Site Preparation, Subbase, and Formwork Layout

Why Site Preparation Matters

A concrete slab is only as good as what lies beneath it. Poor subbase preparation and sloppy formwork are the leading causes of slab cracking, settlement, and costly callbacks. Before a single cubic yard of concrete is ordered, the ground must be graded to the right elevation, the granular fill must be properly compacted, and the forms must hold their position under the weight and pressure of wet concrete. This lab puts all three skills together in a real-work sequence.

Reading Elevations with a Builder's Level

A builder's level (also called an optical level or dumpy level) lets you establish a horizontal line of sight across a jobsite. You set it on a tripod, level it with the plate screws, and read a grade rod held by a partner at various points. Every reading you take is called a rod reading or backsight/foresight, depending on whether you are establishing or checking a benchmark.

The math is straightforward. Start by shooting the instrument height (HI):

> HI = Known elevation + Backsight rod reading

Then, for any other point:

> Point elevation = HI − Foresight rod reading

Shoot at least six points spread evenly across the slab area. Record every reading on a field sketch immediately — memory is not a reliable field tool. Compare each calculated elevation to the design grade shown on the drawings. The difference tells you how much cut (excavate) or fill (add material) is needed at each location.

Subbase Requirements

For residential and light commercial slabs, a compacted granular subbase — typically crushed stone or gravel — provides uniform support, improves drainage under the slab, and reduces capillary rise of moisture. ACI 302.1R recommends a minimum of 4 inches of well-graded granular fill for most interior slabs on grade; exterior slabs and heavier-use floors may require more.

Place the fill in lifts no deeper than the compaction equipment can reach uniformly. For a standard plate compactor, 2-inch lifts are safe; many crews use 2-inch lifts to build a 4-inch total depth, compacting each lift before adding the next. After compaction, probe the surface with a steel rod at random points. The rod should resist penetration uniformly. Soft spots, where the rod sinks easily, indicate loose material that must be removed, corrected, and re-compacted.

Log every compaction pass — number of passes, direction, and any soft-spot locations — in a field log. This documentation protects you and your employer if slab performance is questioned later.

Setting Wood Forms

Forms contain and shape wet concrete until it gains enough strength to stand alone. For a 4-inch slab, 2×4 lumber is the minimum; 2×6 is preferred when the slab is thicker or when extra stiffness is needed against concrete pressure. Key steps:

1. Stake the corners using form stakes driven firmly into undisturbed or compacted soil. Stakes should be set close enough that the form board cannot deflect more than 1/8 inch between them — typically every 4 feet or less.

2. Check squareness with the 3-4-5 triangle method. Measure 3 feet along one side from a corner, 4 feet along the adjacent side, and connect those two points. If the diagonal measures exactly 5 feet, the corner is 90°. For larger slabs, use multiples (6-8-10, 9-12-15).

3. Check level with a 4-foot level placed along the form at multiple locations. The bubble must sit centered. Adjust stake height or add/remove subbase material as needed.

4. Brace the forms by nailing or screwing the form board to the stake on the outside (away from the pour). Duplex (double-headed) nails are standard because they pull out cleanly after the pour.

5. Re-check alignment with a string line stretched tightly along the top edge of the form. Any bow or gap must be corrected before concrete arrives.

The pass criterion for this lab is tight: all six instrument readings must be within 1/8 inch of target elevation and diagonals must match within 1/4 inch to confirm square. These are real-world standards, not just classroom rules.

Plate Compactor Safety

A plate compactor is a high-vibration machine that can cause hand-arm vibration syndrome with prolonged daily exposure. Limit continuous operation and rotate operators when possible.

• Wear hearing protection — compactors commonly exceed 85 dB.
• Wear safety glasses — the plate kicks up debris.
• Wear steel-toed boots — the machine is heavy and may tip.
• Never stand directly behind the machine on a slope; position yourself to the side.
• Keep bystanders clear of the operating radius.
• Inspect the machine before start-up: check fuel, engine oil, and the condition of the base plate and belt/drive.
• Follow lockout/tagout procedures if any maintenance or unclogging is needed — shut the engine off and allow moving parts to stop completely before touching the machine.

Putting It Together

Good flatwork begins at the dirt. Shoot your elevations carefully, compact in lifts and verify with the probe, set forms that are level, square, and braced, and document as you go. These habits keep concrete where it belongs — in the slab, at the right grade, supported by a solid foundation.