How to Document Concrete Slab Elevations for QC and Warranty

Subgrade verification confirms that the prepared base beneath the slab is at the correct elevation before concrete is placed. A subgrade that is too high will produce a slab with insufficient concrete thickness. A subgrade that is too low will produce excess thickness — wasting concrete — and may affect the final slab surface elevation if the concrete depth is not adjusted.

Use an automatic level or digital level set up at a known benchmark. Survey the subgrade surface on the same grid you will use for the post-pour survey — typically 5-foot or 10-foot centers. Log the measured subgrade elevation, the design subgrade elevation, and the variance. Flag any points outside the allowable tolerance (typically ±0.03 ft for subgrade) before proceeding to pour.

Keep the pre-pour subgrade record as part of the project file. If a slab dispute arises later, the subgrade record shows whether the base was properly prepared.

Elevation control during the pour is the most critical intervention point. Corrections made during screeding cost nothing and take seconds. Corrections made after the concrete stiffens cost thousands.

The standard process for laser screed operations:

1. Set the laser transmitter to the design screed elevation. Verify the receiver height on a benchmark before screeding each bay.
2. Check the screed elevation with a rod at the start of each bay and every 10-15 minutes during placement.
3. Log each check: time, location, design elevation, measured elevation, variance.
4. If variance exceeds ±0.01 ft, stop and recheck the laser setup before continuing.

On projects with manual screed and grade pins, verify each grade pin elevation before placement and record pin locations and elevations in the pour log. Grade pins that have been disturbed by foot traffic or vibration before screeding are a common source of slab elevation error.

The post-pour survey must begin within 24 hours of final troweling and be completed within 72 hours. This window is a hard requirement for ASTM E1155 F-Number compliance — measurements taken after 72 hours cannot be used to certify flatwork tolerance compliance.

The step-by-step post-pour documentation process:

1. Set up on a benchmark. Use a benchmark that was established and verified before the pour. Two benchmarks are preferred so you can cross-check. Verify the instrument reads within 0.005 ft of the benchmark before beginning the survey.
2. Walk the survey grid. Take rod readings at each grid point. For a dipstick survey, walk in parallel strips per ASTM E1155 survey line requirements. Log each point with its grid ID, measured elevation, design elevation, and variance.
3. Flag out-of-tolerance points. Any point with a variance exceeding the project specification (e.g., ±0.03 ft for industrial slab) is flagged as a non-conformance in the data record.
4. Calculate F-Numbers. Process the dipstick or profiler data per ASTM E1155. Calculate FF and FL for each survey section and for the overall slab area.
5. Generate the color deviation map. Plot each survey point on the slab plan with a color coding — typically green for within tolerance, yellow for marginal, red for out of tolerance. This map is the most important communication tool for owner review.
6. Compile the acceptance package. Assemble the survey grid drawing, raw data table, deviation map, F-Number report, instrument calibration record, and QC technician certification into a single PDF.

A complete slab elevation acceptance package for owner review includes the following components:

The most common reasons slab documentation packages are rejected or returned for revision:

• Missing benchmark verification: The survey was conducted without verifying the instrument against a known benchmark, making all measurements unverifiable.
• Survey conducted outside the 24-72 hour window: The pour date or survey time was not recorded, making ASTM E1155 compliance unverifiable. Always record exact timestamps.
• Grid spacing inconsistent with specification: The project specification required 5-foot grid spacing and the contractor used 10-foot spacing — valid for some applications but a specification deviation that requires owner acceptance.
• Non-conformances noted without corrective action: Out-of-tolerance points were identified but no corrective action record was included showing how they were resolved or whether an owner waiver was obtained.

Sitemark's field documentation platform prevents these failures by requiring benchmark verification before data collection, timestamping every measurement automatically, and flagging non-conformances for corrective action before the package can be marked complete.