Laser Screed Elevation Drift on a Large Mud Floor Pour
A 1,200 m² agricultural shed mud floor is being placed in 6 m wide strips using a laser screed referenced to a transmitter set on a tripod near the north wall. The specified finish is a uniform slope of 1.5% falling to the south drainage channel, with a tolerance of ±10 mm from design elevation at any point. After three strips, the site engineer's spot check with a rotating laser on a separately established benchmark shows the southern edge of strip 3 is 18 mm high relative to design, while the northern edge is within 4 mm. The screed operator reports the machine's on-board receiver has read 'green' throughout. Weather is overcast, 14 °C, no direct sun. The transmitter has not been moved since setup.
1. Diagnose the most likely cause of the elevation error and state what should be checked before placing strip 4.
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The pattern (error growing toward the south edge, increasing along the pour direction, on-board receiver reading acceptable) points to a reference/benchmark or transmitter setup problem, not a screed mechanical fault. Likely causes to check, in order:
- Transmitter height and level - confirm the transmitter has not settled or been knocked; re-check its height against the independent benchmark. A small tripod settlement over a long radius produces a systematic, direction-dependent error.
- Benchmark agreement - the two benchmarks (screed reference vs engineer's) may disagree. Establish which is authoritative by a closed level run between them.
- Slope setting - verify the entered grade (1.5%) and fall direction in the screed controller; a mis-entered grade compounds with distance from the transmitter.
- Beam interception - check for obstructions (columns, stacked materials, standing water) causing the receiver to pick up a reflected or partial signal.
Do not place strip 4 until the reference is reconciled; the error is systematic and will repeat. The already-placed strips need a survey grid to define the extent of nonconformance.
2. The survey confirms strips 1–3 are progressively high toward the south, up to 18 mm. The floor is still plastic in strip 3 but strip 1 is 36 hours old. Outline a remediation approach consistent with a ±10 mm tolerance.
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Strip 3 (plastic): re-strike to design elevation now, while workable, using a correctly referenced screed or manual strike-off; re-check with the independent benchmark before finishing. This is the cheapest correction.
Strips 1–2 (set): 18 mm and lesser exceedances cannot be corrected by surface grinding without compromising cover and finish. Options:
- Grinding/planing only where the exceedance is small (a few mm) and cover to any embedded items allows it.
- Localized topping is generally not acceptable on a sloped drainage floor because it creates a feathered edge and a bond interface in a wet area.
- Accept by engineering assessment if the high spots still permit positive fall to the channel and do not pond; document with a survey grid and a flow test.
The decision hinges on whether the as-built surface still drains. Produce a contour survey of strips 1–2, run a water test, and refer any ponding to the designer. Record the root cause (reference error) and the corrective action in the quality records; do not simply re-survey against the faulty benchmark to make the numbers pass.