sealing a threshold to concrete

[fredhargis]

I'm thinking Mac's suggestion of the backer rod and some sealant (likely a silicone based product) would work much better than any of the expanding foam stuff, and solve you of the problems that expanding foam might cause.

[mike4244]

I have a very similar issue with one of the doors on my shed. In my case, the builder didn't install a plate below the threshold. So, the threshold flexes and water can/does infiltrate. I've be thinking about sliding a piece of PVC trim under the threshold, machined to the appropriate thickness, then using a good exterior caulk to seal. I'm also interested in other ideas.

To stop the flexing and waterproof at the same time, use hydraulic cement. HD sells it in I believe a 2lb container. It sets up fast, only mix what you can disperse in 5 minutes. This is a powder, you add water. In tight places you can make the mix wet, like pea soup consistency. Where the gaps are larger mix it to a tooth paste consistency. If waterproofing is the only consideration, then caulk will do. 
Use a 2" putty knife to push the cement under the sill. You may need backer rod in the area that has a large gap. The backer rod should be pushed back to the back edge of the sill and cement under as much of the sill as possible.

[dav]

To stop the flexing and waterproof at the same time, use hydraulic cement. HD sells it in I believe a 2lb container. It sets up fast, only mix what you can disperse in 5 minutes. This is a powder, you add water. In tight places you can make the mix wet, like pea soup consistency. Where the gaps are larger mix it to a tooth paste consistency. If waterproofing is the only consideration, then caulk will do. 
Use a 2" putty knife to push the cement under the sill. You may need backer rod in the area that has a large gap. The backer rod should be pushed back to the back edge of the sill and cement under as much of the sill as possible.

Thanks, I'll consider that.