Page B2.2 . 30 May 2007                     
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    SoCal Concrete


    The roof and floor slabs use Type K cement, an expansive cement used to create shrinkage-compensating concrete. The dead-level concrete roof was built without joints, has not cracked, and has never leaked despite the fact that it's been exposed to the elements for more than four decades without any form of roofing membrane or waterproofing.

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    Type K cement was also used in the slabs-on-grade. Ceramic tile flooring, used extensively throughout the house, has not cracked further testimony to the effectiveness of shrinkage-compensating concrete.

    Overcoming Cracking

    Rice was an early proponent of post-tensioning and shrinkage-compensating concrete and used his house to push the envelope of what could be built with these technologies. He wryly admits that, "After more than 40 years, I believe we can say the experiment was a success."

    He is now chairman of the board of CTS Cement Manufacturing Corp., manufacturer of CTS Type K Cement. His pioneering work in post-tensioning was recognized last year when the Post-Tensioning Institute (PTI) feted him as a "Legend of Post-Tensioning."

    Ordinary portland cement concrete slabs can't be relied on for watertight construction due to the material's proclivity to crack. The use of post-tensioning and shrinkage-compensating concrete, however, can overcome two of the major causes of cracking.

    In post-tensioned concrete, high-strength steel strands are cast into a slab. After the concrete cures and before loads are applied, the strands are stretched to put them in tension. The tension in the strands exerts enough compressive force on the concrete to overcome tensile forces that could cause cracking.

    Concrete also cracks due to shrinkage. As concrete dries over a period that can range from weeks to years, its volume decreases. If a concrete slab is unrestrained, shrinkage is not a problem because the slab will simply contract. However, external restraints almost always affect slabs due to either friction with the subgrade or the reactions of adjacent construction. When the internal stresses due to shrinkage are greater than the tensile strength of the concrete, shrinkage cracking occurs.

    Shrinkage-compensating concrete overcomes this problem through the use of a blend of portland cement and expansive hydraulic cement. The drying shrinkage of the portland cement is offset by the crystallization of the expansive component. The two cements are proportioned to result in a slight overall expansion of the concrete. This expansion puts the reinforcing steel in tension and, as a result, prestresses the concrete to keep it under compression. As long as the concrete is in compression, it won't crack due to drying shrinkage.

    Rice House Ingredients

    Rice's roof is a two-way flat plate and is 5 to 7 inches (125 to 175 millimeters) thick depending on the span. No shrinkage or movement control joints were required, even at the inside corner of L-shaped slabs that are especially prone to shrinkage cracking. According to Rice, the absence of joints simplified the seismic design of the building.

    The concrete mixture had a 4000-psi (28-MPa) compressive strength at 28 days and included 658 pounds per cubic yard (390 kilograms per cubic meter) of Type K cement and lightweight expanded shale aggregate. The seven-wire, 1/2-inch- (13-millimeter-) diameter strands, nominally 25 inches (640 millimeters) on center, were post-tensioned to 23,000 pounds (100 kN) to put the concrete under approximately 175 psi (1.2 MPa) compression.

    The tendon anchors were of Rice's own design. According to his citation from PTI, "The Rice Coil Anchor and Twin Cylinder Stressing Jack is considered to be the predecessor of the current anchorage and jack used in all unbonded post-tensioned applications and has revolutionized the use of unbonded post-tensioned concrete construction throughout the world."

    For additional insulation, the concrete was cast onto extruded polystyrene (XPS) set on top of the formwork; the XPS adheres to the concrete and is finished on the underside with plaster. The top of the slab has a nonslip broomed finish, and overhanging eaves have a smooth form finish with an integrally cast drip edge. For skylights, slabs of structural glass were set into rabbetted openings in the roof and glazed with silicone sealant.

    The Beauty of Concrete

    The house was designed by the late Whitney R. Smith, FAIA, one of the architects who participated in the famous Case Study House program. Like the Case Study Houses, the Rice Residence relied on low-maintenance building materials to simplify modern living.

    The 4-inch- (100-millimeter-) high split-faced CMUs used limestone aggregate and were custom tinted so that even an experienced eye can mistake the walls for natural stone. Exterior walls have all but disappeared behind vines that shade the building and minimize its visual mass.

    The house was designed to withstand the extremes of nature, including hillside brush fires and teenage children who used the flat roofs for recreation. The house's incombustibility enables Rice to get the best possible underwriting rates on property insurance; he claims the savings on insurance have more than compensated him for the premium he paid for all-concrete construction.

    Another economy has been on the efficiency of heating and cooling the house. The thermal mass of the concrete helps keep the house comfortable year-round. Not only does this minimize Rice's fuel bill, it also reduced his initial investment for heating and cooling equipment. The only air conditioner in the house, for example, is a small window unit in one room where a large sliding glass door, positioned for views of the ocean, admits the heat of the late afternoon sun.

    Building for the Future

    After 40 years, the Rice home shows no signs of aging other than a comforting patina of time. While some builders shy away from post-tensioned concrete due to a belief that it's difficult to remodel, Rice has made several additions, tying new post-tensioned slabs into the existing construction.

    The future of shrinkage-compensating concrete looks promising. Two recent innovations have increased the versatility and cost efficiency of the material. While Type K cement is a preblended product containing both portland cement and expansive cement, CTS Cement Manufacturing Corp. now offers the expansive cement, branded as Komponent, as a separate ingredient that a concrete producer can blend with portland cement at the batch plant.

    The second opportunity emerges from the use of synthetic fiber reinforcement with shrinkage-compensating concrete to create large joint-free industrial floors without the expense of steel reinforcement.

    Currently, shrinkage-compensating concrete is most frequently used for industrial floors, bridge decks, and containment vessels. According to the National Association of Home Builders, however, the use of above-grade concrete walls for home construction in the United States has increased from a 3 percent market share in 1993 to more than 16 percent in 2004. Rice predicts a continuing increase in the material's use in exposed concrete roof decks for lower maintenance, better energy efficiency, and protection against natural disasters.

    Discuss this article in the Architecture Forum...

    Michael Chusid, RA FCSI is an architect and a Fellow of the Construction Specifications Institute. His Los Angeles, California-based company Chusid Associates investigates and reports on the performance of new building products.

    A version of this article first appeared in the March 2006 issue of Concrete international . It is reprinted here with permission of the author.



    ArchWeek Image

    The piers, like most of the walls in the project, used two wythes of thin CMUs to form a cavity into which concrete was placed to create a composite reinforced concrete wall.
    Photo: CTS Cement Manufacturing Corp.

    ArchWeek Image

    In the spirit of the California Case Study houses, most rooms have direct access into the central courtyard to encourage casual indoor-outdoor living.
    Photo: CTS Cement Manufacturing Corp.

    ArchWeek Image

    Vines were allowed to attach themselves to the solid concrete walls without risking damage to the structure.
    Photo: CTS Cement Manufacturing Corp.

    ArchWeek Image

    South patio.
    Photo: CTS Cement Manufacturing Corp.

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    The Rices in their living room.
    Photo: CTS Cement Manufacturing Corp.

    ArchWeek Image

    The CMUs in the walls used limestone aggregate and were custom tinted to resemble natural stone.
    Photo: CTS Cement Manufacturing Corp.

    ArchWeek Image

    Dining room.
    Photo: CTS Cement Manufacturing Corp.

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    The roof was easily retrofitted with photovoltaic arrays: the units were simply bolted to the exposed concrete deck without concern about penetrating and patching any roofing material.
    Photo: CTS Cement Manufacturing Corp.


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