Whether it is poured monolithically or not should not materially change the need for reinforcement. If you can post a detail, it would make your question clearer.
In theory there's less congestion as you could just have the slab bars provided with a bend at the end. So instead of dowels lapping with he slab rebar, it's just a single piece.
Contractor brings sand bags under the slab on grade. I don't know how they compact sand bags. They usually stack them up to the bottom of slab on grade and install membrane and rebars, and then pour concrete.
The sides of the beams must taper, then, else the pressure from the sandbags would collapse the form. I've encountered using membranes for formwork in past, but, not with this type of cross section.
The contractor's method has the stirrups projecting into the slab, so they do the same job as the dowels, if the dowels are indeed required, which I doubt. But dik's point of constructability of the beams without sloping slides is well taken. It depends on the soil conditions. I fail to understand the sandbags and membrane situation. What membrane?
What is the purpose of the beam? If the slab on grade is non-structural, it doesn't need a beam; it simply rests on the soil. If the slab heaves on a swelling soil or settles on poorly compacted fill, it should be free to do so without hanging up on the beam; it would be better to separate the slab from the beam. This permits the soil to be adequately compacted after the beam is poured and before the slab is poured.
I don't know either, BA, but one fairly common solution for minor structures on swelling soils is to provide a grid of stiffening ribs monolithic with the slab.
Under the contractor's option:
- The side cover for the reinforcement will need to be increased for casting directly against soil (no forms).
- I would usually show the soil casted sides as a rough line that slopes. For sand, 60 degs from plumb is probably the minimum. Who will be watching when the beams and slab are cast to verify that voids in the beam are prevented?
- Integrating the beam and the slab allows a shallower beam and adds the slab as part of the beams compression zone (usually just take it as gravy).
- Better shear transfer between beam and slab
- One pour instead of two over the slab area
- For anything less than a 30" deep grade beam, I'd probably detail it this way.
If you are concerned about differential settlement, you could detail a sheet of rigid insulation along the bottom.
I don't like the idea of using sandbags under a slab on grade. In my mind, the bags will allow voids. If you just put sand down, the sand will provide a uniform sub base.
Is the contractor using larger stirrups and thus a deeper beam than what was specified? does this new section meet acceptable reinforcing requirements (As_min, rho, etc) are the stirrups are the same spacing or further apart (d/2 ?).
