Precast Floor Slabs

Precast hollow-core slabs are one of the most efficient floor systems in commercial construction — and they’re a regular part of our church projects. A single truckload of slabs can cover 100–150 m² of floor area, and a good crane crew can set an entire floor in a day or two. But “fast” doesn’t mean “simple.” Every slab weighs 2–8 tonnes, the bearing details are unforgiving, the grouting has to be right, and the topping — if there is one — ties the whole system together structurally. Get any of those wrong and you’ve got a floor that bounces, cracks, or worse.

This guide covers the full sequence: delivery and site planning, crane erection, bearing and connection details, keyway grouting, composite topping placement, and the inspections that keep everyone honest. Every crew member involved needs to understand it from the ground up — literally.

1. Understanding Hollow-Core Slabs

Hollow-core slabs are precast, prestressed concrete planks with continuous voids (cores) running their length. The cores reduce weight without sacrificing structural depth, and the prestressing strands give the slab its load-carrying capacity. They arrive on site ready to set — no formwork, no shoring, no curing time before you can walk on them.

Common Slab Sizes

The precast engineer selects the slab depth based on span, loading (dead load + live load), and fire rating requirements. For most of our two-storey church projects, 200 mm or 250 mm slabs cover the typical 7–10 m spans in sanctuary and fellowship hall areas. Deeper slabs (305–355 mm) come into play for large open sanctuaries where columns aren’t an option.

Key Terminology

• Cores: The longitudinal voids inside the slab. Usually circular or oval, typically 5–7 per slab depending on width and depth.
• Keyway: The shaped groove along the long edge of each slab. When grouted, adjacent keyways interlock to transfer shear between slabs — making the floor act as a single diaphragm.
• Prestressing strands: High-strength steel strands (typically 12.7 mm / 1/2″ dia.) tensioned before the concrete is cast. They put the bottom of the slab in compression, which is how a 200 mm slab can span 9 metres.
• Bearing: The area where the slab rests on the supporting wall or beam. Minimum bearing length is critical — too short and the slab has no margin for error.
• Composite topping: A cast-in-place concrete layer (typically 50–75 mm) poured on top of the slabs. When bonded properly, it acts compositely with the precast to increase load capacity and create a smooth, level floor.
• Diaphragm: The floor system acting as a horizontal plate to transfer lateral loads (wind, seismic) to the shear walls. Grouted keyways and topping reinforcement are what make this work.

2. Pre-Erection Planning

A precast floor installation is a crane day — and crane days are expensive. Every minute the crane sits idle because something isn’t ready costs money and pushes the schedule. The planning you do in the week before erection determines whether it’s a smooth one-day set or a two-day disaster.

Site Readiness Checklist

1. Bearing surfaces ready: Walls, beams, or ledgers must be at the correct elevation and have reached adequate strength. For masonry bearing walls, the grout in the bond beam course must have cured at least 48 hours. For cast-in-place concrete beams, the engineer specifies minimum strength (typically 75% of f′c) before loading with precast.
2. Bearing surfaces level: Check every bearing point with a laser level or total station. The tolerance is tight — ± 6 mm across any bearing wall. If the wall is out of level, the slabs will rock, and shimming 4-tonne planks is nobody’s idea of fun.
3. Bearing pads placed: Neoprene or fibre bearing pads (typically 75–100 mm wide × slab width × 6 mm thick) must be set at every bearing location before the crane arrives. The pads distribute the slab reaction and prevent point loading on the wall. Set them to the back edge of the bearing area — the slab face should be flush with or slightly recessed from the wall face.
4. Crane setup area clear: Confirm the crane pad location, ground conditions (can it handle the outrigger loads?), swing radius, and any overhead hazards (power lines, trees). The crane operator needs a rigging plan showing slab weights, pick radii, and sequence.
5. Delivery route confirmed: Flatbed trucks are 15–18 m long and heavy. Verify the route from the road to the crane can handle the axle loads and turning radii. Mud season in Ontario has ruined more than one precast delivery day.
6. Openings and blockouts: Verify all slab openings (stairwells, mechanical shafts, elevator pits) against the structural drawings. The precast supplier cuts most openings at the plant, but field openings are sometimes required — and they need to be marked before erection so the crane crew knows which slab goes where.

Erection Sequence Planning

The erection drawing from the precast supplier shows the slab layout, piece marks, and a suggested erection sequence. The sequence matters because:

• The crane can’t reach everywhere from one position. Plan the sequence so the crane sets all slabs within its current radius before repositioning. Repositioning a 100-tonne crane takes 30–45 minutes.
• You need a safe work platform. Set slabs starting from one end and working toward the crane so the crew always has a solid floor behind them. Never leave gaps that someone could fall through — if you must leave an opening, barricade it with guardrails immediately.
• Stairwell and shaft openings are hazards. Set the slabs around openings early so guardrails can go up. An open hole in a floor is an invitation for a fatality.

3. Crane Erection — Setting the Slabs

This is the main event — the day the floor goes up. A well-planned erection with a good crane crew can set 30–50 slabs in a single day, covering 600–1000 m² of floor. The superintendent’s job is to keep the operation moving safely and ensure every slab lands in the right place at the right elevation.

Rigging

Hollow-core slabs are lifted with a spreader beam and slab clamps (also called “tongs” or “lifting clamps”). The clamps grip the bottom edges of the slab at two points, and the spreader beam keeps the sling angle wide enough to prevent the slab from being squeezed. Key points:

• Clamp capacity: Verify the clamp SWL (safe working load) exceeds the heaviest slab by at least 25%. A 250 mm × 1220 mm × 9000 mm slab weighs roughly 3,700 kg.
• Spreader beam length: Must be at least 75% of the slab length to keep the clamp angle reasonable. Too short and the clamps try to squeeze the slab inward — the slab can slip or crack.
• Lifting inserts: Some precast suppliers cast lifting inserts (threaded or loop-type) into the slab top. If present, use them with the matched lifting hardware. Do not use clamps on insert-equipped slabs — it voids the rigging plan.
• Tag lines: Two tag lines (one at each end) are mandatory. The crew on the wall uses them to guide the slab into position. Without tag lines, the slab spins freely in the wind and becomes a 4-tonne pendulum.

Setting Procedure

1. Signal the crane: The designated signaller (one person, qualified, visible to the operator) directs the pick. Standard hand signals per CSA Z150. Radio communication as backup, not primary.
2. Lift and swing: The crane lifts the slab clear of the truck, swings to the building, and brings it over the bearing walls. The slab is oriented with the span direction matching the erection drawing.
3. Guide into position: Using tag lines, the crew guides the slab to within 150 mm of its final position. The slab is lowered slowly until it’s 50–100 mm above the bearing surface.
4. Set on bearing pads: The crew uses pry bars or alignment bars to nudge the slab into exact position as the crane lowers the last 50 mm. The slab should land centred on the bearing pads.
5. Check bearing length: Immediately verify the slab bearing length on both ends. Minimum bearing is specified by the engineer — typically 65 mm on masonry and 75 mm on concrete. If the bearing is short, do not release the crane — lift and reset.
6. Check alignment: The slab edge should be tight to the adjacent slab (or the reference line for the first slab). Gaps between slabs should be consistent — typically 15–25 mm for grout keyways.
7. Release the clamps: Only after bearing and alignment are confirmed. The signaller gives the “all clear” and the crane moves to the next piece.

4. Bearing & Connection Details

The connection between the precast slab and the supporting structure is where the engineering lives. Every detail matters: bearing length, bearing pad type, end anchorage, and how the slab connects to the wall or beam for diaphragm action.

Minimum Bearing Requirements

Bearing Pad Selection

• Neoprene pads (most common): 6 mm thick, 75–100 mm wide, cut to slab width. Duro 50–70 Shore A. They accommodate minor rotation at the slab end and distribute the reaction over a uniform area.
• Fibre-reinforced pads: For higher loads or situations where the pad must resist creep. Cotton-duck or fibreglass reinforced neoprene.
• Hard bearing (no pad): Only when the engineer specifies it — usually for temporary conditions or where the bearing surface is ground perfectly smooth. Not common.

End Anchorage & Diaphragm Connections

For the floor to act as a structural diaphragm (transferring wind and seismic loads to the shear walls), the slabs need to be connected to the walls and to each other. There are several common connection methods:

• Reinforcement in grout keyways: Rebar placed in the grouted joints between slabs, extending into the wall or beam. This is the primary diaphragm connection for most church projects.
• Pour strips: A gap left between the last slab and the wall, filled with cast-in-place concrete and rebar that ties into both the slab cores and the wall reinforcement.
• Welded connections: Embed plates cast into the slab ends, welded to embed plates in the wall or beam. Used for high-seismic or special loading conditions.
• Topping reinforcement: When a composite topping is placed, the mesh or rebar in the topping extends into and over the wall, providing continuity. The topping becomes part of the diaphragm.

5. Keyway Grouting

The grouted keyways between slabs are what turn a floor made of individual planks into a unified structural diaphragm. Without grout in the keyways, the slabs can rock independently, differential deflection creates cracks in the topping and finishes, and the floor can’t transfer lateral loads. Grouting is not optional and it’s not cosmetic — it’s structural.

Grout Mix

• Material: Non-shrink cementitious grout, minimum 30 MPa at 28 days. Pre-bagged non-shrink grout (mixed per manufacturer’s instructions) is standard. Do not use site-batched sand-cement mortar — it shrinks, cracks, and debonds.
• Consistency: Flowable but not soupy. The grout needs to flow into the keyway without leaving voids, but it shouldn’t be so wet that it runs through to the underside. Slump of 200–250 mm (for flow table testing) is typical.
• Temperature: Grout between 10°C and 30°C. Below 10°C, the set time extends dramatically and strength gain stalls. Above 30°C, the grout sets too fast and may not flow properly.

Grouting Procedure

1. Clean the keyways: Blow out all debris, sawdust, ice, and standing water from every keyway with compressed air. Wet debris prevents bond. If it rained since the slabs were set, let the keyways dry or blow them out thoroughly.
2. Pre-wet the keyway surfaces: Dampen (not soak) the concrete faces of the keyway with clean water. Dry concrete sucks the moisture out of the grout before it can hydrate, causing a weak bond. Saturated-surface-dry (SSD) is the target.
3. Place reinforcement: If the structural drawings call for rebar in the keyways (common for diaphragm connections), place it now. Typically 10M or 15M bars, lapped at splices as specified.
4. Mix grout: Follow the manufacturer’s water ratio exactly. Over-watering is the number one cause of grout failure. Use a measured bucket for water, not a hose. Mix in a mortar mixer or with a paddle mixer in a bucket for small quantities.
5. Pour and rod: Pour grout into the keyway from one end, working toward the other. Use a flat steel rod or piece of rebar to push the grout into the bottom of the keyway and eliminate air pockets. Don’t just dump it in from above and hope — rod every metre of keyway.
6. Strike off: Screed the top of the grout flush with the slab surface (if no topping) or leave it 10 mm below the surface (if a topping is coming). A flat trowel works for striking keyways.
7. Cure: Keep the grout moist for a minimum of 24 hours. Wet burlap or a curing compound applied after initial set. Don’t let it dry out in the sun — uncured grout is weak grout.

6. Core Fills & Blockouts

Sometimes you need to fill certain cores with grout or concrete — for anchorage, load distribution, or to create solid sections around openings. This is specified by the precast engineer and shown on the erection drawings.

When Cores Get Filled

• At bearing ends: The first 300–600 mm of cores at each bearing end are often filled with grout to increase shear capacity and provide anchorage for diaphragm reinforcement.
• Around openings: Cores adjacent to field-cut openings are filled to restore the section’s structural integrity. The engineer specifies how many cores on each side and for what length.
• Under concentrated loads: Point loads from columns or heavy equipment may require filled cores to spread the load across the slab section.
• For anchor bolt anchorage: If mechanical equipment anchors are required at specific locations, the core below is filled with grout and a threaded insert or anchor bolt is set.

Core Fill Procedure

1. Break open the core top: Use a hammer drill or chipping hammer to open the top of the core at the location specified. The hole should be large enough to pour grout and rod it — typically 75–100 mm diameter.
2. Plug the core: Push a foam plug or wadded paper down the core to create a dam at the correct fill depth. Verify the plug is at the right depth by measuring with a stick.
3. Pour non-shrink grout: Same grout as the keyways — non-shrink, 30 MPa minimum. Pour in lifts of 300 mm and rod each lift.
4. Set anchors: If an anchor bolt is required, set it into the wet grout at the correct projection and orientation. Template it to the bolt pattern if it’s for equipment mounting.

Field-Cut Openings

Sometimes an opening isn’t shown on the original precast drawings and needs to be cut in the field — for a plumbing chase, duct penetration, or a missed mechanical sleeve. This is structural surgery and requires engineer approval.

• Never cut through prestressing strands without engineer approval. The strands are the slab’s primary reinforcement. Cutting even one strand can reduce the slab’s capacity below the design load.
• Use a concrete saw (diamond blade) for clean cuts. A jackhammer will shatter the slab and damage adjacent areas.
• Fill exposed cores adjacent to the opening as specified by the engineer.
• Add framing if required: Steel angles or channels may be needed to support the slab edges around larger openings. The engineer designs these.

7. Self-Levelling Topping

Most church projects use a thin self-levelling layer over the precast slabs rather than a full structural composite topping. The self-levelling compound — typically about 25 mm (1″) thick — solves the practical problem of creating a flat, smooth surface for finished flooring. It compensates for the natural camber in the prestressed slabs and any differential camber between adjacent planks, without the weight, cost, and complexity of a full concrete topping with reinforcement.

When a Self-Levelling Topping Is Used

• Floor levelness: Precast slabs have a natural camber (upward bow) from the prestressing, and adjacent slabs may have slightly different cambers. The self-levelling layer evens everything out to a flat surface suitable for finished flooring.
• Surface preparation for finishes: Hollow-core slabs have a relatively rough factory finish. Carpet, tile, hardwood, and LVP all need a smooth, flat substrate — the self-levelling compound provides exactly that.
• Speed: Self-levelling compound is pumped and spread quickly, and most products are walkable within 4–6 hours. Compared to a conventional concrete topping that needs days of curing, it keeps the schedule moving.

A full structural composite topping (50–75 mm with welded wire mesh) is sometimes specified by the engineer for added load capacity, diaphragm reinforcement, or fire rating upgrades — but for the majority of church projects, the 25 mm self-levelling layer is all that’s needed.

Self-Levelling Placement Procedure

1. Verify keyway grout has cured: The keyway grout must have reached design strength (typically 48–72 hours) before placing the topping. Loading uncured keyways with wet self-leveller can crack the grout bond.
2. Clean the slab surface: Power-sweep and blow out all debris. The slab surface must be clean and free of any material that would prevent bond — mud, sawdust, drywall dust, coffee cups, and the miscellaneous debris that accumulates on every construction floor.
3. Prime the slab surface: Apply the manufacturer’s recommended primer to the precast surface. The primer improves bond and controls the absorption rate of the concrete substrate. Allow the primer to dry per the product data sheet — typically 1–3 hours. Some products require a second coat; follow the manufacturer’s instructions exactly.
4. Dam perimeter and openings: Install foam backer rod or dam strips at all perimeter edges, stairwell openings, elevator shafts, and floor drains. Self-levelling compound is very fluid and will run through any gap it can find. Seal everything before you start pumping.
5. Set gauge pins or tripods: Set depth gauges (pins or tripod markers) across the slab at 1200–1500 mm centres to control the topping thickness. Set them with a rotary laser to achieve the target elevation. The thickness will vary slightly to accommodate slab camber — thinnest at mid-span, thickest near the bearings.
6. Pump and spread: Mix the self-levelling compound in a continuous mixer or mixing pump per the manufacturer’s water ratio. Pump onto the slab in a continuous pour, working from the far end of the room toward the exit. Use a gauge rake to spread to the correct thickness, then pass a spike roller (porcupine roller) to release trapped air.
7. Allow to cure: Most self-levelling products are walkable in 4–6 hours and ready for floor coverings in 24–48 hours, depending on thickness and conditions. Keep traffic off the surface until it has hardened. Protect from drafts and direct sunlight during initial cure to prevent cracking.

8. Handling Special Conditions

Camber & Differential Camber

Every prestressed slab has camber — an upward bow caused by the eccentric prestressing force. This is normal and expected. The camber decreases over time as the concrete creeps under sustained load. The challenge on site is differential camber: adjacent slabs with different camber values, creating a step or lip at the joint.

• Typical camber values: 10–25 mm for 200 mm slabs; 15–40 mm for 305 mm slabs. The precast supplier provides predicted camber values, but actual camber varies with age, storage conditions, and strand pattern.
• Managing differential camber: If two adjacent slabs differ by more than 6 mm at the joint, it’s visible and can telegraph through thin flooring. The self-levelling topping handles moderate camber differences well, but if the differential is severe (> 12 mm), you may need to grind the high slab before pouring the self-leveller to keep the topping thickness reasonable.
• Matching camber during erection: Ask the precast supplier to group slabs by camber on the truck so adjacent slabs have similar values. This isn’t always possible (the production schedule drives truck loading), but it’s worth requesting.

Cold Weather Grouting

Ontario winters and precast grouting don’t mix well. Grout that freezes before reaching initial set has zero strength — it turns to sand. If you must grout in cold weather:

• Minimum grout temperature at placement: 10°C. Heat the mixing water if needed (not the grout powder).
• Protect after placement: Cover grouted keyways with insulated blankets. Maintain the grout temperature above 10°C for a minimum of 48 hours after placement.
• Don’t add accelerators unless the grout manufacturer specifically allows it. Calcium chloride accelerators can cause corrosion of the prestressing strands.
• Monitor with thermocouples: For critical work, place temperature sensors in the grout and log temperatures every 4 hours until the 48-hour mark.

Services Through Slabs

Running mechanical and electrical services through hollow-core slabs requires coordination with the precast engineer:

• Through the cores: Small pipes and conduit (up to ~50 mm diameter) can run through the hollow cores longitudinally. The cores are accessed by breaking through the slab top or bottom at the entry/exit points. This is common for electrical conduit runs.
• Perpendicular penetrations: Small round penetrations (up to ~150 mm) can be core-drilled through the slab between cores (through the web). Larger penetrations require the engineer’s approval and may need reinforcement.
• Never cut the bottom flange or the prestressing strands. The bottom flange is the tension zone — cutting it is like cutting the bottom chord of a truss. The slab will fail.

9. Inspections & Quality Control

Precast floor installations involve structural connections that get permanently concealed by grout, topping, and finishes. Once covered, nobody can see whether the bearing was adequate, the keyways were grouted, or the connections were made. That’s why the inspection protocol for precast work is rigorous — and why documentation matters more here than almost anywhere else on the project.

Inspection Stages

Documentation

• Erection log: Record every slab set — piece mark, time set, bearing lengths measured (both ends), and any issues noted. This log stays in the QA file.
• Photographs: Photograph every bearing condition before grouting. Wide shots and close-ups. Include a tape measure in the photo showing the bearing length. These photos are your proof that the bearing was adequate — once the grout goes in, the bearing is invisible.
• Engineer’s inspection report: The structural engineer’s sign-off on the erection must be in writing (not verbal) and filed in the project QA records before grouting begins.
• Grout batch records: Record the grout product, batch number, water ratio, air temperature, and placement time for each grouting session.

10. Common Problems & How to Avoid Them

Quick-Reference: Precast Floor Installation Checklist

1. Pre-erection: Bearing walls/beams at elevation? Bearing pads placed? Crane plan approved? Erection drawing distributed?
2. Delivery: Truck access confirmed? Slab piece marks match delivery ticket and erection drawing? Any visible damage?
3. Erection: Each slab — bearing length checked (both ends)? Alignment to adjacent slabs? Keyway gaps consistent? Piece mark logged?
4. Post-erection inspection: Engineer notified? All connections verified? Written sign-off received before grouting?
5. Grouting: Keyways clean and pre-wet? Non-shrink grout mixed to spec? All keyways filled, rodded, and struck off? Core fills complete? Perimeter dammed?
6. Topping (if applicable): Grout cured? Slab surface clean and SSD? Mesh on chairs? Screed rails set to elevation? Concrete ordered for correct volume?
7. Post-pour: Topping cured for 7 days minimum? Control joints cut within 24 hours? Flatness checked?
8. Documentation: Erection log, bearing photos, engineer’s report, grout records — all filed in QA package?

Trade Certification & Continuing Education

Precast concrete erection in Ontario falls primarily under the Ironworker (Structural/Ornamental) trade (420A), since it involves crane rigging, signalling, and setting structural elements. However, on our projects the crew doing the grouting, topping, and finishing work are typically our Cement (Concrete) Finisher (310C) and General Carpenter (403A) apprentices and journeypeople. Understanding precast systems gives your team a significant advantage — the more your people know about how the slabs work structurally, the better they handle the details that keep the floor performing for decades.

• 420A — Ironworker (Structural/Ornamental): Covers crane signalling, rigging, and erection of structural precast and steel.
• 310C — Cement (Concrete) Finisher: Covers topping placement, finishing, and curing — directly applicable to composite toppings.
• CPCI certification: The Canadian Precast/Prestressed Concrete Institute offers education programs on precast design and erection. Recommended for superintendents managing precast installations.