 |
|
|
|
|
|
In this section:
|
|
|
|
|
|
The majority of regular concrete produced is in the density range of 150 pounds per cubic foot (pcf). The last decade has seen great strides in the realm of dense concrete and fantastic compressive strengths (up to 20,000 psi) which mix designers have achieved. Yet regular concrete has some drawbacks. It is heavy, hard to work with, and after it sets, one cannot cut or nail into it without some difficulty or use of special tools. Some complaints about it include the perception that it is cold and damp. Still, it is a remarkable building material - fluid, strong, relatively cheap, and environmentally innocuous. And, it is available in almost every part of the world. Regular concrete with microscopic air bubbles added up to 7% is called air entrained concrete. It is generally used for increasing the workability of wet concrete and reducing the freeze-thaw damage by making it less permeable to water absorption. Conventional air entrainment admixtures, while providing relatively stable air in small quantities, have a limited range of application and aren't well suited for specialty lightweight mix designs. Lightweight concrete begins in the density range of less than 120 pcf. It has traditionally been made using such aggregates as expanded shale, clay, vermiculite, pumice, and scoria among others. Each have their peculiarities in handling, especially the volcanic aggregates which need careful moisture monitoring and are difficult to pump. Decreasing the weight and density produces significant changes which improves many properties of concrete, both in placement and application. Although this has been accomplished primarily through the use of lightweight aggregates, since 1928 various preformed foams have been added to mixes, further reducing weight. The very lightest mixes (from 20 to 60 pcf) are often made using only foam as the aggregate, and are referred to as cellular concrete. The entrapped air takes the form of small, macroscopic, spherically shaped bubbles uniformly dispersed in the concrete mix. Today foams are available which have a high degree of compatibility with many of the admixtures currently used in modern concrete mix designs. Gecko Stone of Hawaii is currently experimenting with one such foam. Foam used with either lightweight aggregates and/or admixtures such as fly ash, silica fume, synthetic fiber reinforcement, and high range water reducers (aka superplasticizers), has produced a new hybrid of concrete called lightweight composite concrete, or LWC. |
|
For the most part, implementation of Lightweight Composite design and construction utilizes existing technology. Its uniqueness, however, is the novel combination drawing from several fields at once: architecture, mix design chemistry, structural engineering, and concrete placement. Given the hoops that any new material or method must go through, implementation of LWC construction can be much at the mercy of any number of bureaucratic standards (licenses, approvals, etc.) including fire ratings, material test data, environmental impacts, as well as opposition from labor unions and existing suppliers supporting the lumber industry. Bureaucratic standards are sometimes easy enough to achieve, but only if one has deep pockets. But these costs are pretty much out of range for the average entrepreneurs in this field. These individuals also have found reluctance within the ready mix industry to take the initiative for R&D... their natural conservatism and relative success in the last four decades only has reinforced their will to keep things the same without added risk. They wait for the entrepreneur's homework. Other technologies, such as synthetic fiber manufacturers, also wait for the entrepreneur. It seems leadership, unfortunately, is not likely to come from the industries with the most available resources, but from those individuals who not only have a vision for the future, but a persistent mission to make it a reality. LWC construction can be a partial solution for several environmental problems. Deforestation could be substantially reduced by relinquishing the demand for timber used in residential construction. Wood can be used for decorative trim rather than structural elements. Steel can be used for what it does best - strength. Plastics can be recycled and used as secondary reinforcement. Used glass and other waste materials such as expanded polystyrene (EPS) can be converted to lightweight aggregate. Structural lumber requires treatment with poisons; steel is a dirty - albeit necessary - industry. The use of structural steel with concrete may be curtailed in the future by the development of high strength plastic-ceramics such as glass, basalt, carbon, and other fibers. Considering all the positive characteristics of LWC, it is surprising so little attention has been given to its possibilities. This may be due to an uniformed public, a tradition to use wood for construction, and a bad rep for a previously cold, stoney product. Yet homes and other buildings can achieve and maintain a higher degree of function and permanence, resulting in greater intrinsic value. The only limitations are the restraint of our imaginations, the reluctance of humanity to try anything new, and the resistance of unions to efficient construction. The latter may influence inordinate restrictions imposed by local, state, and federal building codes. But recent large natural disasters, such as fires and hurricanes, are now prompting people to take a closer look at shelter permanency being of greater value than consumer construction. The potential for the five hundred year house is here. |
|
Strength is a relative term. Concrete mixes should be designed based on end use. High compressive strength is useful where deadload or abrasion are factors, but unnecessary for roofs and non-structural partitions. All concrete is deficient in tensile and shear strengths, however these are supplemented through structural reinforcement. Compressive strength in cellular concrete at 105 pcf has achieved over 7500 psi, far exceeding most structural requirements. | |
|
Innovation has always been met with resistance. In the United States, the concrete industry has been especially notable for this. Also, building codes are designed to favor the existing trades by stipulating redundancy and specifying uniformity to arbitrary standards (read R. Buckminster Fuller's book, "Critical Path"). Just as the chariot axle width eventually dictated contemporary wheeled travel, the commonly accepted 16" modulus for construction was based on multiples of the ordinary brick. Our sense of historical construct is skewed by illogical processes and the use of erroneous paradigms. However, LWC is in common use in many third world countries lacking preconceived notions of how things ought to be. Unfortunately, like most good things in life, you have to seek them out. | |
|
Blame the architects who design ugly things. Designers resort to the box look either because they lack imagination or they just want to get something up as cheaply as possible. Keep in mind, there's no shortage of residential wood boxes or plastic siding either. Concrete can be virtually any shape, color, or texture. People need to shed their preconceived notions of what concrete buildings will look like. | |
|
| |
|
For addtional information on foaming agent or foam generators, select the Information Request Form button, below. For applications of LWC for residential construction, go to the section on Secure Concrete Houses, below. After that, read the section on Permanent Shelter.
|
|
|
|
Includes these topics:
|
|
|
|
|
|
|
HOME