CMU Wall Construction - How Many Courses High Between Mortar Cures?

[Beanbag1]

I am putting together a set of construction specs for a new 15-foot high building with 8-inch CMU exterior load bearing walls, with vertical reinforcing every 32-inches C/C, and I would like to know how many courses high can the blocks be laid before the contractor has to stop and wait for the mortar to cure, before continuing on up. I was told by another contractor that it's 8 courses and then wait 12 hours, but he doesn't recall where he got that information from. Can someone please tell me where in ACI 530 this is conveyed? Thanks.

 

[MrHershey]

You're looking for ACI 530.1, not ACI 530. Specifications would have all those sorts of instructions.

And really has more to do with the procedures used during grouting than mortar, if I remember correctly. Typically it's 5'-0" with no time requirement (we usually allow to 5'-4") unless they want to do a lot of the high lift grouting requirements. If they do, then they can go up to 12'-8" if they wait long enough (4 hours I think), have the right slump, provide cleanouts, etc. Can go even taller if they can still meet the grout space requirements and are willing to do test panels.

 

[concretemasonry]

As with most masonry, mortar strength is not critical in the strength of a wall. - Does it keep masonry units together or keep them apart?

Actually is helps hold the units in relationship to other units to form as a wall which is the real structural element in masonry.

For reinforced/partially reinforced masonry, it(mortar) is used to hold the units in space and allow stability until the wall is grouted (fully or partially) as required.

If you follow the classic ACI/TMS/MJSC standards in the document commonly published as ACI 530, there is no time limit on early grouting as long as the wall is temporarily braced. There have been numerous situations where walls were made and performed well with very, very short-term construction periods (hours or minutes) that were moved and handled after the short wall construction period. In some the walls (8" units in 8'x24' dimensions were made in a plant and moved on rails as the set up prior to being erected in high rise masonry structures. Other common situations were in high rise (7 to 20 story) buildings in the seismic areas of the U.S. that were build one floor per day or every 2 days depending on the construction and speed of erection of cast on site floor slabs or precast floor slabs.

The strength of mortar is not that important in the construction of masonry walls where the masonry units are the critical materials. Block prisms(2 block hollow core) of 4800 psi (f'm) can be made with 2200 psi mortar.

Mortar still has a mystery about itself.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[Beanbag1]

Thank you for the replies. Yes, I agree that grout is addressed in the construction cose but mortar does not seem to be addressed to the point for which I am addressing. To put my question into a hypothetical situation, is it permissable for a contractor to build a 50-foot high x 8-foot wide x 8-inch thick CMU wall in one day, provided it is braced, reinforced, stablized, etc. etc during and after construction, via an existing steel A-frame tower located behind it, for the purpose of installing a large advertisement sign. The steel tower, CMU wall and foundations would be designed to withstand the greatest wind and earthquake loads known to mankind, so they would not be a concern. My question has only to do with high hign can the mason stack the blocks while the mortar is still wet, without the fear of the mortar being crushed and squeezed out due to the weight of the blocks while the mortar is still uncured. I can't seem to find a vertical stacking block limitation (based on mortar) in any construction code or OSHA. Can anyone guide me to the proper construction code that addresses this vertical stack limitation during construction, as it relates to the mortar curing duration?

 

[jayrod12]

I've never come across a specific value. Generally speaking though I've never seen them go more than 12' in one day.

I don't even know if the mason(s) could lay fast enough to put much more than that down in a standard day considering the grouting and bracing and all of that work that is required to keep progressing. Even if you could throw enough workers at a single piece to lay faster there isn't enough room for them all to do their work.

 

[Beanbag1]

Thank you all. If I come across any code that adresses this limitation (if any), I will gladly post it here.

 

[concretemasonry]

There is no limit established to the rate of vertical construction because the lower courses have already obtained different properties in place as the moisture is drawn out of the paste and absorbed by the masonry for bond and curing purposes. Just imagine what the effects of the pressure on a plastic mortar is as the weight of the wall increases and forces the moisture out to be absorbed by the masonry and used in the hydration process that is in effect before the units are even laid. - Also, the heights are limited by logistics.

All too often, mortar and grout are confused. Grout MUST have excess moisture to achieve the 8" to 11" slump necessary to fill all the voids. Mortar must be workable to be used to set the individual courses to the established module (usually 4" or 8") and the guided by the over-all height requirements as the wall is constructed. - The 4', 8' and 12' heights are for walls and not the. - I have seen 20 & 24' high wall build, but it was a very, very long day, but was controlled by the stability of the wall immediately under construction.

Some buildings are properly construction at a floor per day (8' masonry wall and precast slabs). - Just look at the loadbering hotels ans apartments in southern California that were built as long as 40 years ago.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.