09-01-2009, 09:38 PM
(This post was last modified: 09-01-2009, 09:47 PM by jigar0896.)
Could someone let me know if there is a test time restriction for the moisture testing using the RH probes? ASTM 2170 says that the RH and temperature reading should be taken after 72 hrs. Unlike the Calcium Chloride test, the insitu moisture test standard does not specify a not to exceed time for taking the readings. Does it make a difference if the reading is taken at say 84 hrs or 96 hrs given that the environment inside the building shell remains relatively the same during that time.
Thanks in advance for helping me get a solution to my question.
Off the top of my head, I wouldn’t worry about it for two reasons: 1) if it’s equilibrated, it’s equilibrated. It’s not like the Calcium Chloride where it could gain too much moisture to mess up your reading. And 2), the standard doesn’t have a time limit.
Depending on what kind of RH sensor you’re using, I would be very hesitant to leave them on site for very long. Some probes stick out of the hole and could get damaged. The longer they’re there, the more likely they could get damaged.
Temperature swings will greatly affect the humidity readings..particularly where the concrete isn't well insulated...even if the volume of moisture remains unchanged, humidity will rise an approximately 5% for each 2 degree drop in temperature. Relative humidty is NOT a reading of moisture volume, it should be used to determine if condensation is a concern, or where the moisture may be originating.
Anhydrous Calcium Chloride tests actually absorb moisture in a curve... i.e. they do not absorb at a steady rate. Longer tests will slowly absorb less moisture and when extrapolated for time will read lower results.
The whole 72 hours and 1/2 square foot dome with approximately 1 oz of CaCl is a test with no pedigree. Why use 1 oz of CaCl? Why are the CaCl pellets the size they are? Why 72 hours? Why is the dome 2" tall? Why not have a curved dome?
This test is being exposed as a poor indicator of a slab's readiness to receive flooring. And none too soon for me! I have investigated countless failures of flooring placed on slabs with excellent initial MVER's.
RH probes need to equilibrate to the slab, and the slab needs to equilibrate after being drilled. The standard is now 72 hours, but expect this to soon change also as we continue to test and verify results.
A removable probe needs to acclimate to the slab temperature to provide a reliable reading. This can take up to several hours depending on site conditions and probe design. Let the probe set for 45 minutes in the sleeve and take a reading. Without disturbing the probe, allow it to remain in the hole and take a second reading in 5 - 10 minutes. If these two readings are the same you can be reasonably confident of the reading. I say reasonably because I have had far too many erroneous readings with removable probes to bet the farm using them.
12-17-2009, 01:12 PM
(This post was last modified: 12-17-2009, 01:25 PM by RCConcrete Consulting.)
the dome I co-patented does have a curved dome...and the amount of CaCl used is actually approximately 16 ounces rather than one ounce...the prill size of the CaCl (unless in a powdered form) seems to have little effect on the readings. The absorption curve you speak of is outside of the temperature ranges outlined in ASTM F1869, and therefore is of little consequence..but again..if you read humidity in a pore space..and the pore space varies...so does the volume of water...for example, a poorly cured concrete surface can have 20-50% more pore space and the humidity probe cannot make that determination, giving each sampling an equal value, whereas there may be 20-50% higher volume of water...add to that the unhydrated cement particles which has a high hygroscopicity and moisture is taken FROM the pore space, negating any possibility of an accurate moisture reading...BUT it is this very same property that make (in my opinion) humidity probes the best possible method of measuring proper curing for concrete since concrete needs to maintain better than 80% internal humidity to properly hydrate cement....the probes ignore the volume and other factors that lead to confusing results seen in the field versus laboratory testing.
I feel the TRUE value of humidity probes is to gauge new concrete during the critical curing phase.
since Mr. Kanare has the patent for a humidity probe..he has his reasons for producing tests that show probes in a certain way...we tested CaCl kits as as well and found that certain kits have a residual background moisture "pickup" of anywhere from 0.3lb to slightly over a pound when tested over a non permeable aluminum plate..nonetheless...I once again reaffirm that humidity readings cannot predict the amount of moisture in a concrete slab...simply because humidity and moisture content are non-correlative..which again, is WHY humidity tests are preferred for the testing of curing..rather than moisture content...the equalibrium of CaCl is NOT 0% r.h. but rather between 20 and 40%..which is where the humidity under a dome will remain if no other moisture is added..humidity goes up or down by 5% for each 2 degree change in temperature EVEN IF THE MOISTURE CONTENT REMAINS EXACTLY THE SAME! This data is readily available from the local weather stations and is a well-documented fact...the biggest concern with humidity is when the surface is within 5 degrees of dewpoint prior to installing ANY floor covering or adhesive...except when that material is water-based, then even more caution is needed...the SSPC has recommended this precaution for several years...even on non permeable surfaces
I apologize jigar0896 if I have not answered your question. RH probe holes must be in place and sealed for a minimum of 72 hours per ASTM F2170. There is no maximum time the holes or probes may be left in the concrete unless the manufacturer of the probe limits exposure.
I commonly install probes early (after 30 days) and take readings bi-weekly for many months to trend the concrete drying.