| Pervious concrete is a porous skeleton structure formed by slurry cemented aggregate.Because of its good water permeability,it is one of the important materials for the construction of "sponge city".In engineering production and application,it is found that pervious concrete has obvious problems such as insufficient mixing ratio design effectiveness,low strength of prepared concrete and unstable quality.The research group has carried out research on the composition materials and properties of pervious concrete in the early stage,and concluded that the use of aggregates with a particle size of 4.75-9.50 mm to prepare pervious concrete can achieve better mechanical properties on the basis of satisfying the permeable performance;porosity（Under the same conditions,aggregate strength and slurry properties have a significant impact on the skeleton structure and mechanical properties of pervious concrete.In order to find out the main factors affecting the mechanical properties of pervious concrete and its failure mechanism,based on the previous research,this thesis further focuses on the factors such as Paste Coating Thickness（Paste Coating Thickness,PCT）,aggregate strength,paste strength and other factors for pervious concrete.The influence of compressive strength and failure mode is studied,the strength utilization rate of materials and their contribution to concrete strength are analyzed,and an optimal design method of pervious concrete mix ratio based on material strength utilization rate is proposed.In this thesis,a single-graded aggregate（4.75-9.50mm）is used to prepare pervious concrete,and four types of aggregates with the thickness of the slurry wrapping layer（PCT are 300 μm,400 μm,500 μm,and 600 μm respectively）and three strengths（the crushing index respectively）are set up.3.33%（A）,9.63%（B）,14.45%（C））,four strength grades of slurry（compressive strength 110.0MPa（J1）,87.3MPa（J2）,73.3MPa（J3）,48.8 MPa（J4））,a total of48 sets of specimens were designed and tested for mechanical properties and water permeability.The main conclusions are as follows:The thickness of the slurry wrapping layer,PCT,is the most important factor affecting the compressive strength and failure mode of pervious concrete.When the PCT is 500 μm,the porous structure of the skeleton of the pervious concrete can achieve the ideal state of optimal water permeability and mechanical properties.The test results show that: when the PCT is300μm,the compressive strength of the pervious concrete is low,and the specimen shows the failure of the slurry;when the PCT is 400μm,the compressive strength is increased,but the increase is not large,and the specimen is damaged.It is shown as transitional failure（combination of slurry failure and aggregate failure）;when the PCT is 500 μm,the compressive strength increases significantly,and the specimen shows aggregate failure;when the PCT is 600 μm,the specimen also has aggregate failure,but some The bottom of the specimen was blocked with slurry,and the strength did not increase significantly compared with PCT500 um.The compressive strength of the pervious concrete specimen A500-J1 prepared in this thesis can reach as high as 46.6MPa,and the specimen shows the explosive failure of high-strength aggregate,which eliminates the weak point of the bond in the porous structure of the skeleton,and gives full play to the material’s strength.strength.Using different strength aggregates and different strength slurries to prepare pervious concrete,the compressive strength of concrete increases with the increase of aggregate strength and slurry strength,but the increase range is very large under different slurry coating thickness PCT difference.When the PCT is 300 μm,the increase of aggregate strength and slurry strength has little effect on the compressive strength of the specimen.The specimen A300-J1 is only 1.4 MPa higher than that of C300-J1,and the specimen A300-J1 is 1.8 higher than that of A300-J4.MPa;when the PCT is 500 μm,the compressive strength of the specimen increases significantly with the increase of aggregate strength and slurry strength.The specimen A500-J1 is 27.5MPa higher than that of C500-J1,and the specimen A500-J1 is higher than that of A500-J4.21 MPa.Combined with the failure mode of pervious concrete,it can be seen that when the PCT is 300 μm,the failure of the specimen is mainly the failure of the weak link of the paste bond,the material strength cannot be exerted,and the strength utilization rate is low.The strength utilization average rate of the material is only 5.0%,7.0%,and 9.6%,and the strength utilization average rate of the slurry is about 16.9%,15.1%,and14.2%.When the PCT is the optimal 500 μm,the weak point of the porous structure of the skeleton is eliminated,and the test piece For the explosive failure of the aggregate,the strength of the material can be fully exerted,and the strength utilization rate is greatly improved.The strength utilization average rate of the aggregate is 15.2%,14.1%,and 15.1%,respectively,and the strength utilization average rate of the slurry is as high as 48.4% and 29.1%.%,22.0%.It can be seen that the material strength can be fully utilized under the reasonable skeleton porous structure.According to the above research results,an optimal design scheme of pervious concrete mix ratio based on the utilization rate of material strength is proposed: the optimal value range of PCT is determined according to the aggregate size;the aggregate strength is determined according to the target strength of concrete;according to the PCT and aggregate strength,Determine the slurry strength utilization rate;determine the slurry strength according to the slurry strength utilization rate and the strength relationship formula of pervious concrete.This optimized mix ratio design method can not only efficiently design the mix ratio that meets the target strength and water permeability,but also make full use of the material strength to achieve the optimal mix ratio of material performance and economy. |
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