Experiment Study On Fracture Behaviors Of Concrete Exposed To Different Sulfate Enviorments

There exist plenty of sulfate ions in seawater,saline soil,groundwater,decaying organic matter and industrial wastewater.Under such enviormenrs,concrete structures are vulnerable to severe degradation,such as expansion,cracking and spalling,which is prone to result in premature failure.According to field observation,splash zone or tidal zone of offshore concrete structures are more likely to disintegrate more rapidly and severely.Similar scenes can be found in footings and foundations which are partially submerged in saline soil.The varied erosion environments would significantly affect the material properties.Howevere,the diversity of erosion environments and the heterogeneity of concrete materials both make contribution to the complexity of concrete fracture performance after sulfate attack.In fact,the mechanical behavior of concrete and the development of cracks are closely related to the safety and stability of the structure.Therefore,it is essential to investigate the fracture properties of concrete under different sulfate attack environments to provide references for the life-cycle design and risk of fracture assessment.Based on this,the specific contents of this research work and main conclusions are summarized as follows:?1?Experimental investigations on mechanical properties of concrete under different sulfate attack environmentsThe mechanical properties of reference specimens?RS?,long-term immersion?LM?,and dry-wetting cycles?DW?specimens were measured respectively at curing of 28 days,after exposure time of 3,6 and 9 months.The evolution law of compressive strength,splitting tensile strength and elastic modulus are discussed systematically.These experimental measured values are also used as the material parameters for calulations of fracture characteristics and softening constitutive relationships in the below.The results show that degradation of concrete is more serious under wet-dry cycling than under continuous immersion.And the splitting strength appears to be more susceptive to the damage resulted from sulfate attack.?2?Experimental investigations on fracture performances of concrete under different sulfate attack environmentsTo investigate the effects of different sulfate attack environments on the fracture properties of concrete,three-point bending?TPB?tests were conducted on the pre-notched beams of RS,LM,DW and partial submersion?PS?specimens at curing of 28 days,after exposure time of 3,6,9 and 20 months.The initial cracking load(P_ini),critical crack propagation length??a_c?,the peak load(P_max)and the fracture energy?G_f?were measured in the tests,and the unstable fracture toughness(K_IC^un)was calculated accordingly.The test results indicate that the ratio of P_ini to P_max located between 0.8 and 0.99 approximately,which meant the increase in brittleness of concrete.And K_IC^un and G_f of DW and PS specimens remarkably reduce as exposure time increases.?3?Determination of tension softening constitutive relationships of concrete under different sulfate attack environmentsBased on the TPB tests,the energy dissipation was derived using the modified J-integral method.The energy balance during crack propagation can be established,i.e.the energy dissipation is equal to the energy caused by fictitious cohesive force acting on fracture process zone.Then the tension-softening constitutive laws of RS,LM,DW and PS specimens after 9months exposure were respectively established,which were also simplified as bilinear forms for practical applications.Compared with RS,stress-free crack width w₀ decreased from 0.175to 0.161,0.146 and 0.145 after 270 days of exposure under LM,DW and PS conditions.Consequently,a shorter FPZ length could be expected,resulting in the increase in brittleness of concrete.?4?Microscopic tests baesd on SEM&EDS techniques and determination of erosion depths of sulfate attackIn order to explore the microscopic mechanism and damage evolution laws of concrete after sulfate attack,SEM&EDS tests are conducted for RS,LM,DW and PS specimens at curing of 28 days,after exposure time of 3,6,9 and 20 months.Furthmore,the sulfate-ion content profiles in the concrete after 9 months exposure were determined by the modified barium sulfate gravimetric method?chemical titration?.And the erosion depths of concrete after 9 months sulfate attack calculated accordingly.This study suggests that ettringite start forming around the initial pore where the expansion stress of the corrosion product can lead to the formation of cracks in the interior region.When the solution is able to reach the cracked interior zones,it reacts with the hydration products and leads to deeper erosion progressively.The erosion depth in crystalline region of PS specimens was only 3.2 mm inspite of server surface disintegration.And the erosion depth of DW specimens was 18.5 mm,which was the maximum in all groups.