Written By: Mike Hernandez, ASCC Technical Director 

Few words strike fear in a concrete contractor’s heart like: “This floor doesn’t meet tolerance.” A failed floor flatness/levelness (FF/FL) report can trigger owner dissatisfaction, costly remediation or back charges, project delays, and in the worst cases, litigation. Yet what often gets lost in the dispute is whether the measurement itself is performed correctly by a qualified tester.

Recent industry articles—along with an older Concrete International piece and the relevant ACI specifications and ASTM standards—highlight how significant testing practices and interpretation can influence, and even escalate, flatwork disputes. One focuses on common errors made when collecting data with walking floor profilers, another explores the capabilities and limits of laser scanning, the last looks at floor profiler operator qualifications. Taken together, they underscore one point: the credibility of tolerance measurements is critical to validate the reported FF/FL numbers.

Importance of the FF/FL Test Report

Concrete tolerances can have financial and even legal consequences. Owners and designers increasingly specify ACI 117 Floor Flatness (FF) and Floor Levelness (FL) values, as tested per ASTM E1155. If a testing agency reports a failing number, contractors are assumed at fault.

But as Bill Arpin points out in “FF-FL Testing Mistakes: Understanding Concrete Floor Profiler Data Better”, that assumption can be misleading. Even small errors during data collection can swing results from a passing score to a failing one. A quarter inch misread can reduce FF and FL numbers in half, turning an acceptable slab into one deemed defective on paper. If the tests are performed more than 72 hours after placement or worse, after formwork is removed, these tests are not in compliance with the ACI 117 and ASTM E1155 standards and may be impacted by SOG curling or elevated slab deflection.

Potential Errors from the Profiler

Bill Arpin highlights three easy-to-spot mistakes that frequently plague walking profiler testing, (link to read the whole article is below in the References):

Readings taken in the air – If the operator accidently takes a reading before the footpad contacts the surface, the graph will show a sharp vertical step. A single such error can drastically lower the FF/FL results.

Stepping on debris or objects – A stray pebble, dust pile, or even something stuck to the footpad can create spikes in the data, again reducing the test. (Having felt this while pulling an F-meter I can attest that this happens easily in jobsite conditions.)

Calibration issues – If one side of the profiler is miscalibrated or obstructed, a “sawtooth” pattern emerges. Even a 1/16-inch offset can knock a floor into failing territory.

These aren’t hypothetical concerns. Bill provides graphs to show how introducing just one false reading can reduce an FF 65 floor to FF 28—well below most industrial floor requirements. For a contractor facing liquidated damages or repair costs, the distinction between recognizing data error vs actual defects is critical.

Laser Scanning, Technology Solution or Part of the Problem?

If floor profilers are prone to operator error, could 3D laser scanners solve the problem? An interlaboratory study published in Concrete International in Jan 2025 explored this question. Co-authored by Leo Zhang, Hernandez, Dare, Kowalski, and Che, the study examined the precision of terrestrial laser scanning (TLS) for verifying concrete tolerances.

Thirteen participants, many of them ASCC Members, including contractors, surveyors, and manufacturers, scanned a post-tensioned slab and vertical elements on a Conco project in Santa Cruz, CA. The study was partially funded by the ASCC Foundation. The findings were encouraging: repeatability and reproducibility errors were improved when compared to the ASCC 2018 laser scanning study. This showed that TLS is maturing into a reliable tool for construction quality control. It concluded that tolerances greater than ½” can be determined using laser scanning.

Still, the article cautions against assuming laser scans are infallible. Post-processing techniques like smoothing, meshing, or “best-fit” algorithms can introduce their own distortions. Human interpretation remains part of the workflow, and errors in point cloud analysis can misrepresent actual slab performance. Based on the capability to only confirm tolerances > than ½” laser scanners are not suitable for testing the FF/FL of a slab.

Is this Operator Qualified to Test F-Numbers?

An issue that can easily be overlooked is the qualification of the individual who is running the floor profiler. As described in the June 2024 ACI “Concrete Q&A: Operator Qualifications for Determining F-Numbers” by Klinger, Suprenant and Salzano, the operator’s training might be an issue. Training and certification by the floor profiler equipment manufacturer is better than on the job training by a co-worker. The article states “The preconstruction conference specified in ACI 301-20, Section 1.6.1, would be the appropriate time for all stakeholders to confirm all personnel (contractors and inspectors) are properly trained and certified to perform the work.”

Is there an issue with interpretation of a single test line?

An older article from CI, July 2008, “Concrete Q&A: Rejecting Floors based on One Sample Measurement Line” focused on whether a single line could be interpreted as a failing “local minimum”. ASTM E1155 defines a Test Section as “7.2 Test Section—A test section shall consist of any subdivision of a test surface satisfying the following criteria:

7.2.1 No test section shall measure less than 8 ft on a side, nor comprise an area less than 320 sf.

7.2.2 No portion of the test surface shall be associated with more than one test section.

7.2.3 When testing a concrete floor, no test section boundary shall cross any construction joint.”

Therefore, a single placement with its boundaries being the construction joints would be the largest test section. The minimum number of individual readings on lines > 10 ft is A/30 for placements larger than 1,600 sf. So, for a 12,000-sf placement the minimum number of readings is 400.  If the bays are 30’x30’ each bay can be tested with parallel & perpendicular lines or X shaped diagonals. This area would be 13-14 bays. If it is a sawcut slab on grade when testing begins the lines can only be up to 26’ long parallel and perpendicular. Say ~25’ each way or 40’ diagonals. In 13 bays, 16 parallel and perpendicular 25’ lines would be the ASTM minimum, 26 would be expected to cover all full bays, yielding 650 readings. Ten 40’ diagonal lines would be the minimum, skipping several bays. Twenty-six ~40’ lines would cover all bays and provide over 1,000 readings. In this example the minimum local area could be as small as ~900 sf and it would need to be 3/5th of the specified overall, unless a local minimum is already defined in the specification. No single line can define an area so no single line could be used to define a local minimum. Single placements, in this example 12,000 sf, can be used as a “local minimum” if the testing lab only provides the daily FF/FL numbers and does not provide every run with the layout to provide the detail of smaller areas.    

So why do slab flatness disputes persist? Three recurring issues:

Too much trust in reports – When an FF/FL report arrives few stakeholders question the raw data or methods behind it. This can give too much weight to flawed results. The training of the floor profiler operator is rarely questioned and even less often confirmed to have been provided by the equipment manufacturer.

Specification ambiguity – Project documents might not clearly define acceptable test methods, testing time limits, minimum local areas and acceptable methods. This leaves room for inappropriate tools, such as the use of a 10’ straightedge or a laser scanner to check FF/FL compliance.

Technology gap – Contractors are usually more familiar with placing and finishing challenges than the statistical quirks of tolerance measurement. Testing agencies, meanwhile, may not recognize errors in their own reports.

Avoiding the Dispute – Plan, Test and use the Data Appropriately

What can ASCC contractors, owners, designers and testing labs do to reduce the risk of disputes?

Confirm the plan before construction – ACI 301-20 addresses a preconstruction conference in section 1.6.1 with the contractor(s), design team, owner and owner’s representatives, such as a testing agency. At this conference the contractor should confirm the testing method, minimum test area, how soon after placement the tests will be performed and submitted as well as the qualifications of the operator. ACI 117 section 4.8.4.4 and ASTM E1155 both require the FF/FL testing to be performed within 72 hours and before removal of supporting formwork. Before stressing of the post-tension cables is also advisable. Designers should explicitly state whether tolerances will be verified with a walking profiler and if laser scanning will be used for additional QC checks.

Appropriate use of scanning – Laser scanning can be used to generate heat maps to identify potential issues, but this technology has its limitations. The recent ASCC study recommends only using a TLS for tolerances of ½” or more. Supplementing the FF/FL tests with laser scanner heat map can provide data for very high or low spots that might need to be ground or filled to accommodate final finishes but are not appropriate for FF/FL specification acceptance.

Contractors can do their own QC – A walking profiler is ~$10,000 plus the cost to train several operators and time to run the tests. If you need guidance on which companies manufacture them, please contact me at mhernandez@ascconline.org  Multiple ASCC members perform their own FF/FL QC testing immediately after placement and submit this to the GC or design team the next day. This will also provide the ability to discuss the results with the finishers before multiple slabs are placed, and a small problem becomes a large issue. It might be the only record of FF/FL results before the flooring trades arrive on site to look for gaps under a 10’ straightedge.

Request raw graphs – As Bill Arpin advises, insist that testing agencies provide profiler plots with every report. Sharp spikes or sawtooth patterns can reveal errors before results escalate into disputes.

Foster communication – Most importantly, ASCC contractors and testing agencies must establish a dialogue before FF/FL testing begins. Agree on sampling locations, their interpretation of the ASTM E1155 procedures, and how soon it will be reported.

References:

ACI Committee 117-10(15), “Specification for Tolerances for Concrete Construction and Materials”

ACI Committee 301-20, “Specifications for Concrete Construction”

ACI Staff, “Concrete Q&A: Rejecting Floors based on One Sample Measurement Line”, Concrete International, July 2008

Arpin, Bill, “FF/FL Testing Mistakes: Understanding Concrete Floor Profiler Data Better”, ForConstructionPros, https://www.forconstructionpros.com/concrete/equipment-products/flatwork-accessories/article/22909722/somero-matson-group-llc-ff-fl-testing-mistakes-understanding-concrete-floor-profile-graphs-better  Dec 2024

ASTM E1155-23, “Standard Test Method for Determining FF Floor Flatness and FL Floor Levelness Numbers,” ASTM International

Klinger, Suprenant and Salzano, “ACI Q&A: Operator Qualifications for Determining F-Numbers”, Concrete International, June 2024

Zhang, Hernandez, Dare, Kowalski, and Che, “Interlaboratory Study on Precision Statement of Using a Terrestrial Laser Scanner to Verify Concrete Tolerance” Concrete International, Jan. 2025