Gap graded concrete 1

[killswitchengage]

Hi
What are your comments on gap graded grading in regards to concrete properties especially small sized gravel from 4 to 8 mm ? i believe it can decrease the strength since it decreases the density of the concrete and it can increase the risk of shrinkage

 

[cvg]

concrete mixes are generally specified and approved based on demonstrated strength of the mix design. so the strength is not an issue. assuming you have both fine and coarse aggregate, the density may not be affected either.

https://www.nrmca.org/research/D340 AGR report phaseA2.pdf

 

[killswitchengage]

So the strength is mostly determined through C/W ratio alone

 

[cvg]

there is a direct relationship between the amount of cement and the strength of the concrete. Beyond the minimum amount of water required for hydration of the cement, additional water does not increase strength, it reduces it.

 

[killswitchengage]

thanks

 

[Ron]

kse...for a properly prepared mix design, gap grading is common. The combination of gradations and quantities are somewhat self compensating. The compressive strength of concrete is mostly controlled by the water-cement ratio. Yes, the gradation and aggregate type/source have some effect, but mostly the water-cement ratio controls. The lower the w-c ratio, the higher the strength (Abrams law).

As for the smaller aggregate size....when you reduce the coarse aggregate size, the cement content has to be increased because the surface area of the aggregate must be coated with cement paste for adequate bond. Smaller coarse aggregate particles have a higher specific surface than larger coarse aggregate.

 

[killswitchengage]

thanks ron

 

[BigH]

See Neville's tome on gap grading of aggregate for background.