my father has a sports court construction business and I work with him infield and in the office. He usually almost always builds the sports courts using post-tension concrete with 4000 PSI concrete. He also includes turndowns in all the courts he builds. My question to this would be: Is it necessary or beneficial to use a turndown for the slab? The reason he uses post-tension is to avoid cracks which really does work marvelously, but is it necessary to have a turndown when there is no load on the edges of the slab? Can it actually be a negative component of such type of slab since if there is some type of soil erosion on the edges of the slab where the turndown is, this will cause an uneven load distribution of the slab itself on the edges, even though they're tensioned by the PT cables? Or is a turndown the right thing to do in this case? I'm not a civil engineer or anything but wanted to know if anyone had any insight on what would be best in this situation. Thank you!
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2 Answers
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If by turndown you mean a bulkhead at the edge of the slab, that is needed to absorb the forces of the post tension jacks, prventing the shear spliting of the slab.
The bulkhead is also the support for potential fence posts.
A detail would be very helpful.
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$\begingroup$ I do not think it is a bulkhead that I am referring to, rather an edge beam. Here is a foundation plan from the Post-Tension team. archive.org/details/pickleball-court $\endgroup$ Feb 18, 2021 at 20:18
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1$\begingroup$ @chromechris, yes it is a bulkhead. As can be seen in section A they use it to add distance from the tendon to the edge of the slab to help to prevent the shear split of the slab, horizontal cracking of the thickness of the slab. $\endgroup$– kamranFeb 18, 2021 at 20:37
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$\begingroup$ Thank you for explaining that. So then would a bulkhead be the same as an edge beam, just with different purposes? So adding vertical thickness to the edge of the slab helps prevent horizontal craking from the edge-face of the slab? $\endgroup$ Feb 18, 2021 at 20:52
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1$\begingroup$ @chromechris, exactly. 5 inches thinkness divided by 2 and subtracting the diameter of the jack is a cray for splitting. this is the primary reason of the bulkhead. $\endgroup$– kamranFeb 18, 2021 at 20:57
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1$\begingroup$ @chromechris, there are different ways of how to stabilize, keep the slab dry from subterranean moisture and drain the surface water from the deck. Depending on the soils and the expected seasonal rain, there are perforated plastic subterranean drainage and insulation membranes, or top four inches sand under the deck to collecting the surface runoff water and discharging it through swales under the perimeter filled with proper size aggregate. Where one pulls the permit they usually have standards or require a sois report addressing these issues.. $\endgroup$– kamranFeb 18, 2021 at 21:19
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In general, the edge beam (turndown) serves two purposes - prevent the tendency of curling at edges of the slab, and prevent loss of soil under the slab due to erosion. Ideally, the turndown shall be sit on the compacted granular material, that is free to drain, and will not influenced by the freeze-thaw cycles.
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$\begingroup$ Ahh! What kind of sub-base granular material would be the best for about a 10" inch deep by 14" wide turndown? $\endgroup$ Feb 18, 2021 at 20:19
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$\begingroup$ @chromechris: The spec from a local DOT: "Aggregates used as subbase tend to be dense-graded with a nominal maximum size, commonly up to 1 1/2 inches. The percentage of fines (passing No. 200 sieve) in the subbase is limited to 10% for drainage and frost-susceptibility purposes. Typically the thickness is 6", with 4" be the minimum." I suggest to consult with your geotechnical engineer, as the requirement may vary from site to site. $\endgroup$– r13Feb 19, 2021 at 1:15