Jim Klinger, Concrete Construction Specialist, The Voice Newsletter, May 2021
Here at your ASCC Technical Division, much of our work product is created in response to concrete construction substrate acceptance issues disclosed by members via hotline calls or prompted by Email Forum posts.
Sometimes, for example, a concrete floor slab substrate has been deemed unacceptable by a follow-on subcontractor; a flooring installer perhaps. (Whether or not that decision has been rendered to the member in a timely manner might turn out to be a borderline questionable proposition).
At other times, a member might request an interpretation- a second opinion- of ambiguous construction document provisions before submitting a bid. This consultation is requested so the member’s bid can be crafted to anticipate and then meet the owner’s substrate expectations at a mutually agreeable price.
And sometimes the member’s substrate dilemma is totally self-inflicted. In these cases, we attempt to develop a sensible substrate repair plan or even help draft a request for the owner to accept the as-built; arguing that the member’s installation is in a fit-for-purpose, acceptable condition.
The scenarios described above primarily involve acceptance of our completed substrate work by others. On the other hand, at least once on every project, we-- the concrete contractor-- are placed in the position of having to accept a substrate that has either been installed or prepared by others before our work can begin. Based on experience, it’s fair to say that many difficulties surround the mechanics of accepting substrate installed by others.
Below are a few examples that illustrate various substrate issues that have recently come across the transom which feature acceptance--by the concrete contractor—of another contractor’s work product. And, as any veteran concrete contractor can tell you, one of the most hazardous features of a double-edged sword is that it is an equal-opportunity blade that can slice both ways.
Concrete Mat Slab Foundation (Earthwork Substrate)
• Tolerances for top of subgrade substrate elevation are typically found in Division 31 specifications (formerly Division 2), civil/shoring drawings or the project geotechnical report. Rule of thumb: rough grade tolerance of plus or minus 0.1 ft., assuming a mud or working slab will be installed on top of the prepared substrate. In rare cases where the mat is placed directly on top of the subgrade, the plus tolerance would be 0.06 ft (the ACI 117 slab thickness tolerance of minus ¾ inch would govern).
• Prior to accepting the earthwork substrate, insist on receiving a licensed surveyor’s certification drawing confirming the pad has been graded within tolerance, including at all contours, slopes and elevation changes. Also, request certification that the geotechnical engineer has signed off on the pad substrate preparation before taking ownership and starting concrete work.
• For pad substrates with constant elevation, spot-checking can easily be self-performed using typical survey layout instruments. Access to a laser scanner is another efficient way to check the quality of the substrate preparation and, if needed, to run a reasonably accurate volume calculation for actual concrete quantity required for the mud slab. If the scan indicates the substrate is low and will require extra concrete, suggest submitting the scan results to the GC in a timely manner for review by the earthwork sub and/or the surveyor.
Perimeter Sheeting and Shoring (Soldier Pile/Lagging Substrate)
• This condition assumes there are several levels below grade, and that concrete foundation walls are either CIP or shot against a waterproofing membrane attached to the shoring substrate.
• Tolerances for placement of the shoring system are typically included in the general drawing notes prepared by the shoring specialty engineer. For soldier piles, assume a tolerance of 2 inches (away from the structure) and zero tolerance for mislocated substrate encroaching on the nominal design thickness of the structural concrete foundation walls.
• Similar to the earthwork substrate, insist on a certified survey confirming the as-built location of the shoring system before starting work. In cases where the shoring system substrate encroaches on the wall thickness, the structural engineer will need to know straightaway. Changes to the wall reinforcement, concrete strength, or formwork may be required by the LDP. Relocation of the substrate is generally not an option.
• Attempting to spot-check several levels of shoring substrate behind the surveyor is typically impractical to self-perform. This is another time when a laser scanner can quickly determine if the substrate is installed correctly, and-- if needed-- can quickly run a volume calculation for actual concrete quantities required to place the foundation concrete walls.
• In most cases where several levels of substrate below grade require waterproofing, the waterproofing consultant should also certify the waterproofing installation is acceptable before accepting the substrate and starting the concrete scope.
Structural Steel Buildings (Metal Deck Substrate)
• According to ACI 117-10, section 4.4.1, there is actually no tolerance requirement for location of top of concrete slab placed on a metal deck substrate. This is one work scope where almost all of the slab formwork typically installed by the concrete contractor (e.g. soffits and slab edges) are now installed in sheet metal by others. Responsibility and risk associated with tolerances for slab edge locations, opening sizes/locations and other substrate features are also shifted outside of the concrete contractor’s scope.
• In some cases, the timing of acceptance of a metal deck substrate by the concrete contractor can come into play, especially in the case of multi-story buildings. Consider the case of the ASCC member who bid a multi-story deckfill pump-place-finish project assuming the floors would be placed using a 25 percent flyash mix during the summer. After schedule delays pushed the work into the winter, the member’s labor costs to install the work increased dramatically. The cold, plus the fact that the decks below roof level were shaded from direct sunlight, meant longer than anticipated crew days. In this case, the contractor accepted the substrate, but used ASCC Position Statement #15 “Setting Time Expectations for Hard-Troweled Concrete” to substantiate his request for change order to cover costs associated with crew overtime and accelerating admixture.
• Another metal deck substrate booby trap is one that actually needs to be neutralized during bid time, long in advance of the work. This one involves design call-outs in many project specifications requiring metal deck panels to be vented, presumably with the idea that allowing mix water to drip through the deck somehow enhances the application of follow-on flooring. In a Position Statement issued by the Steel Deck Institute, the SDI explains that--in most cases-- vented deck is not required, and slabs on metal deck should be considered similar to slabs-on-grade placed on vapor barrier when assessing potential moisture issues. For the concrete contractor, vented deck adds labor costs to the project for clean-up during and after placements. It is in the owner’s best interest for the concrete contractor to question the need for this particular substrate item during bid time.
Foundation Mat Reinforcing Steel (Waterproofing Substrate)
• On some projects, a protection slab or protection board is specified to protect a waterproofing membrane from damage during installation of reinforcing steel and other follow-on work installed by multiple trades. On other projects, the mat slab reinforcing steel is placed directly on top of the waterproofing membrane substrate. Despite all good intentions, damage to the membrane substrate inevitably occurs when tons of reinforcing steel bars are placed production-style on schedule-driven projects. Suggest writing a qualification clause in bid proposal letters that excludes the cost of such “trade damage” and instead request the owner carry an allowance to cover repairs of incidental damage to the waterproofing membrane substrate. If this approach is unsuccessful, suggest photographing the entire surface of the waterproofing membrane prior to accepting the substrate and then carefully monitoring the work of all trades (rebar, electrical, plumbing) who can possibly contribute trade damage. Since the backcharge costs to repair waterproofing substrate can quickly escalate, this topic should be discussed with all parties—including the waterproofing inspector-- during the preconstruction conference and before the concrete contractor accepts the substrate.