


Concrete Q & A: LowStrength Concrete Masonry Units 

Title: 
Concrete Q & A: LowStrength Concrete Masonry Units 
Author(s): 
Jason Thompson 
Publication: 
Concrete International 
Volume: 
35 
Issue: 
11 
Date: 
November 1, 2013 
ABSTRACT 
I’m trying to resolve an issue on a project with concrete masonry unit (CMU) walls. Although our contract documents specified a compressive strength of masonry (f´m) of 2500 psi (17.2 MPa), the contractor erred and constructed walls assuming an f´m of 1500 psi (10.3 MPa). (While the specified Type S mortar was used, the documentation provided by the CMU producer indicates that the units comply with ASTM C9012,1 which requires a minimum net area compressive strength of 1900 psi [13.1 MPa], rather than the required 3750 psi [25.9 MPa] unit compressive strength indicated in Table 2 of ACI 530.1.2) The contractor has proposed fully grouting these walls with 3500 psi (24.1 MPa) grout. The proposal includes supporting documentation stating that the combination of 3500 psi (24.1 MPa) grout with
1900 psi (13.1 MPa) units will result in walls with an average compressive strength of 2500 psi (17.2 MPa).
The walls were designed as nonloadbearing (flexural loads only) with alternate cells to be grouted and reinforced. ACI 530112 states that the allowable compressive stress available to resist flexure (Fb) is limited to 0.45f´m. ACI 530.1112 also states that the grout strength shall equal or exceed f´m when Table 2 of this standard is used. However, it’s my understanding that the compressive strength to be used for strength calculations is the minimum of the masonry units and the grout compressive strengths, which in this case is 1900 psi (13.1 MPa). Can higher strength grouts be used to compensate for lowerstrength CMUs? Are there any testing requirements that I should consider?

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