Post Tensioned Tennis Court Cracking

[bayhee]

I have been retained as a third party to evaluate a post tensioned tennis court. The slab is five inches thick with tendons spaced at 30 iches o.c.. The concrete is in excess of 5000 psi after testing.
Apparently, the slab was poured in September. A 24 hour initial stress was performed and a final stress 2 days later. No inspection or elongation data is available. The slab has numerous surface cracks which resemble typical shrinkage cracking. However, for the first 4 months no cracks at all were visible. The cracks began to appear after 4 months and have increased in number. Most cracks are directly above tendons and I attribute the cause to insufficient concrete cover.
I still wonder why no cracking appeared during initial curing and showed up months later. Especialy due to the fact an initial stress was performed within 24 hours. I wonder if anyone has seen a similar situation or have further insight.
Is five inches of concrete adequate?
Could there be a problem if no vapor barrier or bedding sand was provided?
Also, there are areas where grease has bled through the cracks to the surface. I find this quite odd and cannot explain it. The tendons were all unbonded.
What do you think?

 

[dik]

Do you have reinforcing in the slab other than post-tensioning cables? Where are the PT cables supposed to be located? Did you use any crack control other than the compression from the PT cables?

Dik

 

[rapt]

bayhee

I would check the design allowing for all the stresses that could be induced.

No sand layer or vapor barrier would increase the friction restraint to shortening and thereforte increase the shrinkage stresses. Also temperature shortening/differential has to be allowed for.

Basiaclly, as all of the possible loadings are either increasing with time (shrinkage) or dependent on the season (shrinkage, temperature), the loading effects could be much worse now than they were when the slab was poured. Plus concrete tensile strength reduces quickly and significantly with time under sustained load and these slabs are under sustained tension load from the restraint to shortening.

Personally, I consider the level of P/A supplied to be far too small to provide a crack free slab but this is common practice in USA/PTI slabs on ground which, from what I have heard and read, has resulted in a high level of failure (cracking) in these types of slabs there.

 

[dik]

Not necessarily reduce shrinkage stresses, but reduce the effective prestress...

Dik

 

[bayhee]

There is no reinforcing steel in the slab other than at anchorages.
Tendons are straight and in center of slab.
No additional crack control.