In the January issue of Concrete Construction, Scott Tarr wrote about silicate densifiers and his feeling that they are not effective curing agents. Here, he goes into more depth on this topic and the concept of “passive” curing.

I think the whole curing capability of silicates may be a big discussion point in ACI Committee 302 as we start the next cycle of the document. I know we've formed a subcommittee to look at it.

As I wrote in the article, I have no doubt the concrete cures at slab mid-depth. But the main purpose of curing is to hydrate the paste at the exposed top surface - and this is really the most important part of the slab! I think we get away without an intentional cure if the ambient conditions are favorable. We know the hydration of cement stops at an RH of 80%. We also know the exposed surface of slabs comes to equilibrium with the ambient air quickly. So if the ambient air is above 80% RH, we get hydration of the surface paste—a passive cure. This is often the case but sometimes it's not.

One slab I’ve learned a bit about in Virginia has a mortar flaking issue. The mortar flakes (as well as aggregate shadows and surface bumps) didn't appear until 18 months after the slab was finished. At the final walk-through, the contractor was literally given a high five and paid in full—no aggregate shadows, bumps, or flakes. There were a few rolled aggregates which the contractor drilled and filled but that was it. Now, the slab designer is blaming the concrete contractor for poor finishing that caused the flakes, shadows, and bumps. I got a call from the concrete guys since they are being asked to grind the entire slab surface at a potential cost of $500,000.

Come to find out, it was an indoor heated winter placement. The owner had just had a bad experience with moisture in the ceiling insulation so asked that the slab not be wet-cured as specified and that the dew point in the building be controlled during the slab placement. They even used heating equipment that dehumidified the air as it heated it. The slab was cured with a lithium silicate which I have used many times as surface hardener/densifiers but not as curing compounds. After the final walk-through, the owner liked the finish but didn't like the white residue left on the surface so the GC had to grind/burnish it off. They went through several rounds before the owner accepted the slab. As a result, the grinding took off a lot of surface paste (since it hadn't hydrated very well) and exposed and polished some coarse aggregate. After this, the slab was very polished and shiny. But the mortar that remained over aggregate had not developed a good paste-aggregate bond due to the lack of cure and started to flake. The grinding/burnishing process also resulted in aggregate shadows and surface bumps (none of which were there prior to the grinding).

The owner’s representative is blaming all of these features on the contractor not embedding the aggregate deeply enough even though they followed the specification and the recommendations of ACI 302. They started with bullfloats and then used pans. The silicate that was specified supposedly had a membrane-forming ingredient that also cures the slab. It may form a membrane but I haven't seen test data to support that it has water retention capability.  For example, evaporation retardants form a membrane but are not effective curing compounds.

Concrete International published an article about a mortar flaking issue on a slab last year in Pennsylvania. In that case, I believe the mortar flakes were the result of dirty aggregate preventing a good paste-aggregate bond. And the mortar flakes showed up within days/weeks of each pour - not 18 months later. They ended up replacing some slabs and drilling/filling thousands of flakes. In the end, when the owner sold the building, the new owner commented on how nice the floor was.

But this article doesn’t mention curing as a factor. Of course it's a factor! The reason the mortar flakes is because there is a weak paste-aggregate bond—period. There is no other reason. If there was a good paste-aggregate bond, the mortar would not flake off. The aggregate may fracture and popouts may happen and crazing may occur, but not mortar flaking. So whatever impacts the paste-aggregate bond, is a potential factor in mortar flaking. Dirty aggregate is a big one. Inadequate curing is another.

Not embedding the aggregate deep enough is a slippery slope. Are we now supposed to measure the depth of the aggregate? How would we do that? Creating a thin mortar layer doesn't cause mortar flaking. We have seen thin mortar layers for decades—that’s what causes aggregate shadows!

My first bout with this was in 1995. My client, the building owner, had me on my hands and knees checking out his "leopard slab" that he did not request or appreciate. I had minimized the paste content by optimizing the aggregates which is now becoming common practice. The mortar layers were, admittedly, paper thin. But, as we have seen, aggregate shadows do not necessarily flake or deteriorate. However, thin mortar layers make the curing more important as the thin paste can dry out even more easily and impact the development of a good paste-aggregate bond. And early drying also results in early shrinkage that potentially shears the mortar paste from the aggregate. I think the combo of thin mortar layers, the lack of curing, and dry ambient conditions is the perfect storm.  Thin mortar layers is included here because the lack of cure impacts the paste-aggregate bond more with thinner mortar layers than thicker - the paste in contact with the aggregate dries and stops hydrating more easily.

The new silicate subcommittee of ACI 302 should be interesting. I think this topic would make a nice article if you have an upcoming issue where it would fit: "Is Intentional Curing Really Necessary?" I like the word "Intentional" there. Usually people just say curing when they are talking about a moist cure or the use of an ASTM C309 curing compound but I think we sometimes get away with passive curing (favorable ambient conditions). It's risky since we are not in control and Mother Nature is not a concrete expert. There are ACI 302 members with whom I obviously disagree on this issue. Some have said they believe we are getting away with near-surface aggregates nine times out of ten. In a way, that's right but it's not the embedment depth of the aggregate we're getting away with, it's the lack of an adequate cure (either intentional or passive).