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Elevated concrete slab cracking[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaint(--nospam--at)seaint.org>
- Subject: Elevated concrete slab cracking
- From: "Albert J. Meyer, Jr." <meyera(--nospam--at)bellatlantic.net>
- Date: Sun, 9 Mar 2003 21:12:29 -0500
I observed a condition in a slab this past Friday that I have not seen before. The slab is an elevated 2 3/4" concrete slab with 6x6-W2.0xW2.0 WWR. The specified design strength for the slab is 3500 psi. The slab is supported by and acting compositely with 14" Hambro joists that are typically spaced at 4'-1 1/4" on center.
For those not familiar with the Hambro system it consists of steel joists which consist of a continuous bent round rod web member that is welded to a double angle bottom chord and a profiled top chord with a cold rolled "S" shaped top portion which is embedded into the slab and a vertical lower portion which is slotted to hold roll bars which serve to support plywood concrete forms.
The joist spacing in my case is 4'-1 1/4" on center. The joist spans in the area in question are about 26'-0". Cracks were observed over the centerline of the joists below but also at 12" on center typically in the field of the slab parallel and perpendicular to the joist span. Some locations exhibited cracks at 6" on center as well. The cracks run exactly parallel and perpendicular to the joists at 90 degrees to one another and appear to be following the wires in the WWR.
Most of the cracks were observed to be full depth through the slab. When the slab is viewed from below, lines of efflorescence matching the crack locations were observed. Crack widths varied from hairline to approximately 1/32". Surface spalls along the cracklines up to 1/8" wide approximately 1/32" deep were also observed.
The slab between joists had midpoint deflections varying from 0" to 5/16" when measured perpendicular to the joist span. Measurements were taken using a 4 foot level placed from the centerline of one joist to the adjacent joist and measuring the gap with a tape measure. All joist chord and web members appeared to be in good condition with no observed distortion. All welds of chords to web members appeared to be good.
Although I don't currently have any information on the concrete tests or the weather at the time of slab placement (9/2002), I will be obtaining it in the next few days. I have done structural framing inspections a number of the buildings in this development and all of the buildings of this type. This is the fifth building of this type and I don't recall seeing any significant cracking in any floor slabs in the buildings constructed to date. The joist manufacturer also performed a site observation and has never seen this condition before.
The amount of construction dust and debris normally on these slabs prior to broom cleaning for framing inspections makes it likely that these cracks and deflections have existed for an extended period of time, but were just recently observed.
I suspect that some of the plywood forms used when this slab was placed were excessively wet. Since wet wood has lower strength and stiffness combined with a greater propensity to creep in wet conditions, under the weight of wet concrete the forms deflected and the concrete cured in the pre-deflected profiles I observed. The plywood forms normally used are 3/8" to 5/8" thick. I don't know at this point what was used for this slab placement. Roll bars at 4'-0" on center are normally used to support the plywood forms.
My suspicion seems reasonable, but the cracking which is following the wires of the WWR is baffling me. The slab was cored in eleven locations and although I don't yet have test information, according to the construction manager the f'c values of the concrete were generally around 5000 psi.
I'd appreciate anyone's thoughts and opinions on this observed condition. Thanks in advance for your help with this.
Albert J. Meyer, Jr., P.E.
Cagley, Harman and Associates
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