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Modified High Performance Concrete

2. MB modifiers influence on properties of concrete and concrete mixtures
2.1. Concrete properties
Strength and deformation	The application of MB modifiers allows to produce high–strength (50…80 MPa) and very high–strength (over 80 MPa) concrete using ordinary materials (cement and aggregates) [3, 4, 9, 10, 11].   Figure 4 shows accessible level of compressive strength for MB–01 concrete depending on cement, modifier and water content.

Figure 4	Compressive Strengthat 28 Days

Naturally, setting kinetics depends on concrete curing temperature (Fig. 5). Low positive temperature (+3…5°C) slows down kinetics at initial stage, but further influences the brands strength (of 28 days) insignificantly. In normal temperature environment (+20°C) a high enough strength could be attained within 24 hours, which provides for an early structure stripping and loading. Increased temperature (+40…42°C) accelerates setting significantly allowing to attain a higher early strength [14, 45].

Figure 5	Kinetics of Concrete Hardening at Different Curing Temperatures (Cement = 480 kg/m3; MB–01 = 48 kg/m3; W/C = 0.32)

At later ages hardening kinetics differs from normal concrete insignificantly and could be estimated similarly to hardening kinetics of concrete containing microsilica, fly ash and superplasticizer [13 24].   Information on different MB type efficiency which has been estimated according to compressive strength criterion for concrete of identical composition is shown in Figure 6. [8, 52].

Figure 6	Relative Efficiency of Various Types of Modifiers Estimated According to Compressive Strength Criterion (Cement=350 kg/m3; W/C=0.42; Slump=16–18 сm)

Standard deformation characteristics of MB concrete classes up to B60 (compressive strength up to 80 MPa) including shrinkage and creep correspond to existing СНиП 2.03.01–84 «Concrete and reinforced concrete structures» and СНиП 2.05.03–84* «Bridges and pipes» [9, 12].   MB concrete classes over B60 (compressive strength over 80 MPa) are characterized by increased modulus of elasticity and high values of prism strength factor (0.75...0.80). Upper limit of micro cracking approaches prism strength. [9, 10, 11].   Relative shrinkage and creep deformations are comparatively small and are stabilized at the age of 70–100 days. General tendency for MB–01 concrete is shown in Figure 7.

Figure 7	Shrinkage (esr) and Creep (С) of Concrete Classes В60–В80 with MB–01 at loading level s/Rb=0.3

Permeability

MB concrete are highly tight for water, gas and chlorides [25]. It should be noted that MB influence on cement paste structure and concrete permeability can be estimated by analogy with concrete containing microsilica, fly ash and superplasticizer [8, 26, 27, 28].   General tendencies of MB–01 concrete permeability alteration depending on penetrating agent type are presented in Figure 8 [45].

Figure 8	Permeability of Concretes with МB–01 (Cement=400 kg/m3 W/C=0.38)

Comparative efficiency of different MB types estimated according to watertightness criterion for concrete of identical composition is shown in Figure 9 [8, 52].

Figure 9	Relative Efficiency of Various Types of Modifiers Estimated According to Permeability (Cement=350 kg/m3 W/C=0.42; Slump=16–18 сm)

Sulfateresistance

MB–01 and MB–30C modifiers introduced into concrete mixture prepared with ordinary medium aluminate Portland cement provide the same level of sulfateresistance as that of concrete prepared with low aluminate (sulfateresistant) Portland cement.   Comparative sulfateresistance for MB–01 concrete with normal Portland cement and concrete with sulfateresistant and normal cement without modifiers is shown in Figure 10.

Figure 10	Relative Deformation of Concretes in Na2SO4 Solution of 5% Concentration (Cement = 350 kg/m3 W/C = 0.42; Slump = 16–18 cm)

Resistance to action of acid solutions	The application of MB–01 and MB–30C modifiers increases concrete strength in acid environment. Acid solutions with pH>3.0 classified as medium and weak aggressive environment for concrete of watertightness brand W8 (Russian norms and rules for designing structures corrosion protection.–СНиП 2.03.11–85) are not aggressive for MB–01 and MB–30C concrete with watertightness brand over W16.
	Note that in some cases when pH of aggressive environment is not lower than 3, the application of modified concrete allows to exclude the so called «secondary» protection of structures (coating isolation).   Table 1 presents information on aggressive environment for MB–01 concrete in comparison with concrete of W8 brand and reflects the need in «secondary» protection [45].

Table 1 Characteristics of concrete: Cement = 480 kg/m3; MB–01 = 50 kg/m3; W/C = 0.32; Slump = 16–18 cm	pH The Degree of aggression The need in «secondary» protection for concrete of watertightness brand W8 for MB concrete of watertightness brand W16 for concrete of watertightness brand W8 for MB concrete of watertightness brand W16 2.0 below 3.0 3.0 below 3.5 3.5 below 4.0 strongly aggressive medium aggressive weak aggressive medium aggressive not aggressive not aggressive yes yes yes yes not not
	The note: The degree of agression of the acid environment in relation to concrete is distributed to inorganic acids with pH>2.0 and organic acids with pH>3.0.

Alkali– aggregate reactions

Presence of MB–01, MB–30C and MB–50C modifiers in concrete prevents expansions associated with alkali – aggregate reaction. Modified concrete corrosion resistance to the indicated interaction is identical to that of concrete with microsilica, fly ash and superplasticizer [13, 17, 24, 29].   Comparative expansion ratio for fine grained MB–01 concrete produced with high alkaline cement and reactive sand is shown in Figure 11.

Figure 11	Expancion of Specimens Prepared on High–Alkaly Cement and Sand, Containing Reactive Silica (cement=500 kg/m3; Na2O+K2O=1.0%)

Corrosion resistance of reinforcement

Steel in concrete is normally in passive state with respect to corrosion due to high pH value in the pore water. With MB modifiers dosages not higher than 20% of cement weight the pH value is decreased to level not lower 11.8...12.0 that is just enough to provide corrosion resistance for all kinds of steel accessories without corrosion inhibitors for relative humidity of gas–air environment within the range of 65…95% [13, 17, 30].   With the dosages over 20% of cement weight effective passivating ability of modifiered concrete for the same humidity is provided at the expense of extra inhibitors introduction [30].

Frostresistance

MB concrete without air entraining or gas forming admixtures are usually of F200–F300 frostresistance brand (resistance to destruction under action of 200–300 cycles of freezing–thawing).   However, the application of MB allows to produce concrete with higher frostresistance and compressive strength and lower permeability [31, 32, 41, 44]. To produce highly frostresistant concrete (of F600–F1000 brands), the following conditions should be observed: to apply Portland cement with mineral admixtures in the amount not more than 5%; to use modifiers doses within the range of 10–11% of cement weight; apart from MB, to use air entraining or gas forming admixtures in optimal dosages.   Figure 12 shows frostresistance variations tendency for MB–01 concrete of B60 class depending on presence of air entraining or gas forming admixtures in concrete mixture [32, 45].

Figure 12	Frostresistance of High–Strength Concretes with МB–01 (Cement=480 kg/m3; МB–01=48 kg/m3; W/C=0.32; Slump=18–20 сm)

Information on different MB types efficiency with respect to its influence on frostresistance of concrete of identical composition is presented in Figure 13 [52].

Figure 13	Relative Efficiency of Various Types of MB Estimated According to Frostresistance of Concrete (Cement=350 kg/m3; Gas forming agent=0.5 kg/m3; W/C=0.42; Slump=16–18 сm)

Heating kinetics

While designing and erecting massive monolith structures with surface modulus (surface area/volume ratio) not less than 3, it should be taken into account that MB concrete heating kinetics differs from that of normal concrete.   Figure 14 describes a general tendency for temperature variations in the process of concrete setting testifying that usually maximal self–heating temperature of concrete with all types of MB modifiers hardening in normal conditions doesn't exceed +60°C. However, note that the total quantity of heat produced by MB concrete while hardening is equal to that of normal concrete.

Figure 14	Heat of Hidration of Cement Systems at Normal Curing (Cement+MB=460 kg/m3; W/(C+МB)=0.4)

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