The ancient Greeks were masters at building in marble. But the Romans came along next and developed a material that allowed them to build bigger, faster and longer lasting. Concrete. Without it, the Coliseum and Pantheon and aqueducts would have been impossible. Now it is ubiquitous - you see it in sky scrapers, sidewalks, bridge abutments and garage floors.
Not every DIYer wants to, or needs to, pour a concrete slab, but concrete is becoming more popular as a less utilitarian, more decorative element. People are making countertops and furniture, vases, sconces and all kinds of other items, small and large. High-end designers and builders are crafting sleek and elegant pieces - like this chair from Hard Goods - out of a material once considered rough and boring.
It can be intimidating to get started – with pictures of either refined design studios or cement mixers and rebar in your head, but there are approachable ways to start getting your hands dirty.
Basically, concrete is a mixture of cement (usually Portland cement), aggregate (sand, gravel or crushed stone) and water. The water turns the cement into a soupy fluid, suspending the aggregate. As it dries, the aggregate binds to the cement, giving it more strength. It also needs to cure, which is more complex than simply drying. It’s a chemical process where the properties of the cement actually transform, becoming even stronger.
The ratio of ingredients changes, depending on what the concrete will be used for. The professional designers and builders using it to make high-end furnishings and decorative elements often create their own mixtures to get the properties they want, maybe substituting fly ash for Portland cement, using smaller or larger aggregate, decorative stone or man-made options like strands of polypropylene. Also becoming more popular is a lighter and stronger material, GFRC (Glass Fiber Reinforced Concrete). It’s not exactly new, but the growth of concrete as a design medium is making it show up in new places.
That all sounds pretty complicated. And where do you get polypropylene strands or GFRC for your project? Not at the home center.
If you discover you really love working with concrete, you can go down the path of seeking out specialty materials. Until then, pick up your basic bag of concrete mix.
They even come in 10-pound bags if you project is small, but the bigger bags are a better value. I usually go for a 60-pounder. You may end up with some left over, but you can pour it into a work glove.
These bags are easy to find, very little money and all the ratios have been figured out but one. How much water? We’ll get to that. The aggregate is chunky, but the pressure against the walls of a form tends to create a smooth layer, pushing the larger pieces below the surface. The sides of this bowl are smooth as glass, where the mold form held it in. The top is rough, where the aggregate wasn’t held back.
If you want your surface to be this smooth, spend a little more money and buy the bag formulated for countertops. It only comes in larger bags, but chances are if you’re being this particular you’re building something bigger. If you try to sand and polish the basic mix you’ll quickly expose the aggregate, but the countertop mix is designed to be polished, so you can make your project shine.
If your project is smaller, but you still want it polished, you can game the system by not really using concrete, but a vinyl concrete repair patch. It has vinyl resin in the mix that adds stability and makes it extremely workable, although it’s more sticky and not going to flow as well as the others.
How Much Water?
Although there is a science to this, there are so many variables it’s best to start slowly adding water, mixing as you go.
When the whole mixture is wet, and has the consistency of sandy mud or soggy oatmeal, stop.
You need something to mix the concrete in and something to mix it with. For small batches you can use a big bowl and a wooden spoon. I like a plastic tub and a hoe. Choose a container larger than you think you’ll need. The small plastic tub you get at the home center fits a 60-pound bag perfectly, leaving no room for error. Get the bigger one and spill less.
You also need a shovel or something else to scoop the wet concrete into your forms and a sturdy, flat trowel to smooth the surface.
What you are going to make out of concrete is limited by your imagination and what you can pour it into. Plastic water bottles are a good place to start - so are milk cartons.
The form needs to be strong and rigid enough not to be pushed apart by the concrete, and water resistant enough not to soak through. Soaking through is a factor not just for the sake of keeping your form together, but also you don’t want the water to be sucked out of the mix too quickly or it won’t cure properly.
Make forms from scratch out of wood, basically building a shallow box. If you want it smooth, use melamine. Seal all the joints with silicone caulk.
Part of the beauty of concrete is it’s easy to experiment with. Make an ice bucket/wine chiller. Replicate a Super Mario powerup mushroom to sit on in the garden.
Mix up a batch to make this table base (WIP) and look around to figure out what to do with the left-overs.
Pack it into a whiskey bottle just to see what happens.
Partially fill a bowl and set a smaller bowl in the center to create a void. When it sets you have a concrete bowl that could also be a dome. Don’t jump in and try to make your dream countertop before you’re familiar with the material, but also don’t be scared away by the technical side. Remember, you’re just making beautiful mud pies.