Poured Concrete vs. Block Walls
DEAR TIM: What is the difference between concrete block and poured concrete foundation walls? Which one is better? The lot I am 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.
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.
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. 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.