Post-tensioned (PT) concrete has grown exponentially in the residential market over the last decade. More than half of all PT strand produced in North America is now used on residential projects. Post-tensioning is a highly efficient structural system that offers many benefits. The efficiency stems from its ability to use high-strength materials to their full potential. PT members typically use about 30 percent less material for a similar structural strength as compared to reinforced concrete members. In homebuilding, Post-tensioning can be efficiently used for slab-on-ground foundations as well as elevated floors. HIGH-STRENGTH MATERIALS
PT concrete is a perfect balance of two materials, that complement each other. Concrete is very strong in compression but relatively weak in tension: Its tensile strength is about 10 percent of its compressive strength. Prestressing steel, on the other hand, yields very strong tension. (It has a tensile strength of 270,000 psi, about four and a half times that of common reinforcing bars.) By combining prestressed steel and concrete, a structural member can effectively resist both compressive and tensile forces. As with reinforced concrete, PT members are made by placing concrete in wooden forms; however, high-strength wires-which are coated with grease and encased in a continuous plastic sheath-are used instead of reinforcing bars. Once the concrete has sufficiently hardened, the high-strength steel wires are elongated with hydraulic jacks, a process known as "stressing
. Once the tendons are stressed, they are anchored to the concrete using specially designed anchors. Stressing imparts precompression to the concrete, greatly increasing its ability to carry load, resulting in greater efficiency.

SLAB-ON-GROUND FOUNDATIONS
As cities grow, extending outside of traditional boundaries, builders face more expansive soil conditions. Engineers must now design foundation systems that withstand larger soil-induced shrink/swell movements to protect structures from consequential damage while containing cost. PT foundation systems are efficient and cost-effective with almost all soil conditions, from stable sites to highly expansive ones, and offer several major advantages. The precompression applied to concrete helps reduce floor slab cracking, which is often unsightly and causes homeowner concern. Cracks and joints are also an access point for water, termites and other insects. Due to the precompression introduced into the concrete by the PT process, shrinkage cracks are less likely to develop, and if they do, they often close as the slab is stressed (squeezed). Standard concrete control joints are often unnecessary with PT. Also, PT slabs have significant structural strength and can typically accommodate movements in the soil without causing cracking to the exterior and interior walls.

The slab-on-ground foundation typically falls into three construction types. First is a thin, flat slab, usually 5 inches thick, with a perimeter thickening; this foundation is used on stable sites where there is little potential for swelling/shrinking of the soils. The second is a "ribbed
foundation that has a 4- to 5-inch thick slab and ribs (beams) at 10- to 15-foot centers. These ribs are generally 10 to 12 inches wide and 20 to 36 inches deep, depending upon the severity of the soil's shrink/swell properties. This type of foundation is commonly used in Texas, where the soils are very stiff and expansive. The third type is a "uniform thickness
foundation that ranges from 7 1⁄2 to 16 inches thick, again depending upon the severity of the expected soil movements. The foundations are most common in Arizona, Nevada and California, where the soils are moderately expansive and the climate is very dry; the uniform typically requires more concrete compared with ribbed but the savings in labor and speed of construction offset the additional concrete cost.

PT is the material of choice for slab-on-ground foundation on expansive and difficult soils. It is estimated that 80 to 90 percent of the houses on expansive soils in the Southern states are built with PT.

LOW-RISE/SINGLE STORY RESIDENTIAL CONSTRUCTION
In conjunction with ICFs, PT is gaining popularity in the residential market. All concrete construction offers the following benefits compared with traditional wood/masonry construction.
>> Structural strength
>> Improved safety
>> Architectural versatility
>> Improved sound insulation
>> Improved serviceability and durability
>> Energy savings
For the same depth, longer spans are possible with PT. This can mean larger, column-free space and greater flexibility in floor space. Spans up to 30 feet are not uncommon with standard 8-inch-thick PT floor systems. This provides large, column-free spaces, which can later be modified easily. PT also reduces building height, and the cost of all related building components, such as plumbing and electrical systems and curtain walls.

IMPROVED SAFETY
Because of the inherent structural redundancy built into concrete houses, they are not only strong but also very resilient. Concrete houses are significantly heavier than traditional wood and metal construction, so it is difficult for hurricanes and tornados to damage them. The walls and floors are rigidly connected, providing a redundant system able to resist very high winds and earthquakes. Ed Rice, a pioneer in the development of PT in North America, lives in Southern California, where his house was built with concrete and PT about 45 years ago. He claims the strength and incombustibility enable him to get the best property insurance rates, a savings he believes more than offsets the cost of all-concrete construction.

ARCHITECTURAL VERSATILITY
Cast-in-place, PT concrete can be easily adapted to accommodate complex geometry and other special design challenges, from curved shapes to nonsymmetrical layouts and unusual design loadings. PT makes longer, slender members with large spaces between supports possible, giving freedom to create functional and aesthetically pleasing structures. PT is also effective for long cantilevers; these features can create illusions of the structure floating in the air with long slender, column-free balconies.

IMPROVED SOUND INSULATION AND ENERGY SAVINGS
Concrete has excellent sound absorption. In conjunction with ICFs, it is possible to isolate the sound transmission between PT buildings that are also considerably more energy efficient. Since the floor system is typically 6 to 8 inches thick, instead of the 2 to 4 feet common in wood-frame construction, the volume of space required to be heated and cooled is considerably smaller. Additionally, the thermal mass of concrete insulates the house from large temperature fluctuations during the day and keeps it comfortable year-round.

IMPROVED SERVICEABILITY AND DURABILITY VERSATILITY
Concrete gains strength with time. Houses built with concrete and stone by the Romans are still standing after hundreds of years in service. Likewise, PT structures are durable and require little maintenance. The compressive forces that are applied to a structure during prestressing result in better crack and deflection/vibration control. Since cast-in-place construction results in monolithic connections between slabs, beams and columns-it eliminates joints that could cause problems and require costly maintenance. Concrete homes easily outlive others, whether wood or steel.

 

A portion of this article first appeared in (Arizona) Homebuilder Magazine, July/August 2006.