What's the time needed before weight applied?

[griffgj]

I'm sure I'm going to start this thread with very vague information (basically due to my lack of knowledge in this subject). Anyway, two concrete runway's were poured for a boat lift on four wheels ("Travel-lift" if you will). The weight of this alone is around 4 ton. It picks up boats in the range of 60 ton. The ramp's diminsions are 50' long x 3' across x 1' deep. The form has lots of "I" beams and rebar piled in for support. It's also adequately supported on the bottom with large wooden pilings. The conrete is a 3000 lb mix with fiber. So finally, my question is: how long does it take the concrete to harden enough so this travel lift is able to go on it and lift boats out of the water? The weather might be approiate to list too. It's south alabama so right now around 80-90 F weather. Thanks for your help and let me know what else I need to say.
Griff

 

[sc]

Generally concrete will take 28 days to acheive 95% of its strength.

The company that supplied the concrete should be able to give you an idea of the expected curing time as they know the mix and the local conditions.

Given the mass of the travel lift and the boats being lifted, I would suggest that it is not used until 28 days has passed since the slab was poured.

regards
sc

 

[jheidt2543]

At the risk of being "nit-picky", I would like to add some clarification to sc's comments. He is correct in saying that generally speaking, 95% of concrete's ultimate strength gain is achieved in 28 days. But, when we talk about 3,000 psi DESIGN STRENGTH, we are talking about the MINIMUM strength at 28 days, generally the concrete's ultimate strength is significantly higher.

The best way to know when a load can be applied to your concrete structure is to have test cylinders broken at various ages, 3, 7, 14, 21 and 28 days are the usual time periods. Not all these are selected all the time, the time period depends on what you are trying to find out. Stripping of forms might be based on 3 and 7 day tests (precaster's use 24 hour tests!).

The point I'm trying to make is that it is not always necessary to wait 28 days. I've seen test reports for a 3,000 psi mix design reach 3,000 psi in 7 days. You should be testing your mix design and the testing should be matched to the conditions of the project.

 

[BigH]

jheidt2543 is right - for low strength mixes, it is quite likely that 20MPa (3000 psi - approximately ) has been reached, if properly cured, within a week to 10 days. Ready mix supplier might have tossed in an extra half bag or so just not to have any hassles with concrete of this class. I would suggest you are likely safe at 14 days - what are the stresses involved anyway - likely much less than 3000psi. The ready mix company supplying the concrete probably has historic records of its own that would give you the estimated strengths at various time intervals for each class of concrete they make.

 

[sc]

jheidt2543

I know quite well that concrete can be cured to the required strength earlier, however from past experience and Murphys law, if a slab is fully loaded (as a 60T will do) early then cracking will occur. Given that the lift is in a marine environment, I would think it prudent that the concrete be allowed the full 28 days if possible so that even minor cracking was restricted. As such I did not advise of the early time possibility.

Earlier loading as stated is definitely possible, but check with the concrete plant for the expected strength at earlier dates as advised.

Regards
sc

 

[bluesman]

Guys
I used to work with several ready mixed concrete suppliers. Take my word for it, don’t trust any data they provide!

What would I like to add to what you guys mentioned is :

Remember that the concrete is mix-designed for average mean strength not the specified (required) strength which means that any concrete supplier (at least any decent one) would have probably designed the required 3000 psi (21 N/mm2) to yield an average of 4450 psi (30 N/mm2) at 28 days. Which means that the 3000psi strength will be reached earlier and at the given temperature (80-90 F) and assuming there's no special concrete or admixtures used, this concrete will achieve the 3000 psi bench mark at 7-8 days!

But remember that strength of concrete is only one of many performance characteristics of good concrete. Let us not forget that at this temperature, the concrete needs to be cured and 7 days is not enough

So back to our question load or not load? It really depends on the importance level of the project!

If you can wait! Then it's "SAFER" to do so! And believe me there’s nothing like peace of mind when it comes to concrete.

If you can't wait! And you like to live on the edge then Test the samples you collected (if you did so) or a couple core tests will do at 7 days.
If cracking, permeability, carbonation and/or any related concrete durability issue are not a major concern you can load it within 7 to 10 days.

Just ask yourself: "is it worth it??? Can I wait??? And if I can’t, did the concrete reached the required strength???"


Drop By !!

http://www.geocities.com/concretesite

 

[griffgj]

Wow, these are awesome answers. I greatly appreciate your input! I'm not exactly sure what we're going to do yet, but at least I know the consequences and it will be my own fault if we go too early.
Thanks again,
Griff