Failed cylinder or cube results are a common occurrence on construction projects, and the standards lay out a clear sequence of checks, from reviewing the test results and non-destructive testing to drilling cores from the actual structure. This article explains what each step involves, which criteria are used to judge it, and what options remain if the concrete still does not pass.

The 5-step investigation sequence when concrete compressive strength fails, with ACI 318-19 criteria for specimen test results and core tests The left side shows the investigation sequence from top to bottom. It starts with a compressive strength test result below the design strength f′c, which is not yet a conclusion that the in-place concrete has failed. Step one: review the acceptance criteria per ACI 318-19 Section 26.12 — the average of three consecutive test results must be at least f′c, and no result may fall below f′c by more than 35 kilograms per square centimeter for f′c not exceeding 350. If both conditions are met, the concrete passes; record it and monitor the next sets. Step two: review specimen sampling, curing and testing per ASTM C31 and C39 together with the concrete batch documents, because improperly handled specimens give lower values than the true strength. Step three: screen with a Rebound Hammer per ASTM C805 and UPV per ASTM C597 to find low-value areas and select coring locations. Step four: drill cores per ASTM C42, at least three cores per area, avoiding reinforcement and critically stressed locations, and correct values for the L/D ratio. Step five: evaluate the cores — they pass when the average is at least 0.85 f′c and no core is below 0.75 f′c. Top right is a worked example of the step-one criteria: f′c 240, three results of 232, 245 and 251, average 242.7, not below 240, and the minimum of 232 is above 205, so both conditions are met. Below that is the core criteria bar and the L/D correction factor table: 2.00 equals 1.00, 1.75 equals 0.98, 1.50 equals 0.96, 1.25 equals 0.93 and 1.00 equals 0.87. At the bottom are two outcomes. Pass: the concrete is acceptable for strength, the core holes are repaired with non-shrink material, and an engineer issues a summary report. Fail: further evaluation by load testing per ACI 318-19 Chapter 27, reanalysis using the actual measured strength, strengthening such as column jacketing or FRP, limiting service loads, or demolition and recasting as a last resort. Every step is decided by the project's engineer of record. Strength failed: check 5 steps before deciding Not to scale Result below f′c Not yet proof the in-place concrete fails 1 Review ACI 318-19 criteria Avg of 3 ≥ f′c and no result < f′c − 35 kg/cm² (f′c ≤ 350) Both met → pass Record + monitor next sets 2 Review sampling, curing, testing ASTM C31 · C39 · tickets · Slump Bad specimens → falsely low 3 Screen with NDT Rebound (C805) · UPV (C597) Find abnormal low areas Pick core spots in suspect areas 4 Core test per ASTM C42 ≥ 3 cores/area · avoid rebar Correct for L/D before judging L/D table At right → 5 Judge cores (ACI 318-19) Avg ≥ 0.85 f′c and no core < 0.75 f′c Step 1 worked example f′c = 240 kg/cm² (example) 180 200 220 240 260 232 245 251 f′c 240 Avg 242.7 f′c−35 = 205 Core criteria (step 5) 0.75 f′c 0.85 f′c 1.00 f′c Any core < 0.75 = fail Avg must be ≥ 0.85 L/D correction (approx.) 2.00 × 1.00 1.75 × 0.98 1.50 × 0.96 1.25 × 0.93 1.00 × 0.87 Short core → overstated strength Pass Fail Pass • Acceptable for strength • Repair holes, non-shrink • Engineer's report Fail — evaluate further, no need to demolish yet Load Test ACI 318 Ch. 27 Reanalyze Actual strength Strengthen Jacket / FRP Limit service loads Demolish/recast Last resort Every step is decided by the engineer of record — this diagram is general guidance
Decision sequence when concrete compressive strength fails — moving from non-destructive methods toward the method that gives the clearest answer (Image: SPN Soil Engineering)

What compressive strength results count as "failing"?

Concrete is considered to fail when it does not meet either of the two conditions in ACI 318-19 Section 26.12. The criteria do not judge on a single specimen; they look at both the average and the minimum at the same time.

  • The average of 3 consecutive test results must be at least the specified compressive strength (f′c) — one "test result" is the average of at least 2 specimens of Ø150 mm or at least 3 specimens of Ø100 mm from the same batch of concrete.
  • No test result may fall below f′c by more than 35 kg/cm² (3.5 MPa) — this applies to concrete with f′c not exceeding 350 kg/cm². If f′c is higher, no test result may fall below f′c by more than 0.10 f′c (equivalent to at least 0.90 f′c).

A single result slightly below the design value is therefore not yet a conclusion that the concrete has failed. The first step in solving the problem is to review all the data, not to rush into drilling the structure.

💡 Worked example of the criteria

Concrete with f′c = 240 kg/cm² gives 3 consecutive test results of 232, 245 and 251 kg/cm². The average is 242.7, which is not below 240, and the minimum of 232 is still above 240 − 35 = 205, so both conditions are met. These figures are an example to explain the criteria only; decisions on an actual project must be made by the responsible engineer.

First check: what needs to be reviewed?

The first step is to separate "the concrete is genuinely weak" from "the test specimens have a problem", because improper sampling, curing and testing of specimens produces values lower than the true strength. There are 4 items to review.

  • Specimen sampling and curing — under ASTM C31, specimens used for acceptance must be cured under standard conditions. If specimens were left in the sun or rain, or not cured for the full period, the results will be abnormally low.
  • Compressive strength test method — under ASTM C39, check that the specimen ends are flat and plane, the testing machine is calibrated, and the loading rate is within the specified range.
  • Concrete batch documents — delivery tickets, time from mixing to placement, slump, and whether water was added on site, which is the most common cause of low strength.
  • Specimen type — cube and cylinder results cannot be used interchangeably. Cylinder strength is generally about 0.8 times cube strength, so results must be compared against the criterion for the specimen type stated in the specification.

If clear deficiencies are found in specimen sampling or curing, those results may not reflect the quality of the concrete in the actual structure, and data must be obtained directly from the structure in the next step.

How does non-destructive testing help?

Non-destructive testing (NDT) is used to screen whether the problem is localized or spread across the whole area, and to select coring locations precisely. It does not replace the actual compressive strength for acceptance decisions.

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MethodStandardWhat it tells youLimitations
Rebound HammerASTM C805Surface hardness and uniformity of the concrete; fast and covers many pointsReads only the outer few centimeters; sensitive to moisture and carbonation. It is a surface hardness index, not a direct compressive strength value
Ultrasonic Pulse VelocityASTM C597Uniformity of the concrete, and locations of internal voids and cracksPulse velocity is not compressive strength; a project-specific correlation is needed to estimate it
Rebar Scanner / GPRBS 1881-204 (cover meter) / ASTM D6432 (GPR)Rebar locations and cover; used to choose core locations that avoid reinforcementDoes not give compressive strength; supporting data for planning the coring

In practice, Rebound Hammer and UPV are usually used first to sweep the area and find zones with abnormally low values, the reinforcement is scanned, and cores are then drilled only in those zones. This means fewer holes to repair and more targeted data. If UPV values come out low, see how to identify the cause in Low UPV Test results: what do they mean?, and read a comparison of the two approaches in NDT vs core test: which to use.

3D model of checking in-place concrete when compressive strength results fail, from sweeping non-destructive test points and scanning reinforcement to drilling three cores from a column A conceptual 3D view of a reinforced concrete column on a floor slab. The left face of the column has a grid of non-destructive test points: green points are normal values, and orange points are lower than the rest, clustered in a suspect zone shaded light orange. A tester holds a Rebound Hammer against the column surface. The right face shows dashed lines for the vertical bars and ties obtained from the scanner. Three core holes are located in the gaps between the bars: two have been drilled, and at the third a core drill anchored to the floor is drilling with a horizontal core barrel. On the floor are two cores labeled C1 and C2. The bottom panel shows three laboratory steps: labeling and measuring, trimming the ends and determining the length-to-diameter ratio, and compression testing with correction for L/D. The right panel summarizes the five-step workflow: sweep the area with NDT, scan the reinforcement, have the engineer approve the core locations, drill at least three cores per ASTM C42, and repair the holes with non-shrink material, together with the ACI 318-19 acceptance criteria: the average of three cores is at least 0.85 f′c and no core is below 0.75 f′c. 3D model: from NDT to coring a real column Not to scale C1 C2 Rebound / UPV grid Green OK · Orange low Suspect zone Batch with low cylinders Rebar lines from scanner Drill in gaps, avoid rebar Core drill Anchored, water-cooled Core hole ≥ 3 cores per area Core samples Label every core location Tester applies Rebound Hammer Surface screening, not true strength In the lab (ASTM C42) Label, measure D and length Trim ends Find L/D ratio L D Compression Correct for L/D Workflow on the structure 1 Sweep area with NDT Rebound · UPV find low zones 2 Scan reinforcement Mark surface before coring 3 Engineer approves spots Avoid critical stress zones 4 Drill ≥ 3 cores per ASTM C42 in suspect zone 5 Repair holes with non-shrink material Criteria (ACI 318-19) Avg of 3 cores ≥ 0.85 f′c and no core < 0.75 f′c Correct L/D before judging NDT picks core spots Not a substitute for true strength Conceptual, not actual design
3D model of the work on the actual structure: sweeping Rebound/UPV points to find low-value zones, scanning reinforcement, then drilling at least 3 cores in the gaps between bars before sending the cores to the laboratory (Conceptual image: SPN Soil Engineering)
SPN technician drilling a concrete core from a structural column with a coring machine for compressive strength testing per ASTM C42
Drilling core samples from the actual structure gives the compressive strength of the concrete in its in-service location, a more direct basis for decisions than cast specimens (Image: SPN Soil Engineering)

What criteria are used to judge a core test?

A core test is the drilling of concrete cores from the actual structure for compressive strength testing per ASTM C42. It is the most direct way to answer what the compressive strength of the concrete at that location actually is.

Under ACI 318-19, at least 3 cores are drilled from the area in question and judged on two conditions together: the average of the 3 cores must be at least 0.85 f′c and no single core may be below 0.75 f′c. If both conditions are met, the concrete in the structure is considered acceptable for strength.

Measured values must be corrected for the length-to-diameter ratio (L/D) before comparing with the criteria, because shorter cores give overstated compressive strength.

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L/D ratioApproximate correction factor
2.001.00
1.750.98
1.500.96
1.250.93
1.000.87

Core locations must avoid reinforcement and must not be in critically stressed areas, and they must always be approved by the responsible engineer first. Core holes must be repaired with non-shrink material as specified. The figure below summarizes all three points in one image: selecting core locations, the L/D correction, and an example of judging 3 cores. For details of the test method, read What is a concrete core test?

2D section of drilling cores from a concrete column while avoiding reinforcement, correcting compressive strength for the L/D ratio, and an example of judging three cores against the 0.85 and 0.75 f′c criteria A three-panel diagram. Panel one is a cross-section of a reinforced concrete column with eight vertical bars and ties. The correct core location is in the gap between vertical bars, with the core barrel entering from the right face, while a location in line with a vertical bar is marked with a red cross, meaning do not drill there because it would cut the reinforcement; scan for rebar first and have the engineer approve the location. Panel two compares two cores of the same diameter: the long core has an L/D ratio of 2.00 and uses a factor of 1.00, while the short core has an L/D of 1.00 and uses a factor of 0.87, because short cores give overstated compressive strength. Both ends of each core must be trimmed flat and square before testing. Panel three is a bar chart with hypothetical values for concrete with f′c of 240 kilograms per square centimeter, with criteria lines at 0.85 f′c equal to 204 and 0.75 f′c equal to 180. Case A gives 222, 205 and 215, an average of 214, not below 204, and a minimum of 205, not below 180, so it is acceptable for strength. Case B gives 232, 172 and 226, an average of 210, which passes the average criterion, but one core at 172 is below 180, so it fails and requires further evaluation. 2D section: core between bars · L/D correction · judge 3 cores Not to scale 1 Column section — core location Core drill Diamond bit Drill in the gap between bars On a bar line Cuts the bar 8 vertical bars + ties (example) Scan rebar first · engineer approves 2 Cores — correct for L/D L D L/D = 2.00 Factor × 1.00 L D L/D = 1.00 Factor × 0.87 Red dashed = trim ends flat, square Short cores read high, so reduce before judging 3 Example: judging 3 cores Hypothetical · f′c = 240 kg/cm² · after L/D 140 160 180 200 220 240 222 205 215 Case A 232 172 226 Case B f′c = 240 0.85 f′c = 204 0.75 f′c = 180 Case A — both conditions met Avg 214 ≥ 204 Min 205 ≥ 180 → Acceptable Case B — avg OK, one core low Avg 210 ≥ 204 Min 172 < 180 → Fails, evaluate further Strength (kg/cm²) →
2D section: (1) drill cores in the gap between vertical bars, not in line with a bar; (2) a core with L/D = 1.00 must be multiplied by 0.87 before comparing with the criteria; (3) hypothetical values for f′c 240 kg/cm²: Case A meets both conditions, while in Case B the average passes but one core is below 0.75 f′c, so it fails
Concrete cores drilled from an actual structure, laid out with location labels before laboratory compressive strength testing
Concrete cores with location labels. Core length affects the measured compressive strength, so values must be corrected for the L/D ratio before comparing with the criteria (Image: SPN Soil Engineering)

If the core test still fails, what are the options?

When core tests confirm that compressive strength is below the criteria, the next step is to evaluate whether the structure can still carry its design loads, not to demolish immediately. There are 5 common options, ordered from least to most disruptive.

  • Reanalysis using actual strength (re-rating) — use the measured compressive strength to recalculate load capacity. In some cases it is found to be still adequate for the intended use, or adequate once loads are limited.
  • Load testing — apply load to the suspect member and measure deflection and recovery. Suitable for accessible slabs and beams. Procedures and acceptance criteria refer to ACI 318-19 Chapter 27 (Strength Evaluation of Existing Structures). See details in Floor load test: testing slab load capacity
  • Restricting use — reduce the allowable load or change the occupancy of the area to a lighter use, with signs posted stating the restriction.
  • Structural strengthening — for example, jacketing columns with concrete or steel, carbon fiber sheets (FRP), or adding load-carrying members. Commonly used when the strength shortfall is small.
  • Demolition and recasting — used when strength is so low that strengthening is not worthwhile, or the damage extends throughout the member. Usually a last resort.

How can this be summarized as a decision table?

This table matches common situations with what to do next. Use it as preliminary guidance when discussing with the designer and the contractor.

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ScenarioWhat to do next
A single result below f′c by no more than 35 kg/cm², and the 3-result average still passesStill passes the criteria; record it and monitor the next sets of results
A single result below f′c by more than 35 kg/cm², or the 3-result average failsReview specimen sampling and curing, and check the concrete batch documents
Review finds no specimen deficienciesScreen the area with non-destructive testing, then drill cores at targeted locations
Core test passes: average ≥ 0.85 f′c and no core below 0.75 f′cAcceptable for strength; repair the core holes and summarize in a report
Core test failsReanalyze using actual strength, load test, or strengthen based on the evaluation
Very low strength throughout the memberConsider demolition and recasting, as advised by the responsible engineer

All steps are general guidance. Decisions on an actual project must be made by the responsible engineer, considering the construction drawings, site conditions and all test results together.

What Are the Costs and Timeline?

Fees depend on the number of test points, the height and accessibility of the coring locations, the core size, and the supporting non-destructive tests used, so there is no fixed lump-sum price. Contact us for a free quote within 24 hours. An itemized breakdown of structural testing prices is available in Structural testing prices 2026

Site work for NDT screening and core drilling usually takes 1–2 days depending on the number of points. The cores then need further time for preparation and laboratory testing before a summary report with the engineer's opinion is issued.

Price note: This price excludes travel and equipment mobilization costs, which are charged extra depending on the distance to site.

Summary

When concrete compressive strength results fail, work from the least to the most intrusive step: start by reviewing the acceptance criteria under ACI 318-19 and the specimen sampling and curing, screen with non-destructive testing, then drill cores per ASTM C42 only where needed. The acceptance criteria are an average of at least 0.85 f′c and no core below 0.75 f′c.

SPN Soil Engineering provides non-destructive structural testing, concrete core drilling and structural load testing, with reports certified by licensed civil engineers registered with the Council of Engineers Thailand (COE), including signature and license number. Contact us for a free quote within 24 hours.