Slab On Grade Problem

[Enginator]

We're trying to assess the integrity, or come up with assumptions for analysis with regard to an existing slab-on-grade at our site.<br><br>Due to radiological contamination reasons, we plan on pouring a new slab over the existing slab.&nbsp;&nbsp;Removing the old slab is not an option.&nbsp;&nbsp;The new slab will have to endure dead loads induced by six 44,000lb pieces of equipment, each having a 5ft. x 5ft. foot print.&nbsp;&nbsp;The existing slab is about 60ft x 50ft, while the new slab will be 10-20 feet larger on one dimension.<br><br>We have absolutely no information on the existing slab regarding reinforcement, actual thickness.&nbsp;&nbsp;We are required to design to UBC, allowing for seismic loads.&nbsp;&nbsp;Any suggestions on valid assumptions that can be made with regard to the existing slab?&nbsp;&nbsp;Any suggestions how to approach the design?&nbsp;&nbsp;For instance, should we embed I-beams into the new slab to span the old slab in order to have sufficient capacity to support the 44,000lb equipment?<br><br><br>

 

[Qshake]

Since you've mentioned the idea of spanning existing slab as an option, I would recommend that alternative.&nbsp;&nbsp;It would eliminate the need for any information whatsoever on the existing slab.<br><br>The new slab could be constructed monolithically with beams oriented parallel to the short dimension for load transfer to grade beams constructed along the perimeter of the old slab on grade.&nbsp;&nbsp;Using a 1/2&quot; or 3/4&quot; sheet of preformed joint filler could act as a bond breaker thereby allowing the new slab and beams to be placed on top of the existing minimizing the profile.<br><br>Of course, depending on the site, this only one alternative.&nbsp;&nbsp;If this is an outside application, perhaps the slab could be demolished and incorporated into the subgrade.&nbsp;&nbsp;After placing and compacting a lift of leveling course (base rock) and new slab could be placed.<br><br>To answer the question of strength for the existing concrete slab...very little concrete was specified, designed, or placed which was lower than 2500 psi with 3000 psi being the most common especially for grade applications.&nbsp;&nbsp;Since concrete gains strength as it ages it is not uncommon for us to use a factor of 1.5 for 20 to 30 year old concrete (ususally we use this for determining the stiffness rather for strength applications).&nbsp;&nbsp;However the conservative approach is to use the 3000 psi.&nbsp;&nbsp;As a last resort you can have the concrete cored and tested.&nbsp;&nbsp;But I don't think that is worth the effort in this application.

 

[Ron]

Look at the existing slab to generally assess its integrity.&nbsp;&nbsp;Is it cracked extensively?&nbsp;&nbsp;Is the crack pattern close (less than 5 feet c-c)?&nbsp;&nbsp;If so, probably some load inducement, otherwise probably just shrinkage.&nbsp;&nbsp;Look at the crack widths.&nbsp;&nbsp;If greater than about 1/8-inch, will likely reflect through topping slab.&nbsp;&nbsp;Check for faulting at the cracks.&nbsp;&nbsp;If faulted or if extensively cracked, would look at doing crack and seat program on existing slab before placement of new slab and provide a geotextile separator between the slabs.&nbsp;&nbsp;Treat the cracked and seated slab as a stiffened subgrade or subbase.<br><br>44,000 lbs is not a great load on a 5'x5' footprint (1760 psf), so unless the load is dynamically enhanced (rotating equipment, etc.) would only be concerned about shear at the edges of the loading (loaded vs. unloaded).<br><br>Agree with Qshake about strength.&nbsp;&nbsp;Probably in 3000 psi range and usually ok to assume so, but if treated as cracked and seated subbase, strength not big issue.&nbsp;&nbsp;Would only use the embedded beams as last resort as this is expensive and probably significant overkill.&nbsp;&nbsp;Haven't run through the numbers, but you are likely looking at a 5 to 6-inch slab thickness.

 

[prex]

Why not simply treating the problem as if the exixting slab was not there?<br>This seems to me a quite safe assumption, as the existing slab should be perfectly seated and presumably free from very large invisible defects; it will therefore behave better than the soil underneath. <p>prex<br><a href=mailto:motori@xcalcs.com>motori@xcalcs.com</a><br><a href=

http://www.xcalcs.com>Xcalcs</a><br>Structural

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[Qshake]

In response to prex and Ron,<br><br>Since the proposed slab exceeds the footprint of the old, the new slab could exhibit reflective cracking due to the differential movement of the old slab and soil.<br><br>Thus it seems best to span or demolish the old slab and include it in the new subgrade.&nbsp;&nbsp;

 

[martinb]

It appears to me that your responses have not taken notice of the fact that the existing slab is contaminated and that in your considered opinion removal/break-up of the existing slab is neither an option nor negotiable. Therefore there is no alternative but to clear span the existing slab and negate the use in any form of making contact with any permiuable fabric between the two elements.<br>Your considered use of integrated beams (in whatever form) appears to me to be your only option.<br>Best of luck from down under<br>

 

[Qshake]

martinb,<br><br>Spanning the existing slab with monolithic beams which would transfer the load to grade beams around the perimeter have been suggested, see above response.<br><br>I don't know why the existing slab couldn't be broken up and incorporated into the immediate subgrade.&nbsp;&nbsp;The contaminated slab or in this case, parts of the slab will remain in the same place.&nbsp;&nbsp;

 

[Enginator]

First, I'd like to thank you all for responding to this question and giving me such informative and useful responses.<br><br>To clear things up regarding radiological contamination concerns, I'd like to say something about incorporating the slab into the grade.&nbsp;&nbsp;Yes, this could be done, but at high cost.&nbsp;&nbsp;The possibility of airborn contamination becomes great, which means a very large containment tent would need to be built and all laborers would need to be on supplied breathing air (per requirments at this site, Hanford Washington).&nbsp;&nbsp;Containment and supplied-air factors can increase cost by 10 times or more. There are similar, though less severe concerns with core-drilling/sampling.<br><br>It looks like I will need to perform a cost-benefits analysis to weigh the cost of core-drilling/sampling against spanning.

 

[Ron]

John,<br>A crack and seat program would not produce significant dust or other debris and is not a process by which the slab is incorporated into the subbase.&nbsp;&nbsp;It is simply running a large compactor or similar heavy equipment over the slab to force cracking to occur and seat the pieces of the slab into the subbase.&nbsp;&nbsp;This technique is commonly used in pavement construction.&nbsp;&nbsp;Reiterating that your loads are relatively low, I suggest that you explore something of this nature as it is very cost effective, does not cause inordinate disturbance to the existing contaminant, and would significantly reduce the amount of time workers would be exposed to contaminants, which should be a consideration in any selected method of remediation.

 

[Qshake]

Often times dust is mitigated with wet demolition. However, I understand the problem and believe that spanning the existing will prove to be the best option.

Good luck.

 

[Ron]

John,
This is a nice, dynamic discussion of your issues. Let us know what you ultimately decide to do, and if you can, your reasons for your decision.
Ron