by Nick Gromicko, CMI®
I was recently asked to look at a new construction issue where the 12,000 psi precision grout under a beam post plate had cracked after being poured.
The way these are normally installed is that the posts are made an inch shorter than the distance from the top of the caisson to the beam, and then a special non-shrink grout is poured under the plate of the beam post and the caisson. This creates an even and complete transfer of load from the beam to the poured concrete caisson, which, in this case, goes down 25 feet to bedrock.
Our worry was that if the grout cracked and a piece got dislodged, the post would not sit on top of the caisson correctly.
We came up with a solution. We welded a 1-inch solid rod to the posts in four locations and painted them.
We then built a concrete form around each post, added a rebar cage, and poured a cube of concrete that surrounded the base of each post.
This accomplished two things:
- The beam post was now hung by the embedded rods on the inside of the concrete cubes, which rested directly on the top of the caissons, and thus was no longer relying on the integrity of the grout.
- The concrete cubes encased the grout so that it could never become dislodged.
The engineers signed off on the plan and we completed the work in one day.
