Preblended Type N Mortar Substituted for Type S
A two-story bearing-wall building uses 8-in. nominal CMU with a specified compressive strength f'm = 2,000 psi. The drawings require Type S mortar by proportion specification and ASTM C90 units with a minimum net-area strength of 2,150 psi. The contractor's submittal lists a preblended Type N mortar, explaining that the supplier's Type S was out of stock and that Type N has more lime and is more workable. The contractor also points to 28-day mortar cube results of 1,900 psi and argues that this exceeds the 1,800-psi property requirement for Type S, so the substitution is equivalent. The structural engineer is on vacation and the masonry is scheduled to start Monday. Cells are grouted at 32 in. on center and at all jambs.
1. Does the Type N mortar support f'm = 2,000 psi? Walk through the determination.
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No, the substitution does not support f'm = 2,000 psi on the basis presented. Under TMS 402, when f'm is determined from the table, it depends on both the unit strength and the mortar type. With 2,150-psi units, Type S mortar gives f'm = 2,000 psi; Type N gives f'm = 1,500 psi. The substitution therefore drops the assumed strength by about 25%, which is not made up by the vertical bars unless the engineer reruns the design. The 1,900-psi cube result does not close the gap: mortar cube strength is not f'm. f'm is a property of the assembled masonry and is established by prism tests (ASTM C1314) or by the table; a cube test measures only the mortar. In addition, 1,900 psi exceeds the Type S property-spec minimum of 1,800 psi, but the property specification also includes water retention and air content, and the project specified the proportion specification. A stronger cube from a Type N mix does not make the mix Type S.
2. What paths could the contractor use to demonstrate compliance, and what would you require before allowing the substitution?
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The contractor has three legitimate routes. First, switch to Type S mortar and resubmit. Second, build and test prisms in accordance with ASTM C1314, at least three prisms made with the same units, mortar, and grout, cured and tested at 28 days, and have the engineer evaluate the results using the TMS 602 prism acceptance criteria. Third, ask the engineer to redesign the wall for f'm = 1,500 psi, checking whether the reinforcement and grouting are adequate. Until one of these is complete, the mason should not lay structural masonry with the Type N product. The inspector should not accept a verbal approval and should confirm that the mortar in the mixer matches the approved submittal; preblended bags make this easy to check but also easy to swap.
3. If the engineer accepts the substitution, what field controls become more important?
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With a lower assumed f'm, workmanship defects matter more. Control joint thickness at about 3/8 in., tool the joints when thumbprint hard, keep mortar out of the grout space, and verify grout slump and consolidation at every lift. Confirm that the units actually delivered meet the 2,150-psi submittal, since the table value depends on unit strength as well. Because the as-built strength assumption has changed, the acceptance should be documented in writing by the engineer, with the prism results or analysis attached, so the file shows why the substitution was allowed.