Experimental Study On The Multi-axial Mechanical Behavior Of Concrete After Freeze-Thaw Cycling Or High Temperature

Freeze-thaw cycling or high temperature can hurt the strength of concrete, and result in more complex deformation. As a kind of bearing material, concrete is widely used in some severe environment. Therefore, suffering freeze-thaw cycling or high temperature is unavoidable for some reinforced concrete structures. Although some experimental studies on the deterioration of mechanical behavior of concrete, due to freeze-thaw cycling or high temperature, could be found at home and abroad, yet, to the best knowledge of the author, these studies are not systematic in some fields, or commonly concentrate on the uniaxialmechanical behavior. Such as systematic studies on the rule of strength decrease of air-entrained concrete after freeze-thaw cycling are lacking, the biaxial mechanical behavior of air-entrained concrete suffered freeze-thaw cycling, the tri-axial mechanical behavior of concrete suffered high temperature, and the multi-axial constitutive models based on corresponding multi-axial tests for concrete after freeze-thaw cycling or high temperature are still not studied. For the purpose of enlarging people's understanding on mechanical characteristics of concrete damaged by such severe conditions, this doctoral dissertation concentrates on corresponding experimental studies on the fields mentioned above, basing on the investigation on the project of National Nature Science Foundation "multi-axial stress and failure criterion of concrete in freeze-thaw environments" (50479059) and the project of educational department of Liao Ning province science foundation "Research on failure criterion and durability of concrete in severe environments"(2023901023). Main research contents are as follows:(1) The freeze-thaw cycling tests are carried out on air-entrained concrete of various strength grades and frost resistance grades. For concrete samples meeting the request of frost resistance, systematic uniaxial compressive tests are followed, and the decrease of strengths with the increase of freezing-thawing cycles is analyzed systematacially. Relationships between uniaxial cube strength, prism strength and freezing-thawing cycles are derived from test results, respectively. The formula of residual strength of air-entrained concrete after freeze-thaw cycling is obtained, using the size converting coefficient derived from contrasting tests. And corresponding tables for engineering application are given.(2) Biaxial compressive and biaxial tensile-compressive experiments are made on the air-entraining concrete suffered freeze-thaw cycling. The relationships between strengths, strains, elastic moduli and freezing-thawing cycles, stress ratios are analyzed respectively. And corresponding biaxial failure criteria is built in principle stress space, based on the varying rule of failure envelopes, with freezing-thawing cycles.(3) Biaxial compressive and biaxial tensile-compressive experiments are conducted on the normal concrete suffered freeze-thaw cycling. The influences of freezing-thawing cycles, stress ratios on strengths, strains and elastic moduli are analyzed respectively. The formula of initial elastic modulus in the maximal principle stress direction is given. According to the varying rule of failure envelops with freezing-thawing cycles, the biaxial failure criterion is built in principle stress space.(4) Biaxial compressive and biaxial tensile-compressive experiments are made on the normal concrete suffered high temperature. The relationships between strengths, strains, elastic moduli and freezing-thawing cycles, stress ratios are analyzed respectively. And corresponding formulas are derived form test results. The biaxial failure criterion is built in principle stress space, following on the shape change of failure envelopes with temperatures.(5) Tri-axial compressive tests are conducted on the normal concrete suffered high temperature. The influences of high temperatures, stress ratios on strengths and strains are analyzed respectively and the tri-axial failure criterion is built. Based on the varying rules of the octahedral normal stresses, shear stresses in meridian plane and deviatoric plane, the tri-axial failure criterion is extended to higher hydrostatic stress state.(6) The varying rule of biaxial stress-strain curves is analyzed, for concrete suffered high temperature. Based on the equivalent uniaxial strains at peak compressive stresses deduced from test results, the biaxial equivalent uniaxial strain constitutive model is built. The yield functions expressed by stress invariants are obtained from test results. Based on the analysis of relationships between effective stresses and effective plastic strains, the biaxial associated plastic hardening constitutive model is built. Using CECFE programmed for concrete suffered severe conditions, the constitutive models are employed for analysis of FEM, and the calculated values are agreement with test results on the whole.(7) Basing on the varying rule of stress-strain curves of normal and air-entrained concrete with freezing-thawing cycles, the corresponding equivalent uniaxial strains at peak compressive stresses, yield functions and hardening parameters are obtained. The biaxial equivalent uniaxial strain constitutive model and the biaxial associated plastic hardening constitutive model are built, respectively for both normal concrete and air-entrained concrete suffered freeze-thaw cycling. The analysis of calculated examples indicates that the agreement of calculated results and test is quite well in general.