Yes, cinder blocks (also known as concrete masonry units or CMUs) used for foundations are manufactured with hollow centers or cells. While the blocks themselves are hollow, a properly built foundation will have many of these hollow cavities filled with steel rebar and concrete grout for structural support.
Foundations Built Using Concrete Blocks
Concrete block foundations are made up of precast concrete foundation blocks stacked on top of each other to form a solid base that supports the weight of the house. Concrete foundation blocks are typically reinforced with rebar and then filled with concrete for added strength.
Almost all traditional cinder and concrete masonry units (CMUs) are manufactured with large, hollow cavities. To verify if your specific block is hollow, or if the hollows have been filled with concrete, use visual inspection, the tap test, or a stud finder.
Concrete blocks can be prone to water seepage.
Concrete blocks can cause seepage damage in areas with high water tables. Concrete blocks are strong but porous and allow water to penetrate. Water is persistent and will seek the path of least resistance. It can eventually make its way through walls and cause damage.
"Poor man's concrete" refers to a budget-friendly, highly compacted mixture used to create a solid, semi-permanent base for driveways, sheds, or patios. It typically consists of a blend of gravel, coarse stone, sand, and stone dust.
If the conditions are right, concrete building blocks have a lifespan of 100 years or more. This makes them a very durable building material. Indeed, concrete created during the Roman Empire can still be found today, which demonstrates how long-lasting it can be.
Cinder block foundations are not inherently bad; they are highly durable and cost-effective when properly constructed. However, they are more susceptible to water intrusion, bowing, and structural cracking from soil pressure than poured concrete walls. Their longevity largely depends on proper reinforcement and maintenance.
The 90-minute rule is a longstanding concrete industry guideline stating that ready-mixed concrete should be completely discharged from the mixing truck within 90 minutes from the time water is first added to the cement. This limit ensures the mixture remains workable and achieves its full structural strength.
Water is the most common enemy of concrete surfaces. When water penetrates the surface of concrete, it can cause it to crack and weaken. This is especially true in colder climates where water can freeze and expand, creating even more damage.
Key Takeaways
For structural foundations (like home additions or basements), standard Concrete Masonry Units (CMUs) are the industry standard. For simple, non-load-bearing structures (like garden sheds or floating decks), specialized deck/foundation blocks are best.
Cracking and Structural Damage
One of the primary issues with cement block foundations is cracking. These cracks can appear due to various factors like settlement, soil movement, or excessive water pressure. Small cracks might seem harmless initially but can exacerbate over time, leading to structural damage.
The strongest foundation for a house is a poured concrete foundation, specifically a deep foundation (like piles) anchored to bedrock or a shallow monolithic slab supported by Insulated Concrete Forms (ICFs).
In most cases, poured concrete walls are cheaper for large, simple builds, while block walls are cheaper for small, intricate, or highly accessible projects. When comparing the two, you must look at the total installed cost (including labor, materials, and heavy equipment) rather than just the price of supplies.
Coca-Cola does three main things to concrete: it slows down the curing process (acts as a retarder), etches the surface to expose decorative aggregate, and removes tough stains. This happens because of the soda's high sugar content and phosphoric acid.
If it rains 12 hours after your pour, your concrete is almost certainly safe. By this point, the concrete has largely hardened, and rain will generally not impact its structural integrity or wash away the surface. In fact, the added moisture can even act as a free "wet cure".
The average cost for a professional to install a 20x20 (400 sq. ft.) concrete slab ranges from $2,400 to $6,500, or about $6 to $16 per square foot. Your final price depends on thickness, site preparation, and finishes.
Building codes. Wood is cheaper. Labor is expensive for block vs poured concrete. Some people don't like the look in a basement vs concrete.
Cinder blocks (and modern concrete masonry units) typically last 50 to 100 years. Their lifespan depends heavily on moisture exposure, structural load, and whether they are reinforced with steel.
Applying a Concrete Sealer Is Important
If you avoid sealing foundation cinder blocks before water issues start, they can and may become unsealable in the future. Efflorescence will loosen and leach from within the matrix of the cinder block, being the first sign that water is present inside the hollow cores.
To dissolve hardened concrete fast, acid-based chemical removers are your best option. These liquids trigger a chemical reaction that breaks the bond of the Portland cement, turning rock-hard chunks into a soft, mushy, rinse-off slurry.
Concrete block walls can lose heat through uninsulated spots called thermal bridges. Adding continuous insulation around the building helps reduce heat loss and cut energy costs.
Manufacturers stopped making actual "cinder blocks" around the mid-20th century, specifically transitioning away from them by the late 1960s. Today, the heavy gray blocks used in construction are Concrete Masonry Units (CMUs), which are made from dense aggregates like gravel and sand rather than coal ash.