**Please note that this article was written in previous years. However, as we are rapidly approaching cold weather season, we found it fitting to share again at this time to help you & your team prepare for cold weather placements & grinding.**
Cold weather concrete placements – its dangers, its advantages? When can you get a grinder on cold weather concrete?
As we are coming out of the winter, I have multiple projects where the concrete was placed during cold temperatures. Although there are ACI standards for cold weather placements, I still experience quite a few slabs that have issues related to cold weather placements. These have a significant impact on exposed concrete surfaces.
Is the structural integrity of the slab negatively affected by cold weather?
The most common issues related to cold weather placement are troweling too early and placing in conditions where the top surface of the slab is allowed to freeze during the placement. The results of these issues are generally not structural, meaning that they will not affect the structural integrity of the slab, but rather create issues with the surface of the slab that would affect any flooring type. This is especially bad for polished or exposed concrete flooring.
Here’s What Happens When Bleed Water Freezes
Let’s talk about freezing and how it affects concrete. Generally, only the water in the top paste layer freezes as the main slab has thermal mass and chemical reaction to keep the temperature up. When the top layer or bleed water, as it is commonly known in the concrete industry, freezes it has several results. Additional bleed water is trapped under the paste layer because it can no longer wick to the surface to evaporate away. This causes blisters of water to form, which after a period when the water eventually does evaporate, lead to surface delamination in areas.
When this situation occurs on the majority of the slab area, the surface layer of the concrete is separate from the main body of the slab. Under normal conditions, a slab placed under stress will break somewhere near the top surface of the large aggregate, approximately 1/4 inch from the surface, but a slab that has a surface failure will separate much easier right at the paste layer.
With traffic, this top layer will separate causing issues for polished concrete, VCT, tile, or even stained concrete floors as the thin top layer delaminates from the main slab body.
The second thing that happens when a slab freezes is that the water in that top paste layer expands as it freezes. When evaporation occurs, you generally see a lot of “craze cracking” due to the shrinkage of this thin layer. These are not structural, but are also not ideal for most decorative concrete applications.
You May Need to Grind or Shotblast Through the Concrete Paste Layer. Exposing the Aggregate Layer of the Slab.
The last major issues is that by freezing, the top layer does not have the full chemical reaction, which, means that is generally soft and chalky. I was on a project recently where we were able to make fairly deep scratches in the slab very easily with a key. If this issue has occurred, the fix is generally to shotblast or grind through the paste layer until you get to a solid substrate layer. This is generally about 1/16th to 1/14th of an inch deep into the slab. As the aggregate is exposed, you will also have a lot of air voids that also become exposed.
The most common fix for these is using an epoxy or grout coat to fill and then grind off the surface residue. As a side note, there is a contractor in the U.S. with the unicorn of floor prep machines. This is a ride-on unit with a massive vacuum that completed 40,000 foot of surface removal at an average of 1/4th of an inch of concrete in just 5 days. For retailers and decorative concrete contractors looking to maintain schedules, this is an invaluable tool.
Scheduling Polished Concrete Related to Concrete Placing and Finishing
Another issue that I have encountered recently is the scheduling of polished concrete related to concrete pours. Almost every project built is on a schedule and they all seem to be behind from day one. I see the schedules getting tighter and tighter. Polished concrete is one area that is always being questions as to the speed that it can be completed. Generally, an experienced crew with two full size grinders and five or six men will be able to complete about 1500 to 2000 feet a day with full polished concrete process.
There are a lot of variables that go into this timetable, though. So, do not count on every job going at that speed.
When Can the Polished Get On a New Concrete Pour with a Grinder?
I get asked all of the time why polished concrete can’t be done earlier. My usual answer is that based on the concrete mix design, the grinding or depth removal of the process can occur as early as 10 days after the pour date. This is beneficial because large projects generally do not have a lot of work going on in the space at this time so the contractor is able to capture time where the slab is not in use by other trades.
Additionally, by doing the main grinds early, contractors can grind under where walls will be with large equipment making edge polishing much easier than later in the project. At this early stage in the slab curing process, the cement is still releasing moisture through the surface, though. This causes issues with any higher grit polishing that is done and the shine will not last.
Additionally, the surface layers, while they may have compressive strength, have not reached full maturity of their tensile strength, which is the side-to-side movement. This is especially important as the finer grits of diamond are used because you can get regular rollout of sand aggregates and small particulates of cement. This leaves the surface pock marked and fractured and this surface profile hinders the clear reflective shine that customers are looking for. Although I do recommend performing main grinds early, I do not recommend polishing within 28 days of concrete placement. It has been my experience that polishing done early does not hold and always has to be redone, from 100 grit resins on up.
