Control Of Restraint Stress And Its Effect On Behavior Of Concrete Under The Attack Of Single-side Freezing And Thawing

Damage caused by freeze-thaw cycles with the salt solution is an important factor affecting the durability of concrete structures in cold regions.The research on salt freezing and thawing of concrete has experienced a process from a single factor to the service state's simulation.The service state of concrete is a restraint state,which means the service state's simulation needs to apply restraint on concrete in the laboratory.The existing researches mainly focus on the salt frozen behavior of concrete with a large water-cement ratio(W/C)under qualitative restraint.Further studies on the restraint stress' characterization and the behavior of restrained concrete with lower W/C are needed.In this paper,a restraint stress test device was designed to test restrained concrete's durability under salt freezing and thawing cycles.The salt frozen behavior of restrained concrete with different water-cement ratios and restrained lightweight aggregate concrete was studied.The damage law and the degradation mechanism of restrained concrete were obtained.The results are as follows:1.The restrained stress test device is designed.Steel rings with different thicknesses d were used to restrain the concrete's expansion,and anchors with restraint length l(Length embedded in concrete)were used to restrain the concrete's shrinkage.Strain gauges were used to collect the steel ring's expansion strain and the anchor's shrinkage strain during all freezing and thawing cycles.The restrained stress provided by the restraint device during the whole test is obtained through calculation.2.The influence of ring thicknesses d and restraint length l on the restrained stress with was obtained.For the first time,the strain and stress in the F/T cycles of concrete under a restrained state can be measured and controlled.A complete restrained stress test method was established.With the increase of d(1 mm,4 mm,8 mm,and 12 mm),the variation of expansion restraint stress(??RE)decreases by 50%,and the restraint ability of steel ring to concrete is enhanced.With the increase of l(28 mm and 33 mm),the variation of shrinkage restraint stress(??RS)increases by 26%,and the restraint ability of anchor is enhanced.Based on the surface scaling(wn),relative dynamic elastic modulus loss(Rn),residual strain(Sn),water absorption of the specimen due to freeze-thaw(?mn,0),and strain-time curve,the appropriate combination of restraint parameters and the number of cycles of concrete under the attack of single-side salt freezing and thawing cycle were determined:d=8 mm,l=28 mm,n=28 cycles.3.Under the proposed restrain condition,the salt frozen behavior of restrained concrete was studied.(1)The influence of the water-cement ratio on salt frozen behavior of restrained concrete was obtained.With the increase of F/T cycles,the surface damage and internal damage of the restrained concrete increase.The surface and internal damage of both restrained and free concrete are reduced at lower W/C.Compared with the free concrete under the same W/C,the restrained concrete has heavier surface damage and lighter internal damage.(2)The salt frozen behavior of restrained lightweight aggregate concrete was obtained.With the increase of F/T cycles,the surface damage and internal damage of the restrained lightweight aggregate concrete increased.Under the same W/C,the surface scaling(w28),relative dynamic elastic modulus loss(R28)and residual strain(S28)of restrained lightweight aggregate concrete are respectively 204%,50%,and 50%of that of the lightweight aggregate concrete in a free state,indicating that restrained lightweight aggregate concrete has heavier surface damage and lighter internal damage.4.The mechanism of surface damage was explored through the calculation and analysis of the tensile stress in the ice layer,and then the reason for the more serious surface of the restained concrete is cleared.When the temperature drops to-1.79?,the test solution starts to freeze,and the ice's internal stress begins to accumulate.When the temperature drops to-10?,the ice layer's internal stress changes into tensile stress gradually,and the risk of cracking increases.Cooling continues,the tensile stress in the ice layer exceeds the ice layer's tensile strength,and cracks appear in the ice layer and develop towards the concrete,resulting in surface damage.Compared with the free concrete,the difference in bulk coefficient of thermal expansion between the ice layer and the restrained concrete increased from 30%to 39%.The tensile stress accumulated by the ice layer increased to 30%,resulting in earlier cracks in the ice layer and heavier surface damage in the restrained concrete than in the free concrete.5.The internal damage mechanism of restained concrete was explored by calculating the deformation stress of concrete and the analysis of the solution migration,pore structure,and crack in the concrete.The restraint can reduce the deformation stress of concrete by 15%(d=1 mm,l=28 mm),20%(d=4 mm,l=28 mm),22%(d=8 mm,l=28 mm)and 21%(d=4 mm,l=28 mm).Further,the solution's migration is limited in concrete,weakening the pore solution's crystallization pressure in concrete and weakening its threat to the pore wall.Restraint reduces the water absorption of concrete by 50%and reduce the damage to pore wall caused by crystallization pressure,so the restrained concrete's internal damage is weak.