Crushed vs Round aggregate in concrete

[ATSE]

Background:
ACI 318-05 and -08 does not appear to reward the designer for specifying crushed aggregate over rounded (usually "river rock") in structural concrete.
Chapter 4 refers to ASTM C33, and leaves it at that.
Certainly crushed rock has better interlock, and must have better shear friction for roughened surfaces. I do not have the data, but I would bet crushed rock is better in most or all performance measures.
The large aggregate (used in ready-mix concrete) in my area - Northern California - is either crushed 1"x#4 - or river rock 3/4". Crushed 3/4" is available at a premium. For high rebar congestion areas, I don't like using 1"x#4.
Most of my work is heavy civil structures, water and wastewater. Not residential, not light commercial.

Question:
Is there industry-accepted, published technical data to justify the specification of crushed rock and not allow rounded river rock? In real terms, this is about $5 to $10/yard difference. Enough for some contractors, with the help of ready mix plants, to fight over (I don't pick the contractors or ready mix plants).

 

[Ron]

From a compressive strength standpoint, there is little discernible difference. Yes, crushed stone provides a bit better aggregate-paste bond, but not so much as to greatly affect the compressive strength. The tensile strength is affected moreso by the aggregate shape.

Durability, cracking and other general parameters are not significantly different.

 

[dik]

Angular aggregate provides more consistent and stronger concrete with less paste... try not to use rounded aggregate.

Dik

 

[concretemasonry]

Here, the rounded aggregate is the stronger, more durable rock that is eons old. The crushed is usually limestone or pricey granite/basalt/trap rock. There is a big difference in placement and consolidation.

There is always a consideration with the bond to smooth, old durable rock if you need unusually high compressive strengths.

It is a local factor, as occurs with most aggregates.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[jdgengineer]

I suggest you read through ACI 221R-96 for additional insight. It is titled "Guide for Use of Normal Weight and Heavyweight Aggregates in Concrete"...

ACI allows both crushed rock and rounded aggregate, but goes over some of there differences in the guide. A couple highlights...

1) Both generally have same compressive strength for same cement content.

2) crushed typically requires higher water requirement than rounded.

3) Rounded aggregate may increase the pumpability of the concrete...

Read the guide for more info...

In the end I think either should be allowed provided they are staying with in your mix design requirements and have previous testing on the mix that show it meets your performance specs.

 

[beej67]

The angularity of stone is quite important when you start dealing with flexible pavements such as asphalt. Roads made of rounded aggregate tend to rut much more quickly than those made of crushed material.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[DST148]

@ATSE: Reference: ACI Education Bulletin E1-07 - Aggregates for concrete:
In very high strength concrete mixtures where coarse aggregate bond is critical, angular cubicle shaped coarse aggregates generally give higher strengths than rounded smooth aggregates. Where extremely high strengths are not required, acceptable concrete can be made with different types of aggregates, with some variation in w/cm required to provide the needed strength.

 

[concretemasonry]

Just keep in mind that the cost portion of the equation depends on the available local aggregates and the ability of the local concrete producers to develop the proper mix design for the application that is based on experience.

There is always the question of durability instead of strength. With our local aggregates the rounded natural aggregates can provide superior results over the local crushed limestone aggregates. Strengths are not a question with either type of aggregates.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[ATSE]

Good comments.
Thanks.

 

[shobroco]

Funny how you don't consider things that don't commonly occur in your area. I live on the Niagara Escarpment in Southern Ontario, in an area with great natural gravel deposits and one of the largest limestone ridges on the continent. The gravel (your rounded aggregate) has not been used here in concrete in more than 60 years other than as fine aggregate; only crushed limestone is used. Early concrete, which I deal with a lot because heritage structures are my pet area, always used gravel & is frequently poor. Modern concrete I never even think to question because the ready-mix companies all use crushed limestone meeting established specs & they wouldn't even think of anything else. We simply spec a strength & durability & they deliver it.

 

[Hoaokapohaku]

From the Portland Cement Association's, "Design and Control of Concrete Mixtures" -

"The bond between cement paste and a given aggregate generally increases as particles change from smooth and rounded to rough and angular. This increase in bond is a consideration in selecting aggregates for concrete where flexural strength is important or where high compressive strength is needed."