Jim Klinger, concrete construction specialist The Voice Newsletter January 2023

ASCC Hotline Question:

We are building a post-tensioned concrete high rise with a 30,000-sffootprint divided into three placements per floor level of 10,000-sf each. One such placement atthe 5th floor level is scheduled for Thursday, December 22. For supply-chain reasons, thisconcrete placement date cannot change. For holiday reasons, the project is shut down untilTuesday, December 27. In other words, the elevated slab will not be stressed until the fifth dayafter placement. The project specifications require stressing to be completed within 96 hours (fourdays) of placement. Weather in our region is normally forecast to range from 40 to 65 degreeswith moderate rain possible each day. The general contractor has acknowledged that stressing onthe industry-standard third day (Sunday, Dec. 25) would be inhumane, but is insisting we breakthe bank and mobilize a crew to stress on Monday, a company payroll double-time holiday. Thissame condition will occur the following week over the New Year's holiday.

The approved PT slab mix design is 5000 psi at 28 days, 3000 psi at three days; the standard"high-early" mix with Orca low-shrink aggregate, which is popular in our market. We have had noissues with abnormal shrinkage cracking on the lower floors of this project, or on other projectsusing this same mix. Other than the post-tensioning steel, the slab is lightly reinforced. Theconcrete slabs are eight in. thick, with a typical bottom mild steel mat of #5 bars at 12 in. OCEWthroughout and top bars provided at columns and other locations. There is no call-out on thestructural drawings for supplemental mild steel shrinkage and temperature reinforcement.

We understand the construction documents require the PT stressing be completed within 96 hoursof placement. Does the ASCC Technical Division know of any reason the slab stressing cannotwait until Tuesday? We would almost be willing to propose adding a layer of #4 bars at 12 inchesOCEW (top) for crack control in lieu of having to open the jobsite on double-time. Please advise.

Answer:

This issue intrigued us because we usually see requests for support of early PTstressing, not the other way around. According to ACI 318-19, section 25.9.4.5.4, the concretestrength must be at least 2500 psi for single-strand tendons before stressing to mitigate shrinkage cracking. Sometimes the LDP will allow stressing at strengths lower than 2500 psi for certain slabson grade and stage-stressing conditions. The Post-Tensioning Institute Journal (PTI, July 2019)tells us that their Post Tensioning Manual recommends PT be stressed within 72 hours and at aminimum strength of 3000 psi. In any event, all such requirements are aimed at closing early top-of-slab shrinkage cracks. Almost all of the PT-type projects we see for review feature requirementsfor stressing to be completed within the 72-to-96-hour timeframe.

Since the mix design has been proven to reach high early strength with low shrinkage and noknown abnormal cracking, ASCC suggested placing the concrete slab, applying a second coat ofcuring compound, and covering the slab with curing blankets over the 96-hour holiday shutdown.On Tuesday, remove the blankets, inspect for cracks, and stress the PT. All costs associated withcuring compound, blankets, and possible crack repair to be by contractor. (N.B. Proposed courseof action was accepted by the LDP and the Owner).
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ASCC Hotline Question:

We have just completed the third concrete placement out of 20 on thelargest contract our company has ever landed. The Owner seems impressed with the quality ofour finish work during job walks, but we have heard from the GC that there have been negativecomments about cracking in the slabs during private meetings. Our scope of work is to furnish andinstall rebar and concrete over a composite steel beam and metal deck substrate erected byothers. We are familiar with the ACI 302 note to Owners and designers regarding the inevitablecracking of certain concrete slabs, but we cannot find specific mention of slabs placed on metaldecks. Is there anything in the ASCC Technical Division library that can help us explain to theOwner that cracking of the metal deck slabs is normal?

Answer:

The design of the composite slabs on metal decks is covered by the Steel Deck Institute.In their design manual titled "C-2011 Standard for Composite Steel Floor Deck-Slabs", thefollowing guidance is given to designers:

"Concrete floor slabs employing Portland cement will start to experience a reduction in volume assoon as they are placed. Where shrinkage is restrained, cracking will occur in the floor. The use ofappropriate types and amount of reinforcement for shrinkage and temperature movement controlis intended to result in a larger number of small cracks in lieu of a fewer number of larger cracks.Even with the best floor design and proper construction, it is unrealistic to expect crack free floors.Every owner should be advised by both the designer and contractor that it is normal to expectsome amount of cracking and that such occurrences do not necessarily reflect adversely on eitherthe adequacy of the floor's design or quality of the construction. Cracking can be reduced whenthe causes are understood, and preventative steps are taken in the design phase. The majorfactors the designer can control concerning shrinkage and cracking include cement type,aggregate type and gradation, water content, water/cement ratio, and reinforcement.”