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Monolithic slab-on-grade joints

Monolithic slab-on-grade joints

Monolithic slab-on-grade joints

(OP)
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.   

RE: Monolithic slab-on-grade joints

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.

RE: Monolithic slab-on-grade joints

(OP)
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?  
 

RE: Monolithic slab-on-grade joints

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.

RE: Monolithic slab-on-grade joints

(OP)
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.   

RE: Monolithic slab-on-grade joints

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.  

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