Durability Study On Fly Ash Concrete Subjected To Carbonation And Freeze-Thaw

With the extension of the application scope increase of fly ash concrete,the durability of fly ash concrete in practical engineering is increasingly prominent.It is mainly manifested as poor carbonation and frost resistance.For the durability of fly ash concrete under the single action of carbonation or freeze-thaw,domestic and foreign scholars have conducted a lot of studies in recent years and obtained some research results.Most of them have been applied and promoted.However,in practical engineering,concrete is not subjected to a single environment factor,and is often affected by multiple environmental factors at the same time.For this reason,the durability of fly ash concrete subjected to carbonation and freeze-thaw is studied in this paper.The deterioration law on the carbonation depth,mass loss,compression strength and dynamic elasticity modulus of the fly ash concrete is explored.Also,the durability degeneration law of the fly ash concrete under the double action of carbonation and freeze-thaw as well as under the single action of carbonation is analyzed.Meanwhile,the effect of carbonation on freeze-thaw is investigated.The test results show that :(1)the external damage of concrete subjected to carbonation and freeze-thaw is greater than that under the single action of freeze-thaw.With the increase of fly ash,the slurry on concrete surface,the damage on edges and corners and the aggregate leakage become more serious.(2)the increase speed of carbonation depth is bigger in the early stage than that in the later stage;Under the same carbonation time,the carbonation depth increases with the increase of fly ash;As regards 28 days,the average carbonation depth of the X4 series concrete is 2.3 times as big as that of the X1 series concrete,and the average carbonation depth of the X3 and X2 series concrete is 1.8 and 1.5 times as big as that of the X1 series concrete,respectively.(3)the strength of fly ash concrete grows slowly;the strength of concrete can be enhanced when the content of fly ash in cementing material is less than30%;Carbonation can improve the strength of the fly ash concrete.After 28 d of carbonation,the cube compression strength of the X1 and X2 series concrete increases by about 10%,and the cube compression strength of the X3 and X4 series concrete increases by about 15% and17%,respectively.(4)the loss of cube compression strength of fly ash concrete under freeze-thaw is bigger than that of normal concrete;with the increase of cycle times and fly ash content,the degeneration speed rate of strength for concrete is raised,therefore the strength loss rate is also enhanced;in the early stage,carbonation slows down the strength loss rate,but carbonation speeds up the strength loss rate with the development of carbonation.(5)the mass loss of the concrete prismatic specimen subjected to the double action of carbonation and freeze-thaw is bigger than that under the single action of freeze-thaw action.With the increase of freeze-thaw cycle times and fly ash content,the mass loss of concrete increases continuously;the mass loss rate of the cube specimen in the same group is greater than that of the prismatic specimen;with the increase of carbonation time,the mass loss rate increases,and carbonation increases the mass loss of concrete after freeze-thaw.(6)after freeze-thaw experiment,the dynamic elastic modulus of concrete decreases.The decrease degree of the prismatic specimen of the X1 and X4 series concrete is greater than that of the X2 and X3 series.When the fly ash content occupies 15%~30% in cementing materials,the fly ash can fine the concrete,and improve the density and therefore slow down the attenuation of the dynamic elasticity modulus.When the fly ash content reaches 45% in cementing materials,the initial porosity of concrete is large and the hydration rate of fly ash is slow,resulting in a rapid decline in the dynamic elasticity modulus after freeze-thaw.The dynamic elasticity modulus under the coupling action of carbonation and freeze-thaw decreases much more greatly than that under the single action of freeze-thaw,and with the development of carbonation,the dynamic elasticity modulus decreases much bigger.(7)the reliability of the experimental data in this paper is verified by comparison with part of the existing literature.