Infilling at Depressed Concrete Slab on Grade 4

[DHKpeWI]

The contractor poured a portion of a slab on ground 2-1/2" too low. He thought it was a depressed slab, but it is not. The existing slab is 6 inches thick with 4x4 W4.0xW4.0 WWF. The rooms are mostly for storage and will have vinyl composition tile.

The contractor’s solution is to apply a bonding agent and infill the 2 -1/2” with a 4,000 psi grout. I have asked that he submit a mix design along with break data.

This seems acceptable to me, what do you think?

 

[Lion06]

I don't think I would use grout. I would use a regular concrete mix, scabble the existing surface, and apply a bonding agent. Make sure to get the new concrete in right after the bonding agent is poured. I would also put mesh in the topping slab and match joint locations in the topping slab to the parent slab below.

 

[Ron]

Lion06 is exactly right. The grout will exhibit too much differential shrinkage and cause debonding. Even though this is a topping and good workability is necessary, keep the water-cement ratio as low as possible. Get your workability by other means (admixtures).

Consider pinning the topping to the existing slab.

 

[DHKpeWI]

Thanks guys.

What would be the difference between 4,000 psi grout and 4,000 psi concrete? Given that the thickness is only 2-1/2" what maximum size of aggregate would you recommend?

Wouldn't a mix similar to that used for composite precast concrete plank be suficient?

 

[Ron]

DHK...the primary difference is shrinkage. The strength is the same, but because the grout is more fluid and contains more cement, it will shrink more.

The coarse aggregate should be #89 stone, although for 2-1/2 inches, you could use #57 stone...but might be a bit difficult to finish.

 

[Lion06]

I agree with Ron, and here is how I typically think about it.
Regardless of w/c ratio, the amount of water in the mix has a direct effect on the shrinkage. The smaller the aggregate, the greater the surface area/volume of the aggregate is. The greater this ratio is, the more cement that is needed. The more cement you have, the more water you have (simply by virtue of the w/c ratio - double the cement, double the water) the more shrinkage you get.

Grout is really just sand, cement, and water. You can't get aggregate smaller than sand, the so the surface area/volume ratio is very high. As Ron noted, this requires a lot of cement, which, in turn, requires a lot of water.

 

[Ron]

Lion06....good to see a structural engineer understand concrete technology...so often that is not the case. Aggregate size and specific area are two concepts that many do not get! The next question on your exam will be the correct welding process to use for a give structural steel application....

 

[MiketheEngineer]

Dumb different suggestion - How about a treated 2x3 with plywood floor??

 

[DHKpeWI]

Thanks guys for your input.