Vertical and Elevated Finishing

Chapter Introduction

Vertical and elevated finishing is a specialized area of the concrete finishing trade. Unlike horizontal slabs, vertical elements (walls, columns, beams) and elevated elements (floor slabs, bridge decks, parking garage slabs) present unique challenges: concrete must be placed against formwork, compacted by vibration, and then finished under conditions where access is limited and gravity works against you. This chapter covers all the knowledge required for the Red Seal exam, including formwork techniques, placement, vibration, mechanical troweling, architectural finishes, and safety requirements.

Fundamental Principles of Vertical Concrete

Behavior of Fresh Concrete in Vertical Elements

Fresh concrete is a thixotropic material: it flows under stress (vibration) and stiffens at rest. In vertical formwork, the lateral pressure of fresh concrete is the critical parameter. It depends on:

The drop height of concrete into the formwork

Concrete temperature (the lower the temperature, the slower the set, therefore the higher the pressure)

The rate of placement (rate of rise of concrete in the formwork)

Consistency (slump)

Vibration method (internal or external)

Maximum lateral pressure is calculated using the ACI 347 formula (adapted to the Canadian context):

P = γ × h (maximum hydrostatic pressure)

Where:

P = lateral pressure (kPa)

γ = unit weight of concrete (≈ 23.5 kN/m³)

h = height of fresh concrete above the point in question (m)

In practice, the pressure never exceeds full hydrostatic pressure. For concrete with normal setting time (temperature 20 °C), the effective pressure can be reduced according to the rate of rise. The following table gives approximate lateral pressures for concrete with a 100 mm slump, internally vibrated:

Rate of Rise (m/h)	Concrete Temperature	Maximum Lateral Pressure (kPa)
1.0	20 °C	48
1.0	10 °C	58
2.0	20 °C	72
2.0	10 °C	88
3.0	20 °C	95
3.0	10 °C	115

Golden rule: the colder the concrete and the faster the formwork is filled, the higher the lateral pressure. Poorly braced or improperly designed formwork can blow out under this pressure.

Formwork for Vertical Elements

Formwork must resist lateral pressure, maintain the geometry of the element, and produce a surface that meets finishing requirements. Common formwork types:

19 mm plywood formwork with wood framing (2×4 or 2×6) — for standard walls

Modular steel formwork — for repetitive walls, smooth surfaces

Insulated concrete forms (ICF) — expanded polystyrene blocks, left in place

Permanent forms — cardboard, fiber, or expanded metal for columns and beams

Tie rods are steel elements that pass through the formwork to resist lateral pressure. Their spacing is calculated based on the allowable load of the tie and the estimated lateral pressure. For a 4,000 lb (17.8 kN) tie, with a pressure of 50 kPa and a vertical spacing of 600 mm, the maximum horizontal spacing is:

Horizontal spacing = Allowable load / (Pressure × Vertical spacing)

Horizontal spacing = 17,800 N / (50,000 Pa × 0.6 m) = 0.59 m → 550 mm in practice

Snap ties are ties with break-off cones that snap off at stripping, leaving a hole to be patched. Water stops are used for foundation walls where watertightness is required.

Placing Concrete in Vertical Elements

Placement Methods

Concrete must be placed in vertical formwork in successive horizontal layers, never in a single pour. The thickness of each layer must not exceed 600 mm (2 feet). This limitation allows:

Effective vibration of each layer

Release of entrapped air

Prevention of segregation

Control of lateral pressure

Placement methods:

41.Direct chute — for low-height walls (less than 1.5 m)

42.Drop chute (hopper) — rubber or plastic tube that extends into the formwork, reducing free fall

43.Concrete pump — with flexible hose, allows precise placement at great heights

44.Bucket — for job sites where a pump is not available

Free-fall rule: the free fall of concrete into vertical formwork must not exceed 1 meter (3 feet) to prevent segregation. Beyond that, use a drop chute or hopper.

Vibrating Vertical Concrete

Vibration is essential for vertical elements. It removes air bubbles, ensures proper cover over reinforcement, and fills all corners of the formwork. Two types of vibrators:

Internal vibrator (needle type) — inserted directly into the concrete

External vibrator (form vibrator) — attached to the outside of the formwork, used for thin elements or heavily reinforced sections

Internal vibrator parameters:

Parameter	Recommended Value
Frequency	12,000 to 14,000 vibrations/min
Needle diameter	25 to 75 mm depending on the element
Radius of action	6 to 10 times the needle diameter
Vibration time per insertion	5 to 15 seconds
Distance between insertions	1.5 times the radius of action

Correct vibration procedure:

53.Insert the vibrator vertically, quickly, to the bottom of the previous layer (penetrating 150 mm into the lower layer)

54.Hold the vibrator in place for 5 to 15 seconds

55.Withdraw slowly (about 75 mm per second) to allow the concrete to fill the void

56.Space insertions systematically, without excessive overlap

Signs of adequate vibration:

Appearance of a film of mortar on the surface

Disappearance of large air bubbles

The sound of the vibrator changes (becomes deeper)

The concrete "closes up" around the needle

Over-vibration: over-vibration causes segregation, accumulation of mortar on the surface, and loss of strength. It is particularly damaging in vertical elements where mortar rises along the reinforcement.

Finishing Vertical Surfaces

Types of Vertical Finishes

The finish of a vertical element depends on its final use. The following table summarizes the types of finishes and their characteristics:

Type of Finish	Description	Typical Use
As-struck (form finish)	Surface as produced by the formwork, without touch-up	Foundation walls, hidden structural elements
Rubbed	Surface smoothed by manual or mechanical rubbing after stripping	Exposed interior walls
Broom finish	Surface textured with a broom, for traction	Stair treads, ramps
Sandblasted	Surface textured by sand projection	Architectural facades
Bush-hammered	Surface hammered to expose aggregates	Decorative walls
Acid-etched	Surface etched with acid to expose aggregates	Architectural elements
Plastered	Application of a finishing mortar	Repair, leveling

Stripping and Curing Vertical Elements

Stripping must not be done before the concrete has reached sufficient strength to support its own weight and stripping stresses. The general rule:

Walls: 12 to 24 hours at 20 °C (minimum strength of 5 MPa)

Columns: 24 to 48 hours

Beams and slabs: 7 to 14 days (depending on span and shoring)

Stripping must be done without impact or forceful prying. Use wooden wedges or stripping jacks to separate the formwork from the concrete.

Curing vertical elements: curing is critical because vertical surfaces dry faster than horizontal surfaces (no puddle of water to compensate for evaporation). Curing methods:

Curing compound — applied by spray, forms a waterproof membrane

Wet burlap — kept moist for 7 days

Polyethylene film — attached to the surface, sealed at joints

Steam curing — for precast elements in the plant

Minimum curing duration: 7 days at 20 °C, or until the concrete reaches 70% of its specified strength.

Elevated Elements: Slabs and Floors

Formwork and Shoring for Elevated Slabs

The formwork of an elevated slab must support the weight of fresh concrete, reinforcement, workers, and equipment, as well as dynamic placement loads. Formwork components:

Shores — adjustable vertical supports in steel or aluminum

Stringers — horizontal members supporting the decking

Decking — formwork surface in plywood or metal panels

Shore spacing: calculated based on total load and the load-bearing capacity of the shores. For a 200 mm thick slab:

Weight of concrete: 0.2 m × 23.5 kN/m³ = 4.7 kN/m²

Weight of formwork: 0.5 kN/m²

Construction load: 2.4 kN/m²

Total load: 7.6 kN/m²

With shores of 30 kN capacity, spaced at 1.2 m in both directions, each shore supports 1.44 m² × 7.6 kN/m² = 10.9 kN — acceptable.

Reshoring rule: after stripping, shores must be replaced under the slab (reshoring) to support construction loads from upper floors. Reshoring must be done as soon as possible after stripping.

Placing Concrete in Elevated Slabs

Concrete is placed in strips or sections, starting from the side farthest from the pump or crane. The placement sequence must avoid creating concentrated loads on any area of the formwork.

Slab thickness: checked using a depth probe or laser level. The typical tolerance is ± 10 mm for a 200 mm slab.

Finishing Elevated Slabs

Finishing an elevated slab follows the same principles as for a slab on grade, but with additional constraints:

Limited access — the finisher works on decking, often several meters above the ground

Setting time — concrete in elevated slabs is often more fluid (100 to 150 mm slump) to facilitate pumping, which extends setting time

Shrinkage — elevated slabs are more prone to plastic shrinkage due to exposure to wind and sun

Finishing sequence for an elevated slab:

103.Screeding — with a vibrating screed or laser screed, as soon as the concrete is placed

104.Floating — with a power trowel after initial set begins

105.Troweling — successive passes of the power trowel, increasing the blade pitch

106.Applying the final finish — according to specifications (troweled, broomed, etc.)

Attention to joints: elevated slabs require contraction joints (saw-cut or formed) spaced at 20 to 30 times the slab thickness. For a 200 mm slab, maximum spacing of 6 m. Joints must be saw-cut within 6 to 24 hours after finishing, before shrinkage causes cracking.

Architectural Vertical Finishes

Architectural Concrete

Architectural concrete is concrete whose surface is intended to remain permanently visible. Requirements are more stringent than for ordinary structural concrete:

Formwork: plywood panels with wood-fiber core, aligned joints, cone-tie ties (for uniform holes)

Concrete: controlled slump (75 to 100 mm), uniformly colored aggregates, water-cement ratio ≤ 0.45

Vibration: careful, without over-vibration, to avoid surface defects

Stripping: after 24 to 48 hours, with extreme care

Common architectural concrete defects:

Defect	Cause	Prevention
Honeycomb	Insufficient vibration, concrete too dry	Vibrate systematically, increase slump
Surface air bubbles	Insufficient vibration, un-oiled formwork	Vibrate longer, oil the formwork
Mortar (laitance)	Excess water, over-vibration	Control slump, avoid over-vibration
Color variation	Cement variation, uneven curing	Standardize sources, uniform curing
Form lines	Misaligned joints, deformed formwork	Check alignment, reinforce formwork

Repairing Surface Defects

Repairs must be made with a repair mortar compatible with the existing concrete. The mortar must have:

A water-cement ratio lower than that of the original concrete

Equal or greater strength

Similar color (preliminary testing)

Repair procedure:

124.Cut out the defective area (honeycomb) down to sound concrete

125.Clean the surface (wire brush, compressed air)

126.Wet the surface (saturated surface dry — SSD)

127.Apply the mortar in thin layers (10 mm maximum per layer)

128.Moist cure for 7 days

Safety for Vertical and Elevated Work

Working at Heights

Work on elevated formwork, scaffolding, or aerial work platforms presents fall hazards. Safety requirements:

Guardrails: installed on any open perimeter more than 1.2 m above the ground

Safety harness: mandatory when collective protection is not possible

Scaffolding: erected according to manufacturer specifications, inspected before use

Aerial work platforms: operator trained and certified

Concrete-Specific Hazards

Chemical burns: fresh concrete is alkaline (pH 12-13), prolonged skin contact causes severe burns. Wear waterproof gloves, rubber boots, and long-sleeved clothing.

Eye irritation: wear safety glasses during placement and vibration.

Silica dust: when saw-cutting, sandblasting, or bush-hammering, wear an approved N95 respirator or better.

Formwork Stability

Formwork must be inspected by a competent person before each pour. Checkpoints:

Vertical shores, well wedged, on stable footings

Ties tightened, without play

Bracing in place

Safe access for workers

No materials or debris on the decking

Useful Calculations and Conversions

Calculating Concrete Volume for a Wall

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

Example: wall 12 m long, 3 m high, 200 mm thick:

V = 12 × 3 × 0.2 = 7.2 m³

Calculating the Number of Ties

Number of ties = (Length / Horizontal spacing) × (Height / Vertical spacing)

For the wall above, with 550 mm horizontal spacing and 600 mm vertical spacing:

N = (12 / 0.55) × (3 / 0.6) = 21.8 × 5 = 109 ties

Pressure Conversion

1 MPa = 1,000 kPa = 145 psi

1 kPa = 20.9 lb/ft²

Calculating the Load on a Shore

Load per shore (kN) = Surface load (kN/m²) × Shore spacing (m) × Shore spacing (m)

For a load of 7.6 kN/m² and shores at 1.2 m × 1.2 m:

Load = 7.6 × 1.2 × 1.2 = 10.9 kN

Applicable Standards and Codes

Vertical and elevated finishing work is governed by several Canadian standards:

Standard	Subject	Key Points
CSA A23.1/A23.2	Concrete: constituents and execution of work	Requirements for concrete, placement, curing
CSA A23.3	Design of concrete structures	Strength, reinforcement, durability
CSA S269.1	Concrete formwork	Design, allowable loads, inspection
CSA S269.3	Slip forms	Specific requirements for slip forms
CSA Z462	Workplace electrical safety	Relevant if electrical equipment is used
Canada Labour Code	Occupational health and safety	General employer obligations

Key points of CSA A23.1 for vertical elements:

Maximum slump for vibrated concrete: 80 mm (unless otherwise specified)

Maximum time between mixing and placement: 90 minutes (or 300 revolutions of the drum)

Minimum concrete temperature at placement: 5 °C (unless with admixtures)

Minimum moist curing: 7 days (or 3 days with high early strength cement)

Pitfalls to Avoid

176.Confusing lateral pressure and hydrostatic pressure: the lateral pressure of fresh concrete is almost always lower than hydrostatic pressure, except for very fluid concrete or very rapid filling. Do not use hydrostatic pressure to size ties, except in extreme cases.

177.Forgetting temperature in pressure calculations: concrete at 5 °C sets much more slowly than concrete at 25 °C. Lateral pressure can be 50% higher. Always check the concrete temperature before calculating tie spacing.

178.Vibrating only at the surface: insert the vibrator to full depth in each layer. Surface vibration gives the illusion of compacted concrete but leaves honeycomb at depth.

179.Withdrawing the vibrator too quickly: rapid withdrawal leaves a void that fills with mortar, creating a local weakness. Withdraw at a maximum of 75 mm per second.

180.Stripping too early: a wall stripped after 8 hours at 10 °C can sag or crack. Check concrete strength (cylinder tests or non-destructive methods) before stripping.

181.Neglecting reshoring: after stripping a slab, shores must be replaced promptly. The concrete has not yet reached full strength, and loads from upper floors can cause cracking.

182.Confusing finish types: the "rubbed" finish is not the same as the "troweled" finish. Know the equivalencies: rubbed = talochée, floated = talochée au bois, troweled = lissée à la truelle.

183.Forgetting contraction joints in elevated slabs: a forgotten joint or one cut too late leads to random cracking. Saw-cut within 6 to 24 hours, before shrinkage exceeds 0.5 mm/m.

184.Using too high a water-cement ratio for architectural concrete: a water-cement ratio above 0.50 produces a porous surface with color variations. Maintain ≤ 0.45 for visible surfaces.

185.Ignoring wind pressure on vertical formwork: tall formwork exposed to wind can be toppled. Provide lateral bracing, even if the formwork seems stable.

Summary

The lateral pressure of fresh concrete in vertical formwork depends on height, temperature, rate of rise, and consistency. It is calculated with P = γ × h and never exceeds hydrostatic pressure.

Concrete must be placed in layers of 600 mm maximum, with free fall limited to 1 meter.

Internal vibration (12,000 to 14,000 vibrations/min) is essential for compacting vertical concrete. Insert to full depth, hold for 5 to 15 seconds, withdraw slowly.

Stripping of walls is done after 12 to 24 hours (strength ≥ 5 MPa); slabs after 7 to 14 days with reshoring.

Curing of vertical elements must be maintained for a minimum of 7 days, using curing compound, wet burlap, or plastic film.

Elevated slabs require shored formwork calculated for construction loads, placement in strips, and contraction joints spaced at 20 to 30 times the thickness.

Architectural concrete requires rigorous control of formwork, vibration, and curing to avoid surface defects.

Applicable standards include CSA A23.1/A23.2 (concrete), CSA S269.1 (formwork), and CSA A23.3 (design of concrete structures).

Safety is paramount: guardrails, harnesses, protection against alkaline burns and silica dust.

Review Questions (Exam-Style)

199.A 3 m high wall is poured at 15 °C with a rate of rise of 1.5 m/h. What is the approximate lateral pressure at the base of the formwork?

a) 35 kPa

b) 50 kPa

c) 70 kPa

d) 110 kPa

(Answer: b — approximately 50 kPa according to the table for 1.5 m/h at 15 °C)

205.What is the maximum penetration depth of the vibrator into the previous layer?

a) 50 mm

b) 100 mm

c) 150 mm

d) 300 mm

(Answer: c — 150 mm)

211.A tie has a capacity of 20 kN. The lateral pressure is 60 kPa and the vertical spacing is 600 mm. What is the maximum horizontal spacing?

a) 450 mm

b) 550 mm

c) 650 mm

d) 750 mm

(Answer: b — 20,000 / (60,000 × 0.6) = 0.556 m)

217.What is the minimum moist curing duration for conventional concrete according to CSA A23.1?

a) 3 days

b) 5 days

c) 7 days

d) 14 days

(Answer: c — 7 days)

223.What is the maximum spacing of contraction joints for an elevated slab 250 mm thick?

a) 4.5 m

b) 6.0 m

c) 7.5 m

d) 10.0 m

(Answer: c — 20 to 30 times the thickness: 250 × 30 = 7,500 mm = 7.5 m)

This chapter covers all the essential knowledge for the Red Seal exam in vertical and elevated finishing. Master the pressure calculations, vibration procedures, and curing requirements — these are the most frequently tested topics.

Vertical and Elevated Finishing