Concrete vs. Block foundation

Old 11-11-03, 12:39 PM
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Question Concrete vs. Block foundation

We are thinking to enclose existing two-season porch into a living space. The contractor told us that the inspector wants to see block foundation installed.

What are the pros and con's on concrete vs. block foundation.

Does MASS building code very from city to city? Or it's inspector preference?

Old 11-11-03, 03:25 PM
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The code can vary and it may well be the preference of the inspector. Overall, there is not much difference between block and poured.

Check your local building permit office, someone there can tell you what is required.
Old 11-15-03, 07:48 AM
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A poured concrete wall is a monolithic, homogenous structure with rebar completely embedded in cement, thus protecting the rebar from the elements. There is a decent to good bond with the footing. Waterproofing (may not be a concern for you) is reasonably straightforward.

An all-voids-filled CMU wall is a latice of concrete with 5"x6" verts and 1" or 2" horiz members surrounded by CMUs. The rebar in the H-members is usually placed at the bottom surface of the H-members and is exposed to moisture and air at the locations of the vert block webbing. If all voids are not filled, the H-rebar in the empty voids is not protected in any way.

The horizontal mortar lines in the CMU wall cause a discontinuity in dimension in the concrete pillars within the block that act like a stitch line in tearoff paper. Ie, it weakens the shear strength of the wall in the vertical plane. The remedy is for the masons to strike off the mortar from the inside of ther blocks as they assemble the wall. This is difficult to do, adds to labor costs and
contributes to the next issue.

Since it is impossible to prevent mortar from falling into the voids, one side of all base blocks must be removed prior to setting so as to allow cleanout. Without cleanout the bottom of the voids will fill with mortar chips and prevent any bond with the footing and leave an opening for water infiltration.

CMUs are very water porous and special care must be taken to insure a complete membrane. I reccommend floating or parging the entire surface with a cementious product before the waterproofing is applied.

That said, if CMU walls are properly done they are comparable to monolithic concrete.


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