Raise The Roof 2

Picture with lintel...

I believe it is drying/plastic shrinkage along the exterior longitudinal bars (acting as restrains), formed in the early stage; also the lack of stirrups to confine its grew in size.

Retred13 probably is right. Likely the concrete used for the lintel was soupy liquid, lots of water in it. In addition to the cracks, the concrete likely is very weak. However the arching action of bricks will save the day for the increased height. I'd not worry about the crummy lintel.

Thank you for looking at the lintel. What about part 2 of my question? Do you think full height channels are overkill?

Gotta disagree on the lintel. Those cracks are in the wrong direction for shrinkage cracks. Older lintels were often made with rebar on top and bottom because they actually had no top or bottom so they could be installed either way. I would bet on water intrusion causing exfoliation of the rebar and thus, expansion of the rebar and cracking of the concrete. The crack follows a natural line consistent with rebar placement.

The lintel and the concrete masonry are obvious repairs. It is likely the wall is a two-wythe brick coursing. Tieing the additional masonry to the original masonry can be done, but will be difficult and how far down would you take the tie-in. Are you planning to continue the brick coursing for the additional height? Is there any attempt to maintain the historical integrity of the structure? Are there any local codes or ordinances that might affect this addition to an old structure?

Since your addition cost will likely exceed the Existing Building structural thresholds, you will likely be required to comply with current codes for structural and roofing considerations. Inside channels would likely be your only choice in that case.

The reason I was reluctant to call it a weathering crack is due to the lack of rust stain, which is usually associated with cracks caused by water and moisture. The crack due to poor concrete quality should look like the sketch below on both faces of the lintel.

After adds on, the wall needs to be checked for the lateral load effects according to design code. I am not sure the channels would work on one side only. You need to throw some numbers to figure it out.

Sketch.

Lintel cracking sure looks like the result rebar corrosion and swelling. I've seem plenty of that and rust stains are not commonly evident.

Regarding the clerestory -
11 or 12 ft tall plus another 3 ft seems awfully tall for 8" thick unreinforced brick. I would expect steel components would probably be needed on interior and exterior faces.
Curious how many windows would be involved and how to tie the roof diaphragm into the walls. Looks like you're in Maryland so... don't know about seismic loads. Seems like something will be needed to "tie" the whole thing together vertically.

Cracks due to rust of steel will likely to have other indications - usually on exterior exposed, or surface subjects to constant/cyclic intrusion of moisture; and uneven surface due to the push out by volume expansion of the rust.

Retired, I have inspected hundreds of cracked/spalled/delaminated concrete and apparent rusting on the surface of the concrete is actually a rarity.

I too would expect plastic shrinkage cracking to be vertical. I'm not entirely convinced that it is due to corroding steel but it does seem the likely culprit. There is a very simple way to check this. Take a standard framing hammer and "hammer sound" the surface of the concrete all over. The concrete should make a "Tinging" sound when you strike it. If it makes a hollow "Tong" sound when you strike it, it means that the concrete has delaminated from the rebar probably due to corrosion.

Yes. Rust expansion causes separation and lose bond. The plastic/drying occurs right after concrete has started to set, usually form pocket directly under the rebars. It should appear on both faces of the beam.