Study On Evaluation Index And Threshold Of Water Stability Of Asphalt Mixture Based On Surface Energy Theory

The water stability of asphalt mixture is closely related to the early water damage of pavement.Accurate and rapid assessment of water stability is beneficial to guiding the design of asphalt mixture,preventing and reducing the occurrence of pavement water damage.Compared with the macroscopic water stability test method,surface energy method with its indexes quantitatively analyze the adhesion ability between asphalt and aggregate from the perspective of microscopic energy,and then evaluate the water stability of asphalt mixture.The evaluation results are not interfered by human factors,and are more accurate,so surface energy method is widely used.However,the existing surface energy evaluation indexes do not consider the influence of macro internal factors such as asphalt amount on material adhesion,and do not explore the requirements of external temperature on water stability,and lack a unified index threshold.The above deficiencies restrict the further development of surface energy method in evaluating the water stability of asphalt mixture.In this regard,based on surface energy method,this article combines surface energy test and macroscopic water stability test,and considers both internal and external factors affecting water stability.In order to directly and accurately predict the water stability of asphalt mixture with surface energy evaluation index and its threshold,in the sequence of index proposal,index verification and threshold determination,the following research results are obtained:(1)A water stability evaluation index SI considering specific surface area of mineral and thickness of asphalt film was proposed.The unit of index SI is m~2/kg,that is,the water stability is evaluated by the total adhesion area of asphalt mixture per unit mass.The larger the calculation result is,the stronger the water stability is.Index SI not only quantitatively analyzes the adhesion by surface energy method,but also reflects the influence of two macro internal factors,mineral gradation as well as asphalt amount,on water stability by specific surface area of mineral and thickness of asphalt film of asphalt mixture.(2)Two kinds of asphalt,70#matrix asphalt and SBS modified asphalt,three kinds of aggregates,limestone,granite and diabase,and two kinds of fillers,limestone and diabase mineral powder,were selected to test the surface energy and calculate the index SI of 12 kinds of asphalt mixture composed of them.After molding mixture specimen,immersion Marshall test,vacuum Marshall test and freeze-thaw split test were carried out.By using the method of controlling variables and classification comparison,it can be verified by the macro water stability tests that,compared with the traditional matching index ER,the goodness of fit between index SI and the three macro test indexes is higher,and the average of R~2is 0.9380,which has a good positive correlation.Besides,the evaluation results of index SI have higher differentiation and stronger applicability.(3)The surface energy of materials were measured in the range of-20℃～50℃,and the variation of surface energy parameters and index SI with temperature was investigated.The cohesion work of asphalt first increases and then decreases with temperature increasing,and the asphalt cohesion also increases and then decreases.The asphalt cohesion is the lowest at-20℃(freeze-thaw cycle),indicating that the freeze-thaw cycle function weakens the asphalt cohesion more seriously than the high temperature alone.Meanwhile,asphalt-aggregate and asphalt-filler adhesion both are the minimum at-20℃(freeze-thaw cycle),indicating that freezing-thawing cycle function seriously weakens the asphalt–mineral adhesion,and the effects of high temperature on adhesion are diversified,depending on the specific temperature.With temperature increasing,the stripping work of asphalt mixture first decreases and then increases,indicating that water is more likely to displace asphalt from the surface of mineral under high temperature,resulting in the stripping of asphalt film,while the effect of freezing-thawing cycle is not significant.Besides,with temperature increasing,index SI of 12 kinds of asphalt mixture first increases and then decreases,indicating that both freeze-thaw cycle and high temperature can weaken water stability.Freeze-thaw cycle weakens the water stability of granite and diabase asphalt mixture more seriously than high temperature,while high temperature damages the water stability of limestone asphalt mixture more severely.(4)The index SI threshold considering temperature factor was determined.The macro test scheme was designed and the test conditions were improved.Four kinds of asphalt mixture were selected to carry out five freezing-thawing cycles splitting test at-20℃,immersion Marshall test at 10℃and 30℃,and immersion Hamburg wheel-tracking test at 50℃.With the help of the above three test indexes and technical requirements,The SI thresholds corresponding to-20℃(freeze-thaw cycle),10℃,30℃and 50℃were determined.For matrix asphalt mixture,the thresholds are 19.43,31.29,27.22 and 16.04,respectively.For modified asphalt mixture,the thresholds are 27.06,38.87,30.47 and 16.04,respectively.(5)The relationship model between threshold and temperature was established.The SI thresholds of both matrix asphalt mixture and modified asphalt mixture increase first and then decrease with temperature increasing.The quadratic function is used to fit the relationship between threshold and temperature,of which the judgment coefficient R~2≥0.9875,indicating a high goodness of fit.At the same time,it is verified by practical engineering cases that compared with the still threshold value,the threshold value of water stability index that changes dynamically with temperature is more applicable,and its evaluation results are more accurate and reasonable.