Aging of concrete and removal of formwork! 1

[engr567]

Hi,
Is there a section in ACI that talks about the percentage compressive strength gaining of concrete over time? As I know 7 days strength is about 70-80% of 28 days compressive strength. Also, how quickly can we remove the form-works and shoring in an elevated slab (not post tensioned slab) per ACI? I would appreciate your help.

 

[rscassar]

Depends on a mix design and the best person to comment on a particular mix is the readymix supplier. ACI-209 and Eurocode2 gives models to predict the strength gain of concrete.

Section 6 of ACI318 talks about the removal of forms and shoring.

Generally the notes will say something like:

ALL SHORING TO REMAIN IN PLACE UNTIL CONCRETE HAS ATTAINED 75% OF ITS 28-DAY COMPRESSIVE STRENGTH.

Refer to this written faq about concrete:

faq507-1572

thread581-120179

Some engineers say shoring can be removed after 14 days, other engineers say 21 days. Personally 21 days is better for the design of the slab because it reduces the contribution of creep and shrinkage to the longterm deflections because the time of first loading has been delayed.

To say they must remain in place until the concrete has attained 75% of it's design strength is ambiguous. It would require and additional cylinder test to ensure the concrete has attained it's particular strength.

Remember to give rowingengineer a good review for the FAQ.

 

[BAretired]

I don't believe there is a definitive answer to the question of when forms can be removed. There are too many variables.

Forms can be safely removed in as little as three days, assuming perfect curing conditions and immediate re-shoring which, in turn, must be maintained until the concrete has gained its 28 day strength.

BA

 

[ishvaaag]

My recent practice here for solid plates as soon as spans come close to the 8 meter range is to order to keep shoring points or lines (not necessarily all planks if the shoring system allows separate removal) 4 weeks; of course these are low to mid-rise buildings, where such shoring times do not affect intolerably to the construction timetable. Not a subject of controversy with contractors over here. It has also shown to be quite effective for the reduction of deflections, most times visually inappreciable.

 

[ron9876]

To remove forms like beam side forms it would be as soon as the concrete has set. With slab shoring normally 2/3 or 3/4 of 28 day strength assuming immediate reshoring. This should be indicated on the drawings.

 

[cvg]

this has been discussed numerous times previously, but in general unless you work for the contractor or are planning to specify a time in the contract, this is means and methods for the contractor to determine. Contractor has to construct the structure and protect it until the project is turned over to the owner. Details of how he does it is up to him. I would not allow a deviation from the specs unless you want to take some liability for the structures.

 

[rscassar]

Adding to what ishvaag wrote about deflections, I have seen engineers say that slabs supporting masonry walls are not to remove props before 28days.

 

[hokie66]

kikflip,
That seems an unusual requirement to me. It would serve to lessen the short term slab deflection, but whether it helps to prevent wall distress depends a lot on the sequence and speed of construction. I think a more common requirement is to require the removal of shoring prior to construction of supported masonry above the slab. That way, the masonry and the slab deflect together as the masonry is constructed.