Tilt-Up Stone Masonry
A Technological Lift to the Ancient Art of Stone Work
by Thomas J. Elpel
Stone masonry is the stuff of castles and cathedrals and of houses that have been around for centuries, or will be. It is the Gold Standard of the construction world.
There would be far more stone houses in the world today, but stonework is considered too labor intensive-and therefore too expensive-to be practical. Builders often settle for a veneer of stone across prominent walls to give the illusion of the real thing.
As a child I dreamed of building my own castle of stone. I drew fantasy pictures of the place where I would someday live. At the age of 21 I moved into a tent with my bride on our five-acre plot and started building our Dream home of stone and log, using free rocks from the roads and hills. We used the slipform method, placing forms on both sides of the walls to guide the stonework, then moved the forms up the walls as we went. It didn't cost much money, just lots of time, but we had plenty of that. We paid for the house out-of-pocket a little at a time, and in a few short years we had a very energy-efficient and beautiful home for the cost of $10 a square foot.
It was also my Dream to make resource-efficient stone houses available and competitive in the marketplace. It simply makes more sense to me to build quality houses that will last for centuries than to toss up structures that need constant repairs to keep them standing. But when we built and sold a small stone house on speculation, we discovered the other side of the labor issue. Sure, it was greatly economical to build our own stone home, but not so economical to build and sell one to someone else. There had to be a way to short-cut the labor and mass-produce stone houses.
It was then that I began to dream of tilt-up stone masonry--that is pouring stone walls flat on the ground and setting them in place with a crane. But I didn't have any time to pursue the idea, while already engaged in writing and publshing books.
Fortunately, my brother Nick grew interested in the idea and decided to figure it out himself. He liked the idea of building with stone, but didn't care for the slipform masonry technique we used. He chose tilt-up stone masonry as a faster way to build, that would also eliminate the cold joints that run throughout slipformed walls. Pouring the walls would simultaneously grout the stonework, insuring an integral bond that would prevent problems with the mortar cracking and falling out later. With tilt-up construction he would be able to bring the stonework up higher without having to lift each individual rock and bucket of concrete. Nick bought a building lot a block away from our place and started building.
Experimentation
As with any new building technique, there were many hurdles to cross to get a working system. Tilt-up construction is hardly new, but the panels are usually faced with coarse, decorative gravel that is simply raked into the forms before the pour.
Nick wanted to securely embed real stones with mortar joints that were neither too shallow nor too deep, while being careful to avoid cement stains on the rock faces. He also needed to find a way to structurally connect the walls to the footings and to the other walls-while artfully hiding the joints between them. We practiced on his garage first, before starting the house.
In 3' x 3' test panels placed face-down, Nick swept sand in between the stones to make a slightly recessed joint, then poured the concrete, but unfortunately, the cement water soaked through the sand and stained the front of the stones. Therefore, to insure high-quality stonework, Nick called in a concrete truck and we poured the walls of the garage face-up , setting the rocks carefully into the fresh mortar. We worked as fast as we possibly could, placing stones and finishing the joints as we went. Working in November in Montana helped to retard the cement, so we could do more work before the concrete set up.
Before the pour, Nick cast concrete trim for the windows and garage door in moulds cut from polystyrene "beadboard" insulation. Wire mesh in the back of the trim enabled us to anchor the trim pieces securely into the wall panels along with the stonework. We mixed concrete as necessary to finish the panels, completing them without any cold joints. The panels consist of approximately 5 inches of reinforced concrete wall, faced with stones 2-3 inches thick, for a total thickness of 7-3/4". The rocks should not be any thicker, as that would compromise the structural wall of concrete and rebar. Although this method worked, setting an entire wall of stones at once was fast-paced and stressful work.
The garage consisted of many small panels-seventeen in all--two for each side of the building, three inside the garage, plus six roof panels. With the aid of a crane, the panels were set onto concrete blocks, then welded together where Nick embedded metal plates into the walls.
Because the project was built into a hill, only two sides of the building were faced with stone. The other walls were plain concrete panels completely hidden after backfilling the site. The concrete roof served as a load-bearing surface which was later covered with earth to make the yard. Rebar protuding from the panels was bonded together with a footing at the bottom and a bond beam at the top. To hide the joints between the panels, Nick left a gap in the stonework, then laid those sections up by hand after the walls were up.
Building the House
While we saved time on the stone panels of the garage, we realized it would be cheaper and easier to pour the concrete walls in the back with conventional concrete forms. We were also unsatisified with the weld plates, bond beam, and the custom stonework at the corners that were required to tie all the parts together. There had to be a better way.
Nick concluded that it would be better to pour the walls with the stones faced-down, so he added bentonite clay powder to the sand between the stones to make the recessed joint. The bentonite swelled when wet and sealed the joints, greatly reducing the problem with cement stains on the rock faces. Pouring the walls face-down also enabled Nick to cast the window trim in place.
I suggested interlocking corner blocks to aesthetically and structurally tie the walls together, and Nick switched to larger panels so there would be fewer joints. This is how the system works from the ground up:
Footings
Bonding the walls to the footing is a unique challenge in tilt-up construction that virtually requires putting up the panels first, then pouring the footing underneath. The solution Nick found was to pour concrete pads at the corners or other joints where two panels come together. The pads should be big enough to support the ends of the two joining panels, usually about two feet square. Rebar should extend out of the concrete pads into the future footings. Rebar should also extend out of the bottom of the wall panels, to bond into the footing.
Pads and footings should be placed below the frost line. That wasn't an issue with Nick's place, since both the house and garage are nestled into the hill. However, on a flat building site, it may be advisable to design a floor plan where the main level is placed below frost line to avoid using extra concrete.
When you pour the pads, you should also pour a "deadman" anchor in the center of the house. It should be a very solid and unmovable pad of concrete, so that you can bolt a temporary brace to it to support the wall.
Forming the Walls
Bonding tilt-up panels to the footing is only half the challenge. The walls must also be bonded to each other, to look and function as a single unit. Weld plates should be pressed into the wet concrete to connect the wall panels together at the joints, but this is only a temporary fix. For true bonding the walls must be permanently connected.
Nick built wooden forms to create the interlocking corners. The corner blocks were made slighlty thicker than the walls (8-1/2" x 15") to accentuate the visual strength of the corners. Oriented-strand board (OSB) was used for the form work, to give a random texture to the concrete blocks. Plywood should not be used, as that would leave a definite grain in the blocks. The corners can be caulked inside the forms for a slightly rounded edge.
Note that the wood moulding used to accentuate the edges of the blocks should be slightly bevel-cut to pop out of the wall easier. Also, since the wood swells up from the water in the concrete, the wood should be allowed to dry and shrink before removing it, to prevent chipping the concrete.
Nick modified a standard rebar bender to wrap the rebar around the beadboard cores in the corners. The cores left a round void (after drilling out the beadboard with a spade bit) to drop a 1-1/4 inch-diameter rebar pin down through the corners. With the rebar also wrapped around the columns, it was much like an interlocking hinge, except that mortar was poured down the hole and troweled between the joints to permanently bond the panels together. Leave a full inch for each mortar joint between the corner blocks, so you have plenty of room to maneuver the panels. Measure everything as many times as necessary to insure accuracy. You cannot afford to get the panels up in the air, only to find they do not fit together!
Keep in mind that the ground is part of the formwork, so it must be perfectly flat, but you don't need a concrete slab to work on. Level the site as much as possible, then stake down the perimeter forms and level them at the corners. Next, rake the ground smooth inside, adding or removing dirt as necessary. Make a screed that fits inside the forms to check for level, and spread a thin layer of sand over the site to make a good surface to work from.
Window and door frames must be included in the formwork before the pour. Be sure to use standard sizes to reduce cost and labor later on. Nick used wooden frames combined with moulded trim made with the aid of custom-cut beadboard molds. The beadboard factory is nearby, so Nick provided a profile of the trim, and the factory used a hot wire to cut that pattern from the insulation. The mold was placed around the window frame and simply filled with concrete to make permanent trim. The beadboard was removed in bits and pieces after the walls were lifted, which left a slightly beady texture in the concrete. Some type of form release on the beadboard might also be helpful. Note that the beadboard is easily injured, so it would be better in the long run to make reusable forms from sheet metal.
Structurally, keep in mind that the concrete and rebar holds this type of wall together as it is lifted and set in place. The stone portion of the wall does not count towards the total strength of the wall during tilt-up, and any stones that penentrate too deep into the concrete could compromise panel strength. Nick's walls were 7-3/4" thick with only 2-3 inches of the total for stonework. Because the stones are thin, they should also have small faces, ideally less than 12 inches across, to bind them securely into the wall. Bigger stones may be prone to popping off of the wall, leaving ugly patches of concrete.
Nick used flat field stone for his projects, collected from the local hills. He felt that gathering the stones was too much trouble, and that it would be far better to have a source of good stones that could easily be scooped into a dump truck and delivered to the site. Fore-thought should be given to the best spot to dump the stones, to avoid moving them again and agian.
Each stone was placed carefully in the forms, leaving space for mortar joints all the way around. Although faster than slipform stone masonry, it still took time to sort through the puzzle pieces to find just the right stones each step of the way.
To make recessed mortar joints, Nick used a mix of 4 parts masonry sand to 1 part bentonite clay powder and sprinkled it about half an inch deep between the rocks. The bentonite swells when damp, making a waterproof barrier to prevent the cement slurry from leaking through the joints and staining the rock faces. He spread the sand/bentonite mix between the rocks, then swept the back of the rocks clean with an air hose and misted the site to dampen the bentonite and start the swelling. Further experimentation would be helpful to find a sand/bentonite mix that seals the joints even better, yet can be removed more easily after the walls are up.
Nick used 1/2" rebar spaced one foot apart throughout the walls, with additional, thicker rebar around windows and doors. Switching to 5/8" rebar would guarantee even stronger walls at nominal extra cost. The rebar should be propped into the middle of the panels for maximum strength. Panels up to about 24 feet wide, tapering to a peaked roof of similar height, can be lifted with a mid-sized crane. Larger panels would require more serious engineering and a much larger crane.
After the rebar, you must imbed coil inserts for pickup points the crane can grab on to. The legs of the pickup points should be placed behind a section of rebar to insure that the pickup point doesn't rip out of the wall in mid air. A plastic plug keeps the threaded hole free of concrete, so the lifting cable can be bolted right to the wall. Note that in a large wall there are four pickup points to evenly spread the load. It is advisable to consult an engineer for optimal placement. Be sure to see the Dayton Superior Tilt Up Construction Handbook (www.daytonsuperior.com) for more details. Anchor bolts should be inserted along the tops of the walls to tie into the roof.
Pouring the concrete is much like pouring a slab. Six-sack cement is recommended for optimal strength. A vibrator helps fill the voids between the rocks. Fill the forms and screed off the top. Then tap the weld plates into the mortar, positioned so that they will nearly touch when two walls come together. This is also the time to add metal brackets to attach floor joists, purlins, or a ridgepole, if those are included in your plans. The concrete must cure for at least one week before lifting, but three or four weeks would be better, just to be on the safe side.
Nick built the forms, set stones and poured the concrete for all four wall panels of the house in about two weeks, working mostly solo.
Lifting the Walls
Lifting the walls is clearly the most exciting aspect of building a tilt-up house, both for the builder and for spectators. People may drive by for weeks not knowing what you are up to, but all of a sudden there are these massive stone walls standing there like they've always been there. Lifting the walls is also the most hazardous aspect of the work, so make safety your top priority.
Hiring a crane can become very costly, so simplicity is required to do the job quickly and efficiently. Ideally the building site should be flat for easy acess, and the walls should be formed right next to the house, such that each wall can be tilted right into place, without having to transport them across the job site. That was not an option for Nick's house, being built into the side of the hill. On both the garage and the house, the walls had to be moved out of the way before putting them back in place, so most walls had to be moved at least twice. Getting the crane in place on the hill and supporting the out-riggers also proved a daunting task, and took more time than setting the house walls in place. In other words, up to three-fourths of the crane cost went towards leveling the crane or moving walls out of the way, rather than setting them in place.
If you absolutely must pour the panels in a stack as Nick did, then be sure to separate them with a layer of sand and not just plastic, which can create a vacuum seal, sticking the panels together like glue.
Before you start a tilt-up house you will have to find out what types of cranes are available and how much they can lift. Each cubic yard of concrete and rock in the walls weighs 2,500-3,000 pounds. Check the phone book, or simply watch for cranes at construction sites, and stop to ask questions. If they can't help you with your project, they can probably lead you to someone else who can. Note that a crane can lift more when the load is close to the rig, such that the boom is almost straight up and down. The lift capacity drops dramatically as the boom is lowered toward a horizontal position. In other words, a crane rated to carry the weight of your panels may not be able to reach far enough with the load to do any good. You will need to describe the panels you are working with and your job site, to make sure that the crane can lift the panels and put them where they belong.
Also note that the crane may not have the proper attachments to lift wall panels. A crane usually comes with a hook, and everything else is purchased or rented as needed. If necessary, show them the pictures here, and they can tell you what they have or need. You may be able to purchase or make the attachments and give them to the operator as partial payment. Or you might want to keep them for your own future projects. Make sure that every little part is ready to go, before the crane shows up and starts billing you $200 or more per hour while you run to the hardware store for extra parts.
After the cables are bolted onto the wall, the lift is all crane work. If necessary, you can stand at the edge of the panel to guide it as it floats through the air. Avoid passing beside the panel, where it could flatten you like a pancake in an accident. Be sure to make guide marks ahead of time on the concrete pads, so that you know exactly where to put the wall. As long as the crane is lifting some of the weight, it is relatively easy to pry the wall in any direction with a prybar. The wall must also be checked for square with the house and plumb along the face and the edges. Plastic shims are available from Dayton Superior, or you can cut some scraps of metal in varying thicknesses for the same purpose.
When the wall is exactly where it belongs, lock it in place with the aid of braces bolted with coil anchors from Dayton Superior between the wall and the deadman concrete pad in middle of the floor. Nick made his own braces, but you can also rent them. Have the crane slack the cables gradually, to make sure everything is okay.
Subsequent panels can be set into place without the need for a brace. Again, check and recheck for level, plumb and square. Weld the panels together with the weld plates imbedded in the concrete at the corners. You will likely need to use metal scraps to bridge the gap between the plates. Make sure these are good welds, as that is the only thing that will be holding the house together until the footings and corners are poured with concrete. Nick also bolted a ridgepole across the house to tie the peaks together.
Expect the panels to be quite messy when you first lift them off the ground. The sand and bentonite mixture really adheres to the wall, and Nich had to scrape it out of every nook and cranny with a screw driver. A pressure hose may be more effective.
Finishing Touches
After the walls are up, it is time to go back to the beginning and pour the footings. Use plenty of rebar and pour a wide footing. Also insert the rebar pins at the corners and pour the cores full of concrete. From that point the rest of the construction can be finished any way you choose.
Since Nick's place is built into the hill, he only tilted up three sides of the house, then used conventional concrete forms to pour the back walls. Rebar from the tilt-up panels extends into the poured concrete walls. Slipform stone masonry was used for the stonework on the porch. The roof and interior walls were framed and insulated with conventional materials.
The eves of the house were stuccoed for fire-proofing. The stucco-screens were pre-bent then attached with a nail-gun. Nick applied a single coat of stucco consisting of 1 part masonry sand to 3 parts masonry of cement.
Nick built a stone stairway from the house down to the garage. He poured concrete risers for the stairs, then set the stones into a sand and gravel base without mortar. Another innovative idea Nick worked with was a wood-fired boiler built into the side of the hill down by the garage. He never has to haul firewood into the house or clean up the mess of it later. Hot water from the boiler is circulated through the garage and a radiant floor in the house. The boiler only has to be used two or three days a week through the winter to keep the house warm.
Conclusion
Tilt-up construction is definitely not for beginners. For one thing, you will not save any money on materials versus the slipform method I described earlier, because there is just as much concrete, and usually more rebar, in a tilt-up stone wall, versus a slipformed stone wall. In addition to the materials cost, you may need to hire an engineer to check your plans and a crane operator to lift the walls, which could cost thousands of dollars before you are done. Also keep in mind that moving concrete panels weighing thousands of pounds each is implicitly hazardous.
Nevertheless, for the experienced builder, or someone who wants to make numerous copies of a single structure, tilt-up construction may be the way to go. With the appropriate building site and a set of plans optimized for that site, that there would be a definite savings with tilt-up stone work. More importantly, you can build a low-maintenance structure that will truly withstand the test of time.
Interesting Stuff?
For more innovative building ideas, check out
Living Homes: Stone Masonry, Log, and Strawbale Construction
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