Slab On Grade Problem
Slab On Grade Problem
(OP)
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.
Due to radiological contamination reasons, we plan on pouring a new slab over the existing slab. Removing the old slab is not an option. 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. The existing slab is about 60ft x 50ft, while the new slab will be 10-20 feet larger on one dimension.
We have absolutely no information on the existing slab regarding reinforcement, actual thickness. We are required to design to UBC, allowing for seismic loads. Any suggestions on valid assumptions that can be made with regard to the existing slab? Any suggestions how to approach the design? 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?
Due to radiological contamination reasons, we plan on pouring a new slab over the existing slab. Removing the old slab is not an option. 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. The existing slab is about 60ft x 50ft, while the new slab will be 10-20 feet larger on one dimension.
We have absolutely no information on the existing slab regarding reinforcement, actual thickness. We are required to design to UBC, allowing for seismic loads. Any suggestions on valid assumptions that can be made with regard to the existing slab? Any suggestions how to approach the design? 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?






RE: Slab On Grade Problem
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. Using a 1/2" or 3/4" 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.
Of course, depending on the site, this only one alternative. If this is an outside application, perhaps the slab could be demolished and incorporated into the subgrade. After placing and compacting a lift of leveling course (base rock) and new slab could be placed.
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. 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). However the conservative approach is to use the 3000 psi. As a last resort you can have the concrete cored and tested. But I don't think that is worth the effort in this application.
RE: Slab On Grade Problem
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).
Agree with Qshake about strength. Probably in 3000 psi range and usually ok to assume so, but if treated as cracked and seated subbase, strength not big issue. Would only use the embedded beams as last resort as this is expensive and probably significant overkill. Haven't run through the numbers, but you are likely looking at a 5 to 6-inch slab thickness.
RE: Slab On Grade Problem
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.
prex
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RE: Slab On Grade Problem
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.
Thus it seems best to span or demolish the old slab and include it in the new subgrade.
RE: Slab On Grade Problem
Your considered use of integrated beams (in whatever form) appears to me to be your only option.
Best of luck from down under
RE: Slab On Grade Problem
Spanning the existing slab with monolithic beams which would transfer the load to grade beams around the perimeter have been suggested, see above response.
I don't know why the existing slab couldn't be broken up and incorporated into the immediate subgrade. The contaminated slab or in this case, parts of the slab will remain in the same place.
RE: Slab On Grade Problem
To clear things up regarding radiological contamination concerns, I'd like to say something about incorporating the slab into the grade. Yes, this could be done, but at high cost. 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). Containment and supplied-air factors can increase cost by 10 times or more. There are similar, though less severe concerns with core-drilling/sampling.
It looks like I will need to perform a cost-benefits analysis to weigh the cost of core-drilling/sampling against spanning.
RE: Slab On Grade Problem
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. 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. This technique is commonly used in pavement construction. 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.
RE: Slab On Grade Problem
Good luck.
RE: Slab On Grade Problem
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