If the walls of the foundation of a house are concrete blocks, are those blocks likely to be solid or hollow?

Question

Most concrete or cinder blocks are hollow. In other words, they're just a shell, about an inch or two thick, not a solid block of concrete. You can hold them with your hands by putting part of each hand in the hollow area, or in both hollow areas for the most common cinder blocks, which have two.

But are all concrete blocks hollow? It seems to me the blocks used in the foundation of a house are more likely to be solid, because they have to hold up a lot of weight. And the top surface of the foundation wall has to be flat if bricks are on top of it. So it makes sense that the concrete blocks would be solid instead of hollow. But are they really?

I'm confused about this issue because I've never actually seen any solid concrete blocks. Just hollow ones. But it doesn't make sense to me that hollow ones would be appropriate for foundation walls. Which way would the holes be aimed? If upward, how would they have a flat surface to build the brick walls of the house on?

Answer 1

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.

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 would find in a poured concrete foundation. The only difference being the size of the gravel used in each application. Typically you will see gravel stones as large as three -quarter inch diameter in a poured concrete foundation. A concrete block may have gravel no larger than the size of a pea.

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

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

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. 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 or stretching force to it. Bending a poured concrete or concrete block wall creates tension. 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.

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 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 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.

Hope it helps.

Answer 2

Concrete Foundation Blocks

Answer 3

Big Dogg has the best answer. I have drilled into and worked around thousands of foundations. The Blocks are made with a controlled process. Poured foundations are more prone to cracking and have a more likely to be poorly done. Though the only con to a block wall is if they are not waterproofed well on the exterior they can prematurely wear out, or leak easier. Though it is rare to see large cracks due to the light weight.

Answer 4

u people r so missed informed......a block wall standing 8' tall can hold up and support over 7000 lbs per sq ft! it is one of if not the strongest foundation materials u can buy......99% of all block foundations r hollow block, except the interior piers, they must be solid or if u use hollow u must fill them with mortar......

the reason they use hollow is because of the weight factor....u would have to increase the size of the concrete footer by 50% if u use the solid block in this application......

if u have a block foundation and it was installed correctly then u have nothing to worry about....

lic. gen. contractor - 28 years

Answer 5

If you are thinking about concrete blocks, which are actually a great building material, why not go all the way and research into concrete homes, which are really the way to go. Hurricane and tornado proof, they are super insulating, save a fortune in heating bills. Just click on the concrete homes ad at the right of this question and get a lot more information. We were going to build one but decided to stay where we were; however if we ever do build a new home, that would be it.

Answer 6

Yes, they are all hollow. Not made for a good foundation. Too many weak spots.

Answer 7

yes they are all hollow,,making a poor foundation in my opinion !!!!

Answer 8

your foundation blocks should be poured solid and if ther not you should have them fixed.

Answer 9

the strength is in the vertical not the horizontal. holes help drainage and bottom row has weep holes to drain.

Answer 10

usually about every 3rd one is filled, with rebar tying them, hope this helps