Curing of Shotcrete

[BigH]

Cheers to all:

Having a bit of an "argument" on site with respect to curing shotcrete. The specifications say to cure unless the ambient weather conditions are such that the Engineer may allow natural curing (e.g., relative humidity >85%).

Notwithstanding the above, does one need to cure shotcrete. We have experienced engineering geologists who say no - they have never done it (even outside of tunnel applications) - it is not "standard" practice.

I am on the side that shotcrete should be cured - it is a thin layer of cementitious material that should the water evaporate quicker than hydration you may not get the strengths that the material had the potential to achieve. I think it may be more important than the curing of a thick section of concrete.

Counter-arugment is that if the cores come out okay, then they don't need to cure. How do they know until the cores are tested? Seems like a "who cares so long as . . ." argument.

From a contractual point of view, the contractor has a cost associated in his tender price for curing and if he doesn't cure, he is "making money" -

NCRs could/can be written but in the end, the closing will depend on the counter-argument noted above.

What is your opinion?

Thanks and

 

[Ron]

BigH....I'm with you...all cementitious materials need some level of "curing". It is often overlooked or neglected in shotcrete. ACI 506.2 (Standard Specification for Shotcrete) requires curing for shotcrete, just as with any concrete material. The "natural" curing, under ACI 506.2, requires an ambient humidity of 95% or higher.

I would cure...even if only done by sprinkling intermittently....some is better than none!

 

[jheidt2543]

What kind and duration of curing are you asking for. If the specifications just says curing that means a lot of different things to each party in the contract - water mist, spray-on curing agent, burlap covering. If "natural curing" is allowed, but it is defined as 95% relative humidity for a certain period of time, it is still curing and I would think someone would have to monitor it. If there is no curing effort made and the shotcrete doesn't come up to strength, then what? The contractor owns it and makes repairs. The contractor should be thinking of the curing as insurance, not unnecessary work IMHO.

 

[PEinc]

For temporary soil nail walls, the shotcrete is rarely cured. It may be protected from cold but is rarely cured. For a permanent shotcrete wall facing, curing should be done as with other concrete walls.

http://www.PeirceEngineering.com

 

[ecanuck]

Hi BigH:

I haven't been around for awhile, but kinda got an urge to have a look at this forum.

Having done a lot of inspections of concrete over the years, it is a very common occurrence to encounter concrete not being cured. There are the various standard replies like "we used a curing compound", or it is not needed since some admixture has been used - latex being a common example.

If you are using shotcrete it seems likely that it might be a vertical wall or hard-to-cover structure eh? (heh a bit of Canadianana there). So why not a curing compound?

Gone are the days when workmanship takes precedence over expedience/dollars.

Oh and I have a question: How is it that geologists are involved with concrete?

Bon vivant,

Tom

 

[BigH]

Tom - as geotechs, as you know, we become jack-of-all-trades in a lot of areas. My last job I was a QA Mgr for a hydro dam so had a lot to do with concrete and shotcrete. Curing compounds are not typically used (if you read the manufacturer's literature) in very hot climates. Had a case in India where they used it on a road slab - and the slab cracked like crazy (plastic shrinkage cracking) - whereas the contractor I could convince to also use water curing had no such problems.

The spec is written that the engineer may give an okay not to cure if the relative humidity was greater than 85% - such as in a tunnel - but outside slope protection - needs curing and they are not doing it - they say they never do it. . . keep on pushing . . .