Pre-Pour Checklist and Subgrade Verification

Purpose

This procedure fixes the moment of no return. Once the plant has a load time, every defect still in the subgrade, the forms or the steel gets cast into a slab that faces the customer for decades: a soft lane over a backfilled utility trench that cracks the panel a season later, mesh lying flat doing nothing, a slab three quarters of an inch thin across a third of its area, water running toward the garage door instead of away. Each costs a rake and an hour with a plate compactor before the pour, and a demolition after. The gate: no truck is ordered until every acceptance value below is written and passing.

Scope

Covers exterior residential and light commercial slab-on-ground flatwork - driveways, patios, walks, garage and shop slabs - from completed excavation through the go/no-go call that releases the order.

Does not cover mix design or delivery, owned by the ready-mix delivery and truck coordination SOP; joint layout, owned by the joint layout and sawcut timing SOP and verified here only as a drawn plan at step 7; interior slabs under moisture-sensitive flooring, where vapor retarder placement per ASTM E1643 and ACI 302.2R is separate; or footings, foundation walls and post-tensioned slabs, engineered work released by the engineer of record.

Roles and responsibilities

Procedure

1. Confirm the locate is live and the excavation clears it. Check the 811 ticket is unexpired for today's date, walk every paint line and flag, and confirm the excavated bottom clears each marked utility by the marked depth plus your base thickness. Acceptance: ticket number and expiry on the record, every marked line visible or hand-exposed, no mechanical excavation inside the tolerance zone. Wrong looks like a faded line, a pulled flag, or a ticket that ran out over a rain delay. Stop rule: stop work and re-ticket; a shop that pours over a stale locate owns the strike. Hazard: inside the tolerance zone hand-dig with a blunt-tipped shovel only; if anyone smells gas everyone walks upwind off the property immediately, no phone used and no engine started on site, then calls the utility emergency line and 911 from across the road.

2. Prove the subgrade is uniform, not just firm. Proof roll the full footprint with a loaded tandem or a ballasted skid steer where access allows, watching for rutting and for pumping (water flushing up under the wheel), and take density tests where a compaction spec is written. Acceptance: no rut deeper than about half an inch under a proof roll pass, no pumping anywhere, and where tested, 95 percent of maximum dry density by ASTM D698 standard Proctor, read by ASTM D6938 nuclear gauge or a spec equivalent. ACI 302.1R ties slab-on-ground failure to differential support, so a stiff pad beside a soft lane is worse than one uniformly medium. Wrong looks like a single soft strip - almost always a utility trench, a stump hole or a removed footing. Stop rule: no-go on the order, excavate the soft zone to firm material, rebuild in lifts, retest that zone plus one point each side. Hazard: stand uphill of a running plate compactor, never on the downhill side of a slope it can run away down.

3. Place and compact the base in lifts you can name. Place granular base in lifts no thicker than the compactor's rated capability, typically 4 in loose for a walk-behind plate and 8 in for a reversible plate, wetting each to near optimum moisture before compacting. Acceptance: lift count and thickness recorded, each lift compacted until the plate stops sinking and rides, total compacted depth within half an inch of the work order figure, probed at four points minimum plus one per 200 sq ft. Wrong looks like one dumped 10 in lift compacted on top, a crust over loose material that reads firm to a boot and fails under a wheel. Stop rule: pull the base back and rebuild in lifts; a base with no stated lift count has been driven on, not compacted. Hazard: dry base releases respirable silica when dumped and bladed, so keep it damp and the crew upwind, and run any dry cutting or grinding of cured concrete on site under 29 CFR 1926.1153.

4. Set and check grade for drainage before anything touches the forms. Shoot or string finish elevations from a fixed benchmark and confirm the fall runs away from every structure. Acceptance: impervious surface within 10 ft of the foundation slopes at least 2 percent away from it, the IRC R401.3 impervious-surface exception, in every jurisdiction that has adopted the IRC; beyond 10 ft the work order governs, with 1 percent a practical floor for a broomed slab that has to shed water. Wrong looks like a flat run, a reverse fall into a doorway, or a plan that drains onto a neighbour's lot. Stop rule: reset elevations before forming; where the site cannot drain, stop and get a written drainage direction from the customer naming the low point. Hazard: laser levels are eye hazards at close range, so keep the head above or below standing eye height and never sight back into the emitter.

5. Set forms so they hold the slab you sold. Stake forms at 4 ft centers maximum, closer on curves, with stake tops cut flush or below the form top so a screed can ride. Acceptance: form top within an eighth of an inch of the marked elevation at every stake, under an eighth of deflection when pushed hard by hand at mid-span, every stake below the screed path, and slab depth from string to base at the step 3 probe points meeting ordered thickness with no negative tolerance. Wrong looks like a form that walks outward under the first load, showing up as a fat edge and a thin middle. Stop rule: add stakes and kickers until the form passes the push test; do not pour against a form you can move. Hazard: driving stakes throws chips, so ANSI Z87.1 eye protection is worn by the person swinging and anyone in the swing radius, and stakes standing at shin or eye height get capped before the crew works around them.

6. Set reinforcement on chairs at the height the design calls for. Support bar or mesh continuously on chairs or bolsters at spacing that keeps it from sagging under a boot, tie laps per the drawing, and set chair height by measurement, not eye. Acceptance: clear cover as the design specifies with no point under the ACI 318 minimum for that exposure; on a 5 in exterior slab with No. 4 bar the working target is 1.75 in clear from the top, putting the bar center 2.0 in down, above mid-depth where shrinkage steel belongs, with a quarter inch over the 1.5 in ACI 318 minimum for No. 5 and smaller exposed to weather. Wrong looks like mesh laid flat with a plan to hook it up during the pour, which no crew does under pressure. Stop rule: no-go until the steel is chaired; hooking is not a method. Hazard: protruding dowels and stakes a worker could fall onto are guarded before the crew works around them, per 29 CFR 1926.701(b).

7. Verify the joint plan, isolation and edge conditions on the ground. Confirm the drawn joint layout is marked on the forms, isolation material is in place at every fixed element the slab abuts (foundation, column, hydrant, stoop, existing slab), and no steel crosses an isolation joint. Acceptance: every panel on the plan matches a marked pair of form marks, isolation strip full depth with no gaps at corners, panel aspect ratio no worse than the plan allows. Wrong looks like an isolation strip stopping short at a re-entrant corner, which is where the slab will crack instead. Stop rule: mark and strip before ordering; joints added after the pour begins produce the odd panels customers photograph. Hazard: none specific beyond the site's own.

8. Check the weather window, then release the order. Read the forecast for the placement window and the first 24 hours after it. Acceptance: no precipitation forecast during placement or finishing, ambient conditions inside your hot and cold weather triggers with protection materials on the truck if you are near either edge, and a written go from the crew leader. Wrong looks like ordering against a forecast that turns during the drive out. Stop rule: no-go, and the office makes the reschedule call the same day - name the value that failed, what it would have caused, and a date, not an apology. Hazard: a hot-weather pour stages a shaded rest area, water and a work/rest cycle before placement, not after someone goes down.

The record this produces

One record per pour, completed on site, photographed and attached to the job before the order is placed. Fields: job and pour identifier; date and crew leader name; locate ticket number and expiry; proof roll result and any density test values with their test location; base lift count, lift thickness and total compacted depth with probe readings; measured fall over the first 10 ft and over the balance; form elevation check and push-test result; chair height, bar size and measured clear cover; isolation and joint-plan verification; forecast summary; and the go/no-go signature with any exception noted. It lands on the job record in the office system, not in a truck: the reader who needs it most is the office answering a warranty call two winters out asking whether that panel was ever compacted.

One pass, with a step that failed

A 24 by 28 ft driveway extension, 672 sq ft, 5 in slab, No. 4 bar at 18 in each way, 6 in compacted base. Step 1 passed: ticket live nine more days, gas and cable hand-exposed and clear by 14 in below the base.

Step 2 failed. The proof roll rutted about an inch and a half in a 4 ft wide strip running the width of the drive at 11 ft out, and the strip pumped water on the second pass. Nuclear gauge readings read 96.8, 97.2 and 96.1 percent of standard Proctor at three locations across the pad and 91.4 percent inside the strip. The gate is 95 percent, so one of four locations failed and the strip was visibly non-uniform besides. The crew leader took the stop rule: no order placed. The strip was the sewer lateral trench, backfilled by the plumber two weeks earlier and never compacted in lifts. The grade hand excavated it to 22 in, rebuilt it in three lifts of roughly 7 in loose, wetted each, and retested: 95.9 percent in the trench, 96.4 and 96.0 percent one form width each side. Passing on the retest, and at the two flanking points that prove the repair did not just create a stiff plug.

The stop cost half a day and a rescheduled truck; skipping it would have cast a settlement crack across a 24 ft panel at the trench line. The office made the step 8 call that afternoon: the trench read 91.4 against a 95 percent requirement, an uncompacted trench settles and cracks the panel above it, we are rebuilding today and pouring Thursday morning.

Steps 3 through 8 then ran clean: base at 6 in in two lifts, probed at six points reading 5.75 to 6.25 in; fall of 2.5 in over the first 10 ft (2 percent, meeting IRC R401.3) and 2.25 in over the remaining 18 ft; forms within an eighth at all eleven stakes; chairs at 2.75 in giving 1.75 in clear cover. Order released at 14:10 for an 07:00 load the next day.

References

• ACI 302.1R, Guide to Concrete Floor and Slab Construction, on uniform subgrade support, in the edition your spec names.
• ACI 318, Building Code Requirements for Structural Concrete, minimum cover by exposure. See related: the ACI 318 minimum cover for residential rebar card.
• ASTM D698 (standard Proctor) and ASTM D6938 (nuclear gauge density), the step 2 acceptance values.
• International Residential Code R401.3, the 2 percent impervious-surface exception within 10 ft of the foundation, as adopted and amended by your jurisdiction.
• 29 CFR 1926.701(b), rebar impalement guarding; 29 CFR 1926.1153, respirable silica in construction.
• See related: the ready-mix delivery and truck coordination SOP, and the joint layout and sawcut timing SOP.