Results Of Experimental Research Of Steel-Concrete Composite Beam Under Cyclic Freezing-Thawing

. One of the ways to improve the efficiency and quality of reinforced concrete structures is the development and implementation of new building structures, the use of which ensures a decrease in material consumption and cost. These effective structures include steel-concrete structures. An important problem when using steel-concrete structures is to ensure the joint work of steel and concrete, which is determined by the adhesion of materials. Reinforced concrete structures must meet the requirements for safety, serviceability and durability. The question of the operation of steel-concrete structures with external reinforcement in the form of a box section remains poorly understood. At the same time, the question of the influence of cyclic freezing and thawing on the adhesion strength between the metal shell and concrete remains open. In order to identify the features of the work of bent steel-concrete elements with external reinforcement at alternating temperatures, experimental studies were carried out, the results of which on strength and deformability are presented in this article. Investigations were carried out in two directions: first, the influence of the CFT on the change in the adhesion strength between the metal shell and concrete under compression; the second influence of CFT on the joint work of concrete and metal in bent steel-concrete elements.


Introduction
One of the most important areas for acceleration of development of building structures manufacturing is creation and use of effective structures with high reliability, low metal use and labor intensity [1][2][3][4][5][6].Steel-concrete composite structures with external reinforcement meet all these requirements.Use of these structures in construction constantly expands both in Russia and foreign countries [7][8][9][10][11][12][13].
Steel-concrete composite structures should meet requirements for safety, usability, and durability.Contemporary Russian normative documents specify regulations for design of steel-concrete composite structures operating in temperature range from 50°С down to -60°С (SP 266.1325800.2016Composite steel and concrete structures.Design rules).However, there is still an open question on considering of alternating temperatures influence on behavior of steel-concrete composite beams.[14][15][16][17][18][19][20].Hence experimental research of cyclic freezing-thawing (CFT) influence on behavior of steel-concrete composite elements with external steel shell was carried out.Influence of CFT on change of bond strength between metal shell and concrete and behavior of bent steel-concrete composite elements was researched.

Experimental research methods
Steel-concrete composite prism elements were used as testing specimens to determine bond between concrete and metal.Figure 1 shows structural concept of testing specimens.Freezing down to -50°С and thawing up to 15°С of steel-concrete composite prisms were done at concrete age of 28 days.Bond strength between concrete and steel shell was determined after respective number of freezing-thawing cycles.Displacement of concrete core [1] relative to metal shell [2] was determined using dial gauges [6,7].
Steel-concrete composite beams with dimensions of 120x200x2200 mm were used as bend testing specimens.Figure 2 shows testing specimen in the shape of welded metal beam with box-shaped cross-section filled with concrete.Beams were subjected to cyclic freezing down to -50°С and thawing up to 15°С at concrete age of 28 days.Steel-concrete composite beams [1] were tested under standard conditions until fracture after 15, 30 and 45 freezing-thawing cycles.Beams were equipped with strain gages and dial gauges [3][4][5].

Results
Figure 3 shows test results of change in bond strength between steel shell and concrete after freezing-thawing cycles.Number of cycles was equal to freeze-thaw resistance concrete grade.Figure 4 shows bond strength of concrete and steel shell.Presented figure shows that there is gradual decrease in bond strength between concrete and steel shell across all tests for steel-concrete composite specimens with lower concrete humidity.Bond strength between metal and concrete decreases by 23% at 4.5% humidity and increases by 4% at 7.0% humidity.This is due to the fact that flow of water to border line between steel and concrete is not enough for creating new adhesive bonds and reconstructing of destroyed ones during CFT in concrete with lower humidity.Formula for determination of bond strength after number of freeze-thaw cycles equal to freeze-thaw resistance concrete grade was acquired as a result of steel-concrete composite prisms tests.
Where -relative humidity of concrete, %; 0  -bond strength between concrete and metal before CFT, MPa.
Change in bond strength under cyclic freezing-thawing based on test data can be interpreted as:    Correlating these results with concrete test results it can be concluded that increased strain properties of concrete acquired due to CFT leads to the fact that on the first stage of steel concrete composite beams behavior (up to 0, 7 ult PP  ) metal shell receives almost all load.Further plastic strain in most stressed fibers of compressed metal leg leads to redistribution of load from metal to concrete.The higher plastic strain in metal, the more intensively compressive area of concrete takes part in behavior of steel-concrete composite beam [19].r -distance between compressed face of cross-section and center of gravity for steel shell.
Height of compressive area of the concrete core was determined using formula [19]: Comparison of test results values for load bearing capacity and calculated values using formula 2 showed that variance lies in the range of ±5%, which confirms acceptability of given formulae for calculation of strength for steel-concrete composite beams with external steel shell.

Conclusions
1. Bond strength of metal to concrete decreases by 13% at 4.5% humidity and increases by 4% at 7.0% humidity after number of freezing-thawing cycles equal to freeze-thaw resistance concrete grade.2. Load bearing capacity of steel-concrete composite beams before and after 45 freezing-thawing was higher than for metal beams by 1.52 and 1.37 times respectively.3. Load bearing capacity of steel-concrete composite beams after 45 freezingthawing cycles decreases by 10% compared to steel-concrete composite beams tested before CFT. 4. Stress-strain behavior of bent steel-concrete composite elements should be assessed considering change of strain and strength properties of concrete after cyclic freezing-thawing.

.
that prism strength and elasticity modulus of concrete decreases by 54% and 78% respectively after 45 freezing-thawing cycles.At the same time compression strains at the diagram vertex increased by 2.7 times.Strain and strength properties of metal shell steel acquired in tests were as follows: Figure4shows test results in "loading level (P/Pult )-curvature" diagrams.

Figure
Figure shows that steel-concrete composite beams tested under standard conditions before CFT have lower deflections compared to metal beams.This suggests that concrete core in steel-concrete composite beams actively contributes to specimen behavior under load and receives significant part of load.Deflections of steel-concrete composite beams tested after 45 freezing-thawing cycles almost match deflections of metal beams up to loading level 0, 7 ult PP  .Afterwards deflections in metal beams start to increase faster, which leads to significant increase in deflection at insignificant load increase at 80 kN load ( 1, 0 ult PP  ).Resistance of steel-concrete composite beams at this load level continues to increase.

5 S
moment of steel shell with box-shaped crosssection; b -cross-section width of compressive area of the concrete core; w t -thickness of steel shell plate; 0,-static moment of cross-section half for steel shell about its geometric axis;

Table 1 .
Shows strain and strength properties of concrete.