Concrete inFocus - Winter 2015 - (Page 16)

best practices The Right Mix: A Guide to Pervious Concrete Mix Design Philip Kresge, NRMCA Senior Director, Local Paving With Tom Evans, Maryland Ready Mix Concrete Association; Scott Erickson, Evolution Pervious Resources; Jim Miller, C2 Products and David Mitchell, Bunyan Industries P roportioning pervious concrete mixtures is different compared to procedures used for conventional concrete. When developing pervious concrete mixtures, the goal is to obtain a target or design void content that will allow for the infiltration of water. The void content of a pervious concrete mixture will depend on the characteristics of the ingredients, how they are proportioned and how the mixture is consolidated. More so than in any other concrete mix design, coarse aggregate properties affect the proportions of all other materials within a pervious concrete mix. Importance of Aggregate on Mix Design All coarse aggregates are not created equal and the characteristics (such as size, 16 ı WINTER 2015 shape and absorption) must be taken into consideration in the mix design process. If the coarse aggregate is of a reasonably uniform size (gap graded), the voids will be large enough to accommodate an adequate quantity of paste while leaving enough interconnecting voids for good permeability. As the gradation of the aggregate becomes more varied, the overall voids are decreased and if too small may not be sufficient for quality pervious concrete. Therefore, the first step in proportioning the mix is determining the void content of the aggregate. This is accomplished by obtaining both the SSD unit weight via ASTM C29 and the aggregate specific gravity, and entering them into a pervious concrete mix proportioning spreadsheet. The density of the aggregate, combined with the unit weight (ASTM C29) is used to determine the volume of solids/ voids in the aggregate. While the suggested void range of pervious concrete is 15 to 25 percent voids, most professional pervious mixes target 18-22% void content. Mix designs must then be proportioned such that the voids available in the aggregate match the percent of paste specified. If the aggregate contains less than 38% voids, the mix must have a paste volume of 20% or less. Mixes with too little void, or with excess paste, will result in low permeability. Cementitious Content and W/C Ratio With traditional concrete, more cement equates to stronger concrete and it seems

Table of Contents for the Digital Edition of Concrete inFocus - Winter 2015

From the President’s Desk
Corporate Suite Column
Enviroscene Column
Smokey Point Concrete
The Right Mix: A Guide to Pervious Concrete Mix Design
Union Threats: The Rules of the Game Have Changed!
Index of Advertisers
2015 Mixer Driver Recruitment and Retention Survey
2015 National Ready Mixed Concrete Association Driver Recruitment and Retention Survey

Concrete inFocus - Winter 2015