3000 psi vs 4000 psi concrete 2

[lisyuse]

It is said that 3000 psi concrete are more ductile than 4000 psi.. by how many percentage? do you use 3000 psi or 4000 psi in your building (ordinary and special moment frames)? I know so many structural engineers who have never use 4000 psi.. they just use 3000 psi.. how about you?

 

[RPMG]

The strength of concrete that you use is regional, industry, and schedule driven. We specify 4,000 psi and receive almost 6,000 psi because contractors want to improve on schedule. My brother, in another region, has worked jobs where 7 day strength was 1,000 psi, and isn't worried because, with the admixtures, it will be 3,000 psi eventually. You should use a strength that is normal for your region because it is economical.

 

[PicoStruc]

7 day strength was 1,000 psi [...] it will be 3,000 psi eventually

Not so sure about that, at 7 days, you reach 36% f'c @ 4 Degree / 62% f'c @ 13 Degree / 74% f'c @23 Degree

 

[rapt]

A design engineer cannot rely on this! What happe3ns if you get a builder who does not think this way, or a supplier who wants to cut his costs to the bone and produces what is actually ordered. A design engineer has to design for the specified properties, not some fanciful hoped for number that might eventuate depending on the whim of the builder or the supplier!

 

[HankTexas]

A little off topic but when I started designing 3000 psi concrete was pretty much the minimum standard. Now days in my area 4000 psi concrete has become the minimum standard due to the minimal cost difference. When I was working in the electric utility industry we used a lot of threaded rebar for anchor bolts and the embedment depth was based on the concrete strength. After having to bust out and re-pour concrete due to low concrete breaks, I started designing anchor bolt embedment for 3000 psi concrete and specifying 4000 psi concrete. Although I had to present the design a few times to justify the lower concrete breaks when it happened, we never had to bust out concrete again. The small increase in concrete price was worth it. I assume engineers often do this for some critical parts of their projects.

 

[BUGGAR]

Like they used to do, specify low strength concrete to avoid special inspection, then specify a w.c. ratio to get the actual strength you need, noting that it's for durability only.