About block-making
Machinery for slag blocks manufacturing
On website of Plant Stroytechnika-Zlatoust LLC you can find all necessary information about manufacturing of paving slabs, machinery for manufacturing of slag blocks, slag blocks manufacturing machinery models, slag block mini plant. Many other important questions are covered in the following article of our honourable author Zaitsev A.N.
The ”Rifey” lines of Stroytechnika-Zlatoust Plant as well as an equal machinery of other companies render possible to manufacture slag blocks, pavers, paving slabs and building materials using dry mixture concrete. Professional constructor knows what the dry mixture concrete and technology of paving slabs manufacturing is, so there is no need for him to read this article. Although our “Rifey” line can be purchased not only by professional constructors but also by people who first meet with building materials and paving slabs manufacture. This article is personally for them.
So, what is the dry mixture concrete? At first let’s find out what is concrete? Concrete is a man-made material resulted from a hardening of rationally assembled, intimate and compacted mixture of mineral adhesive, water, aggregates and sometimes special supplements. A mixture of these components before the beginning of its hardening is called “concrete mixture”. Adhesive and water are active elements of concrete. As a result of chemical interaction between these two elements forms a new compound in the form of viscid paste (cement paste, cement grout). This paste wraps around aggregate grains and then gradually hardening and cement together the grains together moulding the concrete mixture into solid monolithic stone – concrete.
Aggregates (sand, chippings, gravel, etc.) occupy from 80 to 85 % of concrete volume and form its rigid frame which prevents its contraction. By use of aggregates with different features, it becomes possible to produce concretes with various mechanical-and-physical properties such as light, heavy, high-temperature concretes, etc. So, concrete (before the beginning of its hardening) is a concrete mixture - rationally assembled, intimate mixture of mineral adhesive (cement), water, aggregates and, sometimes, special supplements (plasticizing agents and early-strength admixtures). Like any other substance concrete mixture has different mechanical-and-physical properties that considerably preconfigurate a quality and features of a concrete produced from it. Let’s fix upon a few of them. Remolding effort characterizes an ability of concrete mixture to fill a form of concreting unit and firm there under gravity or as a result of external mechanical action. This feature of concrete mixture is measured by medium workability and hardness. Remolding effort of concrete mixture depends on several factors such as kind of cement, amount of water and cement paste, size and shape of aggregate grains, sand content.
Concrete mixtures of the same content but produced with a use of different cements have different molding effort because of difference in water demand of cement. For example, portland-pozzolan cement and blast-furnace cement has more water demand than portland cement and mixtures produced with use of these cements are more dry. Flowability is an ability of concrete mixture to flow under the action of it’s own weight. Workability level “W” is measured by an upset distance (in centimeters) of cone formed from this mixture. Flowability of concrete mixture is measured by a standard cone (height – 300 mm, (высота – 300 мм, diameter of base – 200 mm, diameter of vertex – 100 mm). Cone upset distance (ОК) is measured by a rule. As more is an upset distance as much flowable is a concrete mixture. Almost everybody who dealt with a use of concrete in everyday life worked with flowable (liquid) concrete mixtures, which easily fill every concrete form no matter how difficult it is. Although you should wait a bit before demolding of concrete form because hardening of mixture takes some time. During manufacture of stones using a method of block-making on block-making machine the use of flowable concrete mixtures is impossible because just after finishing of compaction process (vibration compaction) a form of paving slab is demolded from a product.
Hardness of the concrete mix is an ability of concrete mix to flow and fill a form under vibration. Hardness index is measured by indicator that represents a metallic cylinder 0f 240 mm in diameter and 200 mm high. This cylinder establishes to a standard laboratory vibroplatform. Then a standard cone is put into cylinder and filled with concrete mixture at the same way as if you measure a flow. After this the cone is demolded and a steel disk is put to the concrete mixture. Total mass of a disk must be 2750 ± 50g. Then vibroplatform is started and a mixture vibrates till the cement paste begins to low out from 2 ports of the disk. Just at this moment vibrator stops. The time needed for a compacting of mixture in indicator is named a hardness index of concrete mixture (Ж) and works out in seconds.
There is no need to adjust an execution time between hardening and demolding of form for a fix concrete mix. Right after hardening form is demolded and a product holds its shape and is available for the transportation (subject to some measures of precaution).
Paving slabs manufacture (block-making) uses concrete mixtures of Ж3, Ж4, СЖ1 classes. Cohesion is an ability of concrete mixture to hold homogeneous structure i.e. not to segregate during transportation, pouring and hardening. As a result of hardening particles of mixture close on and a part of water which is the lightest component of mixture squeezes upwards forming filamentous channels and hollownesses under grains of coarse aggregate.
Coarse aggregate which compactness differs from a mortar part (mixture of cement, sand and water) also travels through a concrete mix. If the aggregate is compact and heavy for an instance such as a granite chippings, it’s parts fall and light aggregates such as haydite and agloporite float up. These factors impair concrete structure, make it non-homogeneous, increase its water permeability and decrease freeze-thaw resistance. For increase of cohesion and prevention of concrete segregation right setting of fine aggregate in concrete composition and reducing of unit water content by use of plasticizers is needed. Main concrete features are strength, compactness, water resistance, freeze-thaw resistance, shrinkage, expansion, etc.
Compressive strength is a main index of mechanical characteristics of concrete. It is measured by compressive strength of standard samples – cubes of 150х150х150 size or end products (stones) manufactured from this concrete mixture and matured in normal conditions for 28 days. According the compressive strength the following marks are rated for masonry blocks: М25, М35, М50, М75, М100, М125, М150, М200 and the following classes are rated for paving slabs: В22.5, В25, В30, В35. Numbers in marks are corresponding to concrete compressive strength measured in kg/cm² and in classes they are corresponding to the compressive strength measured in MPa.
Concrete compressive strength depends on an activity of cement (strength mark), cement to water ratio, strength and quality of aggregates as well as aggregates grain fineness, time of hardening, environmental temperature and humidity and other factors. The main factors, effect on a concrete strength, are cement activity and cement to water ration in a composition of concrete mixture (water-to-cement ratio (W/C) or backward cement-to-water ratio (C/W)). Aggregates grain fineness also has some effect to a concrete strength.
The strongest concretes are manufactured using coarse-graded aggregates. Grains of coarse-graded aggregates should be strong enough and should have a roughened surface, which provides a good cohesion of cement brick with an aggregate. Strength of concrete depends on right mix of its components in a mixing machine when all grains of aggregate are covered with a cement paste. The main effect to a concrete strength has a degree of concrete mixture compaction and a time and conditions of concrete hardening. Well-compacted concrete in good temperature and humidity conditions is continuously getting strength for several years. Herewith within first 10 days the strength of concrete grows quickly enough but then about 28th day a growth of concrete slows and finally after 1 year it gradually decays. For an instance concrete samples stored in normal conditions in 7-days age have a middle strength equal to 60…70% of a 28-days (mark) strength and at the age of 180 days, 1 year and 2 years their strength is correspondingly 150, 175 and 200% of mark strength.
Environmental temperature has a great effect to a growth of concrete strength. In the atmosphere of saturated vapor (in curing room) where the temperature is 70…85ºС concretes in 10-12 hours get 60…70% of the mark strength. In a low positive environmental temperatures the speed of concrete strength growth slows down and if the temperature is below zero a concrete hardening stops. Consistency – normal concrete is not a homogeneous solid material. Always there are pores appeared as a result of evaporation of extra water and incomplete air bubbles venting during a compaction of concrete mixture.
Concrete consistency grows under a careful selection of grain fineness of aggregates, decrease of water-to-cement ratio, use of plastisizers, which reduce water demand of the mixture with equal flow and in virtue of careful compaction of a concrete mixture. By the growth of concrete consistency grow its features. The highest requirements to the concrete consistency are placed in manufacture of paving slabs and curb stones. Freeze-thaw resistance is an ability of waterlogged material to withstand numerous alternate freezings and thaws. Freeze-thaw resistance mark F means a maximal number of freezing and thaw cycles withstood by material samples without decrease of compressive strength for more than 15%; mass loss must be less than 5%.
Freeze-thaw resistance is a main lifetime pacing factor for exterior structures submitted to action of water and alternating temperature. Design mark of materials by freeze-thaw resistance is assigned according to structure operating conditions and climate. For an instance, for a slag block production F 15, F 25, F 35, F 50 are assigned and for a block-stones production assigned marks are F100, F150, F200, F300. Freeze-thaw resistance of concrete depends on a type of cement, water-to-cement ration, aggregates quality, concrete consistency and other factors.
Concretes with compact structure based on a low-aluminate Portland cement and high quality chippings have a high freeze-thaw resistance. As we can see such qualities of concrete as strength, consistency and freeze-thaw resistance directly depend on water-to cement ratio: the less water the stronger and compacter concrete and the higher its freeze-thaw resistance. And the less water in a concrete mixture the higher its hardness. So, products manufactured by the method of block-making from strong concrete mixtures have higher strength, consistency and freeze-thaw resistance.
