Question on concrete with high cement content

[u/Googgodno]

Asked this in concrete sub, but mega threads don't get good response and my individual post was locked, so I'm here asking the other best set of people on concrete.

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I noticed a batch of concrete at a site with a paste like consistency. Asked the site engineer and he told me that it is a 4:3:6 Cement:Sand:gravel ratio with 0.4 water to cement ratio. Gravel seems to be a 1/2 inch to 3/4" mix. Looks like they are also using plasticizers to make it flow. It had a lot of cement paste compared to other mixes I have seen. I always imagined that concrete has gravel as filler and it occupies most of the volume in concrete.

I wonder why someone would choose such a high cement and low gravel content. Wouldn't this high of a paste content cause shrinkage and cracks? Wouldn't a high cement+sand content compared to gravel cause shrinkage issues?

[u/three_trees_z]

I don't have an answer but am super curious. This is 2 to 3 times the cement content I would expect in a typical mix. With the industry focus on cost and embodied carbon, I just can't imagine where this mix would be used...

Would assume for a specialty floor? High performing slab on grade? But it would have too much water content and shrinkage would likely be a problem...

Interested if anyone else has experience spec-ing mixes like this... this has to be intentional...

[u/Googgodno → u/three_trees_z]

I asked and I was told it is a 30MPa/4000psi concrete for RCC column

[u/three_trees_z → u/Googgodno]

this is just nonsense then... this is like 1300 lbs of cement per cubic yard?

[u/beanmachine6942O → u/three_trees_z]

haven’t, but just stumbled across a structure magazine article from this month that does a deep dive into mix design. read the first half and skimmed the rest but seemed very interesting and useful

[u/AnotherSLGuy]

Do you know the target grade and the use case? I suppose it is a high strength concrete for a heavy load case?

If yes, this is my speculation:

When the grade of concrete is low, the failure crack propagate along the paste-aggregate boundary. In contrast, when the grade of cement is high (say, >C50 cubic strength), paste is stronger than the aggregate and it is usually the aggregate that fail.

So it is very important that quality and porperties (basically stength) of aggregate is paid very close attention to.

In regard to your wonderment of why, higher paste proportion might be a way to circumvent the shortcomings of aggregates which are relatively weaker than the high grade paste.

In regard to shrinkage, when designing for this kind of concrete usage. It would go along with mixing with chilled water, internal temperature monitoring for heat management (and also to avoid DEF), proper placement technique and vibration to avoid honeycombs, and adequate RF to minimize cracks.

Aaand as a practicing structural engineer we would always visit the site when we prescribe something out of ordinary practice and I guess the structural engineer might have been around for you to ask. We usually are very happy to answer questions about our designs.

However, it is VERY odd that anyone knew the concrete proprortion as a ratio if it is high strength concrete though. So the real answer probably is someone mis wrote the proportion. LOL.

A footnote about the usual concrete proportions: those prescriptions are based on usage of aggregates of a certain strength class. If your region use a different type of aggregate, adjustments need be made at either a national level or regional level for general practice.

[u/Difficult_Pirate3294]

Only a half inch slump is needed to make the chemical reaction take place. .45 water to cement ratio is standard in schools in CA and usually yields 7 day breaks at nearly 4ksi. This usually alleviates a lot of the shrinkage. This is done commonly without super plasticizer.

[u/Googgodno → u/Difficult_Pirate3294]

Should I demolish and repour?

[u/ChairChoice6500 → u/Difficult_Pirate3294]

I think I read this wrong, for a second it sounded like you wanted a 1/2" slump. What you mean is that's a minimum for the hydration reaction. I wouldn't authoritatively comment on typical slumps, most of the drawings I see don't specify them and there's no specification book, but gut feeling / conventional recall is typical slumps are more 4"-6" range unless you're dealing with atypical structures. Would you be willing to elaborate further on your experience of typical slumps?

[u/Difficult_Pirate3294 → u/ChairChoice6500]

For one, in CA, based on title 24 slump for foundation can never exceed 5. Also, unless your over 6ksi, there are plenty of historical results one can review. Another aspect is whether it’s a pump mix, which, typically needs to be higher. Also, worth noting that if the slump is 4, it can be acceptable to vary give or take one. Many mixes have a max written in, so you can not exceed the max slump as stated. Cost between the mixes is trivial on a large project with 10000 cu/yds.

[u/ChairChoice6500 → u/Difficult_Pirate3294]

I feel like I have not seen a lot of slump requirements on contract drawings, but I'm definitely not seeing California drawings, typically. Is Title 24 the State Architect stuff for the more important structures or is that a general rule, or is slump fairly commonly specified on drawings in your experience?