Broken Strands

[11822]

Background:

-Cast in-situ viaduct, spans ~42m
-3 webs
-6 anchorage in a web
-31 strands in an anchorage (all fixed at one end)
-Day 1, stress to 50% design stress for all anchorages

-Day 2, 5 strands in an anchorage were broken while stressing in

Any suggestions to this issue?

 

[BigH]

You might want to post this in the structural engineering thread - lots more structural types visit that site. Possibility in that the strands had been knicked/rusted, etc. Even small knick on a wire can reduce its strength considerably. I take it these are the 12.7mm 7-wire strands.

 

[Zambo]

or is it 15.7mm 7 wire strands (this normal Europe). You say stressing from one end, is access difficult at the dead end in which case sometimes wedges not properly fitted and strand can pull out of the anchorage.

If broken then as BigH says small knicks in the wire, or even sparks/molten metal from nearby welding or gouging operations can damage the wires.

Then as also mentioned corrosion can be a serious problem, was the strand delivered in good condition, stored well and stressed immediately after installation?

If all else fails then send a sample of strand for a tensile test and check against the certificates you received.

Zambo

 

[letrab]

How do you stress the tendons? Do you measure elongation and calculate the presumed stress or do you use the jacking pressure to calculate the stress?
Both calcs should give similar results, if not it is likely that the ducts were damaged with consequent leaking of concrete grout into the duct, restricting the elongation of the tendons.
If this is the case stressing the cables to a predetermined elongation will overstress the tendons.
Another possibility is damaged wedges cutting into the outer strands thereby causing failure.