from the passage of that regulation.

I have been to Topsail, North Carolina, which is on the coast here. There are houses on stilts sitting in the middle of water at the north end of Topsail. So while they have not yet passed a regulation on having to use concrete materials they have changed the regulations on that little peninsula as to where you can build.

I have read concrete block is also a good choice for foundations of homes? Is this true? Are there many other choices other than concrete block?

By cafe41 — On Aug 09, 2011

I just wanted to say that here in South Florida we have building regulations that require homes to be built with concrete block construction because of the threat of hurricanes.

This regulation came about after Hurricane Andrew destroyed a ton of homes in the Miami area that were made of wood. It caused the county to change its building codes. All homes built after 1992 have to built with concrete block construction and be able to pass a series of windstorm tests.

Although it is not a requirement in the rest of the state, many builders are applying the building standards of Miami Dade county and building homes with concrete block construction and submitting their projects for certification which is a real selling tool when they market their homes because Miami Dade has among the strictest building codes in the country.

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