S Planning and Design
Transcription
S Planning and Design
How to Build Concrete Homes Using Foam Forms Tips on using one type of stay-in-place block form BY RONALD P. ARDRES AND TIMOTHY S. FISHER tay-in-place foam forms have been accepted as a quick, easy, and efficient way to construct residential foundation walls. Now these systems are quickly catching on as an efficient way to construct entire homes, from footing to eave. In one step, the contractor can form concrete and provide insulation with R-values much higher than conventional concrete walls. Other advantages of these systems when compared to conventional forming methods include: • Faster wall erection (forms are easy to erect and do not require stripping) • Better concrete curing and a longer construction season (the foam insulates the concrete) There are two types of polystyrene foam forms: panel systems and grid systems (see box on page 661). Each type is constructed diff e re n t l y. A grid system produces a post-and-beam wall: a combination of many concrete columns and beams that are spaced apart by sections of foam to resemble a large waffle or a huge window screen. The components of this system are foam blocks with foam webs that act as wall ties and filler between the posts and beams. The post-and-beam grid system block has no top or bottom, making its assembly virtually foolproof. The system also uses less concrete than some other systems, yet provides the same structural strength. Many experienced contractors find this type of system the easiest to construct. Howe ve r, it is also a good system for the novice builder. Following are some hints on using post-and-beam foam block forms. S Planning and Design Planning is probably the most important step in building a concrete home. As with any other form of building construction, a complete set of plans is needed before construction starts. Plans that were originally designed for lumber or concrete block construction can easily be altered to use concrete and stay-in-place forms. No rm a l l y, all that is required is to adjust the home’s overall dimensions to a multiple of the block size. This will make construction easier and less wasteful, since blocks will not have to be cut to fit the original dimensions. To get the most out of an expanded polystyrene (EPS) stay-in-place forming system, designers should consider the following: • Size and location of the walls’ penetrations • Locations of windows and doors • Floor heights • Floor system options EPS systems are very flexible and allow some unique architectural design options. Curved walls, unique finishes, and vaulted ceilings are just a few of the custom features that can be included in a concrete home. Ordering Materials Manufacturers have quantity takeoff formulas that can help contractors order the proper amount of materials. If you know the squarefoot area of wall forms and some other basic information found on the home plans, manufacturers will be able to give you the quantities of materials needed, including blocks, rebar, concrete, scaffolding and bracing, and materials to form window and door openings. Be sure to allow enough time for manufacturing and delivery of the forming system. Local distributors often keep plenty of material in stock, but if they’re short it can take a while for the manufacturer to fill your order. Foundations and Getting Started Width and depth of footings depend on soil-bearing capacities, size and width of blocks, engineering designs, and local codes. Layout and construction of footings are the same as for conventional forming systems. However, extra care should be taken to place the footings flat and level. If the footings are not level, blocks can easily be scribed and cut, or shimmed with liquid foam filling, but a little care when placing concrete will save you a lot of extra work later. Manufacturers normally set tolerances on the levelness of footings, so be sure to follow them. If the home is being constructed on a hilly site, you may need to form stepped footings. St a y- i n place systems can accommodate these footings in a unique way. When forming a step in the footing it is not necessary to form the vertical step face; just keep the height of the step equal to a multiple of the height of the block. For example, if the height of the block is 1 foot, make the height of your step 1 foot, 2 feet, or any other multiple of a foot. When forming up to a step, place a piece of steel horizontally in the block, thus creating a grade beam. Removing the vertical wall in the step will reduce the amount of time needed to form the footings. The footings can be keyed and rebar can be pinned or cast-in-place. With some block systems, it is easy to locate and place rebar before pouring the footings. This is the recommended method whenever possible. With some other block systems, it is more difficult to locate the rebar within the cells of the block because of the number and size of webs holding the block together. A small mistake in calculations will throw the entire layout off, and require bending the rebar so it falls within the cells. To prevent this problem, drill holes in the footing and epoxy the rebar in place when laying out the first course of block. After the footing is placed, the layout of the walls is marked on the footing. Along the outside of the layout lines, nail a wood shoe into the footing. Scrap 2x4 or 5⁄4 lumber works well as a shoe as long as it is straight. The 2x4s should be continuous around the corners to act as a nailer for the corner bracing. Another method is to use a 2-inch, 25-gauge U-channel. Simply fasten the U-channel on the building line using nails for concrete, and insert the block into the U-channel. This will prevent the block from moving off the building lines in any direction. Placing Blocks The first course of block must be level. The forms should be erected following the manufacturer’s instructions, as each system is different. Start from the corners and work t ow a rd the middle by placing any cut pieces in the middle of the wall, much like installing concrete block. Use a 4-foot level to check the block as you pro g re s s. After setting the first course, start on the second course by staggering the joints. Staggered joints strengthen the wall and help keep it aligned. The first couple of courses are called layout courses and are important steps for a straight and true wall. Wall forms should be built and concrete placed one story at a time. When the wall begins to reach onestory height, place a piece of vertical steel that is long enough to lap splice to the next story’s reinforcement. If you are going to continue upward with subsequent pours, be sure to cover the interlocking teeth of the block to keep them free of concrete. Below-grade walls normally require much more reinforcement than above-grade walls. Some systems have ties that act as chairs to hold horizontal bars, and others have special chairs. Since wall construction moves rapidly, be sure not to get carried away: Don’t forget to place bars where needed. Tie the ve rt i c a l bars to the dowels in the footing. Af- Window bucks should be installed before concrete ter vertical bars placement. Additional bracing is required on the inside of are in place, it is large openings to keep the openings square and true. necessary to slide Concrete has already been placed in this wall. the blocks down over the bars. If tems have corner blocks that can be the bars are cut short, you can used instead. In all the systems, anreach over them easily. Howe ve r, gles other than 90 degrees are creatthis will require a lot of bar splicing ed by mitering, gluing, and taping and will quickly eat up your reinblock together. These angles should forcement budget. To use longer be braced for plumb before you bars, you must devise a system to pour the concrete. Most systems get the blocks up and over the bars. can be bent or cut in such a way as One contractor has devised a tool to create a curved wall of almost any that consists of a 1x3 with a nail radius over 4 feet. To accomplish d ri ven through one end. When this, cut pieces of foam out of the placing blocks, he simply stabs the back side of each void. The manublock with the nail and uses this to facturer can supply a chart or forhoist the block up over the bar. One mula that tells you how big the cut manufacturer offers a tie with a must be for a given radius. Curves hole in the center for use with its can also be created by sculpting the system. A few of these ties are foam with a hot-wire cutter or rasp, placed in the wall at different or by gluing pieces of molded foam heights wherever vertical reinforceto the side of the foam wall. ment is needed. When the wall Doors and Windows forming is done, vertical bars are slid into the holes in the ties, alignOpenings of any kind are easily ing them in the center of the block. achieved with polystyrene systems. With some block systems, corConstruct a frame to box out your ners must be created by mitering desired rough opening using a presand then gluing and taping block s u re - t reated 2 x10 or 3⁄4-inch prestogether with duct tape. Other syssure-treated plywood. Build a three- sided form, two legs and a head, to the desired rough opening using the 2x10s or the plywood. Make the bottom of the three-sided form using two 2x4s, leaving a gap in the middle. The gap allows you to pour below the opening and visually check that the area below the opening is filled properly. You should insert the wooden frame as soon as possible and continue to install the block around the frame. Once you have completed the wall, add 1⁄2inch bolts or galvanized spikes through the window frame, and then add a frame of 1x3s around the outer edges of both sides to help support and keep the frame centered in the wall. On larger openings, extra bracing may be needed. Each manufacturer should supply a lintel chart with its product (Figure 1). Be sure to consult this chart for the size and length of reinforcement needed over an opening. Windows can also be installed before pouring the concrete in the walls. To do this, build the frame as described above, wrap the window in plastic to protect it from concrete splatter, and install it. After placing the concrete in the wall, remove the plastic. Doors can be installed similarly. Bracing Start bracing at wall corners by nailing together two 2x4s to form a corner and then bracing it back to the footing. This will plumb the wall in both directions simultaneously (Figure 2). Metal studs also can be used for bracing following the same procedure. If you are going to reuse the bracing, metal corner posts and bracing are much more durable and tend to be straighter than lumber. Additionally, the walls should be braced every 10 to 20 feet with vertical members supported by a kicker securely fastened to the ground or floor. In most cases, you can fasten intermediate bracing to the plastic or metal ties; in cases where this is not possible, a 9-inch shorttail snap tie and hairpins work well. Another method is to use some threaded rod with nuts and washers. If it is below grade, the kicker can be placed horizontally and used to support scaffolding from which pouring can be done. Elevated walls for the second story should be braced from the interior. After floor decking is installed, run braces from the floor to the wall for support. These are minimum bracing requirements and should be adjusted as conditions require. Bracing must be adequate to safely support the wall under all conditions, especially high winds. Scaffold the entire wall system prior to pouring. Several manufacturers offer a bracing/scaffolding system, which can save a lot of time in the long run. If these systems are not available, it is very simple to construct a bracing/scaffolding system using lumber, pipe, and turnbuckles. The turnbuckles allow the wall to be TYPES OF EPS FOAM FORMS There are several stay-in-place foam forming systems for concrete construction on the market today. Expanded polystyrene is a plastic foam made of polystyrene filled with tiny gas bubbles; extruded polystyrene is a seamless foam made by extruding EPS continuously through a die. There are two types of forms and three basic EPS formats: Panel System Grid Systems: Blocks with metal or plastic ties Blocks with foam webs Description R-value Cost This type consists of off-the-shelf sheets of foam held together by plastic ties. Panels are made of two sheets of foam that are normally 2 inches thick, totaling 4 inches of insulation. Ranges from 20 to 21.5 Ranges from $1.80 to $2.50 per square foot These systems are known as “insertion-molded.” They are shipped to the jobsite with the metal or plastic ties fused into the foam block. These blocks form a cavity into which the the concrete is poured. Depending on the system, ranges from 20 to 30 Ranges from $2.00 to $3.00 per square foot This system creates a true postand-beam system, reducing the amount of concrete needed to fill the form. The foam webs add rigidity to the blocks. Ranges from 20 to 30 Ranges from $1.50 to $2.50 per square foot Source: The Portland Cement Association’s Guide To Concrete Homebuilding Systems, published by The McGraw-Hill Companies, 1995. plumbed easily. If job conditions are right, rolling scaffolds work well. When placing concrete from the inside of the wall, set rolling scaffolds on the deck and roll them around the bracing to wherever needed. Floor-to-wall Connections Where continuing up with the form for additional stories, attach a ledger directly to the form to support the floor. Before affixing the ledger board, drill holes approximately 5 inches in diameter in the foam to correspond to the center of every cell. Do not bolt the ledger directly to the foam, since it will compress after you tighten the bolts. Also, a bolt sticking through the foam is not supported and can bend under the weight of the floor. It is best to stagger the holes rather than place them in a straight line. Predrill holes in the ledger corresponding to the holes cut in the foam and set anchor bolts so they fill the core. Support the ledger in place, and when the concrete is poured, it will flow against the ledger, firmly embedding the anchor bolt. Manufacturers also supply details for attaching other types of floor systems. Precast slabs, open-web steel joists, and fire-cut joists are a few other forms of decking that can be attached to the stayin-place concrete wall system. Placing Concrete After the wall is completely erected and all openings are in place, check that the wall is plumb, level, and square. Make any necessary adjustments at this point before placing concrete. When you are ready to pour concrete, any desired placement method can be used. A pump is often the recommended method, since it is the easiest and least timeconsuming. Howe ve r, concrete can be placed by bucket, shovel, or directly from the chute of the truck. When pumping, a hose of 2 or 21⁄2 inches in diameter works best. Use of a larger hose cuts mobility and productivity, unless the hose is controlled or suspended from a boom. If a boom pump is used, c a refully monitor the pump ra t e and use an elbow near the end of the hose to help reduce the force of the falling concre t e. When placing from the chute of the truck, it is often helpful to build a trough to funnel the concrete into the wall. This will cut down on spilled concrete. Always start the pour thro u g h the slots in the window frame. Allow concrete enough time to set so you can continue the pour upward and not have concrete spill out f rom beneath the window. After this is done, start at the corn e r s and work your way around the wall. Be sure to fill both sides of door and window openings evenly to prevent movement of the wooden frames and to equalize the p re s s u re of the concre t e. Do not allow the concrete to drop or create a plopping action in the form; instead, let it flow into the form ( Fi g u re 2). A plopping action increases pressures upward and outward on the forms and will cause floating in some systems. Each m a n u f a c t u rer has a suggested concrete mix design and pouring rate. Lift heights range from 4 to 10 feet, depending on the manufacturer’s recommendations. It is very i m p o rtant that you follow these recommendations in all cases. Figure 1 OPENINGS AND HEADER CONSTRUCTION For most systems, slumps of 4 to 6 inches work best, and 5 inches is the optimum. Also, pea-gra ve l mixes work best, since the forms usually are only 6 inches wide, and with the rebar and ties, space tends to be very tight. The pea-gravel mix will flow much better than larger aggregates around the steel and through the voids, and it will aid in consolidation. When consolidation of concrete becomes necessary, a large flat-plate orbital sander applied to the exterior of the form works well. Or, a concrete vibrator placed on top of the vertical steel will vibrate the steel, and in turn, consolidate the concrete. It is difficult to vibrate within the form, since most of the systems are 6 inches wide, and with the addition of steel, it becomes almost im- possible to insert most vibrators into the concrete. Blowouts. If forms are handled properly, blowouts should not occur. If there is a blowout, the wall will stay intact, but an opening will be created that can easily be fixed. First, remove the piece of wall that has blown out, clean the spilled concrete around the area, and replace the blown-out piece. Us e two pieces of lumber secure d through the wall by lengths of tie wire, snap ties, or threaded rod to keep the patch in place. Re p o u r the blown-out area, and re m ove the lumber after the concrete has set. If the tie system is stuck through a foam web, it can be removed easily and used again. It is helpful to keep a premade blowout kit on hand in case of a failure. Figure 2 BRACING DETAILS AND CONCRETE PLACEMENT Attaching Sills Attaching floors or roofs to the wall system is quite simple. The possibilities are many, and the one you select will depend upon your particular building method. When using these systems for foundation walls only, save a lot of work by cutting, drilling, and installing foundation bolts in your sill plates. Simply set them in the wet concrete after pouring, and tap the top of the bolts or the sill to consolidate the concrete around them. The next day, simply tighten the bolts. By doing this, you will create a sill seal because the concrete will have shrunk slightly, and as you tighten the bolts they will compress the foam into the sill, creating the seal. A sill plate is also placed on top of the wall for the attachment of trusses or conventional framing for the roof. Utilities Place piping with a diameter greater than 3 inches prior to the concrete pour, or use a sleeve to block out a hole for pipe insertion. Piping with a diameter of 3 inches or less can be installed by drilling through the f o rm web after the pour has been completed. For interior piping with a diameter greater than 2 i n c h e s, place the piping in the forms before pouring concrete. After pouring, electrical conduit and plumbing is easily installed in the walls with the use of a hot-wire cutter or router and some common hand tools. First, locate all box and f i x t u re locations. Box e s can be attached with adhesive to the foam or with p ower fasteners to the c o n c rete wall after the foam has been cut away. After the boxes are in p l a c e, a small channel can be cut into the foam with the hot-wire cutter or router. The wires or pipes are then located in the channels and inserted into the boxes or fixtures. Wall Finishes For below-grade walls, it is recommended that a fully adhere d waterproofing membrane system be used. For above-grade exterior walls, a finish must be applied to protect the foam from ultraviolet light. A multitude of options is a va i l a b l e. If using any methods or p roducts that you are unsure of, test them in an inconspicuous area or on an extra form. This will g i ve you a chance to check for qualities such as color and bond, and the compatibility of the finish material with the foam. Some materials are volatile and can destroy the foam, especially petro l e u m based coatings. Test all materials that you are unsure of on a piece of scrap foam and observe it over s e ve ral days. Most systems offer attachment stri p s, usually made of metal or plastic. These stri p s, which run in a pattern on the block, allow you to dri ve nails or s c rews into the block to attach finish materials. E x t e rior finishes. Ex t e rior finishes range from brick or stone veneers to any type of wood, plastic, or aluminum siding. So m e f o rm manufacturers have designed brick shelf blocks to support brick or stone veneer. Others p rovide instructions on how to c reate a brick shelf. Once the shelves are in place, use a waterproofing membrane to at least 18 inches above grade and wrap the shelf. This will take care of any weeping through the brick and the stone. Be sure to provide weep holes for condensation in the bottom of the brick or stone veneers. Then simply finish the veneer as you normally would. Brick ties can be fastened to some of the systems using their plastic or metal ties. Other systems have to be cut so the brick ties are attached to the concrete. Stuccos of all types work very well with these forming systems. When using thin-coat systems, you can add foam moldings or any foam design that can be cove re d with the stucco. Most of the thincoat manufacturers have information on how to apply these moldings and designs. Hard-coat stucco systems can also be applied using a self-furring chicken wire or masons’ lathe attached to the plastic or metal ties or fastened directly into the concrete. Bonding to the foam is enhanced by the addition of a latex modifier in the base course, or scratch coat. Wood, aluminum, or vinyl siding can be fastened quite easily. I recommend installing a wood or metal lathe prior to the siding installation using a combination of mechanical fastening and glue. Using the lathe accomplishes two functions: • It creates an air space, which will increase the R-value, and permit air to circulate and the wood to breathe. This prevents a buildup of moisture when any type of wood siding is used. • In the case of aluminum or vinyl siding, it allows it to expand and contract freely without rubbing the foam. Interior finishes. Sheetrock is applied using a combination of glue and some type of mechanical fastener. The least amount of mechanical fastening is best. It cuts down on spackling and creates a better finished job, since foam shrinks very little. To add moldings, base crowns, and chair rails, attach a strip of thin plastic, light-gauge steel, or aluminum prior to attaching sheetrock to provide a sufficient nailing base for the moldings. Also, many types of foam moldings can simply be glued on. By code, a 15minute thermal barrier must be applied to all interior living spaces. This can be accomplished with 1⁄2inch-thick sheetrock. Ron Ardres is president of ReddiForm Inc., an Oakland, N.J., company that manufactures EPS forming systems. He is also president of the Insulating Concrete Form Association. PUBLICATION # C950659 Copyright © 1995, The AberdeenGroup, All rights reserved