Study On Process And Mechanisms Of Concrete Deterioration Subjected To The Combined Actions Of ASR And Chloride Corrosion

Alkali-silica reaction (ASR) and chloride corrosion are two major hazards to concrete durability. The deterioration of concrete under single destructive action have been broadly investigated, but the degradation evolution and the corresponding mechanisms subjected to coupled actions of ASR and chloride corrosion have not been studied yet .In this experiment, three measures are taken to accelerate concrete damage, including enhancing the alkali level of cement, curing in compound alkali solutions and elevating exposure temperature. In addition, two parameters, the expansion ratio and relative dynamic elasticity modulus (RDEM), are used to characterize concrete damage. When RDEM of concrete suffered from the combined actions of ASR and chloride corrosion has been dropped to a critical point, the concrete is supposed to be completely destructed. The damage evolution and rules of both chloride diffusion and binding are in detail investigated by respectively measuring expansion and RDEM and measuring free and total chloride concentration in different depth during different exposure time. Besides, the failure process of concrete is analyzed, mechanisms of chloride salts accelerating ASR and gangue effectively preventing ASR are further explored by means of optical microscope and SEM-EDAX, XRD et al.After systematically researching various factors affecting ASR expansion, three important conclusions are drawn. Firstly, ASR expansion is increased with the growth of cement alkali content when alkali level varies in a certain range; high concentration chloride solutions can accelerate ASR significantly in a short time, while in the long run, the effects of low concentration solutions on accelerating reaction is more remarkable. Secondly, the addition of gangue can effectively prevent or delay ASR expansion. And finally, The damage evolution of concrete caused by ASR presents some peculiarities, for that in the beginning its RDEM decreases rather quickly, later decreases with a greatly reduced velocity or remains same in a long period, finally gradually decreases to complete failure with a very little speed.In the occurrence of ASR, the diffusion rate of chloride ion in concrete is not proportional to its damage degree as anticipated. Apparent diffusion coefficient decays negatively exponentially with time, while surface chloride concentration increases linearly. As far as this experiment is concerned, the low capacity of concrete in binding chloride ion is presumed to be in relation to low residual C3A contents, high pH value of pore solution and high exposure temperature. According to the cracking observed by optical microscope, a degradation mechanism of concrete subjected to the combined actions of ASR and chloride corrosion is suggested. That is, ASR gels swell when absorbing water in reaction zone or during migration, induce microcracks and damage the concrete. Then the damage is severely aggravated by some nocuous medium such as NaCl solution intruding through microcraks. In combination with others'research results and our microscopic testing, high alkalinity in pore solution resulting from the interaction between chloride ion and ASR gels is considered to be mainly responsible for the acceleration of ASR, the precipitation of Friedel's salt in microcracks also increases the ASR expansion. The reason of the inclusion of gangue to prevent ASR is suggested as follows: the migration of different ions is hampered by inner dense structure and consequently, the reaction is controlled by diffusion.