The California Department of Water Resources has released a technical memorandum laying out its detailed explanation for widespread cracking that has appeared in concrete placed this year during the Oroville Dam spillway rebuilding project.
The report, embedded below, recounts measures taken to prevent cracking in the massive slabs intended to serve as the final, finished concrete surface in two large sections of the spillway. The document was compiled after an Oct. 2 Federal Energy Regulatory Commission letter asked DWR to explain the cracks and document their extent.
The primary factor in the cracking, the report suggests, is due to "the highly restrained nature" of the spillway's concrete slab panels, which are anchored to subsurface rock.
Some of the cracking occurred, the memorandum says, because the contractor on the massive project was slow to begin "water cure" of the newly placed concrete or the water cure process was disrupted. "Water cure" is a process in which newly installed concrete is kept moist -- for instance, by laying wet fabric on a just-finished surface -- to prevent rapid evaporation and allow the concrete to mature fully.
The memo gives a rough idea of the extent of the cracking based on preliminary mapping and evaluation of the spillway's roughly 228 or so newly installed concrete slabs.
The memo says about 75 percent of the 37-by-30-foot slab panels mapped and evaluated as of Nov. 2 contained between 60 to 150 linear feet of cracks. The cracks were .10 of a millimeter wide or less in about half of those panels and between .10 and .20 of a millimeter in the other half. (For those of us wedded to English measurements, .10 millimeter is about 4 one-thousands of an inch.)
About one in 10 of the panels showed more extensive cracking -- more than 150 linear feet and crack widths of .20 to .35 millimeter.
Kiewit Infrastructure West, the general contractor on the $500 million job, began pouring the heavily reinforced, erosion-resistant-concrete slabs in early August. The new concrete was placed in two areas of the 3,000-foot-long spillway -- a roughly 350-foot section at the bottom of the chute and an 870-foot section near the top.
The remainder of the spillway -- a 730-foot length of old concrete that crews patched this year and a 1,000-foot expanse in the structure's midsection -- will be finished next year.
The memorandum outlines three factors that likely played a role in forming the spillway cracks:
- Autogenous shrinkage due to restraint of the slab panels by a thick layer of underlying "leveling" concrete and the use of slab anchors. The document says that most of the cracking appears to be aligned with the anchors -- heavy steel rods drilled through the leveling concrete and grouted into place.
- Thermal shrinkage due to temperature differentials in the curing concrete. While not a primary cause of cracking, the memorandum says, the differential is "likely a contributor of the internal stresses, which caused cracking to occur."
- Plastic shrinkage due to imperfect water cure management. "Several instances occurred where the contractor did not immediately install water cure on the panel following the finishing efforts, as well as instances where the water cure was terminated or disrupted by other onsite operations." The memorandum notes that this type of cracking could pose problems down the road: "Plastic shrinkage cracks begin as shallow cracks but can become full-depth cracks later in the life of the concrete."
The full text of the DWR technical memorandum is below: