10.3. Structure and properties of concrete

The concrete structure. Dense heavy-weight and light-weight concrete, as a rule, include aggregate framework which actively influence on their properties. Such structure is rational, as if it provides low porosity of concrete at the moderate cement content. Structure of light-weight concrete differs from heavy-weight one by the presence of additional pores in grains of aggregates.

The concrete microstructure represents cement stone structure. Cement stone by itself is a conglomerate of cement hydration products, inclusions of the unhydrated grains of clinker, additives and air bubbles. Pores in the cement stone are presented as connected with each other canals of capillaries, disconnected by the products of cement hydration (by cement gel). The pores according to the origin are divided into the gel pores and capillary pores.

Capillary pores, formed by the excessive mechanically bounded water, impair the basic concrete properties, especially frost resistance. Capillary porosity diminishes as far as the water-cement ratio water content declining and increasing of hydration time.

Along with capillary pores, pores and voids, resulting in bad compaction, have negative influence on concrete properties. General pores volume of the cement stone is 25...40 % from the general pores volume of concrete, at that their basic part is the capillary pores. With the increasing of hardening duration, the general porosity and volume of macrocapillaries diminishes, which leads to the improvement of the concrete properties.

Along with the processes of the structure improvement, the destructive, that is ruinous processes, mostly due to aggressive environmental factors, during some time develop in the concrete.

Properties of concrete. The basic property of concrete as structural material is strength. The ultimate strength is defined by control specimens testing, made of the mixture of the set composition or drilled from the concrete structure. At testing on compressive strength cubic specimens (rarely cylinders) with the size of rib from 30 to 7,07 cm depending on the fineness of applied aggregate are used. Standard are the cubic specimens with length of rib 15 cm. For cubic specimens with the other sizes of ribs, experimental scale factor coefficient is accepted.

The basic index of concrete strength which is specified is its class that is strength (in MPa) which is accepted with the assured probability. The probability of strength set, as a rule, 0.95 means that ultimate strength of concrete, which meet the requirements of class numeral value, is achieved at not less than in 95 cases from 100. Compressive strength classes for heavyweight and lightweight concrete are listed in Table 10.8.

In practice at making concrete its strength indexes initially determine as middle results of the separate specimens testing. After the numeral value of average strength and accordingly class of concrete taking into account the coefficient of variation (variability) determine its strength class.

There is dependence between the concrete class (C) and the average strength of consignment ( ) which is controlled:

, (10.5)

де Cv- coefficient of variation of concrete strength.

The coefficient of variation of concrete strength is found according to a formula:

, (10.6)

where S - average square deviation for concrete consignment;

Table 10.8

Compressive strength classes for heavyweight

and lightweight concrete

Heavyweight concrete		Lightweight concrete
Required grade, i.e. required minimum characteristic cube strength, MPa	Specify compressive strength class	Required grade, i.e. required minimum characteristic cube strength, MPa	Specify compressive strength class
10	C8/10	9	LC8/9
15	C12/15	13	LC12/13
20	C16/20	18	LC16/18
25	C20/25	22	LC20/22
30	C25/30	28	LC25/28
35	C28/35	33	LC30/33
37	C30/37	38	LC35/38
40	C32/40	44	LC40/44
45	C35/45	50	LC45/50
50	C40/50	55	LC50/55
55	C45/55	60	LC55/60
60	C50/60	66	LC60/66
67	C55/67	77	LC70/77
75	C60/75	88	LC80/88
85	C70/85
95	C80/95
105	C90/105
115	C100/115

At the high culture of production and stable technological parameters of the variation coefficient of concrete strength achieves at 6...8 %, at the insufficient level of technology - 20...25 %. The standardization of concrete strength on classes allows providing required design reliability regardless of to the coefficient of variation. At the same time, the coefficient of variation diminishing, as ensues from a formula (10.6), allows to decrease the required average strength which in same time conduces the declining of required cement content.

For a transition from the concrete class to the average strength at the normative coefficient of variation 13.5 % (what is accepted as rule at the structural design on the heavy-weight and light-weight concrete) it is possible to use the formula of =C/0.778, where C is a numeral value of concrete class.

The raised tensile strength is the characteristic feature of light-weight concrete. The developed surface of aggregates promotes it, causes the good adhesion with a cement stone. The correlation between tension and compression strength for heavy-weight concrete is 0.05...0.1 and for light is 0.06...0.17.

One of performance criterion of the light-weight concrete is a coefficient of structural quality - the correlation between the ultimate concrete strength to its average density. By density in the dry condition kg/m³ for light-weight concrete grades are set from LC 1.0 to LC 2.0 (with gradation through each 200 kg/m³).

T ime interval (age of concrete) after which class is determined, depends on the type of concrete and row of plant conditions. It is accepted, as a rule, equal 28 days of normal hardening, that at a temperature (20 ± 2)°C and relative humidity of air not below 90 %. If it is required age of concrete can be increased to 90 or 180 days that leads to the economy of cement. At the calculation of reinforced-concrete structures next to cube strength it is required to know prism strength which is determined by the prism compressing with the sizes 20 20 80 cm. The ratio between cube strength of concrete and prism strength is in the range of 0.7...0.8.

There are various non-destructive methods of control of concrete strength directly in element and structures except of determination of concrete strength as a result of control standards testing (Fig. 10.7). Using of non-destructive methods is caused by the necessity of taking into account production factors: deviations from the set concrete composition, conditions of transporting, casting, hardening and other. An important advantage of non-destructive methods is an immediacy and simplicity of the strength control.

T he mechanical methods of non-destructive control are based on principles of resilient rebound or impression. Thus concrete strength is determined by a resiliency which is characterized a rebound of striking bodies (devices of Shmidt and other) or hardness, measured by the diameter of print (devices of Fizdel, Kashkarov, Gubber and other). The mechanical methods allow defining the strength of surface layer; they give a high error at concrete inhomogeneity in a sectional view.

The impulse method, based on measuring of distribution speed of ultrasonic vibrations in material is spread among the physical methods of non-destructive control of concrete strength. The change of speed spreading of ultrasonic vibrations indicates the certain strength changing at the permanent compositions of concrete and the hardening conditions.

The cement strength and water-cement ratio are the main among the known rows of factors which influence on the concrete strength. Under the water-cement ratio is understood the correlation of water mass towards mass the cement in fresh concrete mix.

Dependence between compressive strength concrete (R_c), strength of cement (R_cem) and water- cement ratio (W/C) is possible to present by formula:

. (10.7)

This dependence expresses the basic law of the concrete strength - rule of water- cement ratio, essence of which consists in a fact that at permanent materials, production technique and hardening conditions, the concrete strength depends only on a water-cement ratio (Fig. 10.8).

Next formula is widely used for calculations in relation to a heavyweight concrete:

. (10.8)

The formula (10.8) foresees, that in certain intervals the concrete strength is related to the value, reverse to W/C, in other words a cement-water ratio (C/W rectilinear dependence. In formulas (10.7, 10.8) coefficients A' and A depend on a quality of the initial materials.

The rule of the water- cement ratio is just at comparatively similar conditions, in opposite case the calculated and factual strength can substantially differ, that is explained by the influence of not counted factors.

In the light-weight concrete cement stone and grains of aggregate differ with strength and deformability in a less measure, than in heavy-weight concrete. That is why the strength of porous aggregate influences substantially on concrete strength.

F or approximate prognostication of the concrete strength increasing during the time it is possible to use the logarithmic dependence:

, (10.9)

where - the ultimate concrete compressive strength at the age n days (n > 3); - 28- days concrete strength.

The increasing of the concrete strength on the cement of certain chemical-mineralogical composition during some time is determined with thetemperature-humidity conditions of hardening (Fig. 10.9). At the increasing of temperature, the processes of the cement stone hardening - basic structural component of concrete are accelerated.

The concrete hardening is accompanied with heat release as a result of passing of exothermic processes of the cement hydration. It is required to support high humidity for the normal passing of these processes to prevent the concrete drying. Factors influencing on the hardening intensity, affect also on the heat generation (exothermal reaction) of concrete. The mineralogical composition of cement is the major factor which influence on heat generation. Depending on the intensity of heat generation mіnerals that cement consists of, they are located in a row: C₃A > C₃S > C₄AF > C₂S (Fig. 10.10).

T he thermal effect of hydration of С₃А is almost twice higher, than С₃S and in 5 times higher, than С₂S. The heat generation of concrete is increased at the increasing of cement content, grinding fineness, at increasing of temperature, and also in the presence of hardening accelerators; reduces - at introduction of mineral admixtures (slags, ashes of thermal power stations and other) and surface-active substances (SAS) to the cement or concrete mixture.

The positive role of the heat generation of concrete shows up at the winter concreting, and also heat formation treatment with the purpose of the hardening acceleration.

T he heat generation of concrete plays a negative role for massive structures - dams, foundations under the generating units and other. It causes the considerable increasing of temperature (up to 50°C and higher) in the core of a massive, development of the thermal stresses and cracking The decline of self-heating of concrete in massive constructions is achieved at the use of low-heat cements with the heat generation in 3-day's age not more than 230 J/g. The decline of concrete mixture temperature is effective. The concrete cooling by water which circulates in pipes is also used.

At the negative temperature, hardening of concrete takes place slowly (Fig. 10.11), or does not flow generally, because of free water freezing and stopping of process of cement hydration. The ice formation in the concrete is accompanied by the increasing of pressure which destroys its structure. At the thawing hardening renewals, however the strength and other properties of concrete reduce as a result of the structure deterioration.

The winter concreting, as a rule, is executed by the thermostat method and method of electrical warming. The method of steaming treatment is also used and so-called "cold concrete".

The enclosures and large tents are used in some cases, for example, at the laying of concrete in the blocks of hydraulic buildings at a temperature below of minus 20°C.

The thermos method is used at concreting of massive structures and consists in hardening due to a heat, which is released during the cement hydration and obtained at the warmed components using. The concrete surface is protected by the heat insulator.

The steam- heating is affected in steam jackets or chambers, and also at the steam running through the pipes, filled in a concrete. The concrete hardening is affected also due to the heat which is selected in the body of concrete at the passing of current through metallic electrodes, or as a result of heat-transfer from the heated air at electrical warming. The alternating current of normal frequency is used for the concrete preheating. Direct current causes electrolysis and that’s why it is not used.

"Cold concrete" has an ability to harden at the subzero temperature due to introduction of large amount of chemical admixtures which reduce the mixing water freezing temperature. As antifreeze admixtures are used hydrochloric and ammonium salts, sodium nitrite, ammonium water, potash etc.

The concrete is reliable and durable material in the condition of accordance to its environmental requirements. The rejection and destruction of concrete can be caused by as internal stresses and as external actions. It is possible to take the chemical action of water and substances, which are contained in it, to the number of such factors; turns and multiple wetting and drying; freezing and thawing.

The temperature differences, reaction of cement alkalines with some aggregates, crystallization of salts in the pores of concrete and others like that, destroy the integrity nature of concrete and cause cracks.

The freeze-thaw resistance of concrete is the ability to withstand the certain amount of cycles of alternate freezing and thawing in the saturated water state at the decline of the strength not more than 5 %.

T here are some grades, set according to a frost resistance, depending on the amount of cycles of freezing and thawing, which is established by standards, for the ordinary heavy-weigh concrete – F50, F100, F150, F200, F300; for the hydraulic engineering, except of indicated, in the case of grounding – F400, F500, and higher, and for light-weight – F25 and F35 and higher. The basis for the grades setting according to freeze-thaw durability is a design number of freezing and thawing cycles, which concrete tests during a year in data of supervisions, climatic and operating conditions are also taken into account.

The freeze-thaw longevity of concrete always rises at diminishing of relative volume of capillary pores and increasing of volume of reserved pores, filled with an air.

It is required to reduce water-cement ratio and the water content that are factors which determine capillary porosity, for the concrete production with high density and frost resistance (Fig. 10.12). At the normal hardening of concrete to provide it high frost resistance is recommended, that W/C should not exceed 0.5, and content of water not more than 160 l/m³.

The correct selection of cement and aggregates matters next to the declining of the capillary porosity. The using of cement with minimum content of the tricalcium aluminate (C₃A <8 %) and enhanceable content of minerals-silicates at advantage of the tricalcium aluminate have the positively affect on the freeze-thaw durability of concrete. Introduction of active mineral admixtures to cement in an enhanceable amount conduces to the noticeable decline of freeze-thaw durability

The surface-active substances are widely used to increase the freeze-thaw longevity of concrete. They allow reducing a water-cement ratio and providing the fine air pore formation, to which the frozen water is pushed back from capillaries and they also improve a crystalline structure of the cement a stone.

The water impermeability, which is characterized by the maximum pressure at which there is no water filtration through the specimens, is the important index of concrete quality for the constructions and buildings which work under the action of certain pressure. The grades of water impermeability are appointed depending on the pressure gradient (ratio between maximal water pressure and the thickness of construction) and character of construction functioning. At a pressure gradient to 5 the grade of W4 is recommended; from 5 to 10 - W6; from 10 to 12 - W8; 12 and more than - W12.

The basic ways of water filtration are the capillary pores and especially sediment capillaries, formed as a result of stratification of concrete mix.

The high water impermeability is arrived at the W/C declining, application of expanding, plasticized and hydrophobic cements, pozzolanic and blast-furnace cement. At the sufficient humidity of environment the water impermeability is substantially increased with the hardening duration increasing due to diminishing of volume of opened pores during the cement hydration. The water impermeability and water absorption of concrete are the indexes of its density to which firmness of concrete to the different aggressive actions of circumambience is related (Table 10.9).

Таble 10.9