j-bolts in CMU question.

[boyack123]

FINALLY a permit has been issued will be starting footer soon. Stem wall will be 2' high CMU. Does the strength of concrete used to fill cell and place J- bolts matter? Thought i read somewhere long ago that it did. Cant find anything now.

 

[Hilltopmasonry]

Filling the cells should be grout instead of concrete. I would think that the strength matters in regards to j bolts with pull out resistance. You need to have a proper ratio mix for maximum strength. But i know for a fact that most people usually add more sand to the mix when mixing grout on site to save a bag or two of cement. But i have demo'd walls that were filled with sandier grout and will say that it is pretty strong still!

Always best practice to mix all materials per manufacturers instructions.


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[brownbagg]

not really because the cmu block is only 1800 psi

 

[jack stand]

Good point!

 

[Firebrick43]

Cmu Grout is concrete. Typically using pea gravel vs 3/4" stone but still a sand/gravel/cement mix therefore concrete.

As to Cmu as only 1800 psi, think of them as a form that remains in place. Properly grouted with vertical rebar, they can be extremely strong. Remember to knock out a portion of the web every so many course to do bondbeams. And a 4000 or 4500 psi pea mix concrete. Proper filling is going to need a pencil ******** to eleminate and reduce voids.

But why not a formed wall? By the time you grout fill a Cmu wall your normally at a poured walls cost

 

[Hilltopmasonry]

I have to disagree with grout being concrete. Grout is designed to flow to fill block cavities so any large aggregate would cause the grout to hang up in the block webs and create air pockets. All the grout i have used has essentially been soupy mortar with plasticizers added to promote flow

Of course alot of people have used regular concrete to fill the cells of block especially if you are filling a bond beam since it is a large cavity, throw in a piece of rebar and you have one incredible strong section of wall...and i do agree that the block wall is essentially a form for the concrete so your j bolt pullout strength will depend on whatever your fill material is compared to the CMU strength alone

Either or, the above info is minor details. As long as whatever mix is used isnt too sandy and mixed properly im sure you will be good to go especially with any sort of residential construction




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[rburke65]

IDK, but I'll bet your local concrete supplier could answer your question.

 

[Firebrick43]

I have to disagree with grout being concrete. Grout is designed to flow to fill block cavities so any large aggregate would cause the grout to hang up in the block webs and create air pockets. All the grout i have used has essentially been soupy mortar with plasticizers added to promote flow

Of course alot of people have used regular concrete to fill the cells of block especially if you are filling a bond beam since it is a large cavity, throw in a piece of rebar and you have one incredible strong section of wall...and i do agree that the block wall is essentially a form for the concrete so your j bolt pullout strength will depend on whatever your fill material is compared to the CMU strength alone

Either or, the above info is minor details. As long as whatever mix is used isnt too sandy and mixed properly im sure you will be good to go especially with any sort of residential construction




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You don't use any pea gravel in the "grout" ?

 

[fairway605]

Quikrete makes a pre mixed core fill grout, they have a coarse mix and a fine Mix.

 

[readhead]

The larger issue is the reinforcement. There needs to be proper rebar placement coming up from the footing so that any force applied to the J bolts is transferred back into the footing and not just a stack of blocks.

 

[Hilltopmasonry]

You don't use any pea gravel in the "grout" ?

No, grout is basically a watery mortar. Most of the premixed grout has additives (plasticizers) added to help make it flow

Usually there is a lot of block-lock (wire) and rebar added in the walls that all locks together when the grout is added

Dont get me wrong concrete will do a great job as well and i am just clarifying the terms. Both mortar and concrete use Portland cement the difference is what aggravates are used and mortar has lime added to make it workable


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[flan]

I used bagged concrete and mixed it a bit loose. Then used a long piece of rebar to work out all the air bubbles and voids. Seems to be working good!

 

[Firebrick43]

No, grout is basically a watery mortar. Most of the premixed grout has additives (plasticizers) added to help make it flow

Usually there is a lot of block-lock (wire) and rebar added in the walls that all locks together when the grout is added

Dont get me wrong concrete will do a great job as well and i am just clarifying the terms. Both mortar and concrete use Portland cement the difference is what aggravates are used and mortar has lime added to make it workable


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Ok, many Mason's around here use a soupy pea concrete and refer to it as grout. Must be a regional bastardization of technical terms. Sorry.

 

[Hilltopmasonry]

Ok, many Mason's around here use a soupy pea concrete and refer to it as grout. Must be a regional bastardization of technical terms. Sorry.

No need to apologize, its really is all the same. Actually most people dont know what they used to grout the cells...mortar, concrete, grout....what the hell is the difference it all does the same thing, fill er up!!...haha


*edit*

I just talked to a bricklayer friend of mine about this that worked on a jail and he said that they used grout with pea gravel aggregate on certain walls and some walls were no gravel which was spec'd out by an engineer so i guess we are both right now i am confused as well! lol

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[joes169]

I'm with "Hilltop" for the most part, most grout you'll see in residential is course sand with no stone. I've said it here before, and I'm sure someone will argue with it again, but this is one of those scenarios where "stronger isn't better" and "overkill" is actually ignorance. Really elementary stuff for masonry construction, the strongest part of a masonry assembly should always be the units themselves, not the mortar, grout, etc.....

To give an idea, it's not uncommon on commercial jobs to see a minimum grout strength AND a maximum grout strength in these parts.

not really because the cmu block is only 1800 psi

Not really true, most are 2-3 times stronger than that. Don't be naive and get hung up on generic ASTM standards........

 

[Firebrick43]

Really elementary stuff for masonry construction, the strongest part of a masonry assembly should always be the units themselves, not the mortar, grout, etc.....

can you please enlighten us kindergarteners as to why the Cmu has to be the strongest part? What about the rebar? What about dry stacked CMU and surface bonding cement? What happens when you have a grid core of steel reinforced concrete/grout and why from a structural point is it different than say insulated concrete forms?

To give an idea, it's not uncommon on commercial jobs to see a minimum grout strength AND a maximum grout strength in these parts.

on the same job? Or different specs for different jobs? Is there a reason other than the engineer "engineering" to the strength required?

........

Thank you

 

[joes169]

The cmu doesn't "have" to be the strongest part, but there's decades of conventional practice to prove that using high-strength grout in the wrong application can be detrimental to the entire assembly. One of the biggest issues is that it lends to additional shrinkage cracking through the units, sacrificing the strength of the non-re-enforced block, which sacrifices the assembly as a whole. Also, CMU are close to the perfect environment for curing, so it's not uncommon to see a 2500 psi rated mix exceed 3500-4000 psi in actuality in the wall due to the amount of moisture surrounding the pilaster.

Assuming that 4-4.5k psi pea gravel concrete is an acceptable replacement for grout in cmu walls is irresponsible, as there's plenty of info available out there to debunk the idea. Most importantly, pea gravel concrete will segregate long before grout will at the high slump that is required for grouted assemblies.

As for other other examples, they're not applicable to the OP's situation.

Lastly, YES, I have seen (and been bound to adhere to) minimum & maximum grout compressive strengths on the same job. I would assume the reasoning for those specs is that the engineer had seen his share of high strength grout failures with his own eyes........