Patent Application
20070056479
1. The Field of the Invention
The present invention relates to concrete mixtures and cured concrete obtained therefrom. More particularly, the present invention relates to concrete mixtures incorporating high carbon pozzolans (e.g. high carbon fly ash) and a foam that is stable in the presence of the pozzolans.
2. Related Technology
Concrete mixtures are composite materials that are usually composed of water, cement, and aggregate. Concrete is a well-known structural component with typical compressive strengths greater than about 2500 psi, when cured.
Pozzolans, such as fly ash, blast furnace slag, and natural pozzolans, are a class of materials that can be added to concrete to increase its strength and to impart other desirable characteristics. Pozzolans are a siliceous or siliceous and aluminous material that, in finely divided form, will react with calcium hydroxide in the presence of moisture to form a composition with cementitious properties. Fly ash is a man-made pozzolan resulting from the combustion of coal and typically contains silicon dioxide, aluminum oxide, and ferric oxide.
The use of pozzolanic materials as a partial replacement for Portland cement in concrete has become an increasingly attractive alternative to Portland cement alone. The desire to increase the use of pozzolans in concrete mixtures can be attributed to several factors. These include cement shortages, economic advantages of Portland cement replacement, improvements in the permeability of the concrete product, valued use of an otherwise waste product, and lower heats of hydration during curing.
The chemical composition of pozzolans, and in particular fly ash, can vary greatly depending on the source of the pozzolan and the conditions under which it was produced. Because fly ash is derived from coal, almost all fly ash contains some carbon. It is customary to quantify the amount of carbon in fly ash as a measure of loss of ignition (LOI). High carbon fly ash typically has an LOI greater than about 1.5%.
High carbon fly ash cannot be used in most concrete mixtures that require air entrainment because the carbon has a tendency to destroy the air bubbles. Air entrainment is used to improve the durability of concrete exposed to moisture during cycles of freezing and thawing. In addition, entrained air greatly improves the resistance of concrete to surface scaling caused by chemical deicers. Air entrainment also increases the workability of fresh concrete while eliminating or reducing segregation and bleeding. Because high carbon pozzolans usually hinder air entrainment, the use of high carbon pozzolans would require giving up the beneficial properties of air entrainment. Since the benefits of air entrainment often outweigh the benefits of using high carbon fly ash over low carbon fly ash, high carbon pozzolans are typically not used in concrete mixtures.
The incompatibility of air entraining agents and high carbon pozzolans is due to the chemical properties of air entraining agents and high carbon pozzolans. Air entraining agents are typically surfactants that are used to purposely trap microscopic air bubbles in the concrete. The surfactants and/or the aqueous bubbles formed by most surfactants are not compatible with carbon, which tends to rupture the aqueous bubbles. Consequently, most concrete manufacturers use low carbon pozzolans or no pozzolans.
The present invention relates to concrete mixtures incorporating high carbon pozzolans (e.g. fly ash) and an aqueous foam (i.e. air entrainment). The aqueous foam preferably includes a fluorochemical surfactant that stabilizes the foam bubbles in the presence of the high carbon pozzolans. Surprisingly, the inventors of the present invention have found that this type of aqueous foam is stable in the presence of high carbon fly ash. In an exemplary embodiment, fly ash having an LOI greater than between about 1.5% and 6.0% can be used in the concrete mixtures of the present invention without significantly destroying the air entrainment provided by the foam. Preferably the high carbon fly ash is a type C fly ash, which can increase the strength of the cured concrete and/or reduce the amount of cement needed to achieve a particular strength of cured concrete.
Fluorochemical surfactants used in the foam admixtures of the present invention include fluorochemical surfactants such as those used in making firefighting foams. The aqueous nature and the stability of these stabilized foams make them particularly advantageous for use in the concrete mixtures of the present invention. The use of at least one such foam in a concrete mixture is described in U.S. Pat. No. 6,153,005 to Welker et al., which is incorporated herein by reference.
The foam used in the present invention is made from a foaming concentrate before it is mixed with the cement and/or aggregate. In an exemplary embodiment, the foaming concentrate comprises a combination of foaming agents, solvents and/or viscosity modifiers or other components that when mixed with water and air can form foam. The fluorochemical surfactant, which is used to stabilize the foam, is also part of the foaming concentrate.
In one embodiment, the fluorochemical surfactant comprises an oligomer comprising hydrophilic nonionic monomers and hydrophilic anionic monomers. Foams stabilized using these surfactants provide additional benefits because of the interaction between the oligomer in the foam and certain components of the concrete mixture (e.g. the cement and/or the aggregate). It is believed that the nonionic and anionic monomers are able to better disperse the cement particles and/or the aggregates uniformly in the concrete mixture.
The foam admixture is also preferably stabilized in the concrete using a hydration stabilizer, which inhibits deleterious interactions between the foam and the cement. The hydration stabilizers used to reduce the reaction between the cement and the foam includes a hydration retarder that can slow or stop hydration of the siliceous and/or aluminous component of hydraulic cements.
The stabilizing effect of the fluorochemical surfactant is distinct and in addition to the stabilizing effect of the hydration stabilizer. The fluorochemical surfactant is a component of the foam bubble and provides stability within the bubble. In contrast, the hydration stabilizer is a component of the concrete mixture to prevent deleterious interactions between the cement and the aqueous foam.
Concrete mixtures incorporating the high carbon pozzolans and stabilized foam can surprisingly maintain high percentages of air entrainment and strength. For example, concrete mixtures incorporating high carbon fly ash can maintain greater than 5%, more preferably 10%, air and achieve a compressive strength of greater than about 2500 psi in 28 days, more preferably greater than 3000 psi, and most preferably greater than 4000 psi. Highly cementitious high carbon fly ash can be used with the air entraining foams of the present invention to further increase the strength of the cured concrete.
These and other features of the present invention will become more fully apparent from the following description and appended claims.
The present invention relates to concrete mixtures incorporating high carbon pozzolans and a foam admixture (for air entrainment). The foam bubbles in the foam admixture are stabilized in the presence of the high carbon fly ash by a fluorochemical surfactant. The fluorochemical surfactant preferably comprises an oligomer comprising hydrophilic nonionic monomers and hydrophilic anionic monomers. The high carbon pozzolans preferably have an LOI greater than between about 1.5% and about 6.0% without substantially affecting the durability of the foam bubbles in the concrete mixture.
For purposes of the present invention, the “initial set time” occurs when the concrete reaches a compressive strength of 500 psi.
For purposes of the present invention, the term “air” includes all gases including mixtures of gases and pure gasses, whether obtained from the atmosphere or generated by chemical reaction.
The concrete mixtures of the present invention can have very different compositions depending on the desired use for the concrete. However, in general, the concrete mixtures of the present invention include at least hydraulic cement, a high carbon pozzolans, an aqueous foam admixture, and water (either from the foam or added separately). The concrete mixtures typically include an aggregate, which gives the concrete strength. Dispersants and other admixtures described below can also be included to give the concrete mixtures of the present invention desired properties.
A. Hydraulic Cement and Water
The cement included in the concrete mixtures of the present invention is a hydraulic material such as Portland cement, modified Portland cement, or masonry cement. For purposes of this invention, Portland cement includes all cementitious compositions which have a high content of tricalcium silicate, including Portland cement, cements that are chemically similar or analogous to Portland cement, and cements that fall within ASTM specification C-150-00. Other cementitious materials include ground granulated blast-furmace slag, hydraulic hydrated lime, white cement, slag cement, calcium aluminate cement, silicate cement, phosphate cement, high-alumina cement, magnesium oxychloride cement, and combinations of these and other similar materials.
Water is added to the concrete mixture in sufficient amounts to hydrate the cement. Those skilled in the art will recognize that the amount of water needed will depend on the desired flowability and on the amounts and types of admixtures included in the concrete composition. Many of these admixtures are discussed below. In general, suitable amounts of water for hydrating the cement ranges from about 1% to about 50%, more preferably about 5% to about 50%, and most preferably about 10% to about 25% of the concrete mixture by weight.
B. High Carbon Pozzolans
Any finely divided material that exhibits pozzolanic activity and includes high percentages of carbon can be used in the concrete mixtures of the present invention. In a preferred embodiment, the high carbon pozzolans are cementitious (i.e. they include calcium), in addition to having pozzolanic properties. Suitable sources of pozzolans include fly ash, blast furnace slag, and natural pozzolans.
The amount of carbon in the high carbon pozzolans of the present invention is measured according to the loss of ignition (“LOI”)(also known as loss on ignition). In a preferred embodiment, the high carbon pozzolans have an LOI greater than about 1.5%, more preferably greater than about 2.0%, even more preferably greater than about 4.0%, and most preferably greater than about 6.0%.
A particularly useful source of high carbon pozzolans is fly ash. Suitable fly ashes include Class C and Class F fly ash as defined in the ASTM C618 standard. As explained in the ASTM C618 standard, the Class F fly ash is normally produced from burning anthracite or bituminous coal, whereas Class C fly ash is normally produced from lignite or sub-bituminous coal.
The ASTM C618 standard differentiates Class F and Class C fly ashes primarily according to their pozzolanic properties. Accordingly, the major specification difference between the Class F fly ash and Class C fly ash is the minimum limit of SiO2+Al2O3+Fe2O3 in the composition. The minimum limit of SiO2+Al2O3+Fe2O3 Class F fly ash is 70% and for Class C fly ash it is 50%. Thus, Class F fly ashes are more pozzolanic than the Class C fly ashes.
Although not explicitly recognized in the ASTM C618 standard, Class C fly ashes typically contain high calcium oxide content, which makes Class C fly ash more cementitious. Thus, Class C fly ash is an example of a cementitious pozzolan, since it contains substantial amounts of calcium oxide.
The use of Class C fly ash can be particularly useful to replace a portion of the hydraulic cement in the concrete. Class C fly ash and Portland cement can be blended in weight ratios of ash to cement of about 0/100 to about 150/100, preferably about 75/100 to about 125/100. In a preferred blend of reactive powder, Portland cement is about 40 wt % to about 80 wt % and fly ash is about 20 wt % to about 60 wt %. Alternatively, the ratio of pozzolanic material to Portland cement is about 1.5:1 to about 1:20 by weight, more preferably 1:1.5 to about 1:5.
C. Foam Admixtures
The foams used in the concrete mixtures of the present invention include water, air, and at least one foaming agent (e.g. surfactant). The foam admixtures comprise a plurality of cellular bubbles that trap air in the concrete and provide the uncured concrete with desired properties such as flowability and/or workability and provide the cured concrete with properties such as increased strength and/or resistance to cracking during freeze thaw cycles.
The foams of the present invention can be made from a foaming concentrate. A detailed description of how to make the aqueous foam admixture using a foaming concentrate, air, and water is described below in connection with the methods of the present invention and the examples.
The composition of the foaming concentrate is in large part responsible for the properties of the foam admixture. The following is a description of suitable foaming concentrate composition. The foaming concentrates typically include a foaming agent, solvents, and other surfactants, and/or additives that allow the foaming concentrate to form foam bubbles that can be added to concrete.
1. Foaming Concentrates
(a) Foaming Agents and Solvents
The foaming concentrates include at least one foaming agent suitable for forming an aqueous based foam. Typical foaming agents include ionic, cationic, anionic surfactants, natural and synthetic resins, fatty acids, proteinaceous material, sulfonated hydrocarbons, and the like. In an exemplary embodiment, the foaming concentrate comprises a combination of foaming agents, solvents and/or viscosity modifiers or other components. In a preferred embodiment, the foaming agents can be a non-fluorinated anionic surfactant preferably selected from the group consisting of C8 to C18 anionic surfactants and most preferably, C10 to C18 alpha olefin sulfonates, as well as mixtures of these surfactants.
Suitable solvents include glycol ethers and fatty alcohols of about 8 to about 15 carbons or C2 to C8 aliphatic diols. Specifically preferred as the solvent, if used, is propylene glycol t-butyl ether. The preferred fatty alcohol comprises a mixture of equal parts n-dodecanol, n-tetra decanol and n-hexadecanol. Preferred as the anionic surfactant are mixtures of sodium alkenyl sulfonate, sodium tetradecene sulfonate, and sodium hexadecene sulfonate in a ratio of about 4:1:1.
Viscosity modifiers can also be added to the foam admixture. Suitable viscosity modifiers include those described below as admixtures to be added to the concrete mixture apart from the foam. Preferred viscosity modifiers include polysaccharide gums, preferably selected from the group consisting of rhamsan gums, xanthan gums, guar gums and locust bean gums. Viscosity modifiers such as polysaccharide gums can be useful in foams because they form a membrane on the surface of solvents that protects the rest of the foam from collapsing. The viscosity modifier in the foam can also aid in pumpability of the concrete mixture.
Other ingredients can be employed in the composition of the surfactant formulation to effect specific environmental or shelf-life concerns. Examples of such ingredients are freezing point depressants, such as ethylene glycol, and preservatives, such as that available under the trade name DOWICIDE (Dow Chemical Company).
There are many other known foaming concentrates that can be used with the present invention. Suitable foaming surfactant concentrates include cellulose based concentrates (e.g. “CELLUCON” (Romaroda Chemicals)) and hydrolyzed protein based concentrates (e.g. MEARL (The Mearl Corporation)).
The foam concentrates include surfactants and additives that lead to foams with sufficient mechanical stability to withstand the mixing involved in making concrete compositions. The foregoing concentrates, if needed, can be structurally strengthened (i.e. stabilized) using a fluorochemical surfactant as described below, or other similar stabilizing additive.
(b) Foam Stabilizing Surfactants
In an exemplary embodiment the foam concentrate includes a fluorochemical foam stabilizing surfactant. Fluorochemical foam stabilizing surfactants are well known in the art of fire fighting foams (e.g. U.S. Pat. Nos. 4,472,286; 4,717,744; and 4,859,349; which are incorporated herein by reference). Particularly useful fluorinated surfactants are those described in U.S. Pat. Nos. 4,460,480 and 5,218,021 to Kleine et al., and Clark et al., respectively, which are incorporated herein by reference, and which discloses an oligomer surfactant that includes (i) a fluorinated hydrocarbon monomer, (ii) a hydrophilic nonionic monomer, and (iii) a hydrophilic anionic monomer.
Examples of suitable fluorinated hydrocarbon monomer include (i) straight chain, branched chain, or cyclic perfluoroalkyls of 1 to about 20 carbon atoms, (ii) perfluoroalkyls substituted with perfluoroalkoxy groups of 2 to about 20 carbon atoms, (iii) a perfluoroalkyl oligomers or polymers of greater than 10 carbon atoms, or (iv) monomers of the like and/or combination thereof.
Many non-ionic hydrophilic monomers suitable for use in the present invention are known and commercially available. Particularly useful non-ionic hydrophilic monomers include acrylamide, methacrylamide, diacetone acrylamide, and 2-hydroxyethyl methacrylate. Other examples of such monomers include derivatives of acrylic, methacrylic, maleic, fumaric, and itaconic acids, such as hydroxyalkyl esters of acrylic acids; amides such as N-vinyl-pyrrolidone, N-(hydroxyalkyl)-acrylamides, or N-(hydroxyalkyl)-methacrylamides; and vinyl esters with 1-20 carbons in the ester group such as vinyl acetate, butyrate, laurate, or stearate. The above listed non-ionic hydrophilic monomers can be used alone or in combination with each other as well as in combination with suitable anionic hydrophilic monomers described below. Some non-ionic hydrophilic monomers may require a co-monomer for polymerization, such as di(hydroxyalkyl) maleates with ethoxylated hydroxyalkyl maleates.
Many anionic hydrophilic monomers that co-oligomerize with non-ionic hydrophilic monomers are known and are commercially available. Particularly useful anionic hydrophilic monomers include acrylic and methacrylic acids and salts thereof. Other examples of such monomers include maleic, fumaric, and itaconic acids and salts thereof; acrylamidopropane sulfonic acid and salts thereof; and mono-olefinic sulfonic and phosphonic acids and salts thereof.
Oligomers made using the foregoing fluorinated hydrocarbon monomers and hydrophilic monomers are particularly advantageous when used with foams in the concrete mixtures of the present invention. These oligomers are particularly useful for dispersing and/or suspending the cement and/or aggregates in the concrete mixture. By dispersing and/or suspending these and other components of the concrete mixture, the resulting cured concrete has improved strength and resistance to cracking.
In an exemplary embodiment, the fluorochemical foam stabilizers of the present invention can be characterized by chemical moieties represented by the general formula, Rf-Ea-(S)b-[M1]x-[M2]y-H (Formula I), and mixtures thereof, wherein:
Rf is (i) a straight chain, branched chain, or cyclic perfluoroalkyl of 1 to about 20 carbon atoms, (ii) a perfluoroalkyl substituted with a perfluoroalkoxy of 2 to about 20 carbon atoms, (iii) a perfluoroalkyl oligomer or polymer of greater than 10 carbon atoms (e.g. hexafluoropropylene oxide), or (iv) a mixture of perfluoroalkyl moieties;
Ea is (i) a direct bond, (ii) a branched, straight chain, or cyclic alkylene of 2 to about 20 carbon atoms, (iii) the alkylene of (ii) interrupted by one or more groups selected from the group consisting of —NR—, —C—, —S—, —SO2—, —COO—, —OOC—, —CONR—, —NRCO—, —SO2NR—, —NRSO2—, —SiR2—; or (iii) the alkylene of (ii) terminated at the Rf end with —CONR— or —SO2NR—;
R is independently hydrogen, an alkyl of 1-10 carbon atoms, or a hydroxyalkyl of 2 to 10 carbon atoms; and
a and b are independently 0 or 1; -[M1]- represents a non-ionic hydrophilic monomer unit; -[M2]- represents an anionic hydrophilic monomer unit; and x and y represent the number of monomer units present in the co-oligomers and are both greater than 0; the sum of x andy being between 5 and 200, and y/(x+y) being between 0.01 and 0.98.
Formula (I) does not necessarily depict the actual sequence of the oligomer or macromer units since the units can be randomly distributed throughout. It is also assumed that the monomers for M1 and M2 are known as described above.
(c) Exemplary Foaming Concentrates
Table 1 below sets forth exemplary foaming concentrates according to the present invention. In Table 1, column 1 specifies the useful ranges for each component, column 2 specifies preferred ranges for each of the components and column 3 describes the highly preferred ranges for each of the components. In Table 1, all compositions are in parts by weight. The fluorochemical surfactant is normally supplied as a solution in an alcohol such as tert-butyl alcohol.
A particularly useful foaming concentrate that includes a fluorochemical surfactant is sold by Miracon Technologies, Inc. under the trademark Miracon®.
D. Hydration Stabilization
The hydration stabilizer (also known as an extended set retarder) of the present invention can be used to inhibit the hydration of the hydraulic cement. The hydration stabilizer slows the rate of hydrate formation by tying up (i.e. chelating, complexing, or otherwise binding) calcium ions on the surface of cement particles. The hydration stabilizer includes a hydration retarder that forms a protective barrier around cementitious particles. The hydration retarder bonded to the cement particles acts as a dispersant preventing hydrates from flocculating and setting. This barrier prevents the hydraulic cement from obtaining initial set.
Another feature of the hydration stabilizer is that it degrades and/or is inactivated over time such that hydration of the cement eventually occurs. Preferably the release of the hydraulic cement is progressive over time so as to provide a controlled release of the cement and an ascertainable delay in set time.
The hydration stabilizer preferably comprises a calcium chelating compound such as a polyphosphonic acid or a carboxylic acid that contains hydroxyl and/or amino groups. Polyphosphonic acids and similar compounds can be particularly advantageous because of their controlled degradation in the concrete mixture over an extended period of time that allows for a timed setting of the concrete.
Suitable examples of hydration stabilizers include N-nitrilo tris(methylene phosphonic acid); 1,2-ethanediyl bis[nitrilo di(methylene phosphonic acid)]; 1,6-hexanediyl bis[nitrilo di(methylene phosphonic acid)] and the like.
Another class of suitable phosphonic acid hydration stabilizing compounds include polymethoxy polyphosphonic acids represented by the formula II
wherein x and y are each an integer of from 1-3, and preferably 1, and z is an integer of 0 or 1. It is understood that when z is 0 the radical within the bracket is non-existent and, therefore (OCH2)y is nonexistent. The preferred polymethoxy polyphosphonic acid compounds are represented by the above formula when z=0 and x is 1-3. Other suitable polymethoxy polyphosphoic acid compounds are disclosed in U.S. Pat. No. 5,215,585, which is incorporated herein by reference.
A particularly useful hydration stabilizer is amino tris (methylene phosphonic acid), which is a component of the commercially available hydration stabilizer sold by Master Builders under the trademark Delvo. Illustrative examples of hydration stabilizers, including some of those mentioned above, are set forth in U.S. Pat. Nos. 5,427,617 and 5,203,919, which are incorporated herein by reference. Hydration retarders and accelerators suitable for use as hydration stabilizers are also disclosed in U.S. Pat. No. 6,858,074, which is also incorporated herein by reference.
As mentioned, the hydration stabilizer of the present invention prevents or inhibits setting and then degrades or is released from the cement to provide controlled setting. In some cases, it is necessary that the hydration stabilizer also comprise an accelerator to cause the controlled hydration of the cement. The amount of accelerator that needs to be added depends on several factors, such as the amount of hydration retarder, cement type and reactivity, ambient temperature, concrete mixture proportions, and the presence or absence of certain admixtures in the concrete mixture, such as water reducing polymers.
Accelerators that can be used to activate the hydraulic cement can be selected from conventional cement accelerators such as those classified as ASTM C 494 Type C admixtures. These include alkali metal halides (calcium chloride and the like), alkali metal nitrites (calcium nitrite and the like), alkali metal nitrates (calcium nitrate and the like), alkali metal formates (calcium formate and the like), alkali metal thiocyanates (sodium thiocyanate and the like), triethanolamine and the like. The particular set accelerator to be used will depend on the known nature of the accelerators and side effects of the agent. For example, where metal corrosion is not a problem, calcium chloride might be chosen, while if corrosion is a problem, the nitrite or nitrate salts might be better used. The preferred accelerators are calcium nitrate and the like.
The accelerating agent should be added in amounts which effectively cause the combined cement mixture to set and provide conventional 28 day strength for such compositions (e.g. mortars of about 2000-4000 psi; concrete of about 2,500 to 10,000 psi). The amount, based on cement content, should be from about 0.5 to about 6 weight percent, preferably from about 1 to about 5 percent.
The hydration stabilizer is mixed with the cement mixtures in amounts effective to prevent the hydraulic cement from reacting with the aqueous foam for a desired period of time. The specific effective amount depends on the amount and type of cement and the desired amount of stabilization. Preferably, a sufficient amount of hydration stabilizer is included in the concrete mixture to stabilize substantially all of the cement. Suitable amounts typically require from about 1.5 oz to about 8.0 oz per hundred lbs of cement, more preferably about 3.0 oz to about 6 oz, for a concrete mix having a 28 day cure time. The stabilization can be extended by adding about 4 oz of hydration stabilizer per 100 lbs cement per hour of extension.
Hydration stabilizers are known and used in the concrete industry for waste water reclamation and for reusing concrete mixtures. Currently, hydration stabilizers are added to concrete waste water so that the truck or other mixing machinery does not have to be washed out after use or so that the remaining concrete can be used on another job. The hydration stabilizer prevents setting until the cement can be reused. The inventor of the present invention has found that the properties and concentrations of hydration retarders used in these known hydration stabilizing compositions are surprisingly advantageous for stabilizing foam admixtures according to the present invention. Commercially available hydration stabilizers, in addition to Delvo mentioned above, include Recover (W. R. Grace), Delayed Set (Fritz-Pak Corp.), Stop-Set and Stop-Set L (Axim Italcementi Group), and Polychem Renu (General Resource Technology).
E. Dispersants and/or Water-Reducers
Water reducers are used in concrete mixtures to lower the water content in the plastic concrete (i.e. uncured concrete) to increase its strength and to obtain higher slump without adding water. Water-reducers will generally reduce the required water content of a concrete mixture for a given slump and are useful for pumping concrete and in hot weather to offset the increased water demand. These admixtures disperse the cement particles in the concrete and make more efficient use of the hydraulic cement.. This dispersion increases strength and/or allows the cement content to be reduced while maintaining the same strength. Water-reducers should meet the requirements for Type A in ASTM C 494 Specification.
Another class of water reducers includes mid-range water reducers. These water reducers have a greater ability to reduce the water content of the concrete and are often used because of their ability to improve the finishability of concrete flatwork. Mid-range water reducers should at least meet the requirements for Type A in ASTM C 494.
High range water-reducers (HRWR), also referred to as superplasticizers, are a special class of water-reducer. HRWRs reduce the water content of a given concrete mixture by about 12% to 30%. HRWRs are used to increase strength and reduce permeability of concrete by reducing the water content in the mixture or greatly increase the slump to produce “flowing” concrete without adding water. HRWRs are often used for high strength and high performance concrete mixture that contain higher contents of cementitious materials and mixtures containing silica fume. In a typical concrete mixture, adding a normal dosage of HRWRs to a concrete mixture with a slump of 3 to 4 inches (75 to 100 mm) will produce a concrete with a slump of about 8 inches (200 mm). Exemplary HRWRs that can be used in the present invention are covered by ASTM Specification C 494 and types F and G, and Types 1 and 2 in ASTM C 1017. Particularly advantageous dispersants include the HRWRS described in U.S. Pat. No. 6,858,074, which is incorporated herein by reference.
It is believed that water reducing dispersants may have a particularly beneficial effect on the concrete compositions of the present invention by working in conjunction with the hydration stabilizer to stabilize the foam admixtures of the present invention.
F. Aggregates
Aggregates are usually included in the concrete mixture to add bulk and to give the concrete strength. The aggregate can be a fine aggregate and/or a coarse aggregate. The fine aggregates are materials that pass through a Number 4 sieve (ASTM C125 and ASTM C33), such as silica sand. The coarse aggregate are materials that are retained on a Number 4 sieve (ASTM C125 and ASTM C33), such as silica, quartz, crushed round marble, glass spheres, granite, limestone, calcite, feldspar, alluvial sands, or any other durable aggregate, and mixtures thereof.
Whether an aggregate needs to be added can depend on the desired use of the cured concrete and on the type of aqueous foam admixture that is used. Some aqueous foam admixtures of the present invention are sufficiently stabilized to function as a foam aggregate. For example, the air bubbles in aqueous foams that are stabilized with a fluorochemical surfactant can have sufficient strength to act as a foam aggregate. In particular, foams stabilized with fluorochemical surfactants that include hydrophilic nonionic and hydrophilic anionic monomers are particularly suited to act as foam aggregates. It is believed that the anionic and nonionic monomers are able to disperse the hydraulic cement around the foam bubbles thereby creating a cement matrix similar to the cement matrix that forms around aggregates.
The concrete mixtures of the present invention also include concrete mixtures that include traditional aggregates (i.e. coarse and fine aggregates) in combination with foam aggregates (e.g. aqueous foams stabilized with surfactants having nonionic and anionic monomers). Concrete mixtures of the present invention that incorporate a combination of foam aggregates with fine aggregates and/or coarse aggregates can be made to have superior compressive and flexural strength and/or can include ratios of aggregate sizes that are not possible with traditional concrete mixtures.
For example, ready mixed concrete used in flat work or foundation walls typically has a ratio of fine aggregates to coarse aggregates of 50:50. This ratio can be usually be modified to ratios from 60:40 to 40:60. Using the aqueous foams stabilized with a fluorochemical surfactant, the concrete mixtures of the present invention can be made using ratios of less than 40% of either fine aggregates or coarse aggregates while still maintaining ASTM standards for flexural and compressive strength. In an exemplary embodiment, the aggregate can comprise less than 40% fine aggregate, less than 30% fine aggregate, less than 20% fine aggregate or even substantially no fine aggregate. Alternatively, the aggregate can comprise less than 40% coarse aggregate, less than 30% coarse aggregate, less than 20% coarse aggregate, or even substantially no coarse aggregate. Even with these low percentages of coarse or fine aggregate, a compressive strength of greater than 2500 psi, more preferably greater than 3000 psi, or most preferably greater than 4000 psi can be achieved.
The use of only one size of aggregate is particularly beneficial in areas where both coarse and fine aggregates are not available or a particular size aggregate is in abundance. Also, the concrete mixtures of the present invention are particularly useful for incorporating certain aggregates sizes such as ⅜ inch gravel (i.e. pea gravel), that cannot be used in some concrete mixtures because it leads to lower quality concrete. With the concrete mixtures of the present invention, pea gravel can be used while still maintaining suitable compressive strength (e.g. 3000-4000 psi).
Thus, using the foam aggregates of the present invention, novel combinations of aggregates can be used to make concrete having suitable strength for ready mixed concrete and other applications.
G. Viscosity Modifiers
Viscosity modifiers, also known as viscosity modifying agents (VMA), as Theological modifiers or rheology modifying agents, can be added to the concrete mixture of the present invention. These additives are usually water-soluble polymers and function by increasing the apparent viscosity of the mix water. This enhanced viscosity facilitates uniform flow of the particles and reduces bleed, or free water formation, on the fresh paste surface.
Suitable viscosity modifiers that can be used in the present invention include, for example, cellulose ethers (e.g., hydroxyethyl cellulose (HEC), hydroxyproplmethyl cellulose (HPMC), sodium carboxymethyl cellulose (CMC), carboxymethylhydroxyethyl cellulose (CMHEC), and the like); synthetic polymers (e.g., polyacrylates, polyvinyl alcohol (PVA), polyethylene glycol (PEG), and the like); exopolysaccharides (also known as biopolymers, e.g., welan gum, xanthan, rhamsan, gellan, dextran, pullulan, curdlan, and the like); marine gums (e.g., algin, agar, carrageenan, and the like); plant exudates (e.g., locust bean, gum arabic, gum Karaya, tragacanth, Ghatti, and the like); seed gums (e.g., Guar, locust bean, okra, psyllium, mesquite, and the like); starch-based gums (e.g., ethers, esters, and related derivatized compounds). See, for example, Shandra, Satish and Ohama, Yoshihiko, “Polymers In Concrete”, published by CRC press, Boca Ration, Ann Harbor, London, Tokyo (1994).
Viscosity modifying agents are typically used with water reducers in highly flowable mixtures to hold the mixture together. Viscosity modifiers can disperse and/or suspend components of the concrete thereby assisting in holding the concrete mixture together. This property of viscosity modifiers makes them useful for making self compacting concrete, which requires high flowability.
Some foam admixtures of the present invention (e.g. fluorochemical stabilized foams having nonionic and anionic monomers) can act as a viscosity modifying agent thereby reducing the need for a separate viscosity modifier in the concrete mix. Highly flowable and/or self-compacting concrete can be achieved with these foams of the present invention while substantially reducing the amount of viscosity modifier included separately in the concrete mix. In an exemplary embodiment, viscosity modifier is added to the concrete mix in an amount less than 12 oz/100 wt, more preferably less than 9 oz, even more preferably less than about 5 oz, and most preferably substantially no viscosity modifier is added apart from the foam admixture.
Even with no additional viscosity modifier added to the concrete mixture, the concrete mixtures of the present invention can be highly flowable. Water and Low-range, mid-range, and/or high range water reducers can be added to the concrete mixture to give the concrete mixture a high flowability without the concrete separating because the foam admixture acts as a very good viscosity modifier. Concrete according to the present invention can be manufactured to have a “flow spread” of greater than 24-36 inches (using a 12 inch slump cone). The high spread of the concrete composition of the present invention are particularly advantageous because the spread is homogenous. The foam admixtures of the present invention can suspend the aggregate and other components in the concrete mixture such that mixes that spread greater than about 24-36 inches are spread substantially homogenous.
H. Other Admixtures
Many other types of admixtures can be added to the concrete compositions of the present invention to give the concrete a desired property. As discussed below, other admixtures suitable for use in the concrete mixtures of the present invention include but are not limited to viscosity modifiers, corrosion inhibitors, pigments, wetting agents, water soluble polymers, strength enhancing agents, rheology modifying agents, water repellents, fibers, permeability reducers, pumping aids, fungicidal admixtures, germicidal admixtures, insecticidal admixtures, finely divided mineral admixtures, alkali reactivity reducer, bonding admixtures, and any other admixtures or additive that do not adversely affect the stabilized foam or hydration stabilizers of the present invention.
Corrosion inhibitors in concrete serve to protect embedded reinforcing steel from corrosion due to its highly alkaline nature. The high alkaline nature of the concrete causes a passive and noncorroding protective oxide film to form on the steel. However, carbonation or the presence of chloride ions from deicers or seawater can destroy or penetrate the film and result in corrosion. Corrosion-inhibiting admixtures chemically arrest this corrosion reaction. The materials most commonly used to inhibit corrosion are calcium nitrite, sodium nitrite, sodium benzoate, certain phosphates or flourosilicates, fluoroaluminates, amines, organic based water repelling agents, and related chemicals.
Dampproofing admixtures reduce the permeability of concrete that have low cement contents, high water-cement ratios, or a deficiency of fines in the aggregate. These admixtures retard moisture penetration into dry concrete and include certain soaps, stearates, and petroleum products.
Permeability reducers are used to reduce the rate at which water under pressure is transmitted through concrete. Silica fume, fly ash, ground slag, natural pozzolans, water reducers, and latex can be employed to decrease the permeability of the concrete.
Pumping aids are added to concrete mixes to improve pumpability. These admixtures thicken the fluid concrete, i.e., increase its viscosity, to reduce de-watering of the paste while it is under pressure from the pump. Among the materials used as pumping aids in concrete are organic and synthetic polymers, hydroxyethylcellulose (HEC) or HEC blended with dispersants, organic flocculents, organic emulsions of paraffin, coal tar, asphalt, acrylics, bentonite and pyrogenic silicas, natural pozzolans, fly ash and hydrated lime.
Bacteria and fungal growth on or in hardened concrete may be partially controlled through the use of fungicidal, germicidal, and insecticidal admixtures. The most effective materials for these purposes are polyhalogenated phenols, dialdrin emulsions, and copper compounds.
Fibers can be distributed throughout a fresh concrete mixture to strengthen it. Upon hardening, this concrete is referred to as fiber-reinforced concrete. Fibers can be made of zirconium materials, carbon, steel, fiberglass, or synthetic materials, e.g., polypropylene, nylon, polyethylene, polyester, rayon, high-strength aramid, or mixtures thereof.
The shrinkage reducing agent which can be used in the present invention can include but is not limited to alkali metal sulfate, alkaline earth metal sulfates, alkaline earth oxides, preferably sodium sulfate and calcium oxide.
Alkali-reactivity reducers can reduce the alkali-aggregate reaction and limit the disruptive expansion forces in hardened concrete. Pozzolans (fly ash and silica fume), blast-furmace slag, salts of lithium, and barium are especially effective.
Bonding admixtures are usually added to hydraulic cement mixtures to increase the bond strength between old and new concrete and include organic materials such as rubber, polyvinyl chloride, polyvinyl acetate, acrylics, styrene butadiene copolymers, and other powdered polymers.
Natural and synthetic admixtures are used to color concrete for aesthetic and safety reasons. These coloring admixtures are usually composed of pigments and include carbon black, iron oxide, phthalocyanine, umber, chromium oxide, titanium oxide and cobalt blue.
Air entrainers can be included in the concrete mixtures of the present invention, although this is usually not necessary since the foam admixtures provide an air entraining features. Unlike foam, air entrainers are added to the concrete mixture in a liquid form (i.e. without the air entrapped).
The present invention also includes methods for making foam and methods for incorporating those foams into concrete mixtures.
A. Method of Making Foam Admixtures
As discussed above, in an exemplary embodiment, the foam admixtures are manufactured from a foam concentrate. Foam production is performed by drawing water and the foam concentrate, in proper ratios, and injecting them into a chamber using high pressure air. The mixture is subjected to shearing forces that produce air bubbles (i.e. foam) in the chamber.
The ratio of water, foam concentrate, and air are controlled to produce air bubbles of a desired size and shape. In a preferred embodiment, the water and foam concentrate are mixed to form a diluted concentrate comprising between at least about 80% water, more preferably greater than about 90% water and more preferably greater than about 95% water. The amount of air injected into the diluted foam is controlled by the air pressure and volume of air.
In an exemplary embodiment, air bubbles are formed. having an approximate size of about 0.3 ml to about 1.0 ml, more preferably from about 0.5 ml to about 0.7 ml. In a preferred embodiment, the bubbles are of uniform size and shape. It is believed that the uniform size and shape is beneficial for providing a uniform dispersion of cement and aggregate.
Any number of foam production devices can be used for producing the foam admixtures of the present invention, and the invention is not limited to any specific such device. Such devices are well known in the art. Whatever mechanism used, it should be adequate to produce a stream of bubbles suitable for introduction into the concrete mixtures of the present invention.
The following formula “Concentrate I,” shown in the table below, illustrates a suitable concentrate for manufacturing a foam admixture according to the present invention.
Concentrate I can be used to form a stable and resilient aqueous foam admixture by diluting Concentrate I to 2.5 w/w % water (39 parts water to one part Concentrate I) and then aerating it through a foam generating chamber at about 92 psi, thereby subjecting the diluted Concentrate I to shearing forces that produce an aqueous foam admixture.
B. Method of Mixing Concrete
The concrete mixtures of the present invention are manufacture by mixing proper amounts of a hydraulic cement, a high carbon pozzolanic material, water, and foam admixture. Typically the concrete mixture also includes aggregate. However, for some non-structural applications, aggregate may not be necessary. For concrete that needs a longer lifetime (e.g. greater than 15 minutes) a hydration stabilizer can also be used. Dispersants and other admixtures can be added as needed to give the concrete mixture desired properties.
Typically the hydraulic cement, the high carbon pozzolanic material, water, and foam are mixed together in any order so long as the water is added before the foam. The foam admixture is preferably added last so as to avoid the highest heat of hydration of the hydraulic cement.
It has also been found that the high carbon fly ash adsorbs water. To obtain desired flowability and hydration, additional water and/or water reducer can be added. In an exemplary embodiment, the amount of water reducer is increased by a factor of 1.25-4 as compared with the same concrete mixture that includes a low carbon fly ash.
The amount of foam mixed into the concrete mixture is selected to give the concrete a desired percent of air. In an exemplary embodiment, the foam admixture is included in the concrete mixture in an amount sufficient to provide greater than 5% air in the concrete, more preferably greater than 10% air, and most preferably greater than about 15% air by volume of the concrete mixture.
The limit on the amount of foam that can be added depends on the desired final strength of the concrete and the amount of cement and pozzolans in the mixture. In general, lower percentages of air and higher amounts of cement and pozzolans produce stronger cements. However, the concrete mixtures of the present invention can have very high compressive strengths with percentages of air above 5%, as compared with existing concretes. Using the aqueous foams of the present invention, air can be entrained into the concrete mixture in percentages greater than 5%, 10%, and even 20% while maintaining compressive strengths of greater than about 2500 psi, and more preferably greater than about 3000 psi, in 28 days. For example, concrete mixtures according to the present invention having 12% air and 300 lbs/yd3 of cement can achieve about 3000-4000 psi in 28 days. In another example, concrete mixture according to the present invention having 22% air and 650 lbs/yd cement can achieve a compressive strength of about 6500 psi in 28 days. Even at very high percentages of air, significant compressive strength can be achieved. For example, concrete mixtures of 85% air can achieve 90 psi in 28 days. The addition of cementitious pozzolanic materials also improves compressive strength at 28 days.
Table 2 below provides 3 different exemplary ranges of typical amounts of the components needed to make concrete mixtures according to the present invention.
In the compositions described in Table 2, the foam admixture is manufactured as described above (e.g. Concentrate I diluted to 2.5 w/w % water and foamed at 92 psi). The weight of the foam admixture includes the weight of the water that makes up the foam. Water is also included as a separate component, which does not include the water in the aqueous foam admixture.
Compositions made using the foregoing formulas can maintain air entrainment despite the presence of high carbon pozzolans. It is believed that the stability of the foam makes the foam bubbles resistant to the deleterious effects of carbon on the surfactants in the foam.
If a hydration stabilizer is included in the concrete mixture, a sufficient amount of hydration stabilizer is included to stabilize the foam admixture for a desired amount of time. The sufficiency of the hydration stabilizer can depend in part on the type of concrete being made. For ready mixed concrete, where transportation is often needed, the hydration stabilizer can be added in greater amounts such that the stabilization lasts during transportation and until the concrete has set.
The amount of hydration stabilizer used in the concrete mixture is determined by the need to stabilize the cement with respect to the foam. A sufficient amount of hydration stabilizer will inhibit the reaction between the cement and the foam such that a majority or substantially all of the foam initially mixed into the concrete lasts until the concrete achieves initial set. Typically, less than 2% of the foam collapses within the first 2 hours.
Regardless of whether the concrete is designed to set in a short period (e.g. less than 1 hour) or over along period of time, the hydration stabilizer can be very beneficial to stabilizing the foam admixture. The cement in a concrete mixture is most reactive, and thus most destructive to the foam admixture, when it is first mixed with the water. Consequently, the hydration stabilizer provides its greatest benefit during initial mixing of the cement, foam, and water, although significant benefits can still be realized by including the hydration stabilizer after initial mixing. The use of hydration stabilizer during the initial mixing of the concrete differs from most current practices, which add hydration stabilizer to waste concrete after a job is finished or during a job to preserve the concrete for later use. In the present invention, the hydration stabilizer is added to prevent the reaction of the foam and the cement and/or the cementitious pozzolans. Accelerators can be added anytime during or after mixing to control set time, if needed.
Those skilled in the art will recognize that there are many applications in which the combination of high carbon pozzolans, hydraulic cement, and foam according to the present invention can be easily incorporated into a concrete composition using the foregoing description. The following examples give specific formulations of aqueous foams and concrete composition that employ the concepts of the present invention.
Examples 1-4 describe concrete compositions according to the present invention. Example 1 describes a concrete mixture comprising high carbon, type-C fly ash with an LOI of 4.5%. The cured concrete had a compressive strength of 3131 at 28 days and air entrainment of 7.9%.
Example 2 describes a concrete mixture comprising high carbon, type-F fly ash with an LOI of 4.5%. The cured concrete had a compressive strength of 2103 at 28 days and air entrainment of 11%.
Example 3 describes a concrete mixture comprising high carbon, type-F fly ash with an LOI of 7%. The cured concrete had a compressive strength of 2720 at 28 days and air entrainment of 10.2%.
Example 4 describes a concrete mixture comprising type-F fly ash with an LOI of 1.1%. The cured concrete had a compressive strength of 3342 at 28 days and air entrainment of 8.7%.
The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/715,458, filed Sep. 9, 2005, entitled “Concrete Mixtures Having Aqueous Foam Admixtures,” the disclosure of which is incorporated herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60715458 | Sep 2005 | US |
