Construction Joint provided at location of Maximum shear and Moment's demand 2

[M.IDR]

A contractor has provided construction joint at the location in Raft construction where the shear and moment is maximum at that location. So, any one does have any solution how can it be verified, if this construction joint is feasible or it needs rehabilitation?

By considering shear friction design based on ACI-318-19 sect. 22.9, the bottom steel is not enough to resist the applied shear demand coming at the location of construction joint. The demand value of shear is shown in the figure below

The design section detail is provided in the the section F1-F1

The Calculations are shown below, as the surface of the joint is not roughened intentionally rather it is smooth, so mu= 0.6,
By only considering the Bottom steel (in Tension), the shear friction capacity is not enough to resist the demand,
and i don't think that the top steel is contributing to shear friction.

​

phi Vn < Vu not OK

Malang_wazir

 

[dik]

I'm an L/3 type of guy... but Hokie's 'bang on'... for keyways, I usually bevel the edges of keys... makes it easier to remove them after the concrete has hardened.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[M.IDR]

@r13 The second pour will be cast in a few days. hookie66, Do you know how the epoxy should be used then for proper joint between hardened concrete and fresh concrete to make?

@dik Do you use extra dowel bars as well or just key is enough? any code reference for key dimension?

 

[hokie66]

In my opinion, epoxy in the joint serves no purpose. I think it may be just a sales effort to sell you some epoxy. What possible benefit can it achieve?

 

[rowingengineer]

Are we referring to bonding agents? Are we planning to mechanically scarify?

 

[r13]

Drill and grout dowels to take shear. And, you may chip out a pocket to receive a key in the second lift. I like one or two keys better than many small keys as you have shown previously. Bevel edges of the key to prevent diagonal crack. You should provide reinforcement on the second lift keys. Clean and wet the hardened surface before the second lift.

 

[BridgeSmith]

I agree with one big key, with a height about 1/3 of the thickness, and a depth of 1/6 of the width of the key. If it's going to be cut in and chipped out, or just chipped out, no need to have the edges beveled. For formed keyways, the edges definitely need to be beveled, in order to be able to remove the form.

No need for epoxy or bonding compound. At best, they'll only seal the joint, but won't make it stronger; at worst, they'll weaken it for shear.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[hokie66]

As to scarifying the face of the joint, it just needs to be clean.

 

[M.IDR]

Referring to the type of epoxy having 60-70 MPa compressive and Having more tensile strength than the concrete, while that of concrete strength is 20 MPa. So what would you think about this type of epoxy, won't it work. (Picture 01)

@r13 Can you provide additional information, how the dowels can be drilled and properly anchored? my concern is that concrete is strong and drilling is usually very difficult and while developing the steel would need deep drilling.

For concrete Surface preparation for shear friction where applicable using Guideline No.310.2R–2013-Selecting and Specifying Concrete Surface Preparation for Sealers, Coatings, Polymer Overlays, and Concrete Repair is the right procedure for achieving 1/4" amplitude surface roughness? to give µ(friction coefficient) = 1.0

Concrete surface preparation method for 1/4" amplitude surface roughness, Concrete surface profile (CSP 10) is ok?

 

[hokie66]

I have given you my opinion about bonding agents. They are unnecessary and potentially can function to decrease bond.

Rather than bond, you want to rely on mechanical shear transfer. The key and dowels should suffice at the point of minimal shear. If you want additional insurance, shot blast the surface as in your figure 6.3 or 6.5, and ensure the surface is clean of dust and contaminants.

As to the dowels, cast them in rather than drilling.

Shear friction? Not a concept I support.

 

[r13]

Wazir,

I guess your concrete is in the 3000-5000 psi range, drilling shouldn't be a problem. You can use Hilti reinforcing dowels, with epoxy to provide the required strength. The development length is usually shorter than direct embedment because the greater bond strength.

 

[hokie66]

But r13, what is the point of drilling the bars in when you can install them prior to casting? It makes no sense to post-install bars mid-depth when the top and bottom bars are already cast in and the top bars are in the way of access. And as for development length, we are talking about dowel action, not tension. Even if we were, the capacity of deformed bars in tension is primarily due to mechanical action of the deformations, not bond.

 

[r13]

Hokie66,

If I am not mistaken the situation, half of the mat has been poured without proper joint preparation by the contractor (it wasn't a designated CJ). OP is worried about the shear force demand (138 kips) across the plane, as it is weakened by the unintended joint. Also, with high moment demand, the existing steel can't be accounted for shear friction. So the option now is to add key and dowels, one or both, to make up the deficit in shear.

 

[hokie66]

Sorry, I thought this was a planned joint. I wouldn't like to be the guy with a big drill trying to get in there and drill holes.

 

[r13]

Yeah, that won't be a pleasant job. But the concrete is quite fresh, so could be easier to handle, but the woeful working position.