Monolithic slab-on-grade joints

[McQSE]

I am interested in how other engineers detail their monolithic grade beam/slab-on-grade foundations. I am in a highly expansive clay region and I design my foundations using the WRI method. How do you limit cracking for your monolithic foundations? Sawcut contraction joints would be what I would typically use for a slab-on-grade application if I was not on expansive soil, but it seems this contradicts the idea of a monolithic foundation. ACI 360R-06 recommends a minimum steel ratio of .5% of the slab cross-sectional area in order to eliminate sawcut contraction joints. For a 4” deep slab that would put me at needing 0.24 in2/ft of reinforcement (#4 bars @ 10” o.c.). This seems real high. Any thoughts would be appreciated.

 

[SpiderM]

What part of the country are you in? I am in the DFW area, which has varing levels of expansive soils. I have had success with the equivelent area method in conjunction with saw cuts; which if I recall correctly, is #4's @ 14" for a 4" slab. Of course, some soil treatment was required to stabilize the soil. So, the reinforcement was essentially crack control for shrinkage. This is a difficult subject because most contractors (on residential projects) have quoted the foundation based on #3's or #4's @ 18" o.c.e.w. So, you mostly likely will have a heated discussion with the contractor on the topic. I hope that helps.

 

[McQSE]

I'm in Mississippi. The P.I. can get up to around 60 in this area. This is a relatively small commercial building. The geotechnical report recommends cutting 7’ down and 7’ out and filling back with soil with a maximum P.I. of 24. That seems to be the standard that all the geotechnical firms use around here unless you are going to a deep foundation.

For crack control on a foundation where I was not overly concerned about soil movement, I would typically use sawcut joints and discontinue a portion of the reinforcement at the joint. I don’t think this method would be good for stiffened foundations that have been designed with the anticipation of soil movement.

How do you detail joints for a stiffened foundation? How do you lay them out? Similar to the example in the attached file?

 

[SpiderM]

I appoligize to the tangent, but here in the DFW area there has been a few lawsuits relative to the "remove and replace" method. The ones I specifically know of, had select fill (PI<=15) used to replace the existing. The problem comes down to creating a "bath tub". The select fill would allow water penetration and the moisture would leach into the adjacent expansive soils. As a result, the surrounding areas would swell, causing more surface water to drain toward the building and providing more moisture. The goetechs, typically, have a general statement in their reports to provide adequate subgrade drainage. So, the engineer of the foundation was on the hook for the problem. You might study that a little bit. Just an FYI, perhaps the PI=24 eliminates the problem.

As for our topic, I would refer you to a book called Designing Floor Slabs on Grade: Step-By-Step Procedures, Sample Solutions, and Commentary it was very helpful to me.

I have to go, but if you still need additional information, let me know and I'll find the methods suggested in the book.

 

[McQSE]

I have had that conversation with a few of them in the past and their stand is that specifying the fill to have a PI of 10-24 and more than 50% fines passing the No. 200 sieve will eliminate the issue.

I appreciate your input and I will check out that book.

 

[hokie66]

McQSE,

Removing 7' of expansive material below and out from the building sounds like a solution, and I suppose in your area it may be economical. In other areas, I suggest a deep foundation with void formers under the slab would be preferred.

As to joints, your slab is either a stiff raft or it's not. Sawing joints in a raft makes no sense.