Research On The Pore Distribution Law Of Concrete Based On NMR

The mechanical properties of concrete are closely related to the size of its internal pores and the moisture inside the pores.Accurate analysis of the evolution of the internal pores of concrete with age,environment and load is important to clarify the damage mechanism of concrete materials in complex service environments.Therefore,in this paper,the change law of internal pore space of concrete under different service conditions is investigated by using low-field nuclear magnetic resonance(LF-NMR)technique,through experimental and theoretical analysis,and the mechanism of the influence of pore space change on the mechanical properties of concrete is explored,and the research results can provide reference for the establishment of refined intrinsic structure model of concrete materials.The contents of the paper mainly include:(1)Pore structure and compressive strength tests of concrete at different ages were carried out,focusing on the evolution of pores of different sizes,and the laws of changes in the percentage of harmful and harmless pores with age.The test results showed that the total porosity continued to decrease during the curing age of 3～28d,indicating that the hydration products filled the internal pores of concrete,the percentage of pores smaller than 0.2μm increased,the percentage of pores larger than 0.2μm decreased significantly,and the concrete strength increased gradually.In the age of 28～360d,the percentage of each pore did not change much,and the change of compressive strength of concrete tended to be stable.(2)NMR method was used to analyze the change law of pore water inside the concrete under different drying time,and the evolution law of water inside the pores of different sizes under the drying conditions in the laboratory was analyzed.It was found that at the early stage of drying,the pore water of pore size greater than 0.47μm changed significantly,and the change of water within the smaller pore range was not obvious;at the later stage of drying,the water within the small pore gradually migrated.(3)By analyzing the change law of pore space inside concrete under different stress levels and the number of load repetitions and analyzing the change law of pore space with load at different locations in concrete,the damage evolution law of concrete under load is further clarified from the fine view.The study shows that before 40% peak load,the pores are compressed and the porosity decreases;at 70% to 90% peak load,the porosity keeps increasing.Under the load,the change of multi-damage pores larger than 0.2μm is most obvious.The pore change was most obvious in the middle position of the concrete,followed by the vertical beveled edge direction,and finally the horizontal beveled edge direction.Under different number of load repetitions,the pore variation of concrete changed most significantly at the 1st loading,and the degree of pore variation gradually decreased as the number of load cycles increased.