Experimental Study on Effect of Aggregate Size on the Hydraulic Properties of Pervious Concrete

Abstract

Pervious Concrete is a distinct type of concrete in which narrow gradations of aggregates are used to increase the permeability. Though pervious concrete is green concrete that allows water passage through its pores, it tends to clog due to particles in the runoff. The study aims to evaluate the impact of clogging on pervious concrete mixes and explore a simple method to calculate permeability and clogging using the falling head method in a fabricated unit. The materials used are cementitious materials and aggregates, along with superplasticizers. The cementitious materials used are OPC Grade 53 cement and micro–Ground Granulated Blast Furnace Slag (μGGBS). Two separate narrow aggregate gradations are used - 2.36 - 4.75mm and 4.75 - 6.30mm. The water-binder ratio is taken as 0.25, and the aggregate-binder ratio is taken as 3.33. The clogging materials used are clay and sand with a concentration of 5g/l for six cycles. The pervious concrete's compressive strength, permeability, and clogging potential are calculated. In addition, the image analysis of the top surface of pervious concrete is done in ImageJ open-source software to understand the reduction of pores as the clogging cycle progresses. The R open-source software is used for statistical analysis through split violin plots by comparing the effect of clay and sand on the top surface. The average compressive strength for 2.36 - 4.75mm and 4.75 - 6.30mm is 24MPa and 20MPa respectively. The average permeability for 2.36 - 4.75mm and 4.75 - 6.30mm is 4.78mm/s and 8.16m/s, respectively. The relationship between porosity and permeability is established. The introduction of clay slurry reduces the permeability by 69.8% and 74.9%, respectively, and with sand, it decreases by 74.7% and 71.7%, respectively in its first cycle. The sand particles are retained in the pervious concrete matrix for all clogging cycles in both the aggregate size gradation. The clogging potential of clay and sand are calculated for both the gradations. ii The top surface of the pervious concrete was analyzed for the properties of area, perimeter, aspect ratio, circularity, roundness and solidity through image analysis. For the aggregate size gradation of 2.36 – 4.75mm, in the six clogging cycles of clay, the total percentage area reduced from 14.02% to 8.68%, whereas for sand, the total percentage area reduced from 13.16% to 4.11%. For the aggregate size gradation of 4.75 – 6.30mm, in the six clogging cycles of clay, the total percentage area reduced from 9.54 % to 6.90%, whereas for sand, the total percentage area reduced from 8.89% to 5.50%. It concludes that the clogging cycle follows an exponential curve, and the maximum clogging occurs when the clogging material is introduced to the specimen for the first time. The values indicates that sand clogs the pervious concrete matrix more than clay particles. The clogging of pervious concrete is typically a top surface phenomenon. The degradation of permeability depends on the clogging particle's particulate size and the concrete matrix's pore size. The smaller aggregates in pervious concrete are not recommended in areas of high siltation. The pore analysis indicates that the pores are non-circular and cause delay in the permeability of pervious concrete.