slab-in-grade concrete pad with precast trench

[a10jp]

I am not a structural engineer, but I wish to consult with others if what I have here is a problem.

What I am showing (attached) is a slab in grade pad with a concrete precast u-trench section on top, and the pad is about 347" x 120". The actually rebar dimensions are D16, at 150mm (or 6”) spacing, top to bottom. The way our contractor do this is first pour the bottom part, where I indicated it in red line. Let it cured, then place the precast, and pour the rest. There is actually rebar connection between the top and bottom pour, it just that it is 2 different pours, and some people were extremely concerned now we have a split pad at the horizontal plane, and it is “cold joint”. They claimed there will be a problem if we have an earthquake. And because of that, they want one monolithic pour, instead of 2 separate pours. But my reasoning is, they are joined by the D16 rebars, it would be just like placing a concrete pad on a mud slab, where is there such a big different for a slab in grade. Also, just to clarify, the top slab is elevated above grade by 2”, the rest will be backfilled.

I am not structural engineer, but I just want to understand if there is truly a big issue here. ALso the equipment that will be placed on this is about 50,000lbs.

 

[hokie66]

I don't have a great problem with your contractor's sequence, but I do with the way the reinforcing is detailed. The bars which are bent under the trench should instead go to the bottom of the pad. Use another layer of bars under the trench. You should have some on the face of the pad as well. And all bars should be developed rather than just stopping past a corner.

 

[Teguci]

Any horizontal load in the slab will need to transfer down into the "mudslab" and then transfer back up on the other side of the trench.

Ideally the rebar would be developed on both sides of the control joint.

 

[a10jp]

Actually, that was the design drawing that shows this slab was designed to be monolithically poured, rather than in stages. I am attaching a similar shop drawing which is what the contractor shows in his shop drawing, and it has rebars in vertically. See the lines and dots in red. The contractor did the way he did so that the precast trench can be laid in after the concrete has been formed for a few days, rather than building a form to support the precast.
Again, this is a case that shows the differnece between the design and method of construction.

 

[hokie66]

The contractor's drawing is better than the original design drawing. However, the horizontal leg of that U bar under the trench should not be that high, it should be down below. And the diagonal bar at the setdown should extend to the top.

 

[a10jp]

Thanks. The horizontal bar under the U-trench is currently below the first pour surface by approx 4". In any case, even with the rebars, does anyone feel the contractor proposal, which has more rebar to account for the 2 different pours (or 2 layers), is any more inferior than the original monolithic pour design. Is the cold joint between the 2 layers an issue? And the concern about hte lateral stress due to earthquake? I am assuming the contractor picked this method is because this allows him to sit the U-trench propoerly before he before the top layer. I am not certain if putting up a form to sit the U-trench is easily doable because of the weight of the U-trench, and also if it is easy to make sure the pour underneathh the trnech will be a good mix free if air voids, when pouring underneath the u-trench's form that is not easy to get access to. WHat's your opinionon that?

 

[hokie66]

The contractor's approach is definitely better than the original design.

 

[a10jp]

Hi Hokie66, thanks. I am very happy to hear that opinion.