Verifying frost removal from subgrade for concrete base slab

[NM1010]

Hi,

I am working on project in Canada where we are constructing a large concrete tank on grade. The contractor allowed the subgrade to freeze after preparing the subgrade and placing a protective mudslab. Once the reinforcement for the structural base slab is installed, they plan to heat and hoard the slab for a period of 3-4 days to remove the frost from the beneath the slab. I am wondering what the contractor to do to verify the subgrade has been heated sufficiently that the frost has been completely removed.

 

[oldestguy]

.Sounds like wishful thinking. Thawing frozen ground is very difficult. Before going too far into the details is there any precise measurement of frost heave? Any heave depends in soil type and a little on water available.

As to how to know the frost is gone, I have inserted a thermometer in freshly drilled holes with hand methods. Do a little drilling and then measuring in steps of depth.. The more water there, the shallower the frozen zone, but takes more heat per inch of frozen depth.

My bet is you can't get ground "unfrozen" that quickly and maybe you don't really need to. This is where an experienced geotech engineer of your area should be called in.

 

[HTURKAK]

I understand that a large concrete tank will be constructed.. Without knowing the top soil depth and frost depth , and subgrade material, it is hard to comment..I think the contractor is planning to set up a tent and heat the surface with portable heaters... Heating the surface 3-4 days may work if the existing frost depth is around 300 mm. However this method may work if heating continues up to unfrozen the ground and concrete slab curing time elapsed . The subsurface temperature shall be observed during heating. I wish the subgrade is not frost susceptible soil.

 

[NM1010]

Thanks for the input. A little more information on the site. The subgrade is generally undisturbed material composed of silty river gravels with medium to large cobbles. The site is actively being dewatered with wells that are 6m below grade. Frost depth in our area is typically estimated at 600-800 mm (2-3 ft)

 

[oldestguy]

NM1010 If that site is largely river coarse deposits, no problem with frost. That's where he geotech can clear things up and get moving with the job.

 

[dik]

OG:
If the soil is silty, this is one of the worst materials for frost heave, IMHO.

Dik

 

[r13]

If the silty soil is not excessive, I guess the mix of gravel, M-L cobble, silt can be considered gap graded material. This need to be confirmed with the geotechnical though.

 

[oldestguy]

Dik It depends.
If that site is largely river coarse deposit. Which is it? In my experience high silt content in river deposit unlikely.

 

[dik]

The OP indicated it was silty... don't know how much... if more than about 10%-15%ish, can be a problem, including the source of water. At that amount of silt, the gravel will have little mitigating effect.

Dik

 

[BigH]

Several things come to mind. As OG stated, if the site is granular, then it is no problem. If the site is silty, as also stated, this could be problem but only, too, if there is a source of water that could have been "brought" up into the ground whereby thin ice lenses could have developed.

It would be helpful if one were to know who long the contractor let the ground "freeze". And, what was the temperature fluctuations. If only a few days - and the temperatures were not that severe, it probably will not be critical. If longer, and water could enter into the ground as it was freezing thereby increasing the water content and hence heave, then judgment and experience.

I would suppose one could drill a few short holes through the mud slab and into the ground, recover the core and measure its temperature . . . both before starting heating and after heating.

One of my first jobs with Geocon (Toronto) was a warehouse slab that had been placed in the winter, kept unheated and then when the started to look at installing the rack system tey noted that the clay shale had swelled 50 mm or so. They waited until the spring to install the racks.