Article No: 208

2007-03-28 11:54:41
Concrete Homes: Options for a Squeak-Free Floor
By: Jim Baty, Technical Director, Concrete Foundations Association


Photo courtesy of Lite-Deck, Inc.
 
Part I: This six-part series focuses on details of today’s concrete homes. Over the past two years, this column has provided considerable information on the general benefits of concrete homes, the various methods used for construction and the performance characteristics. We now focus on strengthening the understanding of the decisions, details, and results that can affect the quality achieved in the above-grade concrete home industry delivered by removable forms (RCFs). Since strongly entering the market nearly a decade ago, this method of construction offers an ever-expanding variety of architectural and practical construction solutions for today’s homeowner and designer.

A home is very likely the largest investment a person will make in his/her life. As such, it requires considerable attention to detail to get the desired result. Sophistication will be evident in the systems delivering sound quality, temperature control, and lighting ambiance. But, what happens when the homeowner takes the keys, moves in, and expresses frustration because the home they have planned so carefully has noise emanating from the floor system? You can cure this problem with some of the lumber products on the market … or you could make the decision right now to complete the quality of your new concrete home with concrete floors. 
What are the options for incorporating concrete floors into to the home’s shell? How does this influence the sizes of spaces and the configuration of the mechanical systems throughout the home? The answers depend on the type of structural system selected to do the job. Whether you choose a reinforced slab, structural rib and slab, or precast hollow core plank, the end result will be a floor that cannot move or squeak—it will control sound attenuation throughout the home for the decades of its life. These systems produce monolithic floors that do not rely on mechanical means to secure the assembly.

A thin structural concrete floor
The first floor system we will discuss is a part of the third forming option for RCFs—construction of exterior walls, interior walls, and concrete decks (floors and ceilings). During the forming process, the walls are set around the exterior followed by interior walls. A transition form may be used to create a crown mold or other relief around the top of the walls, followed by the placement of horizontal forms of similar dimension and weight to the wall forms. Once the forms are in place, shoring is brought in to support the span during concrete placement and curing. The structural components and all utilities are placed prior to concrete delivery; this will include the reinforcement and chairs for their support as well as electrical conduit, junction boxes, and fixture boxes. Blockouts for water and HVAC distribution are also placed during this time. The concrete is placed with a normal thickness of 6 to 8 inches depending on the span. The top surface is finished to the design requirements for exposed floor or attic deck. As the concrete cures the shoring is maintained until the engineer approves its removal, although the forms will be removed sometime between 24 hours and three days to allow for reuse on an upper level deck or the next house structure.

Monolithic concrete “joists” and “floorskins”
Next on the list of structural concrete floors is a design that looks similar to taking your wood joists and plywood deck and turning it into monolithic concrete. Structural rib decks differ primarily from reinforced slab decks in thickness and span capacity. They will consist of a profile resembling a series of concrete beams monolithically connected with a shallow concrete floor of 2 to 3 inches. The concrete beams are designed to span in one direction and carry the floor loads to the walls on either end. Forming these floors most often occurs at the same time as the reinforced wall to create structural continuity, although the forming pans could just as easily be set after the walls have been cast. Shoring supports this forming system until the structural engineer approves the removal (concrete attains a sufficient strength for applied construction loads). Unlike the reinforced slab system above, the forming pans remaining in place the entire time. There are unique forming advantages to this system as they allow adjustability for any span and any direction as the spaces are configured for the home. This eliminates a focus on modularity for efficiency. Utilities and chases are incorporated across the ribs by inserting sleeves into the rib section. Reinforcement will be concentrated along the rib sections with crosstie reinforcement in the thinner slab section.

Perhaps the most unique of concrete flooring systems, the structural rib decks can be created one of two ways: they can use a steel pan system and therefore further resemble a reinforced deck system or they can use one of several insulated forming deck systems. The steel pan systems are normally very thin and take advantage of the forming strength of steel combined with their ultra-thin profiles to provide the flexibility of site adjustment to fit any space without a lot of varying form sizes. The insulated forming systems use a structural foam profile that can span between shoring supports. This profile is often a series of wide, thick insulation billets interconnected with narrow, thinner insulation sections very much resembling the reverse of the concrete structure that will result. The concrete deck is consistent in section with the steel pan formed deck, having similar reinforcement patterns and the ability to integrate chases through the floor deck. One primary difference, however, is that the form system is designed to remain as insulation for both thermal and sound control.

Manufactured precast floor
The final option to deliver quiet and movement-free floors is the installation of a precast flooring system. Most often seen in the form of a hollow-core plank, the floor slab would normally consist of an 8-inch-thick concrete slab with holes or cores running in one direction through the plank’s length. These cores lighten the section and concentrate the structural concrete and reinforcement between the cores. One primary difference in this system from the first two described is that it uses pre-stressing to give the concrete structural capacity for the spans. Most precast plank systems in housing are seen in garage slabs over rooms like wine cellars and storage spaces. They can support a much higher load per thickness than the reinforced slab cousin due to the pre-stressing force. The use of precast planks does require considerable planning since they are a manufactured product delivered to the site. Therefore, they are less adaptable to on-site changes in design. All penetrations for chases and utilities are created onsite with sawing or drilling operations if they are to be small diameters that can avoid the pre-stressing cables. However, if ductwork and other systems must pass vertically through the floor system, they need to be designed into concentrated chases that the planks can be fit around.

Final thoughts
The design of the concrete home is as flexible as the design of any home structure. The option for structural systems in the floor is just one example of this variety and selection to fit the homeowner’s interests and budget. The one thing to keep in mind above all others is that the floor can make or break the enjoyment of the space. At the same time, it can contribute significantly to the performance of the space both in thermal and sound control. Many systems can make this argument, but concrete floor systems add the unique variable of durability and storm-resistance that others cannot provide.

Established in 1974 for the purpose of improving the quality and acceptance of cast-in-place concrete foundations, the CFA has a variety of resources on this topic including materials available from the Concrete Homes Council established at the CFA to help contractors’ transition into this market. In addition to providing promotional materials, educational seminars, opportunities for networking, and a telephone network that places members in one-on-one contact with an experienced contractor for assistance in resolving a variety of issues, the CFA and CHC represent the interests of its members and the industry on several code and regulatory bodies, such as the American Concrete Institute’s committee responsible for the creation of the “Residential Concrete Standard.” The CFA has several of its members on the ACI committee responsible for this document and will endeavor to ensure that the interests of the foundation contractor are considered. For more information about CFA, see cfawalls.org or call (319) 895-6940. For more information about CHC, see concretehomescouncil.org or call (319) 895-0761.
 
Jim Baty, jbaty@cfawalls.org, is Technical Director of the Concrete Foundations Association and the Concrete Homes Council.