Characteristics of mechanical and rheological properties of concrete under heating conditions up to 200 ◦ C

The results of experimental research of high up to 200 ◦C temperature influence and scale effect on temperature and shrinkage strain, creep and characteristics of strength and strain properties of high-strength modified fine and heavy concretes under axial compression are presented in the article. The practical way of accounting of the influence of the scale effect on design variables of shrinkage strain and concrete creep is proposed.


INTRODUCTION
Modified concretes based on organic-modifiers characterized by high (55-80 MPa) and ultra-high (over 80 MPa) strength, low permeability, high corrosion resistance and durability have become widespread in the building sphere [1,2].The use of high-strength concrete in construction under the influence of higher technological temperatures seems to be perspective.However, the changes of physical-mechanical and rheological properties of concrete exposed to heating are insufficiently studied.

CONCRETE MIX PROPORTION, TEST SPECIMENS SIZES AND TEST PROCEDURE
The main objects of study are specimens made of heavy and fine high-strength concretes with organicmineral modifiers [1] containing silica fume, fly ash of Donetsk region thermal power-stations, fluidifier and concrete hardening controller.Concrete mixes: heavy concrete with crushed granite (concrete mix N º1) -Cement: Sand: Crushed Stone = 1:1.1:2.2 with W/C = 0.3 with a modifier in the amount of 20% of cement weight (slump = 21 cm), fine concrete (concrete mix N º2) -Cement: Sand = 1:1.8with W/C = 0.26 with a modifier in the amount of 10% of cement weight (slump = 25 cm).Silica sand with the fineness modulus of 1.9 is used for fine concretes.Basic specimens in standard sizes are cubes with the edge of 150 mm and prisms with dimensions of 150 × 150 × 600 mm.Additional specimens are cubes with the edge of 100 mm and prisms with dimensions of 100 × 100 × 400 mm, 250 × 250 × 650 mm and 300 × 300 × 800 mm.
When heating the specimens the thermal response was equal to 15 • C per hour, the first short duration of heating before pressure testing at temperatures of 90, 150 and 200 • C was 15, 13 and 12 hours respectively which corresponds to the time of conventional heavy concrete reaching the minimum strength under similar testing temperature conditions [3].The length of durable heating at the temperatures tested was respectively 40, 30 and 20 days.The loading of all samples was carried out under the press capacity of 2500 kN with block step magnitude of 0.1 of the tolerated stress with a 5-minute interval at each block step.The characteristics of concrete deformation properties, the tangent modulus of elasticity E c and the Poisson's ratio ν, were determined according to the standard method at the level of loading = c /f c ≤ 0.3, the shortening strain cu 1 and extensional strain cu 2 ≈ cu 3according to the values corresponding to the maximum level of stress achieved in the experiments.

THERMAL AND SHRINKING DEFORMATIONS OF CONCRETE
The experimental results indicate a significant dependence of temperature-shrinkage deformation, characteristics of strength and deformation properties of high strength concrete on the size of test specimens, temperature and heating duration (Fig. 1, Fig. 2).
In conditions of normal temperature the deformation shrinkage of high-strength concrete N º1 in the longitudinal direction of the specimens 150 × 150 × 600 mm in size made in average 43.5 • 10 −5 while the specimens 100 × 100 × 400 mm and 250 × 250 × 650 mm in size -125% and 88% respectively of the values of reference prisms (Fig. 1).Fine concrete shrinkage strain of concrete N º2 was on average 14% higher than that of concrete N º1.The shrinkage deformation for the test specimens of all sizes in the transverse direction was 1.6-1.8times higher than the corresponding values in the longitudinal direction.
The average values of the linear thermal strain coefficient bt for the modified concrete in the temperature range of 20-200 • C during the first short-term heating made bt = 10.3 The characteristic data mentioned were numerically close to the corresponding values for the normal-weight concrete of medium strength [4].
The correlation between the relative limiting values of high-strength concrete shrinkage deformations and the open surface modulus M 0 of specimens is shown on Fig. 3.For practical purposes this correlation can be expressed by the relation: cs (M 0 ) -the function of the specimens massiveness effect accounting on their shrinkage strains in comparison with the strains of reference specimens with M 0 = 30 m −1 :

THE STRENGTH AND STRAIN OF CONCRETE UNDER SHORT-TERM AXIAL COMPRESSION
Short-term heating of the concrete N º1 up to a temperatures of 90, 150 and 200 • C has led to the prism strength reduction by 10%, 3% and 4%, the tangent modulus of elasticity -by 21%, 27% and 52%, to the ultimate compressibility increase -by 9%, 17% and 34% compared with the corresponding characteristics at normal temperature (Fig. 4).Long-term heating in comparison with the short-term one at the same temperature results in partial restoration of concrete strength to the levels of 0.95, 1.05 and 0.98 of the unheated concrete strength as well as in reduction of the tangent modulus of elasticity by 25%, 37% and 42% and in increase of the ultimate compressibility by 21%, 38% and 45% compared with the corresponding characteristic values before heating.
Short-term heating of the fine modified concrete N º2 up to temperatures 90, 150 and 200 • C does not practically lead to the strength reduction, but causes decrease of the tangent modulus of elasticity by 20%, 28% and 33% and increase in ultimate compressibility by 18%, 24% and 27% compared with the concrete characteristics before heating.
Deformation curves of high-strength concrete under axial compression under the temperature range from 20 to 200 • C are shown on Fig. 4.
Loading levels corresponding to the elastic deformation amounted for the modified high-strength concrete = 0.7-0.8.The process of the specimens elastic volume reduction changed into its increase (manifestation of the dilation effect) at loading levels over = 0.8.
The values of the Poisson's ratios ν of modified concrete with a crushed stone during shortterm heating up to 90, 150 and 200 • C reduce by 17%, 28% and 42% respectively and do not significantly depend on heating duration.The same characteristics of fine concrete under the same heating temperature reduce by 17%, 34% and 38%.
The changes in strength and strain properties of concretes exposed to elevated temperatures are caused by the manifestation of destructive and constructive factors in the concrete structure.The most significant destructive factors are structural stresses in the contact zone "matrix-aggregate" which in most cases become evident during the first heating of fine concretes.For concrete specimens with the level of long-term compression l = 0.5 and dimentions 150 × 150 × 600 mm (M 0 = 30 m −1 ) the mean limit values of relative linear creep strain of concrete at temperatures of 20, 90, 150 and 200 • C were respectively 90.6 × 10 −5 , 150.1 × 10 −5 , 161.4 × 10 −5 and 216.5 × 10 −5 .For specimens with dimensions 250 × 250 × 650 mm (M 0 = 19 m −1 ), the mean limit values of the linear creep strain of concrete at heating temperatures up to 90 and 150 • C made correspondingly 129 × 10 −5 and 141.4 × 10 −5 that is by 14.2% and 14.4% lower than for specimens of basic size.Reversible creep strain of concrete after complete unloading of basic specimens under the same temperature tests were respectively 9.3 × 10 −5 , 12.9 × 10 −5 , 15 × 10 −5 and 15.8 × 10 −5 (Fig. 5 A, B), and for specimens with dimensions 250 × 250 × 650 mm, after heating at temperatures of 90 and 150 • C were 10 × 10 −5 and 11.5 × 10 −5 respectively (Fig. 5 C, D).

THE CREEP OF CONCRETE
For specimens with dimensions 100 × 100 × 400 mm, 150 × 150 × 600 mm, 250 × 250 × 650 mm and partially moistureproof corner surfaces with the open surface modulus values respectively M 0 = 16, 16 and 8 m −1 the limit creep strain at the level of loading l = 0.5 made respectively cc 1 = 38.9• 10 −5 , cc 1 = 35.6 • 10 −5 and cc 1 = 32.1 • 10 −5 .Thus, for linear relative creep strain there is a tendency for the increase of their limit values with the open surface modulus of concrete M 0 .The values of the specific creep strain (t, t 0 ) = cc 1 / 1 obtained from the results of experimental studies for the specimens of the same size at the level of loading l = 0.3 made respectively 3.3 • 10 −5 , 2.6 • 10 −5 and 2.3 • 10 −5 , at a level l = 0.5 -4.3 • 10 −5 , 4.1 • 10 −5 and 3.7 • 10 −5 , and for partially moistureproof specimens (M 0 = 16, 16 and 8 m −1 ) -3.7 • 10 −5 , 3.7 • 10 −5 and 3.3 • 10 −5 respectively.The values of linear oc and limit C(t, t 0 ) creep strain with the same open surface modulus but with different cross-sectional dimensions are similar in their absolute values (Fig. 7).6.3.The impact of elevated temperatures in comparison with conventional high strength concretes has less influence on the strength of high-strength modified concretes (reduction is not more than 10%) and a more significant impact on the performance of their deformation properties.
The long-term heating insignificantly changes the tangent modulus of concrete elasticity compared with the short-term heating, though it makes for the further increase of the ultimate compressibility compared with the corresponding characteristics of the concrete that is not exposed to heating.The Poisson's ratio of concrete decreases proportionally to the temperature and heating and does not significantly depend on the duration of its action.
6.4.The irregularity of the strength and deformation properties of modified concretes in large-scale prism specimens is caused by different conditions of hardening and drying of internal and external volumes of concrete.Internal volumes of concrete require higher strength values f c (up to 25%), the tangent modulus of elasticity E c (up to 16%) and limit compressibility (12%) compared with the values in external layers.Accounting for the mentioned irregularity of the properties in calculation allows to make the calculated values of strength and deformation structures closer to the tested ones and to use their bearing capacity reserves.

Figure 2 .Figure 3 .
Figure 2. Temperature and shrinkage strains of high-strength heavy modified concrete N º1 under heating up to 200 • C.

Figure 4 .
Figure 4.The effect of short-term heating on deformation curves of high-strain modified concrete under axial compression: A -linear strains; B -relative volume change.