Nonstandard Cylinders, Low Breaks, and a Superintendent Who Wants Acceptance
On a mid-rise project, a field technician molds twelve sets of 4 by 8 in. cylinders for 4,000 psi elevated slabs. The only tamping rod in the truck is the 5/8 in. rod, so he uses it on the 4 by 8 in. molds, filling in two layers at 25 blows per layer and not tapping the outside of the molds afterward. The filled molds ride in an open plastic crate in the bed of a pickup in direct sun for a 40-minute drive to the laboratory, then sit in the same crate overnight in an unheated garage. The afternoon high was 98 degrees F and the cylinders were exposed to it for about five hours; the overnight low was 61 degrees F. They are stripped and placed in the 73 degree F moist room the next morning. The 28-day results range from 3,180 to 3,860 psi, with a project average of 3,560 psi. The superintendent argues the average is within 500 psi of f'c and that the low cylinders were probably just bad breaks.
1. Identify the ASTM C31 requirements that were violated during molding, initial curing, and transport, and explain how each one biases the measured strength.
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Several deviations stack on top of each other.
Rodding: 4 by 8 in. molds take the smaller-diameter rod (typically 3/8 in.), not the 5/8 in. rod used for 6 by 12 in. molds. A 5/8 in. rod in a 4 in. diameter mold concentrates blows in a small area, pushes coarse aggregate to the sides, and produces segregation and voids rather than uniform consolidation. Failing to tap the outside of the molds leaves entrapped air voids on the surface and inside the specimen.
Initial curing temperature: the first 24 hours must be at 60 to 80 degrees F. These molds went through a 98 degree F afternoon in an open crate in direct sun, then a 61 degree F night. Elevated early temperature accelerates hydration and typically raises the 1- to 3-day strength while lowering the 28-day strength; the wide day-night swing causes differential thermal movement and microcracking.
Handling: specimens should remain undisturbed, protected from sun, vibration, and moisture loss through the initial curing period, and should not be carted around in an open pickup bed. Movement before final set damages the internal structure.
Net effect: measured strength is biased low and variably, so the numbers do not represent the concrete in the slab. The superintendent's argument is beside the point - the 500 psi rule applies to valid standard-cured test results, and these are not that.
2. The results look low. What is the correct engineering response before anyone accepts or rejects the slabs, and what criteria decide the outcome?
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Because the specimens do not comply with ASTM C31, the results cannot be used either to reject the concrete or to accept it. The first move is not a debate about the average; it is a validity determination.
Document the deviations in a nonconformance report, retain the break sheets (fracture type, cap condition, individual cylinder values) and the temperature history, and check whether spare cylinders or companion sets from the same trucks exist. If any set was handled correctly, those results may be usable.
If no valid results exist, the engineer investigates the in-place concrete. The standard path is cores per ASTM C42: at least three cores from each area represented by the questionable results, taken away from reinforcement and critical sections, moisture-conditioned and tested per C42, with the average of three cores at least 0.85 f'c (3,400 psi here) and no single core below 0.75 f'c (3,000 psi). If cores meet those thresholds, the concrete can be accepted even though the cylinder results failed. If cores fail, the design professional evaluates structural adequacy - analysis, a load test, or remediation.
Field-cured cylinders can answer a different question (adequacy of curing and protection in the structure), but they must never be substituted for standard-cured specimens in an acceptance decision.
3. List the corrective actions you would require so the next 500 cubic yards do not repeat this, and note how you would verify the fix worked.
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Fix the equipment and the process, then prove it with data.
Equipment and materials: stock the truck with both rod sizes (5/8 in. for 6 by 12 in. molds, 3/8 in. for 4 by 8 in.), a mallet for tapping molds, a proper sampling pan, a slump cone, and a thermometer. Provide an insulated, rigid curing box that holds 60 to 80 degrees F with a recording min-max thermometer, and keep it out of direct sun.
Procedure: mold on a level, stable surface; rod each layer to the specified depth with the correct rod; tap the molds; cover with a nonabsorbent lid to prevent evaporation; keep specimens in the box, undisturbed, for the initial curing period and do not transport them until initial set and the standard curing window allow. Log the box temperature at each set.
People: retrain and re-qualify the technician, and verify certification currency for everyone molding or breaking cylinders. Add a second-person check on the field data sheet (rod size, layers, blows, box temperature).
Verification: mold a check set alongside the next several placements, review the initial curing logs weekly, and watch the within-test variability between paired cylinders. If paired results still scatter widely or strengths stay depressed relative to the mix design, the problem is in the concrete or the laboratory, not just the field technique.