Light weight streinth

[debar99]

We usually let concrete floors we produce sit for 21 days. (until the 21 day break test comes in ) We want to start construction on the floors 7 days after the pour is there any codes that permit this. I have heard that if the 7 day test is returned at 75% then there shouldn't be a problem, We are using light weight with a 3000 PSI

 

[JAE]

This isn't a code issue at all. It is simply your own judgement as an engineer as to what concrete strength is "enough" to allow formwork to be removed, workers to get on top of the concrete, etc.

You should look at the anticipated temporary construction loads on the concrete and perhaps analyze the framing with a reduced f'c to see what level you need.

Usually, f'c doesn't affect flexure all that much, but shear and compression are more sensitive.

We usually require 75% of f'c as a minimum. Have the contractor make additional cylinders (if its not too late) so you can test at 5, 7, etc. days.

 

[archeng59]

I haven't spent time looking it up to see what the exact wording is, but ACI 301 has a statement about not applying construction loads that will damage a concrete member. there is similar verbiage in other ACI documents. typically, the gist is that the contractor can work on a slab as long as the concrete is hard enough to not be damaged and has adequate strength to support the loads. If there isn't adequate strength, then shoring/reshoring is needed for temporary support. for slabs-on-grade, I recommend that the contractor wait 7 days, which is the curing time. usually after 7 days of adequate curing at appropriate temperatures above 40 degrees, the concrete should be a 75% of the 28 day compressive strength. hopefully this helps.

 

[henri2]

Are the cylinders going to be field cured?