Does aggregate make higher or lower F'c 3

[dirtsqueezer]

I've had some time to purge random thoughts about the business, and had noticed that grout (sand only) has significantly higher F'c than concrete. Is this just because of the higher cement content? Does aggregate just fill up space without significantly detracting from the compressive strength? Does it add strength? Just a thought in passing.

PS. I live in the Pacific Northwest, so we're talking durable gravels, not pumice or lightweight. Or that too, if you're interested.

 

[civilperson]

Your assumption is wrong, concrete strengths range from 2500 to 20000 (psi). Grout is rarely over 6500 psi.

 

[Rjeffery]

Aggregate plays a significant role in the strength of concrete. some of the strength comes from the bond developed at the surface of the aggregate with the cement paste. that is why I see only rounded "bank run" gravel used in residential foundations and manufactured crushed stone in structural concrete. The strength of the parent rock is also another factor. The stronger the rock the stronger the concrete, (all other things being held equal)

 

[concretemasonry]

What ASTM methods are you using and what types of concrete and grout are you referring to? f'c is reserved for concrete cylinders and not for other products.

Site cast concrete cylinders are cast in non-absorbant molds to match the procedure and are used as a comparison of the mix materials to the original mix design. They are cured in under laboratory conditions. They are not intended to accurately reflect the actual strength (f'c) of the concrete in place, since this is a affected by the result of the mix, placement and curing.

If you are refering to masonry grout (f'g), the samples are cubes cast in an absorbant "mold" consisting of the units used in the project. The purpose of the test is to provide a measure of the actual grout in place where the units absorb the excess moisture required to place the 8-11" slump grout. Normally, high strength masonry grouts are not necessarily an asset in the structure, because they can provide a misleading measurement of the structure's strength. The controlling strength measuerment for masonry is the prism strength (f'm).

Other types of grout such as non-shrink and grout for anchoring embedment and other uses have different properties and requirements and the test/sampling procedures are not at all similar to concrete cylinders.

You cannot compare apples and oranges since the desired properties, placement and testing procedures are different. Even in some types of cement-based materials, the addition of water will provide more strength than cement on a weight basis.

Dick

 

[shin25]

Also, cube strength, which is used for grout shows higher strength than cylinders used for concrete.

you have to convert cube strength for an equivalent cylinder strength.

 

[msucog]

i'm with concretemasonry...

 

[dirtsqueezer]

Hmm. Well, if I'm not mistaken, masonary grout uses 3/8" aggregate? We are currently using a 3/8" PT mix for columns and shear walls, pours at around an 8. The mix is approved for 8,000psi, so I'm sure it's much stronger than your prisms, but that's due the fact that it's a 7 sack mix. But that's a difference in cement content between your masonary grout and my PT concrete. I'm wondering if something like a 1 1/2" mix proves better than a 3/4" proves better than a 3/8", better than a #4. Do you see what I mean? Does aggregate size make a stronger or a weaker mix? That is my question.

 

[concretemasonry]

The different materials are tested differently, have different size and relative sample dimensions (aspect ratio), formed differently and cured differently.

Masonry grout has two different sizes (fine and coarse) that are controlled by the use. In many cases, 3/8" aggregate would not be acceptable and would be detrimental to construction. Masonry grout has an absolute maximum allowable strength of 5000 psi according to ASTM C476 and normally 2000-3000 psi is specified and preferred. You do not want larger size aggregate in a grout and you are not after high stregths - its the old story of stronger not being better.

Larger aggreagate will give higher strengths with all other things equal. The problem is that large aggregate may be detrimental to the application or may not comply with the specifications. In most situations, not all thing are or cannot be equal.

Dick

 

[dirtsqueezer]

Why are there two different sizes(fine and course) for masonary grout?

 

[Rjeffery]

I can't speak to all situations but generally fine grout is used when the clearance of the bars and CMU cells would not allow the reinforcing to be fully enveloped by the grout. Same reason that you don't see 2-3 inch aggregate when the clearance between the form and the steel is only 1 1/2 inches!

 

[BigH]

Have things changed over the years? When I learned at university (from a pretty reknowned expert), grout was a mix of cement and water; mortar, on the other hand was a mix of cement, sand and water; concrete was a mix of cement, sand, coarse aggregate and water. That was, I thought, the normally accepted standard of use. It always irks me to hear someone say, "He poured that cement footing today."

 

[concretemasonry]

The only one close to pouring is the grout brcause of the required water for proper placement.

The only real problem with grout is not the proportioning and strength variations, but the rate of placement and lack of reconsolidation.

Too often, construction masterials get lumped together just because of the raw materials, while the desired properties design requirements and placement methods that really produce quality construction are more important. Combining plant produced products with site created reults takes good engineering and an understanding of the properties and advantages/disadvantages.

 

[dirtsqueezer]

Thanks.

 

[hokie66]

BigH,

Your university learned definitions for mortar and concrete are still correct, although there are often other things added to improve some property or other.

Grout is the one with lots of meanings. When grouting posttensioning tendons, you use cement and water (and sometimes an admixture). When grouting under a steel column base plate, you use cement, fine aggregate, and water (and in the case of premixed grout, who knows what else). When grouting concrete blocks, you use cement, fine aggregate, generally a rounded small but coarse aggregate, water, and again probably admixtures. I think the reason for calling this particular type of concrete "grout" is that it is used to fill confined spaces.

 

[henri2]

Amongst several other factors, the nominal maximum aggregate size of a concrete mixture does influence the compressive strength.

At the same water-cementitious materials ratio, there is an inverse relationship between MSA and f'c. This is why smaller nominal max aggregate mixes are used for high strength concrete (6000 psi and greater).

ACI materials journal has several papers which focus on this issue...but I think you have to be a member to download their papers. Google scholar is another source.

 

[Dinosaur]

In my experience, aggregate is the weaker link in cured concrete. The paste is stronger than the coarse aggregate. To make high strength concrete, you need the right aggregate supplies and the right blend of materials in the paste.

In school you should have performed some testing on concrete cylinders. In early age concrete, the paste debonds from the face of the coarse aggregates. But once the paste has gained adequate strength, the aggregates fracture and a good test will show an abundance of fractured aggregates.

I am not well versed in masonry and grout.

 

[haynewp]

NIST has a lot of information and modeling programs. I have used an older hydration modeling program from NIST before and it was kind of interesting how varying the aggregate sizes and modifying mixture variables affected the quality of the ITZ. There are other virtual modeling and optimization programs available also.

http://ciks.cbt.nist.gov/~garbocz/monograph/tablecontents.html

 

[henri2]

Great link haynewp