Why can't they place concrete on frozen gran A

[hoobr]

We have been checking a site daily to see if the soil is still frozen in the rooms they are heating. They want to pour the floor slabs as soon as possible, but can't if the floor is still frozen. The soil is 4 - 8 inches of granular A on top of granular B fill or native, fairly clean sand. They compacted everything in the fall to meet or exceed spec (98% standard proctor).

I can't figure out a reason why placing the concrete on the well compacted, but now frozen, granular would be bad. I know why trying to pack frozen soils doesn't usually work.

 

[BigH]

Check out the effects of freeze on the concrete properties.

 

[MotorCity]

The frozen ground surface could freeze some of the water near the bottom of the slab and prevent hydration and proper curing of the slab.

The slab could also experience vertical movement due to frost heave or settling when the soil melts or freezes.

 

[fattdad]

you'd be lucky if the specified degree of compaction remains.

f-d

ípapß gordo ainÆt no madre flaca!

 

[BigH]

fattdad - Granular A (and I am presuming the OR is from Ontario) is a well graded crushed sand and gravel. The freezing of it would have very negligible effect on the material. Granular B is also a well graded crushed sand and gravel (usually). OR hasn't said where the water table is. The main problem I see is the effect on the concrete - not of the fill or underlying soil layers.

 

[aeoliantexan]

I worked in a lab in the Aleutians where the floor slab was placed late in the afternoon while the temperature was dropping, and the concrete apparently froze during the night. I don't know whether the sub grade was frozen or not. There was no spalling, but the concrete was weak and porous. A Schmidt hammer showed about 1000 psi, and I couldn't mop the floor because the water disappeared.

The OP described base and subgrade soils that might be non-frost susceptible, so there might not be vertical movement. I expect the detrimental result would at least be slow strength gain, maybe long-term low strength and durability.

 

[hoobr]

The water table is not an issue. The ground is being thawed by heating the rooms. When I go to check for frozen soil, I am required to dig down to prove that there is no frozen material in the soil. The top four inches usually thaw very quickly, but it takes longer for the soil 6 - 14 inches down to thaw. From what I understand of soil mechanics, It seems the "free-draining/granular" soil frozen 6 to 14 inches below the bottom of the concrete should not cause a problem. The compaction is usually fine without repacking after thaw. I think this process is overly cautious.

Thanks for all your replies

 

[BigH]

Again, hoobr - it is NOT, in my opinion, a "soils" problem but a problem with the concrete and how the frozen ground may affect the concrete properties.

 

[cvg]

if the concrete freezes during initial cure than it should be rejected. if it doesnt freeze but instead is not kept warm, the curing and strength gain could take perhaps months to reach the design strength. neither is a desirable outcome. heat does transfer through soil so just because the top 4 inches is above freezing does not mean you should allow frozen soil below that depth to remain. and it should be well above freezing, not just a little bit.