REMOVING FORMWORK 5

[ALPER356]

Can anyone tell me how long I should wait before removing the forms and wood supports under an 8"x24" concrete beam spanning 12ft between columns for a residential terrace. I poured 3000 psi pump mix down here in South Florida? I hear after 7 days it gets 80% of its full strength but I'm getting different replies from different contractors.

 

[Ron]

Much of the concrete produced here in Florida is blended with Ground Blast Furnace Slag Cement (GBFSC). This material retards the early strength gain of the mix, thus the 7-day compressive strength, assuming no water addition at the site, will be more on the order of 50 percent of the design compressive strength and certainly not 80 percent. Even Type 1 portland cement with no additives or blending historically only achieves about 70 percent of its design strength at 7 days. The 28 day compressive strengths are not usually affected by the GBFSC addition, but it does affect the early strength gain.

You could core the concrete to find its strength, since I assume you did not take test specimens for compressive strength. Don't let the concrete producer or a contractor use a "magic" Swiss Hammer on the concrete to tell you the compressive strength. It doesn't do that, even though there's a scale on it that is shown as compressive strength. It is a poor indicator of strength for a variety of reasons.

Given all that, it would be better to wait a few extra days, to assure yourself of more strength gain, assuming your beam is properly reinforced and the concrete was not modified during placement.

 

[beton1]

In general, 7 days is OK. However, as Ron pointed out (I learned something about Florida concreting), there are factors which can affect this. It also depends on the construction schedule and loading; watch temporary construction loads in particular. If you can wait longer, it is better, especially since 'creep' is another consideration. You may have to consider re-shoring.

 

[kslee1000]

Re-shoring is a cool idea.

Ron, again educated all of us. Thanks.

- a new trick learned today. ^-^

 

[ALPER356]

Thanks alot guys, extremely helpfull. Really great having a wonderful support team like you all.