Control Joints for fuly grouted masonry

[JAE]

Concrete masonry - in our region of the world, we typically provide control joints in concrete block masonry walls at about 24 feet (7300 mm) spacing per a table I once got from a masonry publication. Not sure what NCMA recommends.

We have a one-story building that will be fully grouted with vertical bars in each cell and a bond beam a the top. (tornado shelter). We have heard from someone that if you fully grout all cells, you might get more overall shrinkage in the block walls than say a partially grouted wall (such as vertical bars at 32" o.c. (800 mm).

I can't see how individual grouted cells can add to overall global horizontal shrinkage.

Is there a concern here? Should I shorten up the spacing of CJ to something less?

 

[jike]

I never heard of that either!

 

[steellion]

I've never heard of fully grouted walls shrinking more, and I can't think of how it would impact the wall shrinkage.

FYI, if I remember correctly the NCMA recommendation is 1.5*wall height up to a maximum of 25'.

 

[HereTwoLearn]

I haven't heard of getting "more shrinkage" by fully grouting a wall, nor does that logically make sense to me.

What does logically make sense to me: You're restraining the wall from shrinking/expanding horizontally more than "normal" by fully grouting it. With higher stresses resulting from more restraint, you're more likely to overcome the tensile capacity of the concrete block wall. At least the theory is logical.

 

[concretemasonry]

The block in the wall you are grouted are essentially 100% cured before the wall is built. This combination will shrink less that a similar

The 8-11" slump grout will lose moisture to the block in the wall initially, which will cause a very, very slight expansion that will be dissipated as the wall dries out and the grout cures.

Just don't make the grout too strong. Too much cement does nothing other than cause excessive shrinkage, since the only function of the grout is to transfer the loads to the rebar. Since you are grouting solidly, you can always put in an some intermediate steel (an extra bond beam) for comfort.

Partially grouted walls have many advantages, usually related to cost and weight reduction, since it is always cheaper the use higher strength block (if they are available where the project is).

Since there is a concern for tornadoes, the concept of that amount of steel and grout may be based on the FEMA standards/recommendations and research for projectile protection and protection from projectiles and not purely structural. - The FEMA research is very interesting and based on firing a 12' 2x4 at 150 mph or so at a wall to achieve zero penetration.

 

[JedClampett]

Funny story;
We had some 8'-0" tall CMU cantilever screen walls designed a couple of years ago on a large site. For security reasons, to make them more bullet resistant, we fully grouted them. There was horizontal reinforcing. We spaced the control joints at 24', as usual. Small cracks appeared at about 8' intervals. The mason said it was due to the fully grouted design. He offered to space control joints in subsequent walls at 15' and we agreed. I, too, had never heard of a fully grouted wall being more susceptable to cracking, but figured that there was no downside in spacing the joints more frequently.
Not really a funny story, but it was a case where the mason thought the full grouting caused cracks. I still can't find any technical backup for this.

 

[iceberg187]

NCMA - TEK 10-02B - Control Joints for Concrete Masonry Walls - Empirical Method:

http://www.ncmaetek.org/etek/pdf_brand.asp?pdf=TEK 10-02B.pdf

NCMA - TEK 10-03 - Control Joints for Concrete Masonry Walls - Alternative Engineered Method:

http://www.ncmaetek.org/etek/pdf_brand.asp?pdf=TEK 10-03.pdf

 

[NITTANYRAY]

There is one condition where grouting walls can cause vertical cracking and it also happens in partially grouted walls.
If the walls are grouted in cold weather the grout can freeze and cause vertical cracks at the grouted cores. It took me several winters to figure this out but in almost all cases the vertical cracks were located at cores that had been grouted and records indicate that below freezing temperatures occurred either shortly after the pour or within a few days.
Unless an accelerator is used concrete will not cure below about 40 deg. so a freeze several days after a pour is possible if there were no warm days in between the pour date and the freeze date.

 

[concretemasonry]

NittanyRay -

The problem is that the cold walls were grouted after the walls were built ot they were not protected from freezing between construction and grouting.

In cold weather construction speed is essential to not waste the heat used to build and cure the walls.

Cold weather construction at -0F is fairly common here, but a good contractor or engineer controlling the project will insure that time is not wasted. Once masonry is at a reasonable temperature, it makes little sense to let it cool off before continuing construction. If the conditions warrant, heated grout from a plant is a step in the right direction since it increases the curing rate, absorption into the units of the walls mass and slows any possible freezing.

The classic case of concrete chimneys cracking due to moisture intrusion from above is very common.

Dick

 

[hokie66]

I think the shrinkage of fully grouted walls would depend a lot on the type of block used. In Australia, we tend to fully grout a lot of walls, and we use blocks with half height webs to facilitate complete filling. I have never know this to create added shrinkage cracking, but then we do tend to use a substantial amount of horizontal reinforcement.

 

[southard2]

I think 25 ft o.c. control joints would be alright. I can tell you I have designed several EHPA buildings where we solidly grouted the walls and I still used movement joints at spacing of 25 feet. I haven't had any problems reported to me.

I know another engineer who just adds more bond beam steel and exceeds the 25ft spacing regularly. I haven't tried that but then again I'm conservative.

So if you want to hedge a bit close up your bond beam spacing a bit. I usually try to spacing my bond beams so they run over windows and doors etc... This way my lintel reinforcement can just lap with the bond beam reinforcement. Its my way of trying to also help control cracks around the openings.

At windows and doors I used to try to start my control joints under the lintel and then move up the wall from there just like the books say. I still do that if I have large openings after a long run. But often now I'll just use a straight control joint a few feet away from the openings. I've just had too many masons mess up the detail. Sometimes they extend the control joint through the lintel, sometimes they leave them out entirely, and sometimes they start the joint 8" away from the opening and create an unplanned 8" x 8" column. I've got an iron clad detail that is very easy to read but still I try to make things as simple as I can. Inspection and builders haven't gotten so poor.





John Southard, M.S., P.E.

http://www.pdhlibrary.com

 

[msquared48]

CMU walls are based on 16" modules that lend to the 24 foot control joint dimension.

I agree with the others here that the grout added to an already completely cured form block will not add to longitudinal shrinkage. Any shrinkage seen will be within each cell not adding to the stress from temperature variations already seen.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Ron]

JAE...I think this is probably an erroneous extension of the issue you have when you lay wet block and it shrinks, obviously more than dry block, thus causing more mortar separation. I also think that it does not apply to fully grouted cells. Yes, the block gets wet by the water in the grout, but it has already gone through its autogenous shrinkage and will expand slightly with the moisture then shrink as it dries, but the shrinkage is not likely to be any greater than the initial expansion...thus not an issue.

 

[structuresguy]

See now, I disagree with you guys regarding the shrinkage question. Obviously, if the block is properly cured, it won't shrink on its own. But the grout will shrink at the same, if not higher rate than regular concrete. In a partially grouted wall, the grout shrinking in each cell will cause it to pull on the block around it resulting in some tensile forces trying to shrink that part of the block relative to the surrounding ungrouted areas. But since the grouted cells are spaced out, that pulling force is small once distributed over the length of the wall. But as the grouted cell spacing gets closer and closer, the average pulling force will increase, until max is reached for fully grouted wall. As we know from CIP concrete, shrinkage is a very powerful force. I could easily see it cracking a masonry wall if the joints are too far apart.

Personally, when I do CIP walls, I space joints at 1 * H max. I think I would do the same for fully grouted masonry walls.

 

[structuresguy]

OOps, forgot to mention that the NCMA TEK manual recommends a lot more horizontal steel for fully grouted walls than partially grouted walls. So here they are recognizing the higher potential for shrinkage cracking in fully grouted walls.

Another issue that contributes is the moisture content of the block. For most block that has sat around out in the elements on a job site, the moisture content could be as high as 75% of fully saturated, regardless of how dry it was when it was delivered to the site. Then on top of that, the amount of excess water goes up as the spacing of grouted cells decreases, so you get a much higher (6x) wetting of the wall overall for fully grouted, than for say cells at 48" o.c. So now you have veyr close to saturated block, which starts to dry out after the mortar and grout has hardened. Well, it wants to shrink just from drying out. So that drying shrinkage will add to the grout shrinkage as well.

 

[cntw1953]

If we fully grout one block, would the shrinkage causing movement assuming it sits on frictionless medium?

However, if there are horizontally continuous grouted cells, obviously the shrinkage and joint spacing are crucial factors.

 

[JAE]

Well, if each individual cell, with grout, shrinks, I find it hard to see how the contraction of the grout in each cell creates a compounding effect on the overall wall.

Instead, I see the grout in the cell simply contracting in on itself, creating a small gap between the block and the grout around the inside wall lining of the cell.

Also, I don't know if concrete block shrinks and swells with re-wetting. I thought the shrinkage was due to the initial hydration process and that once cured out there would be a much less amount of shrinkage and certainly not very much wet-induced "swelling" in the block.

For clay based brick, yes - moisture can cause swelling - but not in concrete block, right?

 

[cntw1953]

Agreed. Even it would change a little, shouldn't have much effect.

 

[structuresguy]

JAE: The NCMA literature even references block wetting on site as a contributing factor to shrinkage after construction. And this is the main reason why they got rid of Type 1 and Type 2 block units from the C90 standard a few years back. The Type 1 was supposed to be moisture limited, but once in the field, the manufacturer had no control over it, and typically, the block just sits around in the rain. So the moisture level could not be ensured, hence revision of the C90 standard. Read the NCMA TEK manual, section 10-2 and 10-3 I think is where they mention this, along with their recommendations for control joint spacing and horizontal reinforcing requirements.

Also, you are assuming no bond between the grout and the block cell walls. Since the block is relatively porous, and cement grout bonds pretty well to hardened concrete in general, I think that there would be a pretty good bond between the grout and the block. And any horizontal continuity of the grout would increase that bond.

NCMA doesn't come right out and say that fully grouted shrinks more than partially grouted and thus put your joints closer. Rather, they recommend more horizontal reinforcing in fully grouted walls to compensate for the higher shrinkage, while keeping the joint spacing at the same recommended intervals. Personally, I would rather not have #4 horizontal bars at 8" o.c. in a fully grouted wall. So I put my joints closer and keep horizontal reinforcing the same. IMO, it is 6 of one, 1/2 dozen of another. But the closer joints should be easier and less costly than a bunch more rebar.

 

[JAE]

structuresguy,
I'll take a look at the two NCMA references - thanks.

But I still don't see how vertical "columns" of grout add to horizontal wall shrinkage. And the wetting/drying - I'll have to see what NCMA states but concrete just doesn't swell when wet.