Reinforcing an old unreinforced CMU wall
Reinforcing an old unreinforced CMU wall
(OP)
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?






RE: Reinforcing an old unreinforced CMU wall
Look at your masonry code.
Attached, values of characteristic strength (as above) with the sources for the values
RE: Reinforcing an old unreinforced CMU wall
RE: Reinforcing an old unreinforced CMU wall
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.
RE: Reinforcing an old unreinforced CMU wall
RE: Reinforcing an old unreinforced CMU wall
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Reinforcing an old unreinforced CMU wall
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.
RE: Reinforcing an old unreinforced CMU wall