Concrete Repair 2

[concreteworld]

Hello Gentlemen,

We have a case where bottom 1 meter of concrete wall had late setting and did not achieve required strength at 28 days. Came out only 10% of fck. The bottom 5 meters is absolutely ok. Is there any quick method to remove the bottom 1 meter portion of concrete and replace with new concrete without affecting the stability of top 5meters of wall and such that both top and bottom repaired portion act monolithically.

Thanks in advance

 

[TehMightyEngineer]

Hydrodemolition is the best way to go for large-scale demolition to a prescribed depth. Powder actuated fasteners and/or a bonding agent will allow the replacement coating of concrete to act monolithically.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[KootK]

If this is a wall, I imagine that you'll leave the existing rebar in place when you demo the lower 1m of concrete. As such, the existing rebar itself should assure sufficient monolithic behavior so long as you have a reasonably level demolition joint. The trickier questions is how to pour and consolidate the new concrete properly. If your project can tolerate the lower portion of the wall being 6" thicker than the upper, then that's one alternative. Another is to use self consolidating concrete pumped in from the bottom with some way for air to escape at the top. For a one meter height, shrinkage of the lower bit relative the upper ought not be a problem.

We would need to know more about your situation before commenting on temporary stability of the upper wall. How tall is it? Are there perpendicular walls nearby? Is there a wall above? It might be wise to post an elevation and a section of the wall under consideration. One option might be to make your repair segmentally, similarly to how we underpin foundations.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[concreteworld]

Thanks all for your valuable comments.

The wall dimensions are 6m x 3m x 5m height and thickness is 25cm. The core wall will extend to another 3 floors and adjacent slab is resting on the bearing of this wall. It is used for lift of the building. I have attached the line sketch of the case.

Is it structurally safe to do epoxy injection all along the affected portions of the wall and later take cores to check strength increase? I do not fully endorse such proposal as epoxy injection is more suited where cracks have developed and due to excess voidage in concrete the strength has declined. The proposal of epoxy injecting the whole wall came from the consulting engineer. So, kindly give your comments on this proposal.

Thanks !

 

[KootK]

With the arrangement that you've shown, it should be a relatively simple matter to demo the low strength concrete and cast in new concrete. I'm not all that familiar with this kind of epoxy injection but it raises some concerns for me. The issue here is not crack filling, it's under-strength concrete. Unless the epoxy will somehow strengthen the in situ concrete that will remain, I don't see how the strategy would be effective.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TXStructural]

Has the concrete stopped developing strength? If there is any chance that it will reach sufficient strength with more time, I would probably wait.
Does the design have some latitude? Many times, especially in thick walls, the design may not be governed by concrete strength.
I do not know how epoxy injection would do anything to improve this situation, or even how you would introduce epoxy into an uncracked mass of concrete without voids.
If the concrete is only 10% of required strength, the demo contractor should be able to determine the extent of that weakness as they demo. Do not rely on the rebar alone to hold up the concrete above. You may want to demo and replace in stages, doing one meter wide portions at a time to maintain stability. (Possibly demo and replace in two or three stages.) You can fairly easily remove a "pocket" of concrete. Verify integrity of the reinforcing, install formwork against the face of the remaining concrete. Provide a port on the form into which repair concrete can be pumped and a port at the top for air to escape. Use non-shrink repair concrete (shrinkage compensating, pea-gravel mix with water-reducing admix.) An all sand mortar material can be used, but this could result in more shrinkage.
Once each repair is done and at sufficient strength, demo the next portion.

Make sure the contractor doesn't add water when s/he can't pump the material (ask me how I know that.) Doing so will simply make the mix weak and even less able to be pumped. A good high range water reducing admix (polycarboyxlate) will keep aggregate in suspension and reduce shrinkage, as well as virtually eliminating the need to vibrate the mix once pumped. You will want to beat/vibrate the formwork to assure consolidation at the new surface. Also, you do not need the repair material to bond to the adjacent concrete that will be demo'ed, but you do not want the dry concrete to take moisture from the fresh concrete, so either saturate the existing concrete or selectively apply a sealer. You will want to either apply want a bonding agent above and below or wet the adjacent surfaces to encourage bond. One good method is to install the formwork and then pump it full of water and allow it to sit for a period before draining. Allow it to dry until the surface does not have standing water ("saturated, surface dry").