This invention relates generally to concrete slab forming machines, and has particular reference to an improved machine for forming high quality concrete slabs by a continuous process without the use of conventional casting forms.
Forming slabs by a continuous process without forms requires a concrete mix of a consistency such that the concrete section that is formed will be self-supporting before the concrete has hardened. In concrete technology, such concrete mixes are defined as "zero slump" and "low slump" (up to 3 cm .+-. slump) mixes.
"Zero slump" (dry state) mixes are highly abrasive and cause excessive wear on the forming machinery. An even more serious disadvantage, however, is the lack of cement-sand paste in such mixes which prevents the achievement of a good bond between the concrete and any reinforcing steel there may be.
"Low slump" (plastic state) mixes produce structurally sound concrete with far less wear on the machinery and good reinforcing steel-concrete bonding. However, with such mixes there may be some sagging or slumping in the freshly formed section resulting in an inferior product.
Between these extremes of workable consistency limits, there is a range where the concrete mixes have optimum characteristics and mixes in this range are defined as "zero slump saturated" mixes. These mixes have a practical water variance flexibility and combine the advantages of the "zero slump" stiffness after the forming stage (inside and outside the machinery) and "low slump" workability during the forming stage.
At the present time, concrete slab forming machines can be broadly classified as extrusion type and slip form type machines. The extrusion type machines exert a force on the concrete mix to extrude it through the forming part of the machine. When the extruded concrete section reaches a certain resistance (weight), the reaction of the extrusion force propels the forming machine ahead. Within the extrusion machine classification there are screw type and reciprocating type machines.
Screw type extrusion machines can use only "zero slump" concrete mixes because the mix must be stiff to transmit the longitudinal extrusion force from the cylindrical envelopes of the augers to the rectangular shape of the concrete section. If water were added to the mix to improve its bond with reinforcing steel, the mix would very possibly plug the augers. To achieve the necessary cement-sand paste for good bonding when using this type of machine, water from the floor is vibrated in the bottom of the concrete section. This, however, results in different densities between the top and bottom of the slab which causes excessive camber/deflection problems.
Reciprocating type extrusion machines can use concrete mixes from "zero slump" through "low slump" consistencies. However, because of the stop and go that is necessitated by the feed and extrude cycles, these machines have the disadvantage of relatively low production speed. U.S. Pat. No. 3,159,897 to Ellis discloses a screw type machine, while U.S. Pat. Nos. 3,143,718 and 3,143,782, both to Kalns, disclose reciprocating type machines.
Slip form type slab forming machines, unlike the extrusion type, do not apply any force to extrude, or to pack the concrete mix into the forming part of the machine. These machines are propelled by external means such as a power wheel or winch and the concrete mix is moved by gravity and vibration into the forming part of the machine. To enable the mix to flow, and to minimize the friction between the forms and concrete, the mix must be in the wet range (2 cm .+-. slump) of the low slump consistency resulting in an inferior product. In addition to this disadvantage, the gravity feed frequently is not uniform due to the practical limitations in controlling the concrete mix. Water and aggregate changes influence the flow characteristics of the wet concrete mix and thus impair the uniformity of the product. Concrete mixes of even medium consistency (1 cm .+-. slump) cannot be used in the slip form machinery because of the lack of a force to compensate for the friction between the forms and concrete mix, i.e. such mixes plug up the machinery.
Slip form type slab forming machines are disclosed in U.S. Pat. Nos. 3,401,438 to Dennis; 3,583,046 to Dickinson; 3,608,011 to Jones and 3,647,308 to Yost.