cracking in waffle slab
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Cracking in Waffle FloorWhat could account for the cracking in a waffle floor slab that occurred just before the concrete hardened? The cracks follow the web pattern and we find that they occurred where the section changes in thickness.
This sounds like plastic settlement cracking, which can occur in places where concrete is being differentially restrained in settlement during hardening. This phenomenon is sometimes seen over ties in narrow wall formwork, over reinforcement near the surface, or below the flared head of a column. Often nothing can be done to change the restraint, but something can be done to minimize the amount of bleeding. Anything that will improve the cohesion of the concrete until it has hardened will help, such as using an air-entraining agent or increasing the sand content of the mix. Prompt and efficient curing may minimize the problem. If plastic settlement cracking has already begun it can sometimes be overcome by revibrating the concrete after settlement, perhaps an hour or an hour and a half after placing. Usually plastic settlement cracks don't extend all the way through the section and destroy the structural integrity, but it is usually desirable to seal the cracks to prevent corrosion of the steel.
From: Jerry W. KingTo: seaoc(--nospam--at)seaoc.orgSubject: cracking in waffle slabDate: Monday, January 27, 1997 2:09PM
I am investigating an existing waffle slab that hasnumerous cracks visible from below. The structurewas built in the late 1950's.............
The cracks appear to emanate from the corners ofthe solid heads, and mostly run transversely acrossthe width of the structure. There are also numerouscracks at the interface between the joist and the slabwhen viewed from below. Unfortunately, most of thetop slab is covered with asbestos tile, so we cannotsee the crack patterns from above.
The slab is surrounded on all sides by concretefoundation walls. It is 3 bays @ 21' wide and 5 bays@ 21' long. There is a steel framed structure abovewith moment frames that are supported on thefoundation walls. The structure is located in Fremont,CA. An earlier report indicated that the cracks weredue to seismic activity.
I have had "intimate" and long-term experience with several waffle slaband concrete shear wall buildings constructed in the mid-to-late 1950's. Cracking similar to that which you have described was seen in all(pre-earthquake). These structures are located in the near-field area ofthe 1994 Northridge Earthquake. The waffle slabs cracked across themidpoint of the long direction of the building plan. Typically, thecracks are widest at the top and nearly disappear at the bottom of thewebs (joists). These cracks do not penetrate the heavier edge beamsaround the plan perimeter. It has been long concluded that the cracksare almost entirely due to stresses developed from curing shrinkage.The largest cracks showed some ravelling after the earthquakes, but thecrack width was unchanged. They were all repaired by epoxy injection,
along with the "new" shear wall cracks which were seen onlypost-earthquake.
To help solve the puzzle, look at the shear walls below the slab.Often, the slab shrinkage will generate sufficient forces to open lightdiagonal cracks in the walls near the building corners. Cracks will bemost prominent in the building's long direction. The cracks will slopeupward towards the corners.
If you wish to discuss further, contact me at:
M. Russell Nester, SE, GErnester(--nospam--at)juno.com_________________________________________________________________________________________