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Pretty strong

by Tania Wannenburg
Pretty strong

Coloured concrete has a range of applications – whether used subtly to blend with nature or boldly to make a dramatic design statement.

 

Karien Slabbert looks at how to harness the aesthetic potential of coloured concrete.

Concrete can assume nearly any shape, design, pattern or texture. But the one characteristic that most distinguishes decorative concrete is colour, whether used subtly to blend with nature or boldly to make a dramatic design statement.

According to Architectural Concrete: a guide to achieving high-quality finishes by Daniel van der Merwe, there are many ways to create coloured off-shutter concrete finishes. These include:
• Using white cement.
• Adding a colour pigment to the concrete.
• Applying a coating to the form face that becomes an integral part of the surface finish.
• Applying a surface stain or coating after the concrete has cured.
• Coloured aggregates can also be used if the surface is tooled to expose the aggregates.

According to Van der Merwe, hardened concrete’s colour depends on the colours of the fine particles (cement, sand and pigments) that are used in the mix. Sand colour can influence concrete’s pigment significantly. Therefore it is important to carefully consider the colour of the sand used in coloured concrete work. It is particularly important in the case of light-coloured concrete – this includes colours such as yellow and blue.

Colour uniformity
Colour uniformity plays an important role in the aesthetics of off-shutter concrete. Slight colour variations in cement from different factories may occur due to the differences in raw materials. Therefore one should rather source cement from the same factory when colour plays a key role. However, colour variations on off-shutter concrete’s surfaces are not only due to changes in the colour of the sand or cement.

White cement
Concrete can be produced by using white cement instead of the normal grey cement. Cement’s grey colour is primarily due to its iron content. Off-white and white cement can be produced by reducing the iron content. White cement is imported and there architects should check its availability before it is specified for a project.

White cement is expensive, according to Van der Merwe. Moreover, when the colour is lightened, its effectiveness could easily be lost unless it is used in combination with light-coloured sands. Tests have shown that blends of white and ordinary grey cement are seldom justified economically or aesthetically, since at least 80% of white cement by mass of total cement is required to lighten the shade of concrete significantly.

Trade secret
There are many different shades of white and off-white. If these colours are critical to the off-shutter finish, a colour panel should be constructed to ensure that the colour is acceptable.

 

Pigmented concrete
Pigments are the most common method of colouring the full thickness of concrete. It provides a coloured surface on all faces of a concrete element and eliminates the need to use extra surface coatings and paints. Pigments are available as powders, dissolvable granules or liquids. The ultra-fine particles of pigments disperse as fine solids throughout the concrete matrix and are bound into the concrete in the same way as other aggregates.

Most colours (red, yellow, black and brown) are iron oxides, of which most are now synthetically produced. Hence the name synthetic iron oxides. Mineral and inorganic oxide are permanent and are unaffected by the sun’s ultraviolet (UV) rays. This is because oxides are minerals in their most basic form and they can neither degrade nor change.
Other key characteristics
Oxides are:
• Insoluble, which prevents leaching out.
• Chemically inert, which means that they do not interfere with the cement reaction.
• Alkali-resistant, which makes it ideal for highly alkaline concrete.
• Light-fast, which eliminates fading.
• Harmless to the environment.
• A permanent colouring solution once they are bound into the concrete mix.

Trade secret
Some colours, such as blues and greens, are more expensive than natural yellows, browns and blacks. This is because these special metal-oxide pigments require a special manufacturing process. White or light-coloured aggregates can enhance the brightness of these colours.

Pigment concentration
According to www.concretenetwork.com , the water: cement ratio is one of the most critical factors that affect colour. “Controlling the amount of water added to the concrete mix is critical to producing consistent colour,” states the website. The addition of water permanently changes the concrete, typically lightening the final colour.
According to Architectural Concrete: a guide to achieving high-quality, the normal way to specify the pigment concentration is to relate the amount of pigment to that of cement. This is done by specifying that it should be a percentage by mass of the concrete’s cement content.

The amount of concrete required may vary depending on the colour and its tinting ability. The pigment concentration (or loading) of 5% typically provides good colour intensity. Lower concentrations may not render the intended colour. In turn, adding more than the saturation point is an expensive way of simply adding more fine material into a mix.

Because the pigments concentration and tinting strength affect the colour, it is often better to specify a particular colour from a manufacturer’s range. The pigment manufacturer is then responsible for determining the required pigment concentration for the specific concrete mix. Weathering will have little effect on the colouring if the pigment concentration is correct.

Van der Merwe states that the concentration strength must be kept the same for large areas. Higher-strength concentrations are usually accompanied by a higher level of cementitious concrete. This results in more pigment per cubic metre and possible colour variations.

Cement colour
Grey cement will always take the brilliance out of any colour, which is why colours in concrete that are made with white cement will appear brighter. However, a lot depends on the desired effect and pigment colour. Specifiers should consider white cement when lighter pigments are specified. Variations in the colour of each cement type can also influence the colour of the pigmented concrete.

“Consider breaking up the large areas with bands of different colour and texture. For large pours of one colour, it might be worthwhile considering the use of dry shake colour hardener that takes most colour-related surface variables out of the picture.”

 

Concrete strength
When colour pigments are added at a typical 5% concentration, it will most probably not affect the concrete’s strength. However, adding excessive amounts of pigments that exceed the saturation concentration will increase the water demand and could affect the water: cement ratio, strength and durability.

 

Efflorescence
The formation of efflorescence (a white coating on the surface) can appear to lighten the colour and give the impression that any colour pigments that have been used have faded. However, it will be less visible on lighter-coloured surfaces.

 

Aggregate colour
Concrete aggregates are usually covered by a thin layer of coloured cement paste (cement, sand and pigment). However, the final surface colour will be affected by the aggregates’ colour if the aggregates are not covered completely or become exposed through wearing, weather or subsequent tooling of the surface. The sand will initially change colour, followed by the courser aggregates. The lighter the colour, the more influence the aggregate colour will have. Therefore, it is important to select the appropriate aggregates and aggregate colours.

Where surfaces are treated to highlight the aggregate colours, the pigment can be as low as 0,5% to 1% by cement mass. This will help to complement the aggregate colour rather than the source colour.

Application type Effect
Off-shutter finishes 

• Off-shutter finishes, particularly these with colour pigments, provide a decorative low-maintenance solution.

Coatings 

• Coatings range from various paint finishes to coloured cementituous materials that provide similar results than coloured off-shutter finishes, but with better colour consistency.
• Coatings provide a uniform colour over the entire surface and are generally used when no colour variations are acceptable.
• They conceal the substrate and, depending on the coating type, may provide a moisture or chemical barrier between the concrete and the environment. It may extend the life of the concrete element.
• When using coatings, the maintenance cost should also be considered.

Chemical stains 

• Limited areas of an off-shutter concrete surface can be coloured with chemical stains.
• Stains penetrate the concrete surface. Rather than achieving the same uniform appearance of coatings, stains create distinctly individual effects, depending on the level in which the stain is absorbed into the concrete.  

Coloured aggregates 

• An aggregate’s colour has little or no bearing on its behaviour in concrete. However, the colour property is of special interest to architects who want to harness the aesthetic potential of exposed aggregate concrete.
• There are considerable colour variations between one geological rock type. Because of these variations, it is important to establish whether there are adequate amounts of the desired rock colour present before specifying a specific product. This is particularly important with large buildings that require colour uniformity.

Curing
The proper curing of concrete is important to reduce surface shrinkage cracking and to obtain the proper strength. “It is even more important in coloured concrete because a lack of curing produces inconsistent colour. Slight colour or shade differences in gray concrete are seldom noticed,” states the website www.concretenetwork.com.

Visual verve
Undeniably, coloured concrete adds aesthetic appeal to a building. It gives a new dimension to a building and complements the design. In many ways, it brings a building to life and provides a decorative edge to an otherwise dull material.

Coloured concrete as an additional benefit can assist with reducing maintenance overheads and initial building costs as no paint and plaster, or cladding finishes are required. Coloured concrete will not fade or peel and will provide a permanent and attractive look to buildings which will last many years.

Full acknowledgement and thanks are given to www.concretenetwork.com and “Architectural Concrete: a guide to achieving high-quality finishes” by Daniel van der Merwe from PPC for providing the information to write this article.

For more information you may contact Daniel at: Daniel.VanderMerwe@ppc.co.za, or Tel: 011 386 9000

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