Q.: We frequently are asked to strip slab formwork based on compressive strength, as determined by test cylinders, reaching 80% to 85 % of the 28-day specified compressive strength. Often we get this strength 3 days after placing the concrete.
However, we notice that the American Concrete Institute (ACI) "Guide to Formwork for Concrete"* says (Section 22.214.171.124) that when forms are stripped there must be no excessive deflection or distortion. How can we be sure there will not be excessive deflection? And what's excessive anyway?
A.: As you point out, we have two concerns in deciding when forms can be removed strength and longer term serviceability. The structure must safely carry its own weight and any applied loads; it must also maintain its shape without unsightly or disabling deflections.
The answer to your question depends a lot on the use of the structure and the kind of interior finishes and partitions it will have. The structural engineer usually is in the best position to decide how much deflection is allowable, based on code requirements as well as his or her experience. Because concrete deflection is more severe when loads are applied at very early ages, a time limit before forms can be removed is frequently specified. The exact time depends on the structure's design and on the construction methods.
As to strength and safety, in some flat plate buildings where several levels of shores and reshores may be used, it's possible for the load on a slab to build up to two or more times the service load for which the slab was designed. If the contract documents specify the conditions for form removal, the Occupational Safety and Health Administration (OSHA) permits the contractor to follow these specifications, usually based on testing of the concrete (see Concrete Construction, December 1988, page 1079). If contract documents don't cover this, you must test the compressive strength of the concrete and determine that test results "indicate that the concrete has gained sufficient strength for the concrete to support its weight and superimposed loads." If contractors do not have an engineer on staff, they may want to consult one for help with this determination.
Notice that the new OSHA regulations do not specifically require cylinder tests. Any "appropriate ASTM standard test method" may be used. This opens the way for in-place testing but doesn't rule out cylinders if you want to use them.
Meeting OSHA requirements enables you to remove forms safely. Preventing undesirable deflection in the slabs is more difficult, particularly if you want to maintain a fast construction cycle. Backshoring and preshoring methods explained in the ACI 347 Guide and in an earlier Concrete Construction article (see March 1986, page 307) help control deflection when construction moves at a fast pace.
Forms and shores cannot be removed until concrete is strong enough to support its own weight and any applied loads such as the construction materials seen here. Reshores, backshores, or preshores may be used to help distribute loads among several interconnected slabs, thereby speeding up the construction cycle.