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MikeF. 04-08-2005 07:26 PM

Question on Foundations
Can anyone here address the pros/cons of filled block foundations vs. pre-mix poured to form in terms of load bearing capacity. Is the difference significant? Are there any instances where one is preferred(or required), ie. commercial? Just curious.

pipeguy 04-09-2005 10:02 PM

Here's what I've observed:
The level of inspection on residential foundation jobs is virtually non-existent compared to that of commercial jobs. One reason is that the residential building code seems to require VERY LITTLE steel reinforcing of poured-in-place foundation walls while commercial building codes (design teams) require LOTS of reinforcing.
CMU's (concrete masonry units) have lots of mortar joints. Mortar is mixed on the site in a relatively uncontrolled environment compared to ready-mix concrete that is batch mixed in a plant.
When assembled, the hollow interior core of the CMU wall is anything but a continuos, unobstructed, open cell. Filling that interior core in its entirety is something that takes considerable time and effort. In the abscence of vigorous inspection, failing to fill the interior core in its entirety is something that occurs with considerable frequency. The material with which the wall is filled (ready-mix concrete or site mixed mortar) is also a determining factor in the wall's ultimate strength.
Various factors might impact your choice of specification for
a foundation wall including the cost and availabilty of subcontractors and materials and the anticipated building loads- to name a few. I think it's safe to say that a well reinforced, properly constructed, concrete wall is by far the most substantial of foundation walls when considered alongside a CMU wall or filled CMU wall.
A lightly reinforced concrete wall (such as those seen in residential construction) is probably the next most substantial option provided that appropriate mix designs,batching methods, placement methods and cold weather protection methods are utilized. A properly filled CMU wall is probably more desirable than a shoddily poured, lightly reinforced concrete wall.
Again, my comments are based only on observation, not on first-hand experience.

MikeF. 04-10-2005 02:21 PM

Thanks Pipe, I was just curious. I know very little (read: "nothing") about concrete. I have read a ton about it, but without first hand experience, I really have no understanding. Thanks for your input.

Teetorbilt 04-10-2005 10:10 PM

Another factor in block construction is the nature of the block. Blocks tend to be lightweight and frangible. Casting them into a structure is an artform with many pitfalls. Personally, I would not do it.

DecksEtc 04-11-2005 09:21 AM

Just to add to my 2 cents worth:

MikeF., I don't know what area you live in and I am by no means an expert in concrete but you should read into what pipe & teetor are saying.

IMO, a lot has to do with how your foundation is built - regardless of the material you use. The major reason that concrete block construction has a downside is that there are a lot less highly-skilled masons around that know how to do it right. My parents built an addition where the new foundation "tied" into the existing foundation of the 100+ yr. old main house. This was over 20 yrs. ago and the mason that built that foundation had almost 40 yrs. exp. and learned his craft Germany. Saying he was a perfectionist hardly does him any justice.

Concrete block construction has been primarily replaced by poured for all new construction in my area for awhile now. The quality of the workmanship will determine the best method as much as the materials.

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