Fostering a Deeper Appreciation for Concretes Impact on the Overall Project

This is not going to be an essay steeped in technical esoterica. The purpose of this chat is to bring your focus up out of the forms, the paste, and the aggregate, and look at the concrete being placed on complex commercial projects from a much higher altitude to look at a few of the assemblies that concrete obviously impacts, but for some reason, many dozens, if not hundreds of project teams every year, get surprised when the relationship becomes the focus of everyone’s attention, and usually, not in a good way. Whether it’s slab-on-grade, slab-on-deck, simple slabs, or post-tensioned if you forgive me for forcing the pun, concrete is the foundation for most, if not all, of the subsequent assemblies, and if we are not thinking about this interaction properly, discrepancies can, and will, rise to complicate our otherwise peaceful, and well-organized lives. (Stop laughing.)

One immediate challenge, at least for longer duration projects, hospitals, airports, higher education, etc., is that the slab work is being prepared and executed well before many of the job team members handling the interiors and finishes are assigned to the project, and before late-in-the-project trades are contracted and submittals reviewed. This means that decisions happening at the time of slab placement already dictate conditions for waterproofing, framing, flooring, and other trades. These, in turn, influence glazing, fire-stopping, millwork and other finishes. This is not even to mention the attention needed from the MEP trades for penetration planning and coordination, and in the case of concrete roof slabs, the roofing process itself.

Let’s look at the roof quickly; does your single-ply roof get a vapor barrier? Hot applied or self-adhered? Is it your temp roof, or is your roof following right behind it? There is not enough room in this publication to tackle the issue of roof slab moisture and roofing membranes, so we’ll have to be satisfied with knowing it’s there. Try to get a roof manufacturer to take a firm position on what acceptable roof deck moisture is before covering it up. But getting past that, how did you cure the slab? Hopefully, you moist-cured (my personal preference), but even when we spec moist cure, plan it out, and chant it as a mantra during pre-assembly meetings, it's even money that the concrete will get a chemical cure applied. Did you plan on coordinating that with your vapor barrier? Will the VB stick? And lightweight? My therapist is on vacation, so we can’t go down that moisture-laden rabbit hole.

Let’s move away from roofing: slab penetrations and sleeves. How carefully are you looking at the locations before committing to placement? How many pipes did you have to relocate on your last major project? How many walls were missed? Did you have to create fake columns to hide pipes? Did you properly address the T-Rating at fire-stopping if pipes came out of walls? Do your slab penetrations need a W-rating for water resistance? Have you fully vetted the intended fire-stopping UL detail to see how sleeves impact a W-Rating? Are your firestop submittals even being discussed when your slab is poured?

Are your slab FF/FL’s within tolerance? Are you prefabricating interior framing modules? Back to the question of cure: did you sacrifice the moist cure so you could get back on your schedule that week? Now, think flooring. Are you ready to deal with mechanical prep? Sanding? Access? Reduced trade efficiency during floor prep? Different primers? Pull-tests? How’s that schedule looking now? And pull-tests; try nailing down what is and is not acceptable resistance to removal for sheet flooring pull-testing. In a recent adventure with a manufacturer’s rep testing product compatibility (he had to bring in self-leveling because of floor surface issues, and now the flooring adhesive manufacturer doesn’t like the national brand name self-leveling), but the gist of the story is that he pulled and “found the bond acceptable.” What did that mean? We still have no idea, but we got past it.

Now, let’s talk about planning for tolerance discrepancies. How often have you had to mediate between concrete (and structural steel) and the wall framing? Now, I know that architects habitually build huge safety factors, allowing loads of space for framed walls to make up for concrete or steel tolerance issues (ok, even I couldn’t type that with a straight face.) Dimensions are tight, and the over/under is usually never enough to actually compensate for challenges to columns being plumb or slab edges running true. And even when industry standards are on your side, do you have the budget to unbuild the impinging components? Rarely. You have rooms that are a little smaller or narrower or bump a closet or fountain niche. And then the dominoes start to fall. Are those pipes still in the wall? Will the millwork that is already on order still fit? How about prefabricated tubs or shower surrounds?

All of these later-in-the-game headaches arise from decisions being made, and the attention, or lack thereof, is paid during the concrete phase. And most folks start taking measurements after the slab is in place, possibly not until a trade brings it to your attention that something is a half-inch off if they tell you at all. The point of this little discussion, aside from being personally cathartic, is to emphasize that slowing down a little before the concrete goes in place, the planning, the thinking five moves ahead approach, can save hundreds of man-hours, thousands and thousands of dollars wasted just on the salaries in meetings identifying problems, assigning blame and then, eventually, getting around to finding work-around. No detail is too small, and no trade coordination effort is too early when it comes to the foundation for the success of the rest of your project. A well-planned, well-executed slab can make the rest of the project…well, we’ll stay honest with each other; it makes the project less troublesome than it could have been.