The Use of Clay Bricks for Steel Reinforcing Support

[pricklyPete]

I am currently working on a project where the contractor wants to use clay bricks to support the bottom mat of reinforcing (They are actually shale bricks with test breaks well above 10,000 psi).

I was told early in my career that clay bricks should not be used to support rebar because they will expand if they become wet and crack the concrete. I can understand the concern when the clay bricks are in direct contact with the soil.

However, in this case, the bricks do not sit on the soil. They sit on a lean concrete mud mat that is roughly 2" to 3" in thickness, which should help to protect the bricks from moisture. This foundation is very large and the support bricks are spaced far enough away from the edges such that blowing out the sides of the concrete due to expansion is not a likely possibility.

I am aware that ACI says that reinforcement supports should be of either concrete, steel, or plastic material. What I am wondering is if anyone sees any issues with using clay (or shale) bricks in this particuluar scenario?

 

[concretemasonry]

In realities, the bottom line is compatibility of materials.

If you choose to use solid supports instead of plastic or metal chairs, concrete brick make more sense. The physical properties of a concrete product are closer to the concrete you are pouring/placing. Concrete brick, tested in the same manner (ASTM) are probably 5000+ psi, while the hard and brittle brick will have much higher strength (really meaningless), different absorption, less bond and much different long term stability (expansion instead of shrinkage) than concrete.

Dick

 

[apsix]

I was taught and still think that the main problem with clay bricks is the porosity, resulting in moisture intrusion and therefore corrosion of the reinforcement.
Concrete bricks may also be porous.

I don't think using clay brick in your situation is risk free, I wouldn't allow it.

 

[dik]

Should use concrete bricks, not clay.

Dik

 

[hokie66]

If the clay bricks are solid, they should be less porous than concrete bricks, so I don't see that as an issue, but moisture may be able to get around the edges of either type, so I generally don't like solid supports.

If there is a membrane between the mud mat and slab, then the issue of protection of the membrane makes the solid supports more attractive.

 

[concretemasonry]

hokie -

The existence of cores has nothing to do with porosity. The major reason is the compatibility and similarity of materials of the concrete and the rebar support.

In many areas, concrete reinforcement supports are actually made specifically for that purpose.

Dick

 

[dik]

Concretemasonry 'hit the nail on the head'... Clay usually swells with moisture and time whereas concrete shrinks.

Dik

 

[pricklyPete]

Thanks for responses.

Maybe to ask the question in a different way to try to understand all of the issues better: Even if there is some local cracking around the clay bricks, given that the mud mat is present to provide a barrier to moisture, would these cracks really be that harmful?

The bricks are 3 5/8" x 8" long. So even if they do expand, the total expansion will be very minor. Also since there is a layer of reinforcing directly above the bricks (the reason for the bricks in the first place) and since the mud mat is directly below the bricks, wouldn't this provide a limitation on the volume change of the concrete surrounding the bricks (assuming a majority of the shrinkage has occurred in the mud mat before the foundation is poured).

BIA technical notes indicate a 0.0005 design value for moisture expansion of brick. It doesn't seem like the damage this small volume of brick would do would be worse than a skrinkage crack. Thoughts?

 

[civilperson]

Clay bricks will provide a route for moisture to attach the reinforcement, use approved concrete spacers.

 

[coalmonkey]

I don't think expansion will be a major concern. The main reason it is prohibited in some areas is because clay is an acid. Over time it will react with the steel and the concrete. Moisture would speed the reaction, so the lean concrete mat is a good thing. The reaction will be very slow and may not be an issue depending on the service life of the structure.

 

[csd72]

Porosity, strength and lack of alkalinity are the three major issue I can think of (as people above have said). Dont let them do it.

 

[hokie66]

Seems I am in the minority, but my advice stands. But your assumption that the mud mat will be a moisture barrier is incorrect. If there is a membrane, use the solid supports, be they brick, concrete, whatever. If no membrane, use plastic chairs.

 

[concretemasonry]

The point I was making is that the inherent property of fired clay brick is to expand with time, while concrete has a long term shrinkage. That is the reason relief angles must be provides for brick veneer.

It all depends on the application and what is acceptable for the conditions and use.

There is nothing wrong with using compatible materials (concrete) for the support, especially since it is cheaper. I always get a little crazy when people mistakenly think the high strength "compressive strength" of the clay brick is an asset, when it is really a figment of the imagination because of the testing procedure and sample aspect ratio, which yeilds an unrealistic failure mode. I have seen concrete brick much higher than 12000 psi, but there is no reason to supply them.

Dick

 

[kslee1000]

"given that the mud mat is present to provide a barrier to moisture"

Comment: Mud slab cracks - usually the thickness is not uniform throughout, and the subbase condition is usually poor, otherwise, you wouldn't need a MUD slab, which would cost more than pay for proper rebar supports.

 

[pricklyPete]

Strictly from the standpoint of having moisture permeate through the brick itself. I find it hard to believe that the shale brick would be less resistant to water penetration than the concrete brick. The shale brick had a very dense structure whereas the concrete bricks appear to have the density of CMU units. Perhaps, there is more to it than that, but this is how it appears.

But I'm guessing the real concern is not with moisture permeating through the brick itelf, but rather around the interface between the brick and the concrete. And I believe concretemasonry has made a good point about the bond being less with clay.

Anyone have any further knowledge on the alkalinity or acidity issue that has been mentioned by others?

 

[hokie66]

If the concrete brick looks like a CMU unit, it is. It is very porous. The shale brick would be quite impervious. You are correct in that moisture will penetrate around the interface, but that is also true of concrete (other than CMU) supports. It is a construction joint, and construction joints allow water through.

 

[apsix]

It is generally accepted that hairline cracks (0.3mm/.01")in concrete do not promote corrosion of the reinforcement, due to the protective alkaline nature of the concrete. Therefore the moisture penetration along the interface is not necessarily a problem.

I suspect that the use of clay brick will not provide the same alkaline environment, so I wouldn't use clay brick.

I don't know enough about shale bricks to comment with authority, but I wouldn't risk using them.

 

[kslee1000]

There must be a reason that the slab has double layer of reinforcement. Don't you think the bricks would create weak points all over the bottom face? Does it matter? Certainly does for suspended slab, not sure your application on the mat, couldn't go farther than all concerns have been raised so far.

 

[pricklyPete]

I'm not sure exactly what you mean about the weak points all over the bottom face. But strictly from a strength standpoint, there is no question that these shale bricks are holding up better than the concrete bricks do. There are always numerous concrete bricks that get crushed when they are used. The instances of the shale bricks being crushed were far fewer. So there is at least one upside.

Any reduction in the flexural compression zone due to brick would be negilible. The spacing is roughly on a 5' grid.

 

[kslee1000]

No concerns over tension?