Control Joints for fuly grouted masonry
Control Joints for fuly grouted masonry
(OP)
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?
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?






RE: Control Joints for fuly grouted masonry
RE: Control Joints for fuly grouted masonry
FYI, if I remember correctly the NCMA recommendation is 1.5*wall height up to a maximum of 25'.
RE: Control Joints for fuly grouted masonry
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.
RE: Control Joints for fuly grouted masonry
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.
RE: Control Joints for fuly grouted masonry
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.
RE: Control Joints for fuly grouted masonry
www.nc
NCMA - TEK 10-03 - Control Joints for Concrete Masonry Walls - Alternative Engineered Method:
www.n
RE: Control Joints for fuly grouted masonry
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.
RE: Control Joints for fuly grouted masonry
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
RE: Control Joints for fuly grouted masonry
RE: Control Joints for fuly grouted masonry
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
RE: Control Joints for fuly grouted masonry
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
RE: Control Joints for fuly grouted masonry
RE: Control Joints for fuly grouted masonry
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.
RE: Control Joints for fuly grouted masonry
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.
RE: Control Joints for fuly grouted masonry
However, if there are horizontally continuous grouted cells, obviously the shrinkage and joint spacing are crucial factors.
RE: Control Joints for fuly grouted masonry
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?
RE: Control Joints for fuly grouted masonry
RE: Control Joints for fuly grouted masonry
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.
RE: Control Joints for fuly grouted masonry
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.
RE: Control Joints for fuly grouted masonry
On thinking further here, and I realize that is dangerous,
I think what the concern is here is the horizontal bond beams - the grout in those horizontal ties shrinking, loading up the bond beam steel. I still do not see how the grout in a single isolated cell could affect the whole structure.
As far as I am concerned, the vertical grouting does not enter into play here laterally, maybe vertically, but then you are generally dealing with 4 foot lifts and sequential curing, allowing the shrinkabe to happen gradually..
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Control Joints for fuly grouted masonry
It is like chasing wisps of smoke to try to differential the minor difference between ungrouted, partially grouted and fully grouted when the majority of the wall is composed by the CMUs (by weight or volume).
The location of control joints has been beat to death by many engineers and professionals for 40 years and a method to calculate the spacing not matter what software or assumptions are made. - That is the real reason for the current NCMA and ACI 530 recommendations. This is based on the performance of masonry for many years with all types of construction and aggregate types (which no one on the forum has bothered to think about or consider).
Next someone will start to worry about the temperature variations in different climates and the effects of carbonization of the cement/lime (free and added lime) portion of the wall and moisture over a long period of time, both of which have miniscual effects
Dick
RE: Control Joints for fuly grouted masonry
Wetting the wall after laying the block is significantly not the same as laying wet block and letting it shrink. The effects are totally different.
RE: Control Joints for fuly grouted masonry
Dick
RE: Control Joints for fuly grouted masonry