Early Strength Required 2

[CTY]

I've designed a composite floor/composite floor beams using a 30N/mm2 charecterisic strenth concrete, but the client wants the floor in use soon after the concrete is placed. If I specified a 35N/mm2 concrete (designated mix RC28/35), how soon after placing the concrete could the floor take the full superimposed load. I've seen a chart which indicates that the required strength would be achieved in approximately 14 days, but not found a formula calculate accurately. Any help would be much appreciated.

 

[ishvaaag]

Attache pdf printout of a Mathcad 2000 Professional worksheet that addresses the issue by 3 formulations.

 

[ishvaaag]

Apart from finding the info, that one may seek and find either in codes, general concrete manuals and specific material related to the growth of strength of concrete with age, it is important to note that the structure is normally designed for a final state whence it can be submitted to some specified loads. This means that applying the specified (to be warranted in the end) loads as soon as the average strength attains the characteristic value means not necessarily you are in a code complying situation. Lately, I have seen codes where the safety factors for the load combinations for construction time (o temporary) are akin or the same than those in the final situation, what would mean for your case you would have to wait for the concrete attain the characteristic strength prior to apply safely the nominal loads, at least following such codes' vision.

 

[dik]

Back to the early use of slabs, beams, etc... full loads at an early age will likely increase the creep of the system and may cause permanent deformations...

Dik

 

[shobroco]

Why don't you specify the capacity required at the date required & let the ready-mix supplier meet the specification? Dik is right though, regardless of what you spec or they supply, early loads = more creep.

 

[Ron]

This is not a calculable parameter. Field strength gain in concrete is much too complicated to predict accurately by equations. There are many variables in the field that are not necessarily accommodated in the mix design.

Increasing the base strength of the mix will usually help; however, indiscriminately changing the parameters can often give results you do not want...such as additional cracking, durability issues, etc.

Think this through and let your client know the positives and negatives of THEIR decisions. Do not take responsibility for adverse decisions your client might impose. Get some local concrete technology help as well, even if you consult the concrete supplier. Check with their Quality Assurance department, not their sales department!

 

[racookpe1978]

Can you avoid the problem by keeping the original forms in place or by adding extra bracing under the concrete while the client adds his machinery and loads?

Or add hydraulic bracing to "push up" against the loads with a variable resistance as loads are positioned to allow controlled displacement of the machinery (?) while the supports resist the bending/deformation of the slab and walls as they dry? Expensive, but better than busting the floor and walls.

 

[shobroco]

Ron, I'm not sure why you say it can't be calculated. I have specified high-early strength concrete many times, and the ready-mix companies all have a range of strength vs time mixes. The shorter the time, the more the cost to achieve a given strength, and ultimately the higher final strength, but I've seen plenty of testing to confirm that they deliver what they advertise.

 

[Ron]

shobroco....everything you mentioned is based on empirical data, not calculations. Yes, the curves are available, but they vary with the numerous variables of cement type, water-cement ratio, aggregate type, aggregate gradation, coarse and fine aggregate ratios, admixtures, and a variety of other considerations. Further, those curves apply ONLY to the mix design, not the site-specific conditions.

 

[Hoaokapohaku]

I'm the kind of guy that Ron recommends you talk to. I wear various hats, but one of them is Quality Assurance Manager for a ready mix producer. Ron is correct, there are just too many variables in play to calculate early age strength. And, the earlier the strength requirement is, the more volatility those variables impart to strength test results. A change in a variable that may have negligible effect on 28-day results may have an enormous effect on short term results, such as 24 hour, or 3 days results, much less so for 7 day results.

That being said, shobroco is also correct. Given a specification requirement for a certain high-early concrete, I can almost always come up with a design that will meet the requirement. However, the more difficult the requirement, the more intense the effort to control the variables must be, and the more you are going to pay for it. Often disputes occur, because I don't control all the variables and therefore can't be held responsible when somebody else doesn't do their part.

In my opinion, it is better to use maturity meters to determine the strength of in place concrete when the construction schedule is dependent on achieving a certain concrete strength before the next step can be taken. This takes most of the variables out of play and allows you to know more precisely when the concrete has sufficiently cured for the next construction phase.

 

[demayeng]

I've got a set of 32 MPa concrete cylinder test results in front of me.. it reached 29 MPa after 7 days at 21 deg C, then two tests at 28 days came up at 39.5 and 40 MPa. That concrete was not specified to be high early strength

If you're only worried about strength, you can't go too wrong with testing..

Here is some interesting stuff out of the Australian formwork code.. Table 5.4.2 seems to say that for normal grade 40 concrete you can expect 30 MPa after 7 days

 

[ishvaaag]

Another and empirical approach for the investigation of early strength of concrete.

http://www.construct.org.uk/bpg/BPGEarly_Age.pdf

http://www.ndt.net/article/ndtce2009/papers/219.pdf

 

[dik]

racookpe1978:
Most composite floor in our environs uses metal pan, and is designed not to have an intermediate support... nothing to be left in place and the concrete can be irreversably damaged by early loading. Temporarty shoring, as noted, would be a benefit.

Dik