Block Basements: Part One

With well over 40 plus years of building, renovation and building assessment experience block basements are continuing reminder that they need special care and understanding. Part one in this 6-part series begins with some background.

I shall start by explaining some of the history of blocks, how they are made and some of the “quirks” of a concrete block basement. While it’s generally considered that the Romans invented concrete and used it extensively in their massive buildings, the use of a uniform concrete block did not evolve until the mid 1800’s. Many contractors used hand made molds and poured their own. The first concrete block building in North America is in New York City, it was built in 1837. Thanks to English Inventor Joe Aspidin who invented Portland Cement in 1824 it allowed the development of concrete blocks. In 1868 a company in Chicago called G.A. Frear got the first patent for manufactured concrete blocks. They were designed with a stone chiseled pattern to look like a stone foundation, for a lot less money. By 1900 a gentleman by the name of Harmon Palmer had invented a cast iron machine that mass produced hollow concrete blocks. Coupled with this was the development of reinforced concrete in 1849, by World War 1 the use of steel, concrete and concrete blocks for large scale buildings was well under way.

The actual method of making concrete blocks has not changed dramatically since it original development. They are lighter and stronger now and certainly more uniform in integrity, but they are still made of Portland Cement, sand, and an aggregate of some form. Gravel is used, so is shale and slate in some manufacturing processes. The mix for blocks usually contains more sand than gravel and water, making it a stiffer mix so the blocks hold their shape. Early concrete blocks weighed over 50 lbs a piece, today they average anywhere from 25-35lbs per block. Most modern block manufactures use some manner of compaction to help make the blocks consistent. An average run of blocks from mix to pour, then curing, put in a steam room to reach an even temperature and then they are allowed to dry takes about a day.  We generally see the grey hollow concrete block. There is however many other kinds, including architectural designs used on large buildings for example.

Up until the 1980’s concrete blocks were the basement of choice. They are reasonably priced, fairly simple to lay provided your footings are level and if you need to make some adjustments for square of level it can be “picked up” as you build your rows of blocks. If you are hiring a mason to build your block basement, he will charge you “by the block” to assemble it. Around 1950 we started brushing road tar or bituminous tar onto the blocks in the belief that this would stop water from penetrating the blocks. We thought we had the wet basement issue solved………oh how wrong we were!!

If block basements were so good and reasonably priced, why have we moved to poured concrete and insulated concrete foundations? The answer is longevity, reduced water invasion and better insulation values. We started using basements for living space after WW2 and along came the problems. The first thing we found out was the tar we applied really is just a damp-proofing, not a waterproof coating. It often dried out, did not seal cracks and on average failed in its intended use. Within 3-5 years, depending upon the water concentration in the soil that the basement was sitting in, moisture would begin to invade the blocks.  An average modern concrete block will absorb up to 3-4 lbs of water…per block. The hollow cavities will hold up to a gallon and half of water. We did not use below grade drainage with any consistency until the 60’s and even then, they were a clay tile that quickly filled with silt or tree roots.  Improper drainage around the home, lack of evetrough extensions and poor ground water management, movement of soil due to what is know as the freeze-thaw cycle, lack of proper anchoring at the sill plates, moving heavy equipment too close to the building, it’s a long list of issues that affect block basements. We learned some time ago block basements have limitations as to how large a building they would support. This is one of the reasons builders have moved to poured concrete and ICF foundations.

Block basements were an evolution in modern home assembly. After many years we have learned their shortcomings and fortunately we have developed ways to improve them, unfortunately at some considerable cost in many cases. Next week we look at why block foundations fail.