Concrete Strength Acceptance Dispute on a Silo Mat
A ready-mix supplier has placed 95 m³ of concrete for a mat foundation specified as f′c = 30 MPa at 28 days. Sampling followed ACI 318 frequency requirements, producing ten sets of three 150 × 300 mm cylinders. The 28-day strengths, in MPa, are: 31.5, 29.0, 32.2, 28.1, 30.6, 27.4, 31.0, 29.8, 26.2, and 30.1. The contractor points to the overall average, 29.6 MPa, and argues that because it is close to 30 MPa the mat should be accepted without further testing. The owner’s representative is concerned because one set is far below the others and because the mat supports a 12 m tall silo with concentrated loads. A meeting is scheduled to decide whether to accept the placement, perform cores, or require removal and replacement. The project specification invokes ACI 318 acceptance criteria and does not permit a waiver by the contractor.
1. Apply the ACI 318 strength acceptance criteria to the ten test sets. Show the calculations and state whether the concrete strength level is acceptable.
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The applicable criteria for f′c = 30 MPa are: the average of any three consecutive strength test sets must equal or exceed f′c, and no individual test result may fall below f′c − 3.5 MPa = 26.5 MPa.
Consecutive three-set averages:
- (31.5 + 29.0 + 32.2)/3 = 30.90 MPa - passes
- (29.0 + 32.2 + 28.1)/3 = 29.77 MPa - fails
- (32.2 + 28.1 + 30.6)/3 = 30.30 MPa - passes
- (28.1 + 30.6 + 27.4)/3 = 28.70 MPa - fails
- (30.6 + 27.4 + 31.0)/3 = 29.67 MPa - fails
- (27.4 + 31.0 + 29.8)/3 = 29.40 MPa - fails
- (31.0 + 29.8 + 26.2)/3 = 29.00 MPa - fails
- (29.8 + 26.2 + 30.1)/3 = 28.70 MPa - fails
Six of the eight consecutive averages are below 30 MPa. In addition, the 26.2 MPa set is below the 26.5 MPa individual minimum. The strength level therefore fails both acceptance criteria. The contractor’s overall average of 29.6 MPa is not an acceptance criterion and cannot override the consecutive-average and individual-minimum requirements. The mat should be treated as nonconforming pending core testing or structural evaluation.
2. If the owner authorizes core testing, what acceptance criteria apply, and what minimum number and locations should be selected?
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Core testing should follow ACI 318 requirements for investigating low-strength test results. At least three cores should be taken, normally 100 mm diameter, from the locations represented by the failing test sets rather than from random convenient points. Avoid edges, construction joints, and reinforcement; use a cover meter and coordinate with the structural engineer.
The core acceptance criteria are: the average compressive strength of the three cores must be at least 0.85 f′c = 25.5 MPa, and no single core may be less than 0.75 f′c = 22.5 MPa. Cores should be tested in accordance with ASTM C42 after proper moisture conditioning.
If the cores meet both criteria, the strength level may be accepted even though cylinder tests failed, provided the structural engineer confirms that the lower strength does not adversely affect the silo loads, anchorage, or durability. If the cores fail, the remaining options are a load test or a structural analysis demonstrating adequacy, or removal and replacement. The testing laboratory must be independent of the ready-mix supplier and contractor, and the results must be reported to the engineer of record.
3. The contractor proposes accepting the mat based on the overall average and increasing the silo base plate thickness. Evaluate this proposal.
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The proposal does not satisfy the acceptance criteria. Increasing the base plate thickness addresses only the local bearing and anchorage demands at the column interface; it does not increase the compressive strength of the mat, improve the bond between the mat and the silo anchorage, or correct the variability shown by the 26.2 MPa test set. The mat also resists global bending and punching around the silo, and a thicker base plate does not compensate for a deficient mat section.
A nonconformance report should be issued, and the contractor should be required to propose a compliant remedy: core testing, structural evaluation by the engineer of record, or removal and replacement. Any alternative acceptance must be based on calculated structural adequacy, not on the overall average. The owner’s representative should document the decision and require the engineer of record to seal any evaluation. If the mat is accepted by analysis, the analysis should include the measured concrete strength, not the specified strength, and should confirm anchor capacity and serviceability.