You see them everywhere. Construction sites. Backyard retaining walls. Half-finished strip malls. Those grey, heavy blocks with the big rectangular voids staring back at you. Most people call them cinder blocks or breeze blocks, but if we’re being technical—and since you're reading this, let's be technical—we're talking about concrete bricks with holes, or Concrete Masonry Units (CMU).
Honestly, the holes are the genius part. Without them, we’d be living in a much heavier, more expensive, and sweatier world. A solid concrete block is a beast to carry. It’s also a terrible insulator. By leaving those gaps, manufacturers aren't just saving on raw materials; they’re creating a versatile structural tool that handles everything from vertical reinforcement to thermal regulation.
It’s about air. And physics.
The Secret Anatomy of Concrete Bricks with Holes
Why do they have those holes anyway? It isn't just about making them lighter for the poor soul hauling them up a ladder, though that's a huge perk. These voids are called "cells" or "cores." Typically, a standard block has two or three.
When a mason lays these down, the cells align vertically. This creates a continuous hollow column from the footer of the building all the way to the roof line. If you're building a simple garden wall, you might leave them empty. But for a load-bearing wall in a house or a commercial warehouse, those holes are where the "magic" happens. Contractors drop steel rebar down through those cavities and then pour in grout—a high-slump concrete mix. Suddenly, that stack of brittle bricks becomes a reinforced monolith capable of withstanding hurricane-force winds and seismic shifts.
Weight vs. Strength
There’s a common misconception that "more concrete equals more strength." Not necessarily. Standard ASTM C90 specifications categorize these units into "lightweight," "medium weight," and "normal weight." A lightweight block uses aggregates like expanded shale or clay. Even with those big holes, a "normal weight" block can support incredible compressive loads because the geometry is optimized. The "webs"—the thin parts of the brick connecting the exterior shells—distribute the weight efficiently.
Think about it this way. If you had a solid block of concrete that weighed 50 pounds, it would be a nightmare to handle. By using concrete bricks with holes, you get a unit that weighs maybe 28 to 35 pounds but offers nearly the same structural integrity once the system is fully integrated with steel and grout. It’s a win for the builder's back and the developer's wallet.
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Keeping the Heat Out (and the Quiet In)
Concrete is a thermal sponge. It has high thermal mass, which sounds fancy, but basically means it takes a long time to heat up and a long time to cool down. In places like Arizona or Texas, this is a lifesaver. The air trapped inside concrete bricks with holes acts as a buffer.
Air is a poor conductor of heat.
By having those hollow cores, you’re breaking the path of heat transfer from the scorching outside sun to your air-conditioned living room. If the blocks were solid, the heat would migrate through the material much faster. Some high-end builds actually fill these holes with loose-fill insulation, like perlite or vermiculite, or even custom-fit foam inserts. This turns a basic structural element into a high-performance thermal barrier.
Then there’s the sound. Ever stood next to a highway and noticed those massive grey walls? Most of them are built with hollow-core units. The change in density—moving from solid concrete to an air gap and back to concrete—is great at disrupting sound waves. It’s why masonry homes are notoriously quiet compared to wood-frame houses. You don’t hear the neighbor’s lawnmower nearly as much.
What Most People Get Wrong About "Cinder" Blocks
Let’s clear something up. People use the term "cinder block" as a catch-all for any concrete brick with holes. Technically, a real cinder block uses coal cinders (fly ash) as an aggregate. They are lighter and generally weaker than modern concrete blocks. Nowadays, what you're buying at Home Depot or a local masonry yard is almost certainly a concrete block made from Portland cement, water, and crushed stone or sand.
Actual cinder blocks are becoming a bit of a relic. Modern concrete versions are much more durable and less prone to crumbling over decades of moisture exposure. If you’re doing a DIY project, don't just ask for "cinder blocks" unless you want the old-timers to smirk at you. Ask for CMUs or concrete masonry units.
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The Moisture Trap
Here is a detail that gets overlooked: drainage. Because concrete bricks with holes have those vertical channels, moisture can sometimes find its way inside. If that water gets trapped and freezes, it expands. When it expands, it cracks the concrete. This is why "weep holes" are so vital in professional masonry. These are tiny gaps left in the mortar joints at the bottom of the wall to let any internal condensation or seeped rainwater escape.
If you're building a retaining wall in your backyard and you don't account for the water inside those holes, your beautiful wall will likely bow or crack within five years. Physics doesn't care about your weekend project's aesthetics.
Creative Uses Beyond the Foundation
Lately, these blocks have moved from the crawlspace to the living room. It's a bit of a "brutalist" trend, but it works. People are using concrete bricks with holes to create:
- Outdoor Kitchens: They’re fireproof, which makes them a perfect base for a pizza oven or a charcoal grill station.
- Vertical Planters: Turn the block on its side or use the top-facing holes to plant succulents. The concrete stays cool and holds moisture fairly well for the roots.
- Industrial Furniture: Two stacks of blocks and a piece of reclaimed timber make a surprisingly sturdy (and cheap) bookshelf or TV stand.
- Fire Pits: Because they don't burn, they’re the go-to for DIY fire rings, though you should ideally line them with firebricks to prevent the concrete from cracking under intense heat.
Just be careful with the "raw" look. Concrete is porous. If you’re using them indoors, they can shed dust or absorb oils from your skin. A quick coat of clear matte sealer goes a long way in making them feel like furniture rather than a construction site leftover.
Sorting Through the Sizes
You can't just walk in and grab "a block." They come in specific dimensions, though they’re usually referred to by their "nominal" size. A standard block is called an 8x8x16.
But it’s not actually 8 inches wide.
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It’s 7 5/8 inches. The extra 3/8 of an inch is reserved for the mortar joint. When you add the brick and the mortar together, you get a perfect 8-inch increment. This makes the math way easier for architects. If you’re measuring a space and you forget to account for that mortar gap, your wall is going to end up shorter or longer than you planned. It’s the "nominal vs. actual" trap that gets every first-time builder.
Real-World Advice for Your Next Project
If you’re planning to work with concrete bricks with holes, don't just start stacking. First, check your local building codes. If a wall is over three feet tall, most cities require an engineer to sign off on it because of the lateral pressure from the soil behind it.
Secondly, pay attention to the "ears" of the block. Some blocks have finished ends, while others have indented ends meant to be filled with mortar. Using the wrong one at the corner of your wall will leave you with an ugly, unfinished edge that you'll have to cover up with expensive stucco.
Lastly, think about the finish. Raw concrete is fine for a garage, but for a home, you might want to look into "split-face" blocks. These are manufactured as one double-sized block and then literally snapped in half by a machine. This leaves a rough, stone-like texture on one side that looks much more high-end than the smooth, "parking garage" grey we’re all used to.
Take these steps before you buy:
- Calculate your "actual" dimensions: Remember that 3/8-inch mortar gap for every single block in your run.
- Determine your reinforcement needs: Are you just stacking (gravity wall) or are you using rebar and grout? For anything structural, you need the holes for reinforcement.
- Check the weight: If you’re buying 100 blocks, that’s roughly 3,000 pounds. Your half-ton pickup truck will hate you. Get them delivered.
- Seal the deal: If the blocks will be exposed to rain and freezing temps, buy a silane/siloxane sealer. It soaks in and keeps water from penetrating the "face" of the block while still letting the material breathe.
Concrete is permanent. It’s heavy. It’s unforgiving. But when you use the "holey" version correctly, it's one of the most efficient ways to build something that will literally outlast you. Just make sure you give the water a way out and the steel a way in.