Concrete trowel patch repair bar cover question

[mfstructural]

I have a situation where I'm working with a contractor and owner on repairing concrete at the underside of an existing driveway slab at an apartment building. The concrete under about 60% of the slab had spalled and the exposed bars were corroded with some section loss. it is my understanding coming in now that this has been going on for a while, and repairs were made back in 2014 to the underside of the slab. I estimate the building is about 50 to 60 years old. Again, here we are after 10 years the concrete spalling again...the contractor has removed the spalled/affected concrete around the corroded bars to about even with center of the bars or to the top of the bars. Industry standards and details that I have put together in the past usually include a note that requires at least 1" of concrete above the bars to be removed prior to patch, to be able to properly clean the bars and cover them with the new concrete patch. In this case the slab is 12" thick and corroded bars are visible in both directions along the underside at the exposed areas. the amount of corrosion varies, some bars exhibit surface corrosion while others are extensively corroded. The owner does not want to break the bank on this repair, which is where my dilemma is. There is a decent amount of area to be repaired and they are requesting that the details show a trowelled patch repair where they don't have to remove the concrete 1" above the bar as it will likely take several months and drive up costs significantly. No sagging or cracking of the slab was observed during the inspection, so from a practical standpoint it has not displaced downward in spite of the corroded bars. Based on my inspection of the underside of the slab, I don't think the repair back in 2014 went above the bars either. The concrete exposed just above the top of the bottom most bar is solid with larger aggregate and I don't believe it is a patch. I am debating including the trowel repair on the drawings and noting that the service life of the repair will likely be reduced if the moisture intrusion through the top of the slab is not mitigated with the use of waterproofing. I would also include verbiage that the subject driveway section of concrete shall be monitored at 3 month intervals. As background this issue came up during an inspection of the building by the city and they received violations requiring an engineer to provide repairs and a report.

 

[JAE]

I would agree with removing concrete above the bars - standard procedure for most of my own repair efforts over the years.

The only other thing that you could consider is having them embed many small stainless steel pins that would stick out of the exposed concrete surface and provide at least some mechanical tie-in to the patch.

I've used 9 gage "L" pins before, epoxied into the concrete - for a 3/4" cover (if that's what you have) the pins would end up very close to the finish surface but they are stainless so cover issues might be neglected.

My "L" pin efforts were with columns and 1 1/2" clear situations though.

Also - do you know the condition of the remaining, exposed concrete? Does it have a high pH or chloride content?

 

[mfstructural]

I am showing stainless pins to be installed with a stainless screen along the bottom. My thought is that this will reinforce the repair. I do have petrographic results from testing of 4 cores. In the top two inches the values range between .07 and .126 and decrease to below 0.06% below two inches. Based on ACI published values stating that threshold value for conventional concrete is .06%. So the levels are elevated near the top of the slab.

 

[LittleWheels]

I guess that your petrographic percentages are referring to chloride content by weight of cement. Are the chlorides the result of deicing runoff, salt spray deposition or something else?

Part of the reason to remove concrete around the entire circumference of a corroded bar is for the repair patch to have a mechanical connection but the other reason is to ensure that the entire bar is within similar material. It is fairly common for concrete repairs to spark off rebar corrosion adjacent to the repair areas within 5 years = incipient anode effect.

https://core.ac.uk/download/pdf/288370201.pdf

Carbonation is another concrete degradation mechanism that allows rebar to corrode. Generally not a problem when the concrete is continuously saturated, more so in mostly dry conditions. You didn't find that to be an issue?

It is possible for partial depth patch repairs to spall off as the rebar starts corroding again, due to the difference in alkalinity (or chloride content) between the substrate and repair material.

 

[mfstructural]

That is correct the chloride is by eight of the cement. I would say the result of salting during the winter. The building is in the north. The rooms below the affected area are boiler rooms and laundry. I didn't test the humidity in those rooms. To answer your question I think the top of the slab exposed to the elements is relatively more saturated than the bottom exposed to the interior. I'm not sure to what extent that is affecting carbonation.