Three-phase Structure Characteristics Of Pervious Concrete And Its Impact On Macro Performance

With the rapid advancement of urbanization,more and more urban areas are covered with dense concrete,blocking the exchange of rainwater between the soil and the outside world,resulting in frequent urban waterlogging.To this end,China has proposed the development concept of ’ sponge city ’ as a way to alleviate these problems.In sponge city,pervious concrete is usually used as paving material.Pervious concrete can infiltrate,store and release rainwater due to its rich pore structure,thus allowing rainwater to migrate freely throughout the city.Pervious concrete is mainly composed of aggregate phase,pore phase and cement phase.The complex three-phase structure determines the mechanical properties and water permeability of pervious concrete.How to effectively obtain the three-phase structure characteristics and establish its relationship with the performance of pervious concrete has become the research focus and the key problem to be solved urgently.In this paper,the selfmade pervious concrete section image is used as the initial data set,and the Mask R-CNN framework is used to train the automatic identification model of pervious concrete three-phase structure.The three-phase structure characteristics and influencing factors of pervious concrete were analyzed by using the established three-phase structure identification model.The effects of three-phase structure on the mechanical,permeable and anti-stripping properties of pervious concrete were studied.The main research results are as follows:(1)An automatic identification model of three-phase structure of pervious concrete based on deep learning network Mask R-CNN is established.Nine groups of pervious concrete samples with different aggregate sizes(diabase gravel,2.36mm～4.75 mm,4.75mm～9.5mm,9.5mm～ 16mm)and design porosity(15%,20%,25%)were prepared.At the same time,in order to increase the contrast between the cement phase and the aggregate phase,5% cement dye is added.The specimens were cut and polished,and the pores were filled with white flour to obtain a pervious concrete profile with obvious contrast between the three phases.The initial data set required for model training was obtained by high-definition scanning.The initial data set is preprocessed,labeled and enhanced to obtain the final data set.The three-phase structure identification model training is carried out,and the overall performance and three-phase structure identification accuracy are evaluated.(2)Three-phase structure characteristics and influencing factors of pervious concrete.The three-phase structure characteristics of pervious concrete are analyzed by using the established three-phase structure recognition model,image processing technology and Python programming.The pore phase characteristic parameters include pore size,pore area distribution,pore fractal dimension,porosity,pore shape,etc.The characteristic parameters of aggregate phase include aggregate equivalent particle size,aggregate average free spacing,aggregate fractal dimension,etc.The characteristic parameters of cement phase include slurry layer thickness,slurry layer thickness distribution,etc.In addition,the effects of design porosity,aggregate-cement ratio and aggregate particle size on the three-phase structure characteristics were analyzed.(3)Evaluation of mechanics,water permeability and anti-stripping performance of pervious concrete.The compressive strength,flexural strength,water permeability and spalling resistance of pervious concrete were tested.The experimental results show that the aggregate size and aggregate-cement ratio have a great influence on the mechanical properties of pervious concrete,and the aggregate-cement ratio is an important factor affecting the mechanical properties.The water permeability is mainly affected by porosity and aggregate size,and the aggregate has a greater influence on the pore structure,which in turn affects the water permeability.The anti-stripping performance of pervious concrete is mainly related to the aggregate particle size and the aggregate-cement ratio.When the aggregate particle size is different,except for the large difference in mass loss after the test,the wear of the sample is also different.(4)The influence of three-phase structure on the performance of pervious concrete.The mechanical properties of pervious concrete are mainly affected by the pore size,pore area distribution,pore fractal dimension,aggregate particle size in aggregate phase,average free spacing of aggregate,slurry layer thickness in cement phase,cement phase area ratio and other structural characteristics.The permeability of pervious concrete is mainly related to the structural characteristics such as porosity,pore size and pore area distribution.The aggregate phase will indirectly affect the above characteristics and ultimately affect the permeability.The cement-related characteristics such as the thickness of the slurry layer and the proportion of the cement phase area have little effect on the water permeability;the anti-stripping performance and post-failure morphology of pervious concrete are greatly affected by aggregate particle size.The anti-stripping performance is also related to the structural characteristics such as porosity of pervious concrete and average free spacing of aggregates.