Concrete Pour Shortage: 5 Subgrade Checks

A concrete pour shortage can come from low subgrade, incorrect surface elevations, changed form dimensions, settlement, or uncounted edge details. Compare the final prepared ground with the intended concrete surface before blaming the delivered quantity. Start with the elevation reference, then reconcile the measured cavity with the original takeoff.

Subgrade is the ground supporting the pavement or slab assembly. For quantity measurement, identify the actual bottom of the concrete, which may be the top of a separate granular base or another specified layer. The checks below estimate material volume. They do not establish soil bearing capacity, slab thickness, reinforcement, or structural acceptance.

Five subgrade checks for a concrete pour shortage

Check the elevation reference and finished surface

A slab thickness measurement is the vertical difference between its intended top and the prepared surface below it. A ground elevation alone cannot describe that difference. Start with the approved finished levels, including any planned slope, and confirm that the survey uses the same benchmark as the form-setting work.

Keep the benchmark identifier on the worksheet. Record the instrument, survey date, drawing revision and person responsible for the measurements. If two crews used different temporary reference points, reconcile their readings before averaging anything. A consistent offset across the entire slab can suggest a reference problem rather than scattered excavation errors.

For a sloping surface, calculate the intended top elevation at each measured location. Comparing every ground reading with one flat top elevation can create artificial low spots. If the ground and top surface slope together, the thickness may remain constant even though both elevations change.

Use a tape, level or survey equipment appropriate to the required accuracy and site conditions. Have the responsible professional settle discrepancies in design elevations. A calculator can reproduce the dimensions entered, but cannot determine which benchmark or finish level is correct.

Map low spots by area, not just deepest reading

Uneven concrete depth changes volume in proportion to the area affected. One deep point does not justify applying its depth to the entire pour. Divide the footprint into identifiable zones and take enough readings to describe each zone’s variation, with closer attention around abrupt changes.

An estimator can use a survey grid as a planning worksheet, provided the spacing and interpolation method suit the surface. No universal grid spacing is specified here. Mark additional points at trenches, wheel ruts and transitions. Keep irregular or sharply changing sections separate instead of hiding them within a broad average.

Record the specified thickness beside the measured thickness. Label any zone average as an estimate derived from readings, and retain the individual values. A simple arithmetic mean of survey points represents an area average only when the sampling and weighting support that interpretation.

Enter representative zone dimensions and thicknesses separately in the concrete volume worksheet. Keep the allowance at zero while comparing surveyed geometry with the original plan. This prevents an unexplained percentage from concealing a measurable depth difference.

Check form boundaries and extra concrete pockets

Concrete occupies the final formed cavity, including any approved edge thickening or local depression. Measure inside the forms after bracing and preparation. Compare those dimensions with the takeoff, especially where a perimeter moved, a corner changed or a footing connects to the slab.

Give each additional pocket an identifier. Record its length, width and extra depth below the surrounding slab. If the slab volume already includes the base thickness across that pocket, add only the additional depth. Counting the complete pocket again would duplicate the concrete in the upper part.

For example, a hypothetical 10 ft long by 1 ft wide edge depression extending 6 in below the slab adds 10 × 1 × (6 ÷ 12) = 5 ft³. That is approximately 0.185 yd³. This is geometric arithmetic, not a recommendation for an edge design.

NRMCA’s CIP 8, Discrepancies in Yield identifies actual dimensions exceeding assumed dimensions and form distortion as causes of perceived shortages. Photograph the final boundaries and retain the approved details so the quantity comparison has a traceable basis.

Recheck the prepared surface after disturbance

The survey used for ordering should describe the surface that will actually receive concrete. A reading taken before final preparation may no longer represent the cavity after traffic, excavation changes or settlement. Compare the final inspection with the earlier survey and record what changed.

Pay attention to access routes, service trenches and places where the surface visibly differs from adjacent areas. Mark the footprint of each change before calculating its additional volume. Use the same finished-level reference for both surveys so instrument setup does not masquerade as movement.

NRMCA’s yield guidance also identifies irregular supporting surfaces and settlement before or during placement as possible contributors to a discrepancy. Its advice supports checking the final geometry; it does not give this article authority to approve ground conditions.

If the surface moves under traffic or shows an unresolved preparation defect, refer the condition to the responsible contractor or engineer before pouring. Filling a defect with extra concrete changes material quantity and may leave the underlying performance problem unresolved. Keep any approved repair of the base in a separate material takeoff.

Reconcile measured volume with the original order

A useful shortage calculation compares like quantities: original geometric volume, revised geometric volume, separate contingencies and documented delivered volume. Keep these lines distinct. An additional survey quantity should not be counted once as measured overdepth and again as a generic allowance for the same overdepth.

Calculate the revised concrete volume

Surveyed zones and assumptions

The following hypothetical worksheet represents a 20 ft by 30 ft slab divided into three zones. The depths are illustrative survey inputs, not acceptable construction tolerances. All zones share the same intended top surface, and each representative depth is assumed to describe its full zone.

Convert thickness and volume units

Depth in feet equals depth in inches divided by 12. The nominal volume is 600 × (4 ÷ 12) = 200 ft³. The surveyed total is 100 + 67.5 + 50 = 217.5 ft³. Divide by 27: nominal volume is 7.407407 yd³ and surveyed volume is 8.055556 yd³.

cubic_yards = sum(area_ft2 * (depth_in / 12)) / 27

The geometric increase is 17.5 ft³, or 0.648148 yd³, which is 8.75% above the nominal quantity. Round display values at the end. Ask the supplier about order increments and separately agreed contingencies before converting 8.056 yd³ into a purchase instruction.

Reproduce these zones in the Concrete Calculator and compare the total before adding any allowance. For metric work, multiply each area in square meters by its depth in meters. The same physical cavity should produce matching results after unit conversion.

What if the measured volume does not explain a concrete pour shortage?

Retain delivery tickets, revised measurements and records of incidental concrete use, spillage or returns. Investigating a concrete pour shortage requires separating the measured cavity from the delivered yield. The cavity calculation alone cannot prove that a producer supplied less concrete than ordered. NRMCA distinguishes a geometric shortfall from a yield discrepancy assessed using fresh concrete density and batch information.

Discuss unresolved discrepancies with the producer and qualified testing personnel. Ask what evidence is available and which contractual testing requirements apply. Avoid drawing a conclusion from the finished slab dimensions alone.

When pricing a concrete pour shortage, use the supplier’s quoted rate for additional volume. Identify short-load, transport and pumping charges separately. Do not assume the original unit price includes a second delivery. Keep the measurement record with the purchase revision so the extra order has a stated basis.

Records to send with a revised concrete order

• Final drawing revision and finished-level reference.
• Surveyed zone areas and representative thicknesses, with units.
• Separate additional pockets and any agreed contingency.
• Supplier order increments, delivery timing and quoted extra charges.

Sources

• NRMCA, CIP 8: Discrepancies in Yield, 2016.