Theroretical And Experimental Study On Capillary Water Absorption Of Recycled Aggregate Pervious Concrete

The preparation of recycled aggregate pervious concrete?RAPC?is an effective way to recycle construction waste.RAPC has good permeability,can collect rainwater and seep back,maintain the ecological balance of groundwater resources,and absorb road noise,alleviate the"heat island effect"and"urban drying",so it has great application prospects in"sponge city".In this paper,the capillary water absorption property of RAPC is studied.The particle size of C30 waste pavement concrete block is 9.5-19.0 mm.Based on the water cement ratio of 0.30,12 groups of RAPC are prepared,including 4 kinds of water cement ratio?0.27,0.30,0.33,0.36?,3 kinds of carbon fibers?0.2%,0.4%,0.6%?and Five kinds of double-grain size?1:1,1:2,2:1,2:3,3:2?of recycled coarse aggregate with 4.75-9.5 mm particle size were added to study the capillary water absorption quality and capillary water absorption height of RAPC in the first 8 days under the action of neutral and acidic water absorbent media respectively,and the Allometric model was constructed by combining the strength,porosity and permeability coefficient.From the perspectives of durability and capillary mechanics of RAPC,the law and mechanism of capillary water absorption transmission are studied.By introducing the equivalent pore size of RAPC section,the influence factors of cross-section pore on capillary water absorption transmission law of RAPC are analyzed by image analysis method.On this basis,the weight of each index is established based on grey correlation analysis method,and the main factors affecting capillary water absorption height of RAPC are studied,and then the main influencing factors are selected.Prediction of capillary water absorption height of RAPC was realized by building BP neural network model.The main research results are as follows:?1?Water-cement ratio changes the mechanical properties of RAPC,but also affects its porosity and permeability.Generally,it has the characteristics of high compressive strength and low porosity and permeability coefficient.For example,when water-cement ratio is0.30?standard?,the compressive strength is the highest,which is 9.06 MPa,and the corresponding permeability coefficient is 3.28 mm/s?standard value?.Carbon fibers can modify the properties of RAPC.For example,when the content of carbon fibers is 0.6%,the compressive strength reaches the maximum of 3 kinds of carbon fibers,which is 8.15 MPa.When the content of carbon fibers is 0.4%,the permeability coefficient reaches the maximum,which is 4.24 mm/s,which is 29.3%higher than the standard value value.Regarding gradation,the compressive strength of RAPC decreases first and then increases with the increase of recycled aggregate size from 4.75 mm to 9.5 mm.The maximum compressive strength is 7.92 MPa when gradation is 1:2,and the maximum permeability coefficient is 4.2 mm/s when gradation is 3:2.?2?The capillary water absorption of RAPC can be divided into two stages according to the growth rate of different periods.The first stage is 0-147 sec^1/2?i.e.the first 6 hours?,the second stage is 147-831 sec^1/2?i.e.6 hours-8 days?.The growth rate of the former stage is faster,and the growth rate of the latter stage gradually decreases and tends to be gentle until equilibrium.The capillary water absorption quality and height of RAPC increase with time,showing a power function growth,which conforms to Allometric model and has a high fitting degree.When the water cement ratio is 0.30,the capillary water absorption effect is the best.The capillary water absorption quality reaches 68.25 g and the capillary water absorption height reaches 6.07 mm?the non-linear fitting R² is 0.997?.?3?In neutral and acidic water,the capillary water absorption quality and the capillary water absorption height of RAPC in the first stage are higher than those in the second stage,and the capillary water absorption rate in the first stage is 3-6 times of that in the second stage.The maximum capillary water absorption height of RAPC mixed with carbon fibers in neutral water and acidic water is 5.55 mm and 4.77 mm,respectively.At this time,the content of carbon fibers is 0.6%and 0.4%respectively.The maximum capillary water absorption height of five gradations in neutral water and acidic water occurs at 2:1 and 2:3gradations.The maximum capillary water absorption quality and capillary water absorption height of RAPC occur under neutral water condition.?4?The capillary water absorption of RAPC is related to its porosity,cross-section equivalent pore size and permeability coefficient.BP neural network can be used to predict the capillary water absorption height of RAPC,with an average relative error of 7.83%.