Question:

We are pouring a 4 inch slab for a slab on grade room addition to the back of the house. The slab is the floor only and the walls will be built on foundation and block.

For the slab, the contractor has recommended 4000 psi mixture with pea gravel and fiberglass. Is this mix as good as a typical size gravel with a steel wire mesh? Any difference in expectations for cracking or strength?

Your feedback is appreciated.

 

No expert on the subject, but all the concrete in my house was fiberglass and has held up fine for six years.

 

Using pea gravel and no large aggregate will give you higher shrinkage and more possibilities of cracking for the same strength. The pea rock concrete is easy to slop in and move around.

Fiber mesh does not increase the strength of the concrete and is not a replacement for reinforcing steel.

I like to use both wire mesh (EWWM) -6x6 and fibers. If one had to go, the fibers would be the one to be eliminated.

Dick

 

Fibers reduce micro cracks and make for a better slab. Concrete is poor in tensille strength, and needs the mesh to provide it. Use both or use wire alone.

Pea gravel concrete is good for fenceposts and exposed aggregate and not much else.

 

We had pea gravel done around our last house and a 10' X 200' walkway.

Would never do it again.

We had gravel popping up constantly over three years and when there is heavy dirt, or mold that gets on concrete, it is very difficult to clean with the pressure washer. It is pretty for a while, but the rock does bleach out. Rain and sealant will bring the color back out, but the sealant is expensive, smelly and doesn't last that long (Used a commercial product called Diamond Clear).

The pea gravel lessens strength since the round and smooth rock doesn't adhere to the concrete very well like normal aggregate.

Regarding mesh vs fibercrete, the Rinker guy claims the fibercrete is better since you can rarely find anyone doing the finishing that is going to get the mesh at the right place in the pour. As you can imagine, the ones handling the pour will be stepping on it all the time.

Perhaps an instructive indication is DOT and Florida Building Code which is generally quite tough. They specify mesh, steel or fibercrete. If they accept fibercrete instead of using mesh, I figured they must have some basis for believing the strength is comparable.

We poured 190 yds of concrete with 3000psi fibercrete recently and we do have cracks, but hairline and not separating.

Why anyone is using pea gravel for a slab to be built on is beyond my understanding.

 

The old, "but the wire is never placed right, they walk on it, etc" is a poor argument. After all, if the forms are not done right, the concrete is not mixed right, etc.

Do it right, no matter how you do it. Remember this: When the slab cracks, wire will hold it together; microfibers will not.