Scaling/Peeling Concrete Surface?
Scaling/Peeling Concrete Surface?
(OP)
I am running into one and two-year old concrete slabs (interior and exterior) that have been suceptable to "Freeze/Thaw" conditions that are flaking/peeling the top 1/8" (cream) off after winter's weather. Not all slabs that have been poured, but some. Different concrete contractors have placed and finished these slabs so the finishers haven't been a common denominator. These slabs weren't poured at a slump over 5-5 1/2". Weather has been different, but no rain or water was ever applied. De-icing salts were never applied directly, but I'm sure some salty water dripped from vehicles from time to time. Sealers were applied to several of these but not all (after a 28 day cure time). The Redi-mix plants claim proper air was entrained. FYI: Two of the plants both used Laf**ge and a third used Cem*x. The course aggregate hasn't "popped" at all. Just the cream in various spots. Some slabs were finished by only screeding with a wood 2X4 and bullfloated and then broomed, while others were screeded with an aluminum Power/vibra screed then mag. bullfloat, then steel fresno and then broomed (if exterior) or power/hand trowel (if interior). What's going on? Thanks to all and sorry about the novel.





RE: Scaling/Peeling Concrete Surface?
If the surfaces had been hard troweled, I would have suspected "blistering", but it sounds more like excessive scaling.
A petrographic examination of several cores will tell you the probable cause of the issue.
RE: Scaling/Peeling Concrete Surface?
RE: Scaling/Peeling Concrete Surface?
jryooper - can you post some pictures?
RE: Scaling/Peeling Concrete Surface?
The 28 day cure time is variable depending on the temperatures, exposure and conditions of the concrete and not just the air. The 28 days referred to is for lab cylinders and idealists.
The fact that it is occurring on just some slabs indicate that the problem is isolated to different panels. Unless the panels were very large, it is likely that the concrete in several panels may have been from the same load/batch, so placement and especially finishing was the variable.
If you really want a good analytical analysis, we have a very good local testing lab that works arm-in-arm with an engineering firm that has done significant interpretation of thin slices of concrete under an strong microscope to determine the failure source regarding similar surface problems (and deeper problems that may occur later). - I can provide the contacts.
Incidentally, a person that finishes concrete is a "cement finisher" and not a "mason", which is a totally different trade. - just ask a contractor or a union.
Dick
RE: Scaling/Peeling Concrete Surface?
Three good petrographic labs....
Construction Technology Laboratories in Skokie, Illinois (www.ctlgroup.com)-Multiple offices
Simpson, Gumpertz, and Heger in Boston (Waltham, Massachusetts) (www.sgh.com)-Multiple offices
Wiss, Janney, and Elstner-Chicago
(www.wje.com)-Multiple offices
I don't have a specific contact at CTL, but at SGH you can ask for Paul Kelley (Boston office) or at WJE check with Bill White in their Atlanta office.
RE: Scaling/Peeling Concrete Surface?
RE: Scaling/Peeling Concrete Surface?
Please keep in mind that the comments from your finishers are very common, mostly because unless there is economic loss sufficient to bring them in, when they walk from the job they never see how it performs.
I have seen the problems you describe on numerous occasions. I have seen it result from freeze-thaw cycling, from dry shaking, from retempering the mix, from wind conditions during placement and finishing, from mix design problems, and from combinations of those conditions.
Based on your description of the timing, it is likely that your entrained air content in the surface paste was reduced for some reason and then freeze-thaw took a few cycles to work on it. Just conjecture, though, not having seen photos or the site.
As for the fee estimate by CTL, it seems a bit high, but then I don't know what they proposed.
In general, we charge from $800 to $1200 per core for petrography, depending on what is needed (air content, water-cement ratio, etc.). As for a field visit and coring, it depends on where it is relative to our office. Coring runs about $250 per core, locally. For evaluation and reporting, you're probably looking at somewhere around $2500 to $3000. Given all that, a fee in the range of $5000 to $8000 would be about right, plus travel if it is a remote site (remote to one's office, not remote from civilization!).
You might check around a bit more or if you will tell me your area, I might know someone you could contact. I am not offering our services, I just included that information as a point of reference for you. It would not be appropriate to offer services in the forums, and resultingly, I would not accept an engagement from this even if in my geographic area, but would be glad to help you find someone.
RE: Scaling/Peeling Concrete Surface?
Thanks a lot for all of your input. I greatly appreciate it. If it is possible (within this site) to give me an email address I can contact you with, I will reveal my location and get you some pictures. If not I'll continue to try to explain the sitaution as best I can. Thanks again. I have one of these slab in my driveway and will get some pictures today and try to post them so you can see.
RE: Scaling/Peeling Concrete Surface?
Ron
RE: Scaling/Peeling Concrete Surface?
RE: Scaling/Peeling Concrete Surface?
A similar and increasing problem we have encountered in western Colorado (USA) in the last 5 to 10 years has usually involved concrete placed in the late Fall. Some of the scaling was not severe for 3 to 5 years, indicating a problem near the surface of the concrete mass which was 'waiting' for the proper final conditions to expose the existing weakness. A lot of testing data is available and a fair amount of petrographic analysis, with 'inconsistent' conclusions. I say 'inconsistent' not in a derogatory manner but indicating some of the concrete exhibited problems with entrained air, some was abused by finishers, some was obviously not cured and much of it had little obvious problem other than scaling.
Our concrete was a combination of machine placed (slump <1.5") and hand placed (slump 3"-5"), has 15% to 20% fly ash (in part due to ASR problems) & 6%-8% entrained air. Bleeding progress in our dry climate is often difficult to assess.
Our interpretation of sporatic/inconsistent scaling:
**High air content, combined with troweling may have 'sealed' the surface on some of the hand placed concrete, resulting in large amounts of scaling or even delamination.
**The fly ash was SUBSTITUTED for cement, resulting in relatively low amounts of early heat of hydration, coupled with marginal to no curing & low air temperatures (below 40 degrees F) resulted in low early concrete strengths which did not really improve until the following Spring. Early Spring storms resulted in surface moisture which may or may not have frozen, but surface scaling began.
**Improper sealing/curing materials used or placed too soon which prohibited the removal of moisture from the concrete mass, probably concentrated this water near the surface which froze in the winter, producing scaling in the Spring.
RE: Scaling/Peeling Concrete Surface?
RE: Scaling/Peeling Concrete Surface?
Yes, the concrete with the "issues" is residential (sidewalks, drives, and garages). No initial testing was done for any of the placed concrete. I know I need to have some testing done on these slabs, but the expense of doing that seems unrecoverable and is hardly in the budget right now. Any more information you could pass to me would be greatly appreciated. Thanks a lot.
RE: Scaling/Peeling Concrete Surface?
RE: Scaling/Peeling Concrete Surface?
In your area (I guessed/assumed from your name - I lived in the lower upper) there is substantial freeze-thaw exposure, but possibly not as bad as areas to the south of you that actually may get more cycles.
Freeze/thaw damage is a result of the cycles the concrete has been subjected to. That is why all durability tests are subjected to the ASTM standards (there are several, depending on the type of concrete and exposure) that prescribe the conditions, water type, number of cycles and amount of permissible loss. The key is the number of cycles since no sane person would expect an immediate failure. - Just try to schedule a freeze/thaw test series and you will find out how important the duration and cycles are. It takes time to measure durability since the cycles are important.
There are some general perameters that may give some indication of concrete durability (aggregate durability tests, concrete compression strength and absorption), but they are only indicators because the actual concrete is dependent on the combination of the factors working together or against each other through many cycles. Just as in a chain, the weakest link causes the failure that can be progressive.
As an extreme example, I have seen 3500 psi concrete with high absorption out-perform 8000 psi low absorption concrete by a large margin.
You can do everything correct about the mix design, batching, mixing time, slump, air entrainment, placement, but if the finishing is not correct AND timely, the weak link may be the final uncontrolled step - finishing.
Dick