For commercial construction it's hard to beat the cost, quality, and versatility of tilt-up construction. It can also be the fastest way to build. For these reasons and more, some tilt-up contractors are now starting to market and build residential structures, too. And designers like tilt-up construction because there are so many unique design possibilities.
Top: Forming walls involves planning the space carefully and locating panels for the easiest lift into their final position. Middle: These two-story panels are being constructed on a casting bed. Notice the ledgers for the 2nd floor cast at the midpoint of each panel. Bottom: A “back-picked” panel is being lifted into its final position.
Jerry Daugherty, president of JD Construction, Las Vegas, has probably completed more tilt-up houses than anyone to date. When he and his wife decided to build their new two-story home in Las Vegas they decided that it had to be concrete and it had to be built using tilt-up techniques. But when they compared the footprint of their home to their lot size, it became clear that casting beds would be needed. They located them to optimize the crane location for placing the panels. Because they wanted chemically stained exposed concrete floors, they built the floors after all the panels were in place and the rough construction was complete.
Daugherty says they saved over two months in construction time compared with what's required of homes of comparative value in the Las Vegas area. Now that he's been living in the house, he reports that their heating and cooling costs are well below the house they moved from, which was much smaller. The final costs, he says were a little more than a wood structure, but they got the house of their dreams, and payback in energy savings started the day they moved in.
In Las Vegas, says Daugherty, concrete homes are a hard concept to sell because homeowners have by tradition, lived in wood frame houses. Owners find it difficult to understand why they would want to live in a concrete home. By using his own home as a model to sell others, he believes that the demand will increase as more tilt-up homes are built and that the best application is custom homes.
But there are also some constraints. Bob Long, executive vice president of Composite Technologies, Boone, Iowa, says that the design of a tilt-up home is influenced to some extent by where panels will be cast. One thing separating commercial tilt-up construction from residential is the space available to work, so the decision to build a tilt-up house should be made during the early design process. The size of the building lot and the footprint of the house are important considerations. You have to know where panels can be cast: either on the floor slab or on temporary casting beds. If you cast panels on the floor slab, they must be placed to allow lifting in the right sequence. If casting beds are needed, there must be enough land available. And with either system there must be space for cranes to tilt the panels afterwards. While it's also possible to stack cast, casting panels one on top of the other, the logistics have to be carefully worked out and the cost and time of construction kept in mind.
HOW THE PROCESS WORKS
Once the planning for building tilt-up walls and developing a casting plan are complete, the first step on the jobsite is usually casting the floor for the house. If the plan also requires casting beds, they would be cast at the same time. Casting beds are temporary slabs constructed for casting panels. The concrete in them can be as little as 2 inches thick, but care must be taken so that panels are flat and level. Floors that will be used for casting have the same requirements.
Setting the side forms for the panels is the next step. The forms are secured to the floors or casting beds. If the panels' exterior is to be patterned, formliners are positioned inside the forms. Formliners are made from vacuum-formed thin PVC or elastomeric urethanes so they will hold up under repeated use. If thin brick veneers are specified, plastic gaskets are placed on the casting surface, and the brick units are pressed into the gaskets, which both secure them and provide the "mortar joint." Concrete masonry or rock masonry also can be placed into the forms for casting into the exterior face of the panels. A special bond breaker is then applied to the casting surface to assure a clean release so that neither the panel nor the casting surface is damaged when the panel is tilted up.
The greatest stress a panel will experience is when it's tilted or lifted into position. The engineer of record for a job (the structural engineer) designs the steel reinforcement to handle the in-service loads. Reinforcement to handle the lifting stresses, the location of the lifting hardware, and the locations and requirements for braces are determined by engineers who represent the lifting and bracing supplier chosen by the contractor. While "back-picking" is the normal procedure for lifting tilt-up panels, edge lifts or end lifts can be simpler and more economical for small panels. Wall panels about 10 feet tall are generally more economical to produce from the standpoint of reinforcement and lifting hardware. Lifting 15- or 20-foot tall panels is more complicated because several more lifting points may be required along with more technical crane rigging.
There typically aren't special requirements for panel concrete mix designs. During placement, however, concrete must be carefully consolidated around reinforcing steel and lifting inserts. Most manufacturers specify a minimum strength of 2500 psi before a panel is lifted, with an ultimate goal of 4000 psi. For safety, concrete strength should always be confirmed before lifting.