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3.08 CURING AND PROTECTION A. Protect all concrete work against injury from the elements and defacements of any nature during construction operations. Special curing procedures shall be implemented as described herein to minimize the cracking of concrete in water retaining structures. B. Concrete placed at air temperature below 40 degrees F shall have a minimum temperature of 60 degrees F. When the air temperature is below 40 degrees F and falling, the water and aggregates shall be heated before mixing. Accelerating chemicals shall not be used prevent freezing. All concrete shall be so protected that the temperature at the surface will not fall below 50 degrees F for at least 7 days after placing. The Contractor shall submit for approval by the Engineer the methods he proposes to use against low temperatures. No salt, manure, or other chemicals shall be used for protection. C. All concrete, particularly exposed surfaces, shall be treated immediately after concreting or cement finishing is completed to provide continuous moist curing above 50 degrees F for at least 7 days, regardless of the ambient air temperature. Walls and vertical surfaces may be covered with continuously saturated burlap, or other approved means; horizontal surfaces, slabs, etc., shall be ponded to a depth of 1/2 inch or kept continuously wet by use of wet burlap. 1. Slabs of water retaining structures shall be wet cured continuously with wet burlap or other approved means for a minimum of 14 days if Type II cement is used for 3 days if Type III cement is used. 2. Walls of water retaining structures shall have all their exposed surfaces covered from direct sunlight and forms left in place for a minimum of 3 days. Curing shall commence within four hours after concrete placement. D. In cold weather supplementary continuous warm curing (above 50 degrees F) shall provide a total of 350 -day degrees (i.e., 5 days 70 degrees F, etc.) of heat. E. In hot weather, concrete when deposited shall have a placing temperature which will not cause difficulty from loss of slump, flash set or formation of cold joints. In no case shall the temperature of concrete being placed exceed 90 degrees F. F. Finished surface and slabs shall be protected from the direct rays of the sun to prevent checking and crazing. 3.09 REMOVAL OF FORMS ' A. Except as otherwise specifically authorized by the Engineer, forms shall not be removed before the concrete has cured as specified above in subparagraph 3.08C and the concrete has attained a strength of at least 30 percent of the ultimate strength prescribed by the design, and not before ' reaching the following number of day- degrees (whichever is the longer): 06/06 03300 -11 03- 4138.0