Patent Application
20090178590
1. Field of the Invention
The presently disclosed invention relates to methods for reinforcing cementitious materials and, more particularly, reinforcing cementitious materials that are used for high-temperature applications.
2. Description of the Prior Art
For many years, various types of reinforcing have been used to strengthen various compositions. For example, U.S. Pat. No. 5,636,492 to Dingler describes construction members that are intended for the replacement of wood sheets or boards. According to that patent, metal reinforcing fibers are distributed in plastic that contains 30%-50% olefins. In some cases, reinforcing has been used in cementitious materials such as cement, mortar and concrete. For example, U.S. Pat. No. 4,190,993 is directed to concrete chimney liners that include wire mesh.
In some cases, reinforcing fibers have been used in cementitious materials to retard the tendency of the cementitious material to crack and thus strengthen the reinforced cementitious materials. Fiber-reinforced cementitious composites are made by mixing the fibers with the cementitious material or by placing the fibers in a mold and then infiltrating the mold with cementitious material. U.S. Pat. No. 4,565,840 describes fiber-reinforced concrete having two types of fibers. The fiber types differ as to Young's modulus. The fibers are mixed according to a predetermined ratio and dispersed in the concrete.
To facilitate a higher concentration of fibers in the composite, it is preferred to place the fibers in a mold and then infiltrate the mold with the cementitious material. In U.S. Pat. No. 5,571,628, the metal fibers are organized in pre-forms that have a selected shape such as an I-beam or a mat and then infiltrated with cementitious composition.
Specialized fibers and combinations of fibers with other ingredients have been used to try to optimize the performance of the cementitious composite. For example, U.S. Pat. No. 3,852,930 describes fibers with a three dimensional structure for use with cement. Another example is shown in U.S. Pat. No. 7,178,597 to Reddy et al. which is directed to cement compositions that are used to cement casings and liners in well bores. Down-hole thermal gradients and variations may sometimes break the seal between the cement and the pipe string. The Reddy et al. patent discloses the use of metallic fibers having an aspect ratio in the range of 1.25 to 400 in the cement composition or glass fibers having an aspect ration of 1.25 to 5,000. U.S. Pat. No. 5,503,670 also is directed to particulars concerning fibers that are used in a concrete composite.
Fiber reinforcing has been used in high-temperature applications such as, for example, in connection with steel-making processes. In such high-temperature applications, the reinforcing fibers must maintain their mechanical strength at the applied temperatures. For example, U.S. Pat. No. 4,366,255 is directed to a highly reinforced concrete that is made by infiltrating metal fibers with an aqueous slurry of a refractory material having a superplasticizer. That patent discloses a method for introducing amounts of fibers in excess of 4% by volume into the composition.
In applications where applied temperatures exceed 1500° F./850° C., the use of stainless steel reinforcing fibers is generally recommended. However, the use of stainless steel fibers also increases the cost of the cementitious material in comparison to cementitious materials that are reinforced with less-costly fiber material. For example, the use of certain grades of stainless steel fibers have been found to make the reinforced cementitious composite up to five times more costly than materials with conventional types of reinforcing fibers such as carbon steel fibers.
The relatively high cost of using reinforced cementitious materials has limited their use for high-temperature applications. In some cases, alternative materials or technologies were employed. Accordingly, there was a need in the prior art for a high-temperature fiber-reinforced cementitious composite that would be more cost-competitive with other fiber-reinforced cementitious materials.
In accordance with the presently disclosed invention, a multi-phase cementitious composite is made by locating a first casing inside of a second casing. The two casings define a volume between them as a first zone or a hot zone (hot face). The area inside the first casing is defined as a second zone. High-temperature fibers are fibers for which the maximum service temperature of the fibers is higher than the highest temperature to which the cementitious product is exposed. Thus, the high-temperature fibers will withstand the highest temperature to which the cementitious product is exposed. The high-temperature fibers are placed in the first zone or hot zone. Lower-temperature fibers have a maximum service temperature that is lower than the maximum service temperature of the high-temperature fibers, but higher than the highest temperature that will occur in the second zone at times when the cementitious product is exposed to the highest maximum temperature. Thus, lower-temperature fibers will withstand the highest temperature that occurs within the second zone at times when the cementitious product is exposed to the highest temperature. Lower-temperature fibers are placed in the second zone. The first casing is then removed from the second casing and cementitious material is then poured into the entire mold or second casing which includes both the hot zone and the second zone. When the wet cementitious material has hardened, a two-phase cementitious composite is formed.
Preferably, the high-temperature fibers are made of stainless steel and the lower-temperature fibers are made of carbon steel or a lower grade of stainless steel such as 409 stainless or 430 stainless that contain a lower quantity of chrome and nickel.
Also preferably, the thickness of the first zone is determined according to the highest temperature to which the finished composite will be exposed, the thermal gradient within the finished composite, and the temperature at which the lower-temperature metal fibers begin to lose their mechanical integrity.
Other features, advantages, and objects of the presently disclosed invention will become apparent to those skilled in the art as a description of a presently preferred embodiment thereof proceeds.
Several presently preferred embodiments are described and shown with respect to the accompanying drawings wherein:
Cementitious materials are used in high temperature applications such as steel-making. Typically, the cementitious materials are cast in a desired shape by pouring wet material into a mold and letting it harden. Also in high-temperature applications, various types of fibers are added to the material to strengthen it. Preferably, the fibers are added to the mold and then the liquid cementitious material is poured over the fibers.
Cementitious materials have a relatively high thermal gradient. Thus, when cementitious materials are used in high-temperature applications, the internal temperatures of the cementitious body are lower at locations that are remote from a “hot face” or surface of the cementitious body that is exposed to a high temperature. In the portions of the cementitious material that are adjacent to a hot face or high-temperature surface, high-temperature fibers—i.e., fibers that withstand high temperatures—are needed. However, due to the high thermal gradient of the cementitious material, high-temperature fibers are often not required for portions of the cementitious material that are located at positions that are remote or away from a hot face or high-temperature surface.
In the prior art, the fibers have been homogenously distributed throughout the cementitious material. There has been no commercially practicable method for constructing a metal fiber matrix wherein high-temperature fibers are selectively located in only those regions of the cementitious composites that are adjacent to surfaces that are exposed to high temperatures. Therefore, the high-temperature fibers have been used throughout the cementitious material even though they are only necessary in regions adjacent to the hot face or high-temperature surface.
Accordingly, there was a need in the prior art for a process that would selectively locate high temperature reinforcing fibers in those portions of the cementitious composite that are adjacent to surfaces that are exposed to high temperatures while using less-costly, lower temperature fibers in other portions of the cementitious material.
In the example of the preferred embodiment, walls 14 of casing 10 are equidistant from opposing walls 16 of casing 12 so that primary zone 18 has the same thickness (T) throughout. However, many other geometries of casings 10 and 12 are also possible depending on the particular application for the finished cementitious composite. As explained in greater detail below, the separation between adjacent walls 14 and 16 depends on the maximum applied temperature at the surface of the composite, the thermal gradient of the cementitious material, and the temperature at which the reinforcing fibers lose mechanical strength.
High-temperature reinforcing fibers 22 are made of a first material that retains its mechanical integrity or mechanical strength up to a first rated temperature which is the maximum service temperature for the particular material of reinforcing fibers 22. The first rated temperature or maximum service temperature of reinforcing fibers 22 is higher than the maximum temperature to which the hot surface of the cementitious material will be exposed. Reinforcing fibers 22 are placed in primary zone 18. Reinforcing fibers 24 are made of a second material that will retain mechanical integrity or mechanical strength up to a second rated temperature which is the maximum service temperature for the material of reinforcing fibers 24. The second rated temperature or maximum service temperature of reinforcing fibers 24 is higher than the maximum temperature that is developed in zone 20, but is lower than the first rated temperature for reinforcing fibers 22 made from the first material. Reinforcing fibers 24 are placed in zone 20 inside of walls 14 of casing 10.
The normal distance (“T”) between opposing walls 14 and walls 16 is determined according to the thermal gradient of the cementitious material, the maximum temperatures to which the outermost surface or hot surface of the cast composite will be exposed, and the temperature at which the lower-temperature reinforcing fibers 24 begin to lose mechanical integrity. For a given maximum temperature applied to the hot face or outermost surface of the two-stage composite and a given lower-temperature reinforcing fiber 24 with a given second rated temperature, the normal dimension between walls 14 and 16 is determined according to the thermal gradient of the cementitious material. The normal separation between walls 14 and 16 is selected to be equal to or greater than the distance over which the internal temperature of the cementitious material decreases from the maximum exposure temperature at the hot face located at wall 16 to below the second rated temperature at which the second reinforcing fibers 24 will retain their strength at the location of wall 14.
In the example of the preferred embodiment of
Alternatively, casing 10 can be inserted inside of casing 12 before the layer of fibers 22 is added. In that case, a layer of fibers 22 having thickness “T” is placed over floor 17 inside of casing 10 before fibers 24 are placed inside casing 10.
Also in the example of the preferred embodiment of
High-temperature reinforcing fibers 22 are placed in hot zone 18 to an elevation that is at least as high as the level of lower-temperature reinforcing fibers 24. Casing 10 is drawn out of the top end 25 of casing 12 so that walls 14 of casing 10 are no longer opposite the walls 16 of casing 12. This leaves a two-stage fiber reinforcement with the first, high-temperature fibers 22 in an outer zone or hot zone and second, lower-temperature fibers 24 in a lower-temperature inner zone. The open portion of casing 12 is filled with high-temperature reinforcing fibers 22 to the top 25 of casing 12.
In some cases, casing 10 can be drawn out of the top end of casing 12 in stages or steps as the high-temperature reinforcing fibers 22 are placed in zone 18 and the lower-temperature reinforcing fibers 24 are placed in zone 20 on the inside of casing 10. The rate of withdrawal of casing 10 is such that the lower end of casing 10 remains below an elevation at which fibers 22 fill zone 18 and fibers 24 fill zone 20 on opposite sides of the walls 14 of casing 10. In this way, the cumulative frictional force applied by fibers 22 and fibers 24 is reduced so that casing 10 can be more easily withdrawn from fibers 22 and 24.
The wet cementitious material is then poured over the reinforcing fibers 22 and 24 to fill casing 12. The entire casing 12 is vibrated so that the cementitious material and fibers are compacted. Fibers 22 may be added as necessary to maintain finished part geometry. When the cementitious material cures, the result is a hardened composite of reinforcing fibers and cementitious material. The composite is two-stage in that high-temperature fibers 22 are in outer zone 18 and lower-temperature fibers 24 are in inner zone 20. The finished product has a level top with high-temperature fibers 24 and a hot zone on the top surface of the finished product.
Many other shapes and geometries are also possible with the disclosed method. For example,
In the example of
Alternatively, casing 28 can be inserted inside of casing 30 before the layer of fibers 22 is added. In that case, a layer of fibers 22 having thickness “T” is placed over floor 35 inside of casing 28 before fibers 24 are placed inside casing 28.
In the example of the preferred embodiment of
Casing 28 is then removed from casing 30 by longitudinally withdrawing casing 28 through the open top end of casing 30 until the lower end of casing 28 is positioned at the same elevation as the top end 42 of second casing 30. The balance of casing 30 is then filled with high-temperature reinforcing fibers 22 to the top of casing 30. Casing 28 is also filled with high-temperature reinforcing fibers 22 to an elevation that is the desired height of the central portion of the top of the molded part. This establishes a two-stage fiber reinforcement with the first, high-temperature fibers 22 in an outer zone or hot zone and second, lower-temperature fibers 24 in a lower-temperature inner zone.
Cementitious material is then added to fill casing 30. Cementitious material is also added to fill the lower portion of casing 28 to a level of the desired height of the central portion of the top of the molded part. Casings 28 and 30 are vibrated so that the cementitious material is compacted. Fibers 22 may be added as necessary to maintain finished part geometry. When the cementitious material cures, the result is a hardened composite of reinforcing fibers and cementitious material. The composite is two-stage in that high-temperature fibers 22 are in outer zone 36 and lower-temperature fibers 24 are in inner zone 38. The finished product has a bi-level top with high-temperature fibers 22 and a hot zone on the top surface of the finished product.
While a presently preferred embodiment of the invention is shown and described herein, the presently disclosed invention is not limited thereto, but can be otherwise variously embodied within the scope of the following claims.
