Bonding Strength of Concrete in Tension

Question

Bonding Strength of Concrete in Tension

umrce (Structural)

(OP)

2 Oct 20 16:24

I'm evaluating a demo for a contractor and we're investigating whether or not they could lift a non-composite slab off a precast beam in sections. One concern I have is that the flanges of the PCB are fairly thin so I was going to check for an uplift condition as they "pop-off" the slab. My problem is that I can't find any reference material regarding the bonding of plain concrete in tension to determine this uplift force.

The closest I've come is AASHTO includes cohesion of two concrete surfaces when calculating interface shear (8th Ed., Section 5.7). The cohesion of two smooth clean, concrete surfaces is 0.075 ksi. My only reluctance to using this is value is that it's intended for the two surfaces to be resisting a shear force along the bonded surface as opposed to a tension force acting perpendicular to the bonded surface.

Any help would be greatly appreciated.

r13 (Civil/Environmental) · Answer 1 · 2020-10-02 18:36:42

I believe concrete does adhere to surface with micro roughness/surface irregularities, but I think it is rather difficult to find literature/report on this matter. The investigation should be conduct in a lab with controlled measurements, not in the field over real structures. The concept, IMO, is rather simple - with the slab weight is known, it is therefore the theoretical lift load limit. Every effort beyond, can be contributed to the effect of concrete adhesion. Thus if the calibrated slab weights 1000 lbs, and you intend to give a maximum effort (to stop) of 1100 lbs, the 100 lbs difference can be distributed over the PCB flange to calculate the reinforcing to resist the load. But do you actually need this reinforcement, if only the potential to lift off is the focus of this investigation?

hokie66 (Structural) · Answer 2 · 2020-10-17 11:12:29

I suggest that this is beyond the realm of calculation. The principle which applies is that if you want it to bond, it won't, and if you want it to release, it will bond.

Ron (Structural) · Answer 3 · 2020-10-17 23:14:03

Core through the slab and slightly into the precast beam. Bond a plate of the same diameter as the core and do a pull test on it. Do it in several places and average the values. This method is also mentioned in ACI

BridgeSmith (Structural) · Answer 4 · 2020-11-07 15:23:05

I believe the AASHTO values are all minimum values based on shear loading. I'm not sure if even the cohesion value would be valid, or even close, for max tension.

I suggest that you have them lift the sections from the ends of the sections, adjacent to the girder, at least for the first few sections, until the actual tension force required is established.

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

dik (Structural) · Answer 5 · 2020-11-08 05:40:49

I would find some other way to do it... with a cold joint loaded in tension, I would suspect the 'scatter' of results would be so high as to void any standard deviation modelling... best to use some kind of mechanical fastener. Sort of concur with Hokie... If you're looking for some means of separating the surfaces, I would undertake a test with equipment that could develop a force of a few hundred psi... or did I miss something?

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

-Dik

ChooChooBridgeEngineer (Structural) · Answer 6 · 2020-11-20 15:29:05

Like others have indicated above, I'd avoid this method as well. Depending on what you are picking the slab with, there would likely be a 'pop' or release of energy. That would shock load the rigging you have and whatever you are picking it up with (crane, excavator, etc.). Likely some significant risk associated with that type of demo.

Lomarandil (Structural) · Answer 7 · 2020-12-08 12:17:45

The method can be done -- especially if you do some selective sawcutting or light breaking over the precast girder lines before working to pry up the slabs.

There is some impact to the operation, but a good operator will keep it manageable (<25%). The bond can be substantial though -- it's not uncommon to see a smaller excavator up on the toes of his tracks to pry up the slab (the governing condition you would need to check on the remaining structure).

I would expect damage to the PC girder flanges, pieces to spall off or bond to the deck. How I'd account for that in the analysis would depend a little on whether those flanges contain any integrity reinforcing.

----
just call me Lo.

PEinc (Geotechnical) · Answer 8 · 2020-12-08 13:15:59

If the slab is not doweled or studded onto the precast beams (non-composite), maybe you could first saw cut the slab into sections and then apply a jacking force to the underside of each slab section to remove sections one at a time.

www.PeirceEngineering.com

epoxybot (Structural) · Answer 9 · 2020-12-09 00:49:27

I tried looking at what Tilt-Up contractors do when they get a panel that is stuck to the slab. There don't appear to be any great solutions. They get good results when it is just water that is holding the panel to the base slab but when it is a failure to apply bondbreaker or too little bondbreaker, no good solutions.

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Bonding Strength of Concrete in Tension