Flexural Strength and Aggregate Size

[MatthewMansfield]

Hi all

I am trying to understand why flexural strength of concrete decreases as aggregate size increases in a concrete mix.

Compressive strength of concrete increases as aggregate size increases which I understand why.

But

Why does flexural strength of concrete decrease as aggregate size increase?

I am a bit baffled.

I was wondering if anyone could help?

Thank you.

 

[BridgeSmith]

For practical design, flexural strength is dependent primarily on the amount of reinforcing, with the compressive strength as a minor contributing factor. As far as I know, aggregate size doesn't affect the flexural strength of reinforced concrete.

If you're talking about unreinforced concrete, if the flexural strength (what little it has) is reduced, it's obviously due to a decrease in the tensile strength of the concrete. I would guess more interface area between the cement and aggregate = less tensile strength, but that's just guess, since I never cared to indulge in the useless investigation of the flexural strength of unreinforced concrete.

 

[Ron]

Flexural strength is an indirect measure of tensile strength of the concrete. As HR10 notes, the greater the tensile strength, the greater the flexural strength; however, the issue is more complex than that. Flexural strength is optimized by careful grading of the aggregate, particularly the coarse aggregate.

Decreasing the aggregate size requires more cement, which inceases the cracking potential for the concrete. Not good for flexural strength! So optimizing the mix with flexural strength in mind is important.

I never cared to indulge in the useless investigation of the flexural strength of unreinforced concrete.

I disagree with HR10's comment about "useless investigation". The next time your plane lands on a runway, be thankful that someone did one of those "useless" investigations to make that runway safe. I've done many of them, none of them "useless".

 

[BridgeSmith]

I wasn't aware that airport runways utilized unreinforced concrete. In any case Ron, you're right; in cases where you have to determine the capacity of unreinforced concrete slabs, tensile strength of the concrete matters quite a bit. In bridge design, we don't use unreinforced concrete, and we neglect the tensile strength of concrete for design. I should have said that I hadn't investigated the changes in concrete tensile strength due to changes in aggregate size because it was not something useful to me. The AASHTO bridge design spec. utilizes an estimate of the tensile strength (rupture stress) for concrete based on the compressive strength only.

 

[Ron]

HR10...we're even....I don't do bridge design!

Yep, most concrete pavements use plain concrete. Additionally, in some reinforced concrete applications, a high modulus of rupture (flexural strength) mix is used to enhance the properties of concrete for flexural purposes, such as tilt-up panel construction.

 

[IDS]

The flexural tensile strength of the concrete has a significant effect on deflections, crack widths, and minimum tensile reinforcement area, amongst other things, so I wouldn't say that the investigation of this property was useless.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/