Concrete strength

   / Concrete strength #11  
I would not use it for a beam without a lot more info. See picture of a concrete beam.
 

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   / Concrete strength #12  
I have a chance to get some concrete that is 1 foot square, with 4 pieces of 5/8 rebar in it. It would span 7 feet over a drive way to let water under. Any idea what those concrete pieces would carry in weight. There would be 9 of them but the tires would ride on just two of them.

NOPE, 4 pieces of longitudinal rebar do not a beam make. There needs to be stirrups to prevent shear cracks from causing a beam failure.
 
   / Concrete strength #13  
Is there any rebar installed transverse in the beam
Unless the concrete Is homemade the strength should be 3000 PSI minimum
Aggregate size does not factor in beam strength

It does in rebar placement.
 
   / Concrete strength #16  
Concrete has very poor tensile strength and it's only the rebar that allow such tensile loads, so don't even think of the rebar just strengthening the concrete in this application. It IS the strength. If that was designed as a column, the loading was engineered very differently.
 
   / Concrete strength #17  
Actually if it is actually only a 7' span put them down with some planks on top and you could drive anything over it.
 
   / Concrete strength #18  
Does op know if they have stirrups or just the longitudinal rebar? Who made the columns? What was intended purpose?
 
   / Concrete strength #20  
Concrete has very poor tensile strength and it's only the rebar that allow such tensile loads, so don't even think of the rebar just strengthening the concrete in this application. It IS the strength. If that was designed as a column, the loading was engineered very differently.

Concrete will fail in tension when loaded.
Columns are designed as beam/ columns usually. A combination of compression and bending loads
 

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