Low Prism Breaks, Weak Mortar, and a Grout Crew Tapping the Wall
The project is a five-story, 12 in. fully grouted CMU shear-wall building. The design specifies f'm = 2,500 psi, 3,000 psi grout, and Type S mortar. At the first test interval, three prisms break at 28 days at 1,760, 1,820 and 1,910 psi. The lab report notes that the prisms were field-cured in the site trailer, that they were made from block and mortar sampled from the wall, and that they were capped the morning of the test. At the same interval, ASTM C780 mortar cubes average 1,450 psi and grout cubes average 2,150 psi. The mason added water to the mortar throughout the morning because it was 'stiff in the pan.' Grout was placed in a 12 ft lift through a 4 in. opening and consolidated by a laborer who says he 'tapped the wall with a hammer.' The first two floors are already grouted and the third floor is ready to place.
1. Interpret the results. Can you accept the first two floors? What must be verified before conclusions are drawn?
Read the worked answer
Do not accept the floors, and do not keep retesting until a set passes. A prism set averaging about 1,830 psi against a specified 2,500 psi is a 27% shortfall and is a reportable nonconformance.
Before drawing conclusions, validate the test itself: confirm the prisms were made, cured and capped per ASTM C1314 and that curing matched the wall - a trailer in the sun is not representative job conditioning; confirm the units, mortar and grout sampled are the same as those in the wall; check cap thickness and flatness and the lab's qualifications.
Then notify the engineer of record in writing with the data. Acceptance of the masonry is the EOR's determination, not the lab's and not yours. The EOR may require additional prism sets, cores, or in-place evaluation such as flatjack testing, and may accept the wall if in-place strength is adequate. Until that determination, hold grouting above the second floor - placing unverified work on top of unverified work only enlarges the problem.
2. Using the mortar and grout data, what is the likely cause of the low prism strength?
Read the worked answer
The data point at mortar and consolidation, not grout. Grout cubes at 2,150 psi clear the 2,000 psi minimum for ASTM C476 grout, though they are below the 3,000 psi specified. Mortar at 1,450 psi is below the 1,800 psi minimum for Type S - already the strongest common mortar - so the mortar is the prime suspect.
Adding water to stiff mortar on the board raises the water-cement ratio, reduces strength, increases shrinkage, and is not permitted once the mortar has begun to set. Retempering beyond the specification's time limit (typically 2 1/2 hours after mixing, less in hot weather) means the mortar should have been discarded.
Consolidation matters just as much. Tapping the wall with a hammer is not adequate consolidation for a fully grouted 12 in. wall; grout must be mechanically consolidated or rodded in lifts so it fills the space and encases the steel. Voids lower prism strength directly. Also pull the unit supplier's C90 reports - prism strength tracks the weakest link, and a strong unit with weak mortar and voids will still test low.
3. What corrective actions do you take for the remaining floors and for the QA program?
Read the worked answer
Corrective plan: (1) stop grouting until the EOR accepts or rejects the first two floors; (2) fix the mortar operation - batch to the approved mix, no water on the board, discard at the time limit, and mix in a proper pan rather than working a stiff batch with a trowel; (3) fix grout placement - correct lift heights and access, mechanical consolidation, and a pre-placement meeting with the crew and the inspector; (4) increase verification - additional prism sets from the next lift plus mortar and grout specimens at the specified frequency, and add a preconstruction prism test on the next project to establish the mix before the wall starts; (5) verify the testing lab and require reports to state sampling location, mix and curing method.
Document all of it. For already-grouted floors, the EOR's evaluation decides whether they stay or are repaired. Write the lesson into the QA plan: the 28-day prism told you what the day-one mortar test could have caught.