| 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, ie 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 2 1/2). 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. |
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