I am a general contractor who is trying to get the Terrazzo floor going on a new construction project. My concrete slabs (including moisture barrier) have been down for 11 months. The building has been sealed up and HVAC system going for three months.
My subcontractor has been taking moisture readings using his Rapid RH and getting readings consistently in the 95 - 99% range for three months without change. Because of these high readings he is demanding a $40,000 change order for moisture barrier. I watched the sub take his last set of readings and he stuck the probe in the hole and got a result in about 15 seconds. This seems contradictory to the ASTM acclimation of the probe statement.
So I had a testing company come out and take readings with their Delmhorst equipment. Readings came in at 67% and 77%. I reviewed the testing procedures with the tech and observed his procedures. I can find no fault with his work.
The sub of course does not agree and states the testing company did their tests wrong.
So now I have the sub conducting new tests in new locations and I am going to have the testing company observe.
Can anyone offer any advice as to what I should be looking for?
From what I have read most mistakes in testing procedures lead to high readings, is this correct?
I understand your frustration. I have been in your shoes many times.
First off, Delmhorst is a pretty decent system. When used correctly the readings you get from both of these systems will be very close to each other. Now we have to figure out why they are not.
Lets look at some limitations each system has and see if that's where our trouble is:
The Delmhorst probes are reusable which saves money on equipment, but costs us in other ways. You need to leave the Delmhorst probe in the sleeve for at least 30 minutes before reading it, then watch the reading for 30 minutes for drifting. If the RH percentage drifts less than 1% in 30 minutes the reading is accepted. If it drifts more than 1% you must watch it until it stops drifting for 30 minutes.
The Delmhorst hole must be drilled with a 5/8" drill bit and sealed at the top of the sleeve. The majority of false lows read by RH equipment is due to the probe reading more of the concrete hole than just the very bottom at 40% depth. The Wagner probes have sealing flanges designed to isolate the 40% depth horizon and maximize accuracy and dependability.
All dust must be vacuumed from the dry-drilled holes after drilling, then brushed and vacuumed several times to ensure no dust remains.
You need to verify that each system is drilled to 40% depth (on slabs drying from one direction). Don't guess at this when you have conflicting readings. Drill an examination hole about 12" from your probe hole and verify the exact concrete depth. I wouldn't do this for every hole, but you may be told the slab is 4" thick and find out it is 6" thick.
Probes left in the hole for long periods of time can lag behind a little bit as humidity falls. At 90+% this isn't an issue, they won't lag behind more than a point or two. For the utmost accuracy and to compare readings against the Delmhorst, I'd drill two new holes and use a Rapid RH in one and the Delmhorst in the other. You'll have fresh holes near each other and the readings will be very close to each other as long as the Delmhorst is sealed and allowed to sit for at least an hour in the hole. The difference will be the Delmhorst will be reading more of the hole than the ASTM allows which will result in a lower reading (concrete at the top of the slab is drier than concrete down low).
Ask the Delmhorst operator when his probes were checked for calibration and how much they were off. I've owned dozens of these probes and they were NEVER right on (a sure sign he never checked if he says that) but they should vary less than 2% points. They must be sent back for calibration if they are off.
A quick check to ease your mind is to hold the Delmhorst in the air and read the temperature and RH, then do the same with a Rapid RH probe. Be sure the probes have acclimated to their surroundings for a few minutes. They should both read very close to one another. When you verify they read the same holding them in the air, the only things that can cause them to read vastly different in the holes will be:
Hole prep (vacuuming, drilling straight - no wobble, depth of hole)
Depth of hole - must be 40% in most cases
Design of sleeve - Here Wagner isolates the 40% depth and is the most accurate.
Acclimation period - Wagners can be read in seconds, the Delmhorst requires a 60 minute minimum acclimation.
You can email me if you run into troubles. I get emails on my phone immediately. If we need to talk on the phone that is okay too! What I really want is you to have reliable data to make informed decisions.
You can call Wagner also! Ask for Ron or Ed, Jason is in Vegas. You will get immediate help.
"I watched the sub take his last set of readings and he stuck the probe in the hole and got a result in about 15 seconds."
So he put the wagner probe in, took the reading, then returned a few days later to get another reading? Even if he is using the 15sec later test result it still shows too much of a disparity.
I'd love to know how you resolved the issue. I've seen a LOT of bad Terazzo recently and would love to know what the result was.