The Art of Slipforming

Article Tools

Image Gallery

Print

E-mail

Comments

RSS

concrete

Cement work is always fun because you get to create rock. Cement is made from limestone or chalk, plus shale or clay, powdered and baked in a kiln. Water is chemically bound into the parent materials, and the high heat is required to break the molecular bonds. This type of cement is called Portland Type I & II, and that is what you should ask for at the brickyard.

Adding water to cement powder activates a chemical reaction to bind the cement dust back into rock. Most of the chemical reaction occurs within the first few days, but cement continues to cure noticeably for a month, and becomes still harder for years to come. To allow the chemistry to work properly it is important to keep cement work from drying out in the first day or two. Fresh masonry can be sprayed with water or covered with tarps or blankets in extremely hot, dry weather to keep it from drying out too fast.

Sand and gravel is added to cement to make concrete. The different-sized particles interlock with one another, creating a stronger bond—much as if you interlocked the fingers of both hands to keep someone from pulling them apart. Look in the yellow pages for a source of sand and gravel. The gravel used in concrete is usually a 3/4-inch mesh, meaning that it is run through a screen with holes of that size. I prefer pea gravel (3/8-inch mesh) because it flows better between the stones, but it is sometimes more expensive than regular concrete gravel. You can use either washed or masonry sand to go with the concrete. Masonry sand is finer, and usually more expensive, but is nicer for finish work. In our area sand and gravel costs about $5.50 per yard and up, plus about $6 per yard for shipping (50 miles). We order a full truckload of each at a time (10 to 15 yards). Slipformed walls are composed of approximately equal portions of concrete and stone work.

A power cement mixer is a must for sfipform work, and you can get a good used one for less than $200. We make a concrete mix of one part cement to three parts sand and three parts gravel. That works out to approximately two coffee cans of cement plus one five-gallon bucket of sand and one of gravel.

Laying Stone

Start the first row by laying out a bed of concrete one to two inches thick, and place your biggest rocks into it. Otherwise, for maximum efficiency I recommend laying in as much stone as you possibly can in all the forms. You will often be able to stack the stones several deep before filling in with concrete. Just make sure there is room to work the mortar in around each and every rock. Also work toward a "brick layered" effect, by bridging the joints between the rocks on every level. Place as many rocks as you can and then start mixing cement.

Make the first batch of concrete by mixing the ingredients dry, then add the water. Ideally the mix should be gooey enough to slide in around each stone, but not so soupy that it runs out through the joints and down the rock faces.

RSS | Comments | E-mail | Print