Flatwork and Slab Finishing

Slab-on-Grade and Flat Surface Finishing

Introduction to Slab-on-Grade

Slab-on-grade refers to concrete work cast directly on a soil support, generally flat, designed to form a surface for traffic, parking, or work activities. In the context of the concrete finisher trade, slab-on-grade is distinguished from foundations or vertical structural elements by its geometry: a large horizontal surface, relatively thin thickness (typically 100 mm to 300 mm), and requirements for flatness and surface finish.

Slab-on-grade concrete is subject to specific stresses: plastic and thermal shrinkage, rolling or pedestrian loads, abrasion, freeze/thaw cycles, and exposure to de-icing salts. The finisher must understand these stresses to select the proper placement and finishing techniques.

Distinction Between Slab and Slab-on-Grade

A slab is a load-bearing structural element (often reinforced, designed by an engineer) that forms part of the building frame. Slab-on-grade is a concrete placement on the ground (or on compacted fill) that transfers loads directly to the soil. This distinction is fundamental for the Red Seal exam: tolerances, formwork methods, and curing requirements differ.

Subgrade and Support Preparation

Subgrade Requirements

Before any pour, the subgrade must be prepared and verified. The finisher must ensure that:

The soil is compacted to a specified degree (often 95% of modified Proctor maximum density, per the specifications).

The soil is drained to prevent water accumulation beneath the slab-on-grade.

A vapour barrier (6 to 10 mil polyethylene) is installed beneath interior slab-on-grade, in accordance with recommended practices in the National Building Code of Canada (NBC) , to prevent moisture migration.

The bedding layer (sand or crushed stone) is levelled to a tolerance of ± 20 mm below the membrane.

Formwork

Slab-on-grade formwork is typically wood (2×4, 2×6, 2×8) or metal. It must be:

Solid and well braced to resist the pressure of fresh concrete.

Levelled to the exact finished height (tolerance ± 3 mm).

Oiled or treated with a release agent to facilitate removal.

The grade line (string line) is pulled at each form stake. The finisher uses a rotating laser level or optical level to verify elevations. The formula for calculating form height is straightforward:

Form height = Slab-on-grade thickness + Bedding layer thickness (if applicable)

Joints

Joints are critical elements of slab-on-grade. They control cracking and must be planned before the pour.

Joint Type	Function	Typical Location	Method of Construction
Contraction joint	Allows concrete shrinkage without random cracking	Spacing = 24 to 36 × slab thickness (max. 4.5 m for a 150 mm slab)	Saw-cut to 1/4 to 1/3 of the thickness, performed 4 to 12 hours after finishing
Expansion joint	Allows thermal expansion and differential movement	At junctions with walls, columns, other slabs	Compressible material (fibre, foam) installed before the pour
Construction joint	Separates two successive pours	End of day, work stoppage	Vertical formwork, reinforcing bars or dowels

Rule of thumb: The maximum spacing of contraction joints is 24 to 36 times the slab thickness. For a 125 mm slab, the maximum spacing is 125 mm × 24 = 3,000 mm (3 m). The length-to-width ratio of a slab-on-grade panel must not exceed 1.5:1.

Concrete Placement

Receiving and Inspecting Concrete

Upon truck arrival, the finisher must verify:

29.The delivery ticket: concrete type, specified strength (f'c), slump, admixtures (air entrainment, accelerators, etc.).

30.The slump using the Abrams cone: the value must match the specification (typically 75 mm ± 20 mm for standard slab-on-grade).

31.The temperature of the concrete: between 10 °C and 30 °C at placement (depending on conditions).

32.The elapsed time since batching: must not exceed 90 minutes (or 300 drum revolutions) without authorization.

Water dosing formula: Adding water on site is prohibited unless authorized in writing by the engineer. Each litre of water added per cubic metre of concrete reduces strength by approximately 2 to 3 MPa. The finisher must reject concrete that is too wet.

Placement Techniques

Concrete must be deposited as close as possible to its final position to avoid segregation. Acceptable methods include:

Chute: for small areas, with truck repositioning.

Concrete pump: for large areas or difficult access.

Drop bucket: for confined areas.

Concrete must never be discharged in free fall from more than 1.5 m without a guiding device (tremie pipe, elephant trunk).

Spreading and Screeding

Spreading is done with shovels and rakes. A vibrating screed is used to level the surface. The screed is pulled in a zigzag (sawing) motion along the guides or forms. The concrete height should be slightly higher (10 to 20 mm) than the final height to compensate for vibration settlement.

A concrete vibrator (vibrating needle) is used for thick slab-on-grade (> 200 mm) or heavily reinforced placements. The needle must be inserted vertically, spaced 8 to 10 times its diameter apart, and withdrawn slowly (about 3 seconds per insertion). Vibration must not exceed 5 to 15 seconds per point to avoid segregation.

Surface Finishing

Finishing Operation Sequence

Finishing follows a precise sequence, dependent on the concrete setting time. The finisher must assess the finishing time (when the concrete has lost its surface water and supports a person's weight without sinking more than 5 mm).

Operation	Tool	Timing	Objective
Screeding	Vibrating screed	Immediately after placement	Rough levelling
Floating	Wood or magnesium float	After bleeding (surface water has disappeared)	Close the surface, embed aggregates
Trowelling	Steel trowel	After floating	Dense, smooth surface
Brooming	Broom	After trowelling (or instead of it)	Non-slip surface

Floating

Floating is the first finishing operation after screeding. It serves to:

Embed large aggregates below the surface.

Eliminate irregularities.

Close the surface to reduce evaporation.

A wood float is used for surfaces that will be broomed (textured). A magnesium (or aluminium) float is used for surfaces that will be trowelled (smooth). Floating must begin once the bleed water has disappeared. If you float too early, you trap water and create a weak surface (laitance).

Trowelling

Trowelling is performed with a steel trowel. It is done in two or three passes:

55.First trowelling: trowel held flat (angle of 0 to 10°), to close the surface.

56.Second trowelling: trowel slightly angled (10 to 20°), to densify the surface.

57.Third trowelling (optional): trowel steeply angled (20 to 30°), for a very smooth, shiny finish (burnished finish).

Trowelling too early or excessively can cause a burned finish (powdery, fragile surface) or plastic shrinkage cracks. Excessive trowelling can also close the surface and trap air, creating blisters.

Brooming

Brooming is done with a broom (polypropylene street broom) pulled across the fresh surface. It creates a non-slip texture. The timing of brooming is critical: too early, the texture collapses; too late, the broom tears the surface. The rule is to broom when the concrete is firm enough to hold the broom impression without collapsing.

Special Finishes

Broom finish: for sidewalks, driveways, ramps.

Wood float finish: for interior industrial surfaces.

Trowelled finish: for garage floors, mechanical rooms.

Exposed aggregate finish: washing the surface with water and brushing to expose the aggregates (timing of 2 to 6 hours depending on temperature).

Stamped finish: applying stamps (moulds) on fresh concrete to imitate stone, brick, etc.

Tolerances and Quality Control

Flatness and Levelness

Flatness tolerances are defined in the NBC and standard specifications. For industrial slab-on-grade, the 3 m straightedge method is used: the maximum deviation under a 3 m straightedge must not exceed 6 mm (FF/FL tolerance per ASTM E1155, often cited in specifications).

Slab-on-Grade Type	Flatness Tolerance (3 m straightedge)	Levelness Tolerance (slope)
Interior residential slab	± 10 mm	1:50 (2%)
Light industrial slab	± 6 mm	1:100 (1%)
Heavy industrial slab (VNA)	± 3 mm	1:200 (0.5%)

Fresh Concrete Testing

The finisher must be familiar with the following tests:

Slump test: 300 mm high cone, filled in 3 layers of 25 rod strokes each. Result: slump in mm.

Air content: pressure meter method (for air-entrained concrete). Typical air content for slab-on-grade exposed to freeze/thaw is 5 to 8%.

Cylinder sampling: 150 mm × 300 mm cylinders, moulded and cured per CSA A23.2-3C. The finisher must take cylinders at the time of placement, identify them, and transport them to the laboratory.

Volume Calculation

The volume of concrete required is calculated as follows:

Volume (m³) = Length (m) × Width (m) × Thickness (m)

For a slab measuring 12 m × 8 m × 0.150 m:

V = 12 × 8 × 0.150 = 14.4 m³

A waste factor of 5 to 10% must be added to account for losses, spills, and subgrade irregularities. For the example above: 14.4 × 1.07 ≈ 15.4 m³.

Conversion: 1 m³ = 1,000 L. A standard mixer truck carries 6 to 10 m³.

Concrete Curing

Curing Principles

Curing is the set of measures taken to keep concrete moist and at an adequate temperature during the setting and hardening period. It is essential to:

Allow complete cement hydration.

Reduce plastic shrinkage and cracking.

Increase final strength and durability.

The minimum curing duration is 7 days for ordinary concrete (or until the strength reaches 70% of the specified strength). For high-performance concrete, curing may be extended to 14 days.

Curing Methods

Method	Description	Advantages	Disadvantages
Water curing	Continuous spraying or immersion	Excellent effectiveness	Water consumption, risk of thermal shock
Wet burlap	Burlap kept moist	Simple, effective	Requires constant monitoring
Curing compound	Spray-applied membrane	Practical, uniform	Must be applied at the correct rate (200 to 300 ml/m²)
Polyethylene sheeting	Plastic film placed on the surface	Retains moisture	Risk of discolouration (marbling) if placed too early

Golden rule: Curing must begin immediately after finishing, as soon as the surface can support the operator's weight without damage. For freshly finished concrete, a curing compound is often applied by spraying at a rate of 200 to 300 ml/m².

Temperature and Weather Conditions

In hot weather (> 30 °C): use chilled concrete, wet down the formwork, protect the surface from wind and sun, pour early morning or evening. The evaporation rate must not exceed 1.0 kg/m²/h (per CSA A23.1).

In cold weather (< 5 °C): concrete must be protected from freezing for at least 48 hours. Use insulated forms, heated enclosures, or heaters. Concrete temperature must not drop below 10 °C during curing.

Evaporation formula (CSA A23.1 charts): Evaporation depends on air temperature, concrete temperature, relative humidity, and wind speed. If the evaporation rate exceeds 1.0 kg/m²/h, apply an evaporation retarder (monomolecular film) or protect the surface.

Reinforcement and Fibres

Welded Wire Mesh

Welded wire mesh (typically 150 × 150 mm, 4.8 mm or 6.3 mm wire) is used to control cracking. It must be positioned in the upper third of the slab (about 1/3 of the thickness from the surface) to be effective. The finisher must use chairs to maintain the mesh at the correct height. Walking on the mesh during placement can displace it; lift it as you go using a hook.

Fibres

Fibres (polypropylene, nylon, steel) are added to concrete to reduce plastic shrinkage and improve impact resistance. They do not replace welded wire mesh for structural loads. The typical dosage is 0.6 to 0.9 kg/m³ for polypropylene fibres.

Crack Control

Types of Cracks

Type	Cause	Time of Appearance	Prevention
Plastic shrinkage	Rapid evaporation of surface water	1 to 6 hours after placement	Immediate curing, evaporation retarder, wind protection
Thermal shrinkage	Temperature difference between core and surface	1 to 3 days	Contraction joints, low heat-of-hydration concrete
Settlement	Soil or formwork movement	Variable	Soil compaction, solid formwork
Load	Overload or concentrated load	After hardening	Reinforcement, expansion joints, adequate thickness

Joint Saw-Cutting

Saw-cutting of contraction joints must be done at the right time: early enough to prevent random cracking (4 to 12 hours after finishing), but late enough to avoid edge spalling. The saw-cut depth is 1/4 to 1/3 of the slab thickness. For a 150 mm slab, the saw-cut depth is 40 to 50 mm.

Jobsite Safety

The finisher must know the basic safety rules:

Personal protective equipment (PPE): hard hat, safety glasses, gloves, steel-toed boots, hearing protection.

Handling: fresh concrete is corrosive (pH ≈ 12.5). In case of skin contact, rinse thoroughly with clean water.

Working at heights: guardrails are mandatory above 1.2 m.

Formwork: never stand under formwork during placement.

Vibrating tools: the vibrator must be turned off when moving it around.

Pitfalls to Avoid

115.Floating too early: if bleed water is still present, floating traps the water and creates a weak, powdery surface. Wait until the water disappears.

116.Excessive trowelling: over-trowelling causes a burned finish (fragile surface) and plastic shrinkage cracks. Limit the number of passes.

117.Adding water to concrete: this reduces strength and increases shrinkage. Reject or report any unauthorized water addition.

118.Neglecting curing: insufficient curing reduces strength by 30 to 50% and increases cracking. Curing is not optional.

119.Forgetting joints: excessive spacing of contraction joints causes random cracking. Follow the 24 to 36 times thickness rule.

120.Walking on welded wire mesh: this displaces it and makes it ineffective. Use walkways or lift the mesh as you go.

121.Saw-cutting joints too late: if saw-cutting is done after cracks have appeared, it is useless. Plan saw-cutting within 4 to 12 hours.

122.Confusing slab and slab-on-grade: tolerance and curing requirements are not the same. Read the specifications carefully.

123.Ignoring weather conditions: in hot, windy weather, rapid evaporation causes plastic shrinkage cracks. Plan for protection.

124.Not checking the slump: concrete that is too fluid (slump > 100 mm) is difficult to finish and loses strength. Check every load.

Summary

Slab-on-grade is a concrete placement on the ground, distinct from a structural slab. It transfers loads directly to the soil and requires careful subgrade preparation.

Joints (contraction, expansion, construction) are essential for crack control. Contraction joint spacing is 24 to 36 times the slab thickness.

The finishing sequence is: screeding → floating → trowelling (or brooming). Each operation must be performed at the right time, based on bleeding and concrete setting.

Flatness tolerances are ± 6 mm under a 3 m straightedge for standard industrial slab-on-grade.

Curing must last at least 7 days and begin immediately after finishing. Acceptable methods include water curing, wet burlap, curing compounds, and polyethylene sheeting.

Fresh concrete tests (slump, air content, cylinder sampling) are mandatory and must be performed per CSA A23.2 standards.

Concrete volume is calculated by multiplying length × width × thickness, with a 5 to 10% waste factor.

Safety is paramount: fresh concrete is corrosive, formwork must be solid, and PPE is mandatory.

Normative References

National Building Code of Canada (NBC): requirements for slab-on-grade, vapour barriers, and tolerances.

CSA A23.1: Concrete — Constituent materials and execution of work (includes curing, temperature, and evaporation requirements).

CSA A23.2: Concrete — Testing and analytical methods (includes slump, air content, and cylinder sampling tests).

ASTM E1155: Standard test method for determining floor flatness (FF/FL).

This chapter covers the essential knowledge for the Red Seal exam in concrete finishing. Review the CSA standards and the NBC to deepen your technical understanding. Good luck with your preparation.