Microstructure Evolution Of Concrete Frozen At Early Age

Freeze-thaw cycles have significant impact on the strength and durability of concrete,which reduces the service life of concrete structures.For the early age concrete with incomplete hydration and fragile internal structure,the damage of freeze-thaw cycles is more serious.Reseachers have studied the frost resistance mechanism of concrete for decades.Many hypotheses of frost resistance mechanism have been put forward,however,none of them could explain all freeze-thaw damage phenomena.Therefore,it is necessary to evaluat e the degradation effect of freeze-thaw cycles and reveal the deterioration process of internal microstructure through experiments,which is beneficial to the frost resistance mechanism.In this paper,a series of macroscopic and microscopic experiments on concrete frozen at early age were carried out.Two water to cement ratios(w/c),0.3 and 0.5,were used to represent high strength concrete and ordinary concrete,respectively.Macroscopic tests included compressive strength test and chloride ion permeability test.Compressiv e strength test results showed that freeze-thaw cycles at early age had little effect on the strength of low w/c concrete.However,the strength of high water cement ratio concrete decreased with the advance of freezing age.The strength loss rate of concrete frozen at the age of 7 days was 14.4%,and that of concrete frozen at the age of one day was 27.6%.The chloride ion permeability test showed that the permeability of concrete with low w/c is low whether frozen or not,while the permeability of concrete with high w/c frozen at the age of one day changed from medium level to high level,indicating that the impermeability obviously deteriorate d.Microscopic tests included Mercury Intrusion Porosimetry(MIP)test,Back Scattered electron(BSE)test,Thermogravimetric Analysis(TGA)test and X-ray Computed Tomography(X-ray CT)test.The evolution of of microstructure of frozen concrete is studied through two aspects: pore structure and hydration process.The results of MIP test show ed that the pore size distribution of low w/c paste was a single-peak distribution in the pore size range of 3nm?1um,and the freeze-thaw cycles had little impact.The high w/c paste owned a double-peak distribution.With the advance of freezing age,the porosity increase d from 23% to 37%,and the critical pore size increased from 40.3nm to 678.8nm.BSE test showed that the low w/c paste was not affected by freeze-thaw cycles in the pore size range of 1 to 30 ?m.Abnormal structure was formed in high w/c paste due to freeze-thaw cycle,which contributed to higher porosity in the range of 1 to 3 ?m.X-ray CT test could quantify the pore size distribution of 10 to 200 ?m.Most of the pore size of low w/c paste in this range was less than 60 ?m,while high w/c paste had more pore larger than 160 ?m.In terms of hydration process,TGA test showed that the hydration reaction of high w/c paste was very sensitive to freeze-thaw cycles at early age.The hydration degree of frozen cement paste at the age of one day decreased from 0.74 to 0.37,while that of low w/c paste showed little difference.The hydration degree quantified based on BSE image analysis was similar to that of TGA test,with relative error ranging from 1% to 12%.The three dimensional model of cement paste could be reconstructed based on CT image.The hydration degree quantified from the reconstructed microstructure were in good agreement with that of TGA tests.X-ray CT was able to serve as a supplementary method for quantifying hydration degree,but CT equipment with high resolution is required.