Concrete compositions and methods for the preparation thereof

Details Concrete compositions and methods for the preparation thereof Concrete compositions and methods for the preparation thereof

2000-05-25

2002-05-14

Wood, Elizabeth D. (Department: 1755)

Compositions: coating or plastic

Coating or plastic compositions

Inorganic settable ingredient containing

C106S730000, C106S795000, C106S804000

Reexamination Certificate

active

06387171

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention is directed towards concrete compositions and methods for the preparation thereof. More specifically, the invention is directed in its preferred embodiments towards “lightweight” or low-density concrete compositions that generally comprise a cementitious material and an aggregate material that is relatively less dense than aggregate materials used in conventional concretes.
BACKGROUND OF THE INVENTION
Concrete is a hardened material that is formed by blending a cementitious material, typically Portland cement, with an aggregate material and water to form an uncured concrete composition, and subsequently curing the concrete composition. Conventionally used aggregate materials include gravel, shale, stone, and like materials. Concretes that are formulated with such conventional aggregate materials have a density that typically ranges from about 140 to 150 lbs./cu. ft. Such concrete is suitable for use in connection with many applications, and such concretes are particularly suitable for use in connection with load-bearing structural applications.
The prior art has recognized that in many applications “lightweight” concretes are desirable. “Lightweight” concrete, or low-density concrete, may be defined generally as a concrete with a density of less than the 140-150 lb./cu. ft. of conventional concretes. Such lightweight concretes often have a density of less than about 120 lbs./cu. ft. Lightweight concrete may be used in many applications including, for example, concrete construction blocks; insulating materials; decorative applications such as yard ornaments, patio or walkway blocks, and the like; upper-floor construction in multi-storied buildings; maritime applications, such as docks, floating bridges, boats, aircraft carrier decks, off-shore oil platforms, and the like; floors in steel frame buildings; roof structures or in roofing tiles; wallboard; spray-on applications; insulating and fire-proofing liners; backfill; and in numerous other applications.
The prior art has taught to prepare lightweight concrete using lightweight aggregates, including such materials such as perlite, vermiculite, polystyrene beads, polyethylene beads, pumice, scoria, tuff, or expanded materials such as shale, clay, slate, diatomite, fly ash, dross, or blast furnish slag. Many such aggregates are expensive and of limited availability. For example, polystyrene beads are petroleum-based and are in limited supply. Cinders generated in coal-fired boilers have been employed as aggregate materials, but increasing environmental awareness of the adverse impact of coal-fired boilers suggests a long-term supply problem. Similarly, materials such as pumice or scoria may require substantial labor or fuel costs or expensive and cumbersome processing equipment. Dross, a by-product of the iron and steel industries, can be expanded to form a lightweight aggregate by placing it into contact with water. However, dross is not uniform in its properties and thus may be difficult to process under exacting quality control conditions.
The prior art has further taught to prepare lightweight concretes by employing air voids in the concrete to reduce the overall concrete density. Generally, the prior art teaches to provide such air voids by entraining air in the concrete mix to form “cells” in the concrete matrix. While such methods generally will result in an overall decrease in cured concrete density, entraining air to form such cells often is less than a satisfactory solution. Air bubbles can compress at the bottom of deep concrete pours resulting in smaller cells than desired, thus resulting in a concrete of variable internal density and higher weight than desired. Air bubbles can also coalesce to form larger bubbles, thus resulting in a concrete of variable internal density and relatively reduced strength in the region of the air bubble.
More recently, U.S. Pat. No. 5,595,595 purports to teach the use of “aquagels,” or starch pastes, as an aggregate material in a concrete composition. According to the teachings of this patent, aggregate material is prepared by cooking starch until it gelatinizes, spraying this gelatinized starch into a water-immiscible liquid, washing the aquagel thus formed to remove the water-immiscible liquid, and storing the starch until it is ready to be incorporated into a concrete composition. Alternatively, the gelatinized starch may be retrograded, dried, and ground into appropriately sized particles. When preparing a concrete composition, the dried particles are hydrated to thereby swell the particles to cause an increase in the volume. Upon curing of the concrete composition, water is said to be liberated from the swollen particles, thus causing them to shrink to their unhydrated size thereby providing a plurality of voids, or “cells,” in the concrete.
The concrete compositions and aggregate materials postulated by the aforementioned U.S. Pat. No. 5,595,595 are unsatisfactory in a number of respects. For one thing, the preparation of the aquagel material in the manner taught is time-consuming, cumbersome, and expensive. In addition, the patent teaches that the aquagels preferably are chilled after washing to prevent bacterial growth and degradation, thus potentially entailing additional processing costs. Moreover, the dried and ground particles must be pre-hydrated in situ prior to adding to the other components of the concrete composition. The need for such in situ hydration is inconvenient and unwieldy, and potentially limits the applicability of the concrete composition.
THE INVENTION
The present invention is based upon the discovery that a concrete composition and, particularly, if desired, a lightweight concrete composition, can be prepared using water, a cementitious material, and an aggregate material that comprises granular starch, and more preferably, washed, pre-swollen crosslinked granular starch. Starch is an abundant and renewable natural resource, and is substantially inexpensive relative to other lightweight aggregate materials provided in the prior art. Moreover, especially when the starch is provided in the form of washed, pre-swollen crosslinked granules, there is no need to pre-hydrate the starch granules prior to mixing with the other components of the concrete composition. The invention thus can provide an inexpensive, lightweight composition that incorporates as an aggregate material an abundant, renewable resource.
In accordance with one embodiment of the invention, a curable composition comprising water, a cementitious material, and an aggregate material that includes a plurality of swollen starch granules is provided. It is believed that the starch granules will liberate water upon curing of the concrete composition, thus causing the volume of the granules to decrease. This deswelling provides a cured concrete composition that includes a plurality of cells similar to those formed by entraining air in the concrete, but of uniform size and dispersion in the concrete matrix. Thus, the invention also provides a cured concrete composition that comprises water, a cementitious material, and an aggregate that comprises a plurality of water-liberated swollen starch granules. The cured concrete composition preferably is a lightweight concrete composition, i.e., it has a cured bulk density of less than 120 lbs./cu. ft., more preferably, less than about 115 lb./cu. ft. The invention further encompasses a method for preparing a concrete composition, as well as a concrete composition prepared in accordance with this method.
Preferred features and other embodiments of the invention are set forth hereinbelow and in the appended claims.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The concrete composition of the invention generally comprises water, a cementitious material, and an aggregate material that, generally speaking, comprises granular starch. The water provided in the composition may be fresh water, tap water, sea water, natural or artificial brine, or, in general, any water source as may be determined suitable for use in a

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