We wanted this house even though it was not in great shape. We knew we had to figure out why the slab had cracked so badly and repour a slab that would not have the same damage in the future. We found the WaffleMat solution for expansive soils but their engineer, Greg Carr, strongly recommended an analysis of the swell potential of the underlying soil to engineer the best design for the use of the wafflemat boxes under the concrete. The report came back with relatively good news. The final analysis was that the swell potential was only 1 to 3 inches under existing moisture conditions. However the swell potential was much higher when the samples were wetted under pressure. Vertical expansion was between 2.2 and 8.1 percent. Although the crawlspace floor is recommended for expansive soils, the report found that the WaffleMat slab presented only a low risk of floor heave between 1 and 3 inches.
The recommendations for the new slab included:
1. Separate slab from all bearing walls and columns with expansion joints that allow unrestrained vertical movement.
2. Minimize the use of slab bearing partitions. Drywall should not extend to the floor. Partition walls should be able to move independently of the bearing walls.
3. Control joints in the slab should be used to reduce damage due to shrinkage cracking.
4. Where plumbing lines enter through the floor, a positive bond break should be provided as well as flexible connections for mechanical equipment allowing at least 3″ of vertical movement.
5. Loose, non-compact soil should be removed and replaced with compacted non-expansive granular fill.
6. Excessive drying of the floor slab subgrade should be avoided before and during construction.
7. Install a 6 mil or approved vapor retarder with joints lapped not less than 6 inches between the floor slab and the base course of the subgrade.