Low strength concrete in suspended slab 3

[Hemifun]

I have a small building with CIP concrete walls and a CIP suspended slab. The building is roughly 27'X22' in plan size. The contract called for 3750 psi concrete and the slab was designed with f'c of 3500 psi. The cylinders broke low so we had cores taken. The cores broke at an average of about 1900 psi(56 days). I checked the calcs. and structurally the slab is ok even with the low breaks. I know most will say I need to have the slab removed and replaced but there are dowels all around the perimeter which tie the slab to the walls and replacement would be difficult. What are some cons besides durability for leaving the slab in place? The entrained air is 7%. Any suggestions??

 

[Hemifun]

Ron- Yes, I am using the code found in the 2008 MCP which I believe is a 2003 version of ACI 214.4R. Thanks for the heads up on the new code and the reference to ACI 318.

a2mfk- Yes, bond is affected by concrete strength. And you're right, in the grand scheme of things my 27'X22' slab is relatively minor. lol

 

[a2mfk]

Meant to ask this before, what kind of building goes vertical and is only 27x22, that is small even for a modest house.

 

[Hemifun]

It is a pump building at a wastewater plant.

 

[SkiisAndBikes]

Hemifun,

A lot of great information from others on the concrete mix. I would like to address your concern about removing the slab and affecting the dowels coming out from the walls. I have been involved in many total slab replacement projects in parking structures.

Contractors will usually opt for the following procedure:
- shore walls/columns where necessary to replace any lateral support provided by the slab to be removed.
- install formwork on the underside of the slab
- sawcut slab into squares for removal, keeping sawcuts a minimum of 3 feet away from walls/columns.
- jackhammer concrete in the 3 foot zone beside walls/columns, thus preserving any existing dowels. Use small jackhammers for final chipping to avoid bruised concrete at the bond line.
- level formwork installed previously
- install new steel, lapped onto existing dowels and pour new slab.

A contractor experienced in total slab replacement/rehabilitation would have no problem with this type of removal.

 

[hokie66]

Since it is in a wastewater plant, I think that closes the case. For structures like that, your original specification of 3750 psi concrete is on the low side. Durability is the main issue.

 

[Ron]

Exactly, hokie66.

 

[Hemifun]

It is at a WWTP but will not be in contact with any wastewater. It houses sludge pumps in the basement with dry storage on the first floor. We do, however, use type 2 cement on all structures.

 

[JLNJ]

Shears, anchorage, laps, hooks, develoment lengths, all may not comply if you have 2500 psi concrete.


I don't know if I'm in the "tear it out" camp, but even if they pay you handsomely to re-analyze it there's a lot to think about before accepting responsibility.

 

[JAE]

Keep in mind that per ACI 318, Chapter 5, you can go to calculations and check the structure with the lower compressive strength values (taken from the tests, not the cores) and see if the structure is OK. This should include everything as others above have suggested (flexure, shear, development, durability).

This FAQ is a brief summary of the steps in Chapter 5:
faq507-1575