Reinforcing an old unreinforced CMU wall 1

[whir]

I am reinforcing and an 13' tall unreinforced CMU wall built in the early 60's. The wall was 8" CMU with a flemish course of brick that tied the brick veneer to the wall making the effective depth of the wall 12". We have removed the brick veneer and I have to reinforce the block. I am assuming a net compressive strength of 1500 psi for the masonry. Does anyone know if that assumption is inappropriate for some reason?

 

[ishvaaag]

It depends on what are you talking about; characteristic strength values on the whole section (to be compared with factored stresses got after every structural effect that needs be considered, such buckling effects as well) may range between 1/7th and 1/5th of what you state for concrete block masonry; poor construction or materials, or deterioration by ageing or other cause may require go under such values.

Look at your masonry code.

Attached, values of characteristic strength (as above) with the sources for the values

 

[ishvaaag]

The values above are for plain masonry, on whole area. For a reinforcement scheme 1500 psi on the actual section area sounds reasonably if the blockwork looks well preserved.

 

[concretemasonry]

whir -

The assumed compressive strength is low, but testing the CMUs in place is not easy and justified for a small project. If you wear a belt ans suspenders, the assumed 1500 psi could be very conservative. Depending on the actual CMUs since the block properties (web thickness, face shell thickness and core(s) configuration) can have wide variations, because the antiquated ASTM C90 standards are just minimums. In reality, CMUs are much stronger than the ASTM standards because it is cheaper and easier to make a stronger unit that is more sale able. Usually, a manufacturer will shoot for 30% to 40% over ASTM compressive strengths when exiting a plant after 24 hours or so and curing continues after that. - A company I worked for had several production lines in Spain and it was very cheap to add more water to the CMU mix to increase the strength (the world of zero slump concrete) because the units there were not exposed, the labor cost was fairly low so a high degree of precision beyond the typical standards was not justified to create a usable unit for the loadbearing projects supplied.

If you have a bonded brick veneer, it is likely the bonding tie was a cross course of brick that is every 6th course of brick (assuming modular clay brick) that involves a special 8" CMU unit to accept the 8" deep brick (traditional header) that could interfere with future grouting and reinforcement. The alternate method to create a bonded wall was to use a full "collar" joint to bond the brick to the CMU and transfer the shear. - Some investigation on the "guts" of the walls could be very beneficial.

Dick

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

 

[whir]

Your description is correct. There is a row of CMU that has a continuous brick course sized notch where the brick course runs. From product information that I have found, that CMU block will have 2 webs instead of 1 as a normal block would have. I have typically pumped grout up into CMU cells when reinforcing existing block, but because of these blocks, I am asking the grout manufacturer if there is a stiffer mix that can be used in this application.

 

[msquared48]

If the wall is fully grouted, 1500 psi is reasonable, but if not, then 1350 psi.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[concretemasonry]

whir -

Without knowing your location, it sounds like a structure that is commonly grouted fully irregardless of the needs structurally, since full grouting may be cheaper. Block with only one core are localized and rare in the world. Block traditionally have 3 webs (2 cores) or 4 webs in older traditional construction that is difficult to reinforce or grout properly. The best CMU to use has webs placed/spaced (8" o.c.) on a modular dimension compatible with the reinforcement (usually 8").

The grout should not be "stiff" since the ASTM grout and most other standards call for 8" to 11" slump to insure a good bond to the reinforcement and not to arbitrarily fill holes in a wall where it is a "warm, fuzzy" and supposedly conservative assumption, but eliminate other advantages, despite any possible masonry wall properties based on an assumed f'm according to ACI 530. The higher slump allows the CMUs to absorb the excess moisture, create grout consolidation and provide a good curing environment for the grout since the CMUs are already cured (more or less) and provide thermal stability for curing.

Whatever you do, grouting can give you a confidence in the structure. The assumed masonry strength values are more than actual masonry strength unless you want an f'm of 3500-5000 psi.

Dick



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

 

[whir]

I misspoke on the webs. I was thinking of the internal webs due to the fact that you mentioned where the modified block internal webs will not line up with the typical block webs and create voids that the grout can run through. As I watch them take this wall apart, I am seeing that the horizontal reinforcement was placed half way in the CMU with the exterior wire extending out to be embedded in the brick veneer. The exterior wire is now being destroyed with the removal of the brick. This worries me somewhat as it may open the way to temperature cracking. I can't think of a practical way to replace the horizontal reinforcing though. Any thoughts?