Type N Mortar Substitution in a Fully Grouted Wall
An 8-in. hollow concrete masonry addition of 12,000 sq ft is fully grouted, with No. 5 vertical bars at 32 in. on center. The approved drawings require Type S mortar and a specified compressive strength f'm of 1,500 psi. On the second day of laying, the inspector sees the crew mixing mortar from bags labeled Type N. The foreman says the supplier ran out of Type S, the wall is fully grouted, and "mortar strength barely matters." Joints vary from 3/8 in. to 3/4 in., and no mortar batch records are kept. The contractor offers 28-day mortar cube results of 1,100 psi from an independent lab. The structural engineer of record is reachable by phone but has issued no written change. The project is in Seismic Design Category B and has a special inspection program under the adopted code.
1. Can the inspector accept the Type N mortar on the basis of the 1,100 psi cube results? Explain the code and technical reasoning.
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No. The approved construction documents specify Type S mortar; under TMS 602 and the adopted code, the mortar type is part of the structural design and cannot be changed by field substitution. The 1,100 psi cube result is a mortar property, not the masonry assemblage f'm. The 1,500 psi f'm is verified by prism testing or by the code's unit/mortar tables; replacing Type S with Type N changes the assumed mortar strength and can affect allowable stress, bond, and detailing requirements. The inspector's role is to enforce the approved documents and report the nonconformance, not to perform a design evaluation. The inspector should stop further use of the Type N mortar, issue a nonconformance report, and require a written EOR evaluation and revised documents before any acceptance. If the EOR approves, the approval must be project-specific, sealed, and reflected in the inspection records.
2. What should the inspector do about the 3/4-in. joints and missing batch records?
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Both are nonconformances. TMS 602 requires mortar joints of nominal 3/8 in. thickness, with a permitted range of about 1/4 in. to 1/2 in.; a 3/4-in. joint exceeds that tolerance and may reduce bond and compressive capacity, especially in a fully grouted wall. The inspector should measure and document representative joints, identify locations, and require the contractor to propose correction. Because the wall is already laid, correction may require EOR evaluation and possible removal and replacement, not merely pointing. Missing batch records also prevent verification of mortar proportions and mixing; TMS 602 quality assurance requires documentation of mortar materials and proportions. The inspector should require the contractor to establish a batch log, retain bag labels and delivery tickets, and test mortar in accordance with ASTM C780 if the EOR directs. Do not accept the wall until the EOR resolves both items in writing.