Early Strength Required
Early Strength Required
(OP)
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.






RE: Early Strength Required
RE: Early Strength Required
RE: Early Strength Required
Dik
RE: Early Strength Required
RE: Early Strength Required
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!
RE: Early Strength Required
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.
RE: Early Strength Required
RE: Early Strength Required
RE: Early Strength Required
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.
RE: Early Strength Required
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
RE: Early Strength Required
http://www.construct.org.uk/bpg/BPGEarly_Age.pdf
http://www.ndt.net/article/ndtce2009/papers/219.pd...
RE: Early Strength Required
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