A Stone House in Two Weeks
Stone and concrete walls built flat on the ground and craned into place are fast and maintenance-free, if unconventional.
Synopsis: An innovative building contractor describes his techniques for building homes out of steel-reinforced hybrid wall panels of stone and concrete that are assembled flat on the ground like a slab, allowed to cure, then lifted into place with a crane.
Stonemasonry is the stuff of castles and cathedrals and of houses that have been around for centuries, or will be. Because he’d spent years fixing up old wooden houses, my brother Nick was building a new house in Montana with stone walls. However, he thought there had to be a faster, easier way than laying them up one stone at a time. So he decided to experiment with hybrid walls of stone and concrete. In a nutshell, Nick laid the facing stones of his walls flat on the ground and poured a concrete slab over them; the slab also filled the spaces between the stones to lock them in place. Once the concrete set, the assembly was raised into position with a crane. This method is called tilt-up construction, and it’s common in commercial projects.
Walls are formed much like a slab
Because the ground is essentially the bottom of the formwork and any irregularity there would show in the finished walls, Nick leveled the area so that it was almost perfectly flat. He then formed the walls with 2x8s, much as if he were forming for a concrete slab. Once raised, these walls would have to match up precisely at the corners, so he took extra care levelling the forms and bracing them straight.
One of the challenges to tilt-up construction is bonding the wall panels to each other at the corners so that they look and function as a single unit. Nick achieved this end in three ways. Steel plates pressed into wet concrete were welded together as soon as the walls were raised. The true bond in Nick’s walls, however, comes from interlocking quoins, a concrete version of a woodworker’s box joint.
The quoins were formed with 1 in. of clearance between them to provide a little maneuvering room during the lift. This space was filled with mortar once the walls were permanently in place. Oriented strand board was used for the forms to give the quoins a random texture.
Tying together the corners are 1 1⁄4-in. dia. #10 rebar pins grouted into 3-in. dia. holes cast into the concrete. These holes were formed with carefully placed foam cores (Big Sky Insulations; www.bsiinc.com), which were drilled out with a spade bit once the forms were stripped.
Clay and sand keep stones clean
Nick used flat fieldstone collected from the local hills. Each stone was placed carefully in the forms, leaving space for mortar joints all around. Although faster than traditional stonemasonry, the process required time to sort through the puzzle pieces to find just the right stones.
To recess the mortar joints and to keep the stone faces clean, Nick sprinkled a mix of 4 parts masonry sand to 1 part bentonite clay powder about 1⁄2 in. deep between the rocks. After spreading the sand/bentonite mix, he swept the back of the rocks clean with an air hose and misted everything with a garden hose to dampen the bentonite. Bentonite swells when damp, making a waterproof barrier that mostly prevents the cement slurry from leaking through the joints and staining the rock faces.
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