×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Verifying frost removal from subgrade for concrete base slab

Verifying frost removal from subgrade for concrete base slab

Verifying frost removal from subgrade for concrete base slab

(OP)
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.

RE: Verifying frost removal from subgrade for concrete base slab

.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.

RE: Verifying frost removal from subgrade for concrete base slab

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.

RE: Verifying frost removal from subgrade for concrete base slab

(OP)
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)

RE: Verifying frost removal from subgrade for concrete base slab

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.

RE: Verifying frost removal from subgrade for concrete base slab

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

Dik

RE: Verifying frost removal from subgrade for concrete base slab

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.

RE: Verifying frost removal from subgrade for concrete base slab

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

RE: Verifying frost removal from subgrade for concrete base slab

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

RE: Verifying frost removal from subgrade for concrete base slab

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

eBook - Manufacturing the Cars of Tomorrow
In this ebook, we'll explore how additive manufacturing is going to transform the way cars are made. This includes commentary from thought leaders such as Ford's CTO, Ken Washington, Customer case studies of ways 3D printing is being used today, and a variety of part examples where 3D printing is already impacting how automobiles are made. Download Now
White Paper - Smart Manufacturing for Semiconductor
New technologies and approaches present great opportunities for semiconductor manufacturers to achieve high levels of innovation, yield and improvement. This white paper explores some of these cutting-edge technologies and how they can be applied effectively in the semiconductor industry. Read about how Smart Manufacturing is transforming the semiconductor industry. Download Now
White Paper - Analysis and Simulation in Aircraft Structure Certification
Organizations using simulation and analysis tools effectively see the benefits in their ability to achieve certification faster and with drastically less total cost than those who do not maximize these tools. Read this White Paper to learn about how digital tools such as analysis and simulation help in aircraft structure certification. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close