Patent Application
20070028548
The invention relates generally to the manufacture of concrete blocks. More specifically, the invention relates to dimensional control of the top and bottom surfaces of concrete blocks that are formed front face-up in a mold for use in mortar-less walls.
Modern, high speed, automated concrete block plants and concrete paver plants make use of concrete block molds that are open at the top and bottom. These molds are mounted in machines which cyclically station a pallet below the mold to close the bottom of the mold, deliver dry cast concrete into the mold through the open top of the mold, densify and compact the concrete by a combination of vibration and pressure, and strip the uncured blocks from the mold by a relative vertical movement of the mold and the pallet.
For efficient high-volume production, concrete block molds are typically configured to produce multiple blocks simultaneously. A concrete block mold generally comprises side walls and end walls that define the periphery of a mold cavity. Within this mold cavity, division plates may be used to sub-divide the mold cavity into a plurality of block-forming cavities. Further, movable side walls may be used to form the side faces of the block-forming cavity. The division plates are generally rectangular-shaped plates attached to the side walls of the mold. Further, the side walls of the block cavity and the division plates may be covered with replaceable mold face linings to protect the mold components from abrasive wear.
As disclosed in U.S. Published Patent Application 20030182011, some blocks are now being formed with patterned or other processed front faces while retaining the high-speed, mass production of the blocks. As disclosed in U.S. Published Patent Application 20030182011, the blocks can be formed front face-up in the mold, allowing the front face of the block to be contacted by a stripper shoe which imparts a desired three-dimensional pattern to the front face. When a block is formed front-face-up in the mold, the top and bottom surfaces of the blocks (from the perspective of the block as laid in a wall) are formed by division plates. Because the side surfaces of a block must converge to allow the blocks to be laid up in a curved or radiused wall, the front of the block is typically wider than the rear of the block. In order for a block formed front-face-up to be discharged through the bottom of the mold, the side surfaces of a block must be formed by movable side walls that, in a first position during molding, form the wider front portion and narrower bottom portion of the block, and in a second position during discharge of the block from the mold, move sufficiently out of the way for the wider front portion of the block to pass through the bottom of the mold.
A problem that arises when blocks are formed front-face-up in a conventional block mold is that the blocks are prone to being formed with the top and bottom surfaces not being flat and parallel to each other. Since concrete retaining wall blocks are typically assembled without mortar, there is little ability to accommodate variations in the flatness and parallelism of the top and bottom surfaces during the assembly of a wall. It is very important, therefore, that the top and bottom surfaces of the blocks that engage with other blocks be formed as flat as possible and parallel to each other to allow the blocks to lay flat and level on blocks in a lower course of blocks, as well as to allow blocks in an upper course to lay flat and level.
It is also important during the commercial manufacture of concrete blocks that the manufacturing expenses be minimized. Certain components in the concrete mixture are more expensive, such that increasing the percentage of those components in the concrete mixture increases the manufacturing expense. In particular, cementitious materials are a component of a concrete mixture that is typically more expensive than other components. However, the percentage of cementitious materials in the concrete mixture affects the stability and dimensional control of the resulting concrete block. Therefore, it is desired to minimize the amount of cementitious material required in the concrete block mixture while still maintaining acceptable block properties and dimensional control.
Thus, there is a demand for concrete block manufacturing processes that provide for improved control of the flatness and parallelism of the top and bottom surfaces of concrete blocks formed front face-up in a mold, while minimizing the expense of the concrete mixture.
An improved concrete block manufacturing process provides for improved control of the flatness and parallelism of the top and bottom surfaces of concrete blocks formed front face-up in a mold. The improved manufacturing process incorporates an improved concrete block mold and a modified concrete mixture that operates in cooperation with the concrete block mold. A concrete block mold is provided with a division plate that is secured in the mold in a manner that allows the block to be formed with close control of the top and bottom surfaces. The division plate is secured within channels formed in the side walls of the mold that extend substantially the entire height of the mold cavity so that substantially the entire height of the division plate is secured in the channels, without interfering with the pivoting side wall mechanism. The channels are sized so that there is minimal play between the side edges of the division plate and the channels. A plurality of fasteners secure the division plates to the side walls of the mold. The concrete block mold allows for the use of a concrete mixture with an optimized content of cementitious material, sand, coarse aggregates, and water, where the content of cementitious material is minimized. The concrete mixture is optimized to work in conjunction with the mold, so as to provide a block with sufficient stability prior to being cured that the block adequately retains the geometry formed within the mold.
The present invention provides a division plate for a concrete block mold. The resulting surface that is shaped by the division plate is substantially flat, which aids in the construction of a high quality wall or other structure made from a plurality of the concrete blocks.
The division plate works in concert with an optimized concrete mixture to provide the desired control of the block geometry. The concrete mixture generally comprises coarse aggregate, sand (also called fine aggregate), cementitious material, colorant (also called pigment), and water. The concrete mixture can be made more stable and self-supporting by increasing the content of coarse aggregate material. However, a greater concentration of coarse aggregate material may prevent the formation of fine detail on the front face of the block, which is often desired when forming blocks front-face-up. Similarly, the concrete mixture can also generally be made more stable by increasing the amount of cementitious material in the concrete mixture, within limits. This helps to prevent the block from slumping after being released from the mold and before being cured. However, cementitious material is relatively expensive and therefore it is desired to keep its use to a minimum. Further, increased percentages of cementitious material may make the concrete mixture stickier, which can prevent the concrete mixture from flowing into the mold properly. The present invention allows the use of a concrete mixture that is optimized for sufficient block stability without unnecessarily increasing the expense of the mixture, making the mixture too sticky to flow into the mold, or preventing the formation of sufficient detail on the front face of the blocks.
The invention will be described with respect to the formation of retaining wall blocks front-face-up in a mold as disclosed in U.S. Published Patent Application 20030182011, which is incorporated herein by reference in its entirety. In such a front-face-up orientation, the top and bottom surfaces of the blocks are formed by division plates (or by one division plate and one end of the mold in the outer cavities of the mold). In addition, the division plate that forms the lower surface of the block may be provided with an undercut at the open bottom of the mold in order to form a locator protrusion, for example a flange, as disclosed in U.S. Published Patent Application 20030182011. However, the inventive concepts could be applied to the formation of other blocks in other orientations.
The formation of blocks with top and bottom surfaces that are not flat and parallel is an especially significant problem because it may prevent the block from laying flat or preventing other blocks from laying flat on the block when laid up in a wall. It has been determined that 1/32 of an inch deviation in flatness and parallelism is a suitable maximum value of deviation. Preferably, the production process results in most blocks having less than 1/32 of an inch deviation in flatness and parallelism and only a few having the maximum of 1/32 of an inch deviation. Although a maximum deviation of less than 1/32 of an inch may be desirable, the additional expense and difficulty of further reducing this deviation may not be justified.
A concrete block produced according to the present invention is illustrated in
With reference to
The mold 20 is constructed so that the blocks are formed front-face-up (i.e. with the front faces facing upward) and the rear faces supported on the pallet positioned underneath the mold 20. Further information on front face-up block formation can be found in U.S. Published Patent Application 20030182011. In this orientation, the top and bottom surfaces of the block are formed by two adjacent division plates 26, or by a division plate 26 and a mold end wall 24. Referring to
Oftentimes, the block forming surfaces of the mold cavities 28 are provided with replaceable wear liners that actually contact the concrete in the mold cavities. These liners help prevent wear on the division plates 26, block cavity movable side walls 18, and mold end walls 24, which tend to be expensive to replace. The use of wear liners is known to those having ordinary skill in the art. Therefore, although not illustrated in the drawings, references to the block cavity movable side walls 18, mold end walls 24, and division plates 26 as forming faces of the blocks is meant to include direct formation of the faces by the block cavity movable side walls 18, mold end walls 24, and plates 26, as well as formation of the faces by wear liners attached to the block cavity movable side walls 18, mold end walls 24, and plates 26.
Referring to
Referring now to
The concrete mixture is also an important part of controlling the dimensions of concrete blocks. In forming blocks from dry-cast concrete, the blocks are formed in a mold, removed from the mold, transported to a storage location, and then cured. Thus, when the blocks are removed from the mold they are not yet cured. It is therefore important that the blocks have sufficient stability and rigidity that they can support their own weight until they are cured, without slumping or losing their shape.
A typical concrete mixture comprises cementitious material, sand, coarse aggregates, colorants, and water. Cementitious materials may include such materials as cement, fly ash, slag, silica fume, and other pozzolans, and the methods of properly selecting or combining these constituents are known to those of skill in the art. It is possible to increase the stability of the blocks after they are removed from the mold by increasing the size of the coarse aggregates in the concrete mixture or by increasing the percentage of the mixture consisting of coarse aggregates. Coarse aggregates are a collection of rocky materials that have typically been screened or otherwise mechanically separated, such as by a sieve, to produce a coarse aggregate size distribution that comprises material of a maximum size (typically determined by the size of the openings in the screen or sieve) and smaller materials. For example, the coarse aggregate component may comprise a mixture of aggregates with a characteristic size of 3/16 inch or ¼ inch. Increasing the size of the coarse aggregates or increasing the percentage of the coarse aggregates within the mixture creates a coarser mixture. While a coarser mixture may improve the stability of the block after it is removed from the mold, the coarser mixture may not be desirable because it may prevent the formation of a high level of detail on the front face of the block.
Generally, increasing the content of cementitious material in the mixture will also increase the stability of the blocks after they are removed from the mold. However, at some point, such as about 21 percent cementitious material in the concrete mixture, increasing the amount of cementitious material will not increase the stability of the blocks after they are removed from the mold, and may in fact decrease the stability of the blocks. However, a typical concrete mixture contains around 12 percent cementitious material, such that increases in the amount of cementitious material generally increase the stability of the blocks after they are removed from the mold. Increasing the content of cementitious material in the mixture has the further advantage that it will not limit the amount of detail that can be formed on the front face of the block. However, increasing the content of cementitious material in the mixture will make the mixture more expensive. Additionally, increasing the content of cementitious material will make the mixture stickier, which makes it more difficult to completely and consistently fill the mold cavity. Where greater amounts of cementitious material are used, it may be necessary to add larger amounts of water to the mixture to allow the mixture to flow properly. However, increased quantities of water may tend to reduce the stability of the blocks after they leave the mold. Therefore, determination of the mixture composition is both difficult and critical.
The mold with the division plates of the present invention allows the blocks to be suitably formed with an acceptable concrete mixture. The inventors have determined that the following concrete mixture yields good results when used in conjunction with the mold and division plates of the present disclosure:
It is generally desired to minimize the content of cementitious material in order to minimize costs. A cementitious material content of about 13.5 percent is the minimum that will function properly in accordance with the present disclosure. Although it is generally desirable to use the minimum content of cementitious material, a cementitious material content of 15 percent will still yield acceptable results.
A typical concrete mixture for blocks formed conventionally according to methods known to those of skill in the art may contain around 12 percent cementitious material. When blocks are formed front-face-up according to the methods described in U.S. Published Patent Application 20030182011, they are more prone to having excessive deviations of the upper and lower face flatness, and as such, may require greater amounts of cementitous material to yield acceptable properties when used with molds with standard division plates. However, the use of the division plates of the present invention allows the use of an optimized concrete mixture with only about 13.5 to 15 percent cementitious material while maintaining adequate block geometry. Thus, the division plates of the present invention allow for a substantial reduction in cementitious material content in the concrete mixture, and thereby results in substantial cost savings.
