Possible causes of soft concrete (not fully hardenend) in the invert of a concrete tunnel liner

[Bridgegirl67]

Contractor is blind pouring a concrete tunnel liner (un-reinforced) in approximate 40' sections. It is a 24' diameter (finished diameter) and walls are a minimum of 18" thick (greater in a majority of the locations).
NOTE: There is almost no water infiltration into the tunnel and any that is coming in, is infiltrating from areas beyond the pour.
Contractor is working on forming the 6th pour. We have found a couple locations in the invert of the concrete tunnel in a couple of the previous pours where the concrete is soft. Essentially we can dig into the concrete with a hammer 7+ days after the pour and the 3 day strength tests are coming back at anywhere between 3,800 to 5,600 psi. The areas are isolated (not the full length of the pour) but occur in the invert at the locations discovered to date.
Any ideas as to the cause (how we can limit or prevent this)?
In addition, anyone have experience with how something like this has been repaired in the past. As of right now we are looking at a full depth removal repair.
Thank you in advance.

 

[oldestguy]

My guess is that after the forms were set and before concrete that seepage water entered and remained in the invert. So when along comes the concrete it is placed in a ponded water situation. Thus very high W/C ratio in the invert zone. What was sequence procedure for placing concrete, that possibly trapped water? rather than displacing it?

 

[dik]

OG: similar thoughts...

Dik

 

[Ron]

Core the concrete and have a petrographic examination done. Do cores in the good areas and bad areas. Compare the results.....You'll likely find what OG and dik noted, but it will be confirmed.

 

[cvg]

I dont think any further sampling needs to be done. with 3 day cylinders over 5,000 psi, you should expect much better. However, there should be a consensus on the reason for the bad concrete, either water in the forms or a bad mix was delivered. figure it out and eliminate before continuing the work.

there is no repair, just replace