Presentation on theme: "The tolerances for suspended concrete slabs on metal deck are often misunderstood."— Presentation transcript:
The tolerances for suspended concrete slabs on metal deck are often misunderstood.
ACI “Standard Specifications for Tolerances for Concrete Construction and Materials” gives (2) tolerances for suspended slabs on metal deck: Thickness tolerance: +3/8” and -1/4” for slab thicknesses 12” or less” Flatness tolerance: Ff or gap under an unleveled straightedge
There is no level alignment tolerance specified for suspended slabs. The levelness tolerance Fl does not apply to slabs placed on a supporting structure that is unshored, cambered, or inclined. Levelness depends on the location and placement of the supporting structure and the deflection of the structure after the concrete is placed. The concrete contractor has no control of how the structure responds or how it was designed.
The terms FLAT and LEVEL, as they relate to slab tolerances, are also not understood properly. Flat refers to the smoothness of the top level of the slab. The Ff number is an indicator of how well the finisher – cement mason – worked the surface of the concrete slab. Level refers to how close the floor surface is to horizontal. The Fl number indicates how well the carpenters set the construction joints – side forms – and how well the cement masons / laborers struck off the concrete. The Fl number has nothing to do with the finisher’s workmanship.
Prior to beginning superstructure concrete slabs on metal deck, we require a pre-concrete conference. This should be mandatory for all concrete projects. In fact, there should be (2) or (3) opportunities for a conference: prior to submitting concrete mix designs prior to beginning foundation work prior to beginning superstructure work
The pre-concrete conference allows all members of the construction team to discuss tolerances, review construction options, and choose solutions before problems occur. That’s the best time to air disputes and reach agreement about floor levelness issues.
At the conference, we request the as-built survey of the structural steel placement. We check the survey and verify / confirm elevations of primary girders and pour stops – especially if the steel is intentionally cambered. You see, the AISC Manual of Steel Construction allows for a ½” variation in camber tolerance. There’s also the concern for “unintentional camber”. AISC again permits the natural mill camber for a standard 30ft beam to be as much as ½” at midspan. Ideally, the concrete contractor should be able to place a level slab of uniform thickness (as designed) and the result would be consistent with the EOR design. Given the tolerance of the steel fabrication and the tolerance of the steel erection, however, it is not feasible to pour a slab to a constant thickness.
At the conference, we also request the design deflections for the slabs on metal deck under dead load and construction live loads. We rarely get this information. Our rationale is to establish a correlation between the deflection and the Ff/Fl tolerance - the greater the deflection the lower the Fl results. It’s that simple. Most of the time, the owner is concerned with the rolling of the slab, evident as you look down long corridors. They task the EOR to design an efficient structure = cost effective = flexible design = low Ff /Fl numbers = rolling corridors.
The design team has an innovative way of handling the discrepancy – they pass the liability to the contractor. They provide language in the Specification for the project requiring an Fl tolerance consistent with a level floor. The issue is that they haven’t designed the structure rigid enough to provide a level floor. … & there lies the dilemma …
The slab surface tolerance requirements in the specification for CONCRETE is significantly different than that for FLOORING. The concrete contractors will bid and pour floors to a specified American Concrete Institute (ACI) floor flatness (Ff) requirement under Division 3 of the project specification. The flooring contractors bid and install the specified finish in accord with in plane surface tolerances under Division 9 of the project specification.
Even if the concrete contractor pours the concrete and finishes it to the specified floor flatness requirement, the Ff must be measured within 72 hours of the concrete pour. The concrete, however, will continue to undergo change (e.g. shrinkage, curling, creep, etc…) over time. In other words, even if the concrete slab meets Ff requirements within 3 days after the pour, by the time the flooring contractor mobilizes on site additional surface preparation is likely required. The designer must consider the effects of Ff change and how that change affects the ability of the flooring contractor to install the specified finish. The EOR should include an allowance for surface preparation in the specifications.
The design team, knowing the inherent shortfalls of their design puts the onus on the contractor to adhere to a tolerance better than what was designed. How is that possible, you ask? It’s not! Ironically that means the only contractor likely to get work on a project with this specification is the one that doesn’t understand the issue. The contractor should have a reasonable expectation that the EOR has reviewed the design of the new structure for serviceability – deflection, cracking, etc…, as well as strength.
On our projects, we have the pre-concrete conference, we request the deflection information from the designer, and we prepare a plan showing the extent of all concrete pours (identifying the construction joints) and generally try to involve the construction team in our thought process. We confirm the structural steel contractor’s survey and document the as-built condition of the steel.
Initially, we place the concrete slab to an elevation, consistent with the required tolerances. After the pour, we perform a survey of the top of slab as well as the bottom of structural steel. We review the results with the construction team and if necessary make adjustments in slab thicknesses on subsequent floors. It’s an iterative process and requires the efforts of ALL the construction team.
Unfortunately, it’s rare to get this type of involvement from the construction team. Most times they’re more concerned with mitigating their individual risk. It’s through the pre-concrete conference that the contractors, design team, and construction managers – the construction team – can build trust and learn to work together. It is our responsibility – collectively - to manage our client’s expectations!