Chloride Rich CMU's

[CanuckPE]

I've been asked to assess a degraded CMU wall, and it looks as though it's in poor condition. I can see into the cells in a handful of locations, so that gives you an idea of the condition. The site is subject to high chlorides, and I suspect chlorides are culpable for the degradation. I'm hypothesizing the chlorides break down the cohesiveness of the cement material matrix and the blocks simply crumble away (similar to the concrete degradation mechanism). I'm looking for advice in two specific avenues; 1) what, if any, material testing options do I have to provide an objective basis for repair/replacement recommendations (i.e., coring samples for chloride testing, compressive strength testing), and 2) what repair options would potentially be available.

I've got plenty of experience in testing reinforced concrete for chloride content then providing repair recommendations. I don't have much experience with CMUs, but I trust they have similar characteristics and mechanics. Typically we would take a few cores in the 4" to 6" diameter range for RC elements, chloride test some and compressive strength test the others. CMU cell wall thickness is in the range of 25 to 35mm, and I'm wondering about whether strength testing will be possible considering the typical requirements for diameter to length ratio (from the concrete world). I'm expecting the strength to be low, but also interested in the chloride testing requirements. Does anyone have experience material testing CMUs?

For repairs, I was thinking about filling cells with SC grout. Any others available?

Thoughts?

 

[MotorCity]

If you can see into the cells, I am not sure if there's a test out there that would convince me its ok. I would calculate the wall strength considering the section loss you observed. As for a fix, I think that any attempt to fill the cells or repair/replace the blocks will simply be a bandaid. The chloride would still attack the grout and blocks. If the issue truly is chloride infiltration, the solution is to provide a better barrier or waterproofing.

 

[CanuckPE]

Thanks for your feedback MotorCity. I agree, this is likely a replacement project. I'm planning to perform a strength calculation, but need to test for the compressive strength side of the equations. Any feedback on the diameter of the cores? Have you done it previously?

 

[MotorCity]

If its hollow cmu, I cannot envision how you would be able to extract a core with any "meat" on it. You would probably end up with bits and pieces of faceshells and webs. I have not done it before, but I would probably try to extract an entire block and have it tested in compression to failure like would be done with a newly manufactured block.

 

[dhengr]

CanuckPE:
I’ve seen this a number of times before, and unless there is some strong evidence of chlorides from the outside (surrounding environment), or in the block mix itself, I am more inclined to think it is just some bad conc. blk. from a batch, from a careless supplier, poor QC, and cheapest supplier. My reasoning; I’ll see two or three of these blks. (maybe more) in one immediate area, and the blks. around them are just fine; then 8’ away, there will be another two or three inferior blks., etc. Almost as if the guy mixing mud and distributing the conc. blks. took six or eight blks., off the same pallet, and stacked them every 6-8’ for the mason., and some blks. on one corner of that pallet were bad. If this were environmental chlorides, why only a few discrete blks.? There is no doubt that moisture and below, or at grade seems to exacerbate the situation, the conc. just turns mealy or crumbly, from sand size to .25”-3/8” semi-solid crumbs. Have a lab test some of the crumb for chloride and the general makeup of the mix, and let us know what you find. I’ve certainly heard he chloride idea from some builders and masons before. You can tap the outside shell of the blk. with a hammer and quickly determine the extent of the condition. The shell will have a real rough, eroded texture if it’s a bad one, much of cement paste sorta seems to have leached out. Is the inside shell of the blk. o.k., or bad too?

Depending upon the structural situation of the foundation, I’ve done several different things to remedy the problem. Have a mason completely replace the bad conc. blk. And, that’s a little questionable too, because I always wonder about the quality of the inside shell (inaccessible) bed joint, at least on the top and end blks. If it’s a few blks. here and there, and the inside shell is o.k., I’ve allowed them to saw cut (break) blk. lengthwise, so they are laying outer shells and half cross shells. Then grout the surrounding cells as they work their way up. If it’s mostly just the outer shells, I’ve broken them out of the wall, filled cores/cracks to prevent leakage, formed the outer face and poured a loose conc. mix into the form. The top few inches then become a dry packing effort under the sill pl.

 

[CanuckPE]

Thank you for your response dhengr and MotorCity,

In this case, I think there's strong evidence for chlorides. The wall is at a Canadian potash mine. There are chloride stalactites on the surface of the wall (image attached). The stalactite is on the left
hand side of the image. There's another area with outside shells completely missing exposing the unfilled cells. In any case, the chlorides should have a similar degradation effect compared to bad QC with poor cementation of the material matrix.

I'll have the crumbs tested for chlorides, but you're describing the texture correctly as semi-solid crumbs. It may not be chlorides, but time will tell. The inside of the cells has been viewed from a distance only, but I will report back on the inside condition following a site visit.

Is it safe to remove a full block for testing? I was thinking a core in the 30mm diameter range out of a face shell would leave the wall in better condition during the testing and minimize the effect of sampling on wall performance. A 30mm core will have the D/L ratio close to 1, but I could probably extrapolate the results to a correct D/L sample size. Thoughts on a 30mm diameter core for compressive strength testing? I'm expecting a result of 0 to 1MPa, but I though it would give some objective result upon one could base their conclusions and recommendations.

I like the idea of forming the wall to place a lean concrete mix into the walls from the top. This would certainly firm up the missing faces.

 

[MotorCity]

You could probably extract a 30 mm core (disk) from a face shell and compression test it, but I don't think the results would be very meaningful since thats not a standard sample size or shape (that I am aware of). You need a reference test, and follow specific steps, under specific conditions to compare the results to. Otherwise you are not comparing apples to apples. A full block unit is the sample size that I see most often tested. I'd be fairly confident that you could strategically remove one block from the wall (and replace it of course) without disastrous results.

 

[EdStainless]

If this is one of the locations that I am thinking of they are mining KCl, that is nearly 50% Cl by weight.
I don't what testing for Cl would give you.
Chances are that there is damage from both Cl and from salts crystalizing in the pores of the CMU.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[phamENG]

As others have mentioned, CMU doesn't lend itself well to in place testing like CIP concrete (coring, hammer testing, etc.). You probably need to approach testing like you would new construction. Either cut out a section of wall to test as a prism or remove whole blocks to test them. The prism is probably the better idea as it gives a more accurate picture of the assembly strength. Doing it individually would require sampling grout and mortar for testing as well, and would be hard to get a testable mortar sample.

It would be interesting if you could have several done in different locations and sample the chlorides after breaking the prism and see if you can find a correlation between strength loss and chloride content.

 

[masonrygeek]

Believe it or not there are ASTM standards for the removal of masonry specimens from a wall (ASTM C1532). This method is meant for a section of masonry and not a section of a unit, but it may give you some ideas. You can also use ASTM C140 for testing of masonry coupons (or pieces of CMU) to test for compressive strength (See Appendix A1.3.1.3). pham has some good points about just testing pieces and not a section of masonry, but here you may not care about the f'm and just care about the quality of the block. And there are non-destructive tests to determine the quality of the masonry such as impact echo, and others, but that may be too sophisticated for your project.

We see this issue becoming more of a problem near retail buildings when CMU walls are adjacent to sidewalks where copious amounts of deicing salts are used. There is a test to determine the freeze/thaw resistance of concrete pavers that are subjected to a saline solution (ASTM C1262), but not sure if that would apply here or not.

Once you determine the durability of the existing units, you probably will end up applying a water repellent coating to reduce the absorption of chlorides and moisture. Talk with the coating manufacturer (preferably a tech rep) who can lead you to the right product.

 

[CanuckPE]

That is fascinating masonrygeek! I'll dig into those standards.

phamENG, I agree testing the system is ideal. I'll see if that's possible for this project.

EdStainless, yes it's a highly corrosive process. I've found there's a loose correlation between chloride content and compressive strength from the RC repair world. But, that's a good point regarding value of the chloride content. It's more helpful for estimating corrosion potential of reinforcing steel bars.

 

[dhengr]

CanuckPE:
I agree with PhamENG and Masonrygeek about the testing and general investigation of the blk. work. Your photo kinda shows some of what I was thinking or talking about in my earlier post. The conc. blk. to the left of the vert. joint/crack at the upper left corner of the green soaps/2x’s/ whatever head seems to be in better shape than the blk. immediately above the opening. Although, the staining and stalactite (?, at least some mineral deposit, worth testing) seems to be due to moister, in the wall, from above, maybe salts from within the blks. Check the cap flashing on the wall above. A triangular area of blk. over the opening head, to the top of the photo, seems to be a moisture wash or leakage from above. Then, the worst blk. is the ¾ high course right over the green head pieces, and this would be a place were moisture would collect, stand and seep into the blk., then freeze-n-thaw, and leach and break-up. These all seem to indicate detail problems which I would want to understand and investigate before I put all the blame on external chlorides, although your environment may have a lot of those too. I do agree with Masonrygeek that once you get the walls and details cleaned up, you might want to apply a water repellent coating to the walls.

 

[dik]

I'm not sure of the effects of Cl on CMU walls if they are not reinforced. It's a matter of digging in to some reference material either from a CMU manufacturer. Could the Cl be a result of water penetration; this is not good for masonry. Dunno, will see what I can find.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

Something...

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1644033081/tips/Deterioration-of-Masonry-and-Concrete-Structures-Due-to-Salt-Weathering-State-of-Art_pnmgec.pdf[/url]

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik