1.4. Macrostructure
At the macroscopic level the materials structure is examined, if the sizes of particles are over 10–4 m. Macrostructure is studied with unaided eye or at insignificant magnification. Thus it is possible to define the peculiarities of structure and defects of materials, predefine the processes of their formation, production and performance (e.g., defects of casting origin in metals, defects of wood, bubble and impurity inclusions in glass, cracks and voids in concretes).
The study of conglomerate type materials macrostructure gives the opportunity to determine the relative amount of binding materials and aggregates, and sometimes their mineralogical composition, size and form of grains, character of surface, form and amount of macropores, etc.
In a number of cases complex multicomponent structures can be reduced to macrolevel to the two-component. For example, the macrostructure of concrete can be examined as a system "cement stone-aggregate" (sometimes the system cement-sand mortar – crushed stone is meant by the macrostructure of concrete), and macrostructure of pyrocerams - as a system "glassy adhesive substance - crystalline filler".
Two component structures can be divided into parallel comprised and serial comprised. Thus the most common are mixed parallel-serial structures.
Conglomerate two component structures (Fig. 1.16) divide into three groups depending on the degree of separation of grains of aggregates. If material structure is with basal cementation, aggregates grains do not form contacts between themselves; they “float” in binding matter. Properties of material at such macrostructure are predefined mainly by properties of matrix part. Aggregates, acting as stress concentrator, can impair mechanical properties of conglomerate. With saturation of structure by aggregate grains forms dense framework, glued by a thin layer artificial or natural binder. Such structure is called porous structure. It is favorable both from the point of view of binder discharge and of giving the necessary technical properties to the materials.
C
ontact
structure is characterized by
maximum saturation of a material with an aggregate, when the binder
amount is not enough for filling the voids between aggregate grains
and in a number of cases for formation of continuous cover on their
surface.
The index of macrostructure is a packing coefficient (Kp):
, (1.6)
where Lpr is a projection of distance between the centers of neighboring grains; D is a diameter of grains.
At Kp > 0 aggregate grains shared by the interlayer of binder; at Kp = 0 – they contact; at Kp < 0 – they are anchoring, i.e. overstep each other.
Macrostructures may be differentiated also by their absolute and relative sizes of grains. Maximum sizes of grains for fine-, medium- and coarse-grained structures can be different depending on the material. For example, the structures of mountain rocks belong to fine-grained, if the grain sizes is less than 2 mm, for concretes - 5 mm; to medium-grained — 2...5 mm and 5...20 mm; to the coarse-grained - over 5 and 20 mm correspondingly.
By their relative sizes they are differentiated as even-grained and non even-grained structures. Porphyritic structures are the typical kinds of uneven-grained structures, which are characterized by a presence in material of glassy or fine-grained great bulk where separate large crystals - inclusions are dissipated. Such structures are mainly effusive rocks, a series of conglomerate materials which are artificial by origin.
The structure of construction materials changes with time under the influences of processes, predefined both by their internal nature and environment. The development of new formations during hydration process increases concrete strength, improves a series of other properties, but at the same time under the influence of aggressive environmental conditions destructive processes like corrosion may occur with destructive character. The desired durability of material is achieved by formation of such structure which minimizes destructive processes. The typical example of the directed formation of such structure is formation in concrete air voids by SAS admixtures application, uniformly distributed porosity in all the volume. Such pores prevent growth of water pressure which increases as its freezing, and also diminish capillary water suction as a result of hydrophobization of capillaries surface.
Pores, cracks and others defects of materials structure influence on development of destructive processes. The most dangerous pores are capillaries, filled by water.
The criteria of structure efficiency are the parameters of the main properties of materials, such as strength, thermal conductivity, frost resistance, impermeability, etc.
Thus, the optimization of concrete structure from the strength point of view requires minimization of the volume of opened and closed pores, and from the point of view of frost-resistance, it is important to provide appropriate ratios between volume of closed and the volume of opened pores to be saturated with water.
Properties which are connected with each other definitely and formed under the influence of the same structural parameters acquire minimum or maximum value practically at certain structure which can be considered as an optimum one. At the ambiguous connections of different properties an optimum structure is in a compromise area.
The important parameters are the:- disperse phase content and disperse medium, phase relationships, degree of homogeneity of disperse phase particles distribution in the medium, etc. Optimization of materials structure requires the polystructural approach (-taking into consideration of interrelations of property – structure type, formed at all the levels considered).