Frostwall slab design, is this way acceptable?

[flintstone]

We sell pre-engineered metal buildings for commericial projects. We have designed our foundations based on the type of form work we use. Our forms consist of 24" tall x 8'-0" plywood panels held together with a plastic tie system that is light weight and idiot proof for ease of installation. We pour the main footings for the building columns first then the second pour is the frost wall and piers which are poured down inside the 24" form to 18". The frost walls are re-inforced with 4 #5 bars horizontally and 5' long #5 bars @ 24" Centers which we bend over into the slab about 3'-0" (+/-). I have had really nice jobs from this design (which was stamped by an engineer) and I have had severe cracking problems at the perimeter with basically no change in procedures of installation. Does anyone have any thoughts or suggestions on my design? I really like my form system but cant figure out why it works great sometimes and other times not.
thank you.

 

[RiBeneke]

You have not described exactly where the cracks occur. It could be something related to the concrete material, or it could be a structural issue.

As far as concrete material is concerned, the mix should contain all the right proportions of cement, sand, stone and the water content is especially important. The concrete should also be properly cured. There have been several discussions in the 'Structural Engineering Other Topics' and 'Concrete Engineering' forums on the subject of shrinkage and curing. There are also site practices that could lead to cracking.

As far as the structural issues are concerned, one needs to check placement and lap of reinforcement, physical shock or differential movement while the concrete is young, and the overall design.