Jim Klinger, Concrete Construction Specialist The Voice Newsletter April 2023

Full Disclosure: Every once in a while, we field ASCC Hotline calls that appear straightforward at first but quickly elevate into interesting exercises in onion-peeling. In this case, it takes a little bit of wordsmithing to arrive at truth's upper floor and save our ASCC Hotline callers from making a mistake or taking a bath on a business deal.

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 Question: We have just signed the subcontract for our first high-rise project, 30 stories from street level to roof. The typical floor footprint is a rectangular area comprising roughly 25,000 square feet. The elevator and stair shafts are centrally located in one core. We bid the job assuming the use of a climbing formwork system to build the elevator/stair core walls. Our schedule shows us placing the core walls with the proprietary system three floors ahead of the slabs in a 5-day cycle. (Even though the core will be 3 floors ahead, wall placements will be pumped one lift at a time). Our outside formwork designer has asked us what tolerances he should assume for the design. The contract specifications vaguely tell us to "conform to tolerances per ACI 117.” We believe the tolerances we need to follow are found in ACI 117-10, Section 4--Cast-In-Place Concrete For Buildings. The formwork designer is telling us since we are using a climbing form system, he believes the tolerances that apply to our core wall scope are found in ACI 117-10, Section 7--Cast-In-Place, Vertically Slipformed Building Elements. Sure enough, when we turn to Section 7, the very first call-out we see is Section 7.1--"Deviation from plumb for buildings and cores.” Since we are obviously building a building and core, is our formwork designer's position correct? Does our climbing form constitute a slipform?

Answer: In a word, no. But we see what you mean. There are two hangups. One is the ACI CT-20 definition of "slipform", which is defined as "a form that is pulled or raised as concrete is placed.” This implies the concrete placement is performed in a continuous extrusion instead of the stop-start process you will have when you pour the walls one story-height lift at a time as scheduled. The second hangup can be found in ACI SP-4(14) Formwork for Concrete-8th Edition, Chapter 18: Special Techniques in Concrete Construction, which addresses the various means and methods available for slipforming concrete. SP-4 acknowledges there may be some confusion regarding tolerances as follows:

"ACI 117 does not differentiate between slipformed building cores and other slipformed building elements. However, it would be desirable for the designer and builder to review tolerances and verify the specified values are appropriate for the intended use of the structure. This will facilitate achieving the best speed and economy consistent with needed precision. Slipformed cores and shafts of buildings that tie into other adjacent parts of the building should meet the same tolerances for plumbness as those for building walls.”

Here's a suggested course of action for you and your formwork designer. ACI Committee 117 has published a little-known document called ACI 117.1R-14: Guide for Tolerance Compatibility in Concrete Construction. This comprehensive guide document compares the concrete tolerances specified in ACI 117-10 with tolerances used by most of the follow-on building trades, including elevators, stairs, doors, windows, curtain wall, precast concrete, CMU, stone panels, tile flooring, wood flooring and so on. There is also guidance given regarding the compatibility of ACI 117 concrete tolerances with requirements specified in (and mandated by) the Americans With Disabilities Act and Architectural Barriers Act Accessibility Guidelines (ADA/AB-AG). Have your formwork designer review ACI 117.1R Section 5.2--Elevator cores and hoistways, including Table 5.2.1.1, which compares tolerances followed by the National Elevator Industry, Inc. (NEII-1) and tolerances specified in ACI 117-10, including slipformed core walls. Any questions should then be followed up via RFI (preferred), questions asked on the formwork drawing submittal (semi-acceptable) or items to table at a preconstruction meeting.

(N.B.: Preconstruction tolerance meetings among contractors are mandatory (ACI 117-10, Section 1.1.3). The owner and/or design team cannot be forced to attend. Preconstruction tolerance compatibility meetings to discuss elevator embeds, shaft sizes, pit and sump sizes, and slipforming (if actually used) are similarly suggested in ACI 117.1R-14, Section 5.2.3--Construction Strategies. Suggested attendees include the concrete contractor, construction manager, elevator manufacturer, and elevator installer. Once again, the owner and/or design team cannot be required to attend. ACI 301-20, Section 1.6--Preconstruction conference is the one ACI document that actually does specify mandatory attendance by the Architect/Engineer, owner, or owner's representative but only if the meeting itself is specified. No matter which way you slice it, ASCC contractors should make every effort to seize the opportunity to collaborate with the design team and discuss not only tolerances, but the project in general.

For more information on tolerance compatibility, have a look at ASCC Position Statement #6: Division 3 versus Division 9 Floor Flatness Tolerances and ASCC Position Statement #18: Concrete Tolerance Coordination.

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Question: We placed a slab on grade about a year ago at a condominium project. Thanks to the information contained in ASCC Position Statement #7 "Birdbaths on Concrete Slabs,” we were able to educate the Owner about the birdbath phenomenon. Now the Owner is saying the tenants are complaining the birdbaths are not evaporating fast enough and are headed back to meet with Judge Wapner. We cannot find anything in ACI 302 that addresses birdbath evaporation. Are there any industry guidelines that can help get us out of hot water with this Owner?

Answer: Don't throw the towel in yet. The industry words of encouragement, the ASCC Technical Division can offer come to us from the National Association of Home Builders (NAHB) "Residential Construction Performance Guidelines Consumer Reference, Fifth Edition.” In this document, standing water is covered in Section 12-4-2 Driveways and Sidewalks as follows: "Standing water greater than 3/8 inch in depth should not remain on the surface 24 hours after a rain.” In Section 12-2-2 Concrete Stoops and Steps, the NAHB verdict is "minor amounts of water can be expected to remain on stoops and steps for up to 24 hours after rain.” So, let us call it a day. This witness may step down, your honor.

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Profound thanks from the ASCC Technical Division are hereby extended to ASCC member Oscar Antommattei (Kiewit) for both organizing and hosting the special Technical Symposium held at the ACI Spring Convention last week in San Francisco honoring our very own Dr. Bruce Suprenant. This all-day session featured 21 speakers paying tribute to Bruce in their own way for generously using his technical expertise to educate us all and save us piles of cash and prestige in the process. ACI has somehow recorded the presentations, and the VOICE will let you know when the session tapes become available on the ACI web page.

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Congratulations are hereby extended to ASCC member Lingfeng "Leo" Zhang (Conco) for his recent appointment as chairman of ACI/ASCC Joint Subcommittee 117-L: Laser Scanning. Leo participated in the ASCC laser scanning study described in the Jan. 2019 and Feb. 2020 editions of ACI's Concrete International Magazine (CI), and then followed up as lead author and field investigator with colleagues Suprenant and Klinger in three laser scanning articles published in CI in 2022: "F-numbers and Textured Concrete Surface Finishes" (May 2022), "Slab-on-Ground Thickness Measurement" (July 2022) and "Presenting Laser Scan Results for Slabs-on-Ground" (September 2022). Way to go, Leo!