Jim Klinger, Concrete Construction Specialist The Voice Newsletter November 2023

Question: We are building a large, 6-story reinforced concrete parking structure with a footprint of 113,832 gross square feet (GSF). Typical slab thickness is 7 inches, post-tensioned (PT), normal weight concrete (NWC).

The building geometry is such that there are very few level slabs; typically, those are located only at stair landings and elevator entrance areas. All other floor slabs are either ramps or parking slabs sloped to area drains.

On past projects, we ended up getting burned because we let things sit. So, expensive lessons were learned. On this job, our field crew has stayed on top of work items typically saved for end-of-job punch lists; mostly patching, cleanup and so on. And therein lies our problem.

Despite our efforts to maintain a "zero punch list" job, we were very surprised to learn that our latest monthly progress payment application has been called into question by the Owner. During a recent walk-through by the County inspector, several slab soffits were flagged due to inadequate headroom. The Owner, and now the general contractor (GC) have each taken the position that any apparent headroom problems cited in County inspection reports are solely ours to solve.

We surveyed the soffits in question and agree that the as-built clearance dimensions fall shy of those required by the County parking structure building ordinance. But our as-built survey also indicates all the slabs (soffits and tops) were placed to elevations clearly indicated in the construction documents and well within ACI 117 location tolerances.

We have always prepared our formwork and concrete outline drawings in-house, using detailers who have been trained to check for headroom clearances and generate preconstruction requests for information (RFI’s) if needed. But due to time constraints and the sheer size of the project, we farmed the shop drawing preparation scope out to overseas detailers, who were not instructed to perform a routine headroom check. Unfortunately, we missed this when we reviewed the outside shop drawings before we sent them up the food chain. We do not believe we are in any way responsible for meeting County code requirements when we built to design dimensions shown in our construction documents. Please advise.

Answer: You are correct. Although it is always a good idea for your detailers to perform in-house headroom checks while preparing submittals, it is not your responsibility to flush out and correct design dimensions or details that could potentially conflict with building codes or ordinances. There are many ways to prove that and here are two solid industry references for your use.

The first reference applies to the concrete trade and is found in Chapter 26 of ACI 318-19: Building Code Requirements for Structural Concrete. In Commentary section R26.1-Scope, we read the following:

"This chapter is directed to the licensed design professional responsible for incorporating project requirements into the construction documents. The construction documents should contain all the necessary design and construction requirements for the contractor to achieve compliance with the Code. It is not intended that the Contractor will need to read and interpret the Code. A general reference in the construction documents requiring compliance with this Code is to be avoided because the Contractor is rarely in a position to accept responsibility for design details or construction requirements that depend on detailed knowledge of the design. References to specific Code provisions should be avoided as well because it is the intention of the Code that all necessary provisions be included in the construction documents".

The second reference applies to all construction trades, and is found in the 2021 International Building Code (IBC), section 107.2.1: Information on construction documents as follows:

"Construction documents shall be of sufficient clarity to indicate the location, nature and extent of the work proposed and show in detail that it will conform to the provisions of this code and relevant laws, ordinances, rules and regulations, as determined by the Building Official.”

The Commentary to IBC section 107.2.1 explains further: "General statements on the documents, such as "all work must comply with the International Building Code" are not an acceptable substitute for showing the required information."

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Question: I am a structural design engineer practicing in a large metropolitan area. I am not an ASCC member, but I have a question regarding stud rails, which was the topic in the "Concrete Q&A" column titled "Placement of Headed Shear Stud Reinforcement" that appeared in the January 2022 issue of ACI Concrete International magazine. I just received an urgent call from the Owner's inspector at the jobsite. Here it is, the end of the workday, and the concrete contractor is scheduled to place slab concrete first thing tomorrow morning. During today's inspection, it was revealed that there are stud rails that were not installed at a few columns when the bottom reinforcing steel layers were placed due to supply-chain issues. The contractor has now installed the missing stud rails in the "upside-down" configuration described in the ACI Q&A. I have no problem with that, since the stud itself does not care which end is up. In the ACI Q&A Fig. 1, however, the exact configuration that the contractor has placed is not depicted e.g., an upside-down assembly with only one forged stud anchor head (at the bottom) and no head at the top (but shop-welded to a continuous steel base rail). The project inspector is saying that, since the work in place does not match ACI Q&A Fig.1 exactly, the installed assembly is questionable and is looking to us to provide engineering approval before concrete can be placed. Please advise.

Answer: We must assume here that the stud rail assemblies in question were inspected in the fabrication shop and all materials and welding were found to be in compliance with the requirements given in ASTM A 1044: Standard Specification for Steel Stud Assemblies for Shear Reinforcement of Concrete.

According to ACI 421.1R-20: Guide for Shear Reinforcement for Slabs section 1.1, "To be fully effective, the anchorage should be capable of developing the specified yield strength of the studs. The mechanical anchorage can be obtained by heads or strips connected to the studs by welding. The heads can also be formed by forging the stud ends."

In other words, a headless stud end welded to the steel base rail is a dual-purpose mechanical anchorage and spacer that is interchangeable with an anchor head forged in the manufacturing process. It appears, then, that the stud rail assembly installed in the field should be acceptable as long as the quality control inspection requirements described above have been met.

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Question: We are preparing a bid proposal for a large steel structure featuring 26 floors worth of composite concrete slab on metal deck (SOMD). This will be a hotly contested bid among the competitors here in our local market, partly due to the Owner's reputation for disbursing progress and retention payments in a timely manner. Since the building footprint is approximately 44,000 GSF, we estimate we can complete each floor in two placements of 22,000 SF each, which translates into two 375 cubic yards (CY) pours per floor. The construction documents indicate the steel beam and metal deck soffits are to receive a follow-on spray-on fireproofing application. The metal deck specifications call for the metal deck to be vented. These two details, taken in combination, represent potentially significant cost items that could make our price non-competitive if we include them. How should we approach this in our estimate and proposal letter, so we don't get left holding the bag?

Answer: Even though this is a private job, we believe the best course of action is to write a pre-bid RFI that (in theory, anyway) should be answered by the design team and distributed via a Bulletin to all bidders and stakeholders before bid day.

Assuming you stick to your 52-pour placement plan, you could be looking at up to 104-man days’ worth of spraying the soffits and steel beam substrate with water and dragging visqueen around underneath the slab area to catch the drippings during and after each pour. You can also guarantee receiving that friendly call or email from the GC telling you that you missed a few spots and now the fire proofer is refusing to install his work until you rent a scissor lift and commence to scrubbing cement paste that leaked through the vents in the metal deck.

When you craft your pre-bid RFI, attach a copy of Technical Note TN.4 Venting of Composite Steel Floor Deck prepared by the Steel Deck Institute, which explains why some specifiers erroneously believe letting the cement paste and water slurry drip out of the bottom of structural decks will help the concrete dry out faster. It doesn't. Advise the design team that--if the deck venting really is desired--the Owner should carry a budget allowance to cover all labor and equipment costs associated with cleanup of the leakage. This way, all bidders will be pricing that portion of the work on a level playing field. If the Owner requests a ballpark estimate to help establish the allowance amount, you could start by quoting what 104-man days plus water and equipment rental is worth, and that should discourage any unnecessary dollar leakage right then and there.

The Steel Deck Institute's Tech Note TN.4 is available at no cost.

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Question: We just returned to our trailer from the preconstruction "meet and greet" project kickoff meeting. Our formal concrete pre-pour conference is still a month out yet. Today's meeting included the Owner, the design team, the GC, and the inspection agency representative.

This project features a large footprint framed in structural steel with large expanses of LWC on a metal deck. Access to the jobsite is limited, and logistics dictate that we must stage our concrete pump at one end of the site and pump from there using up to 200 feet of pump hose system.

During the meeting, there was much discussion commensurate with a LWC SOMD scope...the importance of adequate amounts of lightweight aggregate pre-soaking, slab finish expectations, and whether or not the Owner wants to consider covering the premium cost for us to provide temporary steel beams in lieu of us pouring to a constant gauge slab thickness. All parties were on the same page until the inspection agency representative stated they would not allow the cement slurry we use to prime our pump line system to be incorporated into the pours. In other words, we would have to prime the pump and collect the slurry into debris boxes up on the metal decks- which opens a costly, logistical can of worms with potential safety concerns.

Until now, neither the structural engineer nor the Owner had even been aware of this. Now they have expressed concerns about potential side effects. Please advise.

Answer: Since your pre-pour conference is a month away, here is one suggested course of action: have your ready-mix supplier prepare an engineered pump primer mix design (as described below) with appropriate backup and submit a preview copy "to be discussed at the upcoming pre-pour conference." In the meantime, consider the following background information.

The concept of using a lubricating cementitious primer is described in several ACI documents with differing points of view. The Owner's inspector is probably basing his position on ACI 311.7-18 Specification for Inspection of Concrete Construction section 3.2.4, which instructs the concrete inspector to "Verify that grout used to lubricate the pump hose is not incorporated into the placement."

In the case of shotcrete, ACI 506R-16 Guide to Shotcrete section 4.3 tells us, "initially, the material hose will be lubricated with a primer. The primer lubricates the line ahead of the initial plug of concrete and can be a water cement slurry or prepackaged material. The nozzleman will direct the nozzle away from the work until the primer has been completely discharged."

On the other hand, a lubricating cementitious primer labeled by ACI as a "rich grout pre-charge" is described in ACI 543R-12: Guide to Design, Manufacture and Installation of Concrete Piles section 8.6.5.1 when using conventional concrete as follows: "it is frequently specified that a small batch of rich grout (generally one part cement and two parts concrete sand and water) be placed in the pile immediately before the concrete placement. The purpose of the grout is to partially precoat the pile sides and reinforcement with a mortar mixture and supply a charge of rich cement grout to the top of the pile to counteract the segregation of coarse aggregate at the pile tip during the initial charge of concrete."

Translation: It is perfectly fine to prime the cylindrical sides of a structural concrete pile with a cementitious slurry and then leave the primer in place as part of the finished work.

On one recent and noteworthy high-rise project, a pump primer mix design was proportioned similar to the ACI 543R pile slurry mix described above-12 sacks of ASTM C150 cement (1128 lbs.) and 2390 lbs. of ASTM C33 concrete sand- essentially in a 2-to-1 proportion. For water, 50 gallons was proportioned to provide a slump range of 8 to 11 inches, which makes the slurry easy to spread out over a large area of the deck. Trial batches of the pump primer yielded 28-day compressive strength of 7520 psi and 56-day compressive strength of 8250 psi. We rarely see LWC specified for use in composite metal decks with design strength above 6000 psi. Preparing and submitting an engineered primer mix design with appropriate backup should go a long way toward convincing the structural engineer to accept the pump primer incorporated into the work.