Spraying Surface Bonding Cement

[asmith555]

We have been trowling it to a cmu wall. It is very time consuming and backbreaking. I was wondering is there a way to spray the stuff. If so what equipment would you suggest?

[concretemasonry]

No.

Not a better way. There are always cheaper shortcuts.

Dick

[hawkeye70]

I've been researching dry stacked sbc methods, so I know really nothing about it, but I came upon a site www.drystacked.com where this guy talks about spraying it with a homemade stucco type sprayer. Thought I'd pass it on...not to hijack, but maybe somebody with some knowlege could comment on it?

[concretemasonry]

You can spray but it is not as good.

Troweling gives you better orientation of the fibers in the mix.

You could spray and hit with a trowel later.

If this is a wall that will retain soil, make sure that you have enough coverage on the bottom 1/3 especially. This is a critical area for shear problems.

If this is for a vertical load, I hope you realize the a surface bonded wall will not carry as much load as a conventional wall, but it may be OK for what you are doing. Any deviation from vertical will reduce the strength, so use a mud (preferred) joint half way up or use shims (second choice) periodically to maintain plumbness. Brace well until you have permanent lateral support at the top.

Dick

[hawkeye70]

Dick,
Could you make an estimate as to how much less vertical load surface bonded can carry? What if all cores are filled? How do you think this compare to a solid wall?

I'm thinking in terms of basement walls, with say a six foot backfill, pillasters, etc.
Thanks.

[concretemasonry]

The actual vertical strength difference can vary widely because it depends on the ability to maintain a plumb wall and how well the block stack on each other. The surface bonding does not really carry the vertical load because it is so thin. It does stabilize the wall to prevent buckling.

Nothing will replace the ability of the mortar to provide a 100% transfer of load from one block to another. A dry stack wall is carrying all the loads on the point contacts of the individual block. These point contacts will crush because of the high stress and the load is then distributed to the crushed area and to other parts of the block that are now in contact.

Where it is accepted by codes, it is usually based on tests of walls built by people experienced in this construction methods. Actual surface bonded construction does not use mortar, so there is no mortar to make up for variations.

Filling the cores will improve the strength, but not to the degree it helps with conventional masonry.

Surface bonding will increase the flexural strength (bending from a horizontal load). This is the strength increase that is generally advertised. There is a limit to how much flexural strength can be increased since there is still point contacts.

Surface bonding is a good sytem for non-critical structures where is is used as permitted and applied as required. It can be used for load bearing structures within the constraints of the code approval. It will not have the strength of plain convention masonry with the same strength units. Filling the cores of surface bonded block can acheive a compressive strength approaching that of conventional block filled. If the wall is reinforced, any advantages of surface bonding disappear in comparison to conventional block reinforce with the same amount od steel.

For a basement with 6' backfill it will have adequate strength for most approved soil types. without filling it will be far weaker for vertical load, which is not really a factor on a basement. One troublesome area is the shear strength of the lower near where the bottom two or three joints between block would be. I suspect this may due to the extreme care that must be taken when applying the surface bonding in this difficult to work area and the fact that lateral loads are high. In this area, you have high lateral loads that try tp prevent the layers of block from sliding and this is only resisted by a 1/8" or so layer of surface bonding on each side of the wall.

If you throw pilasters into the wall you obviously are substituting the solid, reinforced concrete masonry for the surface bonded wall. Normally, pilasters are not used on normal basements and are discouraged by many people since a few reinforced cores in the wall is more desireable for many reasons.

Surface bonding is a good waterproofing material and some people use it as a waterproofer on conventional block walls as opposed to parging. It still does avoid the problem of the joint between the slab and wall due to the shrinkage of the concrete floor.

[Footing]

Basic info on spraying surface bond cement:
resarch.com/bonsalamerican/PDFdocs/SURFACEB.PDF

“People say believe half of what you see,
Son, and none of what you hear.”
Norman Whitfield and Barrett Strong , 1966.