Low Cylinder Breaks and Site-Added Water on an Elevated Slab
A 4,000 psi normal-weight elevated slab mix contains 560 lb/yd³ of cementitious material with 25% Class F fly ash and a design w/cm of 0.51, placed in 50°F weather. During the third of six placements the finisher added about 5 gal/yd³ of water at the truck to raise the slump from 3 to 6 in. The 28-day cylinder results for the last three consecutive sets are 4,150, 3,980 and 3,620 psi. The 56-day breaks from the same sets are 4,620, 4,390 and 4,120 psi. The specification requires 4,000 psi at 28 days, references ACI 318 acceptance criteria, and does not authorize a 56-day acceptance age. The affected columns carry a tributary dead load that was calculated with the specified 4,000 psi strength.
1. Apply the ACI 318 acceptance criteria to these three test sets and state whether the concrete is acceptable.
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Criterion 1 - average of any three consecutive tests ≥ f'c: (4,150 + 3,980 + 3,620)/3 = 3,917 psi < 4,000 psi → fails.
Criterion 2 - no single test below f'c − 500 psi: Lowest test = 3,620 psi ≥ 3,500 psi → passes.
Because these three sets are consecutive tests from the same mix, the average criterion governs and the concrete does not meet the acceptance criteria. Passing criterion 2 does not rescue it - the two criteria are independent and both must be satisfied.
Note also that the individual test value drives the 3,620 psi set close to the 3,500 psi floor: any further loss on a retest would fail both criteria. The placement is in question and the engineer of record must be notified formally, with a hold on further placements of this mix.
2. Evaluate the site-added water as a root cause. Quantify the expected strength loss and compare it with the test results.
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Water added per cubic yard: 5 gal × 8.34 lb/gal ≈ 42 lb/yd³
Revised water-cementitious ratio:
- Original: 0.51 → water = 0.51 × 560 = 286 lb/yd³
- With added water: (286 + 42)/560 = 328/560 = 0.586
Expected strength (for normal concrete in this range, strength varies roughly inversely with w/cm): 4,000 × 0.51/0.586 = 3,480 psi
The predicted loss is about 520 psi, and the observed low break is 380 psi below the specified strength - a similar order. The water addition explains the shortfall; normal batch-to-batch variability on a well-controlled mix would not.
Judgment: the fly ash is not the culprit. The 25% Class F mix does gain strength more slowly in 50°F weather, but a fly ash effect would show as uniformly low early strengths across all six placements, and here only the batch with added water is low. Adding water on site without the producer's written authorization and a corresponding mix adjustment is a specification violation on its own, independent of the strength result.
3. The contractor points to the 56-day breaks and asks for acceptance on that basis. What is the correct sequence of evaluation and what must be re-checked if the concrete is ultimately accepted?
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Sequence:
- Specification governs first. The 56-day age was not authorized in the specification, so the 56-day results cannot be used as the basis of acceptance without a written, engineered variation approved by the owner and the engineer of record. The 4,120–4,620 psi results are useful evidence - they show the mix is still gaining and the in-place concrete is likely stronger than the 28-day tests suggested - but they do not by themselves close the acceptance issue.
- Core testing per ACI 318 if the 28-day results stand: three cores per test, average ≥ 0.85 f'c = 3,400 psi, and no single core < 0.75 f'c = 3,000 psi. Cores must be taken from the affected placement, at locations representative of the low test and away from reinforcement and edges.
- If the cores meet the criteria, the concrete in place is structurally acceptable and the placement stands; the mix must still be revised (no site water, and a water-reducing admixture or an approved slump adjustment) and the remaining placements held until the revised mix is verified.
- If the cores fail, the engineer of record performs a structural adequacy evaluation for the affected members at the demonstrated strength, and repair or strengthening follows.
What to re-check even if the concrete is accepted: the strength-dependent properties used in design were based on 4,000 psi. Shear strength, development length and lap splices, and the modulus all scale with √f'c. A drop from 4,000 to 3,620 psi is a 10% strength reduction, or about 5% on √f'c - enough to change a borderline shear or splice check in the affected columns and slab, so those members must be re-evaluated at the demonstrated in-place strength rather than at the specified value.