While cleaning the sidewalk in front of my house the other day, I noticed that what at first appeared to be a divot in the concrete was actually the concave side of a fairly substantial shell, in pretty good condition embedded in the walk. I know that concrete contains gravel and the like, but the shell is much bigger than the surrounding aggregate. How did such a thing survive the mixing process without being crushed? There are several other shells in the same orientation nearby. In addition, I found a number of similar shells when digging for a walkway in the back yard. Perhaps they are native to the area, and whoever laid the walk found them locally and added them in on-site?
My guess would be that it was simply a pretty hard shell. (Have you tried picking at it? Could it be fossilized?) The mixing of aggregate with portland cement does not actually involve much “crushing” action. The stuff gets tossed around (rather slowly) with a fair amount of water to create a slurry. Depending on the sources and local methods, the aggregate can either be natural or be a deliberate mixture of “pure” gravel and “pure” sand mixed to a specific texture to be combined with the cement. If the shell had initially been packed into a block of sand, by the time the water and churning had washed the sand off, it could easily have been suspended in a slurry that reduced its chances of being hurled from one side of the mixer to the other with shattering force.
Given your location, I would guess that the sand is taken from the Chesapeake Bay where i would expect to find a lot of shells. You might stroll along the walk and see how many shells are only slightly smaller. This one might be larger than most, but it is still not extraordinarily large. It is shorter than three inches.
Is the Chesapeake Bay salt water or fresh? Salt I’d think, at least for a large part.
I remember an episode of This Old House where they were working on a house in Santa Barbara CA. The foundation was deteriorated because when it was constructed, they used beach sand to mix the concrete and the salt contained in the sand caused the problem of crumbling.
Maybe they can take sand and shells as well, from the bay and wash it with fresh water to remove the salt.
That way you could have the shells as well as non-crumbling concrete.
I could be mistaken, but it looks like asphalt that the shell is in . I’ve never seen concrete with such large chunks in it. Maybe it’s an east-west coast thing ? In any case I don’t believe the processe to make asphalt is the same as concrete.
No, but the source of the aggregate usually is. Around here, gravel for aggregate is mined from river beds and graded according to size. Different mixes of either asphalt or concrete require specific size of aggregate. I wouldn’t be too surprised to see a freshwater clam shell in our aggregates here; possibly your source was from a mine closer to a marine source, and therefore has what looks to me like a mussel shell.
I guess that (the source ) makes a difference. Thanks for fighting my ignorance 
A lot of old buildings on the east coast are made from “tabby block.” It’s concrete with shells for the aggregate. The tabby I saw in Savannah, Georgia was well over a century old.
What strikes me as strange is the shell’s orientation. It almost appears to have been pressed into the concrete as an afterthought since the bivalve edge is essentially flush to the concrete surface. Were it to have been part of the mixed aggregate, you’d think it would have been at least partially submerged and that certainly the inside of the cusp would be filled with concrete matrix too. If, like the OP says, additional ones lay in the same manner then I’d suspect these were laid after the concrete was poured.
As to an asphaltic appearance, it’s probably due to mold/mildew. A good powerwash and it’s concrete nature would probably resurface.
Most amazing though was that his neighborhood uses new quarters as aggregate in their sidewalks. Now that’s classy!
I’d vote for asphalt. The shell being thrown in when it was being rolled. If it’s concrete, it’s the poorest I’ve ever seen.
The quarter appers to be ON the surface as opposed to being in the surface and was included in the photo to give a sense of size.