A compaction tool for a concrete brick machine, comprising a mold formed with cells opened at the top & at the bottom, & filled with a no-slump concrete mix. The mold freely rests either on a pallet or vibrating table, which acts as bottom wall for the mold. Compaction shoes, equal in number to the number of cells in the mold, are sized, configured & positioned above the mold so that they may snugly fit in to the mold cells.
With the mold held stationary, the support table or pallet may thereafter be moved down away from the mold to permit the concrete blocks to be dislodged from the mold by the downward movement of the compaction shoes. Alternatively, the shoes may be held stationary, above the pallet standing underneath & the mold is moved up, leaving on the pallet a row of dislodged blocks.
Present day expert manufacturers use a machine which, broadly speaking, has a feed zone where a hopper pours a no-slump concrete mix of any selected water/cement ratio in to a drawer which is thereafter moved laterally & over the vibrating cellsof a mold standing in an adjoining compaction zone. After the cells are filled with the mix, & initially compacted a predetermined lapse of feed time, the drawer is returned to the feed zone, being provided with suitable means to scrape off the tops ofthe cells, as it moves back to the feed zone. Final compaction, while vibration is going on, is achieved thereafter in the work of a predetermined finishing time, by means of a plurality of fixed compaction shoes, provided at the lower ends of a like number ofsolid rod or tubing of fixed lengths, of which the upper ends are also fixedly mounted on a vertically movable head plate. For final compaction, the head plate is moved toward the mold, or vise versa, & a given pressure is then applied to the headplate, & hence, to the mix in each cell, by the fixed compaction shoes with or without vibration applied to the head plate & naturally transmitted to the compaction shoes, that are solid with the head plate, as aforesaid. After a predeterminedperiod of time of vibration in this last position, done concrete blocks, are moved out of the mold onto a steel plate or on a steel pallet.
A major difficulty with this type of equipment is that, because the compaction shoes can be found at the ends of solid rods or tubings of equal length, & have their flat working surfaces constantly at the same level, that is in the samehorizontal plane, final compaction pressure is unequally applied in the various cells, even with vibration on the head. Indeed, the cells are never equally filled & compacted when the drawer backs up, even so, that the top surfaces of the mix in allthe cells are at the same level. Thereafter, when comes the final compaction with the compaction shoes, the result is that the concrete mix is fully compacted in a number of the cells, & less in others in the work of the period of final vibration & thereforethe density in the resulting blocks is not equal. It follows that, the less dense blocks do not have the expected or necessary physical properties able to meeting the stiff specifications of CAN3-A231,2 or A.S.T.M.C936-82 for concrete paving stones,or other concrete blocks for which utmost density, & hence particularities of physical properties, is a most important requirement. In addition of not making full use of the cement powder, these less compacted or dense elements are liable to failprematurely, or do not have the same properties anticipated. For these reasons, the quality of the done products is variable & some may must be rejected. Likewise, other concrete blocks which are less compacted by present day methods, fail tomeet their anticipated physical properties & specifications.
Extensive research has demonstrated that all physical properties of a concrete block resulting from a no-slump mix, are closely related to the density of the compacted mix & this holds true at all water/cement ratios within the no-slumprange. This includes the lowest W/C ratio where a maximum of water is necessary to complete the hydration of the cement. But, the same does not apply so significantly to a slump, or near slump, concrete mix where maximum or near maximum density is moreeasily obtained in each cell due to the hence better workability of the mix, &, where also the physical properties are more uniform but where compressive strength or any other physical properties are often inadequate for different type of concreteblocks or intended to be exposed to extreme conditions. It may even be verified that a higher W/C ratio mix may give a higher compressive strength than a lower one due to its better workability & greater density, but such is not the case of itsphysical properties.
Abram's law which states that the compressive strength of concrete is inversely proportional to the W/C ratio in the mix, applies to a no-slump concrete at any W/C ratio of the no-slump range but it is verified that the Abram's law holds trueonly if the proper maximum density is reached for all mixes in that ratio range. The compressive strength, flexural strength, durability & permeability are therefore functions of the density at any particular W/C ratio in the no-slump range & anydrop of density from the utmost one appreciably reduces the compressive strength & other physical properties. As well, further drop in density causes the undesired water absorption capacity of the concrete to increase & may render the following blocksless weather resistant. Conclusively, the compressive strength & related physical properties are directly proportional to the density at any W/C ratio.
Another fascinating fact, noted with lower W/C ratio no-slump concrete, is the linear percentage of the loss of weight of the preliminary dry mass found between 25 & 50 cycles when the mass is subjected to freezing & thawing, durability testsconducted under CAN3-A231.2M85 specifications for paving stones of final density. There again, the density of the done concrete blocks is of prime importance & losses in weight in percent of the dry mass increase radically as densitiesdecrease. This is most important &, according to the present invention, a maximum acceptable density can be obtained or found for each mix design & W/C ratio, which will permit a loss of weight of less than 1% under the above specifications &, atthe same time, provide durability of all concrete blocks when, as aforesaid, made in accordance with the present invention.
There thus presently exists an issue of uneven properties in all concrete products when made according to the present day methods, due to the fact that these concrete products do not have the appropriate maximum acceptable density for theircorresponding W/C ratio mix as above explained, with resulting uneven lower compressive strength, permeability, flexural strength, durability & related properties at least regarding an important number of concrete blocks so made.
In trying to alleviate the issue of getting full compaction with existing machine, the industry is trying to work at higher W/C ratio to get a better density & compressive strength but this is achieved only at great expense in waste ofcement & beneficial properties of lower W/C ratio mix. Thus, some devices like heated shoes have been designed. They permit the use of a higher W/C ratio mix & consequently a better workability of the mix but to prevent material from sticking tothe shoes, heat has to be used. Of work, a higher workability obtained by adding water does give a higher density but regrettably, when thinking about Abram's law, a higher W/C ratio gives lower compressive, flexural & shear strengths, higher waterabsorption, higher percentage loss of weight, in durability check due to freezing & thawing, & higher shrinkage.
Also, because the mix in a number of the mold cells, in present day machines, are still not sufficiently compacted, some other devices, like vibration devices on the head and/or on the foot of the block machine, have been used in an try to optimize thedensity. However, the same issue of inacceptable uneven compaction of a number of the elements from different cells & consequently high a disparity in physical properties of finish products persist. Some cities using much de-icing salts are nowconsidering not to make use of definite concrete blocks like paving stones, curbs, etc. in their streets because of their poor durability. The same issue applies to other prefabricated concrete blocks like bricks or pipes where any anticipated physicalproperty such as permeability is not attained because of the uneven density at any W/C ratio mix.