Block Cracked - Brick Not

[vandede427]

Existing one-story building, built in mid-90's, unreinforced 8" CMU stack bond, 4" brick veneer running bond, horiz reinf @ 16" (I believe serving as brick ties too), shallow foundation about 3' below grade, expansive plastic clay soils. CMU mortar strength called out in existing drawings is 750 psi (which seems low to me, I call out 1500 psi).

Interior face of CMU has crack in every single vertical mortar joint. Exterior face of brick has no cracking at all.

To me, the vertical cracking in the stack bond is analogous to the tension cracking in a concrete beam. I'm just extremely surprised that the brick isn't cracked at all.

Anybody offer up some opinions on the mechanism happening here?

 

[ToadJones]

how big are the cracks?

 

[Ron]

If the cracks are only vertical, then they are likely a result of shrinkage, not bending. As TJ noted, how wide are the cracks? Are they wider at top or bottom, or fairly consistent in width top to bottom? How far apart are the cracks. If they are regularly spaced, say every 8 to 12 feet, then they are most likely just drying shrinkage.

The stacked bond is more likely to show such cracks than a running bond.

 

[MiketheEngineer]

Tend to agree - but when you say "expansive plastic clay" soils - I get somewhat nervous as these have been known to do VERY strange things

 

[msquared48]

Are these cracks new?

Alabama had some serious freezing weather this year, something rarely seen there. Could be reading the contraction due to that. If so, they should close when the weather warms up.

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

 

[vandede427]

they are new in relation to the buildings age (last 5 years) but have gotten worse this past 16 months.

I don't think it's drying shrinkage. That would take place early in the structure's life and the vertical cracks are at every mortar joint (16" o.c.).

Alabama did have a cold weather. But I'm convinced it's due to the vertical movement from the expansive clay. We had a terrible drought in 2008 and we've had more than abundent rain this past 6 months.

I just don't understand why the brick isn't cracked at all whereas the block is completely cracked. That's the mechanism I'm trying to figure out.

 

[steellion]

I agree with Ron. CMU and brick would have different contraction rates due to temperature and different mortar types. It's possible that weak mortar cracked when the CMU blocks shrunk and separated.

 

[vandede427]

forget to mention. in addition to the vertical cracks, there is a severe horizontal crack at the bottom of the bond beam at roof truss bearing.

 

[Ron]

Do any of the cracks go through the block or just through the mortar joints? Can you post a couple of photos? What is the crack width, top to bottom? Is there a chance the brick veneer is on a separate foundation?

 

[vandede427]

this is a picture of a gable wall (this room has scissor trusses). horizontal crack is widest. vertical cracks are much smaller but more repetitive, go from floor to horizontal crack which is just below the bond beam elevation at bearing walls.

 

[vandede427]

this is a room in another building adjacent to the previous bldg. similar horizontal crack below bond beam. vertical cracks weren't as prominent. similar roof construction.

 

[Teguci]

The brick could be buckling towards the CMU wall while the CMU deflects vertically to adjust for foundation settlement

 

[ToadJones]

Whats the depth of the footing?
Any Chance you have a frost heave situation ?

 

[Ron]

vandede47...are you in south Alabama or north? Any chance of a wind uplift event? Wouldn't have to be a catastrophic one, just one strong enough to cause a bit of tensile strain in the block to bond beam interface.

Also, is the brick tied at all to the bond beam or does it "float"?

I don't see anything that points to foundation movement yet. Any separations at the floor slab near the block wall?

 

[cvg]

if you suspect shrink swell of the foundation soils, then you need to start your forensics at the foundation level. What type of foundation, how deep, any cracking, any water / drainage issues at the foundation, more details on the type of clay, any large trees planted or cut down recently near the foundation, broken water or sewer line near the foundation etc. Also, remove the drywall - you need to see the cracks.

 

[ToadJones]

I investigated a carport, albeit in northern Ohio, that over the course of a few winters, absolutely disintegrated due to frost heave. The some of the block walls were literally laying in piles and the roof was collapsing.

 

[vandede427]

toad: south alabama frost depth is like 2". footings bear at -48" but i'm not sure what good that does. i have boring logs that show plastic clay 20' deep.

ron: we haven't really had a big wind event lately. it's south AL but still only 100 mph, not coastline.

cvg: yes, water is the clue to this mystery.

i held back this info at first to simplify this thread, but here's something more to complicate the issue.

there are three buildings. the two that have the vertical cracks have very poor water drainage around the perimeter. ponding water that drains back toward bldgs. bldgs do not have gutters. i'm thinking this bldg has heaved due to excessive water.

the third bldg has experienced ~1" settlement at perimeter wall and has pulled slab with it. though the existing drawings show slab poured to inside block wall, this evidence shows slab was poured into a form block and is integral with wall. see this attached picture (wall on left is exterior block, wall on right is interior partition). this building has a sump pump under it and trench drains around the perimeter. i'm thinking the pump and trenches have over drained the soil and caused the settlement.

 

[KootK]

While I don't have anything new to offer regarding an explanation of the CMU cracking, I have a theory regarding the fact that the brick is NOT cracking.

Stack bond sucks; Running bond is great.

If the stack bond CMU wall experiences any tension, all you've got to resist it is the mortar in tension. By the time the horizontal reinforcing is engaged, the crack has already formed.

To crack the running bond brick, you have to break the vertical mortar joints (tension) AND overcome the shear resistance of the mortar at the horizontal joints where bricks are interlocked.

The shear mechanism is much stronger than the tension mechanism.

 

[vandede427]

Stack bond does suck.

 

[concretemasonry]

Stack bond (technically, "other than running bond" by definition) is inappropriate unless it is for architectural purposes. I assume the wall is not a bearing wall. What is the horizontal spacing of vertical control joints?

The mortar strength may not be the problem and the ASTM C270 appendix recommends using the lowest strength possible and still carry the loads. This is because the lower strength mortars can have properties other than COMPRESSIVE strength that are superior to higher strength mortars.

VERY IMPORTANT - When trying to analyze the cause of the problem, keep in mind that clay brick has a long term expansion, while concrete has a long term shrinkage. The block are usually 90% cured when delivered and higher if they were inventoried. Obviously, the mortar for both the brick and block will shrink uniformly, especially if the wythes of the wall are laid at the same time.

What type of reinforcing is in the brick wall at 16" O.C.? I assume it is continuous 4" wire joint reinforcing at 16" O.C. vertically (rare)or is it just the vertical spacing of some sort of wall tie?

It appears the brick veneer is expanding, keeping the mortar in compression (or reducing the tenson), while the mortar in the block wall is shrinking slightly. Any addition stresses due to temperature (outside exposure) or stresses due to soil movement can can either or add to the mortar stresses.

The stack bond is action as perfect crack control mechanism, but allowing uniform cracking to minimize any crack width. If the CMU wall was in stack bond and both wythes had multi-wythe reinforcement (tie wires or eye & pintel brick ties), the wythes would act as one and cracks could be eliminated or at least minimized.

Dick

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