A Study of Durability and Service-Life Prediction Models for Aeolian Sand Powder Concretc

Author:Li Gen Feng

Supervisor:shen xiang dong

Database:Doctor

Degree Year:2019

Download:97

Pages:168

Size:23494K

Keyword:

In the present study,aeolian sand powder samples were prepared using the widely distributed aeolian sand of Inner Mongolia(China),and a principle of "alkali activation"was adopted to stimulate the experimental activities.Meanwhile,aeolian sand powder concrete samples were prepared by replacing the cement-based cementitious material.Then,examinations were conducted regarding the deterioration and damage processes,as well as the deterioration mechanism of the aeolian sand powder concrete under both single and coupled conditions.These included freeze-thaw conditions,salt leaching,dry-wet conditions,carbonization,and sand erosion.The macroscopic properties were observed using such indexes as the relative dynamic elastic modulus,mass losses,carbonization depths,and the corrosion resistance coefficients of the compressive strength.The micro-morphology,micro-mechanics,hydration products and pore variation were studied by field emission scanning electron microscopy(FESEM),nanoindentation technique(NI),energy dispersive spectroscopy(EDS),X-ray diffraction(XRD),semi-quantitative full spectrum analysis(SQFSA)and nuclear magnetic resonance(NMR).Then,on the basis of this study’s macroscopic and microscopic experimental results,the activation mechanism of the aeolian sand powder,along with the durability of the aeolian sand powder concrete,were discussed in detail.The results of this study’s experimental processes revealed the following:1.During the modification tests of the aeolian sand powder,the activation rates of the aeolian sand powder were observed to have increased with the increases in the mass fractions of the activators.Also,with the increases in the alkalinity of the solutions,the dissolution amounts of the SiO2 and other active substances in the aeolian sand powder were found to have gradually increased.It was found that the results of the sodium sulfate powder were superior to those of the sodium hydroxide in regard to the modified effects of the aeolian sand powder.It was also determined that when the aeolian sand powder content was 15%and the sodium sulfate content was 2.0%,along with a pre-curing temperature of 35℃,the ettringite in the aeolian sand powder of the cement mortar specimen was highly developed.It had become fully filled in the intermal pores of the aeolian sand powder-cement gel system,and the intermal pores below 20 nm in the disconnected wool stoma had reached 85.69%.The proportion of the irreducible fluid saturation had also been increased to 94.311%,and the activity index had reached as high as 108.2%.2.With the increases in the number of freeze-thaw cycles,it was observed that the relative dynamic elastic modulus of the aeolian sand powder undergoing the coupling effects of the freeze-thaw and salt leaching conditions had first increased,then stabilized,and then dropped to the law of failure.At the same time,the aeolian sand powder concrete had produced an ettringite(AFt)in the magnesium sulfate solution of 3.0%>6%,gypsum(CaSO4)ean also be formed in 6.0%magnesium sulfate solution,and the needle columnar fibrous products had filled cracks caused by the frost-swelling stress conditions experienced by the concrete samples.Therefore,the porosity and permeability of the aeolian sand powder concrete samples were observed to be lower than that of the ordinary concrete samples.However,the binding fluid saturation was determined to be higher than the ordinary concrete.It was found that in the C35 group,the proportions of the harmless and less harmful holes in the aeolian sand powder concrete were 61.12%,11.79%higher than ordinary concrete group,and the structures were also denser.As a result,the aeolian sand powder concrete had displayed a better frost resistance ability than the ordinary concrete samples in the sulfate solutions.3.In regard to the effects of the freeze-thaw and dry-wet coupling conditions on the aeolian sand powder concrete,the deterioration significance was observed to be higher than those of the freeze-thaw or dry-wet single factor actions.It was determined that the freezing and thawing effects had caused the aeolian sand powder concrete structures to change from dense to loose and porous.Then,for the subsequent dry and wet actions,it was observed that when the salt entry provided more pore channels,the dry and wet actions had become accelerated by the chemical corrosion of the sulfates,and an expansion product(ettringite)had been generated.In addition,the proportions of the multi-damage holes in the aeolian sand powder concrete which had undergone freeze-thaw and dry-wet coupling conditions were found to be higher than those of the dry-wet and freeze-thaw coupling conditions(7.8%).Furthermore,the permeability was 3.7 times higher following the dry-wet and freeze-thaw coupling,the enrichment degree of the ettringite expansion product was observed to be much higher than that of dry-wet and freeze-thaw coupling,and the porosity was determined to be 2.1 times that of the initial value,and the saturation of the bound fluids was found to be 13.64%less than after the coupling of the dry-wet and freeze-thaw conditions.Therefore,the deterioration rate of the degradation under the coupling of the freeze-thaw and dry-wet conditions was confirmed to be higher than that of the dry-wet and freeze-thaw coupling conditions.4.Aeolian sand erosion is known to destroy the surfaces of stone structures made from aeolian sand powder concrete.This erosion can potentially increase the carbonization depth by more than 3.0 times;The concrete becoming loose due to the expansion of the carbonization product itself,the mass loss under the coupling action of carbonation and aeolian erosion is more than 1.6 times higher than that under the single aeolian erosion action;It has been found that under the effects of aeolian sand erosion、carbonization coupling,the deterioration of ordinary concrete will be higher than that of aeolian sand powder concrete.Also,the deterioration caused by carbonization and aeolian sand erosion coupling will be higher than that of aeolian sand erosion and carbonization coupling.Moreover,the deterioration rates will be higher than those caused by aeolian sand erosion or carbonization as single factors.It was observed that the depths of the erosion pits of aeolian sand powder concrete caused by erosion angles of 90°were nearly twice than those of the 450erosion angles.It was found that following the aeolian sand erosion,the carbonization depths had gradually decreased over time,the carbonation depth of C25 aeolian sand powder concrete at 14d age is 6%lower than that of C25 ordinary concrete,and 10.6%at 28d age.At the same time,the decrease of porosity of aeolian sand powder concrete is 7.2%higher than that of ordinary concrete,it was also found that the harmless pore below 20 nm was higher than that of the ordinary concrete(25.15%).The carbonation zone,the change zone of carbonization products(calcium sulfate)and the mixed zone of non-carbonization zone are formed in the range of 10-15 mm along the depth of carbonization.5.Under the actions of the freeze-thaw、carbonization coupling,the decrease of relative dynamic elastic modulus of the aeolian sand powder concrete was found to be lower than that of the ordinary concrete.Furthermore,freeze-thaw and carbonation,carbonation and freeze-thaw coupling compared,the relative dynamic elastic modulus of the aeolian sand powder concrete was observed to be 1.5 times higher than that of the carbonization and freeze-thaw coupling conditions.Also,the effects of the freeze-thaw and carbonization coupling conditions on the carbonization depths of the aeolian sand powder concrete were lower than those observed under the carbonization and freeze-thaw coupling conditions(5%),and the proportion of the harmless holes were 16.85%higher than that observed under the carbonization and freeze-thaw coupling conditions.The percentage rate of the multi-damage holes was 22.5%lower than that observed under the carbonization and freeze-thaw coupling conditions;Meanwhile,the effects of the carbonization and freeze-thaw coupling conditions on the aeolian sand powder concrete binding fluid saturation rate were determined to be 0.34%higher than those of the ordinary concrete,and the permeability was 1.5%higher,the change law of the concrete pore structures of the aeolian sand powder undergoing the effects of the freeze-thaw and carbonization coupling was revealed,as well as the saturation effects of the residual fluids of the aeolian sand powder.The proportion of the harmless holes was determined to be 15.35%higher,and the proportion of the multi-damage holes and the porosity rates were lower than those of the ordinary concrete by 22.25%and 3.06%respectively.Moreover,the permeability was found to be 50 times lower than that of the ordinary concrete.Therefore,the deterioration rates of the aeolian sand powder concrete were confirmed to be significantly lower than those of the ordinary concrete.In addition,the deterioration rates of the aeolian sand powder concrete which had been caused by the effects of the carbonization and freeze-thaw coupling were found to be significantly higher than freeze-thaw and carbonization coupling.6.In the present study,it was found that the deterioration damage mechanisms of the aeolian sand powder concrete and the ordinary concrete samples undergoing different coupling conditions had not only shown a certain consistency,but also had displayed certain differences.For example,the hardness in micromechanical properties and porosity in pore parameters have great influence on both,but the proportions of the harmless holes(<20nm)were found to have greater impacts on the ordinary concrete,and the proportions of the less harmful holes(20 to 50nm)were observed to have greater impacts on the aeolian sand powder concrete.In this study,based on a grey theory and a sulfate erosion damage mechanism,combined with the existing previous results of concrete service life prediction models and indoor acceleration tests,a grey prediction model of the service life of aeolian sand powder concrete was established.The model was based on a carbonization and service life prediction model of weathered aeolian sand powder concrete which had included the examination of the damages caused by sulfate erosion.It was determined that the prediction accuracy of the established model was good.Therefore,the results of the current study provided practical guiding significance for the future applications of aeolian sand powder in water conservancy,construction,and other projects,with the potential for remarkable social and economic benefits.