Decorative and Architectural Concrete

Decorative and Architectural Concrete

Introduction

Decorative and architectural concrete represents a specialized field of practice within the concrete finisher trade. Unlike conventional concrete whose primary objective is structural strength, decorative concrete combines aesthetics, durability, and functionality. For the Red Seal exam, you must master the techniques, materials, mix proportions, application methods, and applicable Canadian standards. This chapter covers all the skills assessed, from basic finishes to advanced techniques such as stamped concrete, polished concrete, exposed aggregate, and architectural coatings.

Definitions and Scope

Decorative concrete refers to any concrete whose appearance is modified through finishing, colouring, texturing, or moulding techniques, without compromising its structural integrity. Architectural concrete is concrete exposed to view, whose surface is treated to meet specific aesthetic requirements, often specified by an architect or engineer.

The main categories include:

Planning and Preparation

Contract Requirements and Documents

Before any work, you must consult the plans and specifications. The documents specify:

Typical tolerances for an interior decorative slab are 3 mm under a 3 m straightedge (i.e., Δ = 3 mm / 3 m). For exterior surfaces, the tolerance is generally 6 mm under 3 m.

Environmental Conditions

The concrete temperature at placement must be between 10 °C and 32 °C according to CSA A23.1/A23.2 (Concrete: Materials and methods of concrete construction/Methods of test). In hot weather (above 30 °C), special measures are required:

In cold weather (below 5 °C), concrete must be protected and maintained above 10 °C during initial curing. The use of insulated forms, heated blankets, or thermal curing is common.

Mixes and Materials

Basic Constituents

Decorative concrete uses the same constituents as conventional concrete, but with more stringent requirements on aggregate quality and consistency.

The water-cement ratio (W/C) is critical: it must be maintained between 0.40 and 0.50 for durable decorative concrete. Excess water causes laitance, which impairs the adhesion of stains and sealers.

Pigments and Colourants

Pigments are classified into two categories:

Approved pigments must comply with ASTM C979 (Pigments for concrete). Iron oxides (red, yellow, black, brown) are the most stable. Titanium dioxide (white) and chromium oxides (green) are also used.

Rule of thumb: never exceed 5% pigment by weight of cement, otherwise mechanical strength decreases.

Specific Admixtures

Decorative Finishing Techniques

Stamped Concrete

Stamped concrete involves imprinting patterns (stone, brick, slate, cobblestone) into fresh concrete using rigid or flexible stamps.

Typical procedure:

Frequent errors:

Polished Concrete

Polished concrete is achieved by progressive mechanical grinding of the hardened surface using diamond discs of increasing grit size.

Polishing sequence:

Polished concrete can be stained before the final polishing. Densifying hardeners (lithium or sodium silicates) are applied to increase hardness and abrasion resistance.

Strength requirements: concrete to be polished must have a minimum strength of 25 MPa at 28 days. Concrete that is too weak will crumble under grinding.

Exposed Aggregate Concrete

Exposed aggregate concrete reveals the aggregates at the surface by removing the superficial cement paste.

Methods:

The depth of aggregate exposure must be controlled: 1/3 of the diameter of the largest aggregate is the usual rule. For 20 mm aggregates, the exposure is therefore approximately 7 mm.

Retarded (Washed-Out) Concrete

Retarded concrete is a variant of exposed aggregate concrete where a surface retarder is sprayed on the fresh surface. This product delays the set at the surface, allowing subsequent high-pressure washing (1500 to 2500 psi) to reveal the aggregates.

Control points:

Formlined Concrete

Formliners are membranes or panels made of polyurethane, rubber, or PVC fixed to the inside of formwork to imprint a pattern (wood, stone, geometric patterns) onto the concrete surface.

Requirements:

Coloured Concrete

Two main methods:

Precautions for acid stains:

Joints and Cutting

Types of Joints

For a 100 mm thick slab, the maximum spacing of contraction joints is 2.4 to 3.6 m. Joints must be made by saw-cutting (after 6 to 24 hours depending on temperature) or by inserting strips (before initial set).

Saw-Cutting Joints

Saw-cutting must be done as soon as the concrete can be cut without raveling, generally 6 to 12 hours after placement in hot weather, and up to 24 hours in cold weather. The cut depth is 1/4 of the thickness (25 mm for a 100 mm slab).

Curing and Protection

Curing is essential for decorative concrete. Inadequate curing causes:

Curing methods:

For decorative concrete, film-forming curing compounds must be compatible with subsequent sealers and stains. Some curing compounds prevent sealer adhesion; they must then be removed by grinding or a non-film-forming curing compound must be chosen.

Sealers and Protectants

Types of Sealers

Applying Sealers

Practical Calculations

Pigment Dosage

Formula: P = C × p / 100

Where:

Example: For 350 kg of cement and a 3% pigment dosage:

P = 350 × 3 / 100 = 10.5 kg

Sealer Quantity

Surface to cover: 45 m². Sealer coverage: 8 m²/L per coat. Two coats required.

Quantity = (45 / 8) × 2 = 11.25 L (allow 10% for waste → 12.4 L)

Decorative Slab Thickness

For a pedestrian slab: 100 mm minimum (per CSA A23.1).

For a slab with light vehicles: 150 mm minimum.

For a slab with heavy vehicles: 200 mm minimum.

Applicable Canadian Standards

CSA A23.1/A23.2 — Concrete: Materials and methods of concrete construction/Methods of test

This standard is the primary reference for all concrete work in Canada. Relevant sections for decorative concrete include:

CSA A23.3 — Design of Concrete Structures

This standard covers structural design. The finisher must know the minimum concrete cover requirements for reinforcement:

CSA A3000 — Concrete Blocks and Masonry Units

Relevant for decorative precast elements (pavers, curbs).

National Building Code of Canada (NBC)

The NBC, in Part 9 (Housing and Small Buildings), specifies requirements for concrete slabs:

Pitfalls to Avoid

Summary

Exam Tips