Poured Concrete vs. Block Walls

Poured Concrete vs. Block Walls TIPS

DEAR TIM: What’s the difference between concrete block and poured concrete foundation walls? Which one is better? The lot I’m going to build upon has a seasonably high water table. Which of the wall systems would insure a dry basement? Diane J., Redwood Falls, MN

DEAR DIANE: I think it might have been easier to answer a question such as: Is a glorious sunrise more beautiful than a magnificent sunset? The truth of the matter is that both building materials – concrete block and poured concrete – can yield superior foundation walls so long as they are installed correctly.

All too often builders and sub-contractors fail to realize the limitations of certain masonry materials. When this happens, foundation failures are a common occurrence.

Concrete Block Is Concrete!

The first thing to realize is that concrete block is made from poured concrete. The primary ingredients in concrete block happen to be: Portland cement, gravel aggregate and sand. These are the same ingredients you’d find in a poured, or cast, concrete foundation.

The only difference between the two finished products s the size of the gravel. Typically you’ll see gravel stones as large as big juicy grapes in a poured concrete foundation. A concrete block may have gravel no larger than the size of a standard green pea or a dried raisin.

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Poured or Cast Concrete

Poured concrete foundations are solid through and through. This happens by default as the plastic concrete flows from the ready-mix trucks into the foundation forms. Poor workmanship can yield hollow spots in poured foundations. The contractors use special vibrating tools to prevent this.

The typical concrete block foundation is not solid. The concrete blocks that are used to build block foundations, by their very nature, are hollow.

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Make Block Walls Solid

When concrete blocks are stacked on top of one another, you can look down through the center of the foundation. After the concrete blocks are laid, the voids can be filled with a cement-based mortar or poured concrete that contains small pea gravel.

The concrete used to fill the block voids must have small gravel so it can flow readily into all the narrow voids. To maximize strength, the voids need to have 1/2-inch steel rods centered in the voids from the bottom of the block walls to the top. Put this steel every 2-feet on center.

If the builder does this, then the filled concrete block walls are nearly identical to poured concrete walls.

Both Weak If Bent

But simply doing this is not enough to satisfy myself, code officials and structural engineers. Concrete and things made from concrete or cement tend to have fantastic compressive strength.

This means when you squeeze concrete it’s very strong. Often it is measured in the thousands of pounds per-square-inch range.

But the same material typically has only one-tenth the strength when you apply a tension to it. Tension is a bending or stretching force.

You can easily picture this. Imagine pouring a 4-inch thick slab that’s 10 feet wide and 40 feet long that’s up three feet in the air and just supported on each end.

It would bend in the center under it’s own weight and crack long before it would if you tried to drive across it!

Massive Soil Pressure

Backfilled soil against a wall produces tension as the soil pushes against the wall. Poor quality soil creates tension on a foundation as more and more weight is added to the foundation as the house is built.

Poor drainage around a house creates pressure as tons of water collects in the soil.

This is why roof water should NEVER be allowed to run on the ground near a foundation.

The use of splash blocks at the base of a full basement is comical. Roof water needs to be piped far away from the house to the lowest spot on the lot or into an approved storm sewer.

Steel = Strength

You can strengthen poured concrete and concrete block walls by including reinforcing steel. Often you will see horizontal steel bars placed in the lower and upper sections of poured foundation walls.

This steel is usually found about one foot up from the bottom of the foundation wall and a foot down from the top.

There are usually two rows of the steel and it’s continuous around the entire foundation. When the steel bars overlap, it should be no less than 16 inches.

This steel often has a tensile strength of 40,000 pounds per square inch. This horizontal steel helps prevent vertical cracks should the foundation drop or heave.

Horizontal wire fabric can also be placed in the mortar joints between layers of concrete block to achieve the same result in a concrete block wall.

Vertical Steel

Vertical reinforcing steel is also very important. This steel can be put in both a poured concrete foundation and a concrete block wall that will have the hollow voids filled with cement grout or pea gravel concrete.

The reinforcing steel should be one-half inch in diameter and these rods should be connected to the poured concrete footer that the foundation rests upon. This steel should be placed every two feet on center.

Not Waterproof

Neither wall system will be waterproof. If you want a dry basement or crawlspace you must apply a foundation waterproofing system to the exterior of the foundation walls after they are built.

There are many different systems, but my favorite when I was building was a rubberized asphalt called Tuff-n-Dry. Once the material was sprayed on the walls it was about 1/8-inch thick. If the foundation did crack, this material could stretch and bridge a crack up to 1/4-inch I believe. The specifications may be different today.

Liquid asphalt is often sprayed on a new foundation wall. It’s like a coat of paint and very thin. This is NOT waterproofing. It’s just a vapor barrier.

Drain Tile

In addition, a drain piping system needs to be at the base of the wall adjacent to the footer.

This pipe is covered with two or three feet of washed gravel. The gravel is then covered with six inches of straw or a sheet of tar paper before dirt is placed over it. The straw and tar paper prevent silt from the fluffed up backfill dirt from clogging the gravel. The water that flows through the soil and makes it to the pipe is then drained to daylight if the house is built on a sloping lot. If the house is built on a level lot, the drain pipe often empties into a sump pit.

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