Urgent - Retardation Restrictions on Conventional Concrete

[BigH]

Friends, (Ron, et al)

Question has come up about maximum retardation time for practice in pouring large concrete pours (say 600 to 1000 m3). Contractor wants to dose for up to 20 hours; I am of the opinion that this is far too long. Any thoughts. There is underlying theme to use retardation as hammer to get the contractor to bring in more appropriate equipment - so (1) is the technical issue and (2) is how (1) can be used to advantage to pressure . . . Any quick help would be super appreciated.

Cheers to all - Happy Hannakah to those who celebrate it - -

 

[dgillette]

I sent your query on to our concrete mix expert. His response:

Dave - it's fairly common to retard concrete for long duration placements. Normally, you're talking around 4-6 hour retardation. there are some new retarders thatt can go up to 24 hr. One question would be if you dose something that much, what would happen if there was a misbatch - it might go for a long time or not set up at all (some conventional retarders will not set up if overdosed too much. In addition, if you retard the mix that much, it leads to higher form pressures. You're talking full hydraulic head at 150 pcf. However, many RCC mixes are retarded for 24 hours due to the cold concrete and high fly ash contents.

Tim

 

[BigH]

Dave - many thanks. The Contractor's QC Manager has worked for Sika for 10+ years so knows their products - but I'm just not comfortable with the kind of retardation that they are requesting. Have a good one.

 

[dgillette]

From Tim's reply, it sounds to me like it's a bit risky to retard that long, but I know approximately nothing about it.

BTW - I celebrate Hannukah, along with Christmas, Kwanzaa, 4th of July, Canada Day, the Queen's Birthday, Bastille Day, Guy Fawkes Day, Eid al Fitr, Cinco de Mayo, New Year's Day, and any others I can think of. Used to celebrate Boxing Day, but eventually found out that it's about gift-giving and not pugilism.

 

[BigH]

Dave - actually boxing day is about throwing out the boxes! I celebrate others - you might try the Islamic New Year (18th/19th Dec), thai New Year (usually in April), Laos New Year (can't remember when), my birthday, etc.

 

[Drumchaser]

Good comments....on the retardation issue.........are you worried about internal temperature(s)... differental between central core and exposed surfaces?

What are you pouring the concrete for?

 

[Ron]

BigH...sorry to take so long to check this..20 hours is way too long, I agree. Some of the problems....excessive bleeding; segregation; loss of air entrainment (if any); if the retarder is calcium chloride, severe corrosion of the rebar can result with excessive dosing; crusting; plastic shrinkage; etc.

I would hold the retarding to no more than twice to three times the normal set time on the cement.