art concrete how-to 1_ materials

8/11/2019 Art Concrete How-To 1_ Materials

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9/27/14 7:11 PMArt Concrete How-to 1: Materials

Page 1 of 5http://makersgallery.com/concrete/howto1.html

Art Concrete How-to1: Materials

THIS PAGEis one of severalexplaining techniques about how to use concrete in small-scale art projects.

Book: Concrete Handbook for Artists: Technical Notes for Small-scale Objects. More information?

Concrete is a mix of portland cement, aggregates, water and otheroptional additives that give the mix certain characteristics.

PORTLAND CEMENT:Portland cement is what holds the aggregates together and is available indifferent grades and colours. The type you can buy at the local hardwareor lumber store is grey in colour. White portland cement has thetetracalcium aluminoferrite removed, resulting in a pure white powder,with no loss of strength. It is more expensive.

AGGREGATES:

STONE:Crushed stone or gravel can rangein size from 1/4" to 1". For small-scale sculpture you probably won'tneed any larger than 1/4" or 5mm.

You may not need it at all unlesscost is a factor: stone is cheaperthan cement. Stone adds strength to larger work.

SAND:Sand should be "sharp", that is have sharpedges, so that the crystals of cement grabit. It is usually called sharp sand, brick

sand or mortar sand. The grains of pit runsand or beach sand are usually too round.

FINE:Stone dust, a waste product fromquarries or stone works, can be added tosmooth mixes for small-scale work. Itadds strength, reduces shrinkage on

setting, and may add to the visualappearance of the concrete. Limestone


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or marble dust are two types of stone dust.

Supplementary cementitious materials. L-R: fly ash, metakaolin, silica fume,

fly ash, slag, and calcined shale. Image courtesy Portland Cement Association.

EXTREMELY FINE:SILICA FUME:Another waste product, this admixture is from metal smelting. Itis a very fine dark grey powder. Particle sizes are 1/100 the sizeof portland cement particles. If used in the right proportions(about 8% by weight of the portland cement) it can double thecompressive strength of the concrete, reduce permeability,increase density. It can be difficult to handle as it is very fine,and makes the wet mix "sticky".METAKAOLIN:Metakaolin (used at about 8% by weight of the portlandcement) can double the compressive strength of the concrete,lowers permeability and increases density but also hasadvantages over silica fume. Metakaolin makes the mix

creamier, less sticky, and is a plain white in colour. Particle sizeis smaller than cement but bigger than silica fume. Metakaolin isa fairly new additive and may be difficult to find. (Trydistributors of plastering materials for the pool industry.)

LIGHTWEIGHT:PERLITE, VERMICULITE, PUMICE:Naturally occurring minerals that expand when heated. Perlite iswhite and crispy, vermiculite is darker. Pumice is a volcanic(glass) rock.


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POLYSTYRENE BEADS:Difficult to mix in and messy to use (staticcling and very lightweight) - it is best to usecoated beads. There is a dry pre-mixavailable in large quantities viafoamliteconcrete.com

PLASTIC FIBRES:Fibre additives, such as FiberMesh and Fiber Ad(polypropylene) are chopped (about 1/2", 1 cm)strands of plastic. They disperse into the wet concretemix and help prevent shrinkage cracks as the concretesets. Shrinkage cracks can later cause problems. On avery small scale the fibers also add a small amount oftensile strength. They are almost invisible in the final product. A new fibrecalled PVA or polyvinyl alcohol, has a much greater bond with the cement

and therefore a much higher tensile strength. Note: You cannot use apolymer additive such as latex or acrylic with PVA fibres or that bond isweakened.

GLASS FIBRES:Glass fibres added to the concrete mix mustbealkali resistant, or the alkalinity of the cement willeventually break them down. Their purpose is to add

tensile strength to the concrete, replacing steelreinforcing in special circumstances. This makesthem a useful addition for art work where theconcrete might be cast in a space where steel reinforcing could not beused. The coated fibres, such as Cem-FIL, are usually 12 mm long, andare added after the wet mix is ready. Also available as a mesh or cloth(see image).

STEEL REINFORCING:Steel is the most commonly used reinforcing to give concrete tensile

strength. In construction projects it use used as rebar, 1/2 to 1" rod.Smaller art projects can use much thinner steel rod. There are alsovarieties of galvanized steel meshes or screens available. One type ofexpanded steel mesh is used as a stucco base and can be cut with metalsnips (carefully). There are other square grid galvanized meshes, 1/4",1/2" and so on. Chicken wire, used for fencing, is also a flexible and easyto use material.

AIR ENTRAINERS:Air entrainers are used in concrete that may be exposed to frost, or

repeated freeze-thaw cycles. They work by creating tiny air bubbles in thewet mix. MicroAir is one brand. Very small quantities are used.


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SUPER PLASTICIZERS:In mixing concrete you try and get all the dry particles in the mix to bewetted. The aim is to use as little water as possible, because water notchemically combined stays in the dry mix and causes weakness.Plasticizers help to wet the surfaces, enabling the mix to use less water.One brand is Pozzolith. Tiny amounts are used. It is also called a "water

reducer."LATEX OR ACRYLIC:Latex or acrylic additive (or admixture) is sometimes used in concrete,adding it to the water first. It acts as a plasticizer (to use less water),increases strength and makes it more waterproof, and increases adhesion(for example, when adding a surface to an older piece of concrete).

PIGMENTS AND DYES:Pigments or dyes can be added to the mix, or applied to the surface of setconcrete. Special dyes are made for this purpose as common pigmentscan be broken down by the alkalinity of the cement. The range of coloursare in the earth tone range: black, grey, brown, ochre, dark red. Bluesand greens are available but costly for large projects. Pigments areavailable as liquids, or as powders. Concrete can also be surface colouredwith acid stains or opaque stains made for outdoor use. The latter has awide range of colours, but some may not be long-lasting.

SEALERS:In some circumstances it is advisable to seal the concrete surface toreduce water penetration, which aids frost resistance and resists acid rain.

ACRYLIC:Most of the readily available sealers are acrylic-based. Some may affectthe surface finish leaving it glossy and they may yellow slightly over time,and need renewing if used outside.SILOXANE:Silane or siloxane sealers combine chemically inside the concrete leavinga natural surface. Siloxane does not change the surface appearance ofthe concrete. The solvents in these sealers demand a lot of ventilation.POTASSIUM OR SODIUM:

Potassium silicate or sodium silicate (water glass) water-based sealerswork by chemically combining with the free limne or excess alkalinity ofthe cement mix. They form a gel which hardens and consequentlydensifies the concrete, decreasing the moisture permeability and porosity.The concrete can be damp (not wet) when it is applied. If the concrete isbad quality or very porous this sealer may not work well and you might bebetter off with the older acrylic or vinyl sealers.

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CONTACT + LEGALLast update: August, 2007.

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