You find out that the slab that was poured last winter, froze within hours of being poured. How did this happen and what can you do to ensure a successful installation?
For the most part, frozen slabs are avoidable. ACI 306 details the methods that should be considered. The last time that I checked, the 306 Standard cost was about $38.00, for an ACI member and over $60.00 for a non-member (your customer or their sub-contractor should be a member). However, for this discussion, the following are the more common and generally accepted principles and methods that could have avoided this issue by the G.C., Const. Mgr. and/or flat work contractor.
- Add 100-lbs of cement powder per cubic yard of concrete
- Use concrete curing blankets
- Specify “high early” concrete
- Use of an accelerator.
- Example… Calcium chloride added up to 02% by weight of the concrete.
- Combustion type heaters can be used, provided they are properly vented to the outside. CO2 can react with concrete and generate carbonation. If they were used, verify that they were, vented properly.
- The concrete should be air entrained to resist the effects of freeze/thaw cycles.
- Concrete slump should be specified at less than 4”. Reference ASTM C143.
- Require that the mix be heated prior to being trucked to the site.
This is up to you the flooring contractor, to make the problem go away. And, oh by the way, there is no extra money left in “Contingency”; so, they don’t want to hear about a change order. Remember, these conditions were not referenced in Division 9 nor outlined in Division 3 and it shouldn’t fall on your shoulders. When confronted with this, I’ve always submitted a change order.
They haven’t considered the fact that, if a pour is allowed to freeze, during the first 24 hours, it can lose up to 50% of its calculated strength, at full cure. Also, a slab that freezes in this early stage, can result in a fracturing of the matrix. These conditions will affect your installation.
Now that concrete with fly ash (and also those with slag) are getting increasingly popular, it should be noted that these slabs will cure more slowly, further exposing them to the elements. Concrete with fly ash, generates substantially less heat. They shouldn’t lose sight of this point.
In many areas, the slab will look okay. Some people will sound out the slab with a wooden broom handle (my favorite). I have also seen people use a .75”- 1” steel ball bearing or even a golf ball. Flooring inspectors seem to prefer to drag a chain. In any event, everything from heavy grinding, shot blasting, sand blasting or scarifying have been successfully used, depending on the severity of the surface condition. The top layer has a low compressive. I have seen compressives as low as 35 p.s.i. and this must be removed. The resulting surface will then need to be skimmed, patched or self-leveled, to repair the damage caused by the remediation.
As with almost every fast track schedule, there may also be issues related to moisture. The products to be used will determine what steps you wish to perform first. Skimming/self-leveling or moisture mitigation. Therefore, it is imperative that you contact your local W. J. Grosvenor Branch for assistance.
About the Author:
Ron has over forty-five years of experience in the industry, including Concrete Restoration, Resinous floors, Surface prep, Floor Covering, Tile & Stone and more.
Certifications & Affiliations include:
o CTC registered with the Ceramic Tile Institute of America
o AIA Registered presenter for over 35 Continuing Education Seminars
o Twenty year member of CSI
o Past member of ICRI