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Event: 12/6 CSI Topic: Floor Slab Moisture
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12-01-2012, 11:28 AM #4
eaadams Concrete Moisture Expert *****
Status: Offline Posts:600 Threads:88 Joined:Jul 2010
Yea blotter layers are very common in the South West. They are still common in California, Nevada, Arizona, New Mexico, etc.... They go away when you get to weather. When you have rain / snow engineers become much better. When you get people building in dessert you get extremely lazy engineers. They never think that a water table 30' below surface could come up through sand.

But remember, I am in School Construction so most of my concrete contractors, GC's, and architects aren't the big guys. They are people who have grown into it and are in the public bid (read low bid) world so there is great resistance to re-writing specs or doing anything that might drive cost up.

As far as I can tell it is mostly due to lazy structural engineers. They pull boiler plate off the shelf. What really sucks is Architects who have in-house engineers. Those people are getting sued left and right for moisture failures now. When they sub out the engineering the moisture failure excuse is usually that the engineer didn't spec the floor and the architect didn't spec the concrete. Where owners get in big trouble is when they do the common design/build route where the owner builds and bids out their own schools and handle each Div as a separate bid package. Then the owner assumes all the risk and then they end up suing all the subs. These construction projects are also where silicates remain a huge problem.

If a A/D will sit down with me when writing specs I will intervene there but at bid time it is too late. I will RFI it out after bid if the GC will get me a contract before concrete pour. But it is a fine line. In California concrete design is an Engineering item. I can't provide "engineering advise or services" without being a licensed engineer. So all I can do is RFI that XYZ does not conform to ACI 302.2R and then, inevitably, I get an INCREDIBLY pissed off engineer on the phone who usually just found out he made a huge mistake and because I had to RFI via a GC the GC communicated it to owner and owner communicates it to architect and architect communicates to engineer. And you can imagine the owner doesn't want to pay to write correct after bid specs to fix such a mistake. However, NO concrete subs will catch this, they will do whatever the plans say without thought. Truly frustrating. I give those GC's who I know do it right or who do their own concrete a significant discount if I know I wont have to do this work after bid.

I think what is most interesting for purposes of this forum about a sand blotter layer is that from what I can tell it is MORE susceptible to producing a false Calcium Chloride result. It would be an interesting study to have a professional (CTL or equal) do and write up. As far as I can tell a slab with a sand blotter layer will show similar Calcium Chloride results but on failure will always have 99% rH. Since I got involved, I don't have paired RH/CaCl testing since we don't do CaCl testing anymore as we don't trust the test & it costs an extra trip of work. I also think the sand blotter layer with a silicate on top is just murder. It really produces a false low for CaCl testing.

Wow ... yea wrong side of the bed, I woke up thinking about this this morning. Sorry for the long post but it is a true scourge in my area.

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