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Concrete Slab Deflection...What?

Recently while researching for information for a current Building Forensics International assignment, I came across a report from an old condominium complex project which had blog fodder written all over it. The project is a pretty simple subterranean parking garage podium slab structure on circular concrete columns with two layers of mild deformed steel reinforcing supporting three stories of wood framing for residential use. Some of the enclosing walls of the subterranean parking were concrete but most were reinforced and fully grouted, waterproofed, concrete masonry units, or CMU construction. Excessive deflection and cracking started the inquiry, but radially pointing cracking outward from the column tops heightened the investigators’ concerns. So here is some food for thought from the original report about concrete slab deflection as it relates to

concrete cracking. The following summarizes portions of the original report:

Initially, a structural engineer performed calculations and a deflection analysis. He recommended invasive testing based on his findings that the actual deflection exceeded the amount expected by the design and that an investigation was justified by the excessive deflection.

The field work generally revealed that virtually all of the upper bars of the concrete slab were lowered than designed. Those misplaced bars were consistently placed too low in the slab, leaving the structure with less flexural and shear strength than had the bars been placed in accordance with the construction documents. This was also found to be responsible for the excessive deflection. Other reinforcing steel was found to be misplaced as well. Concrete cover over the top bars was found to be in violation of contract document provisions and outside of industry tolerances articulated by ACI 117. The average concrete slab thickness was generally found to be in conformance with the plans and specifications, though one core was found to be too thick, also in violation of contract requirements which could have exacerbated to the concrete slab deflection. The concrete itself was found to also be suspect, in that its compressive strength was very low and not in compliance, it displayed an excessive amount of air. Furthermore, the amount of cracking in the elevated concrete slab was found to be excessive. These symptoms suggest that there was an excessive amount of water used, either as planned in the mix, as permitted, or from excessive water additions to the truck after batching.

Four full depth (complete thickness) cores, each of 4” diameter, were taken from the elevated concrete slab. Test were performed on each of the concrete core samples in accordance with ASTM C 174-97, Standard Test Method for Measuring the Thickness of Concrete Elements Using Drilled Concrete Cores. One of the four core samples tested in excess of acceptable tolerance for length. When the slab is thicker than intended, it is also heavier, with additional deflection possible due to the excess weight.

Above the podium deck, conventional wood framing was used. The anchor bolts securing the sill plates to the deck appeared loose. While some were loose from original construction, and some were loose from wood shrinkage of the sill, it appears that the deflection of the floor exceeded the amount of deformation in the sill plates over the passage of time, also contributing to the loosening of the nuts from the anchor bolts.

Based on ACI 117 it was determined during the investigation that cover for the top bars was consistently found to exceed permissible tolerances.

Concrete slab cracking was excessive throughout all surfaces examined, and was particularly excessive in the five specific areas of examination. In contrast, the short spans of the parking area supported by the CMU (Concrete Masonry Unit) wall demonstrated an occurrence of width and frequency of cracking judged to be typical for this type of structure.

It was determined as a result of this investigation that many, if not most of the cracks were due to a combination of drying shrinkage, restraint, flexural stress due to self-weight, applied loading and deflection. The extent of cracking, defined by; width, length, number or pattern of cracking is significant, and greater than what would be expected to be typical for this sort of reinforced concrete construction.

Finally, Compressive Strength of the concrete slab was tested and the findings based on ACI 318 found the concrete did not comply with project requirements for strength. It was recommended that a petrographic examination be performed to measure air content (ASTM C 457). The client chose not to have this test performed. It is likely that the concrete strength, air content and shrinkage are related and lead to at least one, and potentially more, common causes. Excessive water content of the concrete may link these manifestations. This excessive water content could have been due to the provisions of an improper mix design, or due to excessive water batched, or due to excessive water added to the truck prior to discharge of the load, or due to water additions by the pumper. Also, there is the possibility that the incorrect mix was ordered or supplied by the ready mix provider.

Recommendations for strengthening the structure were made, and the owners assumed responsibility for upgrading their structure.

Like I stated in the opening of this blog, “lots of fodder” to consider, but I wanted to share an example of the comprehensive work Building Forensics International is capable of providing clients who find their concrete slab flexing too much or find cracks in their concrete and don’t know why.

BFI Staff Writer -

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