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Identifying Problems with Your Historic Concrete Block Foundation | Wisconsin Historical Society

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Identifying Problems with Your Historic Concrete Block Foundation

Identifying Problems with Your Historic Concrete Block Foundation | Wisconsin Historical Society
EnlargeA concrete block foundation in need of repair.

Waukesha County. A concrete block foundation in need of repair - notice the cracking occurring at the mortar joints. Source: WHS - State Historic Preservation Office.

The foundation of your historic house or building could be made of concrete masonry building blocks, which were once referred to as cinder blocks. Most of the problems that occur with concrete foundations result from a lack of maintenance. You might be able to solve some of the typical problems you will face with your concrete block foundation on your own, while other problems will require the expertise of a professional mason.   

Origins of Concrete Block

EnlargeFoundation wall

Excessive moisture may be the cause of this concrete block foundation cracking at the mortar joints as well as the paint peeling. Source: Bob Yapp

Concrete blocks have been used in one fashion or another since early Roman times. The concrete blocks used in Wisconsin up to about 1900 often had coal clinkers — the cinders or waste created from burning coal — added to them. Modern concrete blocks have sand and gravel added to them. By 1900, cheap Portland cement was in wider use, and it greatly strengthened concrete blocks.

Some early versions of concrete block were entirely solid. Today, concrete blocks are made with hollow voids, but they are designed to carry heavy loads. They have a honeycomb shape that you can see by looking at the top of a block.

Typical Concrete Block Foundation Problems

You may want to hire a preservation masonry professional to identify the cause of any problems you are having with your concrete block foundation. The most common problems you are likely to encounter with your concrete blocks are discussed below. Click the link on each subject to learn how to solve the problem.

Common ProblemLearn More
  • Mortar is missing or crumbling between your concrete blocks.
  • Your concrete blocks are cracked or broken.
  • The mortar between your concrete blocks is showing excessive deterioration, and water is entering your basement. This problem occurs because no exterior waterproofing was ever installed below ground. You should hire a professional mason to dig around the exterior of your foundation and coat the exterior surface with a waterproofing material. Have the mason verify the condition of the mortar while your foundation is exposed, because excessive moisture for an extended period of time can leave mineral deposits that break down the mortar. If this is the case, your foundation may need to be repointed while it is still exposed. The mason should also install tiling around the foundation to channel water away from your foundation.
  • The cement coating (parging) on the exterior of your foundation wall is deteriorating. You should hire a professional mason to dig around the exterior of the foundation and coat the exterior surface with a waterproofing material. Have the mason verify the condition of the mortar when the foundation is exposed, because excessive moisture for an extended period of time can leave mineral deposits that break down the mortar. If this is the case, the foundation may need to be repointed while the foundation is exposed. The mason should also install tiling around the foundation to channel water away from your foundation.

Repairing Mortar on Your Historic Masonry Building

  • Your concrete blocks always seem wet and are deteriorating from excessive moisture. To solve this problem, you need to get the water to move away from your foundation. Grade the ground around your foundation so it is angled away from your foundation and add ground extenders to your gutter downspouts.

Maintaining the Gutters on Your Historic House

  • Your interior foundation walls are parged or painted and appear to be trapping water because the parging or paint is flaking off.

Remove all the loose cement or paint and allow the rest to flake off over time. Any coating on the inside of a concrete block wall will prevent the normal migration of moisture. The trapped moisture freezes inside the wall, causing mortar deterioration and spalling of the concrete blocks.

  • One or more sections of your foundation wall are bowing inward or collapsing from exterior forces such as water and tree roots.
  • Your foundation wall has cracks that look like stair steps.
  • Your foundation shows efflorescence, a white powdery substance leaching in between the bricks.
  • Your foundation wall has sunk (settled) or heaved upwards.
  • Your concrete blocks are dirty or have moss or mold growing on them.

Identifying Problems with Your Historic Brick Building

  • Your concrete blocks show deterioration and excessive wear. 
  • The surface of your concrete blocks is coarse and shedding sand particles excessively.

Exposed concrete naturally erodes over time; however, rarely does this result in the foundation wall being structurally compromised.