Doweling into existing slab 1

[LeonhardEuler]

I have a situation where a new slab is going in between two existing ones. There are dowels going into the left hand side, because those slabs will be sharing the same load. My questions are as follows.

1) Does the new slab have to be doweled into the existing slab on the right? Could surface preparation and vertical dowels through that cantilever section be enough? Or does it need shear dowels to connect the two. Intuition tells me it does, or differential settlement will cause that cantilevered piece to break off.

2) If the slab needs to be doweled in on that side I am concerned that the new dowels will hit the existing stirrups. Is there a good way to word how to avoid this on the design drawing? "Dowel every 12 inches, if existing bar is struck move x inches and continue pattern"?

 

[LeonhardEuler]

Perhaps I should reform the question. I know I need to dowel into the existing slab. I am worried about striking existing bar and damaging the existing pad. How can I limit the damage of existing pad/bar while installing the new dowels? Should I specify abandoned holes to be filled?

 

[Ron]

First, you have too many dowels. You only need one in cross section. It should be one inch in diameter, hardened steel, smooth and one side should be greased to allow lateral movement from shrinkage and thermal movement. As for the cantilevered section....get rid of it!. Sawcut the existing concrete back to a vertical surface and dowel in as on the other side.

 

[LeonhardEuler]

Understood for one dowel. I am not sure why I drew it like this.

The reason I am not cutting the existing concrete to vertical is because there is an operational rail on top of it. This addition stretches a fair distance length-wise and I would need to tear up the rail and replace it if I were to do this.

 

[SlideRuleEra]

How thick is the existing slab (the slab with truck traffic)?
Will the new slab be the same thickness as this existing slab?

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[LeonhardEuler]

It's 3'. It's thicker than the slab on the right by 10".

 

[SlideRuleEra]

How thick is the proposed new slab?

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[MotorCity]

For a slab (more like a mat) that is 3' thick, there must be significantly heavier loads than a semi truck as you show. If that's the case, I would suggest more than 1 dowel to transfer the load.

 

[LeonhardEuler]

I'm sorry. The new slab is 3' and is thicker than the one on the right by 10". The trucks are carrying much heavier cargo than typical applications.

 

[SlideRuleEra]

LeonhardEuler - Per MotorCity's comment, a "slab" that is 3' thick must have tremendous loading. The only commercial applications that I know of may be runways for the heaviest aircraft or a shipping container terminal / intermodal rail yard. IMHO, dowels could do more harm than good for this retrofit project... slab settlement under load could cause any reasonable number of dowels to shear or crack concrete. I would not use any.

Suggest getting an experienced geotech involved to design the subgrade. Even then, any needed excavation / backfill will be a challenge to keep from undermining the existing concrete. Predict the slab's settlement and construct with the finish surface "high" by that amount. To make construction somewhat easier, don't try to place the entire 3' thickness in one placement. If calculations permit, use a substantial overlay for the finish surface. We have used this type foundation / overlay approach on the heavily loaded portions of our electric generating stations.

Expansion / isolation joints allow the new concrete to settle without disturbing the existing concrete on either side. Omit the proposed cantilever.

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