Maximum allowable shrinkage crack width in PT structures?

[Tomfh]

What in your opinion is the maximum allowable crack width in a typical PT slab (say a post tensioned car park deck) before structural integrity may begin to be affected? I.e when is the shear and moment capacity across a crack starting to become compromised due to the gap? When does a shrinkage crack become a so called “structural crack”?

Are there any technical references for this?

 

[LittleWheels]

When the shrinkage forces overwhelm the tendon forces.

I have never had shrinkage forces exceed the post-tensioning forces on the bridge decks and beams that I have worked on. I guess car park decks get designed a little tighter.

I would be very worried about localised corrosion of tendons in a post-tensioned car park if there is any chance of salted roads. I have seen up to 100% loss of reinforcement crossing a 0.5mm through-section crack exposed to road salt runoff (rocker column dowels). Tendons have a greater propensity to corrode compared to rebar.

I would be requiring full depth epoxy injection of any cracks beyond 0.3mm. Aggregate interlock is important for shear transfer across induced pavement joints. Aggregate interlock is noticeably worse across cracks wider than 0.3mm, particularly as the aggregate abrades under traffic loads.

 

[Tomfh]

Aggregate interlock is noticeably worse across cracks wider than 0.3mm, particularly as the aggregate abrades under traffic loads.

Is this something you have observed first hand?

 

[LittleWheels]

There are a few papers regarding concrete slab pavements and induced joint durability and behaviour. I work with port pavements that regularly have to sustain 100 tonne axle loads, so the durability and performance of slab joints is pretty important. There is a lot of overlap with airport pavement design and there is published research on airport pavement joints. Motor vehicle loading is much less onerous but it seems that parking garage slabs are designed quite lean (I have no relevant experience with them). I have seen induced joint aggregate abrasion/ degradation under semi-trailer traffic loads that has required expensive and disruptive replacement, so I guess I am overly sensitive to such things. Maybe revise it to inject >0.5mm cracks.

https://aecom.com/uk/wp-content/upl...maintenance-manual_v1-1_national-highways.pdf

(not written by me) might be a good place to start with aggregate interlock across through-depth cracks, particularly Page 23. If you are using abrasion-resistant aggregate in your prestressed concrete or if traffic loads/ ESA cycles are limited, wider cracks are durable but few specifiers require tight LA Abrasion/ micro-Deval (more applicable for aggregate abrasion resistance than LAA) limits for concrete aggregate.

I would have expected that cracks would measure noticeably wider at the surface of prestressed concrete slabs compared to crack width at the tendons themselves. It seems very conservative to assume that a 0.7mm wide surface crack would be the same width at the tendons. The aggregate interlock would vary across the crack depth.

 

[Tomfh]

Thanks, that’s very useful.

I saw a crack recently in a car park structure that was at least 1mm wide, which had also developed a horizontal step of similar magnitude, suggesting aggregate interlock was beginning to fail. It would see hundreds of light vehicles a day. That appears to correspond with the cracks sizes in that document you linked.

When you say to inject when the crack width exceeds 0.5mm, are you talking just about surface width? Do you assess the crack at different depths?

 

[LittleWheels]

IMHO, a >1mm wide dynamic crack with >1mm horizontal displacement (with post-tensioning crossing the crack) hasn't started to fail aggregate interlock, rather aggregate interlock doesn't exist any more.

You can core through a slab, wash the cored hole thoroughly (otherwise coring slurry fills the crack) and accurately measure the crack width at the reo/ tendon depth within the hole. It is crack width over the majority of the slab's depth that is important but it is quite intrusive to investigate.

I don't have enough experience with the sort of structures you are talking about to give a sensible opinion. Just assume that I am conservative regarding allowable full depth crack widths in heavily-trafficked pavements and know that some folk are more relaxed. Partial interlock (>0.3mm IMHO) allows sufficient movement under traffic loads that joint fatigue/ aggregate abrasion should be considered. A proper assessment may say 0.5mm or 0.7mm is fine.