With the mold held stationary, the support table or pallet may thereafter be moved down away from the mold to allow 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 and the mold is moved up, leaving on the pallet a row of dislodged blocks.
Present day skilled manufacturers use a machine which, broadly speaking, has a feed zone where a hopper pours a no-slump concrete mix of any chosen water/cement ratio into a drawer which is thereafter moved laterally and over the vibrating cellsof a mold standing in an adjacent compaction zone. After the cells are filled with the mix, and 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 during 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, and a given pressure is then applied to the headplate, and hence, to the mix in each cell, by the fixed compaction shoes with or without vibration applied to the head plate and of course 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, finished 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 are located at the ends of solid rods or tubings of equal length, and 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 and 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 some of the cells, and less in others during the period of final vibration and thereforethe density in the resulting blocks is not equal. It follows that, the less dense blocks do not have the expected or required physical properties capable of 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, and 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 prone to failprematurely, or do not have the same properties anticipated. For these reasons, the quality of the finished products is variable and some may have to be rejected. Likewise, other concrete blocks which are less compacted by present day methods, fail tomeet their anticipated physical properties and specifications.
Extensive research has demonstrated that all physical properties of a concrete block resulting from a no-slump mix, are very closely related to the density of the compacted mix and this holds true at all water/cement ratios within the no-slumprange. This includes the lowest W/C ratio where a minimum 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, and, 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 severe conditions. It can also be verified that a higher W/C ratio mix may give a higher compressive strength than a lower one due to its better workability and 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 and permeability are therefore functions of the density at any particular W/C ratio in the no-slump range and anydrop of density from the utmost one appreciably reduces the compressive strength and other physical properties. As well, further drop in density causes the undesired water absorption capacity of the concrete to increase and may render the ensuing blocksless weather resistant. Conclusively, the compressive strength and related physical properties are directly proportional to the density at any W/C ratio.
Another interesting fact, noted with lower W/C ratio no-slump concrete, is the linear percentage of the loss of weight of the initial dry mass found between 25 and 50 cycles when the mass is subjected to freezing and thawing, durability testsconducted under CAN3-A231.2M85 specifications for paving stones of ultimate density. There again, the density of the finished concrete blocks is of prime importance and losses in weight in percent of the dry mass increase radically as densitiesdecrease. This is most important and, according to the present invention, a minimum acceptable density can be obtained or found for each mix design and W/C ratio, which will permit a loss of weight of less than 1% under the above specifications and, atthe same time, provide durability of all concrete blocks when, as aforesaid, made in accordance with the present invention.
There thus presently exists a problem 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 minimum acceptable density for theircorresponding W/C ratio mix as above explained, with resulting uneven lower compressive strength, permeability, flexural strength, durability and related properties at least regarding an important number of concrete blocks so made.
In attempting to alleviate the problem of getting full compaction with existing machine, the industry is trying to work at higher W/C ratio to obtain a better density and compressive strength but this is achieved only at great expense in waste ofcement and 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 and consequently a better workability of the mix but to prevent material from sticking tothe shoes, heat has to be used. Of course, a higher workability obtained by adding water does give a higher density but unfortunately, when considering Abram's law, a higher W/C ratio gives lower compressive, flexural and shear strengths, higher waterabsorption, higher percentage loss of weight, in durability test due to freezing and thawing, and higher shrinkage.
Also, because the mix in some 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 attempt to optimize thedensity. However, the same problem of inacceptable uneven compaction of some of the elements from different cells and consequently too high a disparity in physical properties of end products persist. Some cities using much de-icing salts are nowconsidering not to use certain concrete blocks like paving stones, curbs, etc. in their streets because of their poor durability. The same problem 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.