The Concrete FAQ—Concrete Casting

Concrete is defined as a conglomerate stone of Portland cement and various grades of rock and sand, mixed with water. Portland cement was invented by the Romans, who worked out a mortar material called “pozzolano” containing burned lime and volcanic ash, which set to a concrete consistency. It was very durable—some still survives in their roads and walls. Portland cement was reinvented by the English around 1850. It had a pleasant gray color which looked like the building stone quarried at Portland, England, hence the name.

Basically, Portland cement is a fine powder of clays and limestone where all the molecular water has been driven out by heat, similar to the calcining of gypsum plaster. That cement powder seeks to combine with water, upon which it will recrystalize into something resembling limestone..It will take it from the air’s humidity or your skin, so be careful. Never touch cement. Do not let art teachers tell you to mix it by hand. It is a ferocious alkali and will burn the skin without your being aware until later. Make sure to wear a good dust mask when working with the dry powder—it can just as easily burn you inside as out. Cement comes in 94 lb. bags because that much equals a cubic foot. It is produced in grays, white, and buff (although these latter are harder to find than greenish gray, the cheapest and most common). Compressive strengths of 5000 to 8000 psi are now common for concrete, and can go much higher. It is an exotherm, meaning as it cures it produces heat, the same as plaster and polyester resin. Therefore adding heat accelerates the cure, and vice versa.

The size of the rock is crucial in concrete batch design. Consider a bucket full of fist-sized rocks: the gaps between them are big. The points of contact, where stress is transferred through the mass, are tiny. So let’s add enough gravel to fill the gap volume: now that stress can spread among hundreds of little trails through the bucket. A bigger stone is less likely to shatter from compression now that the gravel surrounds it. Let’s go further and add enough sand to fill the voids between the gravels. Now the bucket feels like a solid mass. And we haven’t added the cement yet! For construction purposes, the best concrete is the strongest for the cheapest. Rock and sand are very cheap compared with cement. Concrete is proportioned by the cubic yard (27 cu. ft.) Concrete mixes are referred to by the number of sacks of cement per cubic yard, i.e. 5 per is normal cheap concrete, and 6 per is good practice, what you’d ask the guys to pour your driveway in. Cast stone, high-grade concrete where appearance is as important as strength, has around 8 per.

You may be thinking, if more is better, then pure cement is best—but you’d be wrong. Cement used “neat” has almost no strength, and it shrinks a lot. Back to batch design: Suppose we start with 6 buckets of rock, 3 buckets of gravel, and 2 buckets of sand—6:3:2 (This is just a for-instance, not an approved formula). How much cement should we use? 11 buckets of aggregate, divided by 27, equals .4, so if you want 6-bag concrete, multiply 6 by .4, which equals 2.4 buckets of cement (call it 2½). See? I don’t care how big your bucket is, just keep using the same size! Weigh that cement before you add it, you’ll need its weight to calculate the amount of water to use. For casting sculpture, you usually want to use the aforementioned 8-bag mix—mixing 4 buckets of aggregate to 1 cement gets pretty close.

Water content is extremely important to your concrete. The less water used, the stronger the concrete will be, but if it’s too dry to flow, your casting will be awful. “Superplasticizer” additives reduce the amount of water needed for the concrete to flow, eliminating hydraulic leakage at mold seams, and water trails. Using too much water leaves tiny voids all through the concrete, which later can absorb rain and shatter when it freezes. There are also air-entrainment additives that will promote the formation of tiny air-bubbles in the mix, which help protect the piece against freeze-thaw deterioration. This will weaken the cement to some extent, however. “Anti-hydro” type additives will make the concrete more waterproof, which can help too. The correct ratio is 100 cement to 40 water by weight. Basically, the water mixes with the cement: the rock products are bystanders here. Since sand is often damp or wet, the water added must be decreased to compensate. But some dry aggregates, like Vermiculite (expanded mica) will absorb water, and must be accounted for by adding more. (Adding Vermiculite will retard the setting of the concrete significantly, so if you use it, allow more time before unmolding.) When the correct amount of water is used, the concrete mix will resemble a thick soup, and the way waves go through the fluid is when it is moved is distinctive. A “slump test” is also used to quantify the viscosity of mixed concrete. Some of the mix is scooped up in a cup, which is then inverted. Observing the way this lump of concrete behaves at this point gives clues to its behavior when poured into your mold or form.

Let’s look at the size of the biggest rock you’re using. Pouring a footing? Or a ballerina? (You’ll do both soon enough!) The rule is, the rock’s diameter should equal 1/3 the width of the void. A 6″ wall gets 2″ rock; a ¾″ ankle gets ¼″ gravel. Chunks of old concrete can be used instead of rock, as long as they follow this rule. You can work with sand mix, using no rock, but it’s weaker. I have put nicely colored sand mix on the surface of my mold, then filled it up with good ol’ Sakrete (premixed dry concrete in sacks), and been very pleased. If you substitute fine marble chips for the sand, and use white cement, you’ll have a mix that, when set, can be smoothed and even polished, for a nice cast stone or “terrazzo” effect (you may have seen this as fancy colored pavement, set off by strips of brass). On the other hand, white pebbles stand out nicely against a darker background. Face-coatings also allow you to capture good detail, since you have direct access to the mold surface and can eliminate air bubbles by painting or splashing in the first layers. Just remember to apply the back-up layers while the face-coat is still fresh.

Mixing sand mix with a small amount of water, just enough so a fistful holds its shape, enables you to “drypack” the mold. This means forcefully compacting the stuff against the mold wall, so rubber molds are not used: they are too bouncy for this. Industrial producers demold immediately, so molds can be recycled very fast. Every cinder block, for instance, is drypacked. For more delicate configurations, let it sit overnight and gain strength before unmolding. You won’t need to acidwash drypack, the texture is always great. Keeping the cast items wet while they cure is still vital. When a drypacked block is fresh, it’s easy to tear into with a chisel—or just a spoon. No money for stone? Cast it this way: make your concrete using limestone dust and small chips (and pale grey cement). This gives you an incredibly nice product, it’s pretty, and using crushed limestone as your aggregate gives a wonderfully carvable mass. Just keep some of the dry mix aside for repairs later.

The way concrete is mixed is very important. Mix the rock products dry with each other, add the dry cement and mix it in, along with any colorant, then add the water last. While it is hard to justify buying a concrete mixer for infrequent use, they can often be rented fairly reasonably. Mixing in a bucket or a big pan with hand tools simply cannot equal what a rotary concrete mixer does. But if you don’t have access to one, you can get decent quality concrete if you mix by hand for a good long time. A hoe with a hole in the blade is used for pan mixing. Big plastic or metal mixing pans are available at tile, masonry and building supply outlets. Small bucket batches can be mixed with a square trowel, or a paddle mixer on a half-inch drill. Clean all concrete off your tools immediately after use—it just gets more difficult as time goes by. Don’t use your sink—this can clog it up very thoroughly. Rinse everything into a bucket, and leave the residue to settle. After a day or so, the water can be poured off and the rest disposed of.

The way to lighten concrete is by substituting a lightweight aggregate for the rock component. Around here we use Arkalite, which is a fired-clay popcorn pebble. It does float on water, so it pops out of the concrete, if mixed too wet. On the West Coast, there is a similar product called expanded shale, made by Port Costa Products of California. You can also use crushed lava, perlite, Vermiculite, and other light-weight rock—experiment with whatever’s available locally. “Hypertufa” is a mix invented for making porous artificial rocks for gardens—they won’t last forever, but they provide a good surface for moss to grow on. The recipe calls for 1 part portland cement,1 part sharp sand,1 or 2 parts peat or peat substitute, and water to mix to a stiff but gloppy consistency.There are also bagged cement specialty products available that are super strong (18,000 psi) and demold in less than twenty minutes. I really like these, and they’re simple to mix—just add water. Quik-crete is one brand available in the West, here we use something called Euco-Speed which is very similar to the Euco-Speed MP on the Euclid Chemical page, (listed under suppliers below). Screen out the larger aggregate in the Euco for casting pieces with thin sections. The yield can also be extended with pea gravel on larger pieces. The product works well as an exterior casting medium.If you’d prefer to mix your own quick-setting concrete from scratch, there are a number of concrete accelerators you could try, the most common being calcium chloride. It’s typically used at 1-3% by weight. It has the unfortunate side-effect of corroding any ferrous reinforcing material, so other accelerants have been developed to deal with this, if it’s a problem.

A huge advantage of concrete over real stone is your ability to add rebar (reinforcing iron rod.) Rebar is called by the number of eighths of an inch of its diameter. Half-inch bar is #4. It is considered fundamental to our modern world that steel and concrete have the same “coefficient of expansion”. Because these materials expand and contract the same amount when the temperature changes, they mate perfectly. The sculptor takes advantage of this by making rebar armatures and setting them in the mold before he pours the concrete. If you do this, remember to reduce the maximum stone size to account for the reduced size of the void you’re filling. For some mold configurations, it is possible to fill the mold first, then wiggle the armature into it afterward. Since you don’t want the steel showing at the surface, you can get little nylon clip-on spacers called “chairs” at builder supply places. Heavy wire mesh (“patio wire”) is used for flatwork.

I won’t deal with bending, cutting or welding rebar here (see the Steel FAQ), but it’s done every day, and rebar is commonly tied together with a soft iron wire (tiewire) which I know you’ll love anyway. In the most critical applications, such as seawalls, where the corrosive effect of the salt water will reach into the concrete to rust the steel inside (blowing apart the concrete, since rust takes up more space than steel), rebar is coated with epoxy—even the cut ends are painted. That is the green colored steelwork you may have seen in bridge construction. Normally, concrete broken open after years have passed shows very little rust on the steel, if it is properly embedded with at least an inch of concrete all around. Good concrete breaks through, not around, its rock. If water does get to the rebar, it rusts and slowly explodes the concrete around it. So inspection and repair of your statuary is good practice.

Before pouring a statue, prewet the rubber mold for low surface tension, which yields fewer bubbles. Some people recommend SmoothOn’s In and Out as a release and mold conditioner for use with concrete. Fiber-Reinforced Polyester and other hard molds are misted or wiped with “form oil”, a variant of diesel oil. Bang and shake the mold as you pour it. Tip it around to dislodge bubbles. Concrete precasters use a vibrating steel table to consolidate the concrete, but these are hard on molds. On-site casters use vibrators run by air or electricity, dipped down into the forms. These typically come with a ¼ horse motor and a 6’+ armored cable. They run about $500 new, but they can often be rented, and will save a lot of touch-up, as they dislodge air bubbles very effectively. Also, without this action your concrete will not attain full strength. You may notice water slowly rising to the top of the mold. Leave it alone—it will be reabsorbed as the concrete sets.

Concrete hardens in a few hours in the summer, and overnight otherwise. Working in freezing weather is bad practice. Adding salts to your water to lower the freezing point is questionable at best. Salt will weaken and ultimately destroy concrete, as well as corrode internal steel reinforcements. (For this reason, you should never use ocean beach sand or pebbles as aggregate.) Take off the mold only when the casting is really hard. Wet the surface then and often, for several days. Wrapping the casting in wet burlap is recommended, as this will help retain the moisture essential to the concrete’s curing. It will keep strengthening over time. Its strength is commonly measured after 28 days (although it will keep getting stronger for years, if kept moist). Pour a test cylinder at the same time you pour a statue if testing will be required.

Concrete is often tinted by adding colorants to the mix. For the most vibrant colors, white cement is best; the more common gray cement will yield muddier tones. Use only permanent colors based on metallic oxides; these are sold in a range of colors especially for coloring concrete. The yellow-orange-red-brown-black pigments are cheap and readily available, while green and blue are costly. The color of your sand and pebbles will also influence the color of the concrete. Cast stone uses the cement’s color as an element, but the cement will cover the cast surface, obscuring the aggregate. Many people dislike this look. You can remove the cement with water and a brush if very fresh, or with muriatic (hydrochloric) acid in water later. (remember, “do as you ougta, add acid to water”, if you’re mixing your own solution, not the other way around) This exposes the sand and some pebbles. The cement color is still there, but you will also be able to see the colors contributed by the aggregate. If there are air holes, called ‘bugholes,’ you may fill them with a thick mix of the cement and fine sand, rubbed in wearing gloves. To add new concrete to old, paint the area with bonding agent, a white-glue relative, right before you place the new concrete. Some of this “admix” added to the concrete will also improve its bonding ability.

Concrete can be decorated in many ways. Hey, we’re artists, this is the fun part! Things can be stuck into the concrete from above (like pennies in the sidewalk) or glued to the form face down to be exposed later. Leaves and cloth can be troweled into the wet concrete surface. Mosaic tiles, marbles, whatever, can be set into it. On the technical side, useful things like threaded anchors can be placed in the mold. Screw-eyes can later be screwed in, the piece erected and set in place, the screw-eyes then being removed. Loops are sometimes tied to the rebar, used to secure the piece temporarily, then cut off and patched-over later. Gluing EPS foam inside the mold creates perfect channels and blockouts. Concrete castings are easily incorporated into brick walls if you created mechanical keys for mortar. Mortar, by the way, is just a thinner, stickier form of concrete—a mixture of cement and sand, with some clay added for plasticity. There are also proprietary “thinset” mortars which are extra-sticky, used for setting tile, etc. Grout, is nearly the same thing as mortar—a sand mix of concrete with a lot of colorant, but no clay. This works well for filling the spaces between tiles. In the industry, “grout” refers to a pumpable sand mix, such as is used for filling the cavities in a cinderblock wall. Pumping is the method used commercially for placing concrete, using a large towable pump and a mix truck, replacing workers with wheelbarrows, shovels, and hoes.

After it’s cast, concrete can be stained or painted. When a concrete casting is fresh, it can be stained using a water solution of neat cement and concrete color. This can be painted on, then rubbed off of the high spots for an “antique” effect. This won’t make it more waterproof, but it is easy to do and not very toxic. Painting concrete presents some problems, since the concrete is porous, and wants to “breathe”. When you seal the surface with paint, the water trapped behind the paint layer tries to push the paint off. We’d all like our paintjobs to become eternal, but in the real world, let me recommend VIP Ombrella paint by the Flood Co. It is intended for concrete walls, but I use it on plaster and almost anything. Acrylic-based, it dries so thin that all my texture shows. I get the “stone white” color and add tinting pigments and bronze powders to vary its appearance. My paint finishes are multi-layer and it dries quickly. I painted a concrete nude a nice bronze color and hung it on a treetrunk in my back yard. The trunk rotted and collapsed, and I rehung it on a fence. The hangerwire rusted and broke (use galvanized tie wire if you want these to last), but the finish was still good-looking! You’ll have to buy 4 gals minimum but other artists will want some from you. Sherwin-Williams can get it for you.

by Dan Spector

Contributions from Tom Battersby and Andrew Werby