Grade Beam with Stepped Under-Pour 2

[waytsh]

I have designed a stepped grade beam alongside an existing strip footing. The top of the grade beam is at the same elevation across its entire length but the bottom makes two steps to match the bottom of existing footing elevation. The grade beam starts at 36" deep, steps down to 44" deep, and then makes a final step down to 60" deep. The contractor is asking if he can split up the pours with the first pour bringing all the steps up to the 36" depth. The second pour would then be a continuous 36" deep pour across the entire length. The reinforcing and 36" depth would work for design across the entire length if no steps were to occur. Do you think there would be a problem if I allowed an un-reinforced under-pour to act as a kind of bearing mat? I would then leave the reinforcing for the 36" depth across the entire length. I have attached a sketch of the proposed grade beam prior to the two pour modification.

 

[jayrod12]

I wouldn't personally be overly comfortable with an underpour like that. If he wants to split up the pours why wouldn't he just do it at the step locations?

 

[KootK]

I'd be fine with it. Better yet, make the lower pour lean mix concrete. I'm assuming that the intent here is simply to take the load down to native soil.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[waytsh]

KootK, yes that was my intent.

 

[BAretired]

So the reinforcement would run straight through as shown on the right hand Section X-X and the fill concrete would be unreinforced, right?

I see nothing wrong with that, but would want to do a sulfate test before recommending low strength concrete. Without further test results, I suggest keeping concrete strength the same as originally specified.

BA

 

[KootK]

@BA: can low strength concrete mixes not be made sulphate appropriate? That sounds like something that I may need to know about.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[waytsh]

BA, that's right. I've attached a revised detail showing how it would look.

 

[BAretired]

It might not be a bad idea to include some nominal dowels between the two layers of concrete. Also, the masonry wall needs to be braced against the lateral pressure from the fresh concrete being poured. Does the existing wall belong to your client or a neighbor? If a neighbor, there may be property line issues you should get sorted out before the pour.

On my very first project in private practice, the contractor poured a two foot thick vault wall against the neighbor's building using the existing wall as a form. The wall collapsed and everyone was sued (the owner was a lawyer). Best to avoid that possibility.

@BA: can low strength concrete mixes not be made sulphate appropriate? That sounds like something that I may need to know about.

Perhaps, I am no expert. But I recall a materials engineer telling me years ago that the best resistance to sulfate attack is high concrete strength. There may be other measures such as blends with fly ash.

BA

 

[waytsh]

The existing wall is a frost wall and is filled on the inside. Finish floor of the existing building is at the top of the existing wall. Original grade was about a course from the top of the wall. The existing wall also belongs to my client. It was a good point though, thank you.

 

[jayrod12]

For sulphate resistant concrete my experience is the same as BA. High concrete strength = low permeability = sulphate resistance. That's how it was explained to me by a mix designer.